Viral escape and T cell exhaustion in hepatitis C virus infection analysed using Class I peptide tetramers.

Hepatitis C virus (HCV) has infected over 170 million people world wide, and in the majority sets up a chronic infection associated with hepatic inflammation. How it evades host immunity, particularly CD8+ T cells (CTL) is unclear, but two major factors are likely to operate, viral escape mutation and T cell exhaustion. We have investigated the role of CTL in control of infection during acute disease using Class I peptide tetramers. Although the immune response is quite diverse and numerous epitopes can be targeted, we observe that, especially during acute disease, one epitope (NS3 1073-81) is commonly recognised in HLA-A2 positive individuals. However, the levels of response to this epitope (and others) are very much lower if persistence is established. We examined in detail whether the cause of this low level of reactivity is due to mutation within the epitope. We find that, in fact this epitope is highly conserved during chronic infection, at a clonal level, between individuals, and over time. Thus, although variation within the epitope does occur, lack of reactivity in peripheral blood against this epitope in chronic disease, and loss of control of virus cannot be explained entirely by viral escape. Escape through mutation probably does play an important role in persistence of HCV, but we also discuss other mechanisms which lead to attenuation of T cell responses which may be important in determining the outcome.