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The multifactorial mechanisms of bacterial infection in decompensated cirrhosis

  • Schalk Van der Merwe
    Correspondence
    Corresponding author. Address: Department of Gastroenterology and Hepatology and Laboratory of Hepatology, 49 Here street, University hospital Leuven, 3000 Leuven, Belgium.
    Affiliations
    Department of Gastroenterology and Hepatology, University hospital, Leuven, Belgium

    Laboratory of Hepatology, University of Leuven, Belgium
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  • Shilpa Chokshi
    Affiliations
    Institute of Hepatology, Foundation for Liver Research, London, UK

    Division of Transplantation, Immunology and Mucosal Biology, Faculty of Life Sciences and Medicine, King’s College, London, United Kingdom
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  • Christine Bernsmeier
    Affiliations
    Department of Biomedicine, University of Basel, Switzerland

    University Centre for Gastrointestinal and Liver Diseases, Basel, Switzerland
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  • Agustin Albillos
    Affiliations
    Department of Gastroenterology and Hepatology, Hospital Universitario Ramón y Cajal, Instituto Ramón y Cajal de Investigación Sanitaria (IRYCIS), CIBEREHD, Universidad de Alcalá, Madrid, Spain
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      Summary

      Infections, due to a dysfunctional immune response, pose a great risk to patients with decompensated cirrhosis and herald the beginning of the terminal phase of this disease. Infections typically result from breaches in innate immune barriers and inadequate clearance by immune cells. This leads to bacterial and bacterial product translocation to the systemic circulation, which is already primed by ongoing hepatic inflammation in patients with cirrhosis, who are particularly prone to developing organ failure in the presence of an infection. Early identification of bacterial infection, along with the prompt use of appropriate antibiotics, have reduced the mortality associated with certain infections in patients with decompensated cirrhosis. Judicious use of antibiotic therapy remains imperative given the emergence of multidrug-resistant infections in the cirrhotic population. Important research over the last few years has identified molecular targets on immune cells that may enhance their function, and theoretically prevent infections. Clinical trials are ongoing to delineate the beneficial effects of targeted molecules from their off-target effects. Herein, we review the mechanisms that predispose patients with cirrhosis to bacterial infections, the clinical implications of infections and potential targets for the prevention or treatment of infections in this vulnerable population.

      Keywords

      Introduction

      Decompensated cirrhosis represents an immunological paradox: patients exhibit a hyperinflammatory state at the clinical and molecular level, but this coexists with profound immunoparesis and increased susceptibility to bacterial infection.
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      Recent work has shown that immune effector cells become activated during the development of cirrhosis, but antibacterial effector functions are switched off, resulting in a disrupted balance between host-induced immunopathology and protective anti-pathogen immunity. In this review, we discuss the clinical implications and underlying mechanisms rendering patients with cirrhosis susceptible to infections. We also discuss current therapies and the development of novel treatment approaches based on our growing understanding of the molecular basis underlying infection susceptibility in cirrhosis.
      Physicians have long recognised that patients with decompensated cirrhosis are at risk of developing bacterial infections which frequently result in hospitalisation, the development of overt sepsis, acute-on-chronic liver failure (ACLF), as well as the need for intensive care unit (ICU) admission and organ support.
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      Clinical implications of infections in cirrhosis

      In a third of patients hospitalised for cirrhosis, infection is the precipitant of acute decompensation (and hence hospitalisation). The most common infections include spontaneous bacterial peritonitis (SBP), urinary infection, pneumonia and soft tissue infections.
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      Once hospitalised, a further 10–15% of patients will develop nosocomial infections.
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      Patients hospitalised with cirrhosis are frequently exposed to invasive procedures such as placement of central venous lines and urinary catheters or may require mechanical ventilation, especially for respiratory failure or advanced grade III or IV encephalopathy. Such interventions breach important innate immune barriers, namely the skin and respiratory mucosa, which may have already been colonised by resistant bacteria. Additional risk factors for nosocomial infections include admission with a primary infection, more advanced liver disease with a model for end-stage liver disease (MELD) score >20, the use of SBP prophylaxis, lactulose, rifaximin or proton pump inhibitor therapy, as well as being hospitalised during the previous 6 months.
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      Nosocomial infections are further associated with adverse outcomes including the development of ACLF, the need for admission to the ICU, organ support and delisting from the waiting list in patients awaiting transplantation.
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      The Intensive Care Unit (ICU) course and outcome in Acute-on-chronic liver failure are comparable to other populations.
      Patients with decompensated cirrhosis are highly susceptible to infection because of cirrhosis-associated immunoparesis involving innate and adaptive immune cells, as well as PAMPs, DAMPs, cytokines, hormones and metabolites that regulate immune cell differentiation.
      An extremely concerning trend over the last years has been the emergence of multidrug resistant infections in the cirrhotic population, especially extended spectrum beta-lactamase-producing Enterobacteriaceae.
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      Not only are multidrug resistant (MDR) and extensively drug resistant (XDR) organisms becoming a major problem across the world, but they directly impact on the care of patients with cirrhosis at various levels: the efficacy of currently recommended empirical antibiotic regimens is extremely low when MDR organisms are present, increasing patients’ risk of ACLF and septic shock, and consequently mortality.
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      Second infections independently increase mortality in hospitalized patients with cirrhosis: the North American consortium for the study of end-stage liver disease (NACSELD) experience.
      ,
      EASL Clinical Practice Guidelines for the management of patients with decompensated cirrhosis.
      Patients with cirrhosis frequently require antibiotic therapy, often empirically in the setting of a decompensation event, because of gastrointestinal bleeding or suspicion of infection. The judicious use of antibiotics in this population remains imperative to reduce the emergence of MDR and XDR bacterial strains.
      Patients with decompensated cirrhosis are more likely, in the setting of infection, to develop sepsis and organ failure. In decompensated cirrhosis, like late-stage sepsis, profound immune tolerance may develop at the molecular level following prolonged inflammation, characterised by the appearance of specific immunomodulatory monocyte subsets in the circulation that show blunted responses to microbial challenge.
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      Although immune tolerance may initially protect against further immune pathology, a deep and prolonged state of immunosuppression may develop, further increasing susceptibility to infections. This often occurs in decompensated cirrhosis, wherein portal hypertension has already altered systemic haemodynamics.
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      Recent advances in clinical practice Albumin in decompensated cirrhosis: new concepts and perspectives.
      Persistent systemic immune activation, continuous release of pathogen-associated molecular patterns (PAMPs) from the gut, and splanchnic and systemic vasodilation lead to the development of arterial hypotension. The subsequent reduction in arterial blood pressure and organ perfusion activate the renin-angiotensin-aldosterone system resulting in fluid retention and consequently volume expansion (Fig. 1). Together with a compensatory increase in cardiac contractility and elevated epinephrine and norepinephrine levels, a precarious steady-state is reached. In the presence of an additional insult, such as bacterial infection, cardiac contractility cannot compensate further and may decrease. Compensatory endogenous vasoconstrictor mechanisms, already at maximum capacity, will fail to prevent further arterial hypotension, predisposing patients to circulatory dysfunction and organ failure.
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      The clinical manifestations that may result from this precipitating event include ACLF, associated with various degrees of organ dysfunction, of which the most important determinants on clinical outcome are acute kidney injury and circulatory failure.
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      Mechanisms of decompensation and organ failure in cirrhosis: from peripheral arterial vasodilation to systemic inflammation hypothesis.
      Figure thumbnail gr1
      Fig. 1Systemic inflammation and haemodynamic changes predispose to organ failure in cirrhosis. During the development of decompensated cirrhosis, inflammation in the liver, resulting from hepatocyte loss and immune cell activation, lead to the release of DAMPs and inflammatory cytokines into the circulation.
      • Bernardi M.
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      Mechanisms of decompensation and organ failure in cirrhosis: from peripheral arterial vasodilation to systemic inflammation hypothesis.
      If liver inflammation is not resolved, continuous stellate cell activation results in progressive liver fibrosis and regenerating nodule formation that predisposes to the development of portal hypertension. Progressive liver dysfunction coincides with intestinal bacterial dysbiosis and progressive intestinal barrier failure. The latter results in increased hyperpermeability, with translocation of bacterial products and bacteria to the systemic circulation, which promote immune cell and systemic inflammation. Portal hypertension results in the release of vasoactive mediators into the circulation causing splanchnic vasodilation and arterial hypotension. Arterial hypotension and baroreceptor-perceived low effective blood volume lead to RAAS and sympathetic nervous system activation, and consequently to renal sodium and water retention. In cirrhosis, a bacterial translocation event or release of inflammatory cytokines may further aggravate hypotension, facilitate organ hypoperfusion and precipitate multiorgan failure. Clinically, this may result in worsening of encephalopathy, acute kidney injury, hyperdynamic circulation and ACLF. ACLF, acute-on-chronic liver failure; CO, Carbon oxide; DAMPs, damage-associated molecular patterns; H2S, Hydrogen sulfide LSEC, liver sinusoidal endothelial cell; NO, Nitric oxide; PAMPs, pathogen-associated molecular patterns; RAAS, renin-angiotensin-aldosterone; SBP, spontaneous bacterial peritonitis; SNS, sympathetic nervous system.

      Specific mechanisms of immune dysfunction

      Decompensated cirrhosis can be viewed as a multi-systemic inflammatory disorder wherein the mechanisms underlying bacterial infection stem from prolonged chronic sterile and infectious inflammation and the loss of tissue/barrier integrity in diverse compartments. Ultimately these processes will result in compartmental and, consequently, systemic immunoparesis. In addition to the liver, inflammation may involve other organs – such as the peritoneum, gut, kidney and brain – and may further increase during the development of ACLF. The compartmental and systemic modulation of immune cell function leads to a shift of pathogens and PAMPs across damaged barriers. Together with an abundance of circulating damage-associated molecular patterns (DAMPs) and messenger molecules such as cytokines, chemokines, hormones and others, the constantly “primed” immune system sets in motion mechanisms to curtail inflammation to prevent organ failure. These adaptations favour infection susceptibility due to attenuated responses to bacterial challenge.
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      • Périanin A.
      Innate immune cells in cirrhosis.
      A growing understanding of the pathophysiologic mechanisms associated with immune dysfunction in decompensated cirrhosis over recent years has led to the identification of potential therapeutic targets that may be exploited for immunomodulation, with the intention of reducing infection risk and preventing decompensation events and disease progression. In the sections to follow, we review recent scientific advances focusing on the molecular and cellular factors that predispose patients with cirrhosis to bacterial infections

