Evasion of influenza A viruses from human T-cell immunity

Abstract : Cellular immunity plays an important role in the control of viral infections, including those caused by influenza viruses. However, viruses can exploit a variety of strategies to evade cellular immunity, like the accumulation of amino acid substitutions in CTL epitopes. It was unclear to what extent these amino acid substitutions affect the influenza virus-specific CTL response. In this thesis, this issue was addressed by assessing the effect of the loss of immunodominant epitopes on the human influenza A virus-specific CTL response in vitro. To this end, recombinant influenza viruses with and without the HLA-B*2705-restricted epitope NP383-391 or the HLA-B*3501-restricted epitope NP418-426 were generated, which were used to induce IFN-gamma production or lytic activity in clonal and polyclonal virus-specific CD8+ T-cell populations (chapter 2 and 7). During this study, it was found that recombinant influenza viruses with a single amino acid substitution at! position 384 of the NP (R384G) could not be rescued. We hypothesized that one or more co-mutations were required to compensate for the detrimental effect of the R384G mutation, which is described in chapters 3 and 4. In line with these results, we hypothesized that influenza A viruses need to overcome functional constraints to accumulate mutations in CTL epitopes and escape from CTL. The inability to overcome these functional constraints may explain the highly conserved nature of most identified influenza A virus CTL epitopes, limiting escape from CTL. To assess the impact of amino acid substitutions in conserved epitopes on viral fitness and recognition by specific CTL, we performed a mutational analysis of CTL epitopes (chapter 5). Since examples of evading mutations in influenza A virus CTL epitopes are limited, we assessed the extent of variation in CTL epitopes using virus-specific CD8+ T-cell clones (chapter 6). In addition to CD8+ T-cell clones, we also observed CD4! + T-cell clones that recognized a variable epitope. To date, no evadin g activities from CD4+ T-cells have been described in influenza virus infection. The variable CD4+ T-cell epitope was located and the amino acid substitution responsible for abrogation of CD4+ T-cell recognition was identified (chapter 8). The studies described in this thesis are discussed in the light of evasion of influenza A virus from human T-cell immunity.

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