Advertisement

The phenotypic fate and functional role for bone marrow-derived stem cells in liver fibrosis

Open AccessPublished:December 14, 2011DOI:https://doi.org/10.1016/j.jhep.2011.09.021

      Summary

      Liver fibrosis is an outcome of chronic liver injury of any etiology. It is manifested by extensive deposition of extracellular matrix (ECM) proteins that produce a fibrous scar in the injured liver. Bone marrow (BM) cells may play an important role in pathogenesis and resolution of liver fibrosis. BM cells contribute to the inflammatory response by TGF-β1 secretion and activation of liver resident myofibroblasts. Moreover, BM itself can serve as a source of collagen expressing cells, e.g. BM-derived fibrocytes and mesenchymal progenitors, which in turn, have a potential to in situ differentiate into fibrogenic myofibroblasts and facilitate fibrosis. Finally, BM cells play an active part in resolution of liver fibrosis after cessation of fibrogenic stimuli. While natural killer (NK) cells are implicated in apoptosis of activated hepatic stellate cells/myofibroblasts, cells of myelo-monocitic lineage secrete matrix metalloproteinases to actively degrade the fibrous scar. The focus of this review is on the current understanding of the role of different subsets of BM cells in the onset, development and resolution of liver fibrosis.

      Introduction

      Liver fibrosis is caused by chronic injury which triggers apoptosis of hepatocytes, damage of the endothelial barrier, recruitment of inflammatory cells, increased secretion of TGF-β1, and activation of myofibroblasts responsible for scar formation [
      • Bataller R.
      • Brenner D.A.
      Liver fibrosis.
      ,
      • Kisseleva T.
      • Brenner D.A.
      Mechanisms of fibrogenesis.
      ]. However, the contribution of bone marrow (BM) cells to liver fibrosis remains controversial [
      • Kallis Y.N.
      • Forbes S.J.
      The bone marrow and liver fibrosis: friend or foe?.
      ,
      • Kisseleva T.
      • Uchinami H.
      • Feirt N.
      • Quintana-Bustamante O.
      • Segovia J.C.
      • Schwabe R.F.
      • et al.
      Bone marrow-derived fibrocytes participate in pathogenesis of liver fibrosis.
      ]. At the onset of fibrosis, BM cells are recruited to the site of injury to facilitate inflammation. It is believed that monocytes and macrophages are the primary source of TGF-β1, the major fibrogenic cytokine that plays a critical role in activation of fibrogenic myofibroblasts.
      Myofibroblasts express type I collagen and other extracellular matrix proteins that constitute the fibrous scar in liver fibrosis. Three sources of myofibroblasts have been identified: hepatic stellate cells (HSCs) in hepatotoxic liver injury [
      • Friedman S.L.
      • Roll F.J.
      • Boyles J.
      • Bissell D.M.
      Hepatic lipocytes: the principal collagen-producing cells of normal rat liver.
      ], portal fibroblasts in cholestatic liver injury [
      • Desmouliere A.
      • Darby I.
      • Costa A.M.
      • Raccurt M.
      • Tuchweber B.
      • Sommer P.
      • et al.
      Extracellular matrix deposition, lysyl oxidase expression, and myofibroblastic differentiation during the initial stages of cholestatic fibrosis in the rat.
      ], and fibrocytes in any inflamed liver (Fig. 1). Most myofibroblasts retain the markers of being originally derived from either fibroblasts (such as Thy1 and elastin), HSCs (such as vitamin A droplets, GFAP, and desmin), or fibrocytes (CD45). Theoretically, myofibroblasts may also be derived directly from a precursor cell, unrelated to stellate cells, fibroblasts, or fibrocytes. Cell fate mapping studies in reporter mice have demonstrated that both hepatic stellate cells and fibroblasts are septum transversum mesenchymal cells that migrate from the mesothelium and submesothelium C [
      • Asahina K.
      • Tsai S.Y.
      • Li P.
      • Ishii M.
      • Maxson Jr., R.E.
      • Sucov H.M.
      • et al.
      Mesenchymal origin of hepatic stellate cells, submesothelial cells, and perivascular mesenchymal cells during mouse liver development.
      ].
      Figure thumbnail gr1
      Fig. 1Possible origins of fibrogenic myofibroblasts. Hepatic myofibroblasts may originate from liver resident mesenchymal cells. These include hepatic stellate cells, which under physiological conditions reside in the space of Disse in a quiescent state, and in response to injury undergo activation into myofibroblasts. Portal fibroblasts may also be a source of myofibroblasts in the fibrotic liver. In addition, BM-derived hematopoietic and mesenchymal cells may contribute to the myofibroblast population. While the role of mesenchymal stem cells in liver fibrosis is not well characterized due to the lack of specific markers and difficulties with their isolation, hematopoietic stem cells contribute to hepatic fibrocytes in response to liver injury.
      Cessation of the injury often causes resolution of liver fibrosis with resorption of the fibrous scar [
      • Iredale J.P.
      Models of liver fibrosis: exploring the dynamic nature of inflammation and repair in a solid organ.
      ,
      • Iredale J.P.
      • Benyon R.C.
      • Pickering J.
      • McCullen M.
      • Northrop M.
      • Pawley S.
      • et al.
      Mechanisms of spontaneous resolution of rat liver fibrosis. Hepatic stellate cell apoptosis and reduced hepatic expression of metalloproteinase inhibitors.
      ]. Under these circumstances, activated myofibroblasts undergo senescence [
      • Krizhanovsky V.
      • Yon M.
      • Dickins R.A.
      • Hearn S.
      • Simon J.
      • Miething C.
      • et al.
      Senescence of activated stellate cells limits liver fibrosis.
      ,
      • Schnabl B.
      • Purbeck C.A.
      • Choi Y.H.
      • Hagedorn C.H.
      • Brenner D.
      Replicative senescence of activated human hepatic stellate cells is accompanied by a pronounced inflammatory but less fibrogenic phenotype.
      ,
      • Schrader J.
      • Fallowfield J.
      • Iredale J.P.
      Senescence of activated stellate cells: not just early retirement.
      ], apoptosis and disappear [
      • Issa R.
      • Zhou X.
      • Constandinou C.M.
      • Fallowfield J.
      • Millward-Sadler H.
      • Gaca M.D.
      • et al.
      Spontaneous recovery from micronodular cirrhosis: evidence for incomplete resolution associated with matrix cross-linking.
      ,
      • Ramachandran P.
      • Iredale J.P.
      Reversibility of liver fibrosis.
      ]. It has been shown that NK (and NKT) cells facilitate aHSCs apoptosis during regression of fibrosis [
      • Radaeva S.
      • Sun R.
      • Jaruga B.
      • Nguyen V.T.
      • Tian Z.
      • Gao B.
      Natural killer cells ameliorate liver fibrosis by killing activated stellate cells in NKG2D-dependent and tumor necrosis factor-related apoptosis-inducing ligand-dependent manners.
      ], while newly recruited monocytes actively degrade extracellular matrix proteins (ECM) [
      • Duffield J.S.
      • Forbes S.J.
      • Constandinou C.M.
      • Clay S.
      • Partolina M.
      • Vuthoori S.
      • et al.
      Selective depletion of macrophages reveals distinct, opposing roles during liver injury and repair.
      ] via upregulation and secretion of matrix metalloproteinases (e.g. MMP13) [
      • Uchinami H.
      • Seki E.
      • Brenner D.A.
      • D’Armiento J.
      Loss of MMP 13 attenuates murine hepatic injury and fibrosis during cholestasis.
      ] and collagenases [
      • Issa R.
      • Zhou X.
      • Trim N.
      • Millward-Sadler H.
      • Krane S.
      • Benyon C.
      • et al.
      Mutation in collagen-1 that confers resistance to the action of collagenase results in failure of recovery from CCl4-induced liver fibrosis, persistence of activated hepatic stellate cells, and diminished hepatocyte regeneration.
      ].
      Stem cell biology has become one of the most intensely studied areas of biomedical research and there is great optimism among scientists and the lay public that stem cells will be used as novel therapies for many incurable chronic diseases. Many institutions, including the State of California, have committed billions of dollars specifically to promote stem cell research with the goal of developing new therapies within a few years. As a result of new insights into stem cells, there is a renewed interest in the role of the bone marrow and its stem cells in liver fibrosis. The information to date is very conflicted, with different studies showing either a contributing effect or a therapeutic effect of bone marrow-derived cells to liver fibrosis.
      Many studies have raised the issue of whether liver myofibroblasts may be derived from bone marrow stem cells, either hematopoietic or mesenchymal stem cells. Due to their well defined cell lineage markers and methodology for hematopoietic stem cell transfer, the contribution of hematopoietic stem cells to the population of liver myofibroblasts may be readily assessed in experimental murine liver fibrosis.
      This review will address three issues: (1) the role of BM-derived macrophage to liver fibrogenesis, (2) the contribution of BM cells to myofibroblasts in the fibrotic liver, and (3) the role of BM stem cells in the resolution of liver fibrosis.

      Inflammation

      Expression of collagen type I marks fibrogenic/hematopoietic cells

      Figure thumbnail fx1

      Alternative mechanisms to clear bacterial debris

      Increased intestinal permeability has a critical role in the pathogenesis of liver fibrosis [
      • Seki E.
      • De Minicis S.
      • Osterreicher C.H.
      • Kluwe J.
      • Osawa Y.
      • Brenner D.A.
      • et al.
      TLR4 enhances TGF-beta signaling and hepatic fibrosis.
      ,
      • Yan A.W.
      • Fouts E.
      • Brandl J.
      • Starkel P.
      • Torralba M.
      • Schott E.
      • et al.
      Enteric dysbiosis associated with a mouse model of alcoholic liver disease.
      ]. Recent studies have demonstrated that in addition to phagocytosis, neutrophils, macrophages, and fibrocytes may utilize an alternative pathway to combat bacteria, by releasing extracellular DNA-based traps enriched in histones and major antimicrobial enzymes, cathelescidin and myeoloperoxidase [
      • Brinkmann V.
      • Reichard U.
      • Goosmann C.
      • Fauler B.
      • Uhlemann Y.
      • Weiss D.S.
      • et al.
      Neutrophil extracellular traps kill bacteria.
      ,
      • Chow O.A.
      • von Kockritz-Blickwede M.
      • Bright A.T.
      • Hensler M.E.
      • Zinkernagel A.S.
      • Cogen A.L.
      • et al.
      Statins enhance formation of phagocyte extracellular traps.
      ,
      • von Kockritz-Blickwede M.
      • Goldmann O.
      • Thulin P.
      • Heinemann K.
      • Norrby-Teglund A.
      • Rohde M.
      • et al.
      Phagocytosis-independent antimicrobial activity of mast cells by means of extracellular trap formation.
      ,
      • Yousefi S.
      • Gold J.A.
      • Andina N.
      • Lee J.J.
      • Kelly A.M.
      • Kozlowski E.
      • et al.
      Catapult-like release of mitochondrial DNA by eosinophils contributes to antibacterial defense.
      ]. It remains unclear why terminally differentiated cells with phagocytic capacity decide to intake or exterminate bacteria [
      • Chow O.A.
      • von Kockritz-Blickwede M.
      • Bright A.T.
      • Hensler M.E.
      • Zinkernagel A.S.
      • Cogen A.L.
      • et al.
      Statins enhance formation of phagocyte extracellular traps.
      ]. This mechanism is activated in Vegenar granulomatosus [
      • Kessenbrock K.
      • Krumbholz M.
      • Schonermarck U.
      • Back W.
      • Gross W.L.
      • Werb Z.
      • et al.
      Netting neutrophils in autoimmune small-vessel vasculitis.
      ] and Lupus nephritis [
      • Hakkim A.
      • Furnrohr B.G.
      • Amann K.
      • Laube B.
      • Abed U.A.
      • Brinkmann V.
      • et al.
      Impairment of neutrophil extracellular trap degradation is associated with lupus nephritis.
      ]. Although the significance of such phenomenon for liver fibrosis still has to be investigated, fibrocyte-like cells from the spleen (CD45+Collagen-α1(I)+ BM-derived cells) may form DNA traps following LPS- or CCl4-induced liver injury [
      • Kisseleva T.
      • von Kockritz-Blickwede M.
      • Reichart D.
      • McGillvray S.M.
      • Wingender G.
      • Kronenberg M.
      • et al.
      Fibrocyte-like cells recruited to the spleen support innate and adaptive immune responses to acute injury or infection.
      ]. Thus, identification and classification of fibrocytes and fibrocyte-like cells recruited to the injured liver may provide new insights into the pathogenesis of liver fibrosis.

