If you don't remember your password, you can reset it by entering your email address and clicking the Reset Password button. You will then receive an email that contains a secure link for resetting your password
If the address matches a valid account an email will be sent to __email__ with instructions for resetting your password
Corresponding author. Address: Department of Gastroenterology and Hepatology and Laboratory of Hepatology, 49 Here street, University hospital Leuven, 3000 Leuven, Belgium.
Institute of Hepatology, Foundation for Liver Research, London, UKDivision of Transplantation, Immunology and Mucosal Biology, Faculty of Life Sciences and Medicine, King’s College, London, United Kingdom
Department of Gastroenterology and Hepatology, Hospital Universitario Ramón y Cajal, Instituto Ramón y Cajal de Investigación Sanitaria (IRYCIS), CIBEREHD, Universidad de Alcalá, Madrid, Spain
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.
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.
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.
Prevalence and in-hospital mortality trends of infections among patients with cirrhosis: a nationwide study of hospitalised patients in the United States.
Bacterial infections set in motion a vicious cycle of recurring infections and progressive hepatic decompensation, culminating in a mortality rate exceeding 60% at 1 year in the absence of liver transplantation.
Second infections independently increase mortality in hospitalized patients with cirrhosis: the North American consortium for the study of end-stage liver disease (NACSELD) experience.
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.
Second infections independently increase mortality in hospitalized patients with cirrhosis: the North American consortium for the study of end-stage liver disease (NACSELD) experience.
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.
Second infections independently increase mortality in hospitalized patients with cirrhosis: the North American consortium for the study of end-stage liver disease (NACSELD) experience.
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.
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.
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.
The main predictors of MDR infections include previous nosocomial infection, long-term norfloxacin prophylaxis, recent hospital admission and ICU care.
Second infections independently increase mortality in hospitalized patients with cirrhosis: the North American consortium for the study of end-stage liver disease (NACSELD) experience.
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.
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.
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.
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.
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.
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.
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.
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
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.
Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
Leukocyte activation in the peripheral blood of patients with cirrhosis of the liver and SIRS. Correlation with serum interleukin-6 levels and organ dysfunction.
is impaired, the migration of certain immunoregulatory monocytes (e.g. CD16+ and human leucocyte antigen [HLA]-DR+Mer receptor tyrosine kinase [MERTK]+ cells) occurs unhindered. This allows these cells to cross repeatedly in and out of inflamed tissue sites.
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.
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.
However chronic exposure to lipopolysaccharide (LPS) and other PAMPs, which may be abundant in the portal and systemic circulation and tissue compartments
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.
Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
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.
However, their role in development of immunoparesis requires further clarification (Fig. 2).
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.
Pro- and anti-inflammatory cytokine levels are elevated in the circulation and certain tissue compartments, such as the peritoneum, in decompensated cirrhosis.
Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
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.
Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
Attenuated antigen-specific T cell responses in cirrhosis are accompanied by elevated serum interleukin-10 levels and down-regulation of HLA-DR on monocytes.
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.
Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
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.
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.
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).
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.
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
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.
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.
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.
Hypochlorhydria present in cirrhosis, even in the absence of proton pump inhibition, is another factor that contributes to altered microbial composition.
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.
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.
Specifically, gut microbiome profiles differed between in- and outpatients, whether patients were systemically infected or not, and in patients with organ failure that died.
These changes mirror the severity of disease and can be observed in stool, sigmoid colon mucosal biopsies, saliva and serum of patients with cirrhosis.
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.
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.
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.
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.
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.
A recent study indicated that subclinical intestinal inflammation is driven by modified microbial composition and progressive barrier dysfunction in advanced 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.
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.
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.
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.
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.
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.
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.
Further, faecal transplantation has been shown to improve cognitive function and reduce hepatic encephalopathy-associated hospital admission in patients with decompensated 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.
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.
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.
However more recent evidence has indicated that the lymphatic route of translocation coexists with portal-venous passage of bacteria and bacterial products.
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.
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,
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.
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
Characterization of human peritoneal monocyte/macrophage subsets in homeostasis: phenotype, GATA6, phagocytic/oxidative activities and cytokines expression.
(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.
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.
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.
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.
Peritoneal level of CD206 associates with mortality and an inflammatory macrophage phenotype in patients with decompensated cirrhosis and spontaneous bacterial peritonitis.
Characterization of human peritoneal monocyte/macrophage subsets in homeostasis: phenotype, GATA6, phagocytic/oxidative activities and cytokines expression.
Bacterial DNA activates cell mediated immune response and nitric oxide overproduction in peritoneal macrophages from patients with cirrhosis and ascites.
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.
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.
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.
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.
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.
Nosocomial infection- Infection diagnosed after more than 48 hours of hospitalization.
Presence of sepsis/septic shock
SBP and bacteremia
Gram 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 infection
Gram 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
Pneumonia
Gram positive:
-
S. pneumonia
-
H. influenza
-
S. aureus
Gram negative:
-
K. pneumonia
-
E. coli
-
P. aeruginosa
Piperacillin/tazobactam or Ceftriaxone + macrolide or Levofloxacin or moxifloxacin
Low 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 infections
Gram positive:
-
S. aureus
-
Streptococcus sp.
Gram negative:
-
E. coli
-
K. pneunomiae
-
P. aeruginosa
Piperacillin/tazobactam or 3rd generation cephalosporin + penicillinase resistant penicillin
Meropenem 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)
Second infections independently increase mortality in hospitalized patients with cirrhosis: the North American consortium for the study of end-stage liver disease (NACSELD) experience.
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.
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.
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.
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.
In patients with ACLF, an inhibitor of glutamine synthetase partially restored the phagocytic and inflammatory capacity of monocytes in vitro and ex vivo.
Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
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.
Target
Substance
Therapeutic concept
References
Bile acid FXR pathway
FXR agonists
Increases bile acid synthesis, epithelial repair, tight junction formation. Increases goblet cell numbers and mucus thickness
Inhibition 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 vivo
Inhibition of glutamine synthetase in monocytes from patients with acute-on-chronic liver failure resuscitates their antibacterial and inflammatory capacity.
Mobilises 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)
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.
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,
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.
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.
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.
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.
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.
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
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.
Recombinant 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.
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.
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.
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.
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.
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.
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.
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.
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.
33816-2&id=gr1.jpg){kind=link}
33816-2&id=gr2.jpg){kind=link}
33816-2&id=gr3.jpg){kind=link}
33816-2&id=gr4.jpg){kind=link}