Oscillating CD8(+) T cell effector functions after antigen recognition in the liver

Costimulatory molecule programmed death-1 in the cytotoxic response during chronic hepatitis C

Hepatitis C virus (HCV)-specific CD8⁺ T cells play an important role in the resolution of HCV infection. Nevertheless, during chronic hepatitis C these cells lack their effector functions and fail to control the virus. HCV has developed several mechanisms to escape immune control. One of these strategies is the up-regulation of negative co-stimulatory molecules such us programmed death-1 (PD-1). This molecule is up-regulated on intrahepatic and peripheral HCV-specific cytotoxic T cells during acute and chronic phases of the disease, whereas PD-1 expression is low in resolved infection. PD-1 expressing HCV-specific CD8⁺ T cells are exhausted with impairment of several effector mechanisms, such as: type-1 cytokine production, expansion ability after antigen encounter and cytotoxic ability. However, PD-1 associated exhaustion can be restored by blocking the interaction between PD-1 and its ligand (PD-L1). After this blockade, HCV-specific CD8⁺ T cells reacquire their functionality. Nevertheless, functional restoration depends on PD-1 expression level. High PD-1-expressing intrahepatic HCV-specific CD8⁺ T cells do not restore their effector abilities after PD-1/PD-L1 blockade. The mechanisms by which HCV is able to induce PD-1 up-regulation to escape immune control are unknown. Persistent TCR stimulation by a high level of HCV antigens could favour early PD-1 induction, but the interaction between HCV core protein and gC1q receptor could also participate in this process. The PD-1/PD-L1 pathway modulation could be a therapeutic strategy, in conjunction with the regulation of others co-stimulatory pathways, in order to restore immune response against HCV to succeed in clearing the infection.

Liver is able to activate naïve CD8+ T cells with dysfunctional anti-viral activity in the murine system

The liver possesses distinct tolerogenic properties because of continuous exposure to bacterial constituents and nonpathogenic food antigen. The central immune mediators required for the generation of effective immune responses in the liver environment have not been fully elucidated. In this report, we demonstrate that the liver can indeed support effector CD8⁺ T cells during adenovirus infection when the T cells are primed in secondary lymphoid tissues. In contrast, when viral antigen is delivered predominantly to the liver via intravenous (IV) adenovirus infection, intrahepatic CD8⁺ T cells are significantly impaired in their ability to produce inflammatory cytokines and lyse target cells. Additionally, intrahepatic CD8⁺ T cells generated during IV adenovirus infection express elevated levels of PD-1. Notably, lower doses of adenovirus infection do not rescue the impaired effector function of intrahepatic CD8⁺ T cell responses. Instead, intrahepatic antigen recognition limits the generation of potent anti-viral responses at both priming and effector stages of the CD8⁺ T cell response and accounts for the dysfunctional CD8⁺ T cell response observed during IV adenovirus infection. These results also implicate that manipulation of antigen delivery will facilitate the design of improved vaccination strategies to persistent viral infection.

Host immunity influences disease progression and antiviral efficacy in humans infected with hepatitis B virus

Hepatitis B virus (HBV) infection can lead to several severe liver diseases, including hepatitis, cirrhosis and hepatocellular carcinoma, although the underlying mechanisms responsible for the clinical outcome have not been well characterized. In this review, we retrospectively examine the history of immunological responses to HBV infection and summarize the current understanding of innate and adaptive immunity in the context of HBV-associated liver disease. Recent data indicate that the interaction between HBV and the host immune response not only substantially drives disease progression, but also significantly influences antiviral efficacy in HBV-infected individuals. Advances in the field have provided insight into the immunopathology of HBV infection. Based on the characteristics of host immune responses in patients with HBV infection, a 'climbing slope hypothesis' is proposed to suggest that therapeutic strategies aimed at modulating the immune activity of the host may represent a complementary approach to antiviral drug treatment for the management of chronically HBV-infected patients.

HBV-Specific Adaptive Immunity

The successful control of HBV infection requires an efficient expansion of distinct elements of the adaptive immune system (B cells, helper and cytotoxic T cells) that, due to the hepatotropic nature of HBV, need to operate in the liver parenchyma. In this respect, we will discuss broad features of HBV immunity in patients with resolved or chronic HBV infection and analyze how the liver environment can directly modulate HBV-immunity.

The size of the viral inoculum contributes to the outcome of hepatitis B virus infection

The impact of virus dose on the outcome of infection is poorly understood. In this study we show that, for hepatitis B virus (HBV), the size of the inoculum contributes to the kinetics of viral spread and immunological priming, which then determine the outcome of infection. Adult chimpanzees were infected with a serially diluted monoclonal HBV inoculum. Unexpectedly, despite vastly different viral kinetics, both high-dose inocula (10(10) genome equivalents [GE] per animal) and low-dose inocula (10 degrees GE per animal) primed the CD4 T-cell response after logarithmic spread was detectable, allowing infection of 100% of hepatocytes and requiring prolonged immunopathology before clearance occurred. In contrast, intermediate (10(7) and 10(4) GE) inocula primed the T-cell response before detectable logarithmic spread and were abruptly terminated with minimal immunopathology before 0.1% of hepatocytes were infected. Surprisingly, a dosage of 10(1) GE primed the T-cell response after all hepatocytes were infected and caused either prolonged or persistent infection with severe immunopathology. Finally, CD4 T-cell depletion before inoculation of a normally rapidly controlled inoculum precluded T-cell priming and caused persistent infection with minimal immunopathology. These results suggest that the relationship between the kinetics of viral spread and CD4 T-cell priming determines the outcome of HBV infection.