      Recruited BM macrophages induce fibrosis

      BM macrophages and Kupffer cells (liver resident macrophages) are the major source of TGF-β1 in liver fibrosis [
      • Seki E.
      • De Minicis S.
      • Osterreicher C.H.
      • Kluwe J.
      • Osawa Y.
      • Brenner D.A.
      • et al.
      TLR4 enhances TGF-beta signaling and hepatic fibrosis.
      ]. T and B lymphocytes are also recruited to the site of injury and further facilitate secretion of fibrogenic cytokines. Ablation of myolo-monocytic CD11b+ cells in mice at the onset of liver fibrosis attenuated activation of fibrogenic myofibroblasts and collagen deposition in liver and kidney fibrosis [
      • Duffield J.S.
      • Forbes S.J.
      • Constandinou C.M.
      • Clay S.
      • Partolina M.
      • Vuthoori S.
      • et al.
      Selective depletion of macrophages reveals distinct, opposing roles during liver injury and repair.
      ,
      • Duffield J.S.
      • Tipping P.G.
      • Kipari T.
      • Cailhier J.F.
      • Clay S.
      • Lang R.
      • et al.
      Conditional ablation of macrophages halts progression of crescentic glomerulonephritis.
      ,
      • Karlmark K.R.
      • Weiskirchen R.
      • Zimmermann H.W.
      • Gassler N.
      • Ginhoux F.
      • Weber C.
      • et al.
      Hepatic recruitment of the inflammatory Gr1+ monocyte subset upon liver injury promotes hepatic fibrosis.
      ].
      Bacterial flora and toll-like receptor (TLRs) signaling are critical in the activation of Kupffer cells/macrophages and TGF-β1 release [
      • Seki E.
      • De Minicis S.
      • Osterreicher C.H.
      • Kluwe J.
      • Osawa Y.
      • Brenner D.A.
      • et al.
      TLR4 enhances TGF-beta signaling and hepatic fibrosis.
      ]. For example, TLR4 mutant and knockout mice are resistant to fibrosis of different etiologies [
      • Inokuchi S.
      • Aoyama T.
      • Miura K.
      • Osterreicher C.H.
      • Kodama Y.
      • Miyai K.
      • et al.
      Disruption of TAK1 in hepatocytes causes hepatic injury, inflammation, fibrosis, and carcinogenesis.
      ]. Moreover, genome wide analysis studies have demonstrated that individuals carrying a low efficiency polymorphism in TLR4 gene are less susceptible to HCV-induced liver fibrosis [
      • Li Y.
      • Chang M.
      • Abar O.
      • Garcia V.
      • Rowland C.
      • Catanese J.
      • et al.
      Multiple variants in toll-like receptor 4 gene modulate risk of liver fibrosis in Caucasians with chronic hepatitis C infection.
      ]. Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns (PAMP) such as lipopolysaccharide (LPS), bacterial cell wall component, peptideglycan, and bacteria-derived unmethylated CpG-DNA [
      • Inokuchi S.
      • Aoyama T.
      • Miura K.
      • Osterreicher C.H.
      • Kodama Y.
      • Miyai K.
      • et al.
      Disruption of TAK1 in hepatocytes causes hepatic injury, inflammation, fibrosis, and carcinogenesis.
      ]. In addition, endogenous ligands (alarmins, e.g. HMGB-1, hyaluronan) can bind TLR4 in the presence of CD14 and LPS-binding protein [LBP) and transduce similar signals [
      • Yang D.
      • Oppenheim J.J.
      Antimicrobial proteins act as “alarmins” in joint immune defense.
      ]. Upon activation of TLRs, recruited BM cells produce inflammatory cytokines, such as TNF-a, IL-6, IL-1, MCP-1, and RANTES [
      • Seki E.
      • Uchinami H.
      • Osawa Y.
      • Brenner D.A.
      • Schwabe R.F.
      TLR4 mediates inflammation and fibrogenesis after bile duct ligation.
      ]. Moreover, microbial products have a significant impact on fibrogenic progression [
      • Yan A.W.
      • Fouts E.
      • Brandl J.
      • Starkel P.
      • Torralba M.
      • Schott E.
      • et al.
      Enteric dysbiosis associated with a mouse model of alcoholic liver disease.
      ], and LPS synergistically facilitates other fibrogenic factors such as TGFβ-1, oxidative stress, and mechanical injury [
      • Albillos A.
      • de la Hera A.
      • Gonzalez M.
      • Moya J.L.
      • Calleja J.L.
      • Monserrat J.
      • et al.
      Increased lipopolysaccharide binding protein in cirrhotic patients with marked immune and hemodynamic derangement.
      ]. TLR4 on BM cells is important in experimental alcoholic liver disease [
      • Inokuchi S.
      • Tsukamoto H.
      • Park E.
      • Liu Z.X.
      • Brenner D.A.
      • Seki E.
      Toll-like receptor 4 mediates alcohol-induced steatohepatitis through bone marrow-derived and endogenous liver cells in mice.
      ], and TL9 on BM cells is important in experimental non-alcoholic steatohepatitis [
      • Miura K.
      • Kodama Y.
      • Inokuchi S.
      • Schnabl B.
      • Aoyama T.
      • Ohnishi H.
      • et al.
      Toll-like receptor 9 promotes steatohepatitis by induction of interleukin-1beta in mice.
      ].

      Antifibrotic effects of macrophages

      Original experiments by Duffield et al. [
      • Duffield J.S.
      • Forbes S.J.
      • Constandinou C.M.
      • Clay S.
      • Partolina M.
      • Vuthoori S.
      • et al.
      Selective depletion of macrophages reveals distinct, opposing roles during liver injury and repair.
      ] and subsequent studies [
      • Mitchell C.
      • Couton D.
      • Couty J.P.
      • Anson M.
      • Crain A.M.
      • Bizet V.
      • et al.
      Dual role of CCR2 in the constitution and the resolution of liver fibrosis in mice.
      ,
      • Xidakis C.
      • Ljumovic D.
      • Manousou P.
      • Notas G.
      • Valatas V.
      • Kolios G.
      • et al.
      Production of pro- and anti-fibrotic agents by rat Kupffer cells; the effect of octreotide.
      ] have demonstrated that, over a period of time, two functionally distinct types of macrophages are recruited to the injured liver. During the injury phase, pro-fibrogenic macrophages mediate recruitment of injury-associated macrophages that promote myofibroblast proliferation and apoptosis [
      • Goerdt S.
      • Orfanos C.E.
      Other functions, other genes: alternative activation of antigen-presenting cells.
      ]. In contrast, during recovery from injury, a population of macrophages predominates that resembles classical macrophages and does not promote HSC survival but mediates matrix degradation. This macrophage population is present during resolution of injury and at a time when pro-fibrogenic and inflammatory mediator levels are decreasing [
      • Duffield J.S.
      • Forbes S.J.
      • Constandinou C.M.
      • Clay S.
      • Partolina M.
      • Vuthoori S.
      • et al.
      Selective depletion of macrophages reveals distinct, opposing roles during liver injury and repair.
      ]. These two functional phenotypes are separated chronologically in experimental liver fibrosis by several days, suggesting that they may represent different populations.
      Figure thumbnail fx2

      Fibrogenic myofibroblasts

      Definition of myofibroblasts

      Myofibroblasts are characterized phenotypically by a stellate shape and expression of stress fibers, abundant pericellular matrix and fibrotic proteins (α-smooth muscle actin (α-SMA), non-muscle myosin, fibronectin, vimentin, and collagen type I) [
      • Eyden B.
      The myofibroblast: phenotypic characterization as a prerequisite to understanding its functions in translational medicine.
      ]. Ultrastructurally, myofibroblasts are defined by prominent rough endoplasmic reticulum (rER), a Golgi apparatus producing collagen, peripheral myofilaments, fibronexus (no lamina) and gap junctions [
      • Eyden B.
      The myofibroblast: phenotypic characterization as a prerequisite to understanding its functions in translational medicine.
      ]. Myofibroblasts are implicated in wound healing and fibroproliferative disorders [
      • Gabbiani G.
      • Ryan G.B.
      • Majne G.
      Presence of modified fibroblasts in granulation tissue and their possible role in wound contraction.
      ,
      • Majno G.
      • Gabbiani G.
      • Hirschel B.J.
      • Ryan G.B.
      • Statkov P.R.
      Contraction of granulation tissue in vitro: similarity to smooth muscle.
      ]. Myofibroblasts are produced in response to fibrogenic stimuli, such as TGF-β1 [
      • Parola M.
      • Marra F.
      • Pinzani M.
      Myofibroblast-like cells and liver fibrogenesis: emerging concepts in a rapidly moving scenario.
      ]. Classic myofibroblasts differentiate from a mesenchymal lineage and, therefore, lack expression of lymphoid markers such as CD45 or CD34. However, subsets of myofibroblasts can express Thy1.1 (CD90) or CD34. It remains unclear whether expression of these genes links (myo)fibroblasts to hematopoietic stem cells, or these antigens have a broader distribution than previously appreciated. Sustained injury may trigger (trans)differentiation of myofibroblasts from other cellular sources, including HSCs [
      • Bataller R.
      • Brenner D.A.
      Liver fibrosis.
      ].
      Figure thumbnail fx3

