Receptors on macrophages for the Fc region of IgG (FcγR) mediate a number of responses important for host immunity. Signaling events necessary for these responses are likely initiated bY the activation of Src-family and Syk-family tyrosine kinases after FcγR cross-linking. Macrophages derived from Syk-deficient (Syk-) mice were defective in phagocytosis of particles bound by FcγRs, as well as in many FcγR-induced signaling events, including tyrosine phosphorylation of a number of cellular substrates and activation of MAP kinases. In contrast, Syk- macrophages exhibited normal responses to another potent macrophage stimulus, lipopolysaccharide. Phagocytosis of latex beads and Escherichia coli bacteria was also not affected. Syk- macrophages exhibited formation of polymerized actin structures opposing particles bound to the cells by FcγRs (actin cups), but failed to proceed to internalization. Interestingly, inhibitors of phosphatidylinositol 3-kinase also blocked FcγR-mediated phagocytosis at this stage. Thus, PI 3-kinase may participate in a Syk-dependent signaling pathway critical for FcγR-mediated phagocytosis. Macrophages derived from mice deficient for the three members of the Src-family of kinases expressed in these cells, Hck, Fgr, and Lyn, exhibited poor Syk activation upon FcγR engagement, accompanied by a delay in FcγR-mediated phagocytosis. These observations demonstrate that Syk is critical for FcγR-mediated phagocytosis, as well as for signal transduction in macrophages. Additionally, our findings provide evidence to support a model of sequential tyrosine kinase activation by FcγR's analogous to models of signaling by the B and T cell antigen receptors.
Crowley, M. T., Costello, P. S., Fitzer-Attas, C. J., Turner, M., Meng, F., Lowell, C., … DeFranco, A. L. (1997). A critical role for Syk in signal transduction and phagocytosis mediated by Fcγ receptors on macrophages. Journal of Experimental Medicine, 186(7), 1027–1039. https://doi.org/10.1084/jem.186.7.1027