When cultured human monocytes (MO) were spread on fibronectin (Fn)-coated surfaces, C3 receptors on the MO exhibited markedly enhanced capacity to promote phagocytosis. The activation of C3 receptors by Fn was mediated by a receptor that recognizes a sequence, Arg-Gly-Asp-Ser (RGDS), present in the cell-binding domain of Fn. Soluble, RGDS-containing peptides inhibited the activation of C3 receptors caused by surface-bound Fn, and surface-bound, RGDS-containing peptides themselves caused activation of the C3 receptors of attached MO. Although soluble, RGDS-containing peptides bound to Fn receptors, such monovalent ligation was insufficient to activate C3 receptors.

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