CTLA4 and animal viral hepatitis: Immunosuppressive checkpoints, such as programmed death 1 (PD-1)/PD-L1, T cell immunoglobulin domain and mucin domain-3 (TIM-3), CTLA-4, play a significant role in immunosuppression in chronic viral hepatitis by suppressing T cell responses (Makarova-Rusher et al., 2015).