IL-18-stimulated GADD45β required in cytokine-induced, but not TCR-induced, IFN-γ production

Interleukin-12 (IL-12) and IL-18 induce synergistic transcription of interferon γ (IFN-γ) that is T cell receptor (TCR)-independent, not inhibited by cyclosporin A and requires new protein synthesis. To characterize this pathway, we screened for genes that are induced in IL-12-and IL-18-treated T helper type I cells. GADD45β, which activates mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase kinase 4 (MEKK4), was induced by IL-18 and augmented by IL-12. GADD45β expression in naïve CD4⁺T cells activated p38 MAPK and selectively increased cytokine-induced, but not TCR-induced, IFN-γ production. Kinase-inactive MEKK4 and inhibition of the p38 MAPK pathway both selectively inhibit cytokine-induced, but not TCR-induced, IFN-γ production. Thus, the synergy between IL-12 and IL-18 may involve GADD45β induction, which can maintain the MEKK4 and p38 MAPK activation that is necessary for cytokine-induced, but not TCR-induced, IFN-γ production.