HCV immunology--death and the maiden T cell

Inhibitory killer cell immunoglobulin-like receptors strengthen CD8+ T cell-mediated control of HIV-1, HCV, and HTLV-1

Killer cell immunoglobulin-like receptors (KIRs) are expressed predominantly on natural killer cells, where they play a key role in the regulation of innate immune responses. Recent studies show that inhibitory KIRs can also affect adaptive T cell-mediated immunity. In mice and in human T cells in vitro, inhibitory KIR ligation enhanced CD8+ T cell survival. To investigate the clinical relevance of these observations, we conducted an extensive immunogenetic analysis of multiple independent cohorts of HIV-1-, hepatitis C virus (HCV)-, and human T cell leukemia virus type 1 (HTLV-1)-infected individuals in conjunction with in vitro assays of T cell survival, analysis of ex vivo KIR expression, and mathematical modeling of host-virus dynamics. Our data suggest that functional engagement of inhibitory KIRs enhances the CD8+ T cell response against HIV-1, HCV, and HTLV-1 and is a significant determinant of clinical outcome in all three viral infections.

Interaction between immunoglobulin allotypes and NK receptor genes in diabetes post-hepatitis C virus infection

Genetic interactions between natural killer (NK) cells immunoglobulin-like receptor (KIR) genes and immunoglobulin allotypes have been previously reported in type 2 diabetes mellitus (DM) patients. Puerto Rican Americans with a history of intravenous drug use who developed DM following HCV infection (n=32) were compared to individuals infected with HCV without diabetes (n=121) and to DM non-infected individuals (n=95). Subjects were genotyped for KIRs and immunoglobulin allotypes. We found interactions of immunoglobulin allotypes KM3/KM3 with NK inhibitory receptors 2DL3/2DL3, 2DL1 in the absence of 2DS4 associated with susceptibility to DM in HCV infected individuals. These data suggest the possibility that a subset of patients with HCV could have an immune-mediated component contributing to the development of DM.

Natural epitope variants of the hepatitis C virus impair cytotoxic T lymphocyte activity

AIM: To understand how interactions between hepatitis C virus (HCV) and the host’s immune system might lead to viral persistence or effective elimination of HCV.

METHODS: Nucleotides 3519-3935 of the non-structural 3 (NS3) region were amplified by using reverse transcription polymerase chain reaction (PCR). PCR products of the HCV NS3 regions were integrated into a PCR^® T7TOPO^® TA vector and then sequenced in both directions using an automated DNA sequencer. Relative major histocompatibility complex binding levels of wild-type and variant peptides were performed by fluorescence polarization-based peptide competition assays. Peptides with wild type and variant sequences of NS3 were synthesized locally using F-moc chemistry and purified by high-performance liquid chromatography. Specific cytotoxic T lymphocytes (CTLs) clones toward HCV NS3 wild-type peptides were generated through limiting dilution cloning. The CTL clones specifically recognizing HCV NS3 wild-type peptides were tested by tetramer staining and flow cytometry. Cytolytic activity of CTL clones was measured using target cells labeled with the fluorescence enhancing ligand, DELFIA EuTDA.

RESULTS: The pattern of natural variants within three human leukocyte antigen (HLA)-A2-restricted NS3 epitopes has been examined in one patient with chronic HCV infection at 12, 28 and 63 mo post-infection. Results obtained may provide convincing evidence of immune selection pressure for all epitopes investigated. Statistical analysis of the extensive sequence variation found within these NS3 epitopes favors a Darwinian selection model of variant viruses. Mutations within the epitopes coincided with the decline of CTL responses, and peptide-binding studies suggested a significant impact of the mutation on T cell recognition rather than peptide presentation by HLA molecules. While most variants were either not recognized or elicited low responses, such could antagonize CTL responses to target cells pulsed with wild-type peptides.

CONCLUSION: Cross-recognition of CTL epitopes from wild-type and naturally-occurring HCV variants may lead to impaired immune responses and ultimately contribute to viral persistence.

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.

