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

Gamma-Chain Receptor Cytokines & PD-1 Manipulation to Restore HCV-Specific CD8+ T Cell Response during Chronic Hepatitis C

Hepatitis C virus (HCV)-specific CD8⁺ T cell response is essential in natural HCV infection control, but it becomes exhausted during persistent infection. Nowadays, chronic HCV infection can be resolved by direct acting anti-viral treatment, but there are still some non-responders that could benefit from CD8⁺ T cell response restoration. To become fully reactive, T cell needs the complete release of T cell receptor (TCR) signalling but, during exhaustion this is blocked by the PD-1 effect on CD28 triggering. The T cell pool sensitive to PD-1 modulation is the progenitor subset but not the terminally differentiated effector population. Nevertheless, the blockade of PD-1/PD-L1 checkpoint cannot be always enough to restore this pool. This is due to the HCV ability to impair other co-stimulatory mechanisms and metabolic pathways and to induce a pro-apoptotic state besides the TCR signalling impairment. In this sense, gamma-chain receptor cytokines involved in memory generation and maintenance, such as low-level IL-2, IL-7, IL-15, and IL-21, might carry out a positive effect on metabolic reprogramming, apoptosis blockade and restoration of co-stimulatory signalling. This review sheds light on the role of combinatory immunotherapeutic strategies to restore a reactive anti-HCV T cell response based on the mixture of PD-1 blocking plus IL-2/IL-7/IL-15/IL-21 treatment.

Innate and Adaptive Immune Responses in Chronic HCV Infection

Hepatitis C virus (HCV) remains a public health problem of global importance, even in the era of potent directly-acting antiviral drugs. In this chapter, I discuss immune responses to acute and chronic HCV infection. The outcome of HCV infection is influenced by viral strategies that limit or delay the initiation of innate antiviral responses. This delay may enable HCV to establish widespread infection long before the host mounts effective T and B cell responses. HCV's genetic agility, resulting from its high rate of replication and its error prone replication mechanism, enables it to evade immune recognition. Adaptive immune responses fail to keep up with changing viral epitopes. Neutralizing antibody epitopes may be hidden by decoy structures, glycans, and lipoproteins. T cell responses fail due to changing epitope sequences and due to exhaustion, a phenomenon that may have evolved to limit immune-mediated pathology. Despite these difficulties, innate and adaptive immune mechanisms do impact HCV replication. Immune-mediated clearance of infection is possible, occurring in 20-50% of people who contract the disease. New developments raise hopes for effective immunological interventions to prevent or treat HCV infection.

Immune control and failure in HCV infection--tipping the balance

Despite the development of potent antiviral drugs, HCV remains a global health problem; global eradication is a long way off. In this review, we discuss the immune response to HCV infection and particularly, the interplay between viral strategies that delay the onset of antiviral responses and host strategies that limit or even eradicate infected cells but also contribute to pathogenesis. Although HCV can disable some cellular virus-sensing machinery, IFN-stimulated antiviral genes are induced in the infected liver. Whereas epitope evolution contributes to escape from T cell-mediated immunity, chronic high antigen load may also blunt the T cell response by activating exhaustion or tolerance mechanisms. The evasive maneuvers of HCV limit sterilizing humoral immunity through rapid evolution of decoy epitopes, epitope masking, stimulation of interfering antibodies, lipid shielding, and cell-to-cell spread. Whereas the majority of HCV infections progress to chronic hepatitis with persistent viremia, at least 20% of patients spontaneously clear the infection. Most of these are protected from reinfection, suggesting that protective immunity to HCV exists and that a prophylactic vaccine may be an achievable goal. It is therefore important that we understand the correlates of protective immunity and mechanisms of viral persistence.

Selection of conserved epitopes from hepatitis C virus for pan-populational stimulation of T-cell responses

The hepatitis C virus (HCV) is able to persist as a chronic infection, which can lead to cirrhosis and liver cancer. There is evidence that clearance of HCV is linked to strong responses by CD8 cytotoxic T lymphocytes (CTLs), suggesting that eliciting CTL responses against HCV through an epitope-based vaccine could prove an effective means of immunization. However, HCV genomic plasticity as well as the polymorphisms of HLA I molecules restricting CD8 T-cell responses challenges the selection of epitopes for a widely protective vaccine. Here, we devised an approach to overcome these limitations. From available databases, we first collected a set of 245 HCV-specific CD8 T-cell epitopes, all known to be targeted in the course of a natural infection in humans. After a sequence variability analysis, we next identified 17 highly invariant epitopes. Subsequently, we predicted the epitope HLA I binding profiles that determine their potential presentation and recognition. Finally, using the relevant HLA I-genetic frequencies, we identified various epitope subsets encompassing 6 conserved HCV-specific CTL epitopes each predicted to elicit an effective T-cell response in any individual regardless of their HLA I background. We implemented this epitope selection approach for free public use at the EPISOPT web server.

