Phosphatidylinositol-4-kinase IIα licenses phagosomes for TLR4 signaling and MHC-II presentation in dendritic cells

Significance Dendritic cells (DCs) play a key role at the interface between innate and adaptive immunity. DCs continuously sample their microenvironment, respond to microbial cues by signaling through pattern-recognition receptors such as Toll-like receptors (TLRs), and present bacterial antigens to adaptive immune cells. We show that the lipid kinase phosphatidylinositol-4-kinase IIα (PI4KIIα) is required to generate a phosphatidylinositol-4-phosphate pool on DC phagosomes that allows binding of the TLR sorting adaptor TIRAP and promotes TLR4 phagosomal signaling to proinflammatory cytokine production, phagosomal membrane tubule formation, and presentation of phagocytosed antigens. PI4KIIα therefore ensures phagosomal identity and autonomous signaling to initiate antimicrobial immune responses in DCs. Toll-like receptor (TLR) recruitment to phagosomes in dendritic cells (DCs) and downstream TLR signaling are essential to initiate antimicrobial immune responses. However, the mechanisms underlying TLR localization to phagosomes are poorly characterized. We show herein that phosphatidylinositol-4-kinase IIα (PI4KIIα) plays a key role in initiating phagosomal TLR4 responses in murine DCs by generating a phosphatidylinositol-4-phosphate (PtdIns4P) platform conducive to the binding of the TLR sorting adaptor Toll-IL1 receptor (TIR) domain-containing adaptor protein (TIRAP). PI4KIIα is recruited to maturing lipopolysaccharide (LPS)-containing phagosomes in an adaptor protein-3 (AP-3)-dependent manner, and both PI4KIIα and PtdIns4P are detected on phagosomal membrane tubules. Knockdown of PI4KIIα—but not the related PI4KIIβ—impairs TIRAP and TLR4 localization to phagosomes, reduces proinflammatory cytokine secretion, abolishes phagosomal tubule formation, and impairs major histocompatibility complex II (MHC-II) presentation. Phagosomal TLR responses in PI4KIIα-deficient DCs are restored by reexpression of wild-type PI4KIIα, but not of variants lacking kinase activity or AP-3 binding. Our data indicate that PI4KIIα is an essential regulator of phagosomal TLR signaling in DCs by ensuring optimal TIRAP recruitment to phagosomes.

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