Hepatitis C virus and ethanol alter antigen presentation in liver cells

Alcoholic patients have a high incidence of hepatitis C virus (HCV) infection. Alcohol consumption enhances the severity of the HCV disease course and worsens the outcome of chronic hepatitis C. The accumulation of virally infected cells in the liver is related to the HCV-induced inability of the immune system to recognize infected cells and to develop the immune responses. This review covers the effects of HCV proteins and ethanol on major histocompatibility complex (MHC) class I- and class II-restricted antigen presentation. Here, we discuss the liver which functions as an immune privilege organ; factors, which affect cleavage and loading of antigenic peptides onto MHC class I and class II in hepatocytes and dendritic cells, and the modulating effects of ethanol and HCV on antigen presentation by liver cells. Altered antigen presentation in the liver limits the ability of the immune system to clear HCV and infected cells and contributes to disease progression. HCV by itself affects dendritic cell function, switching their cytokine profile to the suppressive phenotype of interleukin-10 (IL-10) and transforming growth factor beta (TGFbeta) predominance, preventing cell maturation and allostimulation capacity. The synergistic action of ethanol with HCV results in the suppression of MHC class II-restricted antigen presentation. In addition, ethanol metabolism and HCV proteins reduce proteasome function and interferon signaling, thereby suppressing the generation of peptides for MHC class I-restricted antigen presentation. Collectively, ethanol exposure further impairs antigen presentation in HCV-infected liver cells, which may provide a partial explanation for exacerbations and the poor outcome of HCV infection in alcoholics.

Chemokines in the immunopathogenesis of hepatitis C infection

Chronic infection with the hepatitis C virus, a noncytopathic hepatotropic RNA virus, affects over 170 million people worldwide. In the majority of cases, neither the early innate immune response nor the later adaptive immune response succeeds in clearing the virus, and the infection becomes chronic. Furthermore, in many patients, the ineffective inflammatory response drives fibrogenesis and the development of cirrhosis. It is critical to understand this immune pathology if preventative and curative therapies are to be developed. Chemokines are a superfamily of small proteins that promote leukocyte migration and orchestrate the immune response to viruses, including hepatitis C virus. Chemokines are crucial for viral elimination, but inappropriate persistence of expression in chronic hepatitis C infection can drive tissue damage and inflammation. Here we review the role of chemokines and their receptors in hepatitis C virus infection.

Mechanisms of disease: the evolving understanding of liver allograft rejection

Liver transplantation is a successful treatment for select forms of liver disease and is unrivalled amongst other forms of solid organ transplantation in the ability of the recipient to develop long-term tolerance to the allograft. Much of this success can be attributed to the inherently tolerogenic manner in which antigens are presented in the liver and the presence of specialized regulatory lymphocyte populations. These observations are not universal, however, and a proportion of recipients develop problematic allograft rejection. Anti-allograft responses lead to an influx of effector cells that target hepatic parenchymal cells and induce apoptosis through members of the tumor necrosis factor superfamily. Anti-rejection therapies have traditionally targeted the entire lymphocyte response against the allograft and, whilst these therapies have been efficacious at limiting effector cell responses, they have also had deleterious effects on the regulatory cell populations that are required to promote long-term tolerance. The emergence of newer anti-rejection drugs that have the potential to target the effector response selectively, whilst preserving the much needed tolerogenic responses, has afforded us the opportunity to refine our future immunosuppressant strategies.

T lymphocytes interact with hepatocytes through fenestrations in murine liver sinusoidal endothelial cells

The liver has an established ability to induce tolerance. Recent evidence indicates that this unique property might be related to its distinctive architecture allowing T cells to be activated in situ independently of lymphoid tissues. Unlike lymph node-activated T cells, liver-activated T cells are short-lived, a mechanism that might contribute to the "liver tolerance effect." Although the potential role of hepatocytes as tolerogenic antigen-presenting cells has been demonstrated, the question as to whether these cells are able to interact with CD8(+) T cells in physiological settings remains controversial. Contradicting the immunological dogma stating that naïve T lymphocytes are prevented from interacting with parenchymal cells within non-lymphoid organs by an impenetrable endothelial barrier, we show here that the unique morphology of the liver sinusoidal endothelial cell (LSEC) permits interactions between lymphocytes and hepatocytes. Using electron microscopy, we demonstrate that liver resident lymphocytes as well as circulating naïve CD8(+) T cells make direct contact with hepatocytes through cytoplasmic extensions penetrating the endothelial fenestrations that perforate the LSECs. Furthermore, the expression of molecules required for primary T cell activation, MHC class I and ICAM-1, is polarized on hepatocytes to the perisinusoidal cell membrane, thus maximizing the opportunity for interactions with circulating lymphocytes. In conclusion, this study has identified, at the ultrastructural level, a unique type of interaction between naïve T lymphocytes and liver parenchymal cells in vivo. These results hold implications for the pathogenesis of viral hepatitis in which hepatocytes may represent the main antigen-presenting cell, and for the development of immune tolerance as lymphocytes pass through the liver.

