Activating Interferons with Noncanonical IKKs

Abstract

Toll-like receptors (TLRs) initiate immune responses to bacterial and viral pathogens by recognition of conserved patterns presented by the pathogens. The TLR signaling pathways culminate in activation of inflammatory cytokine genes, including stimulation of the expression of the type 1 interferons (IFNα/β). Although all TLRs can signal through the adaptor MyD88, TLR3 and TLR4 continue to activate gene expression in the absence of MyD88 (see Barton and Medzhitov). Fitzgerald et al. investigated, using overexpression studies and interfering RNA (RNAi) techniques, the roles of the noncanonical inhibitor of κB kinase (IKK) members, IKKε and TANK-binding kinase 1 [(TBK1), also known as NF-κB-activating kinase (NAK) and tumor necrosis factor receptor-associated factor 2-associated kinase (T2K)] in the activation of interferon regulatory factor 3 (IRF3)-mediated gene expression. Overexpression of TBK1 or IKKε stimulated IRF3 translocation to the nucleus (traced by the movement of an IRF3-green fluorescent protein fusion construct) and stimulated IRF3-dependent expression of an IFNβ reporter and a reporter containing the interferon-stimulated regulatory element (IRSE) from the gene IFIT2. In addition, overexpression of TBK1or IKKε stimulated secretion of the cytokine RANTES. Treatment of HEK293 cells or 293 T cells with RNAi against TBK1 or IKKε inhibited stimulation of expression of IFIT2 and the IRF3-dependent IFNβ and IRSE reporters and blocked secretion of RANTES in response to virus infection or double-stranded RNA. Coprecipitation experiments demonstrated an interaction between TBK1, IKKε, and IRF3 or the adaptor TRIF. Furthermore, increased gene expression in response to overexpression of TRIF was inhibited by kinase inactive mutants of TBK1 or IKKε. Thus, the authors suggest that the pathway from TLR to IRF3 proceeds through the TRIF adaptor and involves the noncanonical IKKs, TBK1 and IKKε.G. M. Barton, R. Medzhitov, Linking Toll-like receptors to IFNα/β expression. Nat. Immunol. 4, 432-433 (2003). [Online Journal]K.A. Fitzgerald, S. M. McWhirter, K. L. Faia, D. C. Rowe, E. Latz, D. T. Golenbock, A. J. Coyle, S.-M. Liao, T. Maniatis, IKKε and TBK1 are essential components of the IRF3 signaling pathway. Nat. Immunol. 4, 491-496 (2003). [Online Journal]