Interferon regulatory factor-4 activates IL-2 and IL-4 promoters in cooperation with c-Rel

► Interferon regulatory factor-4 binds to an NF-kappaB family member, c-Rel, in T-cells. ► The N-terminal half of c-Rel bound to the C-terminal IRF association domain of IRF-4. ► IRF-4 and c-Rel bound to the interleukin-4 (IL-4) promoter region. ► Exogenously-expressed IRF-4 and c-Rel enhanced the IL-2 production in T-cells. ► IRF-4 was indispensable for the optimal IL-2 and IL-4 productions in vivo.