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The immunophenotype of antigen presenting cells of the mononuclear phagocyte system in normal human liver – A systematic review

  • Otto Strauss
    Affiliations
    Department of Surgery, Faculty of Medical Health Sciences, University of Auckland, Auckland, New Zealand

    Maurice Wilkins Centre for Molecular Biodiscovery, University of Auckland, Auckland, New Zealand

    School of Biological Sciences, Faculty of Science, University of Auckland, Auckland, New Zealand
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  • P. Rod Dunbar
    Affiliations
    Maurice Wilkins Centre for Molecular Biodiscovery, University of Auckland, Auckland, New Zealand

    School of Biological Sciences, Faculty of Science, University of Auckland, Auckland, New Zealand
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  • Adam Bartlett
    Correspondence
    Corresponding author. Address: Department of Surgery, Faculty of Medical Health Sciences, University of Auckland, Level 12 Support Building, Auckland Public Hospital, Park Road, Auckland, New Zealand. Tel.: +64 2124 14647; fax: +64 9377 9656.
    Affiliations
    Department of Surgery, Faculty of Medical Health Sciences, University of Auckland, Auckland, New Zealand

    Maurice Wilkins Centre for Molecular Biodiscovery, University of Auckland, Auckland, New Zealand
    Search for articles by this author
  • Anthony Phillips
    Affiliations
    Department of Surgery, Faculty of Medical Health Sciences, University of Auckland, Auckland, New Zealand

    Maurice Wilkins Centre for Molecular Biodiscovery, University of Auckland, Auckland, New Zealand

    School of Biological Sciences, Faculty of Science, University of Auckland, Auckland, New Zealand
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Open AccessPublished:October 11, 2014DOI:https://doi.org/10.1016/j.jhep.2014.10.006

      Summary

      The mononuclear phagocytic system (MPS), comprised of monocytes, macrophages, and dendritic cells, is essential in tissue homeostasis and in determining the balance of the immune response through its role in antigen presentation. It has been identified as a therapeutic target in infectious disease, cancer, autoimmune disease and transplant rejection. Here, we review the current understanding of the immunophenotype and function of the MPS in normal human liver. Using well-defined selection criteria, a search of MEDLINE and EMBASE databases identified 76 appropriate studies. The majority (n = 67) described Kupffer cells (KCs), although the definition of KC differs between sources, and little data were available regarding their function. Only 10 papers looked at liver dendritic cells (DCs), and largely confirmed the presence of the major dendritic cell subsets identified in human blood. Monocytes were thoroughly characterized in four studies that utilized flow cytometry and fluorescent microscopy and highlighted their prominent role in liver homeostasis and displayed subtle differences from circulating monocytes. There was some limited evidence that liver DCs are tolerogenic but neither liver dendritic cell subsets nor macrophages have been thoroughly characterized, using either multi-colour flow cytometry or multi-parameter fluorescence microscopy. The lobular distribution of different subsets of liver MPS cells was also poorly described, and the ability to distinguish between passenger leukocytes and tissue resident cells remains limited. It was apparent that further research, using modern immunological techniques, is now required to accurately characterize the cells of the MPS in human liver.

      Abbreviations:

      MPS (mononuclear phagocytic system), KCs (Kupffer cells), DC (dendritic cells), APC (antigen presenting cells), MoDC (monocyte-derived dendritic cells), FACS (fluorescence activated cell sorting), HCC (hepatocellular carcinoma), HSC (haematopoietic stem cell), LMPP (lymphoid-primed multipotent progenitor), CMP (common myeloid progenitor), MDP (macrophage and DC progenitor), CDP (common dendritic cell precursor), pDC (plasmacytoid dendritic cell), LSEC (liver sinusoidal endothelial cell)

      Keywords

      Introduction

      The immune system is precisely balanced between immune activation and tolerance. Within this system antigen presenting cells (APCs) play a critical role in orchestrating the immune response [
      • Trinchieri G.
      Interleukin-12: a cytokine produced by antigen-presenting cells with immunoregulatory functions in the generation of T-helper cells type 1 and cytotoxic lymphocytes.
      ].
      In the normal liver the immunological balance is weighted towards a resting state of tolerance [
      • Crispe I.N.
      The liver as a lymphoid organ.
      ]. This immunological tolerance is seen in transplantation with a reduced rate of transplant rejection, even across MHC-disparate barriers [
      • Sanchez-Fueyo A.
      • Strom T.B.
      Immunologic basis of graft rejection and tolerance following transplantation of liver or other solid organs.
      ]. It is also evident in infectious disease or cancer, with the liver readily harbouring chronic diseases such as hepatitis C, and both primary and secondary malignancies [
      • Protzer U.
      • Maini M.K.
      • Knolle P.A.
      Living in the liver: hepatic infections.
      ]. Despite this functional tolerant state, the liver can still under certain circumstances, such as transplant rejection, induce a potent immune response [
      • Neil D.A.H.
      • Hübscher S.G.
      Current views on rejection pathology in liver transplantation.
      ].
      Animal studies show heterogeneous populations of liver APCs with varying functions that help to explain the liver’s tolerogenic state and have identified these cells as potential therapeutic targets. A thorough understanding of the APCs in human liver will be required to enable their therapeutic manipulation.
      Although a number of cell types within the liver have the potential to present antigens to T cells (broadly reviewed for animal and human liver in [
      • Crispe I.N.
      Liver antigen-presenting cells.
      ] and [
      • Thomson A.
      • Knolle P.
      Antigen-presenting cell function in the tolerogenic liver environment.
      ]) – including stellate cells (reviewed in [
      • Winau F.
      • Quack C.
      • Darmoise A.
      • Kaufmann S.
      Starring stellate cells in liver immunology.
      ]), endothelial cells (reviewed in [
      • Knolle P.A.
      • Limmer A.
      Control of immune responses by savenger liver endothelial cells.
      ]), and hepatocytes [
      • Thomson A.
      • Knolle P.
      Antigen-presenting cell function in the tolerogenic liver environment.
      ,
      • Holz L.E.
      • Warren A.
      • Le Couteur D.G.
      • Bowen D.G.
      • Bertolino P.
      CD8+ T cell tolerance following antigen recognition on hepatocytes.
      ] – the intrahepatic cells of the mononuclear phagocyte system (MPS) play a major role in determining the nature of the immune response [
      • Crispe I.N.
      Liver antigen-presenting cells.
      ,
      • Jenne C.N.
      • Kubes P.
      Immune surveillance by the liver.
      ]. This review therefore focused on APCs within the MPS of the human liver, Table 1.
      Table 1Liver APC subsets, function and areas requiring further research.
      Liver APCs can be divided into monocytes, macrophages, and dendritic cells. Monocytes are the most thoroughly investigated subset regarding accurate functional analysis and have a different proportional composition of monocyte subsets compared to blood. Macrophage heterogeneity remains undetermined and very little functional data exists for macrophages. Dendritic cells appear to represent the subsets found in blood but little functional data is available regarding these subsets.
      The MPS is composed of three major cells types – monocytes, macrophages, and dendritic cells (DCs) – although as a result of phenotypic and functional overlaps the precise boundaries, defining these groups, are not certain. A current theory regarding the ontogeny of the MPS, based on animal evidence, is summarized in Fig. 1.
      Figure thumbnail gr1
      Fig. 1Postulated ontogeny of liver antigen presenting subsets. Liver macrophages can broadly be defined as monocyte derived macrophages or self-replicating yolk-sac derived macrophages. The point at which a monocyte becomes a macrophage is not clearly defined. Liver dendritic cells (DCs) also contain a population of monocyte derived cells, these are important in inflammatory states. Liver DCs are also derived from immature blood dendritic cells that develop from a dendritic cell precursor. HSC, haematopoietic stem cell; LMPP, lymphoid-primed multipotent progenitor; CMP, common myeloid progenitor; MDP, macrophage and DC progenitor; CDP, common dendritic cell precursor; pDC, plasmacytoid dendritic cell.
      Compared to other cells in the body, those of the MPS appear to be superior at sampling their environment through phagocytosis, and presenting antigen to T cells, especially to CD4+ T cells via MHC class II molecules (HLA-DP, -DQ, and -DR) [
      • Geissmann F.
      • Gordon S.
      • Hume D.
      • Mowat A.
      • Randolph G.
      • et al.
      Unravelling mononuclear phagocyte heterogeneity.
      ]. APCs of the MPS appear to have a commensurately increased expression of antigen presentation and co-stimulatory molecules, and are potent secretors of modulatory cytokines [
      • Geissmann F.
      • Gordon S.
      • Hume D.
      • Mowat A.
      • Randolph G.
      • et al.
      Unravelling mononuclear phagocyte heterogeneity.
      ]. They are motile, and in other tissues have been shown to express chemokine receptors which facilitate their transit to draining lymph nodes, prime naïve T cells and establish a systemic immune response [
      • Ginhoux F.
      • Jung S.
      Monocytes and macrophages: developmental pathways and tissue homeostasis.
      ,
      • Jakubzick C.
      • Gautier E.L.
      • Gibbings S.L.
      • Sojka D.K.
      • Schlitzer A.
      • Johnson T.E.
      • et al.
      Minimal differentiation of classical monocytes as they survey steady-state tissues and transport antigen to lymph nodes.
      ,
      • Merad M.
      • Sathe P.
      • Helft J.
      • Miller J.
      • Mortha A.
      The dendritic cell lineage: ontogeny and function of dendritic cells and their subsets in the steady state and the inflamed setting.
      ]. It is for these reasons that cells of MPS have become attractive targets to manipulate for therapy, as well as monitor for changes in disease states [
      • Gadd V.L.
      • Melino M.
      • Roy S.
      • Horsfall L.
      • O’Rourke P.
      • Williams M.R.
      • et al.
      Portal, but not lobular, macrophages express matrix metalloproteinase-9: association with the ductular reaction and fibrosis in chronic hepatitis C.
      ].
      Figure thumbnail fx2

      Methods

      An electronic search was performed of the Medline and EMBASE databases from 1950 to July 2013 and 1980 to July 2013, respectively. Subject headings (MeSH) and truncated word searches were used for the following terms: [antigen present$, kupffer cell, macrophage, monocyte or dendritic cell] and [liver$ or hepat$]. Terms to incorporate the immunophenotype were based on methodology and included [histology, phenotype, immunophenotype, immunohistochemistry, flow cytometry or electron microscopy]. Studies were excluded if (i) the liver was diseased or transplanted, (ii) did not describe use of a normal human liver, or (iii) were not original research (systematic review, narrative review, commentary or editorial), Fig. 2. Articles were identified electronically using the above search strategy and eligible abstracts were screened manually by the primary reviewer (O. Strauss). Selected articles were retrieved and screened in depth for eligibility, and reference lists were manually checked for other potentially papers. Human studies focussing on diseased liver but also describing positive immunophenotypic findings of normal controls in liver tissue were included in the analysis. Duplicate studies were excluded and only articles published in the English language were included.

