Quantitative analysis of the peripheral blood cytotoxic T lymphocyte response in patients with chronic hepatitis C virus infection

Effect of interferon-alpha therapy on epitope-specific cytotoxic T lymphocyte responses in hepatitis C virus-infected individuals

The majority of hepatitis C virus (HCV)-infected individuals fail to resolve the infection and become chronically infected despite the presence of HCV-specific CTL responses directed to different HCV-derived peptide antigens. Only a minority of individuals is able to clear the virus by mounting efficient CTL responses early after acute infection, but at present it is not clear whether viral clearance is associated with CTL responses of defined specificity. To elucidate those responses associated with improvement of the disease, we analyzed CTL responses to 16 different HLA-A2-presented, HCV-derived epitopes in 12 chronically infected patients, 14 chronically infected patients treated with interferon-alpha, and in one patient with acute symptomatic disease. We show here that the majority of chronically infected individuals present CTL responses directed to an NS4-derived peptide antigen (amino acids 1789-1797). Treated patients presented stronger HCV-specific CTL responses and therapy-induced changes in CTL target choice. In particular, 13 out of 14 individuals responded to an NS3-derived epitope (amino acids 1073-1081). By longitudinal analysis we show that five individuals responding to IFN-alpha therapy with decreases in alanine aminotransferase levels presented a strong CTL activity directed to the NS3-derived epitope. One patient that spontaneously resolved the infection presented a generally strong CTL activity specific for HCV-derived epitopes with a dominant response to the NS3-derived peptide antigen. This suggests that CTL responses directed to this NS3-derived antigen may be beneficial for the control of HCV infection. Improvement of these responses may represent a therapeutic intervention in chronic HCV infection.

Quantitative and functional differences in CD8+ lymphocyte responses in resolved acute and chronic hepatitis C virus infection

CD8+ T lymphocyte responses are important in the clearance of viral infections. In chronic infections they may contribute to pathogenesis. To investigate the role of CD8+ T lymphocyte responses in viral clearance and chronic hepatitis C we have compared hepatitis C virus (HCV) specific cytotoxicity and interferon-gamma (IFN-gamma) production in patients with resolved-acute, and chronic HCV infection. CD8+ T cell responses to a panel of 13 HCV T cell peptide epitopes were studied using Elispot assays of IFN-gamma production and chromium release cytotoxicity assays. Responses of seven patients with resolved acute HCV infection were compared with those of 14 chronically infected patients. HCV-specific cytotoxicity differentiated the two populations of patients. The majority (71%) of patients with resolved acute infection tested positive to 42% of relevant peptides compared with the minority (28%) of patients with chronic hepatitis C (P=0.03) who responded to only 8% of relevant peptides (P=0.0009). In contrast, HCV-specific IFN-gamma production was detected in 86% of patients with either resolved or chronic infection in response to 42% and 35%, respectively, of relevant peptides tested (not significant). In patients with chronic infection the magnitude of the HCV-specific IFN-gamma production was inversely correlated to viral load (R2=0.52; P=0.042). Failure to clear HCV infection may be attributable to the presence of noncytolytic IFN-gamma producing CD8+ T lymphocytes in chronically infected patients. However these CD8+ T cells may play a beneficial role in contributing to the control of viral load in chronic hepatitis C.

Immunopathogenesis of hepatitis C virus infection

Hepatitis C virus, a recently identified member of the family Flaviviridae, is an important cause of chronic viral hepatitis and cirrhosis. There are similarities in the nature of the immune response to this pathogen with immunity in other flavivirus and hepatotropic virus infections, such as hepatitis B. However, the high rate of viral persistence after primary hepatitis C infection, and the observation that neutralizing antibodies are not protective, would suggest that there are a number of important differences between hepatitis C, other flaviviruses, and hepatitis B. The phenomenon of quasispecies evolution and other viral factors have been proposed to contribute to immune evasion by hepatitis C virus. In the face of established persistent infection, virus-specific cytotoxic T lymphocytes may exert some control over viral replication. However, these same effectors may also be responsible for the progressive liver damage characteristic of chronic hepatitis C infection. The nature of protective immunity, including the role of innate immune responses early after hepatitis C exposure, remains to be defined.

Cross-reactivity between hepatitis C virus and Influenza A virus determinant-specific cytotoxic T cells

The cellular immune response contributes to viral clearance as well as to liver injury in acute and chronic hepatitis C virus (HCV) infection. An immunodominant determinant frequently recognized by liver-infiltrating and circulating CD8(+) T cells of HCV-infected patients is the HCV(NS3-1073) peptide CVNGVCWTV. Using a sensitive in vitro technique with HCV peptides and multiple cytokines, we were able to expand cytotoxic T cells specific for this determinant not only from the blood of 11 of 20 HCV-infected patients (55%) but also from the blood of 9 of 15 HCV-negative blood donors (60%), while a second HCV NS3 determinant was recognized only by HCV-infected patients and not by seronegative controls. The T-cell response of these healthy blood donors was mediated by memory T cells, which cross-reacted with a novel T-cell determinant of the A/PR/8/34 influenza A virus (IV) that is endogenously processed from the neuraminidase (NA) protein. Both the HCV NS3 and the IV NA peptide displayed a high degree of sequence homology, bound to the HLA-A2 molecule with high affinity, and were recognized by cytotoxic T lymphocytes with similar affinity (10(-8) M). Using the HLA-A2-transgenic mouse model, we then demonstrated directly that HCV-specific T cells could be induced in vivo by IV infection. Splenocytes harvested from IV-infected mice at the peak of the primary response (day 7 effector cells) or following complete recovery (day 21 memory cells) recognized the HCV NS3 peptide, lysed peptide-pulsed target cells, and produced gamma interferon. These results exemplify that host responses to an infectious agent are influenced by cross-reactive memory cells induced by past exposure to heterologous viruses, which could have important consequences for vaccine development.

The outcome of hepatitis C virus infection is predicted by escape mutations in epitopes targeted by cytotoxic T lymphocytes

CD8(+) cytotoxic T lymphocytes (CTL) are thought to control hepatitis C virus (HCV) replication and so we investigated why this response fails in persistently infected individuals. The HCV quasispecies in three persistently infected chimpanzees acquired mutations in multiple epitopes that impaired class I MHC binding and/or CTL recognition. Most escape mutations appeared during acute infection and remained fixed in the quasispecies for years without further diversification. A statistically significant increase in the amino acid replacement rate was observed in epitopes versus adjacent regions of HCV proteins. In contrast, most epitopes were intact when hepatitis C resolved spontaneously. We conclude that CTL exert positive selection pressure against the HCV quasispecies and the outcome of infection is predicted by mutations in class I MHC restricted epitopes.

