Although reverse signaling through CD40L has been reported in the literature (35), our data show that CD8+ T cell-expressed CD40L is likely to act in a cell-extrinsic manner, as evidenced by our observation from the influenza model that adoptively transferred CD40L-expressing CD8+ T cells can promote the bystander activation and differentiation of endogenous CD8+ T cells. This evidence concerns the gene CD8A and influenza.