Evaluating replication-defective vesicular stomatitis virus as a vaccine vehicle

We have generated replication-competent (VSV-C/E1/E2) and nonpropagating (VSVDeltaG-C/E1/E2) vesicular stomatitis virus (VSV) contiguously expressing the structural proteins of hepatitis C virus (HCV; core [C] and glycoproteins E1 and E2) and report on their immunogenicity in murine models. VSV-C/E1/E2 and VSVDeltaG-C/E1/E2 expressed high levels of HCV C, E1, and E2, which were authentically posttranslationally processed. Both VSV-expressed HCV E1-E2 glycoproteins were found to form noncovalently linked heterodimers and appeared to be correctly folded, as confirmed by coimmunoprecipitation analysis using conformationally sensitive anti-HCV-E2 monoclonal antibodies (MAbs). Intravenous or intraperitoneal immunization of BALB/c mice with VSV-C/E1/E2 or VSVDeltaG-C/E1/E2 resulted in significant and surprisingly comparable HCV core or E2 antibody responses compared to those of control mice. In addition, both virus types generated HCV C-, E1-, or E2-specific gamma interferon (IFN-gamma)-producing CD8(+) T cells, as determined by enzyme-linked immunospot (ELISPOT) analysis. Mice immunized with VSVDeltaG-C/E1/E2 were also protected against the formation of tumors expressing HCV E2 (CT26-hghE2t) and exhibited CT26-hghE2t-specific IFN-gamma-producing and E2-specific CD8(+) T-cell activity. Finally, recombinant vaccinia virus (vvHCV.S) expressing the HCV structural proteins replicated at significantly lower levels when inoculated into mice immunized with VSV-C/E1/E2 or VSVDeltaG-C/E1/E2, but not with control viruses. Our data therefore illustrate that potentially safer replication-defective VSV can be successfully engineered to express high levels of antigenically authentic HCV glycoproteins. In addition, this strategy may therefore serve in effective vaccine and immunotherapy-based approaches to the treatment of HCV-related disease.

Do antiviral CD8+ T cells select hepatitis C virus escape mutants? Analysis in diverse epitopes targeted by human intrahepatic CD8+ T lymphocytes

Hepatitis C virus (HCV) is a variable RNA virus that can readily establish persistent infection. Cellular immune responses are important in the early control of the virus. Evidence from animal models suggests that mutation in epitopes recognized by CD8+ T lymphocytes may play an important role in the establishment of persistence but in human persistent infection, equivalent evidence is lacking. We investigated this by analysing a unique resource: viruses from a set of chronically HCV-infected individuals in whom the CD8+ T-cell responses in liver had previously been accurately mapped. Virus was sequenced in seven individuals at 10 epitopes restricted by 10 human leucocyte antigen (HLA) molecules. Two main patterns emerged: in the majority of epitopes sequenced, no variation was seen. In three epitopes, mutations were identified which were compatible with immune escape as assessed using phylogenetic and/or functional studies. These data suggest that - even where specific intrahepatic T cells are detectable - many epitopes do not undergo mutation in chronic human infection. On the contrary, virus may escape from intrahepatic CD8+ T-cell responses in a 'patchy' manner in certain specific epitopes. Furthermore, longitudinal studies to identify the differences between 'selecting' and 'nonselecting' intrahepatic CD8+ T-cell responses are needed in HCV infection.