      Results

      Liver monocytes

      Subsets and phenotype

      Sinusoids of the liver contain circulating cells including monocytes. While some of these cells transit through the liver and return to the systemic circulation, others may adhere to the sinusoidal endothelium and ultimately differentiate into KCs [
      • Zimmermann H.
      • Trautwein C.
      • Tacke F.
      Functional role of monocytes and macrophages for the inflammatory response in acute liver injury.
      ].
      Four papers described the immunophenotype of liver monocytes [
      • Aspinall A.I.
      • Curbishley S.M.
      • Lalor P.F.
      • Weston C.J.
      • Blahova M.
      • Liaskou E.
      • et al.
      CX(3)CR1 and vascular adhesion protein-1-dependent recruitment of CD16(+) monocytes across human liver sinusoidal endothelium.
      ,
      • Antoniades C.G.
      • Quaglia A.
      • Taams L.S.
      • Mitry R.R.
      • Hussain M.
      • Abeles R.
      • et al.
      Source and characterization of hepatic macrophages in acetaminophen-induced acute liver failure in humans.
      ,
      • Liaskou E.
      • Zimmermann H.W.
      • Li K.-K.
      • Oo Y.H.
      • Suresh S.
      • Stamataki Z.
      • et al.
      Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
      ,
      • Zimmermann H.W.
      • Seidler S.
      • Nattermann J.
      • Gassler N.
      • Hellerbrand C.
      • Zernecke A.
      • et al.
      Functional contribution of elevated circulating and hepatic non-classical CD14+CD16+ monocytes to inflammation and human liver fibrosis.
      ]. All the subsets of monocytes found in the blood are found in the liver.
      Three major subclasses of monocytes are currently reported to exist in the blood [
      • Wong K.L.
      • Tai J.J.-Y.
      • Wong W.-C.C.
      • Han H.
      • Sem X.
      • Yeap W.-H.H.
      • et al.
      Gene expression profiling reveals the defining features of the classical, intermediate, and nonclassical human monocyte subsets.
      ]. The “classical” CD14+CD16 subset, the “non-classical” CD14loCD16+ subset, and the “intermediate” (CD14hiCD16+) [
      • Ziegler-Heitbrock L.
      • Ancuta P.
      • Crowe S.
      • Dalod M.
      • Grau V.
      • Hart D.N.
      • et al.
      Nomenclature of monocytes and dendritic cells in blood.
      ] subset that appears to be in a transitional state between classical and non-classical monocytes [
      • Wong K.L.
      • Tai J.J.-Y.
      • Wong W.-C.C.
      • Han H.
      • Sem X.
      • Yeap W.-H.H.
      • et al.
      Gene expression profiling reveals the defining features of the classical, intermediate, and nonclassical human monocyte subsets.
      ]. In vivo monocytes can probably also differentiate into “CD14+ DC” [
      • Collin M.
      • McGovern N.
      • Haniffa M.
      Human dendritic cell subsets.
      ] that are HLA-DRhiCD11c+ but lack other DC markers, with a transcriptional profile closest to in vitro cultured monocyte-derived dendritic cells (MoDC). It is postulated that they are the in vivo equivalent of a MoDC and in keeping with this they possess many phenotypic and genomic characteristics of monocyte derived macrophages [
      • Segura E.
      • Touzot M.
      • Bohineust A.
      • Cappuccio A.
      • Chiocchia G.
      • Hosmalin A.
      • et al.
      Human inflammatory dendritic cells induce Th17 cell differentiation.
      ].
      In the liver compared to blood, there is a decreased proportion of classical CD14hiCD16 monocytes (80% vs. 50%), and an increased level of intermediate CD14hiCD16+ monocytes (9% vs. 27%), whereas the frequency of non-classical monocytes is unchanged [
      • Liaskou E.
      • Zimmermann H.W.
      • Li K.-K.
      • Oo Y.H.
      • Suresh S.
      • Stamataki Z.
      • et al.
      Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
      ]. The increased proportion of intermediate monocytes in the liver is thought to be due in part to the increased ability of CD16+ monocytes to transmigrate across the hepatic endothelium as well as through an increase in local differentiation from classical CD14hiCD16 monocytes as a result of the high levels of IL-10 and transforming growth factor beta. These intermediate (CD14hiCD16+) monocytes in the liver exhibit features of potent T cell stimulators, such as high HLA-DR, CD80, CD83, and CD86 [
      • Liaskou E.
      • Zimmermann H.W.
      • Li K.-K.
      • Oo Y.H.
      • Suresh S.
      • Stamataki Z.
      • et al.
      Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
      ]. They also express CD163, often described as a macrophage cell-surface marker, though it is also rapidly upregulated on the surface of activated monocytes.
      As noted below, immunofluorescence microscopy also confirmed that some CD14+, CD16+, and CD14+CD16+ cells co-expressed CD68 – most commonly associated with macrophages. While the CD68 cells were probably monocytes in transit through the sinusoids, the expression of CD68+ may indicate monocytes differentiating into tissue macrophages within the liver. Given that CD14 is also expressed by sinus-resident cells of the MPS in human lymph nodes [
      • Angel C.E.
      • Chen C.-J.J.J.
      • Horlacher O.C.
      • Winkler S.
      • John T.
      • Browning J.
      • et al.
      Distinctive localization of antigen-presenting cells in human lymph nodes.
      ], it is also possible that CD14 is expressed by at least some MPS cells that are resident in the liver sinusoids, as well as monocytes in transit. The current literature is unable to distinguish between these possibilities.
      The most thorough investigation of liver monocytes reported only performed fluorescence-activated cell sorting (FACS) on CD16+ monocytes (comprising both intermediate and non-classical monocytes) and showed that these cells could efficiently present antigen to autologous T cells [
      • Liaskou E.
      • Zimmermann H.W.
      • Li K.-K.
      • Oo Y.H.
      • Suresh S.
      • Stamataki Z.
      • et al.
      Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
      ]. This is in keeping with the finding that intermediate monocytes expressed high HLA-DR, CD80, CD83, and CD86. All three subsets of monocytes in the normal liver express CCR2 [
      • Antoniades C.G.
      • Quaglia A.
      • Taams L.S.
      • Mitry R.R.
      • Hussain M.
      • Abeles R.
      • et al.
      Source and characterization of hepatic macrophages in acetaminophen-induced acute liver failure in humans.
      ,
      • Liaskou E.
      • Zimmermann H.W.
      • Li K.-K.
      • Oo Y.H.
      • Suresh S.
      • Stamataki Z.
      • et al.
      Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
      ,
      • Zimmermann H.W.
      • Seidler S.
      • Nattermann J.
      • Gassler N.
      • Hellerbrand C.
      • Zernecke A.
      • et al.
      Functional contribution of elevated circulating and hepatic non-classical CD14+CD16+ monocytes to inflammation and human liver fibrosis.
      ], and this is supported by liver tissue gene expression analysis [
      • Zimmermann H.W.
      • Seidler S.
      • Nattermann J.
      • Gassler N.
      • Hellerbrand C.
      • Zernecke A.
      • et al.
      Functional contribution of elevated circulating and hepatic non-classical CD14+CD16+ monocytes to inflammation and human liver fibrosis.
      ]. As has been shown in studies in mice [
      • Zimmermann H.
      • Trautwein C.
      • Tacke F.
      Functional role of monocytes and macrophages for the inflammatory response in acute liver injury.
      ], this suggests that CCL2 (monocyte chemoattractant protein 1) will be a major mediator of monocyte recruitment into the liver in humans. Further to that, an in vitro model assessed the recruitment of human peripheral blood CD16+ monocytes across human liver sinusoidal endothelium and highlighted the expression and importance of CX3CR1 in the transmigration of this subset of monocytes [
      • Aspinall A.I.
      • Curbishley S.M.
      • Lalor P.F.
      • Weston C.J.
      • Blahova M.
      • Liaskou E.
      • et al.
      CX(3)CR1 and vascular adhesion protein-1-dependent recruitment of CD16(+) monocytes across human liver sinusoidal endothelium.
      ], however this has yet to be assessed on intrahepatic monocytes.

      Kupffer cells (KCs)

      Macrophages in the liver are generally described as Kupffer cells [
      • Wake K.
      Karl Wilhelm Kupffer and his contributions to modern hepatology.
      ]. The small amount of human data and growing amount of mouse data, supporting heterogeneity of tissue macrophages in general [
      • Davies L.C.
      • Jenkins S.J.
      • Allen J.E.
      • Taylor P.R.
      Tissue-resident macrophages.
      ] and KCs in particular [
      • Klein I.
      • Cornejo J.C.
      • Polakos N.K.
      • John B.
      • Wuensch S.A.
      • Topham D.J.
      • et al.
      Kupffer cell heterogeneity: functional properties of bone marrow derived and sessile hepatic macrophages.
      ], highlight the historical confusion about the accurate definition of these cells. In 1876 Karl von Kupffer described what he thought was the “phagosome rich” cell of the reticuloendothelial system; in fact he described liver stellate cells, with their large number of vitamin A-containing globules. It was only in 1970, that Wisse et al. used electron microscopy to clearly define the presence of peri-sinusoidal macrophage cells. Despite the misnomer, the term “Kupffer cell”, has become synonymous with that of “liver macrophage” [
      • Wake K.
      Karl Wilhelm Kupffer and his contributions to modern hepatology.
      ].
      This review found sixty-seven papers that described the immunophenotype of human liver macrophages [
      • Antoniades C.G.
      • Quaglia A.
      • Taams L.S.
      • Mitry R.R.
      • Hussain M.
      • Abeles R.
      • et al.
      Source and characterization of hepatic macrophages in acetaminophen-induced acute liver failure in humans.
      ,
      • Liaskou E.
      • Zimmermann H.W.
      • Li K.-K.
      • Oo Y.H.
      • Suresh S.
      • Stamataki Z.
      • et al.
      Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
      ,
      • Zimmermann H.W.
      • Seidler S.
      • Nattermann J.
      • Gassler N.
      • Hellerbrand C.
      • Zernecke A.
      • et al.
      Functional contribution of elevated circulating and hepatic non-classical CD14+CD16+ monocytes to inflammation and human liver fibrosis.
      ,
      • Bardadin K.A.
      • Scheuer P.J.
      • Peczek A.
      • Wejman J.
      Immunocytochemical observations on macrophage populations in normal fetal and adult human liver.
      ,
      • Burgio V.L.
      • Ballardini G.
      • Artini M.
      • Caratozzolo M.
      • Bianchi F.B.
      • Levrero M.
      Expression of co-stimulatory molecules by Kupffer cells in chronic hepatitis of hepatitis C virus etiology.
      ,
      • Dominguez-Soto A.
      • Aragoneses-Fenoll L.
      • Gomez-Aguado F.
      • Corcuera M.T.M.T.
      • Claria J.
      • Garcia-Monzon C.
      • et al.
      The pathogen receptor liver and lymph node sinusoidal endotelial cell C-type lectin is expressed in human Kupffer cells and regulated by PU.1.
      ,
      • Fayyazi A.
      • Scheel O.
      • Werfel T.
      • Schweyer S.
      • Oppermann M.
      • Götze O.
      • et al.
      The C5a receptor is expressed in normal renal proximal tubular but not in normal pulmonary or hepatic epithelial cells.
      ,
      • Fukuda Y.
      • Imoto M.
      • Koyama Y.
      • Miyazawa Y.
      • Nakano I.
      • Hattori M.
      • et al.
      Immunohistochemical study on tissue inhibitors of metalloproteinases in normal and pathological human livers.
      ,
      • Gaweco A.S.
      • Wiesner R.H.
      • Yong S.
      • Krom R.
      • Porayko M.
      • Chejfec G.
      • et al.
      CD40L (CD154) expression in human liver allografts during chronic ductopenic rejection.
      ,
      • Lautenschlager I.
      • Taskinen E.
      • Inkinen K.
      • Lehto V.P.
      • Virtanen I.
      • Hayry P.
      Distribution of the major histocompatibility complex antigens on different cellular components of human liver.
      ,
      • Lefkowitch J.H.
      • Haythe J.H.
      • Regent N.
      Kupffer cell aggregation and perivenular distribution in steatohepatitis.
      ,
      • Leifeld L.
      • Clemens C.
      • Heller J.
      • Trebicka J.
      • Sauerbruch T.
      • Spengler U.
      Expression of urotensin II and its receptor in human liver cirrhosis and fulminant hepatic failure.
      ,
      • Martens J.H.
      • Kzhyshkowska J.
      • Falkowski-Hansen M.
      • Schledzewski K.
      • Gratchev A.
      • Mansmann U.
      • et al.
      Differential expression of a gene signature for scavenger/lectin receptors by endothelial cells and macrophages in human lymph node sinuses, the primary sites of regional metastasis.
      ,
      • Masuda M.
      • Senju S.
      • Fujii S.I.
      • Terasaki Y.
      • Takeya M.
      • Hashimoto S.I.
      • et al.
      Identification and immunocytochemical analysis of DCNP1, a dendritic cell-associated nuclear protein.
      ,
      • McGuinness P.H.
      • Painter D.
      • Davies S.
      • McCaughan G.W.
      Increases in intrahepatic CD68 positive cells, MAC387 positive cells, and proinflammatory cytokines (particularly interleukin 18) in chronic hepatitis C infection.
      ,
      • Monnier J.
      • Piquet-Pellorce C.
      • Feige J.-J.J.
      • Musso O.
      • Clement B.
      • Turlin B.
      • et al.
      Prokineticin 2/Bv8 is expressed in Kupffer cells in liver and is down regulated in human hepatocellular carcinoma.
      ,
      • Nakagawa-Toyama Y.
      • Hirano K.
      • Tsujii K.
      • Nishida M.
      • Miyagawa J.
      • Sakai N.
      • et al.
      Human scavenger receptor class B type I is expressed with cell-specific fashion in both initial and terminal site of reverse cholesterol transport.
      ,
      • Pungercar J.
      • Viyjak A.
      • Ivanovski G.
      • Krizaj I.
      Tissue expression and immunolocalization of a novel human cathepsin P.
      ,
      • Scoazec J.Y.
      • Feldmann G.
      Both macrophages and endothelial cells of the human hepatic sinusoid express the CD4 molecule, a receptor for the human immunodeficiency virus.
      ,
      • Shimada M.
      • Kajiyama K.
      • Hasegawa H.
      • Gion T.
      • Ikeda Y.
      • Shirabe K.
      • et al.
      Role of adhesion molecule expression and soluble fractions in hepatic resection.
      ,
      • Szalowska E.
      • Elferink M.G.L.
      • Hoek A.
      • Groothuis G.M.M.
      • Vonk R.J.
      Resistin is more abundant in liver than adipose tissue and is not up-regulated by lipopolysaccharide.
      ,
      • Tomita M.
      • Yamamoto K.
      • Kobashi H.
      • Ohmoto M.
      • Tsuji T.
      Immunohistochemical analysis on the expressions of maturation associated antigens, Fcγ receptors and CD14 in normal and diseased human liver macrophages.
      ,
      • Tomokiyo R.
      • Jinnouchi K.
      • Honda M.
      • Wada Y.
      • Hanada N.
      • Hiraoka T.
      • et al.
      Production, characterization, and interspecies reactivities of monoclonal antibodies against human class A macrophage scavenger receptors.
      ,
      • Wood G.S.
      • Turner R.R.
      • Shiurba R.A.
      • Eng L.
      • Warnke R.A.
      Human dendritic cells and macrophages. In situ immunophenotypic definition of subsets that exhibit specific morphologic and microenvironmental characteristics.
      ,
      • Yoshioka T.
      • Yamamoto K.
      • Kobashi H.
      • Tomita M.
      • Tsuji T.
      Receptor-mediated endocytosis of chemically modified albumins by sinusoidal endothelial cells and Kupffer cells in rat and human liver.
      ,
      • Zhao P.
      • Hou N.
      • Lu Y.
      Fhit protein is preferentially expressed in the nucleus of monocyte-derived cells and its possible biological significance.
      ,
      • Znoyko I.
      • Sohara N.
      • Spicer S.S.
      • Trojanowska M.
      • Reuben A.
      Expression of oncostatin M and its receptors in normal and cirrhotic human liver.
      ,
      • Baldus S.E.
      • Zirbes T.K.
      • Weidner I.C.
      • Flucke U.
      • Dittmar E.
      • Thiele J.
      • et al.
      Comparative quantitative analysis of macrophage populations defined by CD68 and carbohydrate antigens in normal and pathologically altered human liver tissue.
      ,
      • Tuijnman W.B.
      • Van Wichen D.F.
      • Schuurman H.J.
      Tissue distribution of human IgG Fc receptors CD16, CD32, and CD64: an immunohistochemical study.
      ,
      • Alabraba E.B.
      • Lai V.
      • Boon L.
      • Wigmore S.J.
      • Adams D.H.
      • Afford S.C.
      Coculture of human liver macrophages and cholangiocytes leads to CD40-dependent apoptosis and cytokine secretion.
      ,
      • Arany E.
      • Afford S.
      • Strain A.J.
      • Winwood P.J.
      • Arthur M.J.
      • Hill D.J.
      Differential cellular synthesis of insulin-like growth factor binding protein-1 (IGFBP-1) and IGFBP-3 within human liver.
      ,
      • Bastin J.
      • Drakesmith H.
      • Rees M.
      • Sargent I.
      • Townsend A.
      Localisation of proteins of iron metabolism in the human placenta and liver.
      ,
      • Bauer I.
      • Rensing H.
      • Florax A.
      • Ulrich C.
      • Pistorius G.
      • Redl H.
      • et al.
      Expression pattern and regulation of heme oxygenase-1/heat shock protein 32 in human liver cells.
      ,
      • Bode J.G.
      • Peters-Regehr T.
      • Kubitz R.
      • Haussinger D.
      Expression of glutamine synthetase in macrophages.
      ,
      • Brown K.E.
      • Broadhurst K.A.
      • Mathahs M.M.
      • Kladney R.D.
      • Fimmel C.J.
      • Srivastava S.K.
      • et al.
      Immunodetection of aldose reductase in normal and diseased human liver.
      ,
      • Brown K.E.
      • Brunt E.M.
      • Heinecke J.W.
      Immunohistochemical detection of myeloperoxidase and its oxidation products in Kupffer cells of human liver.
      ,
      • Cope E.M.
      • Dilly S.A.
      Kupffer cell numbers during human development.
      ,
      • Esbach S.
      • Pieters M.N.
      • van der Boom J.
      • Schouten D.
      • van der Heyde M.N.
      • Roholl P.J.
      • et al.
      Visualization of the uptake and processing of oxidized low-density lipoproteins in human and rat liver.
      ,
      • Friedman S.L.
      • Rockey D.C.
      • McGuire R.F.
      • Maher J.J.
      • Boyles J.K.
      • Yamasaki G.
      • et al.
      Isolated hepatic lipocytes and Kupffer cells from normal human liver: morphological and functional characteristics in primary culture.
      ,
      • Funaki N.
      • Arii S.
      • Monden K.
      • Higashitsuji H.
      • Furutani M.
      • Mise M.
      • et al.
      Chemical mediators released by primary-cultured human hepatic macrophages in patients with and without cirrhosis: a study in tumor-bearing patients.
      ,
      • Geuken E.
      • Buis C.I.
      • Visser D.S.
      • Blokzijl H.
      • Moshage H.
      • Nemes B.
      • et al.
      Expression of heme oxygenase-1 in human livers before transplantation correlates with graft injury and function after transplantation.
      ,
      • Hartnell A.
      • Steel J.
      • Turley H.
      • Jones M.
      • Jackson D.G.
      • Crocker P.R.
      Characterization of human sialoadhesin, a sialic acid binding receptor expressed by resident and inflammatory macrophage populations.
      ,
      • Hinglais N.
      • Kazatchkine M.D.
      • Mandet C.
      • Appay M.D.
      • Bariety J.
      Human liver Kupffer cells express CR1, CR3, and CR4 complement receptor antigens. An immunohistochemical study.
      ,
      • Huber H.
      • Douglas S.D.
      • Fudenberg H.H.
      The IgG receptor: an immunological marker for the characterization of mononuclear cells.
      ,
      • Hume D.A.
      • Allan W.
      • Hogan P.G.
      • Doe W.F.
      Immunohistochemical characterisation of macrophages in human liver and gastrointestinal tract: expression of CD4, HLA-DR, OKM1, and the mature macrophage marker 25F9 in normal and diseased tissue.
      ,
      • Kitamura Y.
      • Okazaki T.
      • Nagatsuka Y.
      • Hirabayashi Y.
      • Kato S.
      • Hayashi K.
      Immunohistochemical distribution of phosphatidylglucoside using anti-phosphatidylglucoside monoclonal antibody (DIM21).
      ,
      • Kiyohara H.
      • Egami H.
      • Shibata Y.
      • Murata K.
      • Ohshima S.
      • Ogawa M.
      Light microscopic immunohistochemical analysis of the distribution of group II phospholipase A2 in human digestive organs.
      ,
      • Kleinherenbrink-Stins M.F.
      • van de Boom J.H.
      • Schouten D.
      • Roholl P.J.
      • Niels van der Heyde M.
      • Brouwer A.
      • et al.
      Visualization of the interaction of native and modified lipoproteins with parenchymal, endothelial and Kupffer cells from human liver.
      ,
      • Klockars M.
      • Reitamo S.
      Tissue distribution of lysozyme in man.
      ,
      • Kwekkeboom J.
      • Kuijpers M.A.
      • Bruyneel B.
      • Mancham S.
      • De Baar-Heesakkers E.
      • Ijzermans J.N.M.
      • et al.
      Expression of CD80 on Kupffer cells is enhanced in cadaveric liver transplants.
      ,
      • Le Bail B.
      • Bioulac-Sage P.
      • Senuita R.
      • Quinton A.
      • Saric J.
      • Balabaud C.
      Fine structure of hepatic sinusoids and sinusoidal cells in disease.
      ,
      • Li L.
      • Grenard P.
      • Van Nhieu J.T.
      • Julien B.
      • Mallat A.
      • Habib A.A.
      • et al.
      Heme oxygenase-1 is an antifibrogenic protein in human hepatic myofibroblasts.
      ,
      • Micklem K.J.
      • Stross W.P.
      • Willis A.C.
      • Cordell J.L.
      • Jones M.
      • Mason D.Y.
      Different isoforms of human FcRII distinguished by CDw32 antibodies.
      ,
      • Nusing R.
      • Nüsing R.
      • Sauter G.
      • Fehr P.
      • Dürmüller U.
      • Kasper M.
      • et al.
      Localization of thromboxane synthase in human tissues by Tu 300.
      ,
      • Okino T.
      • Egami H.
      • Ohmachi H.
      • Takai E.
      • Tamori Y.
      • Nakagawa A.
      • et al.
      Immunohistochemical analysis of distribution of RON receptor tyrosine kinase in human digestive organs.
      ,
      • Rodriguez-Agudo D.
      • Ren S.
      • Hylemon P.B.
      • Montañez R.
      • Redford K.
      • Natarajan R.
      • et al.
      Localization of StarD5 cholesterol binding protein.
      ,
      • Roels F.
      • De Prest B.
      • De Pestel G.
      Liver and chorion cytochemistry.
      ,
      • Rullier A.
      • Senant N.
      • Kisiel W.
      • Bioulac-Sage P.
      • Balabaud C.
      • Le Bail B.
      • et al.
      Expression of protease-activated receptors and tissue factor in human liver.
      ,
      • Ueda T.
      • Kohli Y.
      • Abe Y.
      • Katoh T.
      • Ogasawara T.
      • Nojyo Y.
      • et al.
      Electron microscopic study of endogenous peroxidase activity in human liver macrophages.
      ,
      • Wang J.
      • Greene S.
      • Eriksson L.C.
      • Rozell B.
      • Reihnér E.
      • Einarsson C.
      • et al.
      Human sterol 12a-hydroxylase (CYP8B1) is mainly expressed in hepatocytes in a homogenous pattern.
      ,
      • Becker E.L.
      • Forouhar F.A.
      • Grunnet M.L.
      • Boulay F.
      • Tardif M.
      • Bormann B.J.
      • et al.
      Broad immunocytochemical localization of the formylpeptide receptor in human organs, tissues, and cells.
      ,
      • Guo S.
      • Yang C.
      • Mei F.
      • Wu S.
      • Luo N.
      • Fei L.
      • et al.
      Down-regulation of Z39Ig on macrophages by IFN-gamma in patients with chronic HBV infection.
      ,
      • Kaiserling E.
      • Ruck P.
      • Xiao J.-C.
      Immunoreactivity of neoplastic and non-neoplastic hepatocytes for CD68 and with 3A5, Ki-M1P, and MAC387 light- and electron-microscopic findings.
      ,
      • Moestrup S.K.
      • Gliemann J.
      • Pallesen G.
      Distribution of the alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein in human tissues.
      ,
      • Murakami I.
      • Sarker A.B.
      • Hayashi K.
      • Akagi T.
      Lectin binding patterns in normal liver, chronic active hepatitis, liver cirrhosis, and hepatocellular carcinoma. An immunohistochemical and immunoelectron microscopic study.
      ,
      • Osterreicher C.H.
      • Penz-Osterreicher M.
      • Grivennikov S.I.
      • Guma M.
      • Koltsova E.K.
      • Datz C.
      • et al.
      Fibroblast-specific protein 1 identifies an inflammatory subpopulation of macrophages in the liver.
      ,
      • Zeng L.
      • Takeya M.
      • Takahashi K.
      AM-3K, a novel monoclonal antibody specific for tissue macrophages and its application to pathological investigation.
      ].

