Development of a vaccine against hepatitis C virus (HCV) has been hindered by our limited understanding of immune correlates of protection during real-life exposure to the virus. We studied the immune response during HCV reinfection.

We analyzed blood samples from participants in the Montreal Acute Hepatitis C Injection Drug User Cohort Study who were reinfected with HCV from 2009 to 2012. Five patients spontaneously resolved their second infection and 4 developed chronic infections. We monitored the phenotypic and functional dynamics of HCV-specific memory T cell responses in all subjects during natural re-exposure and re-infection.

Populations of CD4⁺ and CD8⁺ T cells with HCV-specific polyfunctional memory were expanded in all 5 individuals who resolved 2 successive HCV infections. We detected CD127^hi HCV-specific memory CD8⁺ T cells before reinfection regardless of a subject’s ability to clear subsequent infections. Protection against viral persistence was associated with the expansion of a CD127^neg, PD1^lo effector memory T cells at the peak of the response. We also observed broadening of T-cell response, indicating generation of de novo T-cell responses. The 4 individuals who failed to clear their subsequent infection had limited expansion of HCV-specific CD4⁺ and CD8⁺ memory T cells and expressed variable levels of the exhaustion marker PD1 on HCV-specific CD8⁺ T cells. Dominant epitope regions of HCV strains isolated from patients with persistent reinfection had sequence variations that were not recognized by the pre-existing memory T cells.

Protection from persistent HCV reinfection depends on the magnitude, breadth, and quality of the HCV-specific memory T-cell response. Sequence homology among viruses and ability of T cells to recognize multiple strains of HCV are critical determinants of protective memory.