FcgammaR-mediated phagocytosis by human macrophages involves Hck, Syk, and Pyk2 and is augmented by GM-CSF

The receptors for the constant region of immunoglobulin G (FcgammaR) are widely expressed on cells of hemopoietic lineage and plays an important role in host defense. We investigated the signaling pathways during FcgammaR-mediated phagocytosis in human monocyte-derived macrophages (MDMs) and examined the effect of granulocyte-macrophage colony-stimulating factor (GM-CSF) on these events. FcgammaR-mediated phagocytosis resulted in enhanced tyrosine phosphorylation of a wide range of cellular proteins and activation of tyrosine kinases Hck, Syk, and Pyk2, as well as the multidomain adapter protein paxillin. Stimulation of MDMs with GM-CSF augmented FcgammaR-mediated phagocytosis and increased the levels of tyrosine phosphorylation in phagocytosing MDM cultures, indicating tyrosine kinase-mediated activation. GM-CSF treatment of MDMs without a phagocytic stimulus did not activate Syk, suggesting that GM-CSF may act either distally to Syk in the FcgammaR-mediated signaling cascade or on a parallel pathway activated by the FcgammaR. This study shows that early signaling events during FcgammaR-mediated phagocytosis in human MDMs involve activation of Syk, Hck, and paxillin. It also provides the first evidence for Pyk2 activation during phagocytosis and a baseline for further studies on the effect of GM-CSF on FcgammaR-mediated phagocytosis.