A promiscuous lipid-binding protein diversifies the subcellular sites of toll-like receptor signal transduction.

The Toll-like receptors (TLRs) of the innate immune system are unusual in that individual family members are located on different organelles, yet most activate a common signaling pathway important for host defense. It remains unclear how this common signaling pathway can be activated from multiple subcellular locations. Here, we report that, in response to natural activators of innate immunity, the sorting adaptor TIRAP regulates TLR signaling from the plasma membrane and endosomes. TLR signaling from both locations triggers the TIRAP-dependent assembly of the myddosome, a protein complex that controls proinflammatory cytokine expression. The actions of TIRAP depend on the promiscuity of its phosphoinositide-binding domain. Different lipid targets of this domain direct TIRAP to different organelles, allowing it to survey multiple compartments for the presence of activated TLRs. These data establish how promiscuity, rather than specificity, can be a beneficial means of diversifying the subcellular sites of innate immune signal transduction.

Toll-like receptor 9 trafficking and signaling for type I interferons requires PIKfyve activity.

Toll-like receptors (TLRs) traffic to distinct membranes for signaling. TLR7 and TLR9 recognize viral nucleic acids in the endosomes and induce robust anti-viral program. Signaling from these TLRs bifurcate at the level of distinct endosomal compartments, namely VAMP3(+) and LAMP(+) endosomes, to mediate the induction of cytokine and type I interferon (IFN) genes, respectively. The formation of the TLR9 endosome competent for IFNs induction requires AP-3. Phosphoinositides (PIs) mark distinct subcellular membranes and control membrane trafficking. However, their role in TLR trafficking and signaling in different dendritic cell (DC) subsets remains unclear. Here, we examined the role of phosphatidylinositol 3P 5-kinase, PIKfyve, in TLR9 trafficking and signaling. We demonstrate that inhibition of PIKfyve activity preferentially blocks TLR9 signaling for type I IFN induction in FLT3L-bone marrow-derived DCs. By confocal microscopy using RAW264.7 cells, we show that trafficking of both TLR9 and CpG to the LAMP1(+) compartment was blocked by PIKfyve inhibitor treatment, whereas their trafficking to the VAMP3(+) endosome remained intact. Further, AP-3 recruitment to TLR9 endosomes was impaired by PIKfyve inhibition. These data indicate that PIKfyve provides critical PIs necessary for the formation of endosome from which TLR9 signals to induce type I IFNs.

Autophagy and selective deployment of Atg proteins in antiviral defense.

Autophagy is an evolutionarily ancient process eukaryotic cells utilize to remove and recycle intracellular material in order to maintain cellular homeostasis. In metazoans, the autophagy machinery not only functions in this capacity but also has evolved to perform a diverse repertoire of intracellular transport and regulatory functions. In response to virus infections, the autophagy machinery degrades viruses, shuttles viral pathogen-associated molecular patterns to endosomes containing Toll-like receptors, facilitates viral-antigen processing for major histocompatibility complex presentation and transports antiviral proteins to viral replication sites. This is accomplished through canonical autophagy or through processes involving distinct subsets of the autophagy-related genes (Atgs). Herein, we discuss how the variable components of the autophagy machinery contribute to antiviral defense and highlight three emerging themes: first, autophagy delivers viral cytosolic components to several distinct endolysosomal compartments; second, Atg proteins act alone, as subgroups or collectively; and third, the specificity of autophagy and the autophagy machinery is achieved by recognition of triggers and selective targeting by adaptors.

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

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.

Different routes to the same destination.

The toll-like receptors that detect viral DNA and viral RNA in cells take different paths from the endoplasmic reticulum to the endosome.