      Physical and phenotypic description

      KCs have also been defined by their peri-sinusoidal location in the liver lobule [
      • Ballardini G.
      • Fallani M.
      • Bianchi F.B.
      • Pisi E.
      Antigen presenting cells in liver biopsies from patients with primary biliary cirrhosis.
      ], and a morphological appearance that demonstrates a lack of the extensive fine cell membrane processes, usually found on dendritic cells. Scanning electron microscopy has demonstrated that KCs possess numerous lamellipodia, and show pronounced membrane ruffling [
      • Friedman S.L.
      • Rockey D.C.
      • McGuire R.F.
      • Maher J.J.
      • Boyles J.K.
      • Yamasaki G.
      • et al.
      Isolated hepatic lipocytes and Kupffer cells from normal human liver: morphological and functional characteristics in primary culture.
      ]. They are described to classically lie tightly attached to the sinusoidal luminal surface [
      • Le Bail B.
      • Bioulac-Sage P.
      • Senuita R.
      • Quinton A.
      • Saric J.
      • Balabaud C.
      Fine structure of hepatic sinusoids and sinusoidal cells in disease.
      ]. Data are conflicting as to changes of KC density within the liver lobule; KCs have been described as being more densely populated in peri-central regions [
      • Gaweco A.S.
      • Wiesner R.H.
      • Yong S.
      • Krom R.
      • Porayko M.
      • Chejfec G.
      • et al.
      CD40L (CD154) expression in human liver allografts during chronic ductopenic rejection.
      ], but were also described as being diffusely pan-lobular, involving both the portal tracts and regions around the central vein [
      • Lefkowitch J.H.
      • Haythe J.H.
      • Regent N.
      Kupffer cell aggregation and perivenular distribution in steatohepatitis.
      ].
      Only a few cellular markers have been described in humans that are consistently expressed by KCs. CD68, a lysosome associated trans-membrane glycoprotein, involved in the metabolism of the low-density lipoprotein, is the most consistent reported marker for determining macrophage populations throughout the body and has been used throughout the literature to define macrophages within the liver. Subsequently the majority of papers (n = 37) used immunohistochemistry or fluorescence microscopy to describe the presence or absence of further cellular components (such as cell surface markers, or tissue distribution) on cells that also stain positive for CD68. Hence, a good working definition for a Kupffer cell in the literature to date is a peri-sinusoidal cell expressing CD68.