Cytokine expression during chronic versus occult hepatitis B virus infection in HIV co-infected individuals

Chronic hepatitis B virus infection is characterized by persistent detectable levels of hepatitis B surface antigen (HBsAg) and HBV DNA in the serum. In contrast, HBsAg is not detectable during occult HBV infection, despite the presence of HBV DNA. An altered host immune response could play a role in the development of occult HBV infection; however, potential differences in immune responses among chronic and occult HBV-infected patients have not been evaluated in vivo. In the current study, we evaluated serum levels of regulatory, apoptotic, and fibrotic/anti-fibrotic cytokines/markers as indicators of immune responses in 25 chronic and 12 occult HBV-infected patients. More than half of the patients in both chronic and occult HBV infection groups had IL-2, IL-4, IL-13, and IFN-gamma levels below detectable limits. In contrast, most patients had detectable levels of IL-8, IL-10, IP-10, sFas, sFasL, and TGF-beta1. Of these, only sFas was significantly different between the two groups, with lower levels observed during occult compared to chronic HBV infection (p=0.01). As a surrogate marker of apoptotic inhibition, decreased sFas during occult HBV infection suggests that apoptosis occurs at different rates in occult compared to chronic HBV infection and therefore, may contribute to persistence of occult HBV infection.

Dynamic decrease in PD-1 expression correlates with HBV-specific memory CD8 T-cell development in acute self-limited hepatitis B patients

BACKGROUND/AIMS

Programmed death-1 (PD-1) upregulation can impair virus-specific CD8 T-cell responses during chronic viral infection. Whether and how PD-1 affects virus-specific memory CD8 T cells in humans with acute viral infection, however, remains largely undefined.

METHODS

The association between PD-1 expression and HBV-specific memory CD8 T-cell responses were longitudinally analyzed in eighteen patients with acute hepatitis B virus (HBV) infection, including ten patients with human leucocyte antigen (HLA)-A201 and eight with other HLA-A2 subtypes.

RESULTS

At clinical onset, PD-1 was significantly up-regulated and subsequently led to the functional suppression of HBV-specific effector CD8 T cells, as blocking PD-1/PD-L1 interactions in vitro enhanced their proliferation and IFN-gamma production. Following disease resolution, HBV-specific effector CD8 T cells developed into memory T cells. During this period, the dynamic PD-1 decrease was numerically correlated with the reduction of HBV-specific CD8 T-cell frequency, phenotypically with an acquisition of CCR7, CD45RA and CD127 expressions, and functionally with the increase in proliferation and IFN-gamma production of the memory T cells.

CONCLUSIONS

PD-1-mediated inhibitory signaling not only attenuates HBV-specific CD8 T-cell effector function during the acute phase of infection but also correlates with the development of HBV-specific memory CD8 T cells following disease resolution.

Dendritic cells and chronic hepatitis B

The immune function disorders in patients with chronic hepatitis B (CHB) lead to persistent damage of their liver cells and have impact on the ability to clear the virus. Dendritic cells (DCs) are the most important antigen-presenting cells in body. They help to generate sufficient immune response by the invasion of pathogens. The quantity and functionality of DCs directly correlate with the normal function of the host's immune response. Not only can DCs be resistant to the virus, but also serve as vehicles for the virus to spread and escape immune injury. Therefore the research on DCs and improving their function under the state of CHB is quite significant.

Analysis of CD8+ T-cell-mediated inhibition of hepatitis C virus replication using a novel immunological model

BACKGROUND & AIMS

Virus-specific CD8+ T cells are required for the control of hepatitis C virus (HCV) infection. We investigated the extent to which different effector functions of CD8+ T cells contribute to the inhibition of viral replication.

METHODS

We developed a novel immunologic model by stably transducing the HLA-A2 gene into the replicon system, matching the epitope sequence of the replicon to the sequence targeted by an HCV-specific CD8+ T-cell clone. Luciferase activity was then measured to quantitate HCV RNA replication.

RESULTS

HCV-specific CD8+ T cells strongly inhibited viral replication, through cytolytic and noncytolytic mechanisms, in a dose-dependent manner. HCV replication was almost completely inhibited at an effector-to-target ratio of 1:1 with significant cytotoxicity; however, >95% viral inhibition occurred at ratios as low as 1:100. Importantly, no cytotoxicity was observed at low effector-to-target ratios, indicating a dominant effect of noncytolytic effector functions that was confirmed by Transwell experiments. Neutralization experiments revealed that interferon gamma mediates the noncytolytic inhibition.

CONCLUSIONS

Only a very few HCV-specific CD8+ T cells are required to inhibit HCV replication; inhibition occurs primarily by noncytolytic effector functions.

The diversity of costimulatory and inhibitory receptor pathways and the regulation of antiviral T cell responses

T cell responses are regulated by integrating positive and negative signals from costimulatory and inhibitory receptors. While the function of specific T cell costimulatory molecules during infections has been appreciated for some time, recent observations have now revealed a crucial role for inhibitory receptors in regulating T cell responses to pathogens, especially during chronic infections. A key emerging principle is that there is considerable diversity in the number and type of inhibitory receptors that can be expressed by T cells during both acute and chronic infections. These distinct inhibitory pathways appear to cooperate in regulating T cell function, could have distinct mechanisms of action, and are likely to provide novel therapeutic targets during persisting infections and other diseases.

Upregulation of indoleamine 2,3-dioxygenase in hepatocyte during acute hepatitis caused by hepatitis B virus-specific cytotoxic T lymphocytes in vivo

BACKGROUND/AIMS

Indoleamine-2,3-dioxygenase (IDO) is a tryptophan-catabolizing enzyme inducing suppression of T-cell function and immune tolerance. In hepatitis B virus (HBV) transgenic (Tg) mice, the adoptive transfer of HBV-specific cytotoxic T lymphocytes (CTL) causes a necroinflammatory liver disease that is histologically similar to acute viral hepatitis in man. The present study aimed to determine IDO expression in the liver and hepatocytes during an acute hepatitis model.

METHODS

Serum l-kynurenine (l-Kyn) concentration in HBV Tg mice administered with HBV-specific CTL was measured over time, together with serum levels of alanine aminotransferase (ALT). Furthermore, we examined the expression of IDO in the total liver and isolated hepatocytes of HBV Tg mice after CTL injection using immunohistochemical analysis and reverse-transcription polymerase chain reaction (PCR).