Activation of Vgamma9Vdelta2 T cells by non-peptidic antigens induces the inhibition of subgenomic HCV replication

Hepatitis C virus (HCV) has evolved complex strategies to evade host immune responses and establish chronic infection. Since human Vγ9Vδ2 T lymphocytes play a critical role in the immune response against viruses, we analyzed their antiviral functions on Huh7 hepatoma cells carrying the subgenomic HCV replicon (Rep60 cells). In a transwell culture system, Rep60 cells were co-cultured with either PBMCs or highly purified γδ T cells stimulated by non-peptidic antigens. Vγ9Vδ2 T cell activation was associated with a dramatic reduction of HCV RNA levels. Neutralizing antibodies targeting IFN-γ revealed a critical role for this cytokine in the inhibition of HCV replication. Interestingly, drugs already in clinical use, such as Phosphostim and Zoledronate, known to activate γδ T cells, were shown to induce the inhibition of HCV replication mediated by Vγ9Vδ2 T cells of HCV patients. Our data suggest that the therapeutic activation of Vγ9Vδ2 T lymphocytes may represent an additional strategy to inhibit HCV replication and to restore a T_h1-oriented immune response in HCV-infected patients.

Ethanol metabolism alters interferon gamma signaling in recombinant HepG2 cells

We previously showed that IFNgamma signal transduction was suppressed by ethanol in recombinant HepG2 cells (VL-17A cells), which express alcohol dehydrogenase (ADH) and CYP2E1. We examined the mechanisms by which STAT1 phosphorylation is blocked by ethanol treatment in VL-17A cells. Cells were exposed to 0 or 100 mmol/L ethanol for 72 hours. STAT1 phosphorylation was determined by Western blot after 1 hour IFNgamma exposure. Reduction of STAT1 phosphorylation by ethanol was prevented in the presence of 4MP, DAS, or uric acid, indicating that the oxidative products from ethanol metabolism were partly responsible for suppression of STAT1 phosphorylation. Ethanol exposure decreased STAT1 tyrosine phosphorylation, whereas serine phosphorylation on the protein was unchanged. These effects of ethanol were mimicked by the peroxynitrite (PN) donor, SIN-1, which also blocked tyrosine, but not serine phosphorylation, on STAT1. When cells expressing either ADH (VA-13 cells) or CYP2E1 (E-47 cells) were exposed to ethanol, both ADH- and CYP2E1-generated products reduced STAT1 phosphorylation. In addition, SOCS1, a negative regulator of IFNgamma signaling and which is degraded by the proteasome, was stabilized by ethanol treatment, presumably because of inhibited proteasome activity. Furthermore, SIN-1 treatment elevated SOCS1 levels in VL-17A cells, indicating that PN has a role in SOCS1 elevation. In conclusion, under conditions of ethanol-elicited oxidative stress, PN prevents STAT1 phosphorylation by stabilization of SOCS1, and possibly by nitration of tyrosine residues in STAT1 protein.

Regulatory T cells suppress in vitro proliferation of virus-specific CD8+ T cells during persistent hepatitis C virus infection

The basis of chronic infection following exposure to hepatitis C virus (HCV) infection is unexplained. One factor may be the low frequency and immature phenotype of virus-specific CD8(+) T cells. The role of CD4(+)CD25(+) T regulatory (T(reg)) cells in priming and expanding virus-specific CD8(+) T cells was investigated. Twenty HLA-A2-positive patients with persistent HCV infection and 46 healthy controls were studied. Virus-specific CD8(+) T-cell proliferation and gamma interferon (IFN-gamma) frequency were analyzed with/without depletion of T(reg) cells, using peptides derived from HCV, Epstein-Barr virus (EBV), and cytomegalovirus (CMV). CD4(+)CD25(+) T(reg) cells inhibited anti-CD3/CD28 CD8(+) T-cell proliferation and perforin expression. Depletion of CD4(+)CD25(+) T(reg) cells from chronic HCV patients in vitro increased HCV and EBV peptide-driven expansion (P = 0.0005 and P = 0.002, respectively) and also the number of HCV- and EBV-specific IFN-gamma-expressing CD8(+) T cells. Although stimulated CD8(+) T cells expressed receptors for transforming growth factor beta and interleukin-10, the presence of antibody to transforming growth factor beta and interleukin-10 had no effect on the suppressive effect of CD4(+)CD25(+) regulatory T cells on CD8(+) T-cell proliferation. In conclusion, marked CD4(+)CD25(+) regulatory T-cell activity is present in patients with chronic HCV infection, which may contribute to weak HCV-specific CD8(+) T-cell responses and viral persistence.

