IRAK and TAK1 are required for IL-18-mediated signaling

Interleukin-18 (IL-18), a pleiotropic cytokine produced by activated macrophages, plays significant roles in the immune response, inducing the secretion of IFN-gamma, TNF-alpha and IL-2, enhancing NK cell activity and potentiating the differentiation of Th1 cells. The intercellular signal transduction pathways through which IL-18 functions have not been thoroughly defined. We have generated a mutant cell line, I1A, that lacks the IRAK protein. In this line which has low or no expression of the other known IRAK family members, we find that the IL-1 receptor-associated kinase (IRAK) is essential for the activation of NFkappaB and JNK in response to IL-18. Furthermore, the death domain, but not the kinase activity of IRAK, is necessary for NFkappaB activation in response to IL-18. Interestingly, the N-proximal undetermined region of IRAK is necessary for NFkappaB activation, but not for JNK activation in response to IL-18, indicating IRAK may be a branchpoint in IL-18 signaling. In addition to IRAK, we implicate two other components in IL-18 signaling, TAK1 (TGF-beta-activated kinase 1) and its activator and substrate TAB1. A dominant negative mutant of TAK1 inhibits the IL-18-mediated NFkappaB activation, while IL-18 stimulation leads to the phosphorylation of TAB1. Finally, analysis of IL-18 signaling in IL-1-unresponsive mutant cell lines suggests that the IL-1- and IL-18-mediated pathways are similar, but may not be identical.