      The origin of fibrogenic myofibroblasts

      Although initial reports have suggested that BM may be a source of fibrogenic myofibroblasts [
      • Forbes S.J.
      • Russo F.P.
      • Rey V.
      • Burra P.
      • Rugge M.
      • Wright N.A.
      • et al.
      A significant proportion of myofibroblasts are of bone marrow origin in human liver fibrosis.
      ,
      • Russo F.P.
      • Alison M.R.
      • Bigger B.W.
      • Amofah E.
      • Florou A.
      • Amin F.
      • et al.
      The bone marrow functionally contributes to liver fibrosis.
      ], most recent studies have reported that the majority of myofibroblasts activated in response to injury are from liver resident cells [
      • Higashiyama R.
      • Moro T.
      • Nakao S.
      • Mikami K.
      • Fukumitsu H.
      • Ueda Y.
      • et al.
      Negligible contribution of bone marrow-derived cells to collagen production during hepatic fibrogenesis in mice.
      ,
      • Kisseleva T.
      • Brenner D.A.
      Fibrogenesis of parenchymal organs.
      ,
      • Kisseleva T.
      • Brenner D.A.
      Hepatic stellate cells and the reversal of fibrosis.
      ,
      • Kisseleva T.
      • Uchinami H.
      • Feirt N.
      • Quintana-Bustamante O.
      • Segovia J.C.
      • Schwabe R.F.
      • et al.
      Bone marrow-derived fibrocytes participate in pathogenesis of liver fibrosis.
      ,
      • Seki E.
      • De Minicis S.
      • Osterreicher C.H.
      • Kluwe J.
      • Osawa Y.
      • Brenner D.A.
      • et al.
      TLR4 enhances TGF-beta signaling and hepatic fibrosis.
      ]. These findings are based on BM transplantation techniques in mice, in which the collagen-α1(I) or collagen-α2(I) promoters drive expression of the GFP reporter only in BM cells [
      • Higashiyama R.
      • Moro T.
      • Nakao S.
      • Mikami K.
      • Fukumitsu H.
      • Ueda Y.
      • et al.
      Negligible contribution of bone marrow-derived cells to collagen production during hepatic fibrogenesis in mice.
      ,
      • Kisseleva T.
      • Uchinami H.
      • Feirt N.
      • Quintana-Bustamante O.
      • Segovia J.C.
      • Schwabe R.F.
      • et al.
      Bone marrow-derived fibrocytes participate in pathogenesis of liver fibrosis.
      ]. Since collagen-α1(I) or collagen-α2(I) fibers are expressed in the same cells to form a triple helix [
      • Stefanovic B.
      • Brenner D.A.
      5′ stem-loop of collagen alpha 1(I) mRNA inhibits translation in vitro but is required for triple helical collagen synthesis in vivo.
      ], these reporter genes are expected to exhibit identical localization. Indeed, similar results were obtained in both mice in response to two models of liver fibrosis [
      • Higashiyama R.
      • Moro T.
      • Nakao S.
      • Mikami K.
      • Fukumitsu H.
      • Ueda Y.
      • et al.
      Negligible contribution of bone marrow-derived cells to collagen production during hepatic fibrogenesis in mice.
      ,
      • Kisseleva T.
      • Uchinami H.
      • Feirt N.
      • Quintana-Bustamante O.
      • Segovia J.C.
      • Schwabe R.F.
      • et al.
      Bone marrow-derived fibrocytes participate in pathogenesis of liver fibrosis.
      ], bile duct ligation and toxic liver injury induced by CCl4, demonstrating that activated myofibroblasts do not originate in the BM but emerge from the liver resident cells, e.g. HSCs and portal fibroblasts. Meanwhile, a small population of collagen type I expressing BM-derived cells, scattered in the liver and spleen of these mice, is composed of fibrocytes [
      • Higashiyama R.
      • Moro T.
      • Nakao S.
      • Mikami K.
      • Fukumitsu H.
      • Ueda Y.
      • et al.
      Negligible contribution of bone marrow-derived cells to collagen production during hepatic fibrogenesis in mice.
      ,
      • Kisseleva T.
      • Uchinami H.
      • Feirt N.
      • Quintana-Bustamante O.
      • Segovia J.C.
      • Schwabe R.F.
      • et al.
      Bone marrow-derived fibrocytes participate in pathogenesis of liver fibrosis.
      ]. Despite differences in experimental approaches and duration of injury, there was no evidence that BM contributes to replenishment of HSCs and portal fibroblasts or liver stem cells.

      Fibrocytes are implicated in fibrogenesis of parenchymal organs

      Fibrocytes are defined as spindle shaped “CD45 and collagen type I (Col+) expressing leukocytes that mediate tissue repair and are capable of antigen presentation to naive T cells” [
      • Bucala R.
      • Spiegel L.A.
      • Chesney J.
      • Hogan M.
      • Cerami A.
      Circulating fibrocytes define a new leukocyte subpopulation that mediates tissue repair.
      ]. Due to their ability to differentiate into myofibroblasts, fibrocytes are implicated in the fibrogenesis of skin, lungs, kidneys, and the liver [
      • Abe R.
      • Donnelly S.C.
      • Peng T.
      • Bucala R.
      • Metz C.N.
      Peripheral blood fibrocytes: differentiation pathway and migration to wound sites.
      ,
      • Kisseleva T.
      • Brenner D.A.
      Fibrogenesis of parenchymal organs.
      ,
      • Strieter R.M.
      • Gomperts B.N.
      • Keane M.P.
      The role of CXC chemokines in pulmonary fibrosis.
      ]. In addition to collagen type I, fibronectin and vimentin, fibrocytes express CD45, CD34, MHCII, MHCI, CD11b, Gr-1, and secrete growth factors (TGF-β1, MCP-1) that promote deposition of ECM [
      • Bellini A.
      • Mattoli S.
      The role of the fibrocyte, a bone marrow-derived mesenchymal progenitor, in reactive and reparative fibroses.
      ,
      • Quan T.E.
      • Cowper S.
      • Wu S.P.
      • Bockenstedt L.K.
      • Bucala R.
      Circulating fibrocytes: collagen-secreting cells of the peripheral blood.
      ]. Upon injury or stress, fibrocytes proliferate in the BM and migrate to the injured organ [
      • Bucala R.
      • Spiegel L.A.
      • Chesney J.
      • Hogan M.
      • Cerami A.
      Circulating fibrocytes define a new leukocyte subpopulation that mediates tissue repair.
      ,
      • Quan T.E.
      • Cowper S.
      • Wu S.P.
      • Bockenstedt L.K.
      • Bucala R.
      Circulating fibrocytes: collagen-secreting cells of the peripheral blood.
      ]. The reported number of recruited fibrocytes varies from 25% (lung fibrosis) [
      • Kisseleva T.
      • Brenner D.A.
      Fibrogenesis of parenchymal organs.
      ,
      • Strieter R.M.
      • Keeley E.C.
      • Hughes M.A.
      • Burdick M.D.
      • Mehrad B.
      The role of circulating mesenchymal progenitor cells (fibrocytes) in the pathogenesis of pulmonary fibrosis.
      ] to ∼3–5% (liver fibrosis, e.g. BDL and CCl4) [
      • Kisseleva T.
      • Brenner D.A.
      Hepatic stellate cells and the reversal of fibrosis.
      ] of the collagen expressing cells, suggesting that the magnitude of fibrocyte differentiation into myofibroblasts depends on the organ and the type of injury. Fibrocytes originate from hematopoietic cells and differentiate in the liver into typical myofibroblasts [
      • Scholten D.
      • Reichart D.
      • Paik Y.H.
      • Lindert J.
      • Bhattacharya J.
      • Glass C.K.
      • et al.
      Migration of fibrocytes in fibrogenic liver injury.
      ]. Mice treated with human serum amyloid protein (hSAP) [
      • Pilling D.
      • Buckley C.D.
      • Salmon M.
      • Gomer R.H.
      Inhibition of fibrocyte differentiation by serum amyloid P.
      ], a natural inhibitor of fibrocyte differentiation and maturation, develop less fibrosis in response to injury. Our data and studies in other parenchymal organs [
      • Castano A.P.
      • Lin S.L.
      • Surowy T.
      • Nowlin B.T.
      • Turlapati S.A.
      • Patel T.
      • et al.
      Serum amyloid P inhibits fibrosis through Fc gamma R-dependent monocyte-macrophage regulation in vivo.
      ,
      • Murray L.A.
      • Rosada R.
      • Moreira A.P.
      • Joshi A.
      • Kramer M.S.
      • Hesson D.P.
      • et al.
      Serum amyloid P therapeutically attenuates murine bleomycin-induced pulmonary fibrosis via its effects on macrophages.
      ,
      • Pilling D.
      • Roife D.
      • Wang M.
      • Ronkainen S.D.
      • Crawford J.R.
      • Travis E.L.
      • et al.
      Reduction of bleomycin-induced pulmonary fibrosis by serum amyloid P.
      ] clearly demonstrate that fibrocytes play an important role in pathogenesis of many fibrogenic disorders, including lungs. Elevated levels of circulating fibrocytes in peripheral blood in patients with lung fibrosis have a poor prognostic value [
      • Moeller A.
      • Gilpin S.E.
      • Ask K.
      • Cox G.
      • Cook D.
      • Gauldie J.
      • et al.
      Circulating fibrocytes are an indicator of poor prognosis in idiopathic pulmonary fibrosis.
      ]. Moreover, hSAP has been successfully tested in limited clinical trials in patients with skin, kidney and lung fibrosis [
      • Castano A.P.
      • Lin S.L.
      • Surowy T.
      • Nowlin B.T.
      • Turlapati S.A.
      • Patel T.
      • et al.
      Serum amyloid P inhibits fibrosis through Fc gamma R-dependent monocyte-macrophage regulation in vivo.
      ,
      • Mathai S.K.
      • Gulati M.
      • Peng X.
      • Russell T.R.
      • Shaw A.C.
      • Rubinowitz A.N.
      • et al.
      Circulating monocytes from systemic sclerosis patients with interstitial lung disease show an enhanced profibrotic phenotype.
      ,
      • Murray L.A.
      • Rosada R.
      • Moreira A.P.
      • Joshi A.
      • Kramer M.S.
      • Hesson D.P.
      • et al.
      Serum amyloid P therapeutically attenuates murine bleomycin-induced pulmonary fibrosis via its effects on macrophages.
      ,
      • Pilling D.
      • Roife D.
      • Wang M.
      • Ronkainen S.D.
      • Crawford J.R.
      • Travis E.L.
      • et al.
      Reduction of bleomycin-induced pulmonary fibrosis by serum amyloid P.
      ].