Genetic immunization of wild-type and hepatitis C virus transgenic mice reveals a hierarchy of cellular immune response and tolerance induction against hepatitis C virus structural proteins

To study the effect of genetic immunization on transgenic expression of hepatitis C virus (HCV) proteins, we evaluated the immunological response of HCV transgenic mice to HCV expression plasmids. FVB/n transgenic mice expressing HCV structural proteins (core, E1, and E2) and wild-type (WT) FVB/n mice were immunized intramuscularly with plasmids expressing core (pHCVcore) or core/E1/E2 (pHCVSt). After immunization, HCV-specific humoral and cellular immune response was studied. Both WT and transgenic mice immunized with either HCV construct produced antibodies and exhibited T-cell proliferative responses against core or envelope. In WT mice immunized with pHCVSt, cytotoxic T-lymphocyte (CTL) activities were detected against E2 but not against core or E1, whereas strong CTL activities against core could be detected in WT mice immunized with pHCVcore. In pHCVSt-immunized, transgenic mice, CTL activities against the core or envelope were completely absent, but core-specific CTL activities could be detected in pHCVcore-immunized transgenic mice. A similar pattern of immune responses was also observed in other mouse strains, including a transgenic line expressing human HLA-A2.1 molecules (AAD mice). Despite the presence of a peripheral cellular immunity against HCV, no liver pathology or lymphocytic infiltrate was observed in these transgenic mice. Our study suggests a hierarchy of CTL response against the HCV structural proteins (E2 > core > E1) in vivo when the proteins are expressed as a polyprotein. The HCV transgenic mice can be induced by DNA immunization to generate anti-HCV antibodies and anticore CTLs. However, they are tolerant at the CTL level against the E2 protein despite DNA immunization.

Determinants of viral clearance and persistence during acute hepatitis C virus infection

The virological and immunological features of hepatitis C virus (HCV) infection were studied weekly for 6 months after accidental needlestick exposure in five health care workers, four of whom developed acute hepatitis that progressed to chronicity while one subject cleared the virus. In all subjects, viremia was first detectable within 1-2 weeks of inoculation, 1 month or more before the appearance of virus-specific T cells. The subject who cleared the virus experienced a prolonged episode of acute hepatitis that coincided with a CD38+ IFN-gamma- CD8+ T cell response to HCV and a small reduction in viremia. Subsequently, a strong CD4+ T cell response emerged and the CD8+ T cells became CD38- and started producing IFN-gamma in response to HCV, coinciding with a rapid 100,000-fold decrease in viremia that occurred without a corresponding surge of disease activity. Chronic infection developed in two subjects who failed to produce a significant T cell response and in two other subjects who initially mounted strong CD4+ T cell responses that ultimately waned. In all subjects, viremia was higher at the peak of acute hepatitis than it was when the disease began, and the disease improved during the viremia. These results provide the first insight into the host-virus relationship in humans during the incubation phase of acute HCV infection, and they provide the only insight to date into the virological and immunological characteristics of clinically asymptomatic acute HCV infection, the commonest manifestation of this disease. In addition, the results suggest that the vigor and quality of the antiviral T cell response determines the outcome of acute HCV infection, that the ability of HCV to outpace the T cell response may contribute to its tendency to persist; that the onset of hepatitis coincides with the onset of the CD8+ T cell response, that disease pathogenesis and viral clearance are mediated by different CD8+ T cell populations that control HCV by both cytolytic and noncytolytic mechanisms, and that there are different pathways to viral persistence in asymptomatic and symptomatic acute HCV infection.

Hepatitis C virus core protein inhibits human T lymphocyte responses by a complement-dependent regulatory pathway

Complement proteins are involved in early innate immune responses against pathogens and play a role in clearing circulating viral Ags from the blood of infected hosts. We have previously demonstrated that hepatitis C virus (HCV) core, the first protein to be expressed and circulating in the blood of infected individuals, inhibited human T cell proliferative response through interaction with the complement receptor, globular domain of C1q receptor (gC1qR). To investigate the mechanisms of HCV core/gC1qR-induced inhibition of T cell proliferation, we examined the effect of core protein on the early events in T cell activation. We found that HCV core inhibited phosphorylation of extracellular signal-regulated kinase (ERK) and mitogen-activated ERK kinase (MEK). HCV core-induced impairment of ERK/MEK mitogen-activated protein kinase resulted in the inhibition of IL-2 and IL-2Ralpha gene transcription, which led to the inhibition of IL-2 production and high-affinity IL-2R expression. Importantly, the ability of anti-gC1qR Ab treatment to reverse HCV core-induced inhibition of ERK/MEK phosphorylation reveals that the interaction between HCV core and gC1qR is linked to the interference of ERK/MEK mitogen-activated protein kinase activation. These results imply that HCV core-induced blockage of intracellular events in T cell activation by a complement-dependent regulatory pathway may play a critical role in the establishment of HCV persistence during the acute phase of viral infection.

Direct ex vivo comparison of the breadth and specificity of the T cells in the liver and peripheral blood of patients with chronic HCV infection

The role of intrahepatic lymphocytes in the control of hepatitis C virus (HCV) infection and the pathology associated with it is not understood; most studies of the immunology of this infection use peripheral blood lymphocyte populations. To address this further, we examined in detail the IHL from HCV-infected patients and controls, focusing on the antigen-specific CD8(+) T lymphocyte component. Individual T cells from needle liver biopsies and peripheral blood were isolated from patients with chronic HCV infection and examined directly ex vivo. We used RT-PCR spectratyping to compare the breadth of the T cell receptor usage in the liver in comparison with the peripheral blood, and applied MHC class I tetramer technology to investigate the numbers of HCV-specific CD8(+) cells in the two compartments. T cell receptor usage in the liver of HCV-infected patients was broad, comparable with that in the peripheral blood of the same patients. A much higher proportion of liver CD8(+) cells expressed receptors specific for HCV antigens compared with paired peripheral blood CD8(+) cells. A greater proportion of the liver tetramer-positive cells expressed the activation marker CD69, compared with those in the periphery or other CD8(+) cells in the liver. In the course of chronic HCV infection, HCV-specific CD8 cells, which have been recently activated, appear to accumulate specifically in the livers of infected patients but are present in much lower numbers in the peripheral circulation. Further studies are needed to determine the function of these cells and their role in protection and immunopathology.

Quantitative analysis of specific Th1/Th2 helper cell responses and IgG subtype antibodies in interferon-alpha-treated patients with chronic hepatitis C

This study aimed to characterise the immune mechanisms relevant to viral clearance in interferon (IFN)-alpha-treated chronic hepatitis C virus (HCV) infection. Proliferative responses of peripheral blood mononuclear cells from sustained complete IFN-alpha therapy responders (n = 8), nonresponders (n = 13), untreated patients (n = 10), and healthy controls (n = 5) were measured retrospectively upon stimulation with recombinant HCV-antigens (core, helicase, NS3, NS4, and NS5) and the secretion of IFN-gamma and interleukins (IL-4, IL-5, IL-10, and IL-12) were tested by ELISA. Furthermore, IFN-gamma as well as IL-10 secreting CD4+ T cells were quantitated by intracellular cytokine staining. Anti-HCV core and NS3-specific IgG subclass antibodies were quantitated in the corresponding patient sera. Sustained therapy responders had more frequent and stronger NS3 and helicase-specific cellular immune responses than nonresponders, untreated HCV patients and healthy controls. Independent from therapy outcome HCV-stimulated T cells in IFN-alpha treated patients secreted preferentially IFN-gamma The Th2 cytokines IL-4 and IL-10 were even decreased in nonresponders, while the IL-12 secretion was not influenced. With respect to the humoral immune response sustained complete responders showed significantly reduced IFN-gamma independent anti-HCV-core and -NS3 IgG1 antibody synthesis. In conclusion, vigorous NS3-specific T-helper cell responses were associated with viral clearance in IFN-alpha recipients; however, the cytokine and antibody analysis argues against a Th1/Th2 imbalance as a major factor that influence the therapy outcome.

