In the setting of acute hepatitis C virus (HCV) infection, robust HCV-specific CD8+ cytotoxic T lymphocyte (CTL) responses are associated with initial control of viremia. Despite these responses, 70–80% of individuals develop persistent infection. Although viral escape from CD8 responses has been illustrated in the chimpanzee model of HCV infection, the effect of CD8 selection pressure on viral evolution and containment in acute HCV infection in humans remains unclear. Here, we examined viral evolution in an immunodominant human histocompatibility leukocyte antigen (HLA)-B8–restricted NS3 epitope in subjects with acute HCV infection. Development of mutations within the epitope coincided with loss of strong ex vivo tetramer and interferon γ enzyme-linked immunospot responses, and endogenous expression of variant NS3 sequences suggested that the selected mutations altered processing and presentation of the variant epitope. Analysis of NS3 sequences from 30 additional chronic HCV-infected subjects revealed a strong association between sequence variation within this region and expression of HLA-B8, supporting reproducible allele-specific selection pressures at the population level. Interestingly, transmission of an HLA-B8–associated escape mutation to an HLA-B8 negative subject resulted in rapid reversion of the mutation. Together, these data indicate that viral escape from CD8+ T cell responses occurs during human HCV infection and that acute immune selection pressure is of sufficient magnitude to influence HCV evolution.

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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. Virusspecific CD8؉ T-cell proliferation and gamma interferon (IFN-␥) 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 peptidedriven expansion (P ‫؍‬ 0.0005 and P ‫؍‬ 0.002, respectively) and also the number of HCV-and EBV-specific IFN-␥-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.