      Heterogeneity

      A number of papers report heterogeneity of KCs but to date there is no comprehensive definition of KC subsets. Morphological differences have been noted, such as macrophage populations with varying cell shapes in the portal compared to the central venous regions of the liver lobule [
      • Ballardini G.
      • Fallani M.
      • Bianchi F.B.
      • Pisi E.
      Antigen presenting cells in liver biopsies from patients with primary biliary cirrhosis.
      ], or two different populations of KCs being described as either “round” or thin” [
      • Burgio V.L.
      • Ballardini G.
      • Artini M.
      • Caratozzolo M.
      • Bianchi F.B.
      • Levrero M.
      Expression of co-stimulatory molecules by Kupffer cells in chronic hepatitis of hepatitis C virus etiology.
      ]. In 1995, Ueda et al. [
      • Ueda T.
      • Kohli Y.
      • Abe Y.
      • Katoh T.
      • Ogasawara T.
      • Nojyo Y.
      • et al.
      Electron microscopic study of endogenous peroxidase activity in human liver macrophages.
      ] described KC functional heterogeneity based on endogenous peroxidase activity that divided the cells into monocytes and four types of macrophages that showed a zonal distribution. These findings suggest that KCs are a collection of cells with varying phenotypes throughout different parts of the healthy liver, which is a concept that has been further supported by work in chronically hepatitis C virus infected livers [
      • Gadd V.L.
      • Melino M.
      • Roy S.
      • Horsfall L.
      • O’Rourke P.
      • Williams M.R.
      • et al.
      Portal, but not lobular, macrophages express matrix metalloproteinase-9: association with the ductular reaction and fibrosis in chronic hepatitis C.
      ].
      The heterogeneous co-expression of other phenotypic markers by CD68+ cells supports KC heterogeneity. As noted above, some CD68+ cells co-express CD14, CD16, and CD163 [
      • Liaskou E.
      • Zimmermann H.W.
      • Li K.-K.
      • Oo Y.H.
      • Suresh S.
      • Stamataki Z.
      • et al.
      Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
      ] though it is not clear whether all these cells are resident macrophages, since activated monocytes can express all these molecules. Some KCs express Mac387, a marker for infiltrating macrophages as opposed to resident macrophages [
      • McGuinness P.H.
      • Painter D.
      • Davies S.
      • McCaughan G.W.
      Increases in intrahepatic CD68 positive cells, MAC387 positive cells, and proinflammatory cytokines (particularly interleukin 18) in chronic hepatitis C infection.
      ]. Similarly, CD68+ liver cells variably co-express a large range of APC markers, including lectins (CD209 [
      • Zimmermann H.W.
      • Seidler S.
      • Nattermann J.
      • Gassler N.
      • Hellerbrand C.
      • Zernecke A.
      • et al.
      Functional contribution of elevated circulating and hepatic non-classical CD14+CD16+ monocytes to inflammation and human liver fibrosis.
      ], and CD299 or LSECtin [
      • Martens J.H.
      • Kzhyshkowska J.
      • Falkowski-Hansen M.
      • Schledzewski K.
      • Gratchev A.
      • Mansmann U.
      • et al.
      Differential expression of a gene signature for scavenger/lectin receptors by endothelial cells and macrophages in human lymph node sinuses, the primary sites of regional metastasis.
      ]), complement receptors (predominantly CR1, CR3, and CR4 [
      • Hinglais N.
      • Kazatchkine M.D.
      • Mandet C.
      • Appay M.D.
      • Bariety J.
      Human liver Kupffer cells express CR1, CR3, and CR4 complement receptor antigens. An immunohistochemical study.
      ] and C5a (CD88) receptor [
      • Fayyazi A.
      • Scheel O.
      • Werfel T.
      • Schweyer S.
      • Oppermann M.
      • Götze O.
      • et al.
      The C5a receptor is expressed in normal renal proximal tubular but not in normal pulmonary or hepatic epithelial cells.
      ]), Fc receptors (such as CD16, CD32, and CD64 [
      • Tuijnman W.B.
      • Van Wichen D.F.
      • Schuurman H.J.
      Tissue distribution of human IgG Fc receptors CD16, CD32, and CD64: an immunohistochemical study.
      ]), and scavenger receptors (such as CD206 [
      • Martens J.H.
      • Kzhyshkowska J.
      • Falkowski-Hansen M.
      • Schledzewski K.
      • Gratchev A.
      • Mansmann U.
      • et al.
      Differential expression of a gene signature for scavenger/lectin receptors by endothelial cells and macrophages in human lymph node sinuses, the primary sites of regional metastasis.
      ], CD163, and CD169 [
      • Hartnell A.
      • Steel J.
      • Turley H.
      • Jones M.
      • Jackson D.G.
      • Crocker P.R.
      Characterization of human sialoadhesin, a sialic acid binding receptor expressed by resident and inflammatory macrophage populations.
      ]). How the variable expression of all these markers in KCs relates to different locations within the liver lobule, and different cellular lineages, remains unclear.
      Intriguingly, some CD68+ KCs in healthy liver express the proliferative marker, Ki67 [
      • Antoniades C.G.
      • Quaglia A.
      • Taams L.S.
      • Mitry R.R.
      • Hussain M.
      • Abeles R.
      • et al.
      Source and characterization of hepatic macrophages in acetaminophen-induced acute liver failure in humans.
      ], suggesting they are self-renewing in situ, rather than terminally differentiated cells derived from blood cells. In this context it is important to note recent evidence in mice (discussed below) for tissue macrophages that derive from the yolk sac and foetal liver rather than the bone marrow [
      • Davies L.C.
      • Jenkins S.J.
      • Allen J.E.
      • Taylor P.R.
      Tissue-resident macrophages.
      ].
      There is very little direct experimental evidence of the function of human KCs, so their roles are largely inferred from histology, electron microscopy, immunohistochemistry, and evidence from murine models.
      Macrophages are classically not considered as potent as DCs at stimulating a T cell response, or as capable of travelling to drain secondary lymphoid tissue to instigate a systemic response [
      • Davies L.C.
      • Jenkins S.J.
      • Allen J.E.
      • Taylor P.R.
      Tissue-resident macrophages.
      ]. However, KCs appear to be heavily involved in both the innate and the acquired immune responses within the liver. KCs express MHC class II, and express varying levels of co-stimulation markers (such as CD40, CD80, and CD86 [
      • Kwekkeboom J.
      • Kuijpers M.A.
      • Bruyneel B.
      • Mancham S.
      • De Baar-Heesakkers E.
      • Ijzermans J.N.M.
      • et al.
      Expression of CD80 on Kupffer cells is enhanced in cadaveric liver transplants.
      ]), as well as the inhibitory markers such as Z39Ig [
      • Guo S.
      • Yang C.
      • Mei F.
      • Wu S.
      • Luo N.
      • Fei L.
      • et al.
      Down-regulation of Z39Ig on macrophages by IFN-gamma in patients with chronic HBV infection.
      ]. In the steady state they act as sentinel scavenging cells to process antigen from the gut. KCs readily phagocytose latex beads [
      • Viñas O.
      • Bataller R.
      • Sancho-Bru P.
      • Ginès P.
      • Berenguer C.
      • Enrich C.
      • et al.
      Human hepatic stellate cells show features of antigen-presenting cells and stimulate lymphocyte proliferation.
      ] and therefore play a major scavenging role in conjunction with the liver sinusoidal endothelial cells (LSECs) [
      • Dominguez-Soto A.
      • Aragoneses-Fenoll L.
      • Gomez-Aguado F.
      • Corcuera M.T.M.T.
      • Claria J.
      • Garcia-Monzon C.
      • et al.
      The pathogen receptor liver and lymph node sinusoidal endotelial cell C-type lectin is expressed in human Kupffer cells and regulated by PU.1.
      ]. The migratory potential of KCs is still uncertain and although monocytes clearly express CCR2 and CX3CR1, expression of chemokine receptors in CD68+ cells is yet to be thoroughly explored [
      • Liaskou E.
      • Zimmermann H.W.
      • Li K.-K.
      • Oo Y.H.
      • Suresh S.
      • Stamataki Z.
      • et al.
      Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
      ].
      Electron microscopy has shown KCs in direct contact with pit cells (liver NK cells), and liver stellate cells (the major collagen producing cell in the liver [
      • Le Bail B.
      • Bioulac-Sage P.
      • Senuita R.
      • Quinton A.
      • Saric J.
      • Balabaud C.
      Fine structure of hepatic sinusoids and sinusoidal cells in disease.
      ]). It is postulated that through this interaction they are key in instigating fibrosis following inflammation [
      • Osterreicher C.H.
      • Penz-Osterreicher M.
      • Grivennikov S.I.
      • Guma M.
      • Koltsova E.K.
      • Datz C.
      • et al.
      Fibroblast-specific protein 1 identifies an inflammatory subpopulation of macrophages in the liver.
      ]. CD68+ KCs also co-express prokineticin 2/Bv8, a molecule strongly implicated in angiogenesis [
      • Monnier J.
      • Piquet-Pellorce C.
      • Feige J.-J.J.
      • Musso O.
      • Clement B.
      • Turlin B.
      • et al.
      Prokineticin 2/Bv8 is expressed in Kupffer cells in liver and is down regulated in human hepatocellular carcinoma.
      ].

      Liver dendritic cells

      Ten papers describe the immunophenotype of liver dendritic cells (DC) in human liver [
      • Aspinall A.I.
      • Curbishley S.M.
      • Lalor P.F.
      • Weston C.J.
      • Blahova M.
      • Liaskou E.
      • et al.
      CX(3)CR1 and vascular adhesion protein-1-dependent recruitment of CD16(+) monocytes across human liver sinusoidal endothelium.
      ,
      • Wood G.S.
      • Turner R.R.
      • Shiurba R.A.
      • Eng L.
      • Warnke R.A.
      Human dendritic cells and macrophages. In situ immunophenotypic definition of subsets that exhibit specific morphologic and microenvironmental characteristics.
      ,
      • Ballardini G.
      • Fallani M.
      • Bianchi F.B.
      • Pisi E.
      Antigen presenting cells in liver biopsies from patients with primary biliary cirrhosis.
      ,
      • Bosma B.M.
      • Metselaar H.J.
      • Mancham S.
      • Boor P.P.C.
      • Kusters J.G.
      • Kazemier G.
      • et al.
      Characterization of human liver dendritic cells in liver grafts and perfusates.
      ,
      • Bamboat Z.M.
      • Stableford J.A.
      • Plitas G.
      • Burt B.M.
      • Nguyen H.M.
      • Welles A.P.
      • et al.
      Human liver dendritic cells promote T cell hyporesponsiveness.
      ,
      • Goddard S.
      • Youster J.
      • Morgan E.
      • Adams D.H.
      Interleukin-10 secretion differentiates dendritic cells from human liver and skin.
      ,
      • Haniffa M.
      • Shin A.
      • Bigley V.
      • McGovern N.
      • Teo P.
      • See P.
      • et al.
      Human tissues contain CD141hi cross-presenting dendritic cells with functional homology to mouse CD103+ nonlymphoid dendritic cells.
      ,
      • Ibrahim J.
      • Nguyen A.H.
      • Rehman A.
      • Ochi A.
      • Jamal M.
      • Graffeo C.S.
      • et al.
      Dendritic cell populations with different concentrations of lipid regulate tolerance and immunity in mouse and human liver.
      ,
      • Kelly A.
      • Fahey R.
      • Fletcher J.M.
      • Keogh C.
      • Carroll A.G.
      • Siddachari R.
      • et al.
      CD141+ myeloid dendritic cells are enriched in healthy human liver.
      ,
      • Prickett T.C.
      • McKenzie J.L.
      • Hart D.N.
      Characterization of interstitial dendritic cells in human liver.
      ].
      Although the boundaries between macrophages and DCs have blurred in recent years, the major human DC subsets express well-defined cell surface markers that allow for their identification, using flow cytometry and multi-colour immunofluorescence microscopy. Much of this work has been carried out using cellular markers common to DCs in blood and other non-hepatic tissue.
      DCs in human blood are HLA-DRhi cells that comprise 3 major non-monocytic subsets: plasmacytoid DCs (pDCs) expressing CD303 (BDCA2) and lacking CD11c; and two myeloid CD11c+ subsets – CD1c+ DCs expressing CD1c (BDCA1); and CLEC9A+ DCs expressing CLEC9A and high levels of CD141 (BDCA3) [
      • Collin M.
      • McGovern N.
      • Haniffa M.
      Human dendritic cell subsets.
      ]. Peripheral and lymphoid tissues also have at least two myeloid CD11c+ migratory DC subsets (e.g. Langerhans cells) that express molecules such as CD207 and CD1a [
      • Angel C.E.
      • Chen C.-J.J.J.
      • Horlacher O.C.
      • Winkler S.
      • John T.
      • Browning J.
      • et al.
      Distinctive localization of antigen-presenting cells in human lymph nodes.
      ,
      • Angel C.E.
      • Lala A.
      • Chen C.-J.J.
      • Edgar S.G.
      • Ostrovsky L.L.
      • Dunbar P.R.
      CD14+ antigen-presenting cells in human dermis are less mature than their CD1a+ counterparts.
      ,
      • Angel C.E.
      • George E.
      • Brooks A.E.S.
      • Ostrovsky L.L.
      • Brown T.L.H.
      • Dunbar P.R.
      Cutting edge: CD1a+ antigen-presenting cells in human dermis respond rapidly to CCR7 ligands.
      ]. CD1c+ DC can secrete IL-10 in response to incubation with whole E. coli and the TLR-4 agonist lipopolysaccharide [
      • Kassianos A.J.
      • Hardy M.Y.
      • Ju X.
      • Vijayan D.
      • Ding Y.
      • Vulink A.J.
      • et al.
      Human CD1c (BDCA-1)+ myeloid dendritic cells secrete IL-10 and display an immuno-regulatory phenotype and function in response to Escherichia coli.
      ] and are therefore thought to have tolerogenic potential.
      It is apparent that all three major classes of blood DCs are present in the liver, in addition to CD16+ monocytes [
      • Bamboat Z.M.
      • Stableford J.A.
      • Plitas G.
      • Burt B.M.
      • Nguyen H.M.
      • Welles A.P.
      • et al.
      Human liver dendritic cells promote T cell hyporesponsiveness.
      ], with CD1c+ DCs being the most prevalent subset [
      • Bamboat Z.M.
      • Stableford J.A.
      • Plitas G.
      • Burt B.M.
      • Nguyen H.M.
      • Welles A.P.
      • et al.
      Human liver dendritic cells promote T cell hyporesponsiveness.
      ]. The hepatic DC populations express similar markers to blood DCs, and when compared to skin and spleen DCs, liver DCs have an immature phenotype, with a relatively low expression of co-stimulatory molecules (CD40, CD80, and CD86), as well as maturation markers like CD83 [
      • Bamboat Z.M.
      • Stableford J.A.
      • Plitas G.
      • Burt B.M.
      • Nguyen H.M.
      • Welles A.P.
      • et al.
      Human liver dendritic cells promote T cell hyporesponsiveness.
      ,
      • Goddard S.
      • Youster J.
      • Morgan E.
      • Adams D.H.
      Interleukin-10 secretion differentiates dendritic cells from human liver and skin.
      ].
      Compared to blood, hepatic DCs were less efficient at antigen uptake, processing and presentation, including allo-stimulatory capacity, and upon TLR4 stimulation they secreted substantial amounts of IL-10, a cytokine associated with a tolerogenic phenotype [
      • Steinbrink K.
      • Wölfl M.
      • Jonuleit H.
      • Knop J.
      • Enk A.H.
      Induction of tolerance by IL-10-treated dendritic cells.
      ]. Kwekkeboom et al. [
      • Kwekkeboom J.
      • Boor P.P.C.
      • Sen E.
      • Kusters J.G.
      • Drexhage H.A.
      • de Jong E.C.
      • et al.
      Human liver myeloid dendritic cells maturate in vivo into effector DC with a poor allogeneic T-cell stimulatory capacity.
      ] found that compared to inguinal lymph node DCs, hepatic DCs were less capable of stimulating T cells, despite higher expression of HLA-DR, CD80, and CD86. Bamboat et al. found increased production of Foxp3+ Treg cells and IL-4 producing T cells (associated with the humoral immune response [
      • Zhu J.
      • Yamane H.
      • Paul W.E.
      Differentiation of effector CD4 T cell populations (∗).
      ]) that were difficult to re-activate if initially activated by hepatic DCs in comparison to blood-derived DCs. Interestingly, liver DCs were also found to have a significantly decreased secretion of IL-12p70, which has been viewed as a pro-inflammatory cytokine [
      • Trinchieri G.
      Interleukin-12: a cytokine produced by antigen-presenting cells with immunoregulatory functions in the generation of T-helper cells type 1 and cytotoxic lymphocytes.
      ].
      Goddard et al. [
      • Goddard S.
      • Youster J.
      • Morgan E.
      • Adams D.H.
      Interleukin-10 secretion differentiates dendritic cells from human liver and skin.
      ] used overnight migration to extract DCs from the liver. They found high HLA-DR, CD86, and CD11b prior to culture. They also found that these hepatic DCs produced larger amounts of IL-4 and IL-10 and lower levels of IL-12p70 compared to DCs from the spleen and skin. They report that the DCs extracted, using overnight migration, expressed low levels of CCR5 and were positive for the chemokine receptor CXCR4 and CCR7. Very little other work on intrahepatic liver DC chemokine receptors has been published.
      In 2012 Haniffa et al. described the presence of CD141hi DCs in the liver, using multi-colour flow cytometry, and also confirmed the presence of CD14+ APCs and CD1c+ DCs. CD141hi DCs in other human tissues also express the definitive marker CLEC9A, and some reports indicate that these cells have some of the characteristics of murine CD8+ or CD103+ DCs [
      • Jongbloed S.L.
      • Kassianos A.J.
      • McDonald K.J.
      • Clark G.J.
      • Ju X.
      • Angel C.E.
      • et al.
      Human CD141+ (BDCA-3)+ dendritic cells (DCs) represent a unique myeloid DC subset that cross-presents necrotic cell antigens.
      ], especially the ability to cross-present antigen to CD8+ T cells. However, no functional data from normal intrahepatic CD141hi DCs have yet been published [

      Haniffa M, Bigley V, McGovern N, Wang X-N, Teo P, Sng D, et al. CD141+ human migratory dendritic cells are homologous to murine CD103+ dendritic cells and excel at exogenous antigen cross presentation. Immunol. Conf. Annu. Congr. Br. Soc. Immunol. 2011 Liverpool United Kingdom. Conf. 20111205 Conf. End 20111208. Conf. Publ. (var.pagings). 135 (pp 58), 2011. Date of Publication December 2011.