Sustained dysfunction of antiviral CD8+ T lymphocytes after infection with hepatitis C virus

Hepatitis C virus (HCV) sets up persistent infection in the majority of those exposed. It is likely that, as with other persistent viral infections, the efficacy of T-lymphocyte responses influences long-term outcome. However, little is known about the functional capacity of HCV-specific T-lymphocyte responses induced after acute infection. We investigated this by using major histocompatibility complex class I-peptide tetrameric complexes (tetramers), which allow direct detection of specific CD8+ T lymphocytes ex vivo, independently of function. Here we show that, early after infection, virus-specific CD8+ T lymphocytes detected with a panel of four such tetramers are abnormal in terms of their synthesis of antiviral cytokines and lytic activity. Furthermore, this phenotype is commonly maintained long term, since large sustained populations of HCV-specific CD8+ T lymphocytes were identified, which consistently had very poor antiviral cytokine responses as measured in vitro. Overall, HCV-specific CD8+ T lymphocytes show reduced synthesis of tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma) after stimulation with either mitogens or peptides, compared to responses to Epstein-Barr virus and/or cytomegalovirus. This behavior of antiviral CD8+ T lymphocytes induced after HCV infection may contribute to viral persistence through failure to effectively suppress viral replication.

Sustained dysfunction of antiviral CD8+ T lymphocytes after infection with hepatitis C virus

Hepatitis C virus (HCV) sets up persistent infection in the majority of those exposed. It is likely that, as with other persistent viral infections, the efficacy of T-lymphocyte responses influences long-term outcome. However, little is known about the functional capacity of HCV-specific T-lymphocyte responses induced after acute infection. We investigated this by using major histocompatibility complex class I-peptide tetrameric complexes (tetramers), which allow direct detection of specific CD8+ T lymphocytes ex vivo, independently of function. Here we show that, early after infection, virus-specific CD8+ T lymphocytes detected with a panel of four such tetramers are abnormal in terms of their synthesis of antiviral cytokines and lytic activity. Furthermore, this phenotype is commonly maintained long term, since large sustained populations of HCV-specific CD8+ T lymphocytes were identified, which consistently had very poor antiviral cytokine responses as measured in vitro. Overall, HCV-specific CD8+ T lymphocytes show reduced synthesis of tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma) after stimulation with either mitogens or peptides, compared to responses to Epstein-Barr virus and/or cytomegalovirus. This behavior of antiviral CD8+ T lymphocytes induced after HCV infection may contribute to viral persistence through failure to effectively suppress viral replication.

Immunogenicity of the E1E2 proteins of hepatitis C virus expressed by recombinant adenoviruses

The E1 and E2 proteins of hepatitis C virus (HCV) are believed to be the viral envelope glycoproteins that are major candidate antigens for HCV vaccine development. We reported previously that the replication-competent recombinant adenovirus encoding core-E1-E2 genes of HCV (Ad/HCV) produces serologically reactive E1 and E2 proteins forming a heterodimer in substantial amounts. Here, we examined immunogenicity of the E1E2 proteins copurified from HeLa cells infected with Ad/HCV virus in mice. Furthermore, we constructed a replication-defective recombinant adenovirus encoding the core-E1-E2 genes of HCV (Ad.CMV.HCV) and examined immunogenicity of the virus in mice. The mice immunized intraperitoneally with the copurified E1E2 proteins induced mainly antibodies to E2, but not to E1 by Western blot analysis. The sera of mice immunized with the E1E2 inhibited the binding of E2 protein to the major extracellular loop of human CD81. E2-specific cytotoxic T cells (CTLs), but not antibodies to the E1E2 antigens were induced in the mice intramuscularly immunized with Ad.CMV.HCV virus. When immunized with both Ad.CMV.HCV virus and the E1E2, mice elicited E2-specific CTLs and antibodies to the E1E2 antigens. The results suggest that immunization of Ad.CMV.HCV virus combined with E2 protein is an effective modality to induce humoral as well as cellular immune response to E2 antigen.

Lack of evidence for the Th2 predominance in patients with chronic hepatitis C

A T helper (Th)1 to Th2 shift has been proposed to be a critical pathogenic determinant in chronic hepatitis C. Here, we evaluated mitogen-induced and hepatitis C virus (HCV) core antigen-induced cytokine production in 28 patients with biopsy-proven chronic hepatitis C. Flow cytometry demonstrated that after mitogenic stimulation the percentage of Th2 cells (IL-4 + or IL-13 +) and Th0 cells (IFN-gamma/IL-4 + or IL-2/IL-13 +) did not differ between patients and controls. In contrast, the percentage of Th1 cells (IFN-gamma + or IL-2 +) was significantly increased in CD4 +, CD8 +, 'naive'-CD45RA + and 'memory'-CD45RO + T-cell subsets from patients versus controls. Similar results were obtained by ELISA testing supernatants from mitogen-stimulated, unfractionated peripheral blood mononuclear cell (PBMC) cultures. Interferon-alpha treatment was associated with a reduction in the mitogen-induced Th1 cytokine response in those patients who cleared their plasma HCV-RNA. Analysis of cytokine expression by CD4 + T cells after HCV core antigen stimulation in a subgroup of 13 chronic hepatitis C patients demonstrated no cytokine response in 74% of these patients and an IFN-gamma-restricted response in 26%. Finally, no Th2 shift was found in lipopolysaccharide-stimulated monocytes. These data indicate that a Th1 to Th2 shift does not occur in chronic hepatitis C.

Impaired allostimulatory function of dendritic cells in chronic hepatitis C infection

BACKGROUND & AIMS

Dendritic cells (DC), which play an essential role in the triggering of primary antiviral immune reactions, may also contribute, in some viral models, to the propagation of viral infection and the pathogenesis of viral disease. During natural infection with hepatitis C virus (HCV), the interactions between the virus and DC may contribute to viral persistence, a general feature of HCV infection.

METHODS

We compared the phenotypical and biological functions of monocyte-derived DC from patients with chronic hepatitis C (HCV-DC; n = 6), seronegative individuals (naive-DC; n = 8), long-term responders to antiviral therapy (LTR-DC; n = 8), and a group of patients with non-HCV-hepatic disorders (n = 11). The presence and the nature of HCV sequences during the DC cultures was assessed by reverse transcription-polymerase chain reaction and the analysis of the viral quasispecies distribution.