      Immune system failure in decompensated cirrhosis

      In all human compartments, the immune system is composed of physical and chemical barriers, and involves molecular, as well as innate and adaptive cellular components. The complexity of immunoparesis in cirrhosis remains incompletely understood. Current evidence suggests that it involves all the aforementioned components of the immune system and spans across diverse tissue compartments. Molecular components such as PAMPs, DAMPs, cytokines, hormones and metabolites regulate compartmental and systemic immune cell differentiation and function, contributing to immunoparesis and infection susceptibility. These factors also promote hepatic decompensation and the development of portal hypertension
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      • Schnabl B.
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      Mechanisms of decompensation and organ failure in cirrhosis: from peripheral arterial vasodilation to systemic inflammation hypothesis.
      (Fig. 1).
      Dysfunctional immune responses are not limited to circulating immune cells but extend to the innate immune barriers of the intestine and the peritoneum.
      Susceptibility to bacterial infection and sepsis in cirrhosis has been associated with modulation of diverse functions of innate immune cells. These cells normally maintain homeostasis by preventing microbe invasion and finely regulating responses to inflammation following exposure to pathogens. However, in decompensated cirrhosis, functional modifications arise that attenuate the inflammatory cytokine responses of monocytes and dendritic cells to bacterial challenge.
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      Recent advances in clinical practice Albumin in decompensated cirrhosis: new concepts and perspectives.
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      Human CD8+HLA-DR+ regulatory T cells, similarly to classical CD4+Foxp3+ cells, suppress immune responses via PD-1/PD-L1 Axis.
      enriched in the liver, peritoneum and peripheral blood, express high levels of programmed cell death protein 1 (PD-1) and TIM-3 (T cell immunoglobulin and mucin domain-containing protein 3) and promote less peripheral blood mononuclear cell proliferation compared to their HLA-DR-negative counterparts.
      • Lebossé F.
      • Gudd C.
      • Tunc E.
      • Singanayagam A.
      • Nathwani R.
      • Triantafyllou E.
      • et al.
      CD8+T cells from patients with cirrhosis display a phenotype that may contribute to cirrhosis-associated immune dysfunction.
      Also, dampened antigen-specific T cell responses toward common viral antigens have been observed and linked to elevated interleukin (IL)-10 levels.
      • Peter J.
      • Frey O.
      • Stallmach A.
      • Bruns T.
      Attenuated antigen-specific T cell responses in cirrhosis are accompanied by elevated serum interleukin-10 levels and down-regulation of HLA-DR on monocytes.
      Changes in soluble components have been identified in the circulation and tissues in decompensated cirrhosis. PAMPs are recognised by pattern recognition receptors on innate immune cells and epithelia. The interaction of PAMPs with these receptors is essential in eliminating bacterial infections but is normally short-lived. The role of PAMPs in the pathophysiology of cirrhosis as a systemic inflammatory disease has recently been extensively covered.
      • Bernardi M.
      • Moreau R.
      • Angeli P.
      • Schnabl B.
      • Arroyo V.
      Mechanisms of decompensation and organ failure in cirrhosis: from peripheral arterial vasodilation to systemic inflammation hypothesis.
      However chronic exposure to lipopolysaccharide (LPS) and other PAMPs, which may be abundant in the portal and systemic circulation and tissue compartments
      • Zapater P.
      • Francés R.
      • González-Navajas J.M.
      • de la Hoz M.A.
      • Moreu R.
      • Pascual S.
      • et al.
      Serum and ascitic fluid bacterial DNA: a new independent prognostic factor in noninfected patients with cirrhosis.
      ,
      • Bernardi M.
      • Moreau R.
      • Angeli P.
      • Schnabl B.
      • Arroyo V.
      Mechanisms of decompensation and organ failure in cirrhosis: from peripheral arterial vasodilation to systemic inflammation hypothesis.
      in decompensated cirrhosis, alters immune responses and predisposes patients to bacterial infection.
      • Nolan J.P.
      The role of endotoxin in liver injury.
      ,
      • Gandoura S.
      • Weiss E.
      • Rautou P.-E.
      • Fasseu M.
      • Gustot T.
      • Lemoine F.
      • et al.
      Gene- and exon-expression profiling reveals an extensive LPS-induced response in immune cells in patients with cirrhosis.
      This phenomenon termed “endotoxin tolerance”
      • Biswas S.K.
      • Lopez-Collazo E.
      Endotoxin tolerance: new mechanisms, molecules and clinical significance.
      implicates dampening of innate immune responses following perpetual exposure to endotoxin and has been demonstrated in decompensated cirrhosis, wherein it may be linked to reprogramming of monocytes/macrophages.
      • Bernsmeier C.
      • Triantafyllou E.
      • Brenig R.
      • Lebosse F.J.
      • Singanayagam A.
      • Patel V.C.
      • et al.
      CD14+ CD15- HLA-DR- myeloid-derived suppressor cells impair antimicrobial responses in patients with acute-on-chronic liver failure.
      ,
      • Bernsmeier C.
      • Pop O.T.
      • Singanayagam A.
      • Triantafyllou E.
      • Patel V.C.
      • Weston C.J.
      • et al.
      Patients with acute-on-chronic liver failure have increased numbers of regulatory immune cells expressing the receptor tyrosine kinase MERTK.
      ,
      • Korf H.
      • du Plessis J.
      • van Pelt J.
      • De Groote S.
      • Cassiman D.
      • Verbeke L.
      • et al.
      Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
      ,
      • Brenig R.
      • Pop O.T.
      • Triantafyllou E.
      • Geng A.
      • Singanayagam A.
      • Perez-Shibayama C.
      • et al.
      Expression of AXL receptor tyrosine kinase relates to monocyte dysfunction and severity of cirrhosis.
      ,
      • Markwick L.J.L.
      • Riva A.
      • Ryan J.M.
      • Cooksley H.
      • Palma E.
      • Tranah T.H.
      • et al.
      Blockade of PD1 and TIM3 restores innate and adaptive immunity in patients with acute alcoholic hepatitis.
      ,
      • Netea M.G.
      • Quintin J.
      • van der Meer J.W.M.
      Trained immunity: a memory for innate host defense.
      ,
      • Saeed S.
      • Quintin J.
      • Kerstens H.H.D.
      • Rao N.A.
      • Aghajanirefah A.
      • Matarese F.
      • et al.
      Epigenetic programming of monocyte-to-macrophage differentiation and trained innate immunity.
      This implies that antibacterial effector functions are switched off to limit tissue damage. However, limiting host-induced immunopathology comes at the cost of increasing susceptibility to bacterial infections. Additionally DAMPs, evolving from sterile inflammation of the liver and other tissues, have been observed to modulate immune cell function towards a tolerogenic milieu in the context of cirrhosis.
      • Zhang Q.
      • Raoof M.
      • Chen Y.
      • Sumi Y.
      • Sursal T.
      • Junger W.
      • et al.
      Circulating mitochondrial DAMPs cause inflammatory responses to injury.
      ,
      • Huebener P.
      • Pradere J.-P.
      • Hernandez C.
      • Gwak G.-Y.
      • Caviglia J.M.
      • Mu X.
      • et al.
      The HMGB1/RAGE axis triggers neutrophil-mediated injury amplification following necrosis.
      However, their role in development of immunoparesis requires further clarification (Fig. 2).
      Figure thumbnail gr2
      Fig. 2Systemic inflammation and immune cell paresis in decompensated cirrhosis. Decompensated cirrhosis is a systemic inflammatory condition associated with “spill over” of DAMPs, cytokines and immune regulatory cells from the chronically inflamed liver and potential other inflamed tissue sites to the systemic circulation. Bacterial translocation occurs in the presence of intestinal barrier disruption in cirrhosis, where bacterial products reach the cirrhotic liver where they may be ineffectively cleared by Kupffer cells or shunted through vascularised septae into the systemic circulation, contributing to inflammation. Over time, immune tolerance may develop, which is characterised by accumulation, in the systemic circulation, of immune cells with immune suppressive- or immune regulatory properties, along with high levels of pro- and anti-inflammatory cytokines, DAMPs, bacterial products as well as PAMPs. In addition, with further disease progression to ACLF, cells with a reduced capacity to repel microbial challenges appear in the circulation. These cells include neutrophils with attenuated phagocytosis, ROS and bacterial killing potential, M-MDSCs, monocytes expressing AXL, and monocytes expressing MERTK.
      • Bernsmeier C.
      • van der Merwe S.
      • Périanin A.
      Innate immune cells in cirrhosis.
      ,
      • Bernsmeier C.
      • Triantafyllou E.
      • Brenig R.
      • Lebosse F.J.
      • Singanayagam A.
      • Patel V.C.
      • et al.
      CD14+ CD15- HLA-DR- myeloid-derived suppressor cells impair antimicrobial responses in patients with acute-on-chronic liver failure.
      ACLF, acute-on-chronic liver failure; AXL, anexelekto; BA, bile acid; CRIg, complement receptor of the immunoglobulin superfamily; DAMPs, damage-associated molecular patterns; FXR, farnesoid X receptor; gMACS, gut-resident macrophages; HLA, human leukocyte antigen; HSC, hepatic stellate cell; ICAM-1, intercellular adhesion molecule 1; IL-, interleukin; iNOS, inducible nitric oxide synthase; KC, Kupffer cell; LSEC, liver sinusoidal endothelial cell; MERTK, Mer receptor tyrosine kinase; MAIT, mucosa-associated invariant T cells; M-MDSCs, monocytic myeloid-derived suppressor cells; Mφ, macrophages; Mo, monocyte; NO, nitric oxide; PAMPs, pathogen-associated molecular patterns; ROS, reactive oxygen species; TIM4, T-cell membrane protein 4; SAM, scar associated macrophages; TLR4, toll-like receptor 4; TREM2, triggering receptor expressed on myeloid cell 2; VEGF, vascular endothelial growth factor.
      Pro- and anti-inflammatory cytokine levels are elevated in the circulation and certain tissue compartments, such as the peritoneum, in decompensated cirrhosis.
      • Bernsmeier C.
      • Pop O.T.
      • Singanayagam A.
      • Triantafyllou E.
      • Patel V.C.
      • Weston C.J.
      • et al.
      Patients with acute-on-chronic liver failure have increased numbers of regulatory immune cells expressing the receptor tyrosine kinase MERTK.
      ,
      • Korf H.
      • du Plessis J.
      • van Pelt J.
      • De Groote S.
      • Cassiman D.
      • Verbeke L.
      • et al.
      Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
      ,
      • Stengel S.
      • Quickert S.
      • Lutz P.
      • Ibidapo-Obe O.
      • Steube A.
      • Köse-Vogel N.
      • et al.
      Peritoneal level of CD206 associates with mortality and an inflammatory macrophage phenotype in patients with decompensated cirrhosis and spontaneous bacterial peritonitis.
      Certain cytokines, such as IL-6 and IL-8, have been associated with the development of ACLF. In addition, elevated IL-10 and IL-1 levels have been correlated with negative outcomes in patients with organ failure.
      • Wasmuth H.E.
      • Kunz D.
      • Yagmur E.
      • Timmer-Stranghöner A.
      • Vidacek D.
      • Siewert E.
      • et al.
      Patients with acute on chronic liver failure display “sepsis-like” immune paralysis.
      ,
      • Berry P.
      • Antoniades C.
      • Carey I.
      • McPhail M.
      • Hussain M.
      • Davies E.
      • et al.
      Severity of the compensatory anti-inflammatory response determined by monocyte HLA-DR expression may assist outcome prediction in cirrhosis.
      ,
      • Korf H.
      • du Plessis J.
      • van Pelt J.
      • De Groote S.
      • Cassiman D.
      • Verbeke L.
      • et al.
      Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
      ,
      • Peter J.
      • Frey O.
      • Stallmach A.
      • Bruns T.
      Attenuated antigen-specific T cell responses in cirrhosis are accompanied by elevated serum interleukin-10 levels and down-regulation of HLA-DR on monocytes.
      ,
      • Monteiro S.
      • Grandt J.
      • Uschner F.E.
      • Kimer N.
      • Madsen J.L.
      • Schierwagen R.
      • et al.
      Differential inflammasome activation predisposes to acute-on-chronic liver failure in human and experimental cirrhosis with and without previous decompensation.
      On the other hand, the production of cytokines in response to bacterial challenge is clearly affected in different immune cell types in decompensated cirrhosis, and further depends on the severity of liver disease and whether ACLF is present.
      • Bernsmeier C.
      • Triantafyllou E.
      • Brenig R.
      • Lebosse F.J.
      • Singanayagam A.
      • Patel V.C.
      • et al.
      CD14+ CD15- HLA-DR- myeloid-derived suppressor cells impair antimicrobial responses in patients with acute-on-chronic liver failure.
      ,
      • Lin C.-Y.
      • Tsai I.-F.
      • Ho Y.-P.
      • Huang C.-T.
      • Lin Y.-C.
      • Lin C.-J.
      • et al.
      Endotoxemia contributes to the immune paralysis in patients with cirrhosis.
      • Berres M.-L.
      • Schnyder B.
      • Yagmur E.
      • Inglis B.
      • Stanzel S.
      • Tischendorf J.J.W.
      • et al.
      Longitudinal monocyte Human leukocyte antigen-DR expression is a prognostic marker in critically ill patients with decompensated liver cirrhosis.
      • Berry P.
      • Antoniades C.
      • Carey I.
      • McPhail M.
      • Hussain M.
      • Davies E.
      • et al.
      Severity of the compensatory anti-inflammatory response determined by monocyte HLA-DR expression may assist outcome prediction in cirrhosis.
      • Bernsmeier C.
      • Pop O.T.
      • Singanayagam A.
      • Triantafyllou E.
      • Patel V.C.
      • Weston C.J.
      • et al.
      Patients with acute-on-chronic liver failure have increased numbers of regulatory immune cells expressing the receptor tyrosine kinase MERTK.
      • Korf H.
      • du Plessis J.
      • van Pelt J.
      • De Groote S.
      • Cassiman D.
      • Verbeke L.
      • et al.
      Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
      • Brenig R.
      • Pop O.T.
      • Triantafyllou E.
      • Geng A.
      • Singanayagam A.
      • Perez-Shibayama C.
      • et al.
      Expression of AXL receptor tyrosine kinase relates to monocyte dysfunction and severity of cirrhosis.
      • Weichselbaum L.
      • Azouz A.
      • Smolen K.K.
      • Das J.
      • Splittgerber M.
      • Lepida A.
      • et al.
      Epigenetic basis for monocyte dysfunction in patients with severe alcoholic hepatitis.
      Elevated circulating concentrations of prostaglandin E2 (PGE2) have been shown to impair monocyte responses to bacteria in acute decompensation.
      • O’Brien A.J.
      • Fullerton J.N.
      • Massey K.A.
      • Auld G.
      • Sewell G.
      • James S.
      • et al.
      Immunosuppression in acutely decompensated cirrhosis is mediated by prostaglandin E2.
      Moreover endothelial dysfunction and increased vascular permeability
      • Bernsmeier C.
      • Pop O.T.
      • Singanayagam A.
      • Triantafyllou E.
      • Patel V.C.
      • Weston C.J.
      • et al.
      Patients with acute-on-chronic liver failure have increased numbers of regulatory immune cells expressing the receptor tyrosine kinase MERTK.
      ,
      • Gujral J.S.
      • Liu J.
      • Farhood A.
      • Hinson J.A.
      • Jaeschke H.
      Functional importance of ICAM-1 in the mechanism of neutrophil-induced liver injury in bile duct-ligated mice.
      ,
      • Wadkin J.C.R.
      • Patten D.A.
      • Kamarajah S.K.
      • Shepherd E.L.
      • Novitskaya V.
      • Berditchevski F.
      • et al.
      CD151 supports VCAM-1-mediated lymphocyte adhesion to liver endothelium and is upregulated in chronic liver disease and hepatocellular carcinoma.
      have been described, which may contribute to changes in the cellular and molecular composition of the compartmental and systemic milieu (Fig. 1, Fig. 2).
      Gut dysbiosis as well as damage to the physical and immunological layers that comprise the intestinal barrier compromise its function in patients with decompensated cirrhosis.
      Of note, the aforementioned general mechanisms of immune regulation and attenuation of defence against pathogens are differentially regulated and compartment specific. This corresponds to the systemic requirement to control inflammation and prevent immunopathology, yet at the same time fend off bacterial invasion. Immunoparesis and susceptibility to infection develops independently of aetiology in decompensated cirrhosis.
      • Jalan R.
      • Fernandez J.
      • Wiest R.
      • Schnabl B.
      • Moreau R.
      • Angeli P.
      • et al.
      Bacterial infections in cirrhosis: a position statement based on the EASL Special Conference 2013.
      • Borzio M.
      • Salerno F.
      • Piantoni L.
      • Cazzaniga M.
      • Angeli P.
      • Bissoli F.
      • et al.
      Bacterial infection in patients with advanced cirrhosis: a multicentre prospective study.
      • Arvaniti V.
      • D’Amico G.
      • Fede G.
      • Manousou P.
      • Tsochatzis E.
      • Pleguezuelo M.
      • et al.
      Infections in patients with cirrhosis increase mortality four-fold and should be used in determining prognosis.
      ,
      • Sargenti K.
      • Prytz H.
      • Nilsson E.
      • Bertilsson S.
      • Kalaitzakis E.
      Bacterial infections in alcoholic and nonalcoholic liver cirrhosis.
      While this has been shown for most of the mechanisms detailed above, innate and adaptive immune responses may be different depending on the underlying aetiology (e.g. alcohol, metabolic dysfunction, viral hepatitis, cholestatic liver disease) which requires further investigation. For instance, in alcohol-related liver disease the effects of alcohol on immune cells and gut permeability may act synergistically with cirrhosis to increase a patients susceptibility to bacterial infection.
      • Markwick L.J.L.
      • Riva A.
      • Ryan J.M.
      • Cooksley H.
      • Palma E.
      • Tranah T.H.
      • et al.
      Blockade of PD1 and TIM3 restores innate and adaptive immunity in patients with acute alcoholic hepatitis.
      ,
      • Artru F.
      • Bou Saleh M.
      • Maggiotto F.
      • Lassailly G.
      • Ningarhari M.
      • Demaret J.
      • et al.
      IL-33/ST2 pathway regulates neutrophil migration and predicts outcome in patients with severe alcoholic hepatitis.
      ,
      • Vergis N.
      • Khamri W.
      • Beale K.
      • Sadiq F.
      • Aletrari M.O.
      • Moore C.
      • et al.
      Defective monocyte oxidative burst predicts infection in alcoholic hepatitis and is associated with reduced expression of NADPH oxidase.
      ,
      • Gustot T.
      • Fernandez J.
      • Szabo G.
      • Albillos A.
      • Louvet A.
      • Jalan R.
      • et al.
      Sepsis in alcohol-related liver disease.

