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Review

Current Advances of Innate and Adaptive Immunity in Acute-on-Chronic Hepatitis B Liver Failure

Wang, Liyuan1; Fan, Yuchen1,2,∗

Editor(s): Zhao, Wei

Author Information
Infectious Diseases & Immunity: April 2022 - Volume 2 - Issue 2 - p 113-121
doi: 10.1097/ID9.0000000000000051
  • Open

Abstract

Introduction

The definition of acute-on-chronic liver failure (ACLF) includes patients with chronic liver disease (CLD) or cirrhosis who develop an acute decompensation of liver function.[1] The definitions of ACLF differ from one another. The Asian definition, proposed by the Asian Pacific Association for the Study of the Liver, refers to a rapidly progressive hepatic insult with high short-term mortality, manifested as jaundice and coagulopathy and complicated within 4weeks by clinical ascites and/or encephalopathy. The definition applies to patients with previously diagnosed or undiagnosed chronic liver disease/ cirrhosis, but not decompensation cirrhosis.[2] However, European Association for the Study of the Liver–Chronic Liver Failure (EASL-CLIF) characterize ACLF as a syndrome which develops in patients with acutely decompensated cirrhosis, with or without prior decompensation, associated with different combinations of organ failures among the six major organ systems (liver, kidney, brain, coagulation, circulation, and respiration).[3] After all, ACLF is characterized by an intra- or extra-hepatic precipitating event, the existence of organ failure(s) and high risks of death.[4] ACLF might be frequent, affecting 30% to 40% of patients hospitalized for an acute decompensation of cirrhosis.[3] Intense systemic inflammation is a major feature of this syndrome.[4]

Both hepatic insults and extrahepatic insults could be the precipitating event leading to ACLF, as well as unrecognized precipitating events.[4] Extrahepatic precipitating events include bacterial infection, surgery and upper gastrointestinal bleeding. Hepatic insults consist of flare-up or exacerbation of HBV, superimposed HAV or HEV infection, active drinking, hepatotoxic drugs and so on. Hepatitis B infection is a common underlying CLD of ACLF in Asia.[2] Flare-up or exacerbation of HBV is a dominant precipitating event for ACLF, accounting for 35.8%.[5] HBV flare-up or exacerbation could be spontaneous, but is more commonly attributable to immunosuppression, nucleos(t)ide analog resistance, or interruption of antiviral therapy.[5] Host and viral factors are both the potential triggers of Acute-on-chronic Hepatitis B liver failure (ACHBLF),[6] and a dysfunctional immune response is the key event in the pathogenesis of ACLF.[7]

Although the comprehension of immuno-pathogenesis in ACHBLF has increased, the exact roles of immune cells underlying disease development are not yet fully understood. Herein, we summarized the roles of innate and adaptive immune cells in ACHBLF patients.

Innate and adaptive immune response in chronic HBV infection

The consequence of chronic HBV infection is determined by a complex interaction between a deregulated immune response and the replicating virus.[8] At the early phase of virus infection, HBV is a “stealth virus” replicating itself in infected hepatocytes and is not detected by the innate immunity, attributing to its replication strategy.[9] HBV persists in the nucleus of infected cells as a covalently closed circular DNA (cccDNA), which is used as the transcriptional template for viral RNAs, and it might not be perceived by the innate DNA sensing cellular machinery. In addition, viral mRNA derived from cccDNA mimical the normal cellular transcripts. What's more, newly produced viral progenies are safeguarded within viral capsid in the cytoplasm.[10]

However, HBV can actually be sensed by the innate immunity, but require exposure to high HBV titers. High HBV titers exposure could activate macrophages with increased expressions of inflammatory cytokines and chemokines.[11] Natural killer (NK) cells activation has been discovered in patients who are at the phase of having detectable HBV DNA and HBsAg. Activated NK cells show enhanced expression of activation markers, such as CD69 and natural killer group 2D (NKG2D), cytotoxic activity and interferon-gamma (IFN-γ) secretion.[12] Similar kinetics of intrahepatic NK cell responses is seen in the woodchuck model of acute HBV infection 4 to 5weeks post Woodchuck hepatitis virus (WHV) infection.[13] Numerous innate immune cells play not only a protective but also a pathogenic role in the process of chronic HBV infection.