      BM mesenchymal stem cells (MSCs)

      MSCs are defined as self-renewable, multipotent progenitor cells with the capacity to differentiate into lineage specific cells that form bone, cartilage, fat, tendon and muscle tissue [
      • Kallis Y.N.
      • Forbes S.J.
      The bone marrow and liver fibrosis: friend or foe?.
      ,
      • Song L.
      • Tuan R.S.
      Transdifferentiation potential of human mesenchymal stem cells derived from bone marrow.
      ]. Unlike hematopoietic stem cells, MSCs are radio- and chemoresistant [
      • Bartsch K.
      • Al-Ali H.
      • Reinhardt A.
      • Franke C.
      • Hudecek M.
      • Kamprad M.
      • et al.
      Mesenchymal stem cells remain host-derived independent of the source of the stem-cell graft and conditioning regimen used.
      ] and do not express progenitor markers (CD45, CD34, 133 [
      • Kallis Y.N.
      • Forbes S.J.
      The bone marrow and liver fibrosis: friend or foe?.
      ]) or myelo-monocytic markers (CD11b, MHCII, and F4/80). Hepatic myofibroblasts may arise from BM-derived mesenchymal progenitors [
      • Forbes S.J.
      • Russo F.P.
      • Rey V.
      • Burra P.
      • Rugge M.
      • Wright N.A.
      • et al.
      A significant proportion of myofibroblasts are of bone marrow origin in human liver fibrosis.
      ,
      • Russo F.P.
      • Alison M.R.
      • Bigger B.W.
      • Amofah E.
      • Florou A.
      • Amin F.
      • et al.
      The bone marrow functionally contributes to liver fibrosis.
      ]. BM-derived mesenchymal progenitors can give rise to myofibroblasts in the injured liver [
      • Baertschiger R.M.
      • Serre-Beinier V.
      • Morel P.
      • Bosco D.
      • Peyrou M.
      • Clement S.
      • et al.
      Fibrogenic potential of human multipotent mesenchymal stromal cells in injured liver.
      ,
      • di Bonzo L.V.
      • Ferrero I.
      • Cravanzola C.
      • Mareschi K.
      • Rustichell D.
      • Novo E.
      • et al.
      Human mesenchymal stem cells as a two-edged sword in hepatic regenerative medicine: engraftment and hepatocyte differentiation versus profibrogenic potential.
      ,
      • Li C.
      • Kong Y.
      • Wang H.
      • Wang S.
      • Yu H.
      • Liu X.
      • et al.
      Homing of bone marrow mesenchymal stem cells mediated by sphingosine 1-phosphate contributes to liver fibrosis.
      ]. BM-derived cells may populate fibrotic lungs [
      • Hashimoto N.
      • Jin H.
      • Liu T.
      • Chensue S.W.
      • Phan S.H.
      Bone marrow-derived progenitor cells in pulmonary fibrosis.
      ] and the liver [
      • Russo F.P.
      • Alison M.R.
      • Bigger B.W.
      • Amofah E.
      • Florou A.
      • Amin F.
      • et al.
      The bone marrow functionally contributes to liver fibrosis.
      ] and contribute to fibrosis by differentiating into tissue myofibroblasts [
      • Iredale J.P.
      Models of liver fibrosis: exploring the dynamic nature of inflammation and repair in a solid organ.
      ,
      • Kisseleva T.
      • Brenner D.A.
      Fibrogenesis of parenchymal organs.
      ,
      • Russo F.P.
      • Alison M.R.
      • Bigger B.W.
      • Amofah E.
      • Florou A.
      • Amin F.
      • et al.
      The bone marrow functionally contributes to liver fibrosis.
      ]. By subfractionating the BM stem cell compartment, the hepatic BM-derived myofibroblast-like cells were reported to be of mesenchymal stem cell origin [
      • Kallis Y.N.
      • Forbes S.J.
      The bone marrow and liver fibrosis: friend or foe?.
      ,
      • Russo F.P.
      • Alison M.R.
      • Bigger B.W.
      • Amofah E.
      • Florou A.
      • Amin F.
      • et al.
      The bone marrow functionally contributes to liver fibrosis.
      ]. Cultured mesenchymal stem cells have the potential to become myofibroblast-like cells when transplanted into mouse livers [
      • Baertschiger R.M.
      • Serre-Beinier V.
      • Morel P.
      • Bosco D.
      • Peyrou M.
      • Clement S.
      • et al.
      Fibrogenic potential of human multipotent mesenchymal stromal cells in injured liver.
      ,
      • di Bonzo L.V.
      • Ferrero I.
      • Cravanzola C.
      • Mareschi K.
      • Rustichell D.
      • Novo E.
      • et al.
      Human mesenchymal stem cells as a two-edged sword in hepatic regenerative medicine: engraftment and hepatocyte differentiation versus profibrogenic potential.
      ].
      Whether circulating mesenchymal progenitors significantly contribute to ECM deposition in the course of experimental liver fibrosis remains to be determined, but they most likely represent a population, distinct from hematopoietic-derived fibrocytes [
      • Kisseleva T.
      • Uchinami H.
      • Feirt N.
      • Quintana-Bustamante O.
      • Segovia J.C.
      • Schwabe R.F.
      • et al.
      Bone marrow-derived fibrocytes participate in pathogenesis of liver fibrosis.
      ]. Unlike hematopoietic stem cells, the definitive markers for mesenchymal stem cells have not been identified, and ablative radiation protocols to establish donor cell transfer have not been standardized. Therefore, a definitive murine liver fibrosis experiment with documented transfer of all bone marrow constituents expressing a myofibroblast specific marker has not been reported. Although initial enthusiasm about the BM origin of myofibroblasts declined in the recent years, further studies are required to re-evaluate this phenomenon.
      Liver fibrosis precedes development of hepatocellular carcinoma [
      • Bataller R.
      • Brenner D.A.
      Liver fibrosis.
      ,
      • Inokuchi S.
      • Aoyama T.
      • Miura K.
      • Osterreicher C.H.
      • Kodama Y.
      • Miyai K.
      • et al.
      Disruption of TAK1 in hepatocytes causes hepatic injury, inflammation, fibrosis, and carcinogenesis.
      ]. Recruitment of BM-derived fibroblasts (including fibrocytes) has been implicated in the pathogenesis of liver cancer, and cancer of other organs [
      • Barth P.J.
      • Schenck zu Schweinsberg T.
      • Ramaswamy A.
      • Moll R.
      CD34+ fibrocytes, alpha-smooth muscle antigen-positive myofibroblasts, and CD117 expression in the stroma of invasive squamous cell carcinomas of the oral cavity, pharynx, and larynx.
      ]. Thus, using collagen-α1(I)-GFP and α-smooth muscle actin (SMA)-RFP mice, BM-derived myofibroblasts were shown to contribute to neoplasia in gut and intestine [
      • Quante M.
      • Tu S.P.
      • Tomita H.
      • Gonda T.
      • Wang S.S.
      • Takashi S.
      • et al.
      Bone marrow-derived myofibroblasts contribute to the mesenchymal stem cell niche and promote tumor growth.
      ]. Since most liver injury models in mice develop within a short period of time, it is possible that these experimental conditions are too short for the recruitment of BM myofibroblasts, as seen in cancer during 8–9 months of development.

      Contribution of BM cells in genetically altered mice

      BM cells may have different roles in different mouse models of genetically-induced liver injury. Since these phenomena are usually not observed in the wild type mice, contribution of BM-derived cells to hepatic cells is discussed separately.
      The classical example is FAH−/− mice, in which a mutation in fumaryl-aceto-acetate hydrolase (FAH) gene causes a metabolic disorder equivalent to hereditary tyrosinemia type 1. Withdrawal of the protective drug 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) from drinking water causes extensive apoptosis of FAH−/− hepatocytes in these mice. Transplantation of wild type BM into these mice results in rescue from fatal liver failure by FAH-deficient hepatocytes. Wild type BM myelo-monocytes cells fuse with damaged hepatocytes to give rise to colonies of functional hepatocytes [
      • Lagasse E.
      • Connors H.
      • Al-Dhalimy M.
      • Reitsma M.
      • Dohse M.
      • Osborne L.
      • et al.
      Purified hematopoietic stem cells can differentiate into hepatocytes in vivo.
      ,
      • Wang X.
      • Willenbring H.
      • Akkari Y.
      • Torimaru Y.
      • Foster M.
      • Al-Dhalimy M.
      • et al.
      Cell fusion is the principal source of bone-marrow-derived hepatocytes.
      ]. Moreover, infusion of myeloid cells alone is sufficient to give rise to functional hepatocytes [
      • Alison M.R.
      • Islam S.
      • Lim S.
      Stem cells in liver regeneration, fibrosis and cancer: the good, the bad and the ugly.
      ,
      • Thorgeirsson S.S.
      • Grisham J.W.
      Hematopoietic cells as hepatocyte stem cells: a critical review of the evidence.
      ,
      • Willenbring H.
      • Bailey A.S.
      • Foster M.
      • Akkari Y.
      • Dorrell C.
      • Olson S.
      • et al.
      Myelomonocytic cells are sufficient for therapeutic cell fusion in liver.
      ]. However, fusion of hepatocytes with macrophages was only rarely observed in wild type mice in response to other types of liver injury (CCl4, BDL), suggesting that hematopoietic cells have a limited contribution to hepatocyte population under physiological conditions or in response to injury [
      • Alison M.R.
      • Islam S.
      • Lim S.
      Stem cells in liver regeneration, fibrosis and cancer: the good, the bad and the ugly.
      ,
      • Thorgeirsson S.S.
      • Grisham J.W.
      Hematopoietic cells as hepatocyte stem cells: a critical review of the evidence.
      ].
      Recruitment of fibrocytes into the injured liver representing a high percentage of myofibroblasts has been observed in Abcb4-deficient mice (Abcb4−/− mice), and has been shown to substantially ameliorate development of liver fibrosis [
      • Roderfeld M.
      • Rath T.
      • Voswinckel R.
      • Dierkes C.
      • Dietrich H.
      • Zahner D.
      • et al.
      Bone marrow transplantation demonstrates medullar origin of CD34+ fibrocytes and ameliorates hepatic fibrosis in Abcb4−/− mice.
      ]. However, only a modest contribution of fibrocytes to liver fibrosis (3–5% of fibrogenic myofibroblasts) has been observed in wild type mice in response to CCl4 and BDL [
      • Kisseleva T.
      • Uchinami H.
      • Feirt N.
      • Quintana-Bustamante O.
      • Segovia J.C.
      • Schwabe R.F.
      • et al.
      Bone marrow-derived fibrocytes participate in pathogenesis of liver fibrosis.
      ,
      • Scholten D.
      • Reichart D.
      • Paik Y.H.
      • Lindert J.
      • Bhattacharya J.
      • Glass C.K.
      • et al.
      Migration of fibrocytes in fibrogenic liver injury.
      ]. Although Abcb4-deficient mice provide a unique opportunity to study recruitment of fibrocytes in great detail, they do not reflect the fibrocyte contribution to the population of myofibroblasts in hepatotoxic or cholestatic injury.