      Failure of the innate immune barrier in decompensated cirrhosis

      Dysfunctional immune responses are not limited to circulating immune cells but extend to the innate immune barriers of the peritoneum and the intestine.

      Intestinal barrier failure

      Dysfunction of the gut barrier and changes in the constitution of the microbiome are linked to the natural history of cirrhosis and become more pronounced with progression from compensated to decompensated cirrhosis. In fact, gut barrier failure and alterations in the microbiome are directly related to the frequency and severity of bacterial product translocation, bacterial infections and encephalopathy. Gut barrier disruption in decompensated cirrhosis arises from abnormalities at all levels of intestinal barrier defence, is independent of the aetiology of liver disease, and is associated with liver insufficiency, reduced bile flow and impaired immunity (Fig. 3).
      Figure thumbnail gr3
      Fig. 3Failure of the intestinal barrier in decompensated cirrhosis. The intestinal epithelial barrier is a multi-layered structure that separates bacterial organisms in the gastrointestinal lumen from the systemic circulation. In healthy individuals the lumen of the gut is continuously surveyed by dendritic cells that present bacterial antigen to T cells. T cells migrate to regional lymph nodes where they are reprogrammed before returning to the lamina propria where they secrete IL-10 to maintain an anti-inflammatory environment. Paneth cells, at the base of the intestinal crypts, synthesise and release antimicrobial peptides that mediate intestinal host-microbe interactions.
      • Teltschik Z.
      • Wiest R.
      • Beisner J.
      • Nuding S.
      • Hofmann C.
      • Schoelmerich J.
      • et al.
      Intestinal bacterial translocation in rats with cirrhosis is related to compromised paneth cell antimicrobial host defense.
      ,
      • Hassan M.
      • Moghadamrad S.
      • Sorribas M.
      • Wiest R.
      • Romagnolo B.
      • De Gottardi A.
      Paneth cells promote angiogenesis and regulate portal hypertension in response to microbial signals.
      In health intestinal macrophages (Mφ), in an IL-10 environment, are specialized in the phagocytosis of pathogens that cross the barrier. (Mϕ) are inert and cannot be activated to secrete inflammatory cytokines. In murine animals, Mφ exist as a subpopulation that specialise in maintaining epithelial repair and gut vascular barrier integrity. In cirrhosis, in the presence of an altered microbiome and an increase in secondary BAs, along with altered abnormal FXR signalling
      • Úbeda M.
      • Lario M.
      • Muñoz L.
      • Borrero M.J.
      • Rodríguez-Serrano M.
      • Sánchez-Díaz A.M.
      • et al.
      Obeticholic acid reduces bacterial translocation and inhibits intestinal inflammation in cirrhotic rats.
      ,
      • Sorribas M.
      • Jakob M.O.
      • Yilmaz B.
      • Li H.
      • Stutz D.
      • Noser Y.
      • et al.
      FXR modulates the gut-vascular barrier by regulating the entry sites for bacterial translocation in experimental cirrhosis.
      lead to intestinal macrophages becoming activated. These CD14+iNOS+Trem1+cells secrete nitric oxide, IL-6 and IL-8, as well as MCP-1, that likely contribute to the enhanced barrier failure in cirrhosis and enable bacterial translocation to the systemic circulation.
      • Du Plessis J.
      • Vanheel H.
      • Janssen C.E.I.
      • Roos L.
      • Slavik T.
      • Stivaktas P.I.
      • et al.
      Activated intestinal macrophages in patients with cirrhosis release NO and IL-6 that may disrupt intestinal barrier function.
      BA, bile acid; CCL13, C-C motif chemokine; DC, dendritic cell; FXR, farnesoid X receptor; gMacs, gut-resident macrophages; IL-, interleukin; iNOS, inducible nitric oxide synthase; MCP-1, monocyte chemoattractant protein-1; NO, nitric oxide; PAMP, pathogen-associated molecular pattern.