Activation of the innate immune pathways results in the recruitment and activation of numerous adaptive immune cells, which is crucial for HBV clearance.[14] CD4+ T cells maintain cognate HBV-specific CD8+ T cells and contribute to viral clearance. It is proposed that HBV-specific CD8+ T cells could clear HBV by both noncytolytic and cytolytic effector function. In chronic HBV infection, immune exhaustion occurs after persistent exposure of viral antigens and it is associated with the succeeding inefficiency of host response.[15] A strong HBV-specific T cell response is discovered in acute HBV infection patients experimentally infected chimpanzees, inversely, the T cell response is weak and narrowly concerning the chronic infection condition.[16] AntibodyspecificforHBsAg (anti-HBs)is produced by specific B cells and is associated with virus control. However, anti-HBs production is defective in patients with chronic Hepatitis B infection.[17] Intrahepatic inflammatory reactions induce multiple suppressive pathways, leading to recruitment of regulatory cells and T cell function suppression.[18] The suppressive events protect the liver from severe damage primed by inflammation, while result in viral persistence.[14] So, a multifaceted interplay of both innate and adaptive immune cells causes immuno-pathogenesis of chronic hepatitis B (CHB).

Dysregulated immune responses in ACHBLF

Systemic inflammation is considered as a central driver of ACHBLF.[19] Exogenous bacteria, endogenous damaged or dying liver cells and extracellular matrix breakdown products could all induce systemic inflammation. The pathogen associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) are recognized by pattern recognition receptors (PRRs), such as NOD-like receptors (NLRs) and Toll-like receptors (TLRs) in innate immune cells, and result in numerous productions of pro-inflammatory as well as anti-inflammatory cytokines.[7,20] Early systemic inflammation can result from the release of DAMPs and PAMPs. As ACLF progresses, compensatory anti-inflammatory response (CARS) develops to overcome systemic inflammatory response syndrome (SIRS).[4] CARS is supposed to favor the development of bacterial infections and organ failure at the late stage of ACLF.[21] Hence, it is necessary to identify the immune phase of ACLF patients, which will facilitate the comprehension of immune cells function. After all, immune imbalance plays a critical role in the onset and progression of ACHBLF [Figure 1]. The current review will discuss new concepts of the inefficient innate and adaptive immune cells in ACHBLF.

F1
Figure 1:
The dysfunctional immune response in acute-on-chronic hepatitis B liver failure is featured by systemic inflammatory response and immune cell paralysis. The recognition of PAMPs and DAMPs by pattern-recognition receptors, such as TLRs and NLRs, of the innate immune system results in the induction of the systemic inflammatory response. Activated KCs in hepatic sinusoids release pro-inflammatory cytokines, ROS, and chemokines. With the help of chemokine receptors CXCR1/CXCR2, neutrophils infiltrate in the liver and produce abundant inflammatory mediators leading to liver injury and cell death. Circulating neutrophils are dysfunctional with impaired phagocytic activity. Decreased HLA-DR expression, escalated MERTK expression, and elevated IL-10 produce in monocytes impairs their antigen presenting role and phagocytic capacity. pDCs and mDCs extensively infiltrate in the liver and produce abundant IFN-α. MDSCs are markedly expanded and display immunosuppressive properties by declining T cell proliferation. T cell function is also impaired by the increased expressions of immune inhibitory receptors, lower production of IFN-γ, and increased production of IL10. CD8+ T cells exhibit exhausted phenotype with decreased perforin and granzyme B. TIPE2 can down-regulate TCR and TLR4 signaling. B cells show enhanced secretion of germline IgG and IgM in the liver. Blue and red represent up-regulated and down-regulated immunity processes, respectively. CARS: Compensatory anti-inflammatory response; CTLA-4: Cytotoxic T-lymphocyte-associated protein 4; DAMPs: Damage-associated molecular patterns; HLA: Human leukocyte antigen; mDCs: Myeloid dendritic cells; MDSCs: Myeloid-derived suppressor cells; MERTK: Tyrosine-protein kinase MER; NLRs: NOD-like receptors; PAMPs: Pathogen-associated molecular patterns; PD-1: Programmed cell death protein 1; pDCs: Plasmacytoid dendritic cells; ROS: Reactive oxygen species; SIRS: Systemic inflammatory response syndrome; Tim-3: T cell immunoglobulin and mucin domain molecule 3; TLRs: Toll-like receptors.