T-cell responses in hepatitis B and C virus infection: similarities and differences

Chronic hepatitis B virus (HBV) and hepatitis C virus (HCV) infection are global health problems affecting 600 million people worldwide. Indeed, HBV and HCV are hepatotropic viruses that can cause acute and chronic liver disease progressing to liver cirrhosis and even hepatocellular carcinoma. Furthermore, co-infections of HBV and HCV with HIV are emerging worldwide. These co-infections are even more likely to develop persistent infection and are difficult to treat. There is growing evidence that virus-specific CD4⁺ and CD8⁺ T-cell responses play a central role in the outcome and pathogenesis of HBV and HCV infection. While virus-specific T-cell responses are able to successfully clear the virus in a subpopulation of patients, failure of these T-cell responses is associated with the development of viral persistence. In this review article, we will discuss similarities and differences in HBV- and HCV-specific T-cell responses that are central in determining viral clearance, persistence and liver disease.

CD8+ T-cell differentiation in chronic HCV infection

HCV has the tendency to persist in the majority of infected patients despite the presence of virus-specific CD8+ T-cell responses. The HCV-specific CD8+ T-cell pool consists of heterogenous subsets that can be elegantly distinguished by CD127 expression. Indeed, HCV-specific CD8+ T cells with high levels of CD127 expression are characterized by features consistent with memory differentiation, while HCV-specific CD8+ T cells with low levels of CD127 expression display an exhausted phenotype characterized by the coexpression of PD-1 and other inhibitory receptors. In this article, the current knowledge about the phenotypes and differentiation profiles of HCV-specific CD8+ T cells is summarized. Clearly, the differentiation of HCV-specific CD8+ T cells in chronic HCV infection results from complex virus–host interactions, and both viral and immunological factors shape the HCV-specific CD8+ T-cell pool.

Adaptive immunity to the hepatitis C virus

The hepatitis C virus (HCV) is a global public health problem affecting approximately 2% of the human population. The majority of HCV infections (more than 70%) result in life-long persistence of the virus that substantially increases the risk of serious liver diseases, including cirrhosis and hepatocellular carcinoma. The remainder (less than 30%) resolves spontaneously, often resulting in long-lived protection from persistence upon reexposure to the virus. To persist, the virus must replicate and this requires effective evasion of adaptive immune responses. In this review, the role of humoral and cellular immunity in preventing HCV persistence, and the mechanisms used by the virus to subvert protective host responses, are considered.

Virological footprint of CD4+ T-cell responses during chronic hepatitis C virus infection

Human and animal model evidence suggests that CD4⁺ T cells play a critical role in the control of chronic hepatitis C virus (HCV) infection. However, despite their importance, the mechanism behind the failure of such populations in chronic disease is not understood and the contribution of viral mutation is not known. To address this, this study defined the specificity and virological footprint of CD4⁺ T cells in chronic infection. CD8⁺ T-cell-depleted peripheral blood mononuclear cells from 61 HCV genotype 1-infected patients were analysed against a panel of peptides covering the HCV genotype 1 core – a region where CD4⁺ T-cell responses may be reproducibly obtained. In parallel, the core region and E2 protein were sequenced. Gamma interferon-secreting CD4⁺ T-cell responses directed against multiple epitopes were detected in 53 % of individuals, targeting between one and four peptides in the HCV core. Viral sequence evaluation revealed that these CD4⁺ T-cell responses were associated with mutants in 2/21 individuals. In these two cases, the circulating sequence variant was poorly recognized by host CD4⁺ T cells. Bioinformatics analyses revealed no overall evidence of selection in the target epitopes and no differences between the groups with and without detectable CD4⁺ T-cell responses. It was concluded that sustained core peptide-specific CD4⁺ T-cell responses may be reproducibly measured during chronic HCV infection and that immune escape may occur in specific instances. However, overall the virological impact of such responses is limited and other causes for CD4⁺ T-cell failure in HCV must be sought.

