Next we addressed whether lung resident CD4 T cells were sufficient for protection or if recruitment of CD4 T cells from the lymph nodes or spleen to the lung was required.15 FTY720-treatment was used to block egress of CD4 T cells from lymph nodes.60 To confirm that M2e-tetramer binding cells were lung resident CD4 T cells we injected labeled anti-CD45.1 Mabs i.v and found isolated cells to be negative, as also reported earlier (Supplementary Fig. S1d).5,61 Indeed, the FTY720-treatment had little effect on protection against a lethal challenge infection with X47 virus (Fig. 1d). This evidence concerns the gene CD4 and infection.