Innate immune cells in ACHBLF

Neutrophils

Neutrophils are constituted by a heterogeneous population that display antimicrobial functions and contribute to tissue damage and immune regulation.[22] The normal function of neutrophils is critical to keep liver homeostasis.[22] The release of cellular contents from necrotic hepatocytes into the systemic circulation could initiate the recruitment of neutrophils into the liver. Intrahepatic accumulation of neutrophils has been found in acetaminophen (APAP)-induced liver injury mouse model.[23] Moreover, depletion of neutrophils chemokine CXC or chemokine receptor 2 (CXCR2)-deficient protects against APAP toxicity, which provides evidence for neutrophils’ pathogenic role in APAP-induced liver injury.[24] Peripheral neutrophils of APAP-induced liver injury mice show an increasing activation status (CD11b expression and reactive oxygen species priming). However, neutrophil phagocytic activity (NPA) has been found significantly impaired in circulating neutrophils of subacute liver failure patients.[25] Upregulated TLR9 but down-regulated TLR4 expression on functionally exhausted neutrophils has been detected in APAP-related acute liver failure patients, and it correlates with the degree of encephalopathy.[26] These studies indicate complex roles of neutrophils in liver failure.

A study of clinical blood count measurements has found leukocytosis, neutrophilia, and lymphopenia in ACLF patients.[27] Further analysis of neutrophil RNA expression using microarray analysis reveals a distinct neutrophil phenotype, including upregulation of granule and glycolysis genes, reduction of cell-cycle and cell-migration genes, and down-regulation of the antimicrobial superoxide anion.[27] For patients with ACHBLF, circulating leucocyte counts are also reported higher than those in the No-ACLF patients.[19] The neutrophil-to-lymphocyte ratio (NLR) is associated with risk of death in ACHBLF patients.[19] NLR has been applied as an important index to predict mortality in ACHBLF patients.[28] A retrospective study with a validation cohort has demonstrated elevated NLR could predict mortality of ACHBLF within 28days as well as 90days. The authors suggest that patients who have NLR< 3.1 are at greater risk of lower mortality; on the contrary, those with NLR>4.78 have high mortality. A retrospective cohort study also has suggested that NLR was correlated to mortality in ACLF patients who were admitted to the intensive care unit.[29] High NLR has also been reported as a critical index to predict mortality in ACHBLF patients treated with artificial liver support system and ACLF patients after liver transplantation.[30]

CXC chemokines are produced by hepatic cells and immune cells, and recruit neutrophil to the liver. The process is mediated by chemokine receptors CXCR1 and CXCR2. The alterations of CXCR1 and CXCR2 expression on neutrophils of ACHBLF patients is controversial. A study reported low expression of CXCR1 and CXCR2 on neutrophils in peripheral blood of ACHBLF patients, compared with healthy subjects and CHB patients. The low expression pattern was associated with disease severity of ACHBLF.[31] However, another study found neutrophils with high expression of CXCR1/CXCR2 receptors in ACHBLF patients.[32] Coculture assays in vitro have demonstrated neutrophils mediate hepatocyte death in the manner of apoptosis and necrosis via contact-dependent and -independent mechanisms. In addition, in vitro blockade of CXCR1/CXCR2 significantly reduce cell death.[32] Therefore, functional and phenotypic alterations of neutrophils are generally considered to be related to the outcomes of ACHBLF.

Monocytes and macrophages/Kupffer cells (KCs)

Monocytes originate from the bone marrow hematological myeloid precursors. Peripheral blood monocytes enter tissues after circulation for several days.[4] Monocytes could differentiate into macrophages and their functions involve phagocytosis, antigen presentation, inflammatory cytokine production, and recruitment and activation of immune effector cells.[33] Monocytes traffic through the sinusoids into the liver.[34] Some migrating monocytes remain as monocytes intrahepatic, acquiring antigen-presenting capability, or differentiate into macrophages.[34,35] Macrophages were thought to polarize into classically activated pro-inflammatory macrophage (M1) or alternatively activated anti-inflammatory/wound-healing macrophage (M2). However, M1/M2 polarization is revealed not adequate to describe the spectrum of macrophage activation states recently.[36] Intrasinusoidal KCs are the specialized population of macrophages in the liver which remove intrahepatic noxious substances.[33] Monocytes and macrophages/ KCs dysfunction contribute to disease progression of ACLF.