      Altered gut microbial composition in decompensated cirrhosis

      Altered gut microbial composition is likely the most important factor leading to failure of the gut-liver axis in cirrhosis. These alterations arise when normal maintenance of a healthy microbiome fails, as can occur when cirrhosis and the presence of ascites affect small bowel motility delaying transit time and allowing for bacterial overgrowth.
      • Chesta J.
      • Defilippi C.
      • Defilippi C.
      Abnormalities in proximal small bowel motility in patients with cirrhosis.
      • Sadik R.
      • Abrahamsson H.
      • Björnsson E.
      • Gunnarsdottir A.
      • Stotzer P.O.
      Etiology of portal hypertension may influence gastrointestinal transit.
      • Gunnarsdottir S.A.
      • Sadik R.
      • Shev S.
      • Simrén M.
      • Sjövall H.
      • Stotzer P.O.
      • et al.
      Small intestinal motility disturbances and bacterial overgrowth in patients with liver cirrhosis and portal hypertension.
      In addition, bile acid abnormalities, including reduced primary and elevated secondary bile acid levels, impair intestinal immunity and, consequently, may further contribute to the expansion of enteropathogens.
      • Lorenzo-Zúñiga V.
      • Bartolí R.
      • Planas R.
      • Hofmann A.F.
      • Viñado B.
      • Hagey L.R.
      • et al.
      Oral bile acids reduce bacterial overgrowth, bacterial translocation, and endotoxemia in cirrhotic rats.
      • Kakiyama G.
      • Pandak W.M.
      • Gillevet P.M.
      • Hylemon P.B.
      • Heuman D.M.
      • Daita K.
      • et al.
      Modulation of the fecal bile acid profile by gut microbiota in cirrhosis.
      • Kakiyama G.
      • Hylemon P.B.
      • Zhou H.
      • Pandak W.M.
      • Heuman D.M.
      • Kang D.J.
      • et al.
      Colonic inflammation and secondary bile acids in alcoholic cirrhosis.
      Hypochlorhydria present in cirrhosis, even in the absence of proton pump inhibition, is another factor that contributes to altered microbial composition.
      • Shindo K.
      • Machida M.
      • Miyakawa K.
      • Fukumura M.
      A syndrome of cirrhosis, achlorhydria, small intestinal bacterial overgrowth, and fat malabsorption.
      ,
      • Llorente C.
      • Jepsen P.
      • Inamine T.
      • Wang L.
      • Bluemel S.
      • Wang H.J.
      • et al.
      Gastric acid suppression promotes alcoholic liver disease by inducing overgrowth of intestinal Enterococcus.
      Changes in intestinal microbiota along with bacterial overgrowth have been recognised in humans and experimental models of cirrhosis for decades. However, the extent of alterations to faecal microbial composition could not be fully characterised until the arrival of metagenomic analysis techniques. In cirrhosis, the gut microbiota can be characterised by reduced microbial diversity, increased relative overgrowth of pathogenic taxa (such as Enterococcaceae, Staphylococcaceae and especially Enterobacteriaceae), and decreased relative abundance of potentially beneficial autochthonous taxa such as Lachnospiraceae and Ruminococcaceae.
      • Bajaj J.S.
      • Heuman D.M.
      • Hylemon P.B.
      • Sanyal A.J.
      • White M.B.
      • Monteith P.
      • et al.
      Altered profile of human gut microbiome is associated with cirrhosis and its complications.
      • Chen Y.
      • Yang F.
      • Lu H.
      • Wang B.
      • Chen Y.
      • Lei D.
      • et al.
      Characterization of fecal microbial communities in patients with liver cirrhosis.
      • Qin N.
      • Yang F.
      • Li A.
      • Prifti E.
      • Chen Y.
      • Shao L.
      • et al.
      Alterations of the human gut microbiome in liver cirrhosis.
      Interestingly, the microbiome profile seems to be independent of the aetiology of cirrhosis,
      • Kakiyama G.
      • Pandak W.M.
      • Gillevet P.M.
      • Hylemon P.B.
      • Heuman D.M.
      • Daita K.
      • et al.
      Modulation of the fecal bile acid profile by gut microbiota in cirrhosis.
      ,
      • Chen Y.
      • Yang F.
      • Lu H.
      • Wang B.
      • Chen Y.
      • Lei D.
      • et al.
      Characterization of fecal microbial communities in patients with liver cirrhosis.
      becomes more distinct in the setting of hepatic decompensation, and is associated with poor outcome.
      • Bajaj J.S.
      • Idilman R.
      • Mabudian L.
      • Hood M.
      • Fagan A.
      • Turan D.
      • et al.
      Diet affects gut microbiota and modulates hospitalization risk differentially in an international cirrhosis cohort.
      While the gut microbiome remains unchanged in stable outpatients, the ratio of pathogenic to autochthonous taxa increases in patients with decompensated cirrhosis, especially those with bacterial infection and encephalopathy.
      • Bajaj J.S.
      • Heuman D.M.
      • Hylemon P.B.
      • Sanyal A.J.
      • White M.B.
      • Monteith P.
      • et al.
      Altered profile of human gut microbiome is associated with cirrhosis and its complications.
      Specifically, gut microbiome profiles differed between in- and outpatients, whether patients were systemically infected or not, and in patients with organ failure that died.
      • Bajaj J.S.
      • Heuman D.M.
      • Hylemon P.B.
      • Sanyal A.J.
      • White M.B.
      • Monteith P.
      • et al.
      Altered profile of human gut microbiome is associated with cirrhosis and its complications.
      These changes mirror the severity of disease and can be observed in stool, sigmoid colon mucosal biopsies, saliva and serum of patients with cirrhosis.
      • Bajaj J.S.
      • Heuman D.M.
      • Hylemon P.B.
      • Sanyal A.J.
      • White M.B.
      • Monteith P.
      • et al.
      Altered profile of human gut microbiome is associated with cirrhosis and its complications.
      ,
      • Santiago A.
      • Pozuelo M.
      • Poca M.
      • Gely C.
      • Nieto J.C.
      • Torras X.
      • et al.
      Alteration of the serum microbiome composition in cirrhotic patients with ascites.
      Moreover, these studies also showed that the greater the abundance of pathogenic taxa, the greater the level of endotoxemia, as an expression of gut barrier dysfunction. An extreme expression of dysbiosis is observed in patients with alcoholic hepatitis, most of whom had underlying cirrhosis.
      • Bajaj J.S.
      Alcohol, liver disease and the gut microbiota.
      In addition to bacterial dysbiosis, alcoholic hepatitis also featured reduced fungal diversity and Candida overgrowth, as well as increased viral diversity, which were both associated with severity of liver disease and liver-related mortality.
      • Chu H.
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      • Lang S.
      • Jiang L.
      • Wang Y.
      • Llorente C.
      • et al.
      The Candida albicans exotoxin candidalysin promotes alcohol-associated liver disease.
      ,
      • Jiang L.
      • Lang S.
      • Duan Y.
      • Zhang X.
      • Gao B.
      • Chopyk J.
      • et al.
      Intestinal virome in patients with alcoholic hepatitis.
      Decompensated cirrhosis is also associated with damage to the physical and immunological layers that comprise the intestinal barrier (Fig. 3). Increased intestinal permeability, as an expression of gut barrier disruption, occurs in both experimental and clinical cirrhosis.
      • Pérez-Paramo M.
      • Munoz J.
      • Albillos A.
      • Freile I.
      • Portero F.
      • Santos M.
      • et al.
      Effect of propranolol on the factors promoting bacterial translocation in cirrhotic rats with ascites.
      • Pijls K.E.
      • Jonkers D.M.A.E.
      • Elamin E.E.
      • Masclee A.A.M.
      • Koek G.H.
      Intestinal epithelial barrier function in liver cirrhosis: an extensive review of the literature.
      • Du Plessis J.
      • Vanheel H.
      • Janssen C.E.I.
      • Roos L.
      • Slavik T.
      • Stivaktas P.I.
      • et al.
      Activated intestinal macrophages in patients with cirrhosis release NO and IL-6 that may disrupt intestinal barrier function.
      Gut barrier disruption in cirrhosis leads not only to the increased passage to the systemic circulation of macromolecules, including bacterial components such as LPS or bacterial DNA, but also of viable bacteria.
      • Albillos A.
      • Lario M.
      • Álvarez-Mon M.
      Cirrhosis-associated immune dysfunction: distinctive features and clinical relevance.
      Intestinal barrier damage parallels cirrhosis progression and is particularly severe when ascites and gut bacterial translocation have developed.
      • Pérez-Paramo M.
      • Munoz J.
      • Albillos A.
      • Freile I.
      • Portero F.
      • Santos M.
      • et al.
      Effect of propranolol on the factors promoting bacterial translocation in cirrhotic rats with ascites.
      ,
      • Muñoz L.
      • Borrero M.
      • Úbeda M.
      • Conde E.
      • del Campo R.
      • Rodríguez-Serrano M.
      • et al.
      Intestinal immune dysregulation driven by dysbiosis promotes barrier disruption and bacterial translocation in rats with cirrhosis.
      ,
      • Muñoz L.
      • Borrero M.
      • Ubeda M.
      • Lario M.
      • Díaz D.
      • Francés R.
      • et al.
      Interaction between intestinal dendritic cells and bacteria translocated from the gut in rats with cirrhosis.
      Experimental cirrhosis is associated with a deficiency in Paneth cell α-defensins and impaired function of dendritic cells,
      • Teltschik Z.
      • Wiest R.
      • Beisner J.
      • Nuding S.
      • Hofmann C.
      • Schoelmerich J.
      • et al.
      Intestinal bacterial translocation in rats with cirrhosis is related to compromised paneth cell antimicrobial host defense.
      ,
      • Úbeda M.
      • Lario M.
      • Muñoz L.
      • Borrero M.J.
      • Rodríguez-Serrano M.
      • Sánchez-Díaz A.M.
      • et al.
      Obeticholic acid reduces bacterial translocation and inhibits intestinal inflammation in cirrhotic rats.
      both being especially apparent in experimental cirrhotic rats with ascites and pathological bacterial translocation.
      Gut barrier disruption leads to translocation of bacterial components, such as LPS and bacterial DNA, but also viable bacteria to the systemic circulation, promoting continuous systemic inflammation, immunoparesis and bacterial infections.
      Abnormalities in intestinal barrier function in cirrhosis have been linked to other structural changes in the intestine, including submucosal oedema, minimal infiltration by immune cells, and disorganisation of inter-epithelial tight junction proteins.
      • Du Plessis J.
      • Vanheel H.
      • Janssen C.E.I.
      • Roos L.
      • Slavik T.
      • Stivaktas P.I.
      • et al.
      Activated intestinal macrophages in patients with cirrhosis release NO and IL-6 that may disrupt intestinal barrier function.
      ,
      • Muñoz L.
      • Borrero M.
      • Úbeda M.
      • Conde E.
      • del Campo R.
      • Rodríguez-Serrano M.
      • et al.
      Intestinal immune dysregulation driven by dysbiosis promotes barrier disruption and bacterial translocation in rats with cirrhosis.
      ,
      • Úbeda M.
      • Lario M.
      • Muñoz L.
      • Borrero M.J.
      • Rodríguez-Serrano M.
      • Sánchez-Díaz A.M.
      • et al.
      Obeticholic acid reduces bacterial translocation and inhibits intestinal inflammation in cirrhotic rats.
      ,
      • Llovet J.M.
      • Bartolí R.
      • Planas R.
      • Cabré E.
      • Jimenez M.
      • Urban A.
      • et al.
      Bacterial translocation in cirrhotic rats. Its role in the development of spontaneous bacterial peritonitis.
      A recent study indicated that subclinical intestinal inflammation is driven by modified microbial composition and progressive barrier dysfunction in advanced cirrhosis.
      • Muñoz L.
      • Borrero M.
      • Úbeda M.
      • Conde E.
      • del Campo R.
      • Rodríguez-Serrano M.
      • et al.
      Intestinal immune dysregulation driven by dysbiosis promotes barrier disruption and bacterial translocation in rats with cirrhosis.
      As cirrhosis progressed to the ascitic stage, the intestinal immune system switched to a type 1 helper T cell (Th1) pattern with expansion of tumour necrosis factor-α (TNF-α)- and interferon-γ (IFN-γ)-expressing lymphocytes, and concomitant Th17 depletion in the lamina propria.
      • Muñoz L.
      • Borrero M.
      • Úbeda M.
      • Conde E.
      • del Campo R.
      • Rodríguez-Serrano M.
      • et al.
      Intestinal immune dysregulation driven by dysbiosis promotes barrier disruption and bacterial translocation in rats with cirrhosis.
      Bowel decontamination restored microbial composition, reduced proinflammatory activation of mucosal immune cells, and diminished intestinal permeability and bacterial translocation, supporting the key role of changes in microbiota in intestinal inflammation.
      • Muñoz L.
      • Borrero M.
      • Úbeda M.
      • Conde E.
      • del Campo R.
      • Rodríguez-Serrano M.
      • et al.
      Intestinal immune dysregulation driven by dysbiosis promotes barrier disruption and bacterial translocation in rats with cirrhosis.
      Bile acids form the backbone of the reciprocal interaction between the gut and the liver and have a key role in shaping the composition of microbiota through MyD88 (myeloid differentiation primary response 88) and farnesoid X receptor (FXR) signalling.
      • Duparc T.
      • Plovier H.
      • Marrachelli V.G.
      • Van Hul M.
      • Essaghir A.
      • Ståhlman M.
      • et al.
      Hepatocyte MyD88 affects bile acids, gut microbiota and metabolome contributing to regulate glucose and lipid metabolism.
      Since bile acids and the microbiome influence each other, it is possible that reduced secretion of bile acids into the intestine contributes to the severity of dysbiosis and an abundance of enteropathogens in cirrhosis. In fact, reduced bile flow, decreased faecal bile acids, and increased serum bile acids are all features which worsen in parallel with cirrhosis severity.
      • Lorenzo-Zúñiga V.
      • Bartolí R.
      • Planas R.
      • Hofmann A.F.
      • Viñado B.
      • Hagey L.R.
      • et al.
      Oral bile acids reduce bacterial overgrowth, bacterial translocation, and endotoxemia in cirrhotic rats.
      ,
      • Kakiyama G.
      • Pandak W.M.
      • Gillevet P.M.
      • Hylemon P.B.
      • Heuman D.M.
      • Daita K.
      • et al.
      Modulation of the fecal bile acid profile by gut microbiota in cirrhosis.
      Liver insufficiency impairs the synthesis and excretion of bile acids, resulting in deficient levels of total bile acids in the gut lumen and augmented levels in serum. The reduction in faecal bile acids mostly relates to secondary rather than primary bile acids, and is due to decreased 7α bile acid-dehydroxylating bacterial populations.
      • Lorenzo-Zúñiga V.
      • Bartolí R.
      • Planas R.
      • Hofmann A.F.
      • Viñado B.
      • Hagey L.R.
      • et al.
      Oral bile acids reduce bacterial overgrowth, bacterial translocation, and endotoxemia in cirrhotic rats.
      ,
      • Kakiyama G.
      • Pandak W.M.
      • Gillevet P.M.
      • Hylemon P.B.
      • Heuman D.M.
      • Daita K.
      • et al.
      Modulation of the fecal bile acid profile by gut microbiota in cirrhosis.
      ,
      • Santiago A.
      • Pozuelo M.
      • Poca M.
      • Gely C.
      • Nieto J.C.
      • Torras X.
      • et al.
      Alteration of the serum microbiome composition in cirrhotic patients with ascites.
      ,
      • Stiehl A.
      • Raedsch R.
      • Rudolph G.
      • Gundert-Remy U.
      • Senn M.
      Biliary and urinary excretion of sulfated, glucuronidated and tetrahydroxylated bile acids in cirrhotic patients.
      As the changes in microbial composition become more pronounced during cirrhosis progression, intestinal inflammation, intestinal barrier damage and hepatic inflammation worsen, which in turn further suppress bile acid secretion. Suppressed FXR receptor signalling, that results from decreased bile flow, disrupts intestinal barrier function by reducing mucus thickness and antibacterial peptide synthesis, further damaging the gut vascular barrier.
      • Sorribas M.
      • Jakob M.O.
      • Yilmaz B.
      • Li H.
      • Stutz D.
      • Noser Y.
      • et al.
      FXR modulates the gut-vascular barrier by regulating the entry sites for bacterial translocation in experimental cirrhosis.
      Taken together, the current data point to a direct influence of altered gut microbial composition on complications and outcomes in cirrhosis, making it a promising potential target for therapy. This rationale is reinforced by the fact that gut microbial diversity was independently associated with a lower risk of 90-day hospitalisation in patients with cirrhosis on a western-diet. In addition, a diet rich in fermented milk, vegetables, cereals, coffee, and tea was found to increase microbial diversity and reduce hospitalisations.
      • Bajaj J.S.
      Alcohol, liver disease and the gut microbiota.
      Further, faecal transplantation has been shown to improve cognitive function and reduce hepatic encephalopathy-associated hospital admission in patients with decompensated cirrhosis.
      • Bajaj J.S.
      • Fagan A.
      • Gavis E.A.
      • Kassam Z.
      • Sikaroodi M.
      • Gillevet P.M.
      Long-term outcomes of fecal microbiota transplantation in patients with cirrhosis.
      Further studies are required to determine the effects of faecal transplantation on bacterial infections before its use can be widely advocated.
      Beyond their role in complications and outcomes in cirrhosis, extensive evidence has also linked enteric bacteria and PAMPs to the pathogenesis of immune cell activation and the systemic inflammatory state in cirrhosis.
      • Albillos A.
      • Lario M.
      • Álvarez-Mon M.
      Cirrhosis-associated immune dysfunction: distinctive features and clinical relevance.
      The causal link between systemic inflammation and the gut microbiota has recently been reinforced in patients with cirrhosis undergoing transjugular intrahepatic portosystemic shunting, demonstrating compartment-specific patterns of circulating bacterial DNA and inflammatory cytokine clusters specific to the abundance of blood microbiome genera in each patient.
      • Schierwagen R.
      • Alvarez-Silva C.
      • Madsen M.S.A.
      • Kolbe C.C.
      • Meyer C.
      • Thomas D.
      • et al.
      Circulating microbiome in blood of different circulatory compartments.
      The peritoneal compartment in decompensated cirrhosis contains large peritoneal macrophages with an inflammatory phenotype that are less susceptible to tolerance induction and release more inflammatory cytokines.

