Mutations within a viral genome often confer advantages in vivo, the evolution of which is driven strongly by immune selection pressures. Immune control of the virus before it is able

to mutate is therefore crucial in determining long-term outcome to infection (see Fig. 5). Selleckchem Fludarabine In HIV and simian immunodeficiency virus (SIV), viral escape mutations within immunodominant epitopes play a critical role in early and late loss of immune control [50–52] and this is also shown to influence long-term outcome in acute HCV infection [53,54]. There is a variation in the degree of escape between different epitopes within the viral genome of such persistent viral infections, where some epitopes are observed to escape while others are often conserved. One explanation which has been proposed for this is that more sensitive T cells are associated with escape (‘driver’ responses), while see more less sensitive cells may be simply ‘passengers’ which have little impact on viral evolution or disease outcome [55]. More sensitive populations are observed to drive viral escape, whereas less sensitive CTLs are associated with epitope stability in both HCV [56] and SIV [57]. In HIV, CTL responses

to the promiscuous epitope TL9-Gag were compared between HLA types within the B7 supertype. B*8101-restricted TL9-Gag responses were found to be of significantly higher functional sensitivity than those restricted by B*4201. Higher TL9-Gag sequence variation is observed in B*8101 compared to B*4201-positive

patients [58]. There is a clear conflict of interest in the outcome of better-quality CTL responses. The immune advantages of improved clearance of the more sensitive responses would appear to be balanced against the disadvantage of driving evolution of the virus in its ability to escape the host immune response. However, viral fitness costs associated with the acquisition of escape mutations may contribute to the protective nature of some HLA class I alleles, such as B57 [3]. CTL dysfunction is seen in a number Sodium butyrate of chronic viral infections in humans [59,60] and animal models [61,62]. The genesis of such dysfunction is not well understood, but is thought to be related to repetitive triggering through the TCR. One possible outcome is that more sensitive cells might become preferentially over-stimulated and anergic in the presence of high antigen load. This is supported by in vivo studies showing the persistence of anergic CTLs with high functional sensitivity under such conditions [63,64]. The distinct sensitivities observed in cells of the acute and chronic phase of HIV-1 appears to be a consequence of deletion of the more sensitive cells, as determined by clonotypic analysis of TCR VB chains by polymerase chain reaction (PCR).