Evolution of adaptive immune response from acute to chronic phase of hepatitis C virus infection

Abstract

The CD8+ T cell responses play a pivotal role in controlling viral replication during HCV infection. HCV evades the immune system by rapid viral evolution affording escape from immune selection pressure including at MHC-I restricted epitopes. However, some CTL epitopes remain conserved well past the time of establishment of chronic infection, implying additional mechanisms immune failure exists. CD8+ T cells exhibiting an exhausted phenotype have been extensively reported during the chronic stage of illness for chronic viral infections, such as HCV and HIV. Additionally, impaired differentiation and trafficking of CD8+ T cells is known to be associated with immune escape and exhaustion of CTLs, but the timing and mechanisms and expression patterns of inhibitory receptors as wells as impairments in differentiation during primary HCV infection remains unclear. HCV-specific CD8+ T cell responses against the transmitted founder virus identified via ELISpot. Immune escape was observed in the NGS data set in ~33% of all ELISpot identified epitopes. The majority of HCV-specific CD8+ responses identified via IFN- ELSPOT in chronic progressors were also characterised by a dominant population of terminally differentiated effector memory cells (CCR7lowCD45ROhighKLRG1highCD127low), and elevated expression of co-inhibitory markers (PD-1 and 2B4) targeting both conserved as well as escaped HCV variants at the peak of immune response (as early as 70-90 days post infection). However, evidence of long-term central memory subpopulations with moderate IFN-γ production was identified in a subset of responses. There was an association of viral escape with the magnitude (IFN- production) of the response, suggesting ongoing evolution of CTLs in response to prolonged viral exposure. Analysis of T-bet expression revealed that T-bet expression on HCV-specific CD8+ T cell was not associated with clearance. Immuno-phenotyping of liver showed that, liver was enriched with T cells expressing the chemokine receptors CCR2, CCR5, CXCR3, and CXCR6. Additionally, the studies revealed preferential expression of CXCR3 on HCV-specific CD8+ T cells in both chronic and acute HCV infection suggesting a key role for CXCR3 in regulation of HCV-specific CD8+ T cell trafficking to the site of infection in the liver. Taken together the studies in this thesis provide both consistent findings with more limited studies in HCV and comparable contexts in HIV, and clear contrasts with previous reports in murine LCMV models. The findings offer novel insights into our understanding of the immunopathgenesis of primary HCV and into HCV vaccine design.