Induction of OVA-specific CTL by cross-priming requires help from CD4 T cells, which use CD154 to signal CD40 on the APC. To further dissect the molecular pathways involved in cross-priming, we examined the role of Rel, an NF-κB family member. c-rel-/- mice failed to generate OVA-specific CTL by cross-priming, but could induce CTL to HSV-1. Using chimeric mice, Rel expression was shown to be required by the APC, but not by the T cells. Notably, the deficiency in Rel could be overcome by triggering CD40, implying that the APC required Rel before receipt of the CD40 signal. These data suggest that the cross-priming APC must receive two signals before it can stimulate CTL. The first signal is Rel dependent and is required before activation of CD4 helper T cells, which then deliver the second signal using CD154 to trigger CD40.