RESULTS

In HBV Tg mice, HBV-specific CTL induced, over the course of several days, a chronic increase in serum l-Kyn levels, which was associated with a sustained enhancement of liver IDO activity. In particular, IDO expression was enhanced in the liver parenchymal cells (hepatocytes) after HBV-specific CTL injection both in immunohistochemical analysis and in reverse-transcription PCR. Moreover, murine recombinant interferon-gamma (IFN-gamma) directly increased the IDO expression in primary hepatocytes in vitro.

CONCLUSIONS

Cytotoxic T lymphocytes transduction results in the upregulation of IDO, which might downregulate T-cell responsiveness. Our findings provide evidence that hepatocyte itself expresses IDO and increases levels of l-Kyn in the blood in acute lethal hepatitis of mice. These data indicate that HBV infection facilitates the induction of IDO in response to proinflammatory cytokines, particularly IFN-gamma.

Selective expansion of a subset of exhausted CD8 T cells by alphaPD-L1 blockade

Programmed death-1 (PD-1) regulates T cell exhaustion during chronic infections. Blocking the PD-1:PD-ligand (PD-L) pathway reinvigorates exhausted CD8 T cells. Exactly how blocking PD-1:PD-L interactions improves T cell immunity, however, remains unclear. PD-1:PD-L blockade could reprogram all exhausted T cells to become antiviral effectors. Alternatively, this blockade might selectively expand a subset of exhausted T cells. We have identified two subpopulations of exhausted CD8 T cells during chronic viral infection in mice. One subset of exhausted CD8 T cells is rescued by alphaPD-L1 blockade, whereas the other subset appears more terminally differentiated and responds poorly to PD-1:PD-L blockade. Blocking PD-1:PD-L interactions reduces spontaneous apoptosis and enhances expansion and protective immunity of the rescuable subset, but not the more terminally differentiated subset of exhausted CD8 T cells. These results have implications for predicting clinical responses to PD-1-based therapeutic interventions and for understanding T cell dynamics during persisting infections.

A subset of IL-10-producing gammadelta T cells protect the liver from Listeria-elicited, CD8(+) T cell-mediated injury

Although gammadelta T cells play a role in protecting tissues from pathogen-elicited damage to bacterial, viral and parasitic pathogens, the mechanisms involved in the damage and in the protection have not been clearly elucidated. This has been addressed using a murine model of listeriosis, which in mice lacking gammadelta T cells (TCRdelta(-/-)) is characterised by severe and extensive immune-mediated hepatic necrosis. We show that these hepatic lesions are caused by Listeria-elicited CD8(+) T cells secreting high levels of TNF-alpha that accumulate in the liver of Listeria-infected TCRdelta(-/-) mice. Using isolated populations of gammadelta T cells from wild-type and cytokine-deficient strains of mice to reconstitute TCRdelta(-/-) mice, the TCR variable gene 4 (Vgamma4)(+) subset of gammadelta T cells was shown to protect against liver injury. Hepatoprotection was dependent upon their ability to produce IL-10 after TCR-mediated interactions with Listeria-elicited macrophages and CD8(+) T cells. IL-10-producing Vgamma4(+) T cells also contribute to controlling CD8(+) T cell expansion and to regulating and reducing TNF-alpha secretion by activated CD8(+) T cells. This effect on TNF-alpha production was directly attributed to IL-10. These findings identify a novel mechanism by which pathogen-elicited CD8(+) T cells are regulated via interactions with, and activation of, IL-10-producing hepatoprotective gammadelta T cells.

Expression and function of PDCD1 at the human maternal-fetal interface

The failure to reject the semiallogenic fetus by maternal T lymphocytes suggests that potent mechanisms regulate these cells. PDCD1 is a CD28 family receptor expressed by T cells, and its ligand CD274 is strongly expressed by trophoblast cells of the human placenta. In this study, we examined whether human maternal T cells express PDCD1. Immunofluorescence examination of uterine tissues revealed PDCD1 expression on CD3+ cells was low in nonpregnant endometrium but increased in first-trimester decidua and remained elevated in term decidua (P < 0.05). In addition, higher relative proportions of term decidual CD8 bright, CD4+, and regulatory T cells expressed PDCD1 in comparison to autologous peripheral blood (P < 0.05). Term decidual T cells also expressed full-length and soluble PDCD1 mRNA isoforms more abundantly than their peripheral blood counterparts (P <or= 0.05). We also examined the effects of PDCD1:CD274 interactions in decidual T cells. Jar choriocarcinoma cells were transfected with CD274 and cocultured with activated decidual CD4+ or CD8 bright T cells for 72 h followed by analysis of cytokine concentration and decidual T cell apoptosis. Compared with empty vector-transfected cells, CD274-transfected Jar cells caused a significant suppression of interferon gamma and tumor necrosis factor alpha production by CD4+ (P < 0.05) but not CD8 bright T cells, while having no effect on secretion of IL10 or T cell apoptosis. These results suggest that the PDCD1:CD274 pathway functions in modification of maternal decidual lymphocyte cytokine secretion during pregnancy.

Dynamic programmed death 1 expression by virus-specific CD8 T cells correlates with the outcome of acute hepatitis B

BACKGROUND & AIMS

Recent studies have shown that programmed death 1 (PD-1) expression can impair virus-specific CD8 T-cell responses during chronic viral infection, including hepatitis B virus (HBV). This study aimed to characterize the PD-1 expression during acute hepatitis B (AHB) and further address whether and how the PD-1-mediated pathway balances antiviral immunity versus immunopathology, possibly contributing to disease progression.

METHODS

Peripheral and intrahepatic PD-1 expression was investigated longitudinally in 23 human HLA-A2-positive patients with acute hepatitis B. Four patients with HBV-related acute liver failure, 13 patients with chronic hepatitis B, and 9 healthy individuals were enrolled as controls. Flow cytometric, immunohistochemical, and immunofunctional assays were performed to analyze the impact of PD-1 expression.

