CD8A and influenza: While the expression of a high level of PD-1 on CD8+ T cells impede the cytotoxicity to influenza-infected cells [146], the expression of PD-1 on follicular helper T cells (TFH cells) is indeed required for an efficient production of influenza-specific antibodies in the germinal centers, although the precise function of PD-1high-expressing TFH cells is obscure [148,149].