      ].
      Further to studies of liver tissue from biopsy samples, liver perfusates from liver transplant procedures have described CD141 expressing DCs that appear to be of a more pro-inflammatory phenotype than CD14+ and CD1c+ MPS in the liver [
      • Kelly A.
      • Fahey R.
      • Fletcher J.M.
      • Keogh C.
      • Carroll A.G.
      • Siddachari R.
      • et al.
      CD141+ myeloid dendritic cells are enriched in healthy human liver.
      ]. However, it is apparent that CD141 is much more widely expressed in liver cells than in blood, where it is largely restricted to CLEC9A+ DC [
      • Jongbloed S.L.
      • Kassianos A.J.
      • McDonald K.J.
      • Clark G.J.
      • Ju X.
      • Angel C.E.
      • et al.
      Human CD141+ (BDCA-3)+ dendritic cells (DCs) represent a unique myeloid DC subset that cross-presents necrotic cell antigens.
      ]. Hence, CLEC9A+ DCs may only be a subset of the CD141+ cells, derived from human liver, so results from sorting on CD141 alone need to be interpreted with some caution [
      • Strauss O.
      • Bartlett A.
      Dendritic cell subset composition in the human liver is more complex than it seems.
      ].
      While the majority of research on liver DCs has been to assess phenotype using flow cytometry, there is a small amount of histological work localising DCs, this has used light microscopy and immunohistochemistry. Myeloid DCs are predominantly located in the portal tract and periportal zones, with particular density around the bile ducts while pDC are found scattered throughout the liver lobule [
      • Hart
      Characterization of interstitial dendritic cells in human tissues.
      ].

      Discussion

      Although similar, monocytes in the liver are different from the blood

      As we noted, three major subsets of blood monocytes are considered to be present in liver, and it appears that liver monocytes are richer in CD16 expression than monocytes in the blood. CD16 monocytes probably derive from CD14 monocytes [
      • Tacke F.
      • Randolph G.J.
      Migratory fate and differentiation of blood monocyte subsets.
      ] and the increased number of CD16 monocytes may relate to the activation of classical monocytes in the liver. Clearly some cells bearing monocyte markers also express markers more commonly associated with KCs such as CD68 and CD163 [
      • Liaskou E.
      • Zimmermann H.W.
      • Li K.-K.
      • Oo Y.H.
      • Suresh S.
      • Stamataki Z.
      • et al.
      Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
      ]. Monocytes are therefore likely to be involved in transient inflammatory responses in the liver, but may also be precursor cells to some DC and KC populations, even in normal liver.

      The literature does not define a organ resident or passenger MPS

      The liver is a unique organ as it is highly vascular, blood filtering, and maintains a tissue resident population of the MPS. We currently lack markers that define cells that are transiently passing through the organ from those that are tissue resident. The fact that cells, bearing monocytic markers, can upregulate molecules more commonly associated with macrophages, such as CD68 and CD163, suggests that MPS precursors may alter their phenotype substantially as they traffic through the liver, and/or seed populations of KCs and DCs within the liver. Monocyte markers themselves are problematic: CD14 is also expressed by sinus-resident APCs in other human lymphoid organs [
      • Angel C.E.
      • Chen C.-J.J.J.
      • Horlacher O.C.
      • Winkler S.
      • John T.
      • Browning J.
      • et al.
      Distinctive localization of antigen-presenting cells in human lymph nodes.
      ], and also by endothelial cells [
      • Liaskou E.
      • Zimmermann H.W.
      • Li K.-K.
      • Oo Y.H.
      • Suresh S.
      • Stamataki Z.
      • et al.
      Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
      ], so it may not be specific for monocytes in the human liver; and CD14lo “non-classical” monocytes, transiting through the sinusoids, may be difficult to distinguish from resident KC populations.
      Analysis of c-Myb expression may be informative in this context. It was recently reported that the majority of tissue macrophages that persist into adulthood in mice appear to be negative for the transcription factor c-Myb, which is present on cells derived from haematopoietic stem cells [
      • Schulz C.
      • Gomez Perdiguero E.
      • Chorro L.
      • Szabo-Rogers H.
      • Cagnard N.
      • Kierdorf K.
      • et al.
      A lineage of myeloid cells independent of Myb and hematopoietic stem cells.
      ]. A series of lineage tracking studies in mice have shown that the majority of tissue macrophages are not replenished from bone-marrow derived monocytes, but are self-replicating macrophages that seed the tissue from the yolk-sac and the foetal liver during embryogenesis [
      • Yona S.
      • Kim K.
      • Wolf Y.
      • Mildner A.
      • Varol D.
      • Breker M.
      • et al.
      Fate mapping reveals origins and dynamics of monocytes and tissue macrophages under homeostasis.
      ]. Small numbers of macrophage populations are derived from monocytes under homeostatic conditions in the adult liver, with further mobilization of these cells only under inflammatory conditions [
      • Hettinger J.
      • Richards D.M.
      • Hansson J.
      • Barra M.M.
      • Joschko A.-C.
      • Krijgsveld J.
      • et al.
      Origin of monocytes and macrophages in a committed progenitor.
      ]. Hence, it should now be feasible to identify markers, capable of unequivocally distinguishing between blood-derived MPS and those resident KCs that do not originate from a bone marrow precursor. Further to that, as in other tissue [
      • Segura E.
      • Touzot M.
      • Bohineust A.
      • Cappuccio A.
      • Chiocchia G.
      • Hosmalin A.
      • et al.
      Human inflammatory dendritic cells induce Th17 cell differentiation.
      ], analysis of transcription factors may improve this.

      Several DC subsets are present in the liver

      Liver DC subsets are more thoroughly described than liver macrophages, due to the more recent application of multi-parameter flow cytometry, detecting definitive cell surface markers.
      The current evidence points to the liver containing similar DC populations as other tissue. All the major subsets found blood, including pDCs, CD1c+ DCs and CLEC9A+ DCs appear to be present. Unfortunately, functional data on these subsets are limited, as is a detailed expression of their phenotype (such as a complete description of the pathogen recognition receptors, CCRs, and lectins). The small amount of published data suggests that they are less mature than DCs found in blood and spleen, poorer at eliciting an antigen specific T cell response or MLR, and are associated with an increased production of IL-4 and IL-10 [
      • Bamboat Z.M.
      • Stableford J.A.
      • Plitas G.
      • Burt B.M.
      • Nguyen H.M.
      • Welles A.P.
      • et al.
      Human liver dendritic cells promote T cell hyporesponsiveness.
      ]. Of particular interest is the relative immunogenicity of CD141+ cells in both normal and diseased liver. Considering the increased expression of CD141 in tissue DC populations [
      • Park S.M.
      • Angel C.E.
      • McIntosh J.D.
      • Brooks A.E.S.
      • Middleditch M.
      • Chen C.-J.J.
      • et al.
      Sphingosine-1-phosphate lyase is expressed by CD68(+) cells on the parenchymal side of marginal reticular cells in human lymph nodes.
      ], it will be important to determine whether this is due to CD141hiCLEC9A+ cells or any other liver cell that happen to express CD141.

      The tolerogenic environment of the liver and the MPS

      Although cells of the MPS are implicated in producing the tolerogenic environment of the liver, there remain large areas of MPS characterisation to be explored. However the limited data available from liver DCs suggest substantive differences in their ability to perform some functions, such as phagocytosis, migration, T cell stimulation, cross-presentation, and specific cytokine secretion.
      Tolerance in the liver appears to be due to many factors, including the nature of T cells, the effect of other non-parenchymal cells, and large amounts of TGF-beta [
      • Tiegs G.
      • Lohse A.W.
      Immune tolerance: what is unique about the liver.
      ]. Production of IL-10 in response to bacterial cell wall components represents a prominent tolerogenic mechanism that appears to occur in all three groups of MPS cells in the liver. Considering the large amounts of bacterial cell wall products that enter the liver through the portal vein in the normal liver [
      • Singh R.
      • Bullard J.
      • Kalra M.
      • Assefa S.
      • Kaul A.K.
      • Vonfeldt K.
      • et al.
      Status of bacterial colonization, Toll-like receptor expression and nuclear factor-kappa B activation in normal and diseased human livers.
      ], this mechanism may be central to the liver’s tolerant state. Studies of liver perfusates [
      • Tu Z.
      • Bozorgzadeh A.
      • Pierce R.H.
      • Kurtis J.
      • Crispe I.N.
      • Orloff M.S.
      TLR-dependent cross talk between human Kupffer cells and NK cells.
      ] have supported these findings and it appears that the loss of this mechanism in cirrhosis may be due to a modification of the MPS [
      • Balmer M.L.
      • Slack E.
      • de Gottardi A.
      • Lawson M.A.E.
      • Hapfelmeier S.
      • Miele L.
      • et al.
      The liver may act as a firewall mediating mutualism between the host and its gut commensal microbiota.
      ].
      An area notably lacking data concerns the physiological response of the liver MPS to the presence of whole bacteria [
      • Singh R.
      • Bullard J.
      • Kalra M.
      • Assefa S.
      • Kaul A.K.
      • Vonfeldt K.
      • et al.
      Status of bacterial colonization, Toll-like receptor expression and nuclear factor-kappa B activation in normal and diseased human livers.
      ]. Translocation of bacteria and bacterial fragments from the gut is a common occurrence [
      • Jenne C.N.
      • Kubes P.
      Immune surveillance by the liver.
      ], and the responses they induce in different liver MPS subsets warrant further investigation as they presumably are part of the physiological role of some liver MPS subsets in vivo. In human blood, whole E. coli appears to promote a further induction of IL-10 expression by CD1c+ DCs [
      • Kassianos A.J.
      • Hardy M.Y.
      • Ju X.
      • Vijayan D.
      • Ding Y.
      • Vulink A.J.
      • et al.
      Human CD1c (BDCA-1)+ myeloid dendritic cells secrete IL-10 and display an immuno-regulatory phenotype and function in response to Escherichia coli.
      ] that is independent of TLR ligation; it is yet to be determined if this occurs in the liver, but may also be a further contributing factor in promoting hyporesponsiveness.
      The liver DCs appear to have a relatively immature phenotype with a lower expression of co-stimulatory markers [
      • Bamboat Z.M.
      • Stableford J.A.
      • Plitas G.
      • Burt B.M.
      • Nguyen H.M.
      • Welles A.P.
      • et al.
      Human liver dendritic cells promote T cell hyporesponsiveness.
      ]. This, coupled with their inferior abilities of antigen uptake, processing and presentation, may be contributing factors to their decreased ability to simulate T cells [
      • Bamboat Z.M.
      • Stableford J.A.
      • Plitas G.
      • Burt B.M.
      • Nguyen H.M.
      • Welles A.P.
      • et al.
      Human liver dendritic cells promote T cell hyporesponsiveness.
      ,
      • Goddard S.
      • Youster J.
      • Morgan E.
      • Adams D.H.
      Interleukin-10 secretion differentiates dendritic cells from human liver and skin.
      ]. When T cells are primed by liver DCs they are then more difficult to re-activate compared to blood DCs (this T cell hyporesponsiveness appears to be partially regulated through an IL-10 dependant mechanism), and are more likely to be Foxp3+ regulatory T cells [
      • Bamboat Z.M.
      • Stableford J.A.
      • Plitas G.
      • Burt B.M.
      • Nguyen H.M.
      • Welles A.P.
      • et al.
      Human liver dendritic cells promote T cell hyporesponsiveness.
      ]. These are all factors that may contribute to a tolerogenic environment in the liver.

      Kupffer cells appear to be heterogeneous

      In general, KCs harbour many of the characteristics of other tissue macrophages. They readily phagocytose latex beads [
      • Viñas O.
      • Bataller R.
      • Sancho-Bru P.
      • Ginès P.
      • Berenguer C.
      • Enrich C.
      • et al.
      Human hepatic stellate cells show features of antigen-presenting cells and stimulate lymphocyte proliferation.
      ] and express a variety of macrophage related scavenger receptor molecules [
      • Martens J.H.
      • Kzhyshkowska J.
      • Falkowski-Hansen M.
      • Schledzewski K.
      • Gratchev A.
      • Mansmann U.
      • et al.
      Differential expression of a gene signature for scavenger/lectin receptors by endothelial cells and macrophages in human lymph node sinuses, the primary sites of regional metastasis.
      ,
      • Hartnell A.
      • Steel J.
      • Turley H.
      • Jones M.
      • Jackson D.G.
      • Crocker P.R.
      Characterization of human sialoadhesin, a sialic acid binding receptor expressed by resident and inflammatory macrophage populations.
      ]. However, there is clearly variability in the expression of these markers and other molecules amongst KCs and light [
      • Burgio V.L.
      • Ballardini G.
      • Artini M.
      • Caratozzolo M.
      • Bianchi F.B.
      • Levrero M.
      Expression of co-stimulatory molecules by Kupffer cells in chronic hepatitis of hepatitis C virus etiology.
      ] and electron [
      • Ueda T.
      • Kohli Y.
      • Abe Y.
      • Katoh T.
      • Ogasawara T.
      • Nojyo Y.
      • et al.
      Electron microscopic study of endogenous peroxidase activity in human liver macrophages.
      ] microscopy findings have identified that KCs are composed of differing subsets of cells, which are distributed through different zones of the liver lobule. These findings indicate that KCs appear to be heterogeneous, but exploration of KC subset composition and function is lacking.
      Through their expression of molecules involved in the presentation of antigen to T cells (MHC class II, T cell co-stimulatory molecules) it seems likely that at least some KCs are involved in presenting antigen to liver-resident T cells but their migratory potential and their capacity to traffic antigen to lymph nodes remain uncertain.
      Macrophages are particularly difficult cells to isolate from human liver, due to the loss of large numbers of cells during isolation and lack of consistent cell-membrane markers that can be used in cell sorting. CD68 has a variable expression on the cell surface [
      • Kunisch E.
      • Fuhrmann R.
      • Roth A.
      • Winter R.
      • Lungershausen W.
      • Kinne R.W.
      Macrophage specificity of three anti-CD68 monoclonal antibodies (KP1, EBM11, and PGM1) widely used for immunohistochemistry and flow cytometry.
      ], and little else beyond CD14, CD16, and CD163 has been considered, despite the obvious overlap with transiting monocytes [
      • Liaskou E.
      • Zimmermann H.W.
      • Li K.-K.
      • Oo Y.H.
      • Suresh S.
      • Stamataki Z.
      • et al.
      Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
      ,
      • Geissmann F.
      • Manz M.G.
      • Jung S.
      • Sieweke M.H.
      • Merad M.
      • Ley K.
      Development of monocytes, macrophages, and dendritic cells.
      ].
      There is therefore a need to define and accurately sort KC subsets, in order to enable functional assays for studies in both healthy liver and disease. Multicolour fluorescence microscopy and flow cytometry may provide sufficient details to accurately and comprehensively assess a molecular phenotype for KCs (as has been the case with characterizing other cell subsets [
      • Feisst V.
      • Brooks A.E.S.
      • Chen C.-J.J.
      • Dunbar P.R.
      Characterization of mesenchymal progenitor cell populations directly derived from human dermis.
      ]). As our understanding of the complexity of macrophages increases, the limits of general terms, such as “Kupffer cell” [
      • Chow A.
      • Brown B.
      • Merad M.
      Studying the mononuclear phagocyte system in the molecular age.
      ], may make it timely to review and revise our terminology.