Liver cell lines for the study of hepatocyte functions and immunological response

BACKGROUND

Liver cell lines closely resembling primary hepatocyte are essential for research on hepatitis viruses and hepatocyte function. Currently used cell lines are derived from hepatic tumours and have altered gene expression.

AIMS

The generation and characterisation of novel human hepatocyte lines (HHLs) derived from healthy human liver, retaining the primary hepatocyte phenotype.

RESULTS

Primary hepatocytes were immortalised with Moloney's mouse leukaemia virus expressing E6 and E7 proteins of human papillomavirus, and cultures propagated long-term. All HHLs contained markers of hepatocyte and biliary phenotype (cytokeratins 7, 8, 18 and 19), Cytochrome P450 and albumin. The HHLs did not express high levels of p53 or alpha-fetoprotein. When grown in a collagen sandwich culture, or at the air-liquid interface, HHLs were maintained as monolayer whereas Huh-7 and HepG2 formed thick layers. All HHLs showed increased capacity to bind recombinant hepatitis C virus-like particles in comparison with Huh-7 and HepG2. We also demonstrate that HHLs contained active gap junctions, and that the cells respond to stimulation with IFN-alpha by upregulation of major histocompatibility complex (MHC)-I and -II.

CONCLUSIONS

These HHLs retain primary hepatocyte phenotype and should be useful for investigating mechanisms of entry and replication of hepatotropic viruses, and should also be valuable in the study of hepatocyte biology and pathology.

CXC chemokine ligand 16 promotes integrin-mediated adhesion of liver-infiltrating lymphocytes to cholangiocytes and hepatocytes within the inflamed human liver

Lymphocyte recruitment to the liver is critical for viral clearance in acute hepatitis and in the pathogenesis of chronic inflammatory liver disease when persistent chronic inflammation leads to fibrosis and cirrhosis. Chemokines regulate leukocyte recruitment and positioning in tissues and are thus critical regulators of chronic inflammation. The chemokine CXCL16, which is found in liver tissue, exists in a transmembrane as well as soluble form, providing a potential mechanism for localization to particular structures. We studied the role of CXCL16 and its receptor CXCR6 in lymphocyte recruitment and retention in the liver. A higher proportion of CXCR6(+) T cells was detected in blood of hepatitis C virus patients compared with healthy subjects, and in chronic inflammatory liver disease >60% of intrahepatic T cells expressed CXCR6, including CD4, CD8, and CD56(+) T cells compared with <30% in matched blood samples. CXCR6(+) lymphocytes were found in association with CXCL16(+) bile ducts in portal tracts and with hepatocytes at sites of interface hepatitis. Analysis of CXCL16 expression and subcellular distribution in cultured human cholangiocytes, sinusoidal endothelial cells, and hepatocytes revealed that all three cell types expressed CXCL16, with the strongest staining seen on cholangiocytes. CXCL16 on the cholangiocyte membrane was able to support lymphocyte adhesion by triggering conformational activation of beta(1) integrins and binding to VCAM-1. Thus, CXCL16 can promote lymphocyte adhesion to epithelial cells and may function to attract and retain effector cells that promote biliary and hepatocyte destruction in inflammatory liver disease.

T cell response in hepatitis C virus infection

Hepatitis C virus (HCV) is a hepatotropic RNA virus that causes acute and chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. It is widely accepted that cellular immune responses play an important role in viral clearance and disease pathogenesis. However, HCV often evades effective immune recognition and has a propensity to persist in the majority of acutely infected individuals (ca. 80%). The immunological and virological basis for the inefficiency of the cellular immune response to clear or control the virus is not known. Recent studies, however, have provided new insights into the mechanisms of viral clearance and persistence that will be discussed in detail.