RESULTS

PD-1 expression was significantly up-regulated on HBV-specific CD8 T cells in the early phase of acute HBV infection, and successful viral clearance correlated with a subsequent decrease in PD-1 expression. Blocking the PD-1-mediated pathway in vitro enhanced HBV-specific CD8 T-cell proliferation and inflammatory cytokine production, while reducing interleukin-10 production and apoptosis, confirming the essential role of PD-1 in tempering the T-cell response during the acute phase of infection. In contrast, delayed PD-1 expression on HBV-specific CD8 T cells was associated with acute liver failure.

CONCLUSIONS

PD-1 up-regulation may efficiently mitigate pathogenic CD8 T-cell responses and liver damage, correlating with disease progression of acute HBV infection. This study therefore shows how this negative signaling pathway functions in such early HBV infection, which will be important for better clinical management, prognosis, and new HBV treatments.

Blockade of PD-1/B7-H1 interaction restores effector CD8+ T cell responses in a hepatitis C virus core murine model

The impaired function of CD8(+) T cells is characteristic of hepatitis C virus (HCV) persistent infection. HCV core protein has been reported to inhibit CD8(+) T cell responses. To determine the mechanism of the HCV core in suppressing Ag-specific CD8(+) T cell responses, we generated a transgenic mouse, core(+) mice, where the expression of core protein is directed to the liver using the albumin promoter. Using a recombinant adenovirus to deliver Ag, we demonstrated that core(+) mice failed to clear adenovirus-LacZ (Ad-LacZ) infection in the liver. The effector function of LacZ-specific CD8(+) T cells was particularly impaired in the livers of core(+) mice, with suppression of IFN-gamma, TNF-alpha, and granzyme B production by CD8(+) T cells. In addition, the impaired CD8(+) T cell responses in core(+) mice were accompanied by the enhanced expression of the inhibitory receptor programmed death-1 (PD-1) by LacZ-specific CD8(+) T cells and its ligand B7-H1 on liver dendritic cells following Ad-LacZ infection. Importantly, blockade of the PD-1/B7-H1 inhibitory pathway (using a B7-H1 blocking antibody) in core(+) mice enhanced effector function of CD8(+) T cells and cleared Ad-LacZ-infection as compared with that in mice treated with control Ab. This suggests that the regulation of the PD-1/B7-H1 inhibitory pathway is crucial for HCV core-mediated impaired T cell responses and viral persistence in the liver. This also suggests that manipulation of the PD-1/B7-H1 pathway may be a potential immunotherapy to enhance effector T cell responses during persistent HCV infection.

Restoration of HCV-specific T cell functions by PD-1/PD-L1 blockade in HCV infection: effect of viremia levels and antiviral treatment

BACKGROUND/AIMS

HCV-specific T cells in acute hepatitis C with subsequent chronic evolution are dysfunctional and most of them express PD-1. The aim of the study was to investigate to what extent the antiviral T cell function can be restored by reversing T cell exhaustion by PD-1/PD-L1 blockade and to assess whether this restoration is favored by IFN-alpha treatment.

METHODS

PD-1 and PD-L1 expression was studied on T cells and dendritic cells, respectively, of 14 patients with acute hepatitis C and different evolutions of infection. The effect of anti-PD-L1 was analyzed on proliferation, cytokine production and cytolytic activity of CD4 and CD8 T cells.

RESULTS

While PD-1 expression dropped concurrently with spontaneous or IFN-alpha induced HCV-RNA decline, PD-L1 levels on dendritic cells increased during IFN-alpha treatment. Anti-PD-L1 antibodies improved expansion and cytokine production but not the cytolytic activity of HCV-specific T cells. This restoration tended to be greater at lower levels of viremia and PD-1 expression and during PEG-IFNalpha treatment.

CONCLUSIONS

PD-1/PD-L1 blockade has an immunoregulatory activity which may synergize with the antiviral effect of IFN-alpha therapy and should be thus explored further in long-lasting chronic HCV infections in the perspective of improving the efficacy of available antiviral treatments.

Novel strategies to eliminate persistent viral infections

Infection with viruses that have the capacity to modulate or evade the immune response can result in persistence, which can lead to a variety of chronic problems including neoplasia, immunosuppression, autoimmune-like syndromes, and selective organ failure. Recently, two promising new treatment approaches that target either the inhibitory receptor programmed cell death 1 (PD-1) or neutralize interleukin-10 (IL-10) during chronic viral infection have been described. We discuss how future combination therapies can be used to inhibit viral synthesis as well as strengthen the antiviral response without increasing immunopathology or the development of autoimmune disease.

The programmed death-1 and interleukin-10 pathways play a down-modulatory role in LP-BM5 retrovirus-induced murine immunodeficiency syndrome

Pathology due to the immune system's response to viral infections often represents a delicate balance between inhibition of viral pathogenesis and regulation of protective immunity. In susceptible C57BL/6 (B6) mice, the murine retroviral isolate LP-BM5 induces splenomegaly, hypergammaglobulinemia, profound B- and T-cell immunodeficiency, and increased susceptibility to opportunistic pathogens and terminal B-cell lymphomas. Here, we report that B6.PD-1 (programmed death-1) and B6.IL-10 knockout mice are substantially more susceptible to LP-BM5-induced disease than wild-type B6 mice. LP-BM5-infected B6.PD-1(-/-) mice developed more severe splenomegaly, hypergammaglobulinemia, and immunodeficiency than infected B6 mice: PD-1(-/-) mice are more susceptible to lower doses of LP-BM5 and show more exaggerated disease early postinfection. LP-BM5-infected B6.IL-10(-/-) mice also develop exaggerated LP-BM5-induced disease, compared to B6 mice, without a significant change in the retroviral load. By reciprocal reconstitution experiments, comparing wild-type versus PD-1(-/-) sources of the requisite cells for LP-BM5 pathogenesis-CD4 T and B cells, PD-1(+) B cells appear to be crucial in the normal limitation of LP-BM5-induced disease in B6 mice. Also, infected B6 mice have increased CD11b(+) spleen cells that express interleukin-10 (IL-10). However, PD-1(-/-) mice, though showing an even greater expansion of CD11b(+) cells after LP-BM5 inoculation, did not show an equivalent increase in IL-10-producing cells. Thus, it appears that PD-1/PD-L interactions and IL-10 are primarily important in moderating the effects of LP-BM5-induced disease in B6 mice.