      Liver microanatomy

      Ultimately, understanding MPS function in the liver will require an appreciation of histological differences in the distribution of these different subsets throughout the different areas of the liver and an improved understanding of their role and interaction with sinusoidal endothelia, stellate cells and other leukocytes. Despite data indicating other cell types, such as hepatocytes [
      • Jungermann K.
      • Katz N.
      Functional specialization of different hepatocyte populations.
      ] and LSECs [
      • Gao Z.
      • Williams G.M.
      Vascular endothelial-cell turnover: a new factor in the vascular microenvironment of the liver.
      ,
      • Lalor P.F.
      • Lai W.K.
      • Curbishley S.M.
      • Shetty S.
      • Adams D.H.
      Human hepatic sinusoidal endothelial cells can be distinguished by expression of phenotypic markers related to their specialised functions in vivo.
      ] that are different in different zones of the liver, little data exist beyond gross morphological observations for zonal distributions of different MPS populations, especially KCs.
      Although it is apparent that KCs are scattered throughout the liver lobule, without sophisticated techniques to determine subsets, based on molecular marker expression, using more than one marker (such as CD68) it is impossible to ascertain any accurate intralobular differences in subset distributions, or examine how MPS function changes with location. It would be reasonable for the composition and function of perisinusoidal KCs to also change across these zones as observed with hepatocytes [
      • Jungermann K.
      • Katz N.
      Functional specialization of different hepatocyte populations.
      ] and LSECs [
      • Gao Z.
      • Williams G.M.
      Vascular endothelial-cell turnover: a new factor in the vascular microenvironment of the liver.
      ,
      • Lalor P.F.
      • Lai W.K.
      • Curbishley S.M.
      • Shetty S.
      • Adams D.H.
      Human hepatic sinusoidal endothelial cells can be distinguished by expression of phenotypic markers related to their specialised functions in vivo.
      ]. KCs are closely associated with the sinusoidal endothelia; and are likely to be able to interact extensively with the slow moving plasma and blood cells as they transit [
      • Jenne C.N.
      • Kubes P.
      Immune surveillance by the liver.
      ].
      In contrast, the portal tract is where the majority of the liver’s dendritic cells reside [
      • Crispe I.N.
      Liver antigen-presenting cells.
      ], and it becomes very heavily populated with leukocytes during liver inflammation [
      • Jain A.
      • Ryan C.
      • Mohanka R.
      • Orloff M.
      • Abt P.
      • Romano J.
      • et al.
      Characterization of CD4, CD8, CD56 positive lymphocytes and C4d deposits to distinguish acute cellular rejection from recurrent hepatitis C in post-liver transplant biopsies.
      ]. The portal tract also houses the lymphatic endothelium, and is hence the conduit through which the MPS will travel to draining lymph nodes. As a result of these features it is thought to be the area of the liver lobule where the majority of antigen presentation to T cells occurs. A thorough understanding of the relationship of each MPS subset to the micro-anatomical structure of the liver will therefore help to inform knowledge of their function.

      Clinical significance

      In the case of transplantation, despite showing superior graft acceptance in comparison to other transplanted organs, graft rejection is still a major problem in the liver transplant setting with rejection rates as high as 20–40% [
      • Shaked A.
      • Ghobrial R.M.
      • Merion R.M.
      • Shearon T.H.
      • Emond J.C.
      • Fair J.H.
      • et al.
      Incidence and severity of acute cellular rejection in recipients undergoing adult living donor or deceased donor liver transplantation.
      ]. The focus of the current therapy is primarily to reduce the presence of a pro-inflammatory state at the time of transplantation through the use of calcineurin inhibitors, such as cyclosporin or tacrolimus in combination with cytolytic agents, such as mycophenolate or azathioprine [
      • Sanchez-Fueyo A.
      • Strom T.B.
      Immunologic basis of graft rejection and tolerance following transplantation of liver or other solid organs.
      ]. These drugs are associated with significant side effects, directly through renal and cardiac toxicity but also through the indirect effects of inducing a broadly dysregulated immunological state, leading to higher rates of cancer, infection, and de novo autoimmune disease [
      • Fischer S.E.
      Recurrent and de novo malignancies following liver transplantation.
      ]. The study of MPS in the liver will therefore improve our understanding of the liver’s tolerogenic state and the nature of the biological processes, involved in the loss of this normal state. Understanding which subsets are most tolerogenic or immunogenic may identify targets for up- or downregulation, depending on the desired tissue response.
      Further to this, appreciating which subsets are most capable of antigen presentation and T cell stimulation allows for the opportunity to improve the efficacy of immunotherapy directed towards cancer and infectious diseases. Primary liver cancers, such as hepatocellular carcinoma (HCC), and metastasis (for example from colon cancer), have a high mortality rate and continue to pose huge burdens on the medical community [
      • Zhong J.-H.H.
      • Ma L.
      • Wu L.-C.C.
      • Zhao W.
      • Yuan W.-P.P.
      • Wu F.-X.X.
      • et al.
      Adoptive immunotherapy for postoperative hepatocellular carcinoma: a systematic review.
      ]. HCC in particular reflects the consequences of long-term liver inflammation through insidious disease and infection, such as hepatitis C and hepatitis B, both of which place a huge burden on global health [
      • Bruix J.
      • Sherman M.
      Management of hepatocellular carcinoma: an update.
      ].

      Conclusion

      Currently strategies to manipulate the liver MPS are impaired by a lack of appreciation of the populations of monocytes, macrophages and DCs present in human liver, and their functional attributes. Modern techniques, already being used to describe MPS populations in other organs, should now be implemented to improve the understanding of the liver MPS. An improved and more accurate understanding of these cells will be vital for the accurate description of cell function, and elucidation of appropriate targets for therapy. This will cover a range of applications from enhancing immunity to cancer or infectious agents, to inducing and maintaining tolerance, such as in liver transplantation and autoimmune disease.

      Financial support

      We are grateful for the funding support from the following grants; the Department of Surgery Lectureship grant, Auckland Medical Research Foundation Doctoral Scholarship, MercyAscot Doctoral Scholarship.

      Conflict of interest

      The authors who have taken part in this study declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.

      Acknowledgement

      We acknowledge the assistance with production of the Figures given by Vivian L. Ward.

