CD4 and infection: We then specifically analyzed the expression of PD-1+ of CD4+/CD8+ T cells at 72 h post infection, compared with the uninfected control, and showed that the percentage of CD4+ T cells expressing PD-1 was gradually increasing and markedly increased at 120 h (Fig. 2B), and the percentage of CD4+ of CD3+ T cells significantly decreased in 72–96 h post-SFTSV infection (Fig. EV3A).