The immunology of hepatitis B

The hepatitis B virus (HBV) is an enveloped, hepatotrophic, oncogenic hepadnavirus that is noncytopathic for hepatocytes. HBV infection results in a variety of outcomes that are determined by the quality, quantity, and kinetics of the host innate and adaptive immune responses. Whether HBV infection is cleared or persists as a progressive or nonprogressive liver disease is determined by both viral and host factors. Replicative intermediates can persist in the liver under immunologic control after resolution of acute or chronic hepatitis B, conferring a risk for reactivation following a course of immunosuppression or chemotherapy.

Immune response and tolerance during chronic hepatitis B virus infection

The hepatitis B virus (HBV) is a hepatotropic non-cytopathic DNA virus that despite the presence of an effective prophylactic vaccine is estimated to infect 300 million people, with a particularly high prevalence in Asia and Africa. It causes liver diseases that vary greatly in severity from person to person. Some subjects control infection efficiently and clear the virus from the bloodstream either without clinically evident liver disease or with an acute inflammation of the liver (acute hepatitis) that can resolve without long-term clinical sequelae. Other patients fail to clear the virus and develop chronic infection. Most chronically infected patients remain asymptomatic without life-threatening liver disease but 10-30% develop liver cirrhosis with possible progression to liver cancer. Outcome of infection and the pathogenesis of liver disease are determined by virus and host factors, which have been difficult tofully elucidate because the host range of HBV is limited to man and chimpanzees. However, the study of animal models of related Hepadnavirus infections and transgenic mouse able to express individual HBV genes or replicate the entire viral genome have clarified several aspects connected to HBV infection. Furthermore, the ability to analyze many immunological phenomena ex vivo through direct quantification of Ag-specific T cells in humans and chimps has considerably increased our knowledge of HBV pathogenesis. Here, we will discuss the distinctions of HBV adaptive immunity between resolved and persistently infected patients and the host/viral factors that can cause and maintain them.

Host and viral factors contributing to CD8+ T cell failure in hepatitis C virus infection

Virus-specific CD8+ T cells are thought to be the major anti-viral effector cells in hepatitis C virus (HCV) infection. Indeed, viral clearance is associated with vigorous CD8+ T cell responses targeting multiple epitopes. In the chronic phase of infection, HCV-specific CD8+ T cell responses are usually weak, narrowly focused and display often functional defects regarding cytotoxicity, cytokine production, and proliferative capacity. In the last few years, different mechanisms which might contribute to the failure of HCV-specific CD8+ T cells in chronic infection have been identified, including insufficient CD4+ help, deficient CD8+ T cell differentiation, viral escape mutations, suppression by viral factors, inhibitory cytokines, inhibitory ligands, and regulatory T cells. In addition, host genetic factors such as the host’s human leukocyte antigen (HLA) background may play an important role in the efficiency of the HCV-specific CD8+ T cell response and thus outcome of infection. The growing understanding of the mechanisms contributing to T cell failure and persistence of HCV infection will contribute to the development of successful immunotherapeutical and -prophylactical strategies.

Induction of tolerance in CD8+ T cells to a transgenic autoantigen expressed in the liver does not require cross-presentation

Cross-presentation of normal self and candidate tumor Ags by bone marrow (BM)-derived APCs that have not been activated has been demonstrated as a major mechanism contributing to acquisition of tolerance by mature T cells that first encounter an Ag in the periphery (cross-tolerance). Following adoptive transfer of naive TCR-transgenic CD8(+) T cells into a host expressing a transgenic Ag that is a potentially targetable tumor Ag in normal hepatocytes as a self-Ag, we found that the majority of Ag-specific CD8(+) T cells were deleted, with the remaining cells rendered anergic. Studies in BM chimeric mice and with purified cell populations demonstrated that these events were not dependent on cross-presentation by BM-derived APCs including Kupffer cells or liver sinusoidal endothelial cells, and apparently can occur entirely as a consequence of direct recognition of Ag endogenously processed and presented by hepatocytes. Direct recognition of Ag-expressing hepatocytes in vivo induced a proliferative response and up-regulation of activation markers in responding CD8(+) T cells, but proliferating cells did not accumulate, with most cells rapidly eliminated, and the persisting T cells lost the capacity to proliferate in response to repeated Ag stimulation. The results suggest that parenchymal tissues may retain the capacity to directly regulate in vivo responses to self-Ags processed and presented in the context of class I MHC molecules.

Liver injury is associated with enhanced regulatory T-cell activity in patients with chronic hepatitis B

Chronic hepatitis B virus (HBV) infection is associated with impairment of HBV-specific immune responses. Recently, it has been shown that regulatory T (Treg) cells downregulate HBV-specific immune responses but their role in chronic hepatitis B is still controversial. We hypothesized that liver injury enhances the influence of Treg cells on HBV-specific immune responses. The frequency of Treg cell and the in vitro expansion of HBV-specific CD8+ T cell detected by the tetramer method were investigated in 79 patients with chronic hepatitis B. Thirty-three healthy volunteers were enrolled to measure the frequency of Treg cell as controls. The results showed that in chronic hepatitis B cases, the frequency of Treg cells in peripheral blood was significantly higher than that in normal volunteers. The higher level of serum transaminase was associated with higher frequency of Treg cells, which both had a linear correlation relationship. HBV-DNA level, HBe status, age and sex had no statistical association with Treg cell frequency. Furthermore, in patients with higher serum transaminase levels, the expansion of HBV-specific CD8+ T cells was higher after removal of Treg cells when compared with patients with lower serum transaminase levels. In conclusion, our data indicate a significant association between serum transaminase level and frequency/activity of Treg cells. Based on this observation, we propose that liver-injury enhances Treg cell frequency/activity in chronic hepatitis B patients.