In ACLF, various DAMPs and PAMPs can activate KCs and initiate liver inflammation.[37] Gut bacteria and PAMPs translocate to the liver through the disrupted gut-blood barrier and activate liver-resident KCs, mediated through PRRs, such as the TLRs. The result of KCs activation is production of numerous cytokines which conduces to hepatic and systemic inflammation.[1] Moreover, macrophages are activated by high HBV DNA and secret increased inflammatory cytokine tumor necrosis factor-alpha (TNF-α) and interleukin (IL)-6.[11] Similar observations have been reported by experiments of HBV infected chimpanzees models.[24] What's more, TNF-α and IL-6 play critical roles in liver repair as well as regeneration. KCs and infiltrating monocyte-derived macrophages are the principal source of TNF-α and IL-6. Their absence resulted in a marked delay in liver repair in acute liver injury.[38]

Activated KCs act as recruiter of effector cells to the impaired liver, which produce reactive oxygen species (ROS), chemokines and pro-inflammatory cytokines, recruit bone-marrow derived cells, such as monocytes and neutrophils, into the liver, and amplify the pro-inflammatory signal.[39] Bone marrow derived monocytes might infiltrate into the liver and evolve into monocyte-derived macrophages (MoMFs), which expand macrophage pool of the liver.[40] The CCL2/CCR2 axis contributes to their recruitment to the liver. Some studies have found that monocytes and MoMFs expand liver inflammation in liver failure. In mice treated with overdose APAP, CCR2+monocytes massively accumulate in injured liver during the early phase of acute liver failure (ALF).[41] Ccr2-deficient (Ccr2–/–) mice display attenuated liver injury. The study used a CCR2 antagonist to inhibit monocyte infiltration pharmacologically and found reduced liver injury and liver cell necrosis following injury. Furthermore, a pro-inflammatory phenotype has been observed in the newly-infiltrated MoMFs, which produce massive proinflammatory cytokines including TNF-α and IL-1β, and less anti-inflammatory cytokine IL-10.[41] Another study using mouse models of CCl4 injury also suggests the important role of monocytes in the induction of early acute liver injury.[42] In contrast, intrahepatic macrophages have been considered as promoters of repair in other studies. Enhanced liver inflammation and impaired tissue regeneration have been found in Ccr2-/- mice model after APAP treatment.[43] A possible speculation is that the function of monocytes and macrocytes depends on the phase of ACLF. In propagation phase of liver failure, intrahepatic recruited monocytes differentiate into macrophages which may promote local tissue injury and provoke SIRS. In the later phages, macrophages reprogram toward a phenotype, favoring resolution responses in a drive to tissue recovery.[33]

Intrahepatic immune cells secret soluble mediators and may affect circulating monocytes. Peripheral monocytosis is a character of ACHBLF patients,[19] however, monocytes in peripheral blood show acquired insufficiency in liver failure.[33] Soluble CD163 (sCD163) shed from activated macrophages. The level of sCD163 increases stepwise with rising grades of ACLF in EASL-CLIF ACLF patients.[44] sCD163 is independently correlated with short as well as long-term mortality and predicts mortality with high predictive accuracy.[44] In ACHBLF patients, circulating monocyte counts increase and the ratio between CD3+ T cells and monocytes (T/M) decrease.[45] T/M ratio has been found negatively correlated with the disease severity, which is scaled by Model for End-Stage Liver Disease (MELD) Score. Depressed T/M ratio is associated with a poor outcome. Dysregulated function of monocytes has been reported in different phage of ACLF patients. Huang et al. define more than grade II hepatic encephalopathy, severe hemorrhage, and/or severe ascites as late-stage ACLF. A stepwise decrease of human leukocyte antigen (HLA)-DR expression was observed within rising stage of ACLF. Low HLA-DR expression is correlated with risk of death of ACLF patients.[46] DAMP molecular IL-33 restores HLA-DR expression on monocytes of ACHBLF patients and promotes monocytic inflammation.[47] Monocyte dysfunction in EASL-CLIF ACLF patients is proposed due to increased expression of MER receptor tyrosine kinase (MERTK). MERTK inhibitors restore production of inflammatory cytokines by monocytes and peritoneal macrophages.[48]

Circulating monocytes dysfunction impairs their anti-infection program and leads to a high risk of sepsis.[33] Monocytes feature increased ratio of IL-10-producing cells, decreased HLA-DR expression and inefficient oxidative burst and phagocytic capacity in ACLF patients. RNA sequencing on monocytes revealed upregulated expression of immunosuppressive genes but compromised expression of antibacterial and antigen presentation genes. In vitro, metabolic rewiring of monocytes using glutamine synthetase as a pharmacological inhibitor could restore their inflammatory and phagocytic capacity.[49]

NK cells

NK cells are innate cytotoxic effector lymphocytes against intracellular pathogens and abnormal cells. They make up one-third of the lymphocytes in the liver, and represent around 15% of lymphocytes in the circulating blood.[50] Studies regarding the number and functional status of NK cells during ACLF have revealed, in part, conflicting results.[51–56] These inconsistencies may be attributed to the complexity and insolubility of NK cell phenotypic characteristics, especially under different stages of ACLF.

