Research ArticleInnate Immunity

The interaction between IKKα and LC3 promotes type I interferon production through the TLR9-containing LAPosome

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Sci. Signal.  01 May 2018:
Vol. 11, Issue 528, eaan4144
DOI: 10.1126/scisignal.aan4144

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LC3 recruits IKKα after TLR9 activation

Toll-like receptor 9 (TLR9) is a pattern recognition receptor involved in innate sensing of extracellular DNA within the endosomes of phagocytes. Hayashi et al. found that IKKα kinase associated with the autophagy protein LC3 deposited on endosomes after TLR9 stimulation of macrophages. Formation of this LC3-IKKα signaling platform through noncanonical autophagy also recruited other effectors involved in type I interferon signaling. Mutagenesis of LC3-interacting regions in IKKα suggested that these effects were mediated by a direct interaction of the kinase with LC3. Together, these data identify IKKα as a critical component of LC3-associated phagocytosis-mediated endosome-based signaling platforms necessary for stimulating type I interferon production after TLR9 engagement.

Abstract

Toll-like receptor 9 (TLR9) recognizes DNA in endosomes and activates distinct signaling pathways to stimulate the production of proinflammatory cytokines and type I interferons (IFNs). The assembly of signaling platforms on microtubule-associated proteins 1A/1B–light chain 3 (LC3)–decorated endosomal vesicles is required to transduce TLR9 signals that stimulate the production of IFN but not interleukin-12 p40 (IL-12p40). LC3-associated phagocytosis (LAP), a form of noncanonical autophagy, is critical for the activation of interferon regulatory factor 7 (IRF7) and for IFN synthesis. We showed that after the stimulation of TLR9 by CpG oligonucleotides, the autophagy protein LC3 and the kinase IKKα were recruited to endosomes that contained TLR9. The recruitment of IKKα and LC3 to such signaling endosomes was not stimulated by catalysts of classical autophagosome formation but involved LAP formation, which required ATG5 but not FIP200. In addition, we found that the LC3-IKKα complex further associated with both TRAF3 and IRF7. We identified three putative LC3-interacting regions (LIRs) in IKKα, and mutagenesis suggested that two of these were critical for direct binding to LC3. Moreover, mutation of the same LIR sequences failed to rescue type I IFN production in IKKα-deficient dendritic cells upon reconstitution. Together, these data suggest a direct link between LAP formation and IKKα recruitment downstream of TLR9 activation that is necessary to facilitate type I IFN production.

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