RESULTS

Although HCV-DC displayed a normal morphology, phenotype, and capacity to capture antigen, their ability to stimulate the proliferation of allogeneic T cells was dramatically impaired in comparison with naive-DC (P = 0.0013). Mixing experiments revealed that HCV-DC did not affect the proliferation of T cells induced by naive-DC. Remarkably, the allostimulatory function of LTR-DC or DC from patients with non-HCV-hepatic disorders did not show any impairment. The presence of HCV genomic sequences could be documented for 5 of 6 HCV carriers either in the cells and/or the supernatants of the DC cultures. The presence of HCV sequences was found in the DC cultures from one patient showing a dramatic allostimulation defect. For that patient, extensive analysis of the viral quasispecies distribution revealed the presence, in the DC cultures, of genomic sequences of a unique nature, distinct from those identified in the patient's mononuclear cells, serum, or liver.

CONCLUSIONS

Overall, these results indicate that chronic infection by HCV is associated with an allostimulatory defect of monocyte-derived DC, possibly because these cells constitute an extrahepatic reservoir for the virus. Although the exact mechanism responsible for such an alteration remains to be unraveled, our observations argue against an active immunosuppression-based mechanism.

Detection of diverse hepatitis C virus (HCV)-specific cytotoxic T lymphocytes in peripheral blood of infected persons by screening for responses to all translated proteins of HCV

Broadly directed hepatitis C virus (HCV)-specific cytotoxic T lymphocytes (CTL) have been identified from liver-infiltrating lymphocytes but have been more difficult to assess in peripheral blood of infected persons. To enhance the detection of CTL from peripheral blood mononuclear cells (PBMC), we cocultured PBMC with autologous Epstein-Barr virus-transformed B-lymphoblastoid cell lines that had been infected with recombinant vaccinia virus constructs so that they expressed the entire translated polyprotein of HCV-H, a type 1a strain. These stimulated cells from HCV-infected as well as exposed seronegative persons were then cloned at limiting dilution and tested for HCV-specific CTL activity using a standard (51)Cr release assay. HCV-specific CTL were detected in PBMC from seven of nine persons with chronic hepatitis, including five of seven in whom CTL had previously been detected from liver biopsy specimens but not PBMC. In a single person with chronic HCV infection, CTL directed against as many as five different epitopes were detected in peripheral blood and were similar in specificity to those detected in liver tissue. This technique was used to evaluate eight subjects identified to be at high risk for HCV exposure due to continued injection drug abuse; no evidence of CTL in PBMC was found. We conclude that CTL can be detected in PBMC from the majority of persons with chronic HCV infection but are present at lower levels or absent in exposed but persistently seronegative persons. The high degree of concordance of HCV epitopes identified from liver and PBMC suggests that this strategy is a reasonable alternative to liver biopsy for characterizing the CTL response to HCV in chronically infected persons.

Possible role of cytotoxic T cells in acute liver injury in hepatitis C virus cDNA transgenic mice mediated by Cre/loxP system

A line of hepatitis C virus (HCV) transgenic mice was established previously that was mediated by Cre/loxP system using HCV cDNA, including core, E1, E2 and NS2 genes. Intravenous infection of a recombinant adenovirus that expresses Cre DNA recombinase (AxCANCre) induced HCV structural protein expression in the liver of transgenic mice. HCV core protein production and transgene recombination in the mouse liver were serially evaluated after AxCANCre infusion. Core proteins were expressed efficiently and transgene was almost completely recombined in the liver of mice after 3 days and then the levels of both core protein production and transgene recombination decreased continuously for 28 days. However, 30.6% of the transgene recombination remained at 28 days and only 2.7% of core production remained at 28 days after infection. Compared with nontransgenic controls, the serum alanine aminotransferase levels in transgenic mice were significantly higher 10, 14, and 21 days after adenovirus infection. Histological scoring also indicated severe pathological changes in the liver of transgenic mice after adenovirus infection. AxCANCre infusion increased CD8+ lymphocyte infiltration into the liver of transgenic mice compared with that of non-transgenic controls. Furthermore, cytotoxic T lymphocytes (CTLs) isolated from transgenic mice during liver injury were specific for the HCV proteins. These results suggest that HCV structural proteins expressed in the liver of transgenic mice enhanced liver injury. HCV-specific CTLs may be to enhance hepatitis. Thus, the present HCV transgenic mouse model provides a useful model of liver injury due to HCV, and the host immune response may play a pivotal role(s) in the pathogenesis of HCV.

DNA vaccination: a potential weapon against infection and cancer

DNA vaccination is a novel approach for inducing immunity against target antigens. It provides a direct link between identification of genes encoding these antigens and incorporation of the gene sequences into a vaccine vehicle. Identification of candidate genes is proceeding very rapidly both for infectious organisms and for cancer cells. One advantage is that DNA appears to activate all pathways of immunity, especially cytotoxic T-cell responses, which have been difficult to induce with protein vaccines. For viruses, including those which have caused problems for blood transfusion, DNA vaccination could be used for prevention. However, for chronic infection, or for cancer, vaccination will be performed in a therapeutic setting. For this situation, it is probable that immune-activating sequences will have to be included in the vaccine. The ease of manipulation of gene sequences, together with the increasing knowledge of the operation of the immune system, means that we now have the tools to take vaccines into the next exciting stage of development.

Selection of and evasion from cytotoxic T cell responses in the central nervous system

Cytotoxic CD8 T lymphocytes (CTLs) are critical for the clearance of noncytopathic viruses from infected cells. This chapter discusses one mechanism used by viruses to persist—namely, the selection of a variant virus in which changes in the sequence of a CTL epitope abrogate recognition. The unique features of cytotoxic CD8 T cell function in the central nervous system (CNS) are discussed. The role of CTL escape mutants in the viral evasion of the immune system and subsequent disease progression in non-CNS infections are summarized. The immune response in the CNS is similar to the response in extraneural tissue, but several aspects of the activation of the immune response, cellular trafficking, and antigen presentation are unique to the CNS. Although the CNS has classically been considered a site of immune privilege, surveillance of the normal CNS by circulating, activated lymphocytes occurs, with a limited number of lymphocytes being present in the normal CNS at any given time. In mice infected with mouse hepatitis virus and in some humans persistently infected with human immunodeficiency virus type1, hepatitis B virus or hepatitis C virus, CTL escape mutants play an important role in virus amplification and disease progression.

Transcriptional activation of the interleukin-2 promoter by hepatitis C virus core protein

Most patients infected with hepatitis C virus (HCV) become chronic carriers. Viruses that efficiently establish persistent infections must have effective ways of evading host defenses. In the case of HCV, little is known about how chronic infections are established or maintained. Besides hepatocytes, several reports suggest that HCV can infect T and B lymphocytes. Since T cells are essential for viral clearance, direct or indirect effects of HCV on T-cell function could influence the outcome of infection. Given that T-cell growth and differentiation require the cytokine interleukin 2 (IL-2), we asked whether HCV might modulate synthesis of IL-2. Portions of the HCV polyprotein were expressed in Jurkat cells under a variety of conditions. We found that the highly conserved HCV core protein, in combination with other stimuli, was able to dramatically activate transcription from the IL-2 promoter. The carboxy-terminal hydrophobic portion of the core protein was required for this activity. Activation was dependent on nuclear factor of activated T cells (NFAT), occurred in cells deficient in the tyrosine kinase p56(lck), and could be blocked by addition of cyclosporin A and by depletion of calcium. These results suggest that the HCV core protein can activate transcription of the IL-2 promoter through the NFAT pathway. This novel activity may have consequences for T-cell development and establishment of persistent infections.

