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Phagocytosis through Fcγ receptor (FcγR) or complement receptor 3 (CR) requires Arp2/3 complex-mediated actin polymerization, although each receptor uses a distinct signaling pathway [1]. Rac and Cdc42 are required for actin and Arp2/3 complex recruitment during FcγR phagocytosis, while Rho controls actin assembly at CR phagosomes [2, 3]. To better understand the role of Rho in CR phagocytosis, we tested the idea that a known target of Rho, Rho-kinase (ROK), might control phagocytic cup formation and/or engulfment of particles. Inhibitors of ROK (dominant-negative ROK and Y-27632) and of the downstream target of ROK, myosin-II (ML7, BDM, and dominant-negative myosin-II), were used to test this idea. We found that inhibition of the Rho → ROK → myosin-II pathway caused a decreased accumulation of Arp2/3 complex and F-actin around bound particles, which led to a reduction in CR-mediated phagocytic engulfment. FcγR-mediated phagocytosis, in contrast, was independent of Rho or ROK activity and was only dependent on myosin-II for particle internalization, not for actin cup formation. While myosins have been previously implicated in FcγR phagocytosis [4–6], to our knowledge, this is the first demonstration of a role for myosin-II in CR phagocytosis.

BACKGROUND: beta2 integrins mediate many aspects of the inflammatory and immune responses, including adhesion of leukocytes to the endothelium, complement-mediated phagocytosis in macrophages and neutrophils, and antigen-specific conjugate formation between cytotoxic T cells and their targets. A variety of inflammatory mediators, such as tumor necrosis factor-alpha (TNF-alpha), platelet-activating factor (PAF), and lipopolysaccharide (LPS) and other bacterial products induce the functional activation of beta2 integrins, but the signaling events that link membrane receptors to integrin activation are poorly understood. RESULTS: We report here that expression of the constitutively active small GTPases Rap1 or R-ras, but not Ras or RalA, is sufficient for functional activation of alphaMbeta2, the complement receptor 3 (CR3), in macrophages, allowing phagocytosis of C3bi- opsonized targets. Inhibition of Rap1, but not other Ras-like or Rho- like small GTPases, abolishes activation of alphaMbeta2 induced by phorbol esters, LPS, TNF-alpha or PAF. Finally, Rap1 activation specifically controls the binding properties of alphaMbeta2 towards its physiological ligand, namely the complement-opsonized phagocytic targets. CONCLUSIONS: In macrophages, the Rap1 GTPase regulates activation of the alphaMbeta2 integrin in response to a wide variety of inflammatory mediators

A genetic pathway controlling the removal of apoptotic corpses has now been identified in the nematode. Mammalian homologs of the components of this pathway are already known to have key roles in cell motility and integrin-mediated signaling, suggesting that the three processes are mechanistically related.