Abstract

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 II (PI4KII) 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 TIRAP. PI4KII is recruited to LPS-containing phagosomes in an adaptor protein AP-3 dependent manner, and both PI4KII and PtdIns4P are also detected on phagosomal membrane tubules. Knockdown of PI4KII - but not of the related PI4KII{beta} - impairs TIRAP and TLR4 localization to phagosomes, reduces proinflammatory cytokine secretion, and impairs phagosomal tubule formation and MHC-II presentation. Phagosomal TLR responses in PI4KII-deficient DCs are restored by re-expression of wild-type PI4KII, but not of variants lacking kinase activity or AP-3 binding. Our data indicate that PI4KII is an essential regulator of phagosomal TLR signaling in DCs by ensuring optimal TIRAP recruitment to phagosomes.