Viral, host and interferon-related factors modulating the effect of interferon therapy for hepatitis C virus infection

The estimated prevalence of hepatitis C virus infection in the US is approximately 1.8%. Although interferon monotherapy and combination therapy of interferon with ribavirin represent mainstay for treating HCV infection, the rate of sustained virologic response remains suboptimal. The growing evidence suggested that the clinical sequence and treatment response of chronic hepatitis C are determined by a dynamic, complex tripartite relationship among HCV infection, the host immune response, and the effect of different interferon regimens. The treatment response is associated with various viral factors including the pretreatment viral level, dynamic change of viral level during treatment, viral genotype quasispecies and nucleotide mutation in nonstructural protein 5A of hepatitis C virus. Host factors that may affect treatment response include age, gender, race, HLA alleles and the host immune responses. Interferon regimens, including type, dose, frequency and duration of treatment and combination of interferon with other anti-HCV agents also alter the therapeutic response. Understanding these complicated interaction may provide better insights into the mechanism(s) of interferon response, leading to more effective clinical application of interferon therapy.

Possible role of cytotoxic T cells in acute liver injury in hepatitis C virus cDNA transgenic mice mediated by Cre/loxP system

A line of hepatitis C virus (HCV) transgenic mice was established previously that was mediated by Cre/loxP system using HCV cDNA, including core, E1, E2 and NS2 genes. Intravenous infection of a recombinant adenovirus that expresses Cre DNA recombinase (AxCANCre) induced HCV structural protein expression in the liver of transgenic mice. HCV core protein production and transgene recombination in the mouse liver were serially evaluated after AxCANCre infusion. Core proteins were expressed efficiently and transgene was almost completely recombined in the liver of mice after 3 days and then the levels of both core protein production and transgene recombination decreased continuously for 28 days. However, 30.6% of the transgene recombination remained at 28 days and only 2.7% of core production remained at 28 days after infection. Compared with nontransgenic controls, the serum alanine aminotransferase levels in transgenic mice were significantly higher 10, 14, and 21 days after adenovirus infection. Histological scoring also indicated severe pathological changes in the liver of transgenic mice after adenovirus infection. AxCANCre infusion increased CD8+ lymphocyte infiltration into the liver of transgenic mice compared with that of non-transgenic controls. Furthermore, cytotoxic T lymphocytes (CTLs) isolated from transgenic mice during liver injury were specific for the HCV proteins. These results suggest that HCV structural proteins expressed in the liver of transgenic mice enhanced liver injury. HCV-specific CTLs may be to enhance hepatitis. Thus, the present HCV transgenic mouse model provides a useful model of liver injury due to HCV, and the host immune response may play a pivotal role(s) in the pathogenesis of HCV.

The HCV core protein acts as a positive regulator of fas-mediated apoptosis in a human lymphoblastoid T cell line

Hepatitis C virus (HCV) is a major human pathogen causing mild to severe liver disease worldwide and is remarkably efficient at establishing persistent infections. Previously, we have shown that the core protein has an immunomodulatory function including the suppression of T lymphocyte responses to viral infection. To investigate the underlying mechanism for the role of core protein in immune modulation, we examined the effect of core on the sensitivity of the human T cell line, Jurkat, to Fas-mediated apoptosis. The transient and stable expression of core protein in Jurkat cells increased the sensitivity of cells to Fas-mediated apoptosis when compared to control cells expressing vector DNA alone. In addition, we demonstrated that the core protein binds to the cytoplasmic domain of Fas which may enhance the downstream signaling event of Fas-mediated apoptosis. The expression of core protein did not alter the cell surface expression of Fas, indicating that the increased sensitivity of core-expressing cells to Fas ligand was not due to upregulation of Fas. Furthermore, we observed the augmentation of caspase-3 activity in core-expressing cells. These results suggest that the core protein may promote the apoptosis of immune cells during HCV infection via the Fas signaling pathway, thus facilitating HCV persistence.

How can the cellular immune response control hepatitis B virus replication?

In this review we focus on aspects of the virus-specific cellular immune response, although we should point out that all the components of the innate and adaptive immune response are likely to play a role in successful control of hepatitis B virus (HBV) infection. We concentrate particularly on the relevance of the polyclonality and multispecificity of the HBV-specific cytotoxic T cell response to its antiviral activity. In this context, we discuss the possible role of viral escape mutations and highlight evidence from other models of the benefit of multispecificity in antiviral responses. We stress the contribution of CD4 help for effective CD8 responses and raise the possibility that HBV may produce factors inhibiting the antiviral response.

Why do cytotoxic T lymphocytes fail to eliminate hepatitis C virus? Lessons from studies using major histocompatibility complex class I peptide tetramers

Hepatitis C virus (HCV) infection is a major public health problem, affecting an estimated 3% of the world's population, and over 10% in some countries. Infection in most cases becomes persistent, and can lead to hepatic inflammation, fibrosis and liver failure. The T lymphocyte reponse, in particular that mediated by cytotoxic T lymphocytes (CTLs), is likely to be involved in determining the outcome of infection, although its overall role is not clear. The use of major histocompatibility complex (MHC) class I peptide tetrameric complexes (tetramers) to study antiviral CTL responses has revolutionized our approach to the study of human infection. We have used a panel of MHC class I tetramers to analyse immune responses in HCV-infected individuals at various stages of disease. We find that the CTL response against HCV is vigorous in its early phases but dwindles over time both in terms of lymphocyte number and function. A number of potential explanations for this 'CTL failure' are discussed.

Vigorous peripheral blood cytotoxic T cell response during the acute phase of hepatitis C virus infection

After infection by hepatitis C virus (HCV), a minority of patients develop acute symptomatic disease and some of them are able to clear the virus. In this study, we analyzed peripheral blood mononuclear cells from nine patients with acute symptomatic disease with respect to their cytotoxic T lymphocyte (CTL) response using a panel of HCV-derived peptides in a semiquantitative secondary in vitro culture system. We could detect early CTL responses in 67% of these patients. The CTL responses were directed against multiple viral epitopes, in particular within the structural (core 2-9, core 35-44, core 131-140, and core 178-187) and nonstructural regions of the virus (NS3 1073-1081, NS3 1406-1415, NS4 1807-1816, NS5 2252-2260, and NS5B 2794-2802). We compared the CTL responses displayed by recently and chronically infected HLA-A2-positive patients. Virus-specific CTLs were detectable in chronic carriers but the percentage of positive peptide-specific CTL responses was significantly higher in recently infected patients (P = 0.002). Follow-up of recently infected patients during subsequent disease development showed a significant decrease in the values and proportions of positive peptide-specific CTL responses (P = 0.002 and 0.013, respectively). Patients with limited viral replication exhibited significantly more vigorous early responses (P = 0.024). These data suggest a protective role for the early antiviral CTL response in HCV infection.