      References

        • Trinchieri G.
        Interleukin-12: a cytokine produced by antigen-presenting cells with immunoregulatory functions in the generation of T-helper cells type 1 and cytotoxic lymphocytes.
        Blood. 1994; 84: 4008-4027
        • Crispe I.N.
        The liver as a lymphoid organ.
        Annu Rev Immunol. 2009; 27: 147-163
        • Sanchez-Fueyo A.
        • Strom T.B.
        Immunologic basis of graft rejection and tolerance following transplantation of liver or other solid organs.
        Gastroenterology. 2011; 140: 51-64
        • Protzer U.
        • Maini M.K.
        • Knolle P.A.
        Living in the liver: hepatic infections.
        Nat Rev Immunol. 2012; 12: 201-213
        • Neil D.A.H.
        • Hübscher S.G.
        Current views on rejection pathology in liver transplantation.
        Transpl Int. 2010; 23: 971-983
        • Crispe I.N.
        Liver antigen-presenting cells.
        J Hepatol. 2011; 54: 357-365
        • Thomson A.
        • Knolle P.
        Antigen-presenting cell function in the tolerogenic liver environment.
        Nat Rev Immunol. 2010; 10: 753-766
        • Winau F.
        • Quack C.
        • Darmoise A.
        • Kaufmann S.
        Starring stellate cells in liver immunology.
        Curr Opin Immunol. 2008; 20: 68-74
        • Knolle P.A.
        • Limmer A.
        Control of immune responses by savenger liver endothelial cells.
        Swiss Med Wkly. 2003; 133: 501-506
        • Holz L.E.
        • Warren A.
        • Le Couteur D.G.
        • Bowen D.G.
        • Bertolino P.
        CD8+ T cell tolerance following antigen recognition on hepatocytes.
        J Autoimmun. 2010; 34: 15-22
        • Jenne C.N.
        • Kubes P.
        Immune surveillance by the liver.
        Nat Immunol. 2013; 14: 996-1006
        • Geissmann F.
        • Gordon S.
        • Hume D.
        • Mowat A.
        • Randolph G.
        • et al.
        Unravelling mononuclear phagocyte heterogeneity.
        Nat Rev Immunol. 2010; 10: 453
        • Ginhoux F.
        • Jung S.
        Monocytes and macrophages: developmental pathways and tissue homeostasis.
        Nat Rev Immunol. 2014; 14: 392-404
        • Jakubzick C.
        • Gautier E.L.
        • Gibbings S.L.
        • Sojka D.K.
        • Schlitzer A.
        • Johnson T.E.
        • et al.
        Minimal differentiation of classical monocytes as they survey steady-state tissues and transport antigen to lymph nodes.
        Immunity. 2013; 39: 599-610
        • Merad M.
        • Sathe P.
        • Helft J.
        • Miller J.
        • Mortha A.
        The dendritic cell lineage: ontogeny and function of dendritic cells and their subsets in the steady state and the inflamed setting.
        Annu Rev Immunol. 2013; 31: 563-604
        • Gadd V.L.
        • Melino M.
        • Roy S.
        • Horsfall L.
        • O’Rourke P.
        • Williams M.R.
        • et al.
        Portal, but not lobular, macrophages express matrix metalloproteinase-9: association with the ductular reaction and fibrosis in chronic hepatitis C.
        Liver Int. 2013; 33: 569-579
        • Ziegler-Heitbrock L.
        • Ancuta P.
        • Crowe S.
        • Dalod M.
        • Grau V.
        • Hart D.N.
        • et al.
        Nomenclature of monocytes and dendritic cells in blood.
        Blood. 2010; 116: e74-e80
        • Knolle P.A.
        • Gerken G.
        Local control of the immune response in the liver.
        Immunol Rev. 2000; 174: 21-34
        • Zimmermann H.
        • Trautwein C.
        • Tacke F.
        Functional role of monocytes and macrophages for the inflammatory response in acute liver injury.
        Front Physiol. 2012; 3: 56
        • Aspinall A.I.
        • Curbishley S.M.
        • Lalor P.F.
        • Weston C.J.
        • Blahova M.
        • Liaskou E.
        • et al.
        CX(3)CR1 and vascular adhesion protein-1-dependent recruitment of CD16(+) monocytes across human liver sinusoidal endothelium.
        Hepatology. 2010; 51: 2030-2039
        • Antoniades C.G.
        • Quaglia A.
        • Taams L.S.
        • Mitry R.R.
        • Hussain M.
        • Abeles R.
        • et al.
        Source and characterization of hepatic macrophages in acetaminophen-induced acute liver failure in humans.
        Hepatology. 2012; 56: 735-746
        • Liaskou E.
        • Zimmermann H.W.
        • Li K.-K.
        • Oo Y.H.
        • Suresh S.
        • Stamataki Z.
        • et al.
        Monocyte subsets in human liver disease show distinct phenotypic and functional characteristics.
        Hepatology. 2013; 57: 385-398
        • Zimmermann H.W.
        • Seidler S.
        • Nattermann J.
        • Gassler N.
        • Hellerbrand C.
        • Zernecke A.
        • et al.
        Functional contribution of elevated circulating and hepatic non-classical CD14+CD16+ monocytes to inflammation and human liver fibrosis.
        PLoS One. 2010; 5: e11049
        • Wong K.L.
        • Tai J.J.-Y.
        • Wong W.-C.C.
        • Han H.
        • Sem X.
        • Yeap W.-H.H.
        • et al.
        Gene expression profiling reveals the defining features of the classical, intermediate, and nonclassical human monocyte subsets.
        Blood. 2011; 118: e16-e31
        • Collin M.
        • McGovern N.
        • Haniffa M.
        Human dendritic cell subsets.
        Immunology. 2013; 140: 22-30
        • Segura E.
        • Touzot M.
        • Bohineust A.
        • Cappuccio A.
        • Chiocchia G.
        • Hosmalin A.
        • et al.
        Human inflammatory dendritic cells induce Th17 cell differentiation.
        Immunity. 2013; 38: 336-348
        • Angel C.E.
        • Chen C.-J.J.J.
        • Horlacher O.C.
        • Winkler S.
        • John T.
        • Browning J.
        • et al.
        Distinctive localization of antigen-presenting cells in human lymph nodes.
        Blood. 2009; 113: 1257
        • Wake K.
        Karl Wilhelm Kupffer and his contributions to modern hepatology.
        Comp Hepatol. 2004; 3: S2
        • Davies L.C.
        • Jenkins S.J.
        • Allen J.E.
        • Taylor P.R.
        Tissue-resident macrophages.
        Nat Immunol. 2013; 14: 986-995
        • Klein I.
        • Cornejo J.C.
        • Polakos N.K.
        • John B.
        • Wuensch S.A.
        • Topham D.J.
        • et al.
        Kupffer cell heterogeneity: functional properties of bone marrow derived and sessile hepatic macrophages.
        Blood. 2007; 110: 4077-4085
        • Bardadin K.A.
        • Scheuer P.J.
        • Peczek A.
        • Wejman J.
        Immunocytochemical observations on macrophage populations in normal fetal and adult human liver.
        J Pathol. 1991; 164: 253-259
        • Burgio V.L.
        • Ballardini G.
        • Artini M.
        • Caratozzolo M.
        • Bianchi F.B.
        • Levrero M.
        Expression of co-stimulatory molecules by Kupffer cells in chronic hepatitis of hepatitis C virus etiology.
        Hepatology. 1998; 27: 1600-1606
        • Dominguez-Soto A.
        • Aragoneses-Fenoll L.
        • Gomez-Aguado F.
        • Corcuera M.T.M.T.
        • Claria J.
        • Garcia-Monzon C.
        • et al.
        The pathogen receptor liver and lymph node sinusoidal endotelial cell C-type lectin is expressed in human Kupffer cells and regulated by PU.1.
        Hepatology. 2009; 49: 287-296
        • Fayyazi A.
        • Scheel O.
        • Werfel T.
        • Schweyer S.
        • Oppermann M.
        • Götze O.
        • et al.
        The C5a receptor is expressed in normal renal proximal tubular but not in normal pulmonary or hepatic epithelial cells.
        Immunology. 2000; 99: 38-45
        • Fukuda Y.
        • Imoto M.
        • Koyama Y.
        • Miyazawa Y.
        • Nakano I.
        • Hattori M.
        • et al.
        Immunohistochemical study on tissue inhibitors of metalloproteinases in normal and pathological human livers.
        Gastroenterol Jpn. 1991; 26: 37-41
        • Gaweco A.S.
        • Wiesner R.H.
        • Yong S.
        • Krom R.
        • Porayko M.
        • Chejfec G.
        • et al.
        CD40L (CD154) expression in human liver allografts during chronic ductopenic rejection.
        Liver Transplant Surg. 1999; 5: 1-7
        • Lautenschlager I.
        • Taskinen E.
        • Inkinen K.
        • Lehto V.P.
        • Virtanen I.
        • Hayry P.
        Distribution of the major histocompatibility complex antigens on different cellular components of human liver.
        Cell Immunol. 1984; 85: 191-200
        • Lefkowitch J.H.
        • Haythe J.H.
        • Regent N.
        Kupffer cell aggregation and perivenular distribution in steatohepatitis.
        Mod Pathol. 2002; 15: 699-704
        • Leifeld L.
        • Clemens C.
        • Heller J.
        • Trebicka J.
        • Sauerbruch T.
        • Spengler U.
        Expression of urotensin II and its receptor in human liver cirrhosis and fulminant hepatic failure.
        Dig Dis Sci. 2010; 55: 1458-1464
        • Martens J.H.
        • Kzhyshkowska J.
        • Falkowski-Hansen M.
        • Schledzewski K.
        • Gratchev A.
        • Mansmann U.
        • et al.
        Differential expression of a gene signature for scavenger/lectin receptors by endothelial cells and macrophages in human lymph node sinuses, the primary sites of regional metastasis.
        J Pathol. 2006; 208: 574-589
        • Masuda M.
        • Senju S.
        • Fujii S.I.
        • Terasaki Y.
        • Takeya M.
        • Hashimoto S.I.
        • et al.
        Identification and immunocytochemical analysis of DCNP1, a dendritic cell-associated nuclear protein.
        Biochem Biophys Res Commun. 2002; 290: 1022-1029
        • McGuinness P.H.
        • Painter D.
        • Davies S.
        • McCaughan G.W.
        Increases in intrahepatic CD68 positive cells, MAC387 positive cells, and proinflammatory cytokines (particularly interleukin 18) in chronic hepatitis C infection.
        Gut. 2000; 46: 260-269
        • Monnier J.
        • Piquet-Pellorce C.
        • Feige J.-J.J.
        • Musso O.
        • Clement B.
        • Turlin B.
        • et al.
        Prokineticin 2/Bv8 is expressed in Kupffer cells in liver and is down regulated in human hepatocellular carcinoma.
        World J Gastroenterol. 2008; 14: 1182-1191
        • Nakagawa-Toyama Y.
        • Hirano K.
        • Tsujii K.
        • Nishida M.
        • Miyagawa J.
        • Sakai N.
        • et al.
        Human scavenger receptor class B type I is expressed with cell-specific fashion in both initial and terminal site of reverse cholesterol transport.
        Atherosclerosis. 2005; 183: 75-83
        • Pungercar J.
        • Viyjak A.
        • Ivanovski G.
        • Krizaj I.
        Tissue expression and immunolocalization of a novel human cathepsin P.
        Pflugers Arch. 2000; 439: R119-R121
        • Scoazec J.Y.
        • Feldmann G.
        Both macrophages and endothelial cells of the human hepatic sinusoid express the CD4 molecule, a receptor for the human immunodeficiency virus.
        Hepatology. 1990; 12: 505-510
        • Shimada M.
        • Kajiyama K.
        • Hasegawa H.
        • Gion T.
        • Ikeda Y.
        • Shirabe K.
        • et al.
        Role of adhesion molecule expression and soluble fractions in hepatic resection.
        J Am Coll Surg. 1998; 186: 534-541
        • Szalowska E.
        • Elferink M.G.L.
        • Hoek A.
        • Groothuis G.M.M.
        • Vonk R.J.
        Resistin is more abundant in liver than adipose tissue and is not up-regulated by lipopolysaccharide.
        J Clin Endocrinol Metab. 2009; 94: 3051-3057
        • Tomita M.
        • Yamamoto K.
        • Kobashi H.
        • Ohmoto M.
        • Tsuji T.
        Immunohistochemical analysis on the expressions of maturation associated antigens, Fcγ receptors and CD14 in normal and diseased human liver macrophages.
        Int Hepatol Commun. 1994; 2: 245-249
        • Tomokiyo R.
        • Jinnouchi K.
        • Honda M.
        • Wada Y.
        • Hanada N.
        • Hiraoka T.
        • et al.
        Production, characterization, and interspecies reactivities of monoclonal antibodies against human class A macrophage scavenger receptors.
        Atherosclerosis. 2002; 161: 123-132
        • Wood G.S.
        • Turner R.R.
        • Shiurba R.A.
        • Eng L.
        • Warnke R.A.
        Human dendritic cells and macrophages. In situ immunophenotypic definition of subsets that exhibit specific morphologic and microenvironmental characteristics.
        Am J Pathol. 1985; 119: 73-82
        • Yoshioka T.
        • Yamamoto K.
        • Kobashi H.
        • Tomita M.
        • Tsuji T.
        Receptor-mediated endocytosis of chemically modified albumins by sinusoidal endothelial cells and Kupffer cells in rat and human liver.
        Liver. 1994; 14: 129-137
        • Zhao P.
        • Hou N.
        • Lu Y.
        Fhit protein is preferentially expressed in the nucleus of monocyte-derived cells and its possible biological significance.
        Histol Histopathol. 2006; 21: 915-923
        • Znoyko I.
        • Sohara N.
        • Spicer S.S.
        • Trojanowska M.
        • Reuben A.
        Expression of oncostatin M and its receptors in normal and cirrhotic human liver.
        J Hepatol. 2005; 43: 893-900
        • Baldus S.E.
        • Zirbes T.K.
        • Weidner I.C.
        • Flucke U.
        • Dittmar E.
        • Thiele J.
        • et al.
        Comparative quantitative analysis of macrophage populations defined by CD68 and carbohydrate antigens in normal and pathologically altered human liver tissue.
        Anal Cell Pathol. 1998; 16: 141-150
        • Tuijnman W.B.
        • Van Wichen D.F.
        • Schuurman H.J.
        Tissue distribution of human IgG Fc receptors CD16, CD32, and CD64: an immunohistochemical study.
        APMIS. 1993; 101: 319-329
        • Alabraba E.B.
        • Lai V.
        • Boon L.
        • Wigmore S.J.
        • Adams D.H.
        • Afford S.C.
        Coculture of human liver macrophages and cholangiocytes leads to CD40-dependent apoptosis and cytokine secretion.
        Hepatology. 2008; 47: 552-562
        • Arany E.
        • Afford S.
        • Strain A.J.
        • Winwood P.J.
        • Arthur M.J.
        • Hill D.J.
        Differential cellular synthesis of insulin-like growth factor binding protein-1 (IGFBP-1) and IGFBP-3 within human liver.
        J Clin Endocrinol Metab. 1994; 79: 1871-1876
        • Bastin J.
        • Drakesmith H.
        • Rees M.
        • Sargent I.
        • Townsend A.
        Localisation of proteins of iron metabolism in the human placenta and liver.
        Br J Haematol. 2006; 134: 532-543
        • Bauer I.
        • Rensing H.
        • Florax A.
        • Ulrich C.
        • Pistorius G.
        • Redl H.
        • et al.
        Expression pattern and regulation of heme oxygenase-1/heat shock protein 32 in human liver cells.
        Shock. 2003; 20: 116-122
        • Bode J.G.
        • Peters-Regehr T.
        • Kubitz R.
        • Haussinger D.
        Expression of glutamine synthetase in macrophages.
        J Histochem Cytochem. 2000; 48: 415-421
        • Brown K.E.
        • Broadhurst K.A.
        • Mathahs M.M.
        • Kladney R.D.
        • Fimmel C.J.
        • Srivastava S.K.
        • et al.
        Immunodetection of aldose reductase in normal and diseased human liver.
        Histol Histopathol. 2005; 20: 429-436
        • Brown K.E.
        • Brunt E.M.
        • Heinecke J.W.
        Immunohistochemical detection of myeloperoxidase and its oxidation products in Kupffer cells of human liver.
        Am J Pathol. 2001; 159: 2081-2088
        • Cope E.M.
        • Dilly S.A.
        Kupffer cell numbers during human development.
        Clin Exp Immunol. 1990; 81: 485-488
        • Esbach S.
        • Pieters M.N.
        • van der Boom J.
        • Schouten D.
        • van der Heyde M.N.
        • Roholl P.J.
        • et al.
        Visualization of the uptake and processing of oxidized low-density lipoproteins in human and rat liver.
        Hepatology. 1993; 18: 537-545
        • Friedman S.L.
        • Rockey D.C.
        • McGuire R.F.
        • Maher J.J.
        • Boyles J.K.
        • Yamasaki G.
        • et al.
        Isolated hepatic lipocytes and Kupffer cells from normal human liver: morphological and functional characteristics in primary culture.
        Hepatology. 1992; 15: 234-243
        • Funaki N.
        • Arii S.
        • Monden K.
        • Higashitsuji H.
        • Furutani M.
        • Mise M.
        • et al.
        Chemical mediators released by primary-cultured human hepatic macrophages in patients with and without cirrhosis: a study in tumor-bearing patients.
        Hepatology. 1996; 23: 1353-1358
        • Geuken E.
        • Buis C.I.
        • Visser D.S.
        • Blokzijl H.
        • Moshage H.
        • Nemes B.
        • et al.
        Expression of heme oxygenase-1 in human livers before transplantation correlates with graft injury and function after transplantation.
        Am J Transplant. 2005; 5: 1875-1885
        • Hartnell A.
        • Steel J.
        • Turley H.
        • Jones M.
        • Jackson D.G.
        • Crocker P.R.
        Characterization of human sialoadhesin, a sialic acid binding receptor expressed by resident and inflammatory macrophage populations.
        Blood. 2001; 97: 288-296
        • Hinglais N.
        • Kazatchkine M.D.
        • Mandet C.
        • Appay M.D.
        • Bariety J.
        Human liver Kupffer cells express CR1, CR3, and CR4 complement receptor antigens. An immunohistochemical study.
        Lab Invest. 1989; 61: 509-514
        • Huber H.
        • Douglas S.D.
        • Fudenberg H.H.
        The IgG receptor: an immunological marker for the characterization of mononuclear cells.
        Immunology. 1969; 17: 7-21
        • Hume D.A.
        • Allan W.
        • Hogan P.G.
        • Doe W.F.
        Immunohistochemical characterisation of macrophages in human liver and gastrointestinal tract: expression of CD4, HLA-DR, OKM1, and the mature macrophage marker 25F9 in normal and diseased tissue.
        J Leukoc Biol. 1987; 42: 474-484
        • Kitamura Y.
        • Okazaki T.
        • Nagatsuka Y.
        • Hirabayashi Y.
        • Kato S.
        • Hayashi K.
        Immunohistochemical distribution of phosphatidylglucoside using anti-phosphatidylglucoside monoclonal antibody (DIM21).
        Biochem Biophys Res Commun. 2007; 362: 252-255
        • Kiyohara H.
        • Egami H.
        • Shibata Y.
        • Murata K.
        • Ohshima S.
        • Ogawa M.
        Light microscopic immunohistochemical analysis of the distribution of group II phospholipase A2 in human digestive organs.
        J Histochem Cytochem. 1992; 40: 1659-1664
        • Kleinherenbrink-Stins M.F.
        • van de Boom J.H.
        • Schouten D.
        • Roholl P.J.
        • Niels van der Heyde M.
        • Brouwer A.
        • et al.
        Visualization of the interaction of native and modified lipoproteins with parenchymal, endothelial and Kupffer cells from human liver.
        Hepatology. 1991; 14: 79-90
        • Klockars M.
        • Reitamo S.
        Tissue distribution of lysozyme in man.
        J Histochem Cytochem. 1975; 23: 932-940
        • Kwekkeboom J.
        • Kuijpers M.A.
        • Bruyneel B.
        • Mancham S.
        • De Baar-Heesakkers E.
        • Ijzermans J.N.M.
        • et al.
        Expression of CD80 on Kupffer cells is enhanced in cadaveric liver transplants.
        Clin Exp Immunol. 2003; 132: 345-351
        • Le Bail B.
        • Bioulac-Sage P.
        • Senuita R.
        • Quinton A.
        • Saric J.
        • Balabaud C.
        Fine structure of hepatic sinusoids and sinusoidal cells in disease.
        J Electron Microsc Tech. 1990; 14: 257-282
        • Li L.
        • Grenard P.
        • Van Nhieu J.T.
        • Julien B.
        • Mallat A.
        • Habib A.A.
        • et al.
        Heme oxygenase-1 is an antifibrogenic protein in human hepatic myofibroblasts.
        Gastroenterology. 2003; 125: 460-469
        • Micklem K.J.
        • Stross W.P.
        • Willis A.C.
        • Cordell J.L.
        • Jones M.
        • Mason D.Y.
        Different isoforms of human FcRII distinguished by CDw32 antibodies.
        J Immunol. 1990; 144: 2295-2303
        • Nusing R.
        • Nüsing R.
        • Sauter G.
        • Fehr P.
        • Dürmüller U.
        • Kasper M.
        • et al.
        Localization of thromboxane synthase in human tissues by Tu 300.
        Virchows Arch. 1992; 421: 249-254
        • Okino T.
        • Egami H.
        • Ohmachi H.
        • Takai E.
        • Tamori Y.
        • Nakagawa A.
        • et al.
        Immunohistochemical analysis of distribution of RON receptor tyrosine kinase in human digestive organs.
        Dig Dis Sci. 2001; 46: 424-429
        • Rodriguez-Agudo D.
        • Ren S.
        • Hylemon P.B.
        • Montañez R.
        • Redford K.
        • Natarajan R.
        • et al.
        Localization of StarD5 cholesterol binding protein.
        J Lipid Res. 2006; 47: 1168-1175
        • Roels F.
        • De Prest B.
        • De Pestel G.
        Liver and chorion cytochemistry.
        J Inherit Metab Dis. 1995; 18: 155-171
        • Rullier A.
        • Senant N.
        • Kisiel W.
        • Bioulac-Sage P.
        • Balabaud C.
        • Le Bail B.
        • et al.
        Expression of protease-activated receptors and tissue factor in human liver.
        Virchows Arch. 2006; 448: 46-51
        • Ueda T.
        • Kohli Y.
        • Abe Y.
        • Katoh T.
        • Ogasawara T.
        • Nojyo Y.
        • et al.
        Electron microscopic study of endogenous peroxidase activity in human liver macrophages.
        Histochem Cell Biol. 1995; 103: 11-17
        • Wang J.
        • Greene S.
        • Eriksson L.C.
        • Rozell B.
        • Reihnér E.
        • Einarsson C.
        • et al.
        Human sterol 12a-hydroxylase (CYP8B1) is mainly expressed in hepatocytes in a homogenous pattern.
        Histochem Cell Biol. 2005; 123: 441-446
        • Becker E.L.
        • Forouhar F.A.
        • Grunnet M.L.
        • Boulay F.
        • Tardif M.
        • Bormann B.J.
        • et al.
        Broad immunocytochemical localization of the formylpeptide receptor in human organs, tissues, and cells.
        Cell Tissue Res. 1998; 292: 129-135
        • Guo S.
        • Yang C.
        • Mei F.
        • Wu S.
        • Luo N.
        • Fei L.
        • et al.
        Down-regulation of Z39Ig on macrophages by IFN-gamma in patients with chronic HBV infection.
        Clin Immunol. 2010; 136: 282-291
        • Kaiserling E.
        • Ruck P.
        • Xiao J.-C.
        Immunoreactivity of neoplastic and non-neoplastic hepatocytes for CD68 and with 3A5, Ki-M1P, and MAC387 light- and electron-microscopic findings.
        Int Hepatol Commun. 1995; 3: 322-329
        • Moestrup S.K.
        • Gliemann J.
        • Pallesen G.
        Distribution of the alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein in human tissues.
        Cell Tissue Res. 1992; 269: 375-382
        • Murakami I.
        • Sarker A.B.
        • Hayashi K.
        • Akagi T.
        Lectin binding patterns in normal liver, chronic active hepatitis, liver cirrhosis, and hepatocellular carcinoma. An immunohistochemical and immunoelectron microscopic study.
        Acta Pathol Jpn. 1992; 42: 566-572
        • Osterreicher C.H.
        • Penz-Osterreicher M.
        • Grivennikov S.I.
        • Guma M.
        • Koltsova E.K.
        • Datz C.
        • et al.
        Fibroblast-specific protein 1 identifies an inflammatory subpopulation of macrophages in the liver.
        Proc Natl Acad Sci U S A. 2011; 108: 308-313
        • Zeng L.
        • Takeya M.
        • Takahashi K.
        AM-3K, a novel monoclonal antibody specific for tissue macrophages and its application to pathological investigation.
        J Pathol. 1996; 178: 207-214
        • Ballardini G.
        • Fallani M.
        • Bianchi F.B.
        • Pisi E.
        Antigen presenting cells in liver biopsies from patients with primary biliary cirrhosis.
        Autoimmunity. 1989; 3: 135-144
        • Viñas O.
        • Bataller R.
        • Sancho-Bru P.
        • Ginès P.
        • Berenguer C.
        • Enrich C.
        • et al.
        Human hepatic stellate cells show features of antigen-presenting cells and stimulate lymphocyte proliferation.
        Hepatology. 2003; 38: 919-929
        • Bosma B.M.
        • Metselaar H.J.
        • Mancham S.
        • Boor P.P.C.
        • Kusters J.G.
        • Kazemier G.
        • et al.
        Characterization of human liver dendritic cells in liver grafts and perfusates.
        Liver Transplant. 2006; 12: 384-393
        • Bamboat Z.M.
        • Stableford J.A.
        • Plitas G.
        • Burt B.M.
        • Nguyen H.M.
        • Welles A.P.
        • et al.
        Human liver dendritic cells promote T cell hyporesponsiveness.
        J Immunol. 2009; 182: 1901-1911
        • Goddard S.
        • Youster J.
        • Morgan E.
        • Adams D.H.
        Interleukin-10 secretion differentiates dendritic cells from human liver and skin.
        Am J Pathol. 2004; 164: 511-519
        • Haniffa M.
        • Shin A.
        • Bigley V.
        • McGovern N.
        • Teo P.
        • See P.
        • et al.
        Human tissues contain CD141hi cross-presenting dendritic cells with functional homology to mouse CD103+ nonlymphoid dendritic cells.
        Immunity. 2012; 37: 60-73
        • Ibrahim J.
        • Nguyen A.H.
        • Rehman A.
        • Ochi A.
        • Jamal M.
        • Graffeo C.S.
        • et al.
        Dendritic cell populations with different concentrations of lipid regulate tolerance and immunity in mouse and human liver.
        Gastroenterology. 2012; 143: 1061-1072
        • Kelly A.
        • Fahey R.
        • Fletcher J.M.
        • Keogh C.
        • Carroll A.G.
        • Siddachari R.
        • et al.
        CD141+ myeloid dendritic cells are enriched in healthy human liver.
        J Hepatol. 2014; 60: 135-142
        • Prickett T.C.
        • McKenzie J.L.
        • Hart D.N.
        Characterization of interstitial dendritic cells in human liver.
        Transplantation. 1988; 46: 754-761
        • Angel C.E.
        • Lala A.
        • Chen C.-J.J.
        • Edgar S.G.
        • Ostrovsky L.L.
        • Dunbar P.R.
        CD14+ antigen-presenting cells in human dermis are less mature than their CD1a+ counterparts.
        Int Immunol. 2007; 19: 1271-1279
        • Angel C.E.
        • George E.
        • Brooks A.E.S.
        • Ostrovsky L.L.
        • Brown T.L.H.
        • Dunbar P.R.
        Cutting edge: CD1a+ antigen-presenting cells in human dermis respond rapidly to CCR7 ligands.
        J Immunol. 2006; 176: 5730-5734
        • Kassianos A.J.
        • Hardy M.Y.
        • Ju X.
        • Vijayan D.
        • Ding Y.
        • Vulink A.J.
        • et al.
        Human CD1c (BDCA-1)+ myeloid dendritic cells secrete IL-10 and display an immuno-regulatory phenotype and function in response to Escherichia coli.
        Eur J Immunol. 2012; 42: 1512-1522
        • Steinbrink K.
        • Wölfl M.
        • Jonuleit H.
        • Knop J.
        • Enk A.H.
        Induction of tolerance by IL-10-treated dendritic cells.
        J Immunol. 1997; 159: 4772-4780
        • Kwekkeboom J.
        • Boor P.P.C.
        • Sen E.
        • Kusters J.G.
        • Drexhage H.A.
        • de Jong E.C.
        • et al.
        Human liver myeloid dendritic cells maturate in vivo into effector DC with a poor allogeneic T-cell stimulatory capacity.
        Transplant Proc. 2005; 37: 15-16
        • Zhu J.
        • Yamane H.
        • Paul W.E.
        Differentiation of effector CD4 T cell populations (∗).
        Annu Rev Immunol. 2010; 28: 445-489
        • Trinchieri G.
        Interleukin-12: a cytokine produced by antigen-presenting cells with immunoregulatory functions in the generation of T-helper cells type 1 and cytotoxic lymphocytes.
        Blood. 1994; 84: 4008-4027
        • Jongbloed S.L.
        • Kassianos A.J.
        • McDonald K.J.
        • Clark G.J.
        • Ju X.
        • Angel C.E.
        • et al.
        Human CD141+ (BDCA-3)+ dendritic cells (DCs) represent a unique myeloid DC subset that cross-presents necrotic cell antigens.
        J Exp Med. 2010; 207: 1247-1260
      1. Haniffa M, Bigley V, McGovern N, Wang X-N, Teo P, Sng D, et al. CD141+ human migratory dendritic cells are homologous to murine CD103+ dendritic cells and excel at exogenous antigen cross presentation. Immunol. Conf. Annu. Congr. Br. Soc. Immunol. 2011 Liverpool United Kingdom. Conf. 20111205 Conf. End 20111208. Conf. Publ. (var.pagings). 135 (pp 58), 2011. Date of Publication December 2011.