      Resolution of liver fibrosis

      Disappearance of myofibroblasts

      Reversal of fibrosis is associated with increased collagenase activity, activation of macrophages/Kupffer cells that secrete matrix metalloproteinases, e.g. MMP-13, and matrix degradation [
      • Fallowfield J.A.
      • Mizuno M.
      • Kendall T.J.
      • Constandinou C.M.
      • Benyon R.C.
      • Duffield J.S.
      • et al.
      Scar-associated macrophages are a major source of hepatic matrix metalloproteinase-13 and facilitate the resolution of murine hepatic fibrosis.
      ,
      • Uchinami H.
      • Seki E.
      • Brenner D.A.
      • D’Armiento J.
      Loss of MMP 13 attenuates murine hepatic injury and fibrosis during cholestasis.
      ]. Senescence and apoptosis of activated HSCs play a significant role in resolution of liver fibrosis by eliminating the cell type responsible for producing the fibrotic scar [
      • Iredale J.P.
      • Benyon R.C.
      • Pickering J.
      • McCullen M.
      • Northrop M.
      • Pawley S.
      • et al.
      Mechanisms of spontaneous resolution of rat liver fibrosis. Hepatic stellate cell apoptosis and reduced hepatic expression of metalloproteinase inhibitors.
      ,
      • Krizhanovsky V.
      • Yon M.
      • Dickins R.A.
      • Hearn S.
      • Simon J.
      • Miething C.
      • et al.
      Senescence of activated stellate cells limits liver fibrosis.
      ]. Several mechanisms are implicated in the apoptosis of activated HSC: (1) activation of death receptor-mediated pathways (Fas or TNFR-1 receptors) and caspases 8 and 3; (2) upregulation of pro-apoptotic proteins (e.g., p53, Bax, caspase 9); and (3) decrease of pro-survival genes (e.g., Bcl-2) [
      • Kisseleva T.
      • Brenner D.A.
      Mechanisms of fibrogenesis.
      ]. A population of liver-associated natural killer (NK) cells and NKT cells mediate apoptosis of activated HSCs [
      • Radaeva S.
      • Sun R.
      • Jaruga B.
      • Nguyen V.T.
      • Tian Z.
      • Gao B.
      Natural killer cells ameliorate liver fibrosis by killing activated stellate cells in NKG2D-dependent and tumor necrosis factor-related apoptosis-inducing ligand-dependent manners.
      ]. Kupffer cells and BM macrophages actively participate in clearance of apoptotic cells and degradation of extracellular matrix proteins.
      Studies in culture suggest that aHSCs, at least in part, can revert to a quiescent phenotype. Therefore, the disappearance of activated α-SMA+ Col+ HSCs in the course of fibrosis reversal may indicate that activated HSCs return to their quiescent state, which is associated with expression of lipogenic genes (Adfp, Adipor1, CREBP, PPAR-γ) [
      • She H.
      • Xiong S.
      • Hazra S.
      • Tsukamoto H.
      Adipogenic transcriptional regulation of hepatic stellate cells.
      ] and storage of vitamin A in lipid droplets. Depletion of peroxisome proliferator-activated receptor gamma (PPAR-γ) constitutes a key molecular event for HSC activation, and ectopic over-expression of this nuclear receptor results in the phenotypic reversal of activated HSC to quiescent cells in culture [
      • She H.
      • Xiong S.
      • Hazra S.
      • Tsukamoto H.
      Adipogenic transcriptional regulation of hepatic stellate cells.
      ]. The treatment of activated HSCs with an adipocyte differentiation cocktail, over-expression of SREBP-1c, or culturing on basement membrane-like ECM [
      • Gaca M.D.
      • Zhou X.
      • Issa R.
      • Kiriella K.
      • Iredale J.P.
      • Benyon R.C.
      Basement membrane-like matrix inhibits proliferation and collagen synthesis by activated rat hepatic stellate cells: evidence for matrix-dependent deactivation of stellate cells.
      ,
      • Wells R.G.
      The role of matrix stiffness in regulating cell behavior.
      ] result in up-regulation of adipogenic transcription factors and cause morphologic and biochemical reversal of activated HSCs to quiescent cells [
      • Tsukamoto H.
      Adipogenic phenotype of hepatic stellate cells.
      ,
      • Tsukamoto H.
      Fat paradox in liver disease.
      ]. Although these results suggest that activated HSCs can revert to a quiescent state, these findings have only been documented in cultured cells.

      Therapy

      Many studies have demonstrated that transplantation of bone marrow cells reduces experimental liver fibrosis ([
      • Sakaida I.
      • Terai S.
      • Yamamoto N.
      • Aoyama K.
      • Ishikawa T.
      • Nishina H.
      • et al.
      Transplantation of bone marrow cells reduces CCl4-induced liver fibrosis in mice.
      ,
      • Sun C.K.
      • Chen C.H.
      • Kao Y.H.
      • Yuen C.M.
      • Sheu J.J.
      • Lee F.Y.
      • et al.
      Bone marrow cells reduce fibrogenesis and enhance regeneration in fibrotic rat liver.
      ] and others). The mechanism is not trans-differentiation of bone marrow cells into hepatocytes. More likely, hematological stem cells may contribute to the reversal of liver fibrosis via macrophages that produce collagenases [
      • Thomas J.A.
      • Pope C.
      • Wojtacha D.
      • Robson A.J.
      • Gordon-Walker T.T.
      • Hartland S.
      • et al.
      Macrophage therapy for murine liver fibrosis recruits host effector cells improving fibrosis, regeneration, and function.
      ] and phagocytose dead parenchymal cells [
      • Popov Y.
      • Sverdlov D.Y.
      • Bhaskar K.R.
      • Sharma A.K.
      • Millonig G.
      • Patsenker E.
      • et al.
      Macrophage-mediated phagocytosis of apoptotic cholangiocytes contributes to reversal of experimental biliary fibrosis.
      ]. More unexpectedly, mesenchymal stem cells, even though they have the potential to become myofibroblasts, also have functions that may contribute to the reversal of fibrosis. Cultured mesenchymal stem cells secrete agonists that inhibit hepatocyte apoptosis, induce hepatocyte proliferation, and increase hepatocyte specific gene expression [
      • van Poll D.
      • Parekkadan B.
      • Cho C.H.
      • Berthiaume F.
      • Nahmias Y.
      • Tilles A.W.
      • et al.
      Mesenchymal stem cell-derived molecules directly modulate hepatocellular death and regeneration in vitro and in vivo.
      ]. Also, mesenchymal stem cells may be induced in culture to become endothelial progenitor cells (EPCs). Transplantation of EPCs reverses hepatic fibrosis and improves survival in CCl4-induced cirrhosis in rats [
      • Nakamura T.
      • Torimura T.
      • Sakamoto M.
      • Hashimoto O.
      • Taniguchi E.
      • Inoue K.
      • et al.
      Significance and therapeutic potential of endothelial progenitor cell transplantation in a cirrhotic liver rat model.
      ].

      BM cells for anti-fibrotic therapy

      The improvement of liver function following transplantation of hematopoietic progenitors in mice and rats with injured livers provided the basis for clinical trials [
      • Fitzpatrick E.
      • Mitry R.R.
      • Dhawan A.
      Human hepatocyte transplantation: state of the art.
      ]. Clinical studies with adoptive transfer of autologous CD133+ BM cells in patients have been reported to stimulate liver regeneration [
      • am Esch 2nd, J.S.
      • Knoefel W.T.
      • Klein M.
      • Ghodsizad A.
      • Fuerst G.
      • Poll L.W.
      • et al.
      Portal application of autologous CD133+ bone marrow cells to the liver: a novel concept to support hepatic regeneration.
      ]. Similar to that, autologous infusion of CD34+ blood cells, or even monocytes, improved biochemical parameters and stimulated liver regeneration [
      • Gordon M.Y.
      • Levicar N.
      • Pai M.
      • Bachellier P.
      • Dimarakis I.
      • Al-Allaf F.
      • et al.
      Characterization and clinical application of human CD34+ stem/progenitor cell populations mobilized into the blood by granulocyte colony-stimulating factor.
      ]. Within the limits of these small, uncontrolled clinical trials, evidence is starting to accumulate that transplantation of hematopoietic progenitors may be beneficial in patients. However, the mechanism of their action remains to be defined. Such improvement may result from release of cytokines and growth factors by transplanted hematopoietic cells, or occur due to infusion of scar-resorbing monocytes. In concordance with these observations, treatment with granulocyte-colony stimulating factor (G-CSF) was used to mobilize the BM cells and demonstrated a positive histological effect in patients with alcoholic steatohepatitis [
      • Gaia S.
      • Smedile A.
      • Omede P.
      • Olivero A.
      • Sanavio F.
      • Balzola F.
      • et al.
      Feasibility and safety of G-CSF administration to induce bone marrow-derived cells mobilization in patients with end stage liver disease.
      ].
      Mesenchymal stem cells serve as another potential target for the liver stem cell therapy. In addition, mesenchymal cells are readily available (for example, from fat tissue) and relatively easy to expand in vitro. A recent study investigated the ability of purified hematopoietic stem cells (HSCs), mesenchymal stem cells (MSCs), and mononuclear cells to engraft and contribute to liver regeneration in response to injury in mice [
      • Cho K.A.
      • Ju S.Y.
      • Cho S.J.
      • Jung Y.J.
      • Woo S.Y.
      • Seoh J.Y.
      • et al.
      Mesenchymal stem cells showed the highest potential for the regeneration of injured liver tissue compared with other subpopulations of the bone marrow.
      ]. However, only a low level of engraftment with the MSCs and reconstitution of the liver mass has been reported [
      • Banas A.
      • Teratani T.
      • Yamamoto Y.
      • Tokuhara M.
      • Takeshita F.
      • Osaki M.
      • et al.
      Rapid hepatic fate specification of adipose-derived stem cells and their therapeutic potential for liver failure.
      ].
      In concordance with this notion, injection of MSC-derived conditioning media into a liver-assist device decreased hepatocyte apoptosis and increased their proliferation [
      • van Poll D.
      • Parekkadan B.
      • Cho C.H.
      • Berthiaume F.
      • Nahmias Y.
      • Tilles A.W.
      • et al.
      Mesenchymal stem cell-derived molecules directly modulate hepatocellular death and regeneration in vitro and in vivo.
      ,
      • Yagi H.
      • Parekkadan B.
      • Suganuma K.
      • Soto-Gutierrez A.
      • Tompkins R.G.
      • Tilles A.W.
      • et al.
      Long-term superior performance of a stem cell/hepatocyte device for the treatment of acute liver failure.
      ]. However, recent studies have raised a safety question on MSCs transplantation, demonstrating that MSCs can give rise to myofibroblasts in mice in response to liver injury. For example, BM-derived MSCs contributed to the development of liver fibrosis in chimeric mice that received bone marrow transplantation with an enriched BM mesenchymal fraction, and subjected to the CCl4-liver injury [
      • Russo F.P.
      • Alison M.R.
      • Bigger B.W.
      • Amofah E.
      • Florou A.
      • Amin F.
      • et al.
      The bone marrow functionally contributes to liver fibrosis.
      ]. Taken together, both hematopoietic and mesenchymal stem cells demonstrate a limited, if any, contribution to hepatocyte replenishment, but may stimulate liver function by providing soluble growth factors or cytokines [
      • Alison M.R.
      • Islam S.
      • Lim S.
      Stem cells in liver regeneration, fibrosis and cancer: the good, the bad and the ugly.
      ,
      • Kisseleva T.
      • Brenner D.A.
      Hepatic stellate cells and the reversal of fibrosis.
      ,
      • Thorgeirsson S.S.
      • Grisham J.W.
      Hematopoietic cells as hepatocyte stem cells: a critical review of the evidence.
      ].
      A few clinical trials have been performed in patients with end-stage liver disease caused by hepatitis B, hepatitis C, alcoholic liver disease, and cryptogenic fibrosis. These patients were transplanted with autologous MSCs harvested from the iliac crest. The tested parameters (albumin, creatinine) demonstrated a modest but significant improvement without severe adverse effects, suggesting that MSCs might be useful for the treatment of end-stage liver disease with satisfactory tolerability [
      • Kharaziha P.
      • Hellstrom P.M.
      • Noorinayer B.
      • Farzaneh F.
      • Aghajani K.
      • Jafari F.
      • et al.
      Improvement of liver function in liver cirrhosis patients after autologous mesenchymal stem cell injection: a phase I–II clinical trial.
      ].