      The anatomical route of bacterial translocation in cirrhosis

      It remains uncertain how bacteria and bacterial products gain entry into the systemic circulation in cirrhosis. The classical concept suggests that pathological bacterial translocation involves the passage of viable bacteria from the gut lumen to the mesenteric lymph nodes and from there to the systemic circulation via the thoracic duct.
      • Pérez-Paramo M.
      • Munoz J.
      • Albillos A.
      • Freile I.
      • Portero F.
      • Santos M.
      • et al.
      Effect of propranolol on the factors promoting bacterial translocation in cirrhotic rats with ascites.
      ,
      • Wiest R.
      • Lawson M.
      • Geuking M.
      Pathological bacterial translocation in liver cirrhosis.
      However more recent evidence has indicated that the lymphatic route of translocation coexists with portal-venous passage of bacteria and bacterial products.
      • Sorribas M.
      • Jakob M.O.
      • Yilmaz B.
      • Li H.
      • Stutz D.
      • Noser Y.
      • et al.
      FXR modulates the gut-vascular barrier by regulating the entry sites for bacterial translocation in experimental cirrhosis.
      Portal-venous passage of bacteria led to vascular hyperpermeability, which was independent of the lymphatic route as well as of portal hypertension, since it was only present in models of liver injury incorporating hepatic insufficiency. Interestingly, in this model, obeticholic acid was able to restore ileal FXR signalling, improve the mucus machinery and stabilise the gut vascular barrier. This supports the notion that the nuclear receptor FXR is important in maintaining the mucus and vascular barrier in health.
      • Sorribas M.
      • Jakob M.O.
      • Yilmaz B.
      • Li H.
      • Stutz D.
      • Noser Y.
      • et al.
      FXR modulates the gut-vascular barrier by regulating the entry sites for bacterial translocation in experimental cirrhosis.
      Additionally, obeticholic acid and other FXR agonists can reconstitute microbial composition, improve intestinal innate defence mechanisms, reduce intestinal inflammation and decrease bacterial translocation and endotoxemia in experimental cirrhosis,
      • Duparc T.
      • Plovier H.
      • Marrachelli V.G.
      • Van Hul M.
      • Essaghir A.
      • Ståhlman M.
      • et al.
      Hepatocyte MyD88 affects bile acids, gut microbiota and metabolome contributing to regulate glucose and lipid metabolism.
      ,
      • Sorribas M.
      • Jakob M.O.
      • Yilmaz B.
      • Li H.
      • Stutz D.
      • Noser Y.
      • et al.
      FXR modulates the gut-vascular barrier by regulating the entry sites for bacterial translocation in experimental cirrhosis.
      ,
      • Schwabl P.
      • Hambruch E.
      • Seeland B.A.
      • Hayden H.
      • Wagner M.
      • Garnys L.
      • et al.
      The FXR agonist PX20606 ameliorates portal hypertension by targeting vascular remodelling and sinusoidal dysfunction.
      ,
      • Verbeke L.
      • Farre R.
      • Verbinnen B.
      • Covens K.
      • Vanuytsel T.
      • Verhaegen J.
      • et al.
      The FXR agonist obeticholic acid prevents gut barrier dysfunction and bacterial translocation in cholestatic rats.
      wherein reduced ileal FXR signalling has been linked to primary bile acid depletion and elevated secondary bile acid levels.
      • Úbeda M.
      • Lario M.
      • Muñoz L.
      • Borrero M.J.
      • Rodríguez-Serrano M.
      • Sánchez-Díaz A.M.
      • et al.
      Obeticholic acid reduces bacterial translocation and inhibits intestinal inflammation in cirrhotic rats.
      ,
      • Sorribas M.
      • Jakob M.O.
      • Yilmaz B.
      • Li H.
      • Stutz D.
      • Noser Y.
      • et al.
      FXR modulates the gut-vascular barrier by regulating the entry sites for bacterial translocation in experimental cirrhosis.
      (Fig. 3)

      Failure of the peritoneal barrier

      Spontaneous bacterial peritonitis (SBP) is the most frequent bacterial infection occurring in decompensated cirrhosis. It results from intestinal and peritoneal barrier failure, which enables viable bacteria to cross into the sterile peritoneal cavity. The normal peritoneal niche is mainly populated by a heterogeneous population of macrophages that are crucial for immunological surveillance, resolution of inflammation, and recruitment and activation of other immune cells. Recently phenotypically and functionally different subpopulations of large and small CD14+ peritoneal macrophages were described; they could be characterised by complement receptor of the immunoglobulin superfamily (CRIg)high and CRIglow expression, respectively.. Large CRIghigh macrophages displayed a high phagocytic and antimicrobial activity and their presence was associated with a lower MELD score.
      • Irvine K.M.
      • Banh X.
      • Gadd V.L.
      • Wojcik K.K.
      • Ariffin J.K.
      • Jose S.
      • et al.
      CRIg-expressing peritoneal macrophages are associated with disease severity in patients with cirrhosis and ascites.
      In line with their murine counterpart (F4/80+ GATA binding protein (GATA)6+), large CRIghigh peritoneal macrophages can be considered resident immune cells, shaped by local niche factors and optimised for gate keeping
      • Ruiz-Alcaraz A.J.
      • Carmona-Martínez V.
      • Tristán-Manzano M.
      • Machado-Linde F.
      • Sánchez-Ferrer M.L.
      • García-Peñarrubia P.
      • et al.
      Characterization of human peritoneal monocyte/macrophage subsets in homeostasis: phenotype, GATA6, phagocytic/oxidative activities and cytokines expression.
      ,
      • Bou Ghosn E.E.
      • Cassado A.A.
      • Govoni G.R.
      • Fukuhara T.
      • Yang Y.
      • Monack D.M.
      • et al.
      Two physically, functionally, and developmentally distinct peritoneal macrophage subsets.
      (Fig. 4). However, since the human peritoneal niche has not yet been completely elucidated, it can be anticipated that additional subpopulations of peritoneal macrophages exist. A recent study identified a phenotypically, transcriptionally, and functionally distinct population of CD14+ large peritoneal macrophages in the ascitic fluid of patients with cirrhosis, which expressed CD206, exhibited features of inflammatory priming, and remained a significant source of cytokine production after repeated exposure to bacterial products.
      • Stengel S.
      • Quickert S.
      • Lutz P.
      • Ibidapo-Obe O.
      • Steube A.
      • Köse-Vogel N.
      • et al.
      Peritoneal level of CD206 associates with mortality and an inflammatory macrophage phenotype in patients with decompensated cirrhosis and spontaneous bacterial peritonitis.
      Interestingly, large CD206+ peritoneal macrophages in cirrhotic ascitic fluid behaved differently to those in peritoneal dialysis: those obtained from cirrhotic fluid had higher ex vivo proinflammatory activity. This supports the notion that large CD206+ peritoneal macrophages in cirrhotic ascitic fluid are unique to the dysregulated innate immune state observed in decompensated cirrhosis.
      • Stengel S.
      • Quickert S.
      • Lutz P.
      • Ibidapo-Obe O.
      • Steube A.
      • Köse-Vogel N.
      • et al.
      Peritoneal level of CD206 associates with mortality and an inflammatory macrophage phenotype in patients with decompensated cirrhosis and spontaneous bacterial peritonitis.
      Decompensated cirrhosis represents a paradoxical immunological landscape characterised by a highly activated immune response with injurious proinflammatory responses that is simultaneously unable to defend against bacterial pathogens.
      Figure thumbnail gr4
      Fig. 4Failure of the peritoneal barrier: Immune responses in the peritoneal niche in patients with cirrhosis and portal hypertension. In the peritoneal fluid from patients with decompensated cirrhosis, portal hypertension and ascites, 2 distinct populations of macrophages have been identified: LPMs and SPMs, which differ in granularity, phenotype and function. LPMs have an inflammatory phenotype (CD14+CD206+CCR2-CRIg+MERTK+), are less susceptible to tolerance induction, and release more inflammatory cytokines (TNF-α) than SPMs. In the context of spontaneous bacterial peritonitis (SBP) activation of PMs altered CD206 expression on the surface of LPMs leading to the release of soluble CD206 into the ascetic fluid. Loss of LPMs occur in early phases of SBP, but are reconstituted after treatment.
      • Stengel S.
      • Quickert S.
      • Lutz P.
      • Ibidapo-Obe O.
      • Steube A.
      • Köse-Vogel N.
      • et al.
      Peritoneal level of CD206 associates with mortality and an inflammatory macrophage phenotype in patients with decompensated cirrhosis and spontaneous bacterial peritonitis.
      ,
      • Irvine K.M.
      • Banh X.
      • Gadd V.L.
      • Wojcik K.K.
      • Ariffin J.K.
      • Jose S.
      • et al.
      CRIg-expressing peritoneal macrophages are associated with disease severity in patients with cirrhosis and ascites.
      ,
      • Ruiz-Alcaraz A.J.
      • Carmona-Martínez V.
      • Tristán-Manzano M.
      • Machado-Linde F.
      • Sánchez-Ferrer M.L.
      • García-Peñarrubia P.
      • et al.
      Characterization of human peritoneal monocyte/macrophage subsets in homeostasis: phenotype, GATA6, phagocytic/oxidative activities and cytokines expression.
      ,
      • Francés R.
      • Muñoz C.
      • Zapater P.
      • Uceda F.
      • Gascón I.
      • Pascual S.
      • et al.
      Bacterial DNA activates cell mediated immune response and nitric oxide overproduction in peritoneal macrophages from patients with cirrhosis and ascites.
      ,
      • Francés R.
      • Rodríguez E.
      • Muñoz C.
      • Zapater P.
      • De La Sen M.L.
      • Ndongo M.
      • et al.
      Intracellular cytokine expression in peritoneal monocyte/macrophages obtained from patients with cirrhosis and presence of bacterial DNA.
      CCR2, C-C motif chemokine receptor 2; CRIg, complement receptor of the immunoglobulin superfamily; DAMPs, damage-associated molecular patterns, IL-6, interleukin 6; LPM, large peritoneal macrophage; MERTK, Mer receptor tyrosine kinase; NO, nitric oxide; PAMPs, pathogen-associated molecular patterns; SAM, scar associated macrophage; SBP, spontaneous bacterial peritonitis; SPM, small peritoneal macrophage; TNF-α, tumour necrosis factor-α; WBC, white blood cell.
      Spontaneous bacterial peritonitis is characterised by a relatively low microbial load that provokes an intense inflammatory response, which correlates with systemic inflammation and clinical outcomes such as renal failure.
      • Navasa M.
      • Follo A.
      • Filella X.
      • Jiménez W.
      • Francitorra A.
      • Planas R.
      • et al.
      Tumor necrosis factor and interleukin-6 in spontaneous bacterial peritonitis in cirrhosis: relationship with the development of renal impairment and mortality.
      Recent evidence indicated an association between altered peritoneal macrophage function, systemic inflammation and clinical outcomes in SBP. During SBP, large peritoneal macrophages secreted a cleaved protein soluble mannose receptor CD206 (sCD206). The concentrations of sCD206 in ascitic fluid, but not in serum, correlated with 90-day mortality as well as with peritoneal (TNF-α and sCD163) and systemic (C-reactive protein, white blood cell count) inflammation, but not with microbiological and cytological response to antibiotic therapy at day 1.
      • Stengel S.
      • Quickert S.
      • Lutz P.
      • Ibidapo-Obe O.
      • Steube A.
      • Köse-Vogel N.
      • et al.
      Peritoneal level of CD206 associates with mortality and an inflammatory macrophage phenotype in patients with decompensated cirrhosis and spontaneous bacterial peritonitis.
      Additionally, in SBP, the peritoneal cavity was flooded with peripheral immune cells, mainly monocytes and neutrophils, and depleted of large peritoneal macrophages.
      • Stengel S.
      • Quickert S.
      • Lutz P.
      • Ibidapo-Obe O.
      • Steube A.
      • Köse-Vogel N.
      • et al.
      Peritoneal level of CD206 associates with mortality and an inflammatory macrophage phenotype in patients with decompensated cirrhosis and spontaneous bacterial peritonitis.
      The reason for the depletion of CD206 macrophages and how this contributes to the pathogenesis of SBP is unknown. Clearly more research is required to understand the unique role of large resident macrophages in homeostasis as well as in SBP.

