A Novel IRAK1-IKKε Signaling Axis Limits the Activation of TAK1-IKKβ Downstream of TLR3

IRAK1 is involved in the regulation of type I IFN production downstream of TLR3. Previous work indicated that IRAK1 negatively regulates TRIF-mediated activation of IRF3 and IRF7. We report that IRAK1 limits the activation of the TLR3-NF-kappa B pathway. Following TLR3 stimulation, IRAK1-deficient macrophages produced increased levels of IL-6 and IFN-beta compared with wild type macrophages. Pharmacological inhibition of TAK1 reduced this increase in IFN-beta, together with the heightened activation of IRF3 and p65 found in TLR3-ligand stimulated IRAK1-deficient macrophages. Recently, IKK epsilon and TANK-binding kinase 1 (TBK1) were reported to limit activation of the NF-kappa B pathway downstream of IL-1R, TNFR1, and TLRs. We show that TBK1 has a positive role in the TLR3-NF-kappa B pathway, because we detected reduced levels of IL-6 and reduced activation of p65 in TBK1-deficient macrophages. In contrast, we show that IKK epsilon limits the activation of the TLR3-NF-kappa B pathway. Furthermore, we show that IRAK1 is required for the activation of IKK epsilon downstream of TLR3. We report impaired activation of ERK1/2 in IRAK1- and IKK epsilon-deficient macrophages, a novel finding for both kinases. Importantly, this work provides novel mechanistic insight into the regulation of the TLR3-signaling pathway, providing strong evidence that an IRAK1-IKK epsilon-signaling axis acts to limit the production of both type I IFNs and proinflammatory cytokines by regulating TAK1 activity. The Journal of Immunology, 2013, 190: 2844-2856.