Analysis of successful immune responses in persons infected with hepatitis C virus

Although hepatitis C virus (HCV) infection is very common, identification of patients during acute infection is rare. Consequently, little is known about the immune response during this critical stage of the disease. We analyzed the T lymphocyte response during and after acute resolving HCV infection in three persons, using interferon (IFN)-gamma enzyme-linked immunospot (ELISPOT) and human histocompatibility leukocyte antigen (HLA) peptide tetramer assays. Acute infection was associated with a broadly directed T helper and cytotoxic T lymphocyte (CTL) response, which persisted after resolution of clinical hepatitis and clearance of viremia. At the earliest time point studied, highly activated CTL populations were observed that temporarily failed to secrete IFN-gamma, a "stunned" phenotype, from which they recovered as viremia declined. In long-term HCV-seropositive persons, CTL responses were more common in persons who had cleared viremia compared with those with persistent viremia, although the frequencies of HCV-specific CTLs were lower than those found in persons during and after resolution of acute HCV infection. These studies demonstrate a strong and persistent CTL response in resolving acute HCV infection, and provide rationale to explore immune augmentation as a therapeutic intervention in chronic HCV infection.

Activation of nuclear factor kappaB in hepatitis C virus infection: implications for pathogenesis and hepatocarcinogenesis

The hepatitis C virus (HCV) core protein is a multifunctional protein. It may bind to the death domain of tumor necrosis factor receptor 1 (TNFR1) and to the cytoplasmic tail of lymphotoxin-beta receptor, implying that it may be involved in the apoptosis and anti-apoptosis signaling pathways. In vitro studies have been inconclusive regarding its ability to inhibit or enhance TNF-alpha-induced apoptosis. To address this issue, electrophoretic mobility shift assay (EMSA) and immunohistochemical studies were used to show the activation of nuclear factor kappaB (NF-kappaB) in HCV-infected liver tissues and in HCV core-transfected cells. The activation of NF-kappaB was correlated with the apoptosis assays. The results showed that NF-kappaB activation could be shown in HCV-infected livers and HCV core-transfected cells. The data of EMSA correlated with those of immunohistochemical studies, which revealed a higher frequency of NF-kappaB nuclear staining in HCV-infected than in normal livers. NF-kappaB activation conferred resistance to TNF-alpha-induced apoptosis in HCV core-transfected cells. Inhibition of NF-kappaB activation by pyrrolidine dithiocarbamate sensitized them to TNF-alpha-induced apoptosis. These findings suggest that HCV infection may cause anti-apoptosis by activation of NF-kappaB and implicate a mechanism by which HCV may evade the host's immune surveillance leading to viral persistence and possibly to hepatocarcinogenesis.

Pathogenesis, natural history, treatment, and prevention of hepatitis C

Approximately 4 million persons in the United States and probably more than 100 million persons worldwide are infected with hepatitis C virus. The virus has the unique ability to cause persistent infection in susceptible hosts after parenteral or percutaneous transmission, and its underlying mechanisms are not well understood. The immunologic correlates of protection and viral clearance and the pathogenesis of liver injury are yet to be defined, but recent studies suggest the importance of cell-mediated immune responses. Although 70% to 80% of infected persons become chronic carriers, most have relatively mild disease with slow progression. However, chronic and progressive hepatitis C carries significant morbidity and mortality and is a major cause of cirrhosis, end-stage liver disease, and liver cancer. Development of an effective hepatitis C virus vaccine is not imminent, but recent advances in technology and basic knowledge of molecular virology and immunology have engendered novel approaches to the fundamental problems encountered in vaccine development. Current therapy for hepatitis C, although effective in some patients, is problematic and still evolving. Advances in modern biology and immunology promise new therapies for this important disease.

Possible mechanisms of action and reasons for failure of antiviral therapy in chronic hepatitis C

The known mechanisms of hepatitis C virus (HCV) clearance and their failure in persistent infection are discussed. Interferon-alpha is the main treatment in chronic HCV but has shown poor sustained virological response rates when used as a monotherapy. The effects of interferon-a may include inhibition of HCV virion production by an effect on viral RNA and protein synthesis, enhancement of immune lysis of HCV infected cells, inhibition of hepatic fibrosis by an effect on TGFbeta, and an effect on HCV induced carcinogenesis. Mathematical modelling studies have provided insights into the mechanisms of action of interferon-alpha in chronic HCV. The two-phase plasma HCV RNA disappearance curve may reflect the presence of an interferon-resistant second site of HCV replication either within or outside the liver. Clinical observations and cerebral magnetic resonance scans provide evidence of functional cerebral impairment in HCV infected patients, raising the issue of the central nervous system (CNS) as a site for HCV replication. Recent studies using ribavirin in combination with interferon suggest that this approach doubles the sustained response rates obtained without having a major effect on the initial rate of HCV clearance (see Zeuzem paper). The potential mechanisms of action of ribavirin, although not yet fully understood, include inhibition of synthesis of GTP by an effect on inosine monophosphate dehydrogenase thereby limiting viral RNA synthesis, and enhancement of TH1 responses, which may assist viral clearance. There is no significant effect on HCV RNA polymerase activity. It is possible that ribavirin may have activity at extrahepatic sites of HCV infection, thus explaining the marked reduction in relapse rates with combination therapy, without an appreciable effect on initial antiviral response.

Increased frequency of interferon-gamma-producing peripheral blood CD4+ T cells in chronic hepatitis C virus infection

OBJECTIVES

To determine the profile of cytokine secretion by CD4+ T helper (Th) cells in chronic hepatitis C virus (HCV) infection, we used flow cytometry to determine the percentage of interferon (IFN)-gamma and interleukin (IL)-4 producing cells from CD4+ T lymphocytes in peripheral blood obtained from patients chronically infected with HCV.

METHODS

Peripheral blood mononuclear cells isolated from 89 HCV infected subjects (22 asymptomatic carriers, 56 patients with chronic hepatitis, and 11 patients with liver cirrhosis) and 24 healthy controls were stained with surface CD4 and intracellular IFN-gamma and IL-4. Serum soluble IL-2 receptor (sIL-2R) levels were analyzed by ELISA.

RESULTS

The frequency of IFN-gamma producing CD4+ cells in asymptomatic HCV carriers, patients with chronic hepatitis, and patients with liver cirrhosis were significantly higher than those of healthy controls (p<0.01, respectively). In contrast, the percentages of IL-4-producing CD4+ cells were very low, and there were no significant correlations with disease progression. A significant elevation in serum sIL-2R levels was found in chronic HCV infection compared to healthy controls, and serum sIL-2R levels significantly correlated with the frequency of IFN-gamma-producing cells.

