Virus-specific T cells and neutralizing antibodies are independent correlates of long-term protective immunity against hepatitis C virus

ABSTRACT Virus-specific T cells and broadly neutralizing antibodies (NAbs) are associated with spontaneous clearance of acute primary hepatitis C virus (HCV) infection and reinfection. Following spontaneous clearance, HCV-specific memory T cells are long-lived, but NAbs are less durable. How these two arms contribute to long-term protective immunity upon re-exposure is unknown. Herein, we compared the magnitude and breadth of memory HCV-specific T cells and NAbs in a cohort of HCV-spontaneously resolved people who inject drugs with high-risk exposure to the virus but who remained free of observed reinfections (FOR, n = 34) or who got reinfected ( n = 22). Among FOR, 73.5% ( n = 25) exhibited high frequencies of gamma interferon (IFN-γ)-producing T cells, while 23.5% ( n = 8) showed high neutralization breadth (>25% neutralization of HCV pseudoparticles [HCVpp]) and/or potency (geometric mean neutralization >50% against seven HCVpp). Notably, some subjects had detectable NAbs several years after clearance. Multivariate random forest analysis demonstrated that the frequency of IFN-γ-producing T cells and the NAb response against only one difficult-to-neutralize HCVpp (1b58) were significant predictors of protection against reinfection ( P < 0.05). Longitudinal analysis of the immune response in subjects with multiple episodes of infection showed that strong and broad HCV-specific T cell responses were associated with spontaneously cleared episodes. In contrast, NAbs did not protect from chronicity upon re-exposure in the absence of memory T cells, or if they were unable to recognize the infecting virus due to imprinting by strains of the previous episodes. In conclusion, T cell responses and NAbs targeting specific isolates are independent predictors of long-term protective immunity against HCV. IMPORTANCE Hepatitis C virus (HCV) is a blood-borne virus that disproportionately affects people who inject drugs. Most HCV infections become persistent, leading to liver fibrosis and cancer. Although effective antiviral therapies are available, around 50 million people remain persistently infected with HCV and can transmit the virus. Currently, there is no available vaccine to control the spread of HCV. However, around one in four people infected can naturally clear the virus, becoming partially protected upon re-exposure, suggesting it is possible to develop a vaccine that induces similar protection. In this study, we investigated the contribution of T cells and neutralizing antibodies to protection against observed reinfection in a cohort of people who inject drugs and have naturally cleared HCV, yet remain at high-risk exposure. Our findings suggest that strong T cell responses and neutralizing antibodies targeting hard-to-neutralize isolates could predict protective immunity upon reinfection.