Decreased peripheral numbers of cytotoxic CD56dim CD16bright NK cells have been reported in ACHBLF patients. The activating receptors NKG2D, NKp30, NKp44, NKp46 and the inhibitory receptor CD158a increased, though another inhibitory receptor CD158b decreased in ACHBLF patients. Moreover, the cytotoxic function and killing activity of NK cells are both downregulated.[53] In the contrast, another study observed higher frequency of CD3-CD56+NK cells in ACHBLF patients. The ratio of NKG2A (an inhibitory receptor) to NKG2D (a stimulatory receptor) on NK cells is upregulatedin the ACLF patients.[52] Peripheral NKG2D+ NK cells frequencies have also been found higher in ACHBLF patients. When they are co-cultured with HepG2 HBV-replicating liver cells line, more TNF-α, IFN-γ, granzyme B and perforin are secreted into supernatant. These are triggered by NKG2D, which is massively increased in the peripheral blood and liver of ACHBLF patients.[51] NK cell mediated cytotoxicity is promoted through tumor necrosis factor-related apoptosis inducing ligand (TRAIL) pathway too.[56]

The number of intrahepatic NK cells increase in a fulminant hepatic failure (FHF) mouse model as well as ACHBLF patients. After infection of murine hepatitis virus strain 3 (MHV-3), NK cells number increase markedly, peak at 48 hours post-infection, and remain at a high level. Upregulated level of CD69, elevated production of intracellular TNF-α and IFN-γ, as well as increased cytotoxic activity have all been observed in intrahepatic NK cells at 48 hours post-infection. Increased cytotoxic activity is mediated by NKG2D/NKG2D ligand and Fas/Fas ligand.[55] An experiment in vitro demonstrates highly expressed KCTD9 in peripheral circulating and liver NK cells of ACHBLF patients, which is hypothesized be involved in NK cell activation.[54]

Dendritic cells (DCs)

DCs are versatile and professional antigen-presenting cells. Circulating DCs are divided into two subsets, myeloid and plasmacytoid DCs (mDCs and pDCs), according to phenotypic and functional characterizations. Currently, pDCs are proven to mediate responses by their cytokine secretion, but not antigen presentation.[57] Several studies illustrate their role in ACLF. Zhang et al. first reported both pDCs and mDCs massively infiltrated in the liver and expressed mature phenotypes in ACHBLF patients. The intrahepatic mature pDCs secrete abundant IFN-α. Meanwhile, a dramatically lower ratio of peripheral pDCs with reduced IFN-α production, as well as a decreased numbers of mDC with lower function have been observed in the ACHBLF patients, which are associated with ACHBLF prognosis.[58] Khanam et al. also reported decreased DCs in non-survivors of ACLF.[59] Notably, improved prognosis is correlated with dendritic cell restoration in ACLF patients who receive methylprednisolone or granulocyte colony-stimulating factor (G-CSF) therapy.[59]

Purified monocytes from peripheral blood are induced into monocyte-derived dendritic cells (MoDCs) in vitro. A study has discovered upregulated IL-23 and IL-23R levels in MoDCs from ACHBLF patients compared to CHB patients. The level of IL-23 expression is associated with disease severity markers of ACHBLF patients and IL-23 levels are significantly upregulated in non-survivors.[60] TLR3/IFN-β signaling also contributes to MoDC impairment of ACHBLF patients. Decreased levels of TLR3 and IFN-β expression are observed in MoDCs, which is significantly lower in non-survivors.[61]

Mucosal-associated invariant T (MAIT) cells

MAIT cells are a newly-discovered population of innate-like lymphocytes, who play a role in antimicrobial and immune regulatory.[62] MAIT cells participate in liver disease, including ACHBLF. Circulating MAIT cells are significantly decreased in ACHBLF patients. ACHBLF patients are divided into early stage and middle/late stage according to the level of PTA. The proportion and number of MAIT cells are lower in patients with middle/late-stage liver failure, in contrast to those with early-stage liver failure. The same downtrend is observed in the dead/liver transplantation group compared with the survival group and the decreased circulating MAIT cells predict poor overall survival.[63] However, the precise role and mechanism of MAIT cells in ACLF need extended investigations.