      Conclusions

      The literature provides evidence that bone marrow cells might contribute to increase or to inhibit experimental liver fibrosis (Fig. 2). Although there is clearly a need for additional, better defined studies, some conclusions can be made from our current information. Hematological stem cells are the source of monocytes, Kupffer cells and recruited macrophage. Overall, these cells contribute to the initial inflammation in the injured liver that progresses to liver fibrosis. However, recruited macrophages may also secrete agonists such as IL-10 that inhibit stellate cell activation as well as collagenases that cause regression of fibrosis. Hematological stem cells are also the source of fibrocytes, which are recruited to the injured liver and function in the innate immune response as well as differentiate into myofibroblasts. Mesenchymal stem cells have the capacity to become myofibroblasts, but studies to follow their cell fate in vivo are limited by the lack of specific markers.
      Figure thumbnail gr2
      Fig. 2Potential roles of BM-derived progenitor populations in liver injury. BM is the source of hematopoietic and mesenchymal stem cells, which may participate in the response to liver injury.
      Most, but not all, studies using BM transplantation have demonstrated a beneficial effect on experimental liver fibrosis. The mechanism for this benefit is unclear, and in particular BM-derived cells do not constitute a significant source of hepatocytes in the injured liver. However, both mesenchymal stem cells and hematopoietic stem cells are reported to contribute to the regression of liver fibrosis. On the basis of these studies, small, mostly uncontrolled clinical studies have treated cirrhotic patients with autologous transplantation of BM derived cells. Although these studies have established the feasibility of this approach, the mechanism and long term benefit of transplantation of BM-derived cells in cirrhosis is unknown.

      Conflict of interest

      The authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript. The underlying research reported in the study was funded by the NIH Institutes of Health.