        • Strauss O.
        • Bartlett A.
        Dendritic cell subset composition in the human liver is more complex than it seems.
        J Hepatol. 2014; 60: 1097-1098
        • Hart
        Characterization of interstitial dendritic cells in human tissues.
        Transplant Proc. 1989; 21: 401-403
        • Tacke F.
        • Randolph G.J.
        Migratory fate and differentiation of blood monocyte subsets.
        Immunobiology. 2006; 211: 609-618
        • Schulz C.
        • Gomez Perdiguero E.
        • Chorro L.
        • Szabo-Rogers H.
        • Cagnard N.
        • Kierdorf K.
        • et al.
        A lineage of myeloid cells independent of Myb and hematopoietic stem cells.
        Science. 2012; 336: 86-90
        • Yona S.
        • Kim K.
        • Wolf Y.
        • Mildner A.
        • Varol D.
        • Breker M.
        • et al.
        Fate mapping reveals origins and dynamics of monocytes and tissue macrophages under homeostasis.
        Immunity. 2013; 38: 79-91
        • Hettinger J.
        • Richards D.M.
        • Hansson J.
        • Barra M.M.
        • Joschko A.-C.
        • Krijgsveld J.
        • et al.
        Origin of monocytes and macrophages in a committed progenitor.
        Nat Immunol. 2013; 14: 821-830
        • Park S.M.
        • Angel C.E.
        • McIntosh J.D.
        • Brooks A.E.S.
        • Middleditch M.
        • Chen C.-J.J.
        • et al.
        Sphingosine-1-phosphate lyase is expressed by CD68(+) cells on the parenchymal side of marginal reticular cells in human lymph nodes.
        Eur J Immunol. 2014; 44: 2425-2436
        • Tiegs G.
        • Lohse A.W.
        Immune tolerance: what is unique about the liver.
        J Autoimmun. 2010; 34: 1-6
        • Singh R.
        • Bullard J.
        • Kalra M.
        • Assefa S.
        • Kaul A.K.
        • Vonfeldt K.
        • et al.
        Status of bacterial colonization, Toll-like receptor expression and nuclear factor-kappa B activation in normal and diseased human livers.
        Clin Immunol. 2011; 138: 41-49
        • Tu Z.
        • Bozorgzadeh A.
        • Pierce R.H.
        • Kurtis J.
        • Crispe I.N.
        • Orloff M.S.
        TLR-dependent cross talk between human Kupffer cells and NK cells.
        J Exp Med. 2008; 205: 233-244
        • Balmer M.L.
        • Slack E.
        • de Gottardi A.
        • Lawson M.A.E.
        • Hapfelmeier S.
        • Miele L.
        • et al.
        The liver may act as a firewall mediating mutualism between the host and its gut commensal microbiota.
        Sci Transl Med. 2014; 6: 237ra66
        • Kunisch E.
        • Fuhrmann R.
        • Roth A.
        • Winter R.
        • Lungershausen W.
        • Kinne R.W.
        Macrophage specificity of three anti-CD68 monoclonal antibodies (KP1, EBM11, and PGM1) widely used for immunohistochemistry and flow cytometry.
        Ann Rheum Dis. 2004; 63: 774-784
        • Geissmann F.
        • Manz M.G.
        • Jung S.
        • Sieweke M.H.
        • Merad M.
        • Ley K.
        Development of monocytes, macrophages, and dendritic cells.
        Science. 2010; 327: 656-661
        • Feisst V.
        • Brooks A.E.S.
        • Chen C.-J.J.
        • Dunbar P.R.
        Characterization of mesenchymal progenitor cell populations directly derived from human dermis.
        Stem Cells Dev. 2014; 23: 631-642
        • Chow A.
        • Brown B.
        • Merad M.
        Studying the mononuclear phagocyte system in the molecular age.
        Nat Rev Immunol. 2011; 11: 788-798
        • Jungermann K.
        • Katz N.
        Functional specialization of different hepatocyte populations.
        Physiol Rev. 1989; 69: 708-764
        • Gao Z.
        • Williams G.M.
        Vascular endothelial-cell turnover: a new factor in the vascular microenvironment of the liver.
        Trends Immunol. 2001; 22: 421-422
        • Lalor P.F.
        • Lai W.K.
        • Curbishley S.M.
        • Shetty S.
        • Adams D.H.
        Human hepatic sinusoidal endothelial cells can be distinguished by expression of phenotypic markers related to their specialised functions in vivo.
        World J Gastroenterol. 2006; 12: 5429-5439
        • Jain A.
        • Ryan C.
        • Mohanka R.
        • Orloff M.
        • Abt P.
        • Romano J.
        • et al.
        Characterization of CD4, CD8, CD56 positive lymphocytes and C4d deposits to distinguish acute cellular rejection from recurrent hepatitis C in post-liver transplant biopsies.
        Clin Transplant. 2006; 20: 624-633
        • Shaked A.
        • Ghobrial R.M.
        • Merion R.M.
        • Shearon T.H.
        • Emond J.C.
        • Fair J.H.
        • et al.
        Incidence and severity of acute cellular rejection in recipients undergoing adult living donor or deceased donor liver transplantation.
        Am J Transpl. 2009; 9: 301-308
        • Fischer S.E.
        Recurrent and de novo malignancies following liver transplantation.
        Diagn Histopathol. 2012; 18: 290-296
        • Zhong J.-H.H.
        • Ma L.
        • Wu L.-C.C.
        • Zhao W.
        • Yuan W.-P.P.
        • Wu F.-X.X.
        • et al.
        Adoptive immunotherapy for postoperative hepatocellular carcinoma: a systematic review.
        Int J Clin Pract. 2012; 66: 21-27
        • Bruix J.
        • Sherman M.
        Management of hepatocellular carcinoma: an update.
        Hepatology. 2011; 53: 1020-1022