IL-1 receptor-associated kinase 4 is essential for IL-18-mediated NK and Th1 cell responses. 
IL-18 is an important cytokine for both innate and adaptive immunity.  NK T cells and Th1 cells depend on IL-18 for their divergent functions.  The IL-18R, IL-1R, and mammalian Toll-like receptors (TLRs) share homologous intracellular domains known as the TLR/IL-1R/plant R domain.  Previously, we reported that IL-1R-associated kinase (IRAK)-4 plays a critical role in IL-1R and TLR signaling cascades and is essential for the innate immune response.  Because TLR/IL-1R/plant R-containing receptors mediate signal transduction in a similar fashion, we investigated the role of IRAK-4 in IL-18R signaling.  In this study, we show that IL-18-induced responses such as NK cell activity, Th1 IFN-gamma production, and Th1 cell proliferation are severely impaired in IRAK-4-deficient mice.  IRAK-4(-/-) Th1 cells also do not exhibit NF-kappaB activation or IkappaB degradation in response to IL-18.  Moreover, AP-1 activation which is triggered by c-Jun N-terminal kinase activation is also completely inhibited in IRAK-4(-/-) Th1 cells.  These results suggest that IRAK-4 is an essential component of the IL-18 signaling cascade.