The site of primary T cell activation is a determinant of the balance between intrahepatic tolerance and immunity

Hepatic immunobiology is paradoxical: although the liver possesses unusual tolerogenic properties, it is also the site of effective immune responses against multiple pathogens and subject to immune-mediated pathology. The mechanisms underlying this dichotomy remain unclear. Following previous work demonstrating that the liver may act as a site of primary T cell activation, we demonstrate here that the balance between immunity and tolerance in this organ is established by competition for primary activation of CD8+ T cells between the liver and secondary lymphoid tissues, with the immune outcome determined by the initial site of activation. Using a transgenic mouse model in which antigen is expressed within both liver and lymph nodes, we show that while naive CD8+ T cells activated within the lymph nodes were capable of mediating hepatitis, cells undergoing primary activation within the liver exhibited defective cytotoxic function and shortened half-life and did not mediate hepatocellular injury. The implications of these novel findings may pertain not only to the normal maintenance of peripheral tolerance, but also to hepatic allograft tolerance and the immunopathogenesis of chronic viral hepatitis.

Limited T cell receptor diversity of HCV-specific T cell responses is associated with CTL escape

Escape mutations are believed to be important contributors to immune evasion by rapidly evolving viruses such as hepatitis C virus (HCV). We show that the majority of HCV-specific cytotoxic T lymphocyte (CTL) responses directed against viral epitopes that escaped immune recognition in HCV-infected chimpanzees displayed a reduced CDR3 amino acid diversity when compared with responses in which no CTL epitope variation was detected during chronic infection or with those associated with protective immunity. Decreased T cell receptor (TCR) CDR3 amino acid diversity in chronic infection could be detected long before the appearance of viral escape mutations in the plasma. In both chronic and resolved infection, identical T cell receptor clonotypes were present in liver and peripheral blood. These findings provide a deeper understanding of the evolution of CTL epitope variations in chronic viral infections and highlight the importance of the generation and maintenance of a diverse TCR repertoire directed against individual epitopes.

Analysis of 'driver' and 'passenger' CD8+ T-cell responses against variable viruses

Variable viruses, such as human immunodeficiency virus (HIV) and hepatitis C virus (HCV), persist despite host immune responses directed against them. Numerous lines of evidence have suggested that antiviral CD8+ T-cell responses are key among these immune responses, but these vary widely in their ability to contain virus. We propose that only a proportion of responses may exert significant antiviral pressure ('driver' responses), leading to control over viral replication (protection) and/or, ultimately, selection of escape mutants. Another set of responses may exert only weak pressure on the virus ('passenger' responses): these neither protect nor select. To examine this we have analysed (using established databases of HIV and HCV sequences and cytotoxic T-lymphocyte (CTL) epitopes, and published experimental datasets) two important features--predicted binding of the epitope to major histocompatibility complex molecule and observed variability of the epitope--that might distinguish such responses. We find that a high predicted binding estimate could only explain a limited set of 'driver' responses associated with protection or selection. There is statistical evidence that readily defined (and non-protective) CTL responses target regions associated with lower levels of viral variability. Taken together, this suggests that a large number of well-documented responses may represent 'passengers' and we propose a mechanism that might explain their presence.

Pervasive influence of hepatitis C virus on the phenotype of antiviral CD8+ T cells

Recent studies using MHC class I tetramers have shown that CD8(+) T cell responses against different persistent viruses vary considerably in magnitude and phenotype. At one extreme, hepatitis C virus (HCV)-specific CD8(+) T cell responses in blood are generally weak and have a phenotype that is perforin low and CCR7 high (early memory). At the other, specific responses to CMV are strong, perforin high, and CCR7 low (mature or effector memory). To examine the potential mechanisms behind this diversity, we compared CMV-specific responses in HCV-infected and healthy individuals. We find a striking difference in the phenotype of CMV-specific CD8(+) T cells between these groups. In the HCV-infected cohort, CMV-specific CD8(+) T cells lost markers associated with maturity; they had increased expression of CCR7 and reduced expression of Fas and perforin. They nevertheless responded to Ag in vitro in a manner similar to controls, with strong proliferation and appropriate acquisition of effector memory markers. The reduction in mature CD8 T cells in HCV-infected individuals may arise through either impairment or regulation of T cell stimulation, or through the early loss of mature T cells. Whatever the mechanism, HCV has a pervasive influence on the circulating CD8(+) T cell population, a novel feature that may be a hallmark of this infection.