Myeloid-derived suppressor cells (MDSCs)

MDSCs are a heterogeneous subset of immature myeloid cells, that function as negative regulators of immune responses. MDSCs perform this inhibitory effect through various mechanisms, including suppression of T cells proliferation, antibody and IFN-γ production, and induction of regulatory T (Treg) cell populations. MDSCs are important in the development of tumors and various autoimmune disease.[64] Circulating CD14+CD15-CD11b+HLA-DR- M-MDSCs are dramatically increased in ACLF patients. These M-MDSCs down-regulate immune response by reducing T cell proliferation and declining the production of IL-6 and TNF-α in answer to TLR activation. The increase of M-MDSCs recede antimicrobial responses and are associated with incidence of infectious complications and disease severity in patients with ACLF.[65] Arginase production by MDSC depletes arginine, downregulates the expression of the TCR ζ chain (CD3 ζ chain) and impairs T-cell signal transduction and function.[66] Moreover, Hepatitis B surface Ag (HBsAg) efficiently promotes polarization of monocytes into M-MDSCs in vitro. ERK/IL-6/STAT3 signaling feedback and mediate differentiation of M-MDSCs, which suppresses T cell activation. Importantly, an MDSC-targeted drug restores HBV-specific CD8+ and CD4+ T cell responses in peripheral blood mononuclear cells (PBMCs) from CHB patients and prevents increase of viral load in mouse model.[67] A recent study has reported increased peripheral CD14+CD33+CD11b+HLA-DR-/low MDSCs in ACHBLF patients. The percentage of MDSCs is closely associated with liver injury and short-term prognosis of ACHBLF.[66]

Adaptive immune cells in ACHBLF

T cells

Besides innate immune response dysregulation, defective adaptive immune system is also discovered in ACHBLF. Reduced numbers of CD4+ T cells and CD8+ T cells have been found in ACLF patients.[27,68] Peripheral CD4+ and CD8+ T cell frequency as well as CD3 ζ chain expression in T cells are both decreased in ACHBLF patients.[66] Further subtyping of T cell populations shows decrease of naïve and effector CD4+ T cells, not decreased or even enhanced central memory and effector memory CD4+ T cells, whereas massively higher regulatory T cells in patients with ACLF comparing to healthy subjects. Concerning to the CD8+ T cell, a decrease of naïve CD8+ T cells, not significantly altered proportion of effector CD8+ T cells, central memory CD8+ T cells, but massively higher percentage of activated CD8+ T cells have been observed in ACLF patients.[68] What's more, T cell function is also down-regulated by lower generation of IFN-γ, increased production of IL-10, and the enhanced expressions of immune inhibitory receptors, such as T cell immunoglobulin and mucin domain molecule 3 (Tim-3), programmed cell death protein 1 (PD-1), and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) in T cells.[45,69] Tumor necrosis factor-α-induced protein 8-like 2 (TIPE2) is supposed to be a negative regulator especially highly expressed in T cells and monocytes.[70] Our recent study has found TIPE2 exhibits a distinct expression pattern in the acute exacerbation of liver function and during the onset of ACHBLF.[71] The TIPE2 mRNA level is dramatically elevated in ACHBLF patients. Notably, TIPE2 mRNA would serve as an independent index to predict 3-month mortality in ACHBLF and the combination of MELD scores and TIPE2 mRNA could further improve the diagnosis value.[72] Due to reduced CD3+ T cells counts and enhanced monocyte counts, the T/M ratios are reduced in ACHBLF patients. Moreover, T/M ratios are negatively associated with severity of ACLF and elevated T/M ratios are discovered in patients with good prognosis.[45]