      References

        • Abe R.
        • Donnelly S.C.
        • Peng T.
        • Bucala R.
        • Metz C.N.
        Peripheral blood fibrocytes: differentiation pathway and migration to wound sites.
        J Immunol. 2001; 166: 7556-7562
        • Albillos A.
        • de la Hera A.
        • Gonzalez M.
        • Moya J.L.
        • Calleja J.L.
        • Monserrat J.
        • et al.
        Increased lipopolysaccharide binding protein in cirrhotic patients with marked immune and hemodynamic derangement.
        Hepatology. 2003; 37: 208-217
        • Alison M.R.
        • Islam S.
        • Lim S.
        Stem cells in liver regeneration, fibrosis and cancer: the good, the bad and the ugly.
        J Pathol. 2009; 217: 282-298
        • am Esch 2nd, J.S.
        • Knoefel W.T.
        • Klein M.
        • Ghodsizad A.
        • Fuerst G.
        • Poll L.W.
        • et al.
        Portal application of autologous CD133+ bone marrow cells to the liver: a novel concept to support hepatic regeneration.
        Stem Cells. 2005; 23: 463-470
        • Asahina K.
        • Tsai S.Y.
        • Li P.
        • Ishii M.
        • Maxson Jr., R.E.
        • Sucov H.M.
        • et al.
        Mesenchymal origin of hepatic stellate cells, submesothelial cells, and perivascular mesenchymal cells during mouse liver development.
        Hepatology. 2009; 49: 998-1011
        • Baertschiger R.M.
        • Serre-Beinier V.
        • Morel P.
        • Bosco D.
        • Peyrou M.
        • Clement S.
        • et al.
        Fibrogenic potential of human multipotent mesenchymal stromal cells in injured liver.
        PLoS One. 2009; 4: e6657
        • Banas A.
        • Teratani T.
        • Yamamoto Y.
        • Tokuhara M.
        • Takeshita F.
        • Osaki M.
        • et al.
        Rapid hepatic fate specification of adipose-derived stem cells and their therapeutic potential for liver failure.
        J Gastroenterol Hepatol. 2009; 24: 70-77
        • Barth P.J.
        • Schenck zu Schweinsberg T.
        • Ramaswamy A.
        • Moll R.
        CD34+ fibrocytes, alpha-smooth muscle antigen-positive myofibroblasts, and CD117 expression in the stroma of invasive squamous cell carcinomas of the oral cavity, pharynx, and larynx.
        Virchows Arch. 2004; 444: 231-234
        • Bartsch K.
        • Al-Ali H.
        • Reinhardt A.
        • Franke C.
        • Hudecek M.
        • Kamprad M.
        • et al.
        Mesenchymal stem cells remain host-derived independent of the source of the stem-cell graft and conditioning regimen used.
        Transplantation. 2009; 87: 217-221
        • Bataller R.
        • Brenner D.A.
        Liver fibrosis.
        J Clin Invest. 2005; 115: 209-218
        • Bellini A.
        • Mattoli S.
        The role of the fibrocyte, a bone marrow-derived mesenchymal progenitor, in reactive and reparative fibroses.
        Lab Invest. 2007; 87: 858-870
        • Brinkmann V.
        • Reichard U.
        • Goosmann C.
        • Fauler B.
        • Uhlemann Y.
        • Weiss D.S.
        • et al.
        Neutrophil extracellular traps kill bacteria.
        Science. 2004; 303: 1532-1535
        • Bucala R.
        • Spiegel L.A.
        • Chesney J.
        • Hogan M.
        • Cerami A.
        Circulating fibrocytes define a new leukocyte subpopulation that mediates tissue repair.
        Mol Med. 1994; 1: 71-81
        • Castano A.P.
        • Lin S.L.
        • Surowy T.
        • Nowlin B.T.
        • Turlapati S.A.
        • Patel T.
        • et al.
        Serum amyloid P inhibits fibrosis through Fc gamma R-dependent monocyte-macrophage regulation in vivo.
        Sci Transl Med. 2009; 1: 5ra13
        • Cho K.A.
        • Ju S.Y.
        • Cho S.J.
        • Jung Y.J.
        • Woo S.Y.
        • Seoh J.Y.
        • et al.
        Mesenchymal stem cells showed the highest potential for the regeneration of injured liver tissue compared with other subpopulations of the bone marrow.
        Cell Biol Int. 2009; 33: 772-777
        • Chow O.A.
        • von Kockritz-Blickwede M.
        • Bright A.T.
        • Hensler M.E.
        • Zinkernagel A.S.
        • Cogen A.L.
        • et al.
        Statins enhance formation of phagocyte extracellular traps.
        Cell Host Microbe. 2010; 8: 445-454
        • Corbin B.D.
        • Seeley E.H.
        • Raab A.
        • Feldmann J.
        • Miller M.R.
        • Torres V.J.
        • et al.
        Metal chelation and inhibition of bacterial growth in tissue abscesses.
        Science. 2008; 319: 962-965
        • Desmouliere A.
        • Darby I.
        • Costa A.M.
        • Raccurt M.
        • Tuchweber B.
        • Sommer P.
        • et al.
        Extracellular matrix deposition, lysyl oxidase expression, and myofibroblastic differentiation during the initial stages of cholestatic fibrosis in the rat.
        Lab Invest. 1997; 76: 765-778
        • di Bonzo L.V.
        • Ferrero I.
        • Cravanzola C.
        • Mareschi K.
        • Rustichell D.
        • Novo E.
        • et al.
        Human mesenchymal stem cells as a two-edged sword in hepatic regenerative medicine: engraftment and hepatocyte differentiation versus profibrogenic potential.
        Gut. 2008; 57: 223-231
        • Duffield J.S.
        • Forbes S.J.
        • Constandinou C.M.
        • Clay S.
        • Partolina M.
        • Vuthoori S.
        • et al.
        Selective depletion of macrophages reveals distinct, opposing roles during liver injury and repair.
        J Clin Invest. 2005; 115: 56-65
        • Duffield J.S.
        • Tipping P.G.
        • Kipari T.
        • Cailhier J.F.
        • Clay S.
        • Lang R.
        • et al.
        Conditional ablation of macrophages halts progression of crescentic glomerulonephritis.
        Am J Pathol. 2005; 167: 1207-1219
        • Evseenko D.
        • Schenke-Layland K.
        • Dravid G.
        • Zhu Y.
        • Hao Q.L.
        • Scholes J.
        • et al.
        Identification of the critical extracellular matrix proteins that promote human embryonic stem cell assembly.
        Stem Cells Dev. 2008; 18: 919-928
        • Eyden B.
        The myofibroblast: phenotypic characterization as a prerequisite to understanding its functions in translational medicine.
        J Cell Mol Med. 2008; 12: 22-37
        • Fallowfield J.A.
        • Mizuno M.
        • Kendall T.J.
        • Constandinou C.M.
        • Benyon R.C.
        • Duffield J.S.
        • et al.
        Scar-associated macrophages are a major source of hepatic matrix metalloproteinase-13 and facilitate the resolution of murine hepatic fibrosis.
        J Immunol. 2007; 178: 5288-5295
        • Fitzpatrick E.
        • Mitry R.R.
        • Dhawan A.
        Human hepatocyte transplantation: state of the art.
        J Intern Med. 2009; 266: 339-357
        • Forbes S.J.
        • Russo F.P.
        • Rey V.
        • Burra P.
        • Rugge M.
        • Wright N.A.
        • et al.
        A significant proportion of myofibroblasts are of bone marrow origin in human liver fibrosis.
        Gastroenterology. 2004; 126: 955-963
        • Friedman S.L.
        • Roll F.J.
        • Boyles J.
        • Bissell D.M.
        Hepatic lipocytes: the principal collagen-producing cells of normal rat liver.
        Proc Natl Acad Sci USA. 1985; 82: 8681-8685
        • Gabbiani G.
        • Ryan G.B.
        • Majne G.
        Presence of modified fibroblasts in granulation tissue and their possible role in wound contraction.
        Experientia. 1971; 27: 549-550
        • Gaca M.D.
        • Zhou X.
        • Issa R.
        • Kiriella K.
        • Iredale J.P.
        • Benyon R.C.
        Basement membrane-like matrix inhibits proliferation and collagen synthesis by activated rat hepatic stellate cells: evidence for matrix-dependent deactivation of stellate cells.
        Matrix Biol. 2003; 22: 229-239
        • Gaia S.
        • Smedile A.
        • Omede P.
        • Olivero A.
        • Sanavio F.
        • Balzola F.
        • et al.
        Feasibility and safety of G-CSF administration to induce bone marrow-derived cells mobilization in patients with end stage liver disease.
        J Hepatol. 2006; 45: 13-19
        • Gebhardt C.
        • Nemeth J.
        • Angel P.
        • Hess J.
        S100A8 and S100A9 in inflammation and cancer.
        Biochem Pharmacol. 2006; 72: 1622-1631
        • Goerdt S.
        • Orfanos C.E.
        Other functions, other genes: alternative activation of antigen-presenting cells.
        Immunity. 1999; 10: 137-142
        • Gordon M.Y.
        • Levicar N.
        • Pai M.
        • Bachellier P.
        • Dimarakis I.
        • Al-Allaf F.
        • et al.
        Characterization and clinical application of human CD34+ stem/progenitor cell populations mobilized into the blood by granulocyte colony-stimulating factor.
        Stem Cells. 2006; 24: 1822-1830
        • Hakkim A.
        • Furnrohr B.G.
        • Amann K.
        • Laube B.
        • Abed U.A.
        • Brinkmann V.
        • et al.
        Impairment of neutrophil extracellular trap degradation is associated with lupus nephritis.
        Proc Natl Acad Sci USA. 2010; 107: 9813-9818
        • Hashimoto N.
        • Jin H.
        • Liu T.
        • Chensue S.W.
        • Phan S.H.
        Bone marrow-derived progenitor cells in pulmonary fibrosis.
        J Clin Invest. 2004; 113: 243-252
        • Higashiyama R.
        • Inagaki Y.
        • Hong Y.Y.
        • Kushida M.
        • Nakao S.
        • Niioka M.
        • et al.
        Bone marrow-derived cells express matrix metalloproteinases and contribute to regression of liver fibrosis in mice.
        Hepatology. 2007; 45: 213-222
        • Higashiyama R.
        • Moro T.
        • Nakao S.
        • Mikami K.
        • Fukumitsu H.
        • Ueda Y.
        • et al.
        Negligible contribution of bone marrow-derived cells to collagen production during hepatic fibrogenesis in mice.
        Gastroenterology. 2009; 137 (el): 1459-1466
        • Inokuchi S.
        • Aoyama T.
        • Miura K.
        • Osterreicher C.H.
        • Kodama Y.
        • Miyai K.
        • et al.
        Disruption of TAK1 in hepatocytes causes hepatic injury, inflammation, fibrosis, and carcinogenesis.
        Proc Natl Acad Sci USA. 2010; 107: 844-849
        • Inokuchi S.
        • Tsukamoto H.
        • Park E.
        • Liu Z.X.
        • Brenner D.A.
        • Seki E.
        Toll-like receptor 4 mediates alcohol-induced steatohepatitis through bone marrow-derived and endogenous liver cells in mice.
        Alcohol Clin Exp Res. 2011; 35: 1509-1518
        • Iredale J.P.
        Models of liver fibrosis: exploring the dynamic nature of inflammation and repair in a solid organ.
        J Clin Invest. 2007; 117: 539-548
        • Iredale J.P.
        • Benyon R.C.
        • Pickering J.
        • McCullen M.
        • Northrop M.
        • Pawley S.
        • et al.
        Mechanisms of spontaneous resolution of rat liver fibrosis. Hepatic stellate cell apoptosis and reduced hepatic expression of metalloproteinase inhibitors.
        J Clin Invest. 1998; 102: 538-549
        • Issa R.
        • Zhou X.
        • Constandinou C.M.
        • Fallowfield J.
        • Millward-Sadler H.
        • Gaca M.D.
        • et al.
        Spontaneous recovery from micronodular cirrhosis: evidence for incomplete resolution associated with matrix cross-linking.
        Gastroenterology. 2004; 126: 1795-1808
        • Issa R.
        • Zhou X.
        • Trim N.
        • Millward-Sadler H.
        • Krane S.
        • Benyon C.
        • et al.
        Mutation in collagen-1 that confers resistance to the action of collagenase results in failure of recovery from CCl4-induced liver fibrosis, persistence of activated hepatic stellate cells, and diminished hepatocyte regeneration.
        FASEB J. 2003; 17: 47-49
        • Kallis Y.N.
        • Forbes S.J.
        The bone marrow and liver fibrosis: friend or foe?.
        Gastroenterology. 2009; 137: 1218-1221
        • Karlmark K.R.
        • Weiskirchen R.
        • Zimmermann H.W.
        • Gassler N.
        • Ginhoux F.
        • Weber C.
        • et al.
        Hepatic recruitment of the inflammatory Gr1+ monocyte subset upon liver injury promotes hepatic fibrosis.
        Hepatology. 2009; 50: 261-274
        • Kessenbrock K.
        • Krumbholz M.
        • Schonermarck U.
        • Back W.
        • Gross W.L.
        • Werb Z.
        • et al.
        Netting neutrophils in autoimmune small-vessel vasculitis.
        Nat Med. 2009; 15: 623-625
        • Kharaziha P.
        • Hellstrom P.M.
        • Noorinayer B.
        • Farzaneh F.
        • Aghajani K.
        • Jafari F.
        • et al.
        Improvement of liver function in liver cirrhosis patients after autologous mesenchymal stem cell injection: a phase I–II clinical trial.
        Eur J Gastroenterol Hepatol. 2009; 21: 1199-1205
        • Kisseleva T.
        • Brenner D.A.
        Fibrogenesis of parenchymal organs.
        Proc Am Thorac Soc. 2008; 5: 338-342
        • Kisseleva T.
        • Brenner D.A.
        Hepatic stellate cells and the reversal of fibrosis.
        J Gastroenterol Hepatol. 2006; 21: S84-S87
        • Kisseleva T.
        • Brenner D.A.
        Mechanisms of fibrogenesis.
        Exp Biol Med (Maywood). 2008; 233: 109-122
        • Kisseleva T.
        • Uchinami H.
        • Feirt N.
        • Quintana-Bustamante O.
        • Segovia J.C.
        • Schwabe R.F.
        • et al.
        Bone marrow-derived fibrocytes participate in pathogenesis of liver fibrosis.
        J Hepatol. 2006; 45: 429-438
        • Kisseleva T.
        • von Kockritz-Blickwede M.