      Existing and potential future therapies in decompensated cirrhosis

      Therapies to improve innate immune function

      Current guidelines for the prevention of bacterial infection in cirrhosis do not include any direct immunomodulatory treatment approaches as these are currently under investigation.
      EASL Clinical Practice Guidelines for the management of patients with decompensated cirrhosis.
      ,
      • Simonetto D.A.
      • Piccolo Serafim L.
      • Gallo de Moraes A.
      • Gajic O.
      • Kamath P.S.
      Management of sepsis in patients with cirrhosis: current evidence and practical approach.
      Indirect approaches involve the use of antibiotics for the treatment of current bacterial infection (Table 1), prevention of bacterial infection following variceal bleeding, use of norfloxacin in SBP prophylaxis, and the regulation of gut microbiota using rifaximin in the context of hepatic encephalopathy. It must be stated that the use of norfloxaxin in SBP prophylaxis is based on a small study where 68 patients with advanced decompensated cirrhosis at high risk of SBP were randomised to either norfloxacine 400 mg daily or placebo. This study showed that norfloxacine prophylaxis decreased the risk of hepatorenal syndrome and improved 3- and 12-month survival.
      • Fernández J.
      • Navasa M.
      • Planas R.
      • Montoliu S.
      • Monfort D.
      • Soriano G.
      • et al.
      Primary prophylaxis of spontaneous bacterial peritonitis delays hepatorenal syndrome and improves survival in cirrhosis.
      Table 1Recommendations on the use of empirical antibiotic therapy for community acquired and nosocomial infections in cirrhosis.
      Type of infectionLikely bacterial sppRecommended antibiotics in the community acquired setting
      Community acquired infection- Infection diagnosed within 48 hours of hospitalization.
      Recommended antibiotics: Nosocomial
      Nosocomial infection- Infection diagnosed after more than 48 hours of hospitalization.
      Presence of sepsis/septic shock
      SBP and bacteremiaGram negative:
      • -
        E. coli
      • -
        K. pneumonia
      Gram positive:
      • -
        S. pneumonia
      • -
        S. viridans
      • -
        Enterococcus faecalis
      • -
        Enterococcus faecium
      First line therapy:
      • -
        IV 3rd generation cephalosporin (Cefotaxime, Ceftriaxone)
      Other options:

      Uncomplicated SBP*:
      • -
        IV ciprofloxacin or oral levofloxacin
      *Cannot be considered in patients previously exposed to norfloxacin prophylaxis

      *Not indicated if vomiting; shock; HE grade III, IV; creatinine >3 mg/dl

      High prevalence MDR:

      Piperacillin/tazobactam
      Low prevalence MDR:
      • -
        Piperacillin/tazobactam
      High prevalence of MDR:
      • -
        Meropenem ± vancomycin
      No sepsis: prevention acute kidney injury/HRS
      • -
        Albumin 1 g/kg max 100 g day 0,3
      Sepsis/septic shock:

      Reconsider diagnosis: source control, secondary bacterial peritonitis?

      Fluid resuscitation to maintain organ perfusion

      Indication for ICU care

      Indication for organ support:
      • -
        Vasopressor support (terlipressin, norepinephrine)
      • -
        Mechanical ventilation
      • -
        Renal replacement therapy
      Urinary tract infectionGram negative:
      • -
        E. coli
      • -
        K. pneumonia
      Gram positive:
      • -
        Enterococcus faecalis
      • -
        Enterococcus faecium
      Uncomplicated infection:
      • -
        Oral ciprofloxacin or co-trimoxazole


      Complicated infection with evidence of SIRS or sepsis:
      • -
        IV 3rd generation cephalosporin
      • -
        Piperacillin/tazobactam
      Uncomplicated infection:
      • -
        Fosfomycin or nitrofurantoin
      Complicated infection with evidence of SIRS or sepsis:

      Low prevalence MDR:
      • -
        Piperacillin/tazobactam
      High prevalence MDR:
      • -
        Meropenem ± vancomycin
      Sepsis/septic shock:

      Reconsider diagnosis: Pyelonephritis, renal abscess

      Fluid resuscitation to maintain organ perfusion

      Indication for ICU care

      Indication for organ support:
      • -
        Vasopressor support (terlipressin, norepinephrine)
      • -
        Mechanical ventilation
      • -
        Renal replacement therapy
      PneumoniaGram positive:
      • -
        S. pneumonia
      • -
        H. influenza
      • -
        S. aureus
      Gram negative:
      • -
        K. pneumonia
      • -
        E. coli
      • -
        P. aeruginosa
      Piperacillin/tazobactam or Ceftriaxone + macrolide or Levofloxacin or moxifloxacinLow prevalence MDR:
      • -
        Piperacillin/tazobactam
      High prevalence of MDR:

      Meropenem ± vancomycin or Ceftazidime + vancomycin

      §Linezolid in cases of high risk for MRSA

      §Ventilator associated pneumonia, previous antibiotic therapy, positive nasal swab for MRSA
      Reconsider diagnosis: atypical, viral or fungal pneumonia

      Fluid resuscitation to maintain organ perfusion

      Indication for ICU care

      Indication for organ support:
      • -
        Vasopressor support (terlipressin, norepinephrine)
      • -
        Mechanical ventilation
      • -
        Renal replacement therapy
      Skin and soft tissue infectionsGram positive:
      • -
        S. aureus
      • -
        Streptococcus sp.
      Gram negative:
      • -
        E. coli
      • -
        K. pneunomiae
      • -
        P. aeruginosa
      Piperacillin/tazobactam or 3rd generation cephalosporin + penicillinase resistant penicillinMeropenem or ceftazidime + penicillinase resistant penicillin or glycopeptide#

      # IV Vancomycin or teicoplanin should be used in areas with a high prevalence of MRSA or vancomycin susceptible enterococci. Linezolid or daptomycin in areas with high prevalence of VRE
      Reconsider diagnosis: incomplete source control consider debridement

      Resuscitation to maintain organ perfusion

      Indication for ICU care

      Indication for organ support:
      • -
        Vasopressor support (terlipressin, norepinephrine)
      • -
        Mechanical ventilation
      • -
        Renal replacement therapy
      Compiled and modified from references
      • Piano S.
      • Singh V.
      • Caraceni P.
      • Maiwall R.
      • Alessandria C.
      • Fernandez J.
      • et al.
      Epidemiology and effects of bacterial infections in patients with cirrhosis worldwide.
      ,
      • Bajaj J.S.
      • O’Leary J.G.
      • Reddy K.R.
      • Wong F.
      • Olson J.C.
      • Subramanian R.M.
      • et al.
      Second infections independently increase mortality in hospitalized patients with cirrhosis: the North American consortium for the study of end-stage liver disease (NACSELD) experience.
      ,
      • Fernández J.
      • Prado V.
      • Trebicka J.
      • Amoros A.
      • Gustot T.
      • Wiest R.
      • et al.
      Multidrug-resistant bacterial infections in patients with decompensated cirrhosis and with acute-on-chronic liver failure in Europe.
      • Fernández J.
      • Acevedo J.
      • Castro M.
      • Garcia O.
      • Rodríguez de Lope C.
      • Roca D.
      • et al.
      Prevalence and risk factors of infections by multiresistant bacteria in cirrhosis: a prospective study.
      EASL Clinical Practice Guidelines for the management of patients with decompensated cirrhosis.
      ,
      • Simonetto D.A.
      • Piccolo Serafim L.
      • Gallo de Moraes A.
      • Gajic O.
      • Kamath P.S.
      Management of sepsis in patients with cirrhosis: current evidence and practical approach.
      ,
      • Fernández J.
      • Navasa M.
      • Planas R.
      • Montoliu S.
      • Monfort D.
      • Soriano G.
      • et al.
      Primary prophylaxis of spontaneous bacterial peritonitis delays hepatorenal syndrome and improves survival in cirrhosis.
      .
      HE, hepatic encephalopathy; HRS, hepatorenal syndrome; ICU, intensive care unit; MDR, multidrug resistance; MRSA, methicillin-resistant Staphylococcus aureus; SBP, spontaneous bacterial peritonitis; SIRS, systemic inflammatory response syndrome; VRE, vancomycin resistant enterococci.
      Definition of sepsis: life-threatening organ dysfunction caused by a dysregulated host response to infection. Organ dysfunction is defined as an increase in the Sequential Organ Failure Assessment (SOFA) score of ≥2 points. Finally, septic shock is identified by the requirement of vasopressors to maintain a mean arterial pressure (MAP) of ≥65 mm Hg and a serum lactate level >2 mmol/L.
      a Community acquired infection- Infection diagnosed within 48 hours of hospitalization.
      b Nosocomial infection- Infection diagnosed after more than 48 hours of hospitalization.
      The use of albumin is strongly recommended in addition to antibiotic therapy for the treatment of SBP to prevent hepatorenal syndrome and has been associated with immunomodulatory effects.
      • Bernardi M.
      • Angeli P.
      • Claria J.
      • Moreau R.
      • Gines P.
      • Jalan R.
      • et al.
      Recent advances in clinical practice Albumin in decompensated cirrhosis: new concepts and perspectives.
      In addition, the ANSWER trial randomised patients with decompensated cirrhosis to weekly albumin infusions or standard of care. Weekly albumin infusions were associated with reduced all-cause mortality as well as reduced incidence of SBP and other infections.
      • Caraceni P.
      • Riggio O.
      • Angeli P.
      • Alessandria C.
      • Neri S.
      • Foschi F.G.
      • et al.
      Long-term albumin administration in decompensated cirrhosis (ANSWER): an open-label randomised trial.
      Ongoing studies, such as the ATTIRE and PRECIOSA trials, are currently assessing which patients with decompensated cirrhosis will most benefit from albumin.
      A molecule which has been intensively studied in clinical trials recently, is granulocyte colony-stimulating factor (G-CSF). This glycoprotein releases bone marrow-derived CD34+ haematopoietic stem cells. By replacing dysfunctional circulating innate immune cells (granulocytes, monocytes, dendritic cells) with functional counterparts, G-CSF may improve both innate immune responses and liver regeneration in decompensated cirrhosis and ACLF. Up to now, the use of G-CSF remains controversial and restricted to ongoing clinical trials.
      • Garg V.
      • Garg H.
      • Khan A.
      • Trehanpati N.
      • Kumar A.
      • Sharma B.C.
      • et al.
      Granulocyte colony-stimulating factor mobilizes CD34(+) cells and improves survival of patients with acute-on-chronic liver failure.
      • Kedarisetty C.K.
      • Anand L.
      • Bhardwaj A.
      • Bhadoria A.S.
      • Kumar G.
      • Vyas A.K.
      • et al.
      Combination of granulocyte colony-stimulating factor and erythropoietin improves outcomes of patients with decompensated cirrhosis.
      • Verma N.
      • Kaur A.
      • Sharma R.
      • Bhalla A.
      • Sharma N.
      • De A.
      • et al.
      Outcomes after multiple courses of granulocyte colony-stimulating factor and growth hormone in decompensated cirrhosis: a randomized trial.
      Current guidelines propose indirect immunomodulation with antibiotics. Translational research has identified potential drug targets which could directly regulate immune cell function and thereby augment favourable immune responses to bacterial challenges.
      Several other concepts developed using in vitro and ex vivo models merit attention and further investigation: Potent toll-like receptor (TLR) 7/8 agonists (CL097, R848) have been shown to improve the function of neutrophils obtained from patients with decompensated cirrhosis ex vivo and may therefore restore antimicrobial responses in vivo.
      • Rolas L.
      • Boussif A.
      • Weiss E.
      • Lettéron P.
      • Haddad O.
      • El-Benna J.
      • et al.
      NADPH oxidase depletion in neutrophils from patients with cirrhosis and restoration via toll-like receptor 7/8 activation.
      Ex vivo data supports the potential use of agents targeting TAM receptors (MERTK, AXL)
      • Bernsmeier C.
      • Pop O.T.
      • Singanayagam A.
      • Triantafyllou E.
      • Patel V.C.
      • Weston C.J.
      • et al.
      Patients with acute-on-chronic liver failure have increased numbers of regulatory immune cells expressing the receptor tyrosine kinase MERTK.
      ,
      • Brenig R.
      • Pop O.T.
      • Triantafyllou E.
      • Geng A.
      • Singanayagam A.
      • Perez-Shibayama C.
      • et al.
      Expression of AXL receptor tyrosine kinase relates to monocyte dysfunction and severity of cirrhosis.
      or M-MDSCs.
      • Bernsmeier C.
      • Triantafyllou E.
      • Brenig R.
      • Lebosse F.J.
      • Singanayagam A.
      • Patel V.C.
      • et al.
      CD14+ CD15- HLA-DR- myeloid-derived suppressor cells impair antimicrobial responses in patients with acute-on-chronic liver failure.
      Among these candidates is the well-known drug metformin, which reduced AXL-expression and enhanced the cytokine responses of monocytes in vitro.
      • Brenig R.
      • Pop O.T.
      • Triantafyllou E.
      • Geng A.
      • Singanayagam A.
      • Perez-Shibayama C.
      • et al.
      Expression of AXL receptor tyrosine kinase relates to monocyte dysfunction and severity of cirrhosis.
      Importantly metformin has recently been reported to be safe in patients with cirrhosis.
      • Kaplan D.E.
      • Serper M.
      • John B.V.
      • Tessiatore K.M.
      • Lerer R.
      • Mehta R.
      • et al.
      Effects of metformin exposure on survival in a large national cohort of patients with diabetes and cirrhosis.
      The TLR3 agonist (polyI:C) has been shown to reduce the proportions of suppressive M-MDSCs and augment their antimicrobial function.
      • Bernsmeier C.
      • Triantafyllou E.
      • Brenig R.
      • Lebosse F.J.
      • Singanayagam A.
      • Patel V.C.
      • et al.
      CD14+ CD15- HLA-DR- myeloid-derived suppressor cells impair antimicrobial responses in patients with acute-on-chronic liver failure.
      In patients with ACLF, an inhibitor of glutamine synthetase partially restored the phagocytic and inflammatory capacity of monocytes in vitro and ex vivo.
      • Korf H.
      • du Plessis J.
      • van Pelt J.
      • De Groote S.
      • Cassiman D.
      • Verbeke L.
      • et al.
      Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
      Targeting PGE2 using cyclooxygenase inhibitors or albumin improved TNF-α production by monocytes in vitro.
      • O’Brien A.J.
      • Fullerton J.N.
      • Massey K.A.
      • Auld G.
      • Sewell G.
      • James S.
      • et al.
      Immunosuppression in acutely decompensated cirrhosis is mediated by prostaglandin E2.
      Further studies are required to identify potential further benefits and off-target effects of these therapies in decompensated cirrhosis (for an overview see Table 2).
      Table 2Potential therapeutic targets to prevent bacterial infections in decompensated cirrhosis.
      TargetSubstanceTherapeutic conceptReferences
      Bile acid FXR pathwayFXR agonistsIncreases bile acid synthesis, epithelial repair, tight junction formation. Increases goblet cell numbers and mucus thicknessSorribas et al. J Hepatol 2019
      • Sorribas M.
      • Jakob M.O.
      • Yilmaz B.
      • Li H.
      • Stutz D.
      • Noser Y.
      • et al.
      FXR modulates the gut-vascular barrier by regulating the entry sites for bacterial translocation in experimental cirrhosis.
      CCR2/CCR5CenicrivirocInhibition of CCR2/5 reduced hepatic recruitment of Ly-6C+ MoMF (murine models) and fibrosis (human NAFLD)Krenkel et al., Hepatology 2018
      • Krenkel O.
      • Puengel T.
      • Govaere O.
      • Abdallah A.T.
      • Mossanen J.C.
      • Kohlhepp M.
      • et al.
      Therapeutic inhibition of inflammatory monocyte recruitment reduces steatohepatitis and liver fibrosis.
      TAM receptors (AXL, MERTK)UNC569, BGB324Inhibition of MERTK (UNC569) restored LPS-induced pro-inflammatory response in ACLF ex vivo; inhibition of AXL (BGB324) restored pro-inflammatory cytokine responses in cirrhosis ex vivoBernsmeier et al., Gastro 2015;
      • Bernsmeier C.
      • Pop O.T.
      • Singanayagam A.
      • Triantafyllou E.
      • Patel V.C.
      • Weston C.J.
      • et al.
      Patients with acute-on-chronic liver failure have increased numbers of regulatory immune cells expressing the receptor tyrosine kinase MERTK.
      Brenig et al., Life Sci Alliance 2019
      • Brenig R.
      • Pop O.T.
      • Triantafyllou E.
      • Geng A.
      • Singanayagam A.
      • Perez-Shibayama C.
      • et al.
      Expression of AXL receptor tyrosine kinase relates to monocyte dysfunction and severity of cirrhosis.
      M-MDSCPolyI:CTLR-3 agonist poly(I:C) reduced proportions of immunesuppressive M-MDSC and augmented their antimicrobial function in vitroBernsmeier et al., Gut 2018
      • Bernsmeier C.
      • Triantafyllou E.
      • Brenig R.
      • Lebosse F.J.
      • Singanayagam A.
      • Patel V.C.
      • et al.
      CD14+ CD15- HLA-DR- myeloid-derived suppressor cells impair antimicrobial responses in patients with acute-on-chronic liver failure.
      AXLMetformineReduces AXL expression; increases cytokine productionBrenig et al., Life Sci Alliance 2019
      • Brenig R.
      • Pop O.T.
      • Triantafyllou E.
      • Geng A.
      • Singanayagam A.
      • Perez-Shibayama C.
      • et al.
      Expression of AXL receptor tyrosine kinase relates to monocyte dysfunction and severity of cirrhosis.
      Glutamine synthaseMethionine sulfoximineInhibition of glutamine synthase restored TNF-α production and phagocytosis in ACLF-plasma conditioned monocytes in vitroKorf et al., Gut 2019
      • Korf H.
      • du Plessis J.
      • van Pelt J.
      • De Groote S.
      • Cassiman D.
      • Verbeke L.
      • et al.
      Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
      PGE2