CD8+T cells

Although several studies demonstrate no significant difference in the proportion of total CD8+ T cells between ACLF patients and CHB patients,[52] the results are supposed to be questionable. The peripheral blood CD8+ T cell counts are found significantly decreased in ACHBLF patients compared with CHB patients and healthy subjects in other studies,[73,74] CD8+ T cell counts are dramatically reduced in non-survivors of ACHBLF patients.[73] An investigation studied dysregulated adaptive immunity in liver cirrhosis preceding the development of ACLF, and has found simultaneous increase of co-stimulatory (e.g. CD40L, OX40, CD69, GITR, TIM-1) and inhibitory immune checkpoints (e.g. PDPN, PROCR, 2B4, TIGIT) on CD8+ and CD4+ T cells.[68] Exhausted phenotype is a main characteristic of CD8+ T cells in peripheral blood of ACLF patients, with increased CTLA-4, PD-1 and lymphocyte activation gene-3 (LAG-3) but decreased perforin, granzyme B and FasL.[75] Serum concentration of immunotolerance cytokine IL-35 has been found elevated in ACLF patients, which is supposed to suppress not only cytolytic but also non-cytolytic activity of CD8+ T cells.[75] Besides, the level of PD-1 expression is elevated in CD8+ T cells from the ACHBLF group, and the functions of CD8+ T cells decrease after PD-1/PD-L1 is stimulated in vitro. These results demonstrate PD-1-induced T lymphocyte dysfunction.[74] Deferred PD-1 expression on HBV-specific CD8+ T cells contribute to acute deterioration of liver function in HBV-related acute liver failure.[76]

Th17 cells

Th17 cells are a notable subtype of CD4+ T lymphocytes. Cytokine IL-17A, but also IL-17F, IL-21, IL-22 and CXCL8 (IL-8) is produced by Th17 cells with specific transcription factors such as retinoic acid receptor-related orphan nuclear receptor γt (RORγt).[77] A landmark study that was published in 2010 reported peripheral and intrahepatic Th17 cells were elevated in ACHBLF patients.[78] Th17 cells accelerate the activation of DCs and monocytes, promote the production of pro-inflammatory cytokines, including IL-1β, IL-6, TNFα, and IL-23, and promote liver damage progression.[78] In ACHBLF patients, peripheral Th17 cells frequencies are increased in non-survivors compared with survivors, and are associated with disease severity of ACHBLF.[78] A frequency over 5.9% of peripheral Th17 cells at admission predicts risk of death of ACHBLF patients.[79] Proportion of circulating Th17 cells is positively associated with serum IL-27 level.[80] The stimulation of mTOR/STAT3 pathway upon IL-6 activation may conduce to the elevated Th17 cell response in ACHBLF.[81] In addition, TLR2 signaling also contribute to Th17 cell differentiation and disease progression of ACHBLF. The Th17 responses are associated with TLR2 level in CD4+ T cells. TLR2 ligands stimulation promotes Th17 cell differentiation and production of IL-17A, IL-22, and TNF-α in vitro.[82]

Regulatory T cells (Tregs)

Tregs are a subtype of CD4+FOXP3+ T lymphocytes, which specifically suppress immune responses and play key roles in promoting tissue homeostasis and repairment. The proportion of peripheral CD4+ CD25+ Tregs is elevated in ACHBLF patients.[83] Dynamic trend of Tregs is observed in a rat model of ACLF. The frequency of circulating Treg cells decrease significantly first and finally become stable in ACLF rat.[84] Moreover, correlation of Tregs and Th17 is notable in ACLF.[84–86] An imbalance of Tregs/Th17 cells ratio is observed in the remission stage of ACHBLF patients.[86] Tregs/Th17 cells ratio is associated with disease aggravation and restoring the ratio may alleviate ACHBLF.[86] Protective properties against hepatic inflammatory injury by restoration of the Treg/Th17 imbalance have been confirmed in vivo.[87]