Loss of viral fitness and cross-recognition by CD8+ T cells limit HCV escape from a protective HLA-B27-restricted human immune response

There is an association between expression of the MHC class I molecule HLA-B27 and protection following human infection with either HIV or HCV. In both cases, protection has been linked to HLA-B27 presentation of a single immunodominant viral peptide epitope to CD8+ T cells. If HIV mutates the HLA-B27-binding anchor of this epitope to escape the protective immune response, the result is a less-fit virus that requires additional compensatory clustered mutations. Here, we sought to determine whether the immunodominant HLA-B27-restricted HCV epitope was similarly constrained by analyzing the replication competence and immunogenicity of different escape mutants. Interestingly, in most HLA-B27-positive patients chronically infected with HCV, the escape mutations spared the HLA-B27-binding anchor. Instead, the escape mutations were clustered at other sites within the epitope and had only a modest impact on replication competence. Further analysis revealed that the cluster of mutations is required for efficient escape because a combination of mutations is needed to impair T cell recognition of the epitope. Artificially introduced mutations at the HLA-B27-binding anchors were found to be either completely cross-reactive or to lead to substantial loss of fitness. These results suggest that protection by HLA-B27 in HCV infection can be explained by the requirement to accumulate a cluster of mutations within the immunodominant epitope to escape T cell recognition.

The association between the genetic polymorphism of HLA-DQA1, DQB1, and DRB1 and serum alanine aminotransferase levels in chronic hepatitis C in the Chinese population

BACKGROUND AND AIM

To investigate a possible association between HLA genes with serum alanine aminotransferase (ALT) levels and evaluate whether the HLA-DQA1, DQB1, and DRB1 genes could influence the development of liver damage in chronic hepatitis C.

METHODS

A total of 145 patients with chronic hepatitis C virus (HCV) infection (36 patients with persistently normal ALT values; 109 patients with elevated ALT levels) and 160 uninfected healthy controls were examined for HLA-DQA1, DQB1, and DRB1 molecules by using polymerase chain reaction-sequencing based typing (PCR-SBT).

RESULTS

Among the patients chronically infected with HCV, the frequencies of DQA1*0501, DQB1*0301, and DRB1*0401 alleles were significantly increased in the normal ALT group compared with those with abnormal ALT levels, whereas that of DQB1*0201 was significantly lower. As compared to uninfected healthy controls, DQA1*0501, DQB1*0301, and DRB1*0401 allele frequencies were also statistically higher in the normal ALT group, whereas that of DQB1*0201 was the inverse. The haplotype frequencies of DQA1*0301-DQB1*0301, DQA1*0501-DQB1*0301, and DRB1*1101-DQB1*0301 were found to be significantly higher in the normal ALT group. Multivariate logistic regression indicated that female sex, and the DQB1*0301 allele and DRB1*0401 allele were independently associated with normal ALT values, whereas DQB1*0201 allele was the inverse.

CONCLUSIONS

These results suggest that particular HLA alleles may have an influence on the serum ALT level of chronic HCV infection as a host genetic factor in the Chinese population. The DQA1*0501, DQB1*0301, and DRB1*0401 alleles, and the DQA1*0301-DQB1*0301, DQA1*0501-DQB1*0301, and DRB1*1101-DQB1*0301 haplotypes seem to be associated with low hepatitis activity; whereas DQB1*0201 allele is closely correlated with the progression of liver injury in chronic HCV infection.

Flying Under the Radar: The Immunobiology of Hepatitis C

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

T cells and viral persistence: lessons from diverse infections

Persistent virus infections create specific problems for their hosts. Although the dynamics of immune responses after acute infection are well studied and very consistent, especially in mouse models, the patterns of responses noted during persistent infection are more complex and differ depending on the infection. In particular, CD8(+) T cell responses differ widely in quantity and quality. In this review we examine these diverse responses and ask how they may arise; in particular, we discuss the function of antigen re-encounter and the CD4(+) T cell responses to and the escape strategies of specific viruses. We focus on studies of four main human pathogens, cytomegalovirus, Epstein-Barr virus, human immunodeficiency virus and hepatitis C virus, and their animal models.