Phagocytosis is a pivotal process by which macrophages eliminate microorganisms after recognition by pathogen sensors. Here we unexpectedly found that the self ligand and cell surface receptor SLAM functioned not only as a costimulatory molecule but also as a microbial sensor that controlled the killing of Gram-negative bacteria by macrophages. SLAM regulated activity of the NADPH oxidase NOX2 complex and phagolysosomal maturation after entering the phagosome, following interaction with the bacterial outer membrane proteins OmpC and OmpF. SLAM recruited a complex containing the intracellular class III phosphatidylinositol kinase Vps34, its regulatory protein kinase Vps15 and the autophagy-associated molecule beclin-1 to the phagosome, which was responsible for inducing the accumulation of phosphatidylinositol-3-phosphate, a regulator of both NOX2 function and phagosomal or endosomal fusion. Thus, SLAM connects the Gram-negative bacterial phagosome to ubiquitous cellular machinery responsible for the control of bacterial killing.
MyD88 is an adaptor protein for the TLR family of proteins that has been implicated as a critical mediator of innate immune responses to pathogen detection. In this study, we report that MyD88 plays a crucial role in killing Gram-negative bacteria by primary macrophages via influencing NADPH oxidase function. Peritoneal macrophages from MyD88−/− mice exhibited a marked inability to kill Escherichia coli (F18) or an attenuated strain of Salmonella typhimurium (sseB) in vitro. This defect in killing was due to diminished NADPH oxidase-mediated production of superoxide anion in response to bacteria by MyD88−/− phagocytes as a consequence of defective NADPH oxidase assembly. Defective oxidase assembly in MyD88-deficient macrophages resulted from impaired p38 MAPK activation and subsequent phosphorylation of p47phox. Together these data demonstrate a pivotal role for MyD88 in killing Gram-negative bacteria via modulation of NADPH oxidase activity in phagocytic cells.
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