Th22 cells

Th22 cells are a recently identified CD4+T lymphocytes subpopulation, which produce specific cytokines such as IL-22, IL-13 and TNF-α, but not others, such as IL-4, IL-17 or IFN-γ. Th22 cells also express chemokine receptors CCR4, CCR6 and CCR10.[88] Th22 cells participate in the immunopathogenesis of autoimmune diseases and human viral diseases.[88] A study reported the proportion of Th22 cells in peripheral blood and plasma IL-22 were upregulated in ACHBLF patients. Elevated Th22 cells frequency and IL-22 level are correlated with prognosis of ACHBLF.[89] Activation and expansion of Th22 lymphocytes are correlated with minimal hepatic encephalopathy in cirrhotic patients.[90] IL-22 is the most important effector cytokine of Th22 cells. Studies in preclinical liver injury models reveal IL-22's remarkable role of anti-bacterial infection, anti-fibrosis, anti-apoptosis, anti-oxidation and protecting against liver injury by regenerative stimulation.[91] A novel mouse model of ACLF has been reported by Xiang et al. The model simulates the pathological process of ACLF, which uses chronic carbon tetrachloride [CCl4] injection to induce chronic injury, a next double dose of CCl4 injection to induce acute hepatic insult, and the last viable bacterial infection.[92] In this model, the proregenerative IL-6/STAT3 pathway is down-regulated, however the anti-regenerative IFN-γ/STAT1 pathway is activated, resulting to a severely impaired liver regeneration. When this model mice were treated with IF-22Fc, the STAT1/STAT3 pathway imbalance was reversed and many antibacterial genes amplify. Afterall, IL-22Fc intervention could improve survival in ACLF mice.[92] A clinical trial has revealed higher serum level of IL-22 in ACLF patients, compared with that in stable decompensated liver cirrhosis patients or acute decompensation liver cirrhosis patients. Higher level of IL-22 is strongly associated with mortality.[93]

T follicular helper (Tfh) cells

Tfh cells are a unique CD4+ T lymphocytes subpopulation, which play an important role in accelerating germinal center reactions, promoting cognate B cell differentiation and driving antibody secretion.[94] The proportion of CD4+CXCR5+ Tfh cells is elevated in peripheral blood and liver tissue of ACHBLF patients. Moreover, the percentage of Tfh cells is correlated well with MELD score and reduced proportion of Tfh cells is found in improving ACLF patients. Naïve CD4+ T cells from healthy controls differentiate into Tfh cells when they are treated with ACHBLF patients’ serum, and the induced Tfh cells can promote the proliferation of B cells and production of IgG in vitro.[95]

B cells

The importance of T cells in ACHBLF is well established, however, the influence of B cells has not been investigated in detail. B cell frequencies are decreased in ACLF patients than healthy controls and compensated cirrhosis patients.[68] B-cell compartment abnormalities are found as the hallmark of ACLF,[96] and especially decreased proportions of memory lymphocytes[27] and higher frequencies of double-negative (DN) B cells.[96] A study use next generation sequencing (NGS) to investigate B-cell receptor (BCR) heavy chain repertoires in ACHBLF patients. They found a dramatically higher extent of clonal expansion for B cells and abnormally expressed BCR heavy chain CDR3 of the V, D, J and V-J combinations of subfamily genes in ACHBLF patients. These results indicate there is a link between the BCR repertoire and ACHBLF.[97]

In HBV-associated ALF, a dramatical B cell response significantly centers in the liver, which is characterized by gather of plasma cells accompanied by IgG and IgM secretion and complement deposition.[98] The extensive intrahepatic IgG and IgM are unmutated in germline configuration. They represent high affinity to hepatitis B core antigen (HBcAg), which can bind HBcAg on the HBV-infected hepatocyte surface and trigger complement-mediated cell lysis.[99] Recently, a mass of VH genes in germline configuration with identical VDJ sequences in the IgG and IgM repertoires are confirmed via NGS in these patients, revealing an anomalous T cell-independent humoral response in the immunopathogenesis of HBV-associated ALF.[100]

Conclusions

Although the precise immunologic and pathogenic mechanisms of ACHBLF remains poorly understood, immune dysregulation has been supposed to participate in the pathogenesis of ACHBLF and contribute to disease onset and development. Recent knowledge of the potential role of innate and adaptive immune cells in ACHBLF has been expanding rapidly. The multimodal innate and adaptive immune responses drive immune-pathogenesis and are responsible for disease progression. Extensive phenotypic and functional variation in the immune cells conduces to systemic inflammation, promotes hepatocyte death, and induces organ failures. A deeper understanding of the immune-pathogenesis in ACHBLF may result in the development of novel immunotherapies that aim to remedy the immune cell defects. We hope that this review will contribute to current research into the immune-pathogenesis of ACHBLF.

Funding

This work was supported by the National Natural Science Foundation of China (No. 81970522, No. 82000564), Shandong University multidisciplinary research and innovation team of young scholars (2020QNQT11), Shandong Provincial Natural Science Foundation (ZR2019PH027), China Postdoctoral Science Foundation (2020M672074), and the Young Taishan Scholars (tsqn202103169).

Conflicts of Interest

None.

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Keywords:

Acute-on-chronic liver failure; Adaptive immunity; Hepatitis B virus; Immune cells; Innate immunity

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