        • Reichart D.
        • McGillvray S.M.
        • Wingender G.
        • Kronenberg M.
        • et al.
        Fibrocyte-like cells recruited to the spleen support innate and adaptive immune responses to acute injury or infection.
        J Mol Med. 2011; 89: 997-1013
        • Krizhanovsky V.
        • Yon M.
        • Dickins R.A.
        • Hearn S.
        • Simon J.
        • Miething C.
        • et al.
        Senescence of activated stellate cells limits liver fibrosis.
        Cell. 2008; 134: 657-667
        • Lagasse E.
        • Connors H.
        • Al-Dhalimy M.
        • Reitsma M.
        • Dohse M.
        • Osborne L.
        • et al.
        Purified hematopoietic stem cells can differentiate into hepatocytes in vivo.
        Nat Med. 2000; 6: 1229-1234
        • Li C.
        • Kong Y.
        • Wang H.
        • Wang S.
        • Yu H.
        • Liu X.
        • et al.
        Homing of bone marrow mesenchymal stem cells mediated by sphingosine 1-phosphate contributes to liver fibrosis.
        J Hepatol. 2009; 50: 1174-1183
        • Li Y.
        • Chang M.
        • Abar O.
        • Garcia V.
        • Rowland C.
        • Catanese J.
        • et al.
        Multiple variants in toll-like receptor 4 gene modulate risk of liver fibrosis in Caucasians with chronic hepatitis C infection.
        J Hepatol. 2009; 51: 750-757
        • Majno G.
        • Gabbiani G.
        • Hirschel B.J.
        • Ryan G.B.
        • Statkov P.R.
        Contraction of granulation tissue in vitro: similarity to smooth muscle.
        Science. 1971; 173: 548-550
        • Marra F.
        • Aleffi S.
        • Galastri S.
        • Provenzano A.
        Mononuclear cells in liver fibrosis.
        Semin Immunopathol. 2009; 31: 345-358
        • Mathai S.K.
        • Gulati M.
        • Peng X.
        • Russell T.R.
        • Shaw A.C.
        • Rubinowitz A.N.
        • et al.
        Circulating monocytes from systemic sclerosis patients with interstitial lung disease show an enhanced profibrotic phenotype.
        Lab Invest. 2010; 90: 812-823
        • Mitchell C.
        • Couton D.
        • Couty J.P.
        • Anson M.
        • Crain A.M.
        • Bizet V.
        • et al.
        Dual role of CCR2 in the constitution and the resolution of liver fibrosis in mice.
        Am J Pathol. 2009; 174: 1766-1775
        • Miura K.
        • Kodama Y.
        • Inokuchi S.
        • Schnabl B.
        • Aoyama T.
        • Ohnishi H.
        • et al.
        Toll-like receptor 9 promotes steatohepatitis by induction of interleukin-1beta in mice.
        Gastroenterology. 2010; 139 (e327): 323-334
        • Moeller A.
        • Gilpin S.E.
        • Ask K.
        • Cox G.
        • Cook D.
        • Gauldie J.
        • et al.
        Circulating fibrocytes are an indicator of poor prognosis in idiopathic pulmonary fibrosis.
        Am J Respir Crit Care Med. 2009; 179: 588-594
        • Murray L.A.
        • Rosada R.
        • Moreira A.P.
        • Joshi A.
        • Kramer M.S.
        • Hesson D.P.
        • et al.
        Serum amyloid P therapeutically attenuates murine bleomycin-induced pulmonary fibrosis via its effects on macrophages.
        PLoS One. 2010; 5: e9683
        • Nakamura T.
        • Torimura T.
        • Sakamoto M.
        • Hashimoto O.
        • Taniguchi E.
        • Inoue K.
        • et al.
        Significance and therapeutic potential of endothelial progenitor cell transplantation in a cirrhotic liver rat model.
        Gastroenterology. 2007; 133 (e101): 91-107
        • Parola M.
        • Marra F.
        • Pinzani M.
        Myofibroblast-like cells and liver fibrogenesis: emerging concepts in a rapidly moving scenario.
        Mol Aspects Med. 2008; 29: 58-66
        • Pilling D.
        • Buckley C.D.
        • Salmon M.
        • Gomer R.H.
        Inhibition of fibrocyte differentiation by serum amyloid P.
        J Immunol. 2003; 171: 5537-5546
        • Pilling D.
        • Fan T.
        • Huang D.
        • Kaul B.
        • Gomer R.H.
        Identification of markers that distinguish monocyte-derived fibrocytes from monocytes, macrophages, and fibroblasts.
        PLoS ONE. 2009; 4: e7475
        • Pilling D.
        • Roife D.
        • Wang M.
        • Ronkainen S.D.
        • Crawford J.R.
        • Travis E.L.
        • et al.
        Reduction of bleomycin-induced pulmonary fibrosis by serum amyloid P.
        J Immunol. 2007; 179: 4035-4044
        • Popov Y.
        • Sverdlov D.Y.
        • Bhaskar K.R.
        • Sharma A.K.
        • Millonig G.
        • Patsenker E.
        • et al.
        Macrophage-mediated phagocytosis of apoptotic cholangiocytes contributes to reversal of experimental biliary fibrosis.
        Am J Physiol Gastrointest Liver Physiol. 2010; 298: G323-G334
        • Quan T.E.
        • Cowper S.
        • Wu S.P.
        • Bockenstedt L.K.
        • Bucala R.
        Circulating fibrocytes: collagen-secreting cells of the peripheral blood.
        Int J Biochem Cell Biol. 2004; 36: 598-606
        • Quante M.
        • Tu S.P.
        • Tomita H.
        • Gonda T.
        • Wang S.S.
        • Takashi S.
        • et al.
        Bone marrow-derived myofibroblasts contribute to the mesenchymal stem cell niche and promote tumor growth.
        Cancer Cell. 2011; 19: 257-272
        • Radaeva S.
        • Sun R.
        • Jaruga B.
        • Nguyen V.T.
        • Tian Z.
        • Gao B.
        Natural killer cells ameliorate liver fibrosis by killing activated stellate cells in NKG2D-dependent and tumor necrosis factor-related apoptosis-inducing ligand-dependent manners.
        Gastroenterology. 2006; 130: 435-452
        • Ramachandran P.
        • Iredale J.P.
        Reversibility of liver fibrosis.
        Ann Hepatol. 2009; 8: 283-291
        • Roderfeld M.
        • Rath T.
        • Voswinckel R.
        • Dierkes C.
        • Dietrich H.
        • Zahner D.
        • et al.
        Bone marrow transplantation demonstrates medullar origin of CD34+ fibrocytes and ameliorates hepatic fibrosis in Abcb4−/− mice.
        Hepatology. 2010; 51: 267-276
        • Russo F.P.
        • Alison M.R.
        • Bigger B.W.
        • Amofah E.
        • Florou A.
        • Amin F.
        • et al.
        The bone marrow functionally contributes to liver fibrosis.
        Gastroenterology. 2006; 130: 1807-1821
        • Sakaida I.
        • Terai S.
        • Yamamoto N.
        • Aoyama K.
        • Ishikawa T.
        • Nishina H.
        • et al.
        Transplantation of bone marrow cells reduces CCl4-induced liver fibrosis in mice.
        Hepatology. 2004; 40: 1304-1311
        • Schnabl B.
        • Purbeck C.A.
        • Choi Y.H.
        • Hagedorn C.H.
        • Brenner D.
        Replicative senescence of activated human hepatic stellate cells is accompanied by a pronounced inflammatory but less fibrogenic phenotype.
        Hepatology. 2003; 37: 653-664
        • Schnoor M.
        • Cullen P.
        • Lorkowski J.
        • Stolle K.
        • Robenek H.
        • Troyer D.
        • et al.
        Production of type VI collagen by human macrophages: a new dimension in macrophage functional heterogeneity.
        J Immunol. 2008; 180: 5707-5719
        • Scholten D.
        • Reichart D.
        • Paik Y.H.
        • Lindert J.
        • Bhattacharya J.
        • Glass C.K.
        • et al.
        Migration of fibrocytes in fibrogenic liver injury.
        Am J Pathol. 2011; 179: 189-198
        • Schrader J.
        • Fallowfield J.
        • Iredale J.P.
        Senescence of activated stellate cells: not just early retirement.
        Hepatology. 2009; 49: 1045-1047
        • Seki E.
        • De Minicis S.
        • Osterreicher C.H.
        • Kluwe J.
        • Osawa Y.
        • Brenner D.A.
        • et al.
        TLR4 enhances TGF-beta signaling and hepatic fibrosis.
        Nat Med. 2007; 13: 1324-1332
        • Seki E.
        • Uchinami H.
        • Osawa Y.
        • Brenner D.A.
        • Schwabe R.F.
        TLR4 mediates inflammation and fibrogenesis after bile duct ligation.
        Hepatology. 2005; 42: 265A-266A
        • She H.
        • Xiong S.
        • Hazra S.
        • Tsukamoto H.
        Adipogenic transcriptional regulation of hepatic stellate cells.
        J Biol Chem. 2005; 280: 4959-4967
        • Song L.
        • Tuan R.S.
        Transdifferentiation potential of human mesenchymal stem cells derived from bone marrow.
        FASEB J. 2004; 18: 980-982
        • Stefanovic B.
        • Brenner D.A.
        5′ stem-loop of collagen alpha 1(I) mRNA inhibits translation in vitro but is required for triple helical collagen synthesis in vivo.
        J Biol Chem. 2003; 278: 927-933
        • Steinbakk M.
        • Naess-Andresen C.F.
        • Lingaas E.
        • Dale I.
        • Brandtzaeg P.
        • Fagerhol M.K.
        Antimicrobial actions of calcium binding leucocyte L1 protein, calprotectin.
        Lancet. 1990; 336: 763-765
        • Strieter R.M.
        • Gomperts B.N.
        • Keane M.P.
        The role of CXC chemokines in pulmonary fibrosis.
        J Clin Invest. 2007; 117: 549-556
        • Strieter R.M.
        • Keeley E.C.
        • Hughes M.A.
        • Burdick M.D.
        • Mehrad B.
        The role of circulating mesenchymal progenitor cells (fibrocytes) in the pathogenesis of pulmonary fibrosis.
        J Leukoc Biol. 2009; 86: 1111-1118
        • Suh H.N.
        • Han H.J.
        Collagen I regulates the self-renewal of mouse embryonic stem cells through alpha2beta1 integrin- and DDR1-dependent Bmi-1.
        J Cell Physiol. 2011; 226: 3422-3432
        • Sun C.K.
        • Chen C.H.
        • Kao Y.H.
        • Yuen C.M.
        • Sheu J.J.
        • Lee F.Y.
        • et al.
        Bone marrow cells reduce fibrogenesis and enhance regeneration in fibrotic rat liver.
        J Surg Res. 2011; 169: e15-e26
        • Thomas J.A.
        • Pope C.
        • Wojtacha D.
        • Robson A.J.
        • Gordon-Walker T.T.
        • Hartland S.
        • et al.
        Macrophage therapy for murine liver fibrosis recruits host effector cells improving fibrosis, regeneration, and function.
        Hepatology. 2011; 53: 2003-2015
        • Thorgeirsson S.S.
        • Grisham J.W.
        Hematopoietic cells as hepatocyte stem cells: a critical review of the evidence.
        Hepatology. 2006; 43: 2-8
        • Tsukamoto H.
        Adipogenic phenotype of hepatic stellate cells.
        Alcohol Clin Exp Res. 2005; 29: 132S-133S
        • Tsukamoto H.
        Fat paradox in liver disease.
        Keio J Med. 2005; 54: 190-192
        • Uchinami H.
        • Seki E.
        • Brenner D.A.
        • D’Armiento J.
        Loss of MMP 13 attenuates murine hepatic injury and fibrosis during cholestasis.
        Hepatology. 2006; 44: 420-429
        • van Poll D.
        • Parekkadan B.
        • Cho C.H.
        • Berthiaume F.
        • Nahmias Y.
        • Tilles A.W.
        • et al.
        Mesenchymal stem cell-derived molecules directly modulate hepatocellular death and regeneration in vitro and in vivo.
        Hepatology. 2008; 47: 1634-1643
        • von Kockritz-Blickwede M.
        • Goldmann O.
        • Thulin P.
        • Heinemann K.
        • Norrby-Teglund A.
        • Rohde M.
        • et al.
        Phagocytosis-independent antimicrobial activity of mast cells by means of extracellular trap formation.
        Blood. 2008; 111: 3070-3080
        • Wang X.
        • Willenbring H.
        • Akkari Y.
        • Torimaru Y.
        • Foster M.
        • Al-Dhalimy M.
        • et al.
        Cell fusion is the principal source of bone-marrow-derived hepatocytes.
        Nature. 2003; 422: 897-901
        • Watanabe T.
        • Niioka M.
        • Hozawa S.
        • Kameyama K.
        • Hayashi T.
        • Arai M.
        • et al.
        Gene expression of interstitial collagenase in both progressive and recovery phase of rat liver fibrosis induced by carbon tetrachloride.
        J Hepatol. 2000; 33: 224-235
        • Wells R.G.
        The role of matrix stiffness in regulating cell behavior.
        Hepatology. 2008; 47: 1394-1400
        • Willenbring H.
        • Bailey A.S.
        • Foster M.
        • Akkari Y.
        • Dorrell C.
        • Olson S.
        • et al.
        Myelomonocytic cells are sufficient for therapeutic cell fusion in liver.
        Nat Med. 2004; 10: 744-748
        • Xidakis C.
        • Ljumovic D.
        • Manousou P.
        • Notas G.
        • Valatas V.
        • Kolios G.
        • et al.
        Production of pro- and anti-fibrotic agents by rat Kupffer cells; the effect of octreotide.
        Dig Dis Sci. 2005; 50: 935-941
        • Yagi H.
        • Parekkadan B.
        • Suganuma K.
        • Soto-Gutierrez A.
        • Tompkins R.G.
        • Tilles A.W.
        • et al.
        Long-term superior performance of a stem cell/hepatocyte device for the treatment of acute liver failure.
        Tissue Eng Part A. 2009; 15: 3377-3388
        • Yan A.W.
        • Fouts E.
        • Brandl J.
        • Starkel P.
        • Torralba M.
        • Schott E.
        • et al.
        Enteric dysbiosis associated with a mouse model of alcoholic liver disease.
        Hepatology. 2011; 53: 96-105
        • Yang D.
        • Oppenheim J.J.
        Antimicrobial proteins act as “alarmins” in joint immune defense.
        Arthritis Rheum. 2004; 50: 3401-3403
        • Yousefi S.
        • Gold J.A.
        • Andina N.
        • Lee J.J.
        • Kelly A.M.
        • Kozlowski E.
        • et al.
        Catapult-like release of mitochondrial DNA by eosinophils contributes to antibacterial defense.
        Nat Med. 2008; 14: 949-953