Given hypothesized risk of triggering exacerbations of autoimmune diseases and/or chronic viral infections, clinical trials (including regulatory studies) evaluating checkpoint blocking antibodies PD-1 and CTLA-4 have excluded patients with autoimmune diseases, chronic hepatitis B/C virus (HBV/HCV), and/or human immunodeficiency virus (HIV) infections. Here, CTLA4 is linked to chronic hepatitis B virus infection.