Abstract
The integrin CD11b/CD18 plays a central role in neutrophil phagocytosis. Although CD11b/CD18 binds a wide range of ligands, including C3bi and beta-glucan, and transmits outside-in signaling, the mechanism of this signaling responsible for phagocytosis remains obscure. Here, we report that lactosylceramide (LacCer)-enriched lipid rafts are required for CD11b/CD18-mediated phagocytosis of nonopsonized zymosans (NOZs) by human neutrophils. Anti-CD11b and anti-LacCer antibodies inhibited the binding of NOZs to neutrophils and the phagocytosis of NOZs. During phagocytosis of NOZ, CD11b and LacCer were accumulated and colocalized in the actin-enriched phagocytic cup regions. Immunoprecipitation experiments suggested that CD11b/CD18 was mobilized into the LacCer-enriched lipid rafts during phagocytosis of NOZs. DMSO-treated, neutrophil-like HL-60 cells (D-HL-60 cells) lacking Lyn-coupled, LacCer-mediated signaling showed little phagocytosis of NOZs. However, loading of D-HL-60 cells with C24 fatty acid chain-containing LacCer (C24-LacCer) reconstructed functional Lyn-associated, LacCer-enriched lipid rafts, and restored D-HL-60 cell NOZ phagocytic activity, which was inhibited by anti-LacCer and anti-CD11b antibodies. Lyn knockdown by small interfering RNA blocked the effect of C24:1-LacCer loading on D-HL-60 cell phagocytosis of NOZs. CD11b/CD18 activation experiments indicated phosphorylation of LacCer-associated Lyn by activation of CD11b. Taken together, these observations suggest that CD11b activation causes translocation of CD11b/CD18 into Lyn-coupled, LacCer-enriched lipid rafts, allowing neutrophils to phagocytose NOZs via CD11b/CD18.