title: CD44-mediated phagocytosis induces inside-out activation of complement receptor-3 in murine macrophages.
abstract: Diverse receptors, including Fcgamma receptors and beta(2) integrins (complement receptor-3 [CR3], CD11b/CD18), have been implicated in phagocytosis, but their distinct roles and interactions with other receptors in particle engulfment are not well defined. CD44, a transmembrane adhesion molecule involved in binding and metabolism of hyaluronan, may have additional functions in regulation of inflammation and phagocytosis. We have recently reported that CD44 is a fully competent phagocytic receptor that is able to trigger ingestion of large particles by macrophages. Here, we investigated the role of coreceptors and intracellular signaling pathways in modulation of CD44-mediated phagocytosis. Using biotinylated erythrocytes coated with specific antibodies (anti-CD44-coated erythrocytes [Ebabs]) as the phagocytic prey, we determined that CD44-mediated phagocytosis is reduced by 45% by a blocking CD11b antibody. Further, CD44-mediated phagocytosis was substantially (42%) reduced in CD18-null mice. Immunofluorescence microscopy revealed that CD11b is recruited to the phagocytic cup. The mechanism of integrin activation and mobilization involved activation of the GTPase Rap1. CD44-mediated phagocytosis was also sensitive to the extracellular concentration of the divalent cation Mg(2+) but not Ca(2+). In addition, buffering of intracellular Ca(2+) did not affect CD44-mediated phagocytosis. Taken together, these data suggest that CD44 stimulation induces inside-out activation of CR3 through the GTPase Rap1.
authors.affiliation: Division of Respirology, Department of Medicine, University of Toronto, Toronto, ON, Canada.
authors.forename: Eric
authors.initials: E
authors.lastname: Vachon
authors.forename: Raiza
authors.initials: R
authors.lastname: Martin
authors.forename: Vivian
authors.initials: V
authors.lastname: Kwok
authors.forename: Vera
authors.initials: V
authors.lastname: Cherepanov
authors.forename: Chung-Wai
authors.initials: CW
authors.lastname: Chow
authors.forename: Claire M
authors.initials: CM
authors.lastname: Doerschuk
authors.forename: Jonathan
authors.initials: J
authors.lastname: Plumb
authors.forename: Sergio
authors.initials: S
authors.lastname: Grinstein
authors.forename: Gregory P
authors.initials: GP
authors.lastname: Downey
mesh_terms: D000818:Animals; D039481:CD11b Antigen; D018821:CD18 Antigens; D018960:Hyaluronan Receptors / Q000276:immunology*; D016177:Macrophage-1 Antigen / Q000276:immunology*; D008264:Macrophages / Q000276:immunology*; D051379:Mice; D010587:Phagocytosis / Q000276:immunology*; D015398:Signal Transduction; D020827:rap1 GTP-Binding Proteins / Q000378:metabolism