We then interrogated the expression of PD-1 on young and aged CD4+ T cells during influenza infection since increased PD-1 expression on aged TFH was described recently by Sage et al.19, who suggested that increased PD-1 expression could be responsible for the reduced function of aged TFH. The total aged CD4+ T cell population (Supplemental Fig. 2A–C) as well as the total aged TFH population (Supplemental Fig. 2D,E) do indeed express higher levels of PD-1 when compared to young. Here, CD4 is linked to influenza.