Adsorbed IgG: A potent adhesive substrate for human macrophages

Previous reports from our laboratory have demonstrated qualitatively that preabsorbed IgG can enhance long-term macrophage adhesion in vitro. This investigation further characterizes and quantifies the biological effect of adsorbed human IgG on human macrophages and probes the potential mechanisms. Ten-day human monocyte/macrophage cultures on Plastek M (PM), a normally poor cellular substrate for macrophages, confirmed the ability of preabsorbed IgG to dramatically enhance long-term macrophage adhesion. An adsorption solution concentration of 200 microg/mL of IgG was necessary to provide a consistent, optimal cellular response. (125)I adsorption studies indicated Langmuir-style IgG adsorption at low concentrations; however, no adsorption maximum was observed. Additional adsorption analysis revealed that the IgG fragments Fab, F(ab')(2), and Fc adsorb at levels only 20-40% that of the whole molecule. Despite the lower adsorption levels, both preabsorbed Fab and F(ab')(2) were shown to be as effective as whole molecule IgG at enhancing long-term macrophage adhesion. Surprisingly, the preabsorbed Fc fragment demonstrated no IgG-like activity, thereby eliminating the possibility of an Fc receptor-based mechanism. Other possible mechanisms, such as macrophage lectins, novel macrophage Fab receptors, and complement activation by adsorbed IgG and IgG fragments, are discussed.