One of our previous studies investigated the levels of PD-1 and CTLA-4 expression in CD4+ or CD8+ T cells from patients infected with hepatitis A. We found that both CD4+ and CD8+ T cells from patients with hepatitis A infection had significant levels of PD-1 and CTLA-4 during the symptomatic phase compared to those from the healthy controls, which was the first report to present the highly upregulated expression of PD-1 and CTLA-4 in hepatitis A [79]. Here, CTLA4 is linked to hepatitis A virus infection.