The function of programmed cell death 1 and its ligands in regulating autoimmunity and infection

The programmed cell death 1 (PD-1) surface receptor binds to two ligands, PD-L1 and PD-L2. Studies have shown that PD-1-PD-L interactions control the induction and maintenance of peripheral T cell tolerance and indicate a previously unknown function for PD-L1 on nonhematopoietic cells in protecting tissues from autoimmune attack. PD-1 and its ligands have also been exploited by a variety of microorganisms to attenuate antimicrobial immunity and facilitate chronic infection. Here we examine the functions of PD-1 and its ligands in regulating antimicrobial and self-reactive T cell responses and discuss the therapeutic potential of manipulating this pathway.

PD-1:PD-L1 interactions contribute to the functional suppression of virus-specific CD8+ T lymphocytes in the liver

Mechanisms contributing to the development of chronic viral infections, including chronic hepatitis B virus (HBV) infections, are not well understood. We have shown recently that production of IFN-gamma, an important antiviral cytokine, by HBV-specific CTLs is rapidly induced when they enter the liver of HBV transgenic mice, and then rapidly suppressed, despite the continued presence of Ag. Suppression of IFN-gamma production by the CTLs coincides with the up-regulation of programmed cell death (PD)-1, a cell surface signaling molecule known to inhibit T cell function. To determine whether PD-1 plays a role in the functional suppression of IFN-gamma secretion by CTLs, we treated HBV transgenic mice with blocking Abs specific for PD ligand (PD-L)1, the most widely expressed PD-1 ligand, and adoptively transferred HBV-specific CTLs. Treatment with anti-PD-L1 Abs resulted in a delay in the suppression of IFN-gamma-producing CTLs and a concomitant increase in the absolute number of IFN-gamma-producing CTLs in the liver. These results indicate that PD-1:PD-L1 interactions contribute to the suppression of IFN-gamma secretion observed following Ag recognition in the liver. Blockade of inhibitory pathways such as PD-1:PD-L1 may reverse viral persistence and chronic infection in cases in which the CTL response is suppressed by this mechanism.

Flying Under the Radar: The Immunobiology of Hepatitis C

The hepatitis C virus (HCV) is a remarkably successful pathogen, establishing persistent infection in more than two-thirds of those who contract it. Its success is related to its abilities to blunt innate antiviral pathways and to evade adaptive immune responses. These two themes may be related. We propose that HCV takes advantage of the impaired innate response to delay the organization of an effective adaptive immune attack. The tolerogenic liver environment may provide cover, prolonging this delay. HCV's error-prone replication strategy permits rapid evolution under immune pressure. Persistent high levels of viral antigens may contribute to immune exhaustion. Finally, the virus may benefit from the efficient enlistment of memory T and B cells in the pursuit of a moving target.

HBV pathogenesis in animal models: recent advances on the role of platelets

Hepatitis B virus (HBV) causes acute and chronic necroinflammatory liver diseases and hepatocellular carcinoma (HCC). HBV replicates noncytopathically in the hepatocyte, and most of the liver injury associated with this infection reflects the immune response. While the innate immune response may not contribute significantly to the pathogenesis of liver disease or viral clearance, the adaptive immune response, particularly the cytotoxic T lymphocyte (CTL) response, contributes to both. Recent observations also reveal that antigen-nonspecific inflammatory cells enhance CTL-induced liver pathology and, more surprisingly, that platelets facilitate the intrahepatic accumulation of CTLs, suggesting that the host response to HBV infection is a highly complex but coordinated process. The notion that platelets contribute to liver disease and viral clearance by promoting the recruitment of virus-specific CTLs into the liver is a new concept in viral pathogenesis, which may prove useful to implement treatments of chronic HBV infection in man.

B7-1 mediated costimulation regulates pancreatic autoimmunity

Costimulation by B7-1 and B7-2 molecules results in divergent biological effects. This is particularly striking in the NOD mouse, since the lack of B7-2 leads to complete protection from diabetes whereas the B7-1 deficiency causes exacerbation of disease. We tested the hypothesis that B7-1 costimulation suppresses pancreatic autoimmunity. We describe that the lack of B7-1 not only causes aberrant thymocyte maturation but also significantly enhances expansion, survival, and effector function of islet specific T cells in periphery. We also observed a significant reduction in the proportion of T-regulatory (T-regs) cells. Immunophenotypic analysis of T and APCs revealed a significantly lower frequency of T cells expressing the negative costimulatory receptor PD-1 in B7-1KO mice whereas the proportion of B7-H1 positive APCs was found to be significantly higher. Blocking studies in B7-1KO mice suggest that B7-H1 provides negative signals for anti islet CD4 and CD8 T-cell expansion but is differentially required for their priming. Our data demonstrate that deficiency of B7-1 mediated costimulation causes multitude of immunological defects, which involve reduction in T-regs and a concomitant enhancement of expansion, survival and effector potential of auto reactive T cells.

Pathogenesis of hepatitis B virus infection

Infection with hepatitis B virus (HBV) leads to a wide spectrum of clinical presentations ranging from an asymptomatic carrier state to self-limited acute or fulminant hepatitis to chronic hepatitis with progression to cirrhosis and hepatocellular carcinoma. Infection with HBV is one of the most common viral diseases affecting man. Both viral factors as well as the host immune response have been implicated in the pathogenesis and clinical outcome of HBV infection. In this review, we will discuss the impact of virus-host interactions for the pathogenesis of HBV infection and liver disease. These interactions include the relevance of naturally occurring viral variants for clinical disease, the role of virus-induced apoptosis for HBV-induced liver cell injury and the impact of antiviral immune responses for outcome of infection.