      Toll-like receptor signalling
      AlbuminProstaglandin E2 levels decreased in vivo and improved TNF-α production in conditioned MoMF;

      Albumin modulates endosomal TLR signalling
      O’Brien et al. Nature Med 2013;
      • O’Brien A.J.
      • Fullerton J.N.
      • Massey K.A.
      • Auld G.
      • Sewell G.
      • James S.
      • et al.
      Immunosuppression in acutely decompensated cirrhosis is mediated by prostaglandin E2.
      China et al., Clin Gastro Heptol 2019;
      • China L.
      • Maini A.
      • Skene S.S.
      • Shabir Z.
      • Sylvestre Y.
      • Colas R.A.
      • et al.
      Albumin counteracts immune-suppressive effects of lipid mediators in patients with advanced liver disease.


      Casulleras et al. Sci Trans Med 2020
      • Casulleras M.
      • Flores-Costa R.
      • Duran-Güell M.
      • Alcaraz-Quiles J.
      • Sanz S.
      • Titos E.
      • et al.
      Albumin internalizes and inhibits endosomal TLR signaling in leukocytes from patients with decompensated cirrhosis.
      Toll-like receptor: TLR 7/8CL097, R848Restore deficient degranulation, ROS production and bacterial killing by neutrophilsRolas et al.
      • Rolas L.
      • Boussif A.
      • Weiss E.
      • Lettéron P.
      • Haddad O.
      • El-Benna J.
      • et al.
      NADPH oxidase depletion in neutrophils from patients with cirrhosis and restoration via toll-like receptor 7/8 activation.
      Bone marrow derived stem cellsG-CSFMobilises CD34+ haematopoietic stem cells from the bone marrow, with the capacity to differentiate into multiple cell lineages and replace dysfunctional circulating innate immune cells (granulocytes, monocytes, dendritic cells)Garg et al., Gastro 2012;
      • Garg V.
      • Garg H.
      • Khan A.
      • Trehanpati N.
      • Kumar A.
      • Sharma B.C.
      • et al.
      Granulocyte colony-stimulating factor mobilizes CD34(+) cells and improves survival of patients with acute-on-chronic liver failure.
      Kedarisetty et al., Gastro 2015;
      • Kedarisetty C.K.
      • Anand L.
      • Bhardwaj A.
      • Bhadoria A.S.
      • Kumar G.
      • Vyas A.K.
      • et al.
      Combination of granulocyte colony-stimulating factor and erythropoietin improves outcomes of patients with decompensated cirrhosis.
      Verma et al. Hepatology 2018;
      • Verma N.
      • Kaur A.
      • Sharma R.
      • Bhalla A.
      • Sharma N.
      • De A.
      • et al.
      Outcomes after multiple courses of granulocyte colony-stimulating factor and growth hormone in decompensated cirrhosis: a randomized trial.
      Newsome et al., Lancet 2018
      • Newsome P.N.
      • Fox R.
      • King A.L.
      • Barton D.
      • Than N.-N.
      • Moore J.
      • et al.
      Granulocyte colony-stimulating factor and autologous CD133-positive stem-cell therapy in liver cirrhosis (REALISTIC): an open-label, randomised, controlled phase 2 trial.
      Stool microbial compositionFermented milk, vegetables, cereals, tea, coffeeExpansion of microbial diversityBajaj. Nat Rev Gastro Hepatol 2019
      • Bajaj J.S.
      Alcohol, liver disease and the gut microbiota.


      Albillos et al. JHep 2020
      • Albillos A.
      • de Gottardi A.
      • Rescigno M.
      The gut-liver axis in liver disease: pathophysiological basis for therapy.
      Stool microbial compositionDonor stoolRestitution of microbial diversity and reduction of enteropathogensBajaj et al. Gastro 2019
      • Bajaj J.S.
      • Fagan A.
      • Gavis E.A.
      • Kassam Z.
      • Sikaroodi M.
      • Gillevet P.M.
      Long-term outcomes of fecal microbiota transplantation in patients with cirrhosis.
      EnteropathogensBacteriophage therapySelective elimination of enteropathogens: proof of principle demonstrated in cystic fibrosisNg et al. 2021
      • Ng Renee R.N.
      • Tai Anna A.S.
      • Chang Barbara B.J.
      • Stick Stephen S.M.
      • Kicic Anthony A.
      Overcoming challenges to make bacteriophage therapy standard clinical treatment practice for cystic fibrosis.
      ACLF, acute-on-chronic liver failure; AXL, anexelekto; CCR, C-C motif chemokine receptor; FXR, farnesoid X receptor; G-CSF, granulocyte colony-stimulating factor; LPS, lipopolysaccharide; M-MDSCs, monocytic myeloid-derived suppressor cells; MERTK, Mer receptor tyrosine kinase; MoMF, monocyte-derived macrophage; NAFLD, non-alcoholic fatty liver disease; PGE2, prostaglandin E2; TLR, toll-like receptor; TNF, tumour necrosis factor.

      Targeting cytokines

      Cytokines are the grassroot effectors in the immunopathogenesis of decompensated cirrhosis (Table 3). Elevations in proinflammatory cytokines underlie immunopathology, whilst increased levels of immunosuppressive cytokines hinder antibacterial immunity.
      • Claria J.
      • Stauber R.E.
      • Coenraad M.J.
      • Moreau R.
      • Jalan R.
      • Pavesi M.
      • et al.
      Systemic inflammation in decompensated cirrhosis: characterization and role in acute-on-chronic liver failure.
      • Couper K.N.
      • Blount D.G.
      • Riley E.M.
      IL-10: the master regulator of immunity to infection.
      • Dirchwolf M.
      • Podhorzer A.
      • Marino M.
      • Shulman C.
      • Cartier M.
      • Zunino M.
      • et al.
      Immune dysfunction in cirrhosis: distinct cytokines phenotypes according to cirrhosis severity.
      • Fischer J.
      • Silva T.E.
      • Soares E.S.P.E.
      • Colombo B.S.
      • Silva M.C.
      • Wildner L.M.
      • et al.
      From stable disease to acute-on-chronic liver failure: circulating cytokines are related to prognosis in different stages of cirrhosis.
      Blockade of proinflammatory mediators has been proposed as a ‘silver-bullet’ to halt ongoing tissue damage and promote wound-healing processes in decompensated cirrhosis. Trials investigating TNF-α blockers, such as infliximab in alcoholic hepatitis, have shown clinical benefits but often at the cost of increasing the risk of infection,
      • Mookerjee R.P.
      • Sen S.
      • Davies N.A.
      • Hodges S.J.
      • Williams R.
      • Jalan R.
      Tumour necrosis factor alpha is an important mediator of portal and systemic haemodynamic derangements in alcoholic hepatitis.
      • Sharma P.
      • Kumar A.
      • Sharma B.C.
      • Sarin S.K.
      Infliximab monotherapy for severe alcoholic hepatitis and predictors of survival: an open label trial.
      • Spahr L.
      • Rubbia-Brandt L.
      • Frossard J.L.
      • Giostra E.
      • Rougemont A.L.
      • Pugin J.
      • et al.
      Combination of steroids with infliximab or placebo in severe alcoholic hepatitis: a randomized controlled pilot study.
      • Tilg H.
      • Jalan R.
      • Kaser A.
      • Davies N.A.
      • Offner F.A.
      • Hodges S.J.
      • et al.
      Anti-tumor necrosis factor-alpha monoclonal antibody therapy in severe alcoholic hepatitis.
      which probably reflects the significant antimicrobial and immunoregulatory functions of TNF-α.
      • Delgado M.E.
      • Brunner T.
      The many faces of tumor necrosis factor signaling in the intestinal epithelium.
      • Keane J.
      • Gershon S.
      • Wise R.P.
      • Mirabile-Levens E.
      • Kasznica J.
      • Schwieterman W.D.
      • et al.
      Tuberculosis associated with infliximab, a tumor necrosis factor alpha-neutralizing agent.
      • Zganiacz A.
      • Santosuosso M.
      • Wang J.
      • Yang T.
      • Chen L.
      • Anzulovic M.
      • et al.
      TNF-alpha is a critical negative regulator of type 1 immune activation during intracellular bacterial infection.
      Manipulation of the IL-1/IL-1R pathway has also garnered much interest. This family of 11 cytokines consists of both proinflammatory and protective/anti-inflammatory members that represent potentially crucial therapeutic targets.
      • Dinarello C.A.
      Overview of the IL-1 family in innate inflammation and acquired immunity.
      Results of trials assessing the clinical utility of recombinant IL-Ra (anakinra) or blockade of the IL-1/IL- receptor (IL-1R) pathway with a neutralising antibody directed against IL-1β (canakinumab) in alcoholic hepatitis are eagerly awaited (clinicaltrials.gov number: NCT01809132 and NCT03775109 respectively). However, the IL-1 network plays a pivotal role in host defence and its component cytokines are key inducers of antimicrobial-protein expression, so targeting this pathway may inadvertently increase the risk of bacterial infection.
      • Dinarello C.A.
      Overview of the IL-1 family in innate inflammation and acquired immunity.
      ,
      • Kolls J.K.
      • McCray Jr., P.B.
      • Chan Y.R.
      Cytokine-mediated regulation of antimicrobial proteins.
      In parallel, directly targeting antimicrobial cytokines, such as IL-17, may in fact promote detrimental tissue injury cascades.
      • Cooper A.M.
      IL-17 and anti-bacterial immunity: protection versus tissue damage.
      Cytokine therapy with effector molecules that have antimicrobial as well as hepatoprotective properties may reveal promising candidates, and IL-22 is an attractive option, due to its multifaceted anti-oxidant, anti-apoptotic and antimicrobial functions.
      • Pan C.X.
      • Tang J.
      • Wang X.Y.
      • Wu F.R.
      • Ge J.F.
      • Chen F.H.
      Role of interleukin-22 in liver diseases.
      Clinical trials of F-652, a recombinant fusion protein containing human IL-22, in patients with alcoholic hepatitis have been undertaken (clinicaltrials.gov number: NCT02655510); however, there are concerns regarding the anti-apoptotic nature of IL-22 and its active role in hepatocarcinogenesis, particularly in the context of cirrhosis.
      • Carmo R.F.
      • Cavalcanti M.S.M.
      • Moura P.
      Role of Interleukin-22 in chronic liver injury.
      ,
      • Jiang R.
      • Tan Z.
      • Deng L.
      • Chen Y.
      • Xia Y.
      • Gao Y.
      • et al.
      Interleukin-22 promotes human hepatocellular carcinoma by activation of STAT3.
      Overall, because of their multifunctional properties, selectively preventing the detrimental effects of targeting cytokines remains challenging. Upstream cellular effectors and/or immunoregulatory switches may be more favourable targets. It must also be stressed that most ongoing clinical trials investigating these molecules are focused on alcoholic hepatitis and not necessarily on other aetiologies of liver disease, nor on more advanced disease, e.g. ACLF and multi-organ failure.
      Table 3Potential future therapeutic targeting of cytokines in decompensated cirrhosis.
      TargetTherapeuticMechanism of actionActive clinical trials in liver disease (clinicaltrials.gov identifier)Potential adverse consequences of therapy in decompensated cirrhosisRef
      TNF-αTNF-α blockers.