CONCLUSIONS

In HCV infected subjects, both serum sIL-2R and IFN-gamma are increased in chronic HCV infection no matter the stage of disease, meaning they are no different in asymptomatic carriers, patients with chronic hepatitis, and patients with liver cirrhosis, and that Th1 cytokine or Th1 cells may participate in the pathogenesis of liver damage in chronic HCV infection.

Hepatitis C virus structural proteins induce liver cell injury in transgenic mice

To develop an animal model of hepatitis C virus (HCV) infection, transgenic mice carrying part of the HCV cDNA (C980) encoding HCV-core and envelope proteins under control of the mouse class I major histocompatibility complex gene (H-2K) regulatory region were produced. HCV-C980 RNA and HCV-core protein were present in livers from line H36 as determined by RNase protection assay and immunostaining, respectively. More than 40 animals from line H36 were examined histologically. Most of these H36 mice after 10 months of age developed spontaneous focal infiltration of lymphocytes, hepatocyte necrosis, degeneration, and altered foci with mitotic hepatocytes. These pathological lesions were absent in livers from the age-matched control littermates. Liver cells from these H36 mice were sensitive to damage induced by intravenous administration of an anti-Fas antibody. It is suggested that HCV-C980 proteins by themselves may be one causative agent of liver cell injury in subjects with HCV infection.

Quasispecies in viral persistence and pathogenesis of hepatitis C virus

Hepatitis C virus (HCV) is an important cause of chronic liver disease worldwide. This RNA virus circulates as a quasispecies and its genetic heterogeneity has been implicated in the lack of protective immunity against HCV and in its persistence following infection. HCV might escape from immune surveillance by developing mutations in proteins that are subject to immune pressure.

Pathogenesis of chronic hepatitis C: immunological features of hepatic injury and viral persistence

The immune response to viral antigens is thought to be responsible for viral clearance and disease pathogenesis during hepatitis C virus (HCV) infection. In chronically infected patients, the T-cell response to the HCV is polyclonal and multispecific, although it is not as strong as the response in acutely infected patients who display a more vigorous T-cell response. Importantly, viral clearance in acutely infected patients is associated with a strong CD4(+) helper T-cell response. Thus, the dominant cause of viral persistence during HCV infection may be the development of a weak antiviral immune response to the viral antigens, with corresponding inability to eradicate infected cells. Alternatively, if clearance of HCV from the liver results from the antiviral effect of T-cell-derived cytokines, as has been demonstrated recently for the hepatitis B virus, chronic HCV infection could occur if HCV is not sensitive to such cytokines or if insufficient quantities of cytokines are produced. Liver cell damage may extend from virally infected to uninfected cells via soluble cytotoxic mediators and recruitment and activation of inflammatory cells forming the necroinflammatory response. Additional factors that could contribute to viral persistence are viral inhibition of antigen processing or presentation, modulation of the response to cytotoxic mediators, immunological tolerance to HCV antigens, mutational inactivation of cytotoxic T lymphocyte (CTL) epitopes, mutational conversion of CTL epitopes into CTL antagonists, and infection of immunologically privileged tissues. Analysis of the basis for viral persistence is hampered because the necessary cell culture system and animal model to study this question do not yet exist.

The viral clearance in interferon-treated chronic hepatitis C is associated with increased cytotoxic T cell frequencies

BACKGROUND/AIMS

Cytotoxic T lymphocytes have been demonstrated in peripheral blood and liver tissue of patients with chronic hepatitis C virus infection, but their significance for viral clearance is unknown. Therefore, we analyzed hepatitis C virus-specific cytotoxic T lymphocyte precursor frequencies in chronic hepatitis C virus carriers during interferon-alpha treatment.

METHODS

Blood mononuclear cells or CD8+ T cells from HLA-A2 positive and negative patients and controls were analyzed in chromium-release assays using a panel of 18 synthetic peptides from the HCV core, E1 and NS4 antigens bearing HLA-A2 binding motifs. Specific cytotoxic T lymphocyte precursor frequencies were studied within CD8+ T cells derived from interferon-alpha-treated patients using a TNF-alpha-based ELISPOT assay and compared to viremia levels.

RESULTS

T cells from 16 of 24 HLA-A2+ but none of the six HLA-A2- patients with chronic hepatitis C and six HLA-A2+ healthy controls lysed targets pulsed with peptide cocktails. Fine specificity revealed four very immunogenic epitopes in the core (C36-44, C132-140) and the envelope regions (E332-340, E363-372). Cytotoxic T lymphocyte precursor frequencies were prospectively analyzed in 11 interferon-alpha-treated HLA-A2+ hepatitis C virus patients. Four sustained and two transient therapy responders showed lower pretreatment viremia levels and significantly higher specific cytotoxic T lymphocyte precursor frequencies during viral clearance compared to five therapy non-responders and untreated controls.

CONCLUSIONS

The quantitative induction of HLA-class I restricted responses by interferon-alpha could contribute to a beneficial outcome of hepatitis C virus infections. Furthermore, it appears that the balance between viral load and specific cellular immune responses is critical for successful viral clearance.

Molecular mimicry of human cytochrome P450 by hepatitis C virus at the level of cytotoxic T cell recognition

Hepatitis C virus (HCV) is thought to be involved in the pathogenesis of autoimmune hepatitis (AIH) type 2, which is defined by the presence of type I antiliver kidney microsome autoantibodies directed mainly against cytochrome P450 (CYP)2D6 and by autoreactive liver infiltrating T cells. Virus-specific CD8(+) cytotoxic T lymphocytes (CTLs) that recognize infected cells and contribute to viral clearance and tissue injury during HCV infection could be involved in the induction of AIH. To explore whether the antiviral cellular immunity may turn against self-antigens, we characterized the primary CTL response against an HLA-A*0201-restricted HCV-derived epitope, i.e., HCV core 178-187, which shows sequence homology with human CYP2A6 and CYP2A7 8-17. To determine the relevance of these homologies for the pathogenesis of HCV-associated AIH, we used synthetic peptides to induce primary CTL responses in peripheral blood mononuclear cells of healthy blood donors and patients with chronic HCV infection. We found that the naive CTL repertoire of both groups contains cross-reactive CTLs inducible by the HCV peptide recognizing both CYP2A6 and CYP2A7 peptides as well as endogenously processed CYP2A6 protein. Importantly, we failed to induce CTLs with the CYP-derived peptides that showed a lower capacity to form stable complexes with the HLA-A2 molecule. These findings demonstrate the potential of HCV to induce autoreactive CD8(+) CTLs by molecular mimicry, possibly contributing to virus-associated autoimmunity.

Cellular immune response to the hepatitis C virus

Hepatitis C virus (HCV) infection is most often clinically inapparent and rarely associated with symptoms of acute hepatitis. Most patients, however, fail to resolve the acute infection and proceed to develop chronic hepatitis with the risk of liver cirrhosis and hepatocellular carcinoma later in life. Since the kinetics of the earliest events of virus-host interaction are likely to determine the outcome of infection, research has focused on the characterization of the strength and kinetics of the antiviral immune response in different stages of disease. The identification of the immunological correlates of viral clearance is pivotal for the development of vaccines and efficient therapies.