The level of viral antigen presented by hepatocytes influences CD8 T-cell function

CD8 T cells exert their antiviral function through cytokines and lysis of infected cells. Because hepatocytes are susceptible to noncytolytic mechanisms of viral clearance, CD8 T-cell antiviral efficiency against hepatotropic viruses has been linked to their capacity to produce gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha). On the other hand, intrahepatic cytokine production triggers the recruitment of mononuclear cells, which sustain acute and chronic liver damage. Using virus-specific CD8 T cells and human hepatocytes, we analyzed the modulation of virus-specific CD8 T-cell function after recognition peptide-pulsed or virally infected hepatocytes. We observed that hepatocyte antigen presentation was generally inefficient, and the quantity of viral antigen strongly influenced CD8 T-cell antiviral function. High levels of hepatitis B virus production induced robust IFN-gamma and TNF-alpha production in virus-specific CD8 T cells, while limiting amounts of viral antigen, both in hepatocyte-like cells and naturally infected human hepatocytes, preferentially stimulated CD8 T-cell degranulation. Our data document a mechanism where virus-specific CD8 T-cell function is influenced by the quantity of virus produced within hepatocytes.

Hepatitis B virus promotes hepatocarcinogenesis in transgenic mice

HBV is a major risk factor for hepatocellular carcinoma (HCC). However, whether HBV can directly cause HCC or only indirectly via the induction of chronic liver inflammation has been controversial. By using transgenic mice carrying the entire HBV genome as a model, we now demonstrate that HBV by itself is an inefficient carcinogen. However, it can efficiently promote hepatocarcinogenesis initiated by the carcinogen diethylnitrosamine (DEN). This effect of HBV does not involve chronic liver inflammation, is apparently due to enhanced hepatocellular apoptosis and compensatory regeneration following DEN treatment, and does not require the HBV X protein.

CONCLUSION

Our results demonstrate a direct role of HBV in a hepatocarcinogenesis pathway that involves the interaction between this virus and a dietary carcinogen.

Cellular and molecular mechanisms of liver tolerance

The liver exhibits a distinctive form of immune privilege, termed liver tolerance, in which orthotopic liver transplantation results in systemic donor-specific T-cell tolerance, while antigens introduced either into hepatocytes or via the portal vein also cause tolerance. Here we argue that the fundamental mechanism driving liver tolerance is likely to be the continuous exposure of diverse liver cell types to endotoxin, derived from the intestinal bacteria. This exposure promotes the expression of a set of cytokines, antigen-presenting molecules, and costimulatory signals that impose T-cell inactivation, partly via effects on liver antigen-presenting cells. The evidence favors clonal deletion mechanisms and is consistent with a role for regulatory T cells but does not support either anergy or immune deviation as important factors in liver tolerance.

Liver-infiltrating lymphocytes in chronic human hepatitis C virus infection display an exhausted phenotype with high levels of PD-1 and low levels of CD127 expression

The majority of people infected with hepatitis C virus (HCV) fail to generate or maintain a T-cell response effective for viral clearance. Evidence from murine chronic viral infections shows that expression of the coinhibitory molecule PD-1 predicts CD8+ antiviral T-cell exhaustion and may contribute to inadequate pathogen control. To investigate whether human CD8+ T cells express PD-1 and demonstrate a dysfunctional phenotype during chronic HCV infection, peripheral and intrahepatic HCV-specific CD8+ T cells were examined. We found that in chronic HCV infection, peripheral HCV-specific T cells express high levels of PD-1 and that blockade of the PD-1/PD-L1 interaction led to an enhanced proliferative capacity. Importantly, intrahepatic HCV-specific T cells, in contrast to those in the periphery, express not only high levels of PD-1 but also decreased interleukin-7 receptor alpha (CD127), an exhausted phenotype that was HCV antigen specific and compartmentalized to the liver, the site of viral replication.

Immunopathogenesis of hepatitis B virus infection

Hepatitis B virus (HBV) infection is a non-cytopathic hepatotropic virus that can lead to severe liver disease including acute hepatitis, cirrhosis and hepatocellular carcinoma. Successful clearance of the virus as well as the establishment of liver disease is largely driven by a complex interaction between the virus and the host immune response. In this review, the immunological events, including both the innate and adaptive immune response are discussed in the setting of both acute and chronic HBV infection and liver disease.

The immune response during hepatitis B virus infection

Hepatitis B virus (HBV) is a major cause of chronic liver inflammation worldwide. Recent knowledge of the virological and immunological events secondary to HBV infection has increased our understanding of the mechanisms involved in viral clearance and persistence. In this review, how the early virological and immunological events might influence the development of a coordinate activation of adaptive immunity necessary to control HBV infection is analysed. The mechanism(s) by which high levels of viral antigens, liver immunological features, regulatory cells and dendritic cell defects might maintain the HBV-specific immunological collapse, typical of chronic hepatitis B patients, is also examined.

Transient inhibition of Th1-type cytokine production by CD4 T cells in hepatitis B core antigen immunized mice is mediated by regulatory T cells

The non-cytopathic hepatitis B virus (HBV) can induce chronic infections characterized by weak and limited T cell responses against the virus. The factors contributing to the failure to clear HBV and subsequent development of chronic HBV infections are not clearly understood, but a strong interferon-gamma (IFN-gamma) response by CD4+ T cells against the nucleocapsid hepatitis B core antigen (HBcAg) of the virus appears to be important for viral clearance. The present study documents depressed numbers of CD4+ T cells secreting IFN-gamma and interleukin-2 (IL-2) in enzyme-linked immunospot assay (ELISPOT) assays restimulated for 24 hr with antigen following both primary and secondary immunizations of mice with recombinant hepatitis B core antigen (rHBcAg). The kinetics of these responses showed that the depression occurred following a peak response and lasted approximately 2 weeks before returning to the previous peak levels. The depression was abrogated by depletion of CD25+ cells prior to culture in the ELISPOT assay, suggesting inhibition by regulatory T cells. This inhibition of IFN-gamma and IL-2 production was also reversed by in vitro restimulation of the test cells for 48 hr rather than 24 hr in the assay. No such transient, reversible inhibition was detected in the production of IL-5, a Th2-type cytokine. The inhibition in cytokine production did not appear to correlate with the number of antibody-secreting cells or the isotypes produced. This delay by regulatory T cells of Th1-type cytokine production could contribute to viral persistence in chronic HBV infection by interfering with the critical role IFN-gamma plays in protection against viral infections.

