A protective HLA-B*27 restricted epitope contributes to viral control in chronic HCV infection despite presence of viral escape mutations and is characterized by a unique T cell phenotype

Introduction and aim: The HLA class I allele B*27 is associated with spontaneous HCV clearance. We have previously linked this protective effect of HLA-B*27 to an immunodominant HLA-B*27 restricted CD8+ T cell epitope located in NS5B. Viral escape within this CD8+ T cell epitope follows a complicated pathway of several mutations, thus, in most HLA-B*27+ subjects, HCV is cleared by the immune response before viral escape can occur. In the current study, we analyzed the impact of the HLA-B*27 restricted CD8+ T cell response in viral control in persistent infection.

Methods: We determined the ex vivo HLA class I tetramer frequency of CD8+ T cells specific for the HLA-B*27 epitope in 25 HLA-B27+ patients with chronic HCV infection and correlated the results with viral loads of the patients. In a subset of patients we determined the phenotype and expression of inhibitory receptors on virus-specific CD8+ T cells by multicolor HLA class I tetramer staining. In addition, autologous viral sequences corresponding to the epitope region were analyzed.

Results: Patients with a substantial ex vivo frequency of CD8+ T cells specific for the HLA-B*27 epitope (tetramer+ cells in CD8+ T cells > 0.05%) had significantly lower viral loads compared to patients with a weak or absent response. In addition, these patients had also lower viral loads compared to patients that were negative for HLA-B*27 (p < 0.001 for both observations). A similar association between epitope-specific CD8+ T cells detectable ex vivo and viral loads was not observed for HLA-A*02+ patients. CD8+ T cells specific for the HLA-B*27-restricted CD8+ T cell epitope showed high expression of CD127 as well as a low expression of the inhibitor markers 2B4, CD160, KLRG1, and Tim-3. This memory-like phenotype was in agreement with the observation that all patients displayed viral escape mutations within the targeted epitope region. Strikingly, however, the HLA-B*27 restricted CD8+ T cells displayed a very high (> 90%) expression of PD-1 that is unusual for memory-like CD8+ T cells. Indeed, we did not observe a similar CD8+ T cell phenotype e.g. for HLA-A*02 restricted CD8+ T cells responses. Importantly, when we performed PD-1 blocking experiments, we did not observe an increase in proliferative capacity of the HLA-B*27 restricted CD8+ T response. These results indicate that PD-1 was not a marker of exhaustion in this case, but rather a marker of activation and likely partial ongoing recognition of viral antigen, despite the presence of viral escape mutations within the epitope.

Conclusion: We demonstrated that the immunodominant HLA-B*27 epitope is not only associated with spontaneous clearance of HCV in acute infection, but also with low viral loads in persistent infection. Viral control occurs despite the presence of viral escape mutations and is associated with a unique CD8+ T cell phenotype that resembles memory-like CD8+ T cells but is distinct in a high expression of PD-1.