      Infliximab adalimumab, certolizumab

      etanercept
      Dampen TNF- α mediated inflammation and related immunopathologyNo active trialsTNF-α has significant anti-microbial and immunoregulatory functions and blockade of these pathways increases risk of infection and liver injury
      • Dirchwolf M.
      • Podhorzer A.
      • Marino M.
      • Shulman C.
      • Cartier M.
      • Zunino M.
      • et al.
      Immune dysfunction in cirrhosis: distinct cytokines phenotypes according to cirrhosis severity.
      • Fischer J.
      • Silva T.E.
      • Soares E.S.P.E.
      • Colombo B.S.
      • Silva M.C.
      • Wildner L.M.
      • et al.
      From stable disease to acute-on-chronic liver failure: circulating cytokines are related to prognosis in different stages of cirrhosis.
      • Mookerjee R.P.
      • Sen S.
      • Davies N.A.
      • Hodges S.J.
      • Williams R.
      • Jalan R.
      Tumour necrosis factor alpha is an important mediator of portal and systemic haemodynamic derangements in alcoholic hepatitis.
      • Sharma P.
      • Kumar A.
      • Sharma B.C.
      • Sarin S.K.
      Infliximab monotherapy for severe alcoholic hepatitis and predictors of survival: an open label trial.
      • Spahr L.
      • Rubbia-Brandt L.
      • Frossard J.L.
      • Giostra E.
      • Rougemont A.L.
      • Pugin J.
      • et al.
      Combination of steroids with infliximab or placebo in severe alcoholic hepatitis: a randomized controlled pilot study.
      • Tilg H.
      • Jalan R.
      • Kaser A.
      • Davies N.A.
      • Offner F.A.
      • Hodges S.J.
      • et al.
      Anti-tumor necrosis factor-alpha monoclonal antibody therapy in severe alcoholic hepatitis.
      • Delgado M.E.
      • Brunner T.
      The many faces of tumor necrosis factor signaling in the intestinal epithelium.
      • Keane J.
      • Gershon S.
      • Wise R.P.
      • Mirabile-Levens E.
      • Kasznica J.
      • Schwieterman W.D.
      • et al.
      Tuberculosis associated with infliximab, a tumor necrosis factor alpha-neutralizing agent.
      • Zganiacz A.
      • Santosuosso M.
      • Wang J.
      • Yang T.
      • Chen L.
      • Anzulovic M.
      • et al.
      TNF-alpha is a critical negative regulator of type 1 immune activation during intracellular bacterial infection.
      IL-1RAIL-1 receptor antagonist.

      Anakinra
      Block action of IL-1α and IL-1β to prevent inflammation and assembly of inflammasome.
      • Anakinra (plus Zinc) or prednisone in patients with severe alcoholic hepatitis.
      • Phase 2 Multicentre, randomized, double blinded, placebo-controlled clinical trial.
      • Primary objective to determine clinical efficacy and safety.
      • NCT04072822
      The IL-1 network, however, plays a pivotal role in host defence and its component cytokines are key inducers of antimicrobial-protein expression, so targeting this pathway may inadvertently increase the risk of bacterial infection
      • Dinarello C.A.
      Overview of the IL-1 family in innate inflammation and acquired immunity.
      ,
      • Kolls J.K.
      • McCray Jr., P.B.
      • Chan Y.R.
      Cytokine-mediated regulation of antimicrobial proteins.
      IL-1BHuman monoclonal anti-IL-1β antibody.

      Canakinumab
      Neutralisation of IL-1β inflammatory activity by blocking its interaction with IL-1 receptors.
      • IL-1 signal inhibition in alcoholic hepatitis.
      • Phase II multicentre, double blind, randomized (1:1), placebo controlled trial.
      • Primary endpoint of the trial is histological improvement of alcoholic hepatitis on liver biopsy after 28 days of treatment compared to baseline.
      • NCT03775109
      IL-17Human monoclonal antibody that binds to the protein IL-17A.

      Secukinumab
      Selectively neutralizes IL-17A to reduce activation of profibrogenic pathways
      • A study of secukinumab treatment in patients with plaque psoriasis and coexisting non-alcoholic fatty liver disease (NAFLD)
      • Phase 3. Randomized, double-blind, multicenter, 24-week study.
      • Aims to assess therapeutic efficacy of secukinumab on the psoriatic skin and to explore the anti-inflammatory (reduction of hepatic inflammation and cell damage), anti-steatotic (reduction of hepatic triglyceride content) and anti-fibrotic (reduction of hepatic fibrosis) effects of secukinumab.
      • NCT04237116
      IL-17A plays an important protective role in the host immune response bacterial, fungal, parasitic, and viral infections. Blockade may increase susceptibility to infections in an immunocompromised cohort.
      • Cooper A.M.
      IL-17 and anti-bacterial immunity: protection versus tissue damage.
      IL-22Recombinant fusion protein containing human IL-22.

      F-652
      Anti-microbial, anti-inflammatory and hepatoprotective effects.
      • Recent interventional phase II trial.
      • An Open-Label, cohort dose escalation study to assess the safety and efficacy of F-652 in patients with alcoholic hepatitis
      • Study found F-652 to be safe and associated with a high rate of improvement as determined by Lille and MELD scores, reductions in markers of inflammation and increases in markers of hepatic regeneration
      • NCT02655510
      The anti-apoptotic nature of IL-22 and its active role in hepatocarcinogenesis is of concern in patients with cirrhosis.
      • Pan C.X.
      • Tang J.
      • Wang X.Y.
      • Wu F.R.
      • Ge J.F.
      • Chen F.H.
      Role of interleukin-22 in liver diseases.
      • Carmo R.F.
      • Cavalcanti M.S.M.
      • Moura P.
      Role of Interleukin-22 in chronic liver injury.
      • Jiang R.
      • Tan Z.
      • Deng L.
      • Chen Y.
      • Xia Y.
      • Gao Y.
      • et al.
      Interleukin-22 promotes human hepatocellular carcinoma by activation of STAT3.
      IL-, interleukin; TNF, tumour necrosis factor.

      Checkpoint inhibitors

      As already stated, decompensated cirrhosis represents an immunological paradox wherein a hyperinflammatory state and profound immunoparesis co-exist with an increased susceptibility to bacterial infection.
      • Bernardi M.
      • Moreau R.
      • Angeli P.
      • Schnabl B.
      • Arroyo V.
      Mechanisms of decompensation and organ failure in cirrhosis: from peripheral arterial vasodilation to systemic inflammation hypothesis.
      Immune effector cells are primed and activated, but antibacterial effector functions are switched off, reflecting a disrupted balance between protective anti-pathogen immunity and host-induced immunopathology. Preservation of this homeostatic equilibrium is maintained through multifaceted immunoregulatory networks, of which checkpoint receptors are a major component.
      • Robinson M.W.
      • Harmon C.
      • O’Farrelly C.
      Liver immunology and its role in inflammation and homeostasis.
      ,
      • Chen L.
      • Flies D.B.
      Molecular mechanisms of T cell co-stimulation and co-inhibition.
      Checkpoint receptors constitute a complex array of receptors that act, with their ligands, to suppress or activate key signal transduction pathways to modulate effector cell functions and fine-tune the magnitude, spread and breadth of the immune response.
      • Riva A.
      • Chokshi S.
      Immune checkpoint receptors: homeostatic regulators of immunity.
      Best known for their involvement in suppressing antitumour immunity, blockade with nivolumab (anti-PD-1) has obtained FDA approval in multiple cancers including hepatocellular carcinoma.
      • Hargadon K.M.
      • Johnson C.E.
      • Williams C.J.
      Immune checkpoint blockade therapy for cancer: an overview of FDA-approved immune checkpoint inhibitors.
      ,
      • Xie Y.
      • Xiang Y.
      • Sheng J.
      • Zhang D.
      • Yao X.
      • Yang Y.
      • et al.
      Immunotherapy for hepatocellular carcinoma: current advances and future expectations.
      Checkpoint receptors are involved in the immunopathogenesis of liver diseases; in alcoholic hepatitis, the hyper-expression of membrane-bound PD-1 and TIM3 on antibacterial T cells impairs their functionality.
      • Rajkovic I.A.
      • Williams R.
      Abnormalities of neutrophil phagocytosis, intracellular killing and metabolic activity in alcoholic cirrhosis and hepatitis.
      ,
      • Cooksley H.
      • Riva A.
      • Katzarov K.
      • Hadzhiolova-Lebeau T.
      • Pavlova S.
      • Simonova M.
      • et al.
      Differential expression of immune inhibitory checkpoint signatures on antiviral and inflammatory T cell populations in chronic hepatitis B.
      ,
      • Evans A.
      • Riva A.
      • Cooksley H.
      • Phillips S.
      • Puranik S.
      • Nathwani A.
      • et al.
      Programmed death 1 expression during antiviral treatment of chronic hepatitis B: impact of hepatitis B e-antigen seroconversion.
      Moreover, ex vivo blockade using neutralising antibodies leads to reconstitution of bacteria-specific innate and adaptive immunity, without exacerbating the production of cytokines associated with systemic inflammation.
      • Rajkovic I.A.
      • Williams R.
      Abnormalities of neutrophil phagocytosis, intracellular killing and metabolic activity in alcoholic cirrhosis and hepatitis.
      ,
      • Bataller R.
      • Mandrekar P.
      Identifying molecular targets to improve immune function in alcoholic hepatitis.
      Novel therapies should aim to restore the equilibrium between protective anti-pathogen immunity and host-induced immunopathology. Molecular switches that regulate activation, shut-down and each facet of immune function and fate are promising candidates.
      Pre-clinical and clinical studies have also described a role for checkpoint receptors including PD-1 and TIM3 in sepsis and septic shock, wherein increased immune cell expression has been associated with a higher rate of nosocomial infections and mortality.
      • Patil N.
      • Guo Y.
      • Luan L.
      • Sherwood E.
      Targeting immune cell checkpoints during sepsis.
      ,
      • Guignant C.
      • Lepape A.
      • Huang X.
      • Kherouf H.
      • Denis L.
      • Poitevin F.
      • et al.
      Programmed death-1 levels correlate with increased mortality, nosocomial infection and immune dysfunctions in septic shock patients.
      Moreover, blockade of PD-1 or its ligand PD-L1 with neutralising antibodies can reverse immune dysfunction and improve survival in experimental models of sepsis.
      • Guignant C.
      • Lepape A.
      • Huang X.
      • Kherouf H.
      • Denis L.
      • Poitevin F.
      • et al.
      Programmed death-1 levels correlate with increased mortality, nosocomial infection and immune dysfunctions in septic shock patients.