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

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 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.

Monitoring Phosphoinositide Fluxes and Effectors During Leukocyte Chemotaxis and Phagocytosis

“…In the phagolysosomal membrane, WASH and actin-rich regions have also been reported to co-localize with PtdIns(4)P (Levin-Konigsberg et al, 2019 ) and may serve to propel the initial extension of resorption tubules. In this regard it is interesting to note that PI4K2A and late phagosomal PtdIns(4)P have recently been described to support Toll-like receptor signaling and antigen presentation in dendritic cells (López-Haber et al, 2020 ).…”

Better Together: Current Insights Into Phagosome-Lysosome Fusion

“…By extension, LC3-recruited Plekhm1 on phagosomes could serve to tether HOPS- and RILP-containing vesicles during PL formation. Additionally, the LC3 isoforms γ-aminobutyric acid receptor-associated protein (Gabarap) and Gabarap-like 2 can recruit PI4KIIα ( 201 ), which generates PI4P on phagosomes and lysosomes to promote HOPS recruitment ( 114 , 202 ). However, direct evidence to support these hypotheses in phagocyte LAPosomes has yet to be substantiated and awaits further investigation.…”

Role of Apoptotic Cell Clearance in Pneumonia and Inflammatory Lung Disease

“…This may act to enhance the recovery of amino acids, lipids, sugars and proteins from the apoptotic cell, while simultaneously limiting immunogenic loading of autoantigens onto MHC II [ 132 , 133 ]. While the specific mechanism which allows Rab17 to be persistently recruited to the efferosome remains unknown, it is established that MIIC formation requires TLR signaling, and Rab17 knockdown enables MHC II recruitment to efferosomes, suggesting that Rab17 may antagonize MIIC formation and is displaced from phagosomes via a TLR-dependent signaling mechanism [ 132 , 134 , 135 ]. It is currently unknown how Rab17 prevents MHC II recruitment to the phagosome, with this process currently being under investigation in our lab.…”