Comparison of cytotoxic T-lymphocyte responses to hepatitis C virus core protein in uninfected and infected individuals

Cytotoxic T lymphocytes have been implicated in the control of hepatitis C virus (HCV) infection. Recognition by cytotoxic T lymphocytes of epitopes within HCV core protein has been defined previously by in vitro stimulation with synthetic peptides. The aim of this study has been to examine cytotoxic T-lymphocyte responses generated against peptides produced naturally following intracellular processing of viral protein. Antigen-specific cytotoxic T-lymphocyte lines were generated from both HCV uninfected and infected individuals by culturing CD8+ T cells with autologous dendritic cells loaded intracytoplasmically with recombinant HCV core protein. Analysis of the epitopes recognized by core protein-specific cytotoxic T lymphocytes used synthetic peptides that were selected based on their predicted binding to HLA-A*0201 molecules. Core protein-specific cytotoxic T lymphocytes derived from HCV uninfected and infected individuals were able to lyse autologous target cells pulsed with each of 5 predicted epitopes. Generation of HCV-specific cytotoxic T lymphocytes using dendritic cells as antigen presenting cells provides a method of comparing the potential repertoire of cytotoxic T-lymphocyte responses to the responses that occur in chronically infected individuals. No evidence of a qualitatively different response by patient cytotoxic T lymphocytes was apparent which might explain persistence of the virus.

Role of the specific T-cell response for clearance and control of hepatitis C virus

T cells are believed to be the main players in antiviral defence. To investigate the role of the specific CD4+ T cell response for clearance and control of the hepatitis C virus we studied patients with acute hepatitis C (AHC) during the phase of spontaneous viral clearance and during follow up after elimination of the virus and resolution of disease. Symptomatic AHC has a self-limited course in 50% of patients, whereas the other half show virus persistence and develop chronic course of disease. Patients who were able to mount a vigorous, polyclonal, multispecific, TH1 lymphokine dominated CD4+ T-cell response showed viral clearance and a self-limited course of disease. In contrast, absence of this T-cell response in patients with AHC invariably led to viral persistence and chronic hepatitis. The characteristics of the T-cell response were as follows: it was mainly directed against nonstructural proteins of the virus, it was multispecific and demonstrated immunodominant epitopes, and the majority of T-cell clones established from our patients responded to a single peptide (NS3 amino acid 1248-1261) within the helicase region of HCV. Presentation of the peptide was HLA DR specific, the peptide showed promiscuous binding, and it had high binding affinity to 10 of the most common 13 HLA DR alleles, thus patients with diverse HLA DR backgrounds could mount an immune response. Furthermore, the epitope was conserved in 100% of 33 HCV strains published in databases. This strong initial CD4+ T-cell response is not sufficient for a definitive recovery from AHC, it has to be maintained to control the hepatitis C virus. Loss of the response after initial resolution of disease is followed by relapse. Even 20 years after an episode of self-limited AHC with elimination of HCV, we have observed a significant virus-specific CD4+ T-cell response. Our data indicate the decisive role of the virus-specific CD4+ T-cell response for clearance and control of HCV, and contribute to our understanding of immune mechanisms by which the host defends the HCV virus. This is a prerequisite for the development of new strategies to efficiently defend the virus by manipulating or modulating the immune response.

Presence of Effector CD8+ T Cells in Hepatitis C Virus-Exposed Healthy Seronegative Donors

CTL responses against multiple hepatitis C virus (HCV) epitopes were detected in 7 of 29 (24.1%) healthy family members (HFM) persistently exposed to chronically HCV-infected patients (HCV-HFM). These precursor CTL were at very low or undetectable frequencies, as determined by limiting dilution analysis. However, when HCV-specific effector CD8+ T cells, freshly isolated from PBMC of HCV-HFM, were assessed by a sensitive enzyme-linked immunospot assay, their frequencies were severalfold higher than those of precursor CTL. These results indicate that the two assays detect two functionally distinct T cell populations and that the effector cells are not assayed by the 51Cr-release assay. Furthermore, the combination of cell depletion and enzyme-linked immunospot analyses showed that the effector cells were confined into a CD8+ CD45RO+ CD28− population. The persistence of effector CD8+ T cells specific for both the structural and nonstructural viral proteins in uninfected HCV-HFM, suggest that: 1) an immunological memory is established upon a subclinical infection without any evidence of hepatitis, in a large cohort of HCV-exposed individuals; 2) because these cells required neither restimulation nor the addition of particular cytokines in vitro for differentiating in effectors, they should be capable of prompt HCV-specific effector function in vivo, possibly providing antiviral protection; and 3) the maintenance of effector T cell responses may be sustained by persisting low-level stimulation induced by inapparent infections.

Serum levels of soluble tumor necrosis factor receptors and effects of interferon therapy in patients with chronic hepatitis C virus infection

OBJECTIVE

The aim of this study was to understand the significance of the tumor necrosis factor receptor (TNFR)-mediated signaling pathway in the pathophysiology of chronic hepatitis C.

METHODS

The serum levels of soluble TNFRs (sTNFRs; sTNFR p55 and sTNFR p75) were measured in 84 patients with chronic hepatitis C virus (HCV) infection (24 sustained responders and 25 nonresponders to interferon [IFN] therapy and 35 patients with persistent normal blood chemistries) and 20 healthy controls, then compared with clinical parameters.

RESULTS

The serum levels of sTNFRs increased in proportion to the severity of liver disease. The levels of sTNFRs revealed significant correlations with the serum levels of alanine aminotransferase, aspartate aminotransferase, gamma-glutamyl transpeptidase, and gamma-globulin, but not with the serum levels of HCV-core protein. In the sustained responder group, the levels of sTNFR p75 showed a significant decrease (p < 0.0002) 1 yr after IFN therapy, although the levels of sTNFR p55 did not. The levels of sTNFR p75 were correlated with the serum levels of macrophage-colony stimulating factor both before and after IFN therapy. In the nonresponder group, the levels of both sTNFRs were unaltered after IFN therapy.

CONCLUSIONS

The TNF alpha-TNFRs system, especially the TNFR p75-mediated pathway, is involved in the hepatic inflammation-fibrosis process in chronic hepatitis C. The serum levels of sTNFR p75, but not sTNFR p55, were correlated with the serum levels of macrophage colony stimulating factor in this process.

Analysis of a successful immune response against hepatitis C virus

To investigate the type of immunity responsible for resolution of hepatitis C virus (HCV) infection, we monitored antibody and intrahepatic cytotoxic T lymphocyte (CTL) responses during acute (<20 weeks) infection in chimpanzees. Two animals who terminated infection made strong CTL but poor antibody responses. In both resolvers, CTL targeted at least six viral regions. In contrast, animals developing chronic hepatitis generated weaker acute CTL responses. Extensive analysis of the fine specificity of the CTL in one resolver revealed nine peptide epitopes and restriction by all six MHC class I allotypes. Every specificity shown during acute hepatitis persisted in normal liver tissue more than 1 yr after resolution. These results suggest that CD8+CTL are better correlated with protection against HCV infection than antibodies.