Fcγ-receptors Induce Mac-1 (CD11b/CD18) Mobilization and Accumulation in the Phagocytic Cup for Optimal Phagocytosis

Abstract

Functional interactions between Fcγ-receptors (FcγR) and the β2 integrin Mac-1 (CD11b/CD18) have been described, but the molecular basis of this relationship remains unclear. Although the glycosylphosphatidylinositol-linked receptor FγRIIIB of human neutrophils is constitutively associated with Mac-1, we found no evidence for direct physical association between Mac-1 and the FcγR of mouse macrophages, which are transmembrane proteins. Nevertheless, Mac-1 accumulated in the phagocytic cup following engagement of FcγR by IgG-opsonized particles. Blocking the CD18 chains of β2 integrins by using specific antibodies reduced Mac-1 accumulation in the cup. These antibodies or the addition of the recombinant CD11b 1-domain inhibited the ingestion of IgG-opsonized particles. FcγR cross-linking stimulated cell adhesion to surfaces coated with Mac-1 ligands and in addition enabled macrophages to bind C3bi-opsonized particles, indicating that FcγR-derived signals induce activation of Mac-1. Measurements of fluorescence recovery after photobleaching revealed that whereas most (>80%) of Mac-1 is immobile in resting cells, stimulation of FcγR markedly increases the mobile fraction of the integrin. Activation of Mac-1 by FcγR required the activity of Src family tyrosine kinases, phosphatidylinositol 3-kinase and phospholipase C, with the release of diacylglycerol and stimulation of protein kinase C. Because elevated cytosolic Ca2+ was not required, we suggest that novel protein kinase C isoforms are involved in Mac-1 activation. These results suggest that FcγR stimulation promotes Mac-1 clustering into high avidity complexes in phagocytic cups by releasing the integrin from cytoskeletal constraints and enhancing its lateral diffusion. FcγR can enhance host defense by activating Mac-1 (and possibly other integrins), having a synergistic effect on pathogen engulfment and promoting the adherence of phagocytes at sites of infection.