Reviving exhausted T lymphocytes during chronic virus infection by B7-H1 blockade

Cytotoxic T lymphocytes (CTLs) are killer cells that are crucial in the control of viral pathogens and cancers. They can become exhausted during chronic viral infection, a phenomenon that consists of a reduction in both number and functionality of CTLs. Recently, Barber and colleagues demonstrated that B7-H1 (also called PD-L1), a cell-surface molecule that is widely distributed in tissues, was necessary for the maintenance of T-cell exhaustion in a chronic-infection mouse model of lymphocytic choriomeningitis virus (LCMV). PD-1, the receptor of B7-H1, was greatly upregulated on CTLs in response to LCMV, and its expression was maintained during chronic infection. Blockade of the B7-H1-PD-1 pathway by a monoclonal antibody restored CTL function and reduced viral burden. These results suggest a new strategy for the treatment of chronic viral infection.

Expression of the interleukin-7 receptor alpha chain (CD127) on virus-specific CD8+ T cells identifies functionally and phenotypically defined memory T cells during acute resolving hepatitis B virus infection

Virus-specific CD8+ T cells play a central role in the outcome of several viral infections, including hepatitis B virus (HBV) infection. A key feature of virus-specific CD8+ T cells is the development of memory. The mechanisms resulting in the establishment of T-cell memory are still only poorly understood. It has been suggested that T-cell memory may depend on the survival of virus-specific CD8+ T cells in the contraction phase. Indeed, a population of effector cells that express high levels of the interleukin-7 receptor alpha chain (CD127) as the precursors of memory CD8+ T cells has recently been identified in mice. However, very little information is currently available about the kinetics of CD127 expression in an acute resolving viral infection in humans and its association with disease pathogenesis, viral load, and functional and phenotypical T-cell characteristics. To address these important issues, we analyzed the HBV-specific CD8+ T-cell response longitudinally in a cohort of six patients with acute HBV infection who spontaneously cleared the virus. We observed the emergence of CD127 expression on antigen-specific CD8+ memory T cells during the course of infection. Importantly, the up-regulation of CD127 correlated phenotypically with a loss of CD38 and PD-1 expression and acquisition of CCR7 expression: functionally with an enhanced proliferative capacity and clinically with the decline in serum alanine aminotransferase levels and viral clearance. These results suggest that the expression of CD127 is a marker for the development of functionally and phenotypically defined antigen-specific CD8+ memory T cells in cleared human viral infections.

Rejuvenating exhausted T cells during chronic viral infection

In a recent paper in Nature, show that the immunoreceptor PD-1 is upregulated by "exhausted" T cells during the chronic phase of viral infection in mice. Remarkably, blocking the interaction between PD-1 and its ligand, PD-L1, reactivates these T cells and reduces viral load.

Pathogenetic and antiviral immune responses against hepatitis B virus

Hepatitis B virus (HBV) is a noncytopathic virus that causes liver disease of variable duration and severity. It is widely assumed that during HBV infection the host immune response is responsible for both hepatocellular damage and viral clearance. Whereas there is considerable evidence that the innate immune response does not play a significant role in these processes, the adaptive immune response, particularly virus-specific cytotoxic T lymphocytes (CTLs), seems to contribute to nearly all of the liver injury associated with HBV infection. By killing infected cells and producing antiviral cytokines capable of purging HBV from viable hepatocytes, CTLs are also thought to eliminate the virus. Although liver damage is initiated and mediated by the CTLs, antigen-nonspecific inflammatory cells can worsen CTL-induced immunopathology and platelets may facilitate the accumulation of CTLs in the liver. The mechanisms responsible for disease pathogenesis and viral clearance during HBV infection are the subject of this review.

The liver as an immunological organ

The liver is a unique anatomical and immunological site in which antigen-rich blood from the gastrointestinal tract is pressed through a network of sinusoids and scanned by antigen-presenting cells and lymphocytes. The liver's lymphocyte population is selectively enriched in natural killer and natural killer T cells which play critical roles in first line immune defense against invading pathogens, modulation of liver injury and recruitment of circulating lymphocytes. Circulating lymphocytes come in close contact to antigens displayed by endothelial cells, Kupffer cells and liver resident dendritic cells in the sinusoids. Circulating lymphocytes can also contact hepatocytes directly, because the sinusoidal endothelium is fenestrated and lacks a basement membrane. This unique anatomy of the liver may facilitate direct or indirect priming of lymphocytes, modulate the immune response to hepatotrophic pathogens and contribute to some of the unique immunological properties of this organ, particularly its capacity to induce antigen-specific tolerance.

Restoring function in exhausted CD8 T cells during chronic viral infection

Functional impairment of antigen-specific T cells is a defining characteristic of many chronic infections, but the underlying mechanisms of T-cell dysfunction are not well understood. To address this question, we analysed genes expressed in functionally impaired virus-specific CD8 T cells present in mice chronically infected with lymphocytic choriomeningitis virus (LCMV), and compared these with the gene profile of functional memory CD8 T cells. Here we report that PD-1 (programmed death 1; also known as Pdcd1) was selectively upregulated by the exhausted T cells, and that in vivo administration of antibodies that blocked the interaction of this inhibitory receptor with its ligand, PD-L1 (also known as B7-H1), enhanced T-cell responses. Notably, we found that even in persistently infected mice that were lacking CD4 T-cell help, blockade of the PD-1/PD-L1 inhibitory pathway had a beneficial effect on the 'helpless' CD8 T cells, restoring their ability to undergo proliferation, secrete cytokines, kill infected cells and decrease viral load. Blockade of the CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) inhibitory pathway had no effect on either T-cell function or viral control. These studies identify a specific mechanism of T-cell exhaustion and define a potentially effective immunological strategy for the treatment of chronic viral infections.