Inhibition of Complement-dependent Phagocytosis by Monocytes Following Pre-incubation with Immune Complexes and Interaction with Plasma

Normal monocytes have been pre-incubated with soluble immune complexes (IC) (BSA-anti-BSA) or heat-aggregated human IgG (AHG) (ICs) at pathophysiological concentrations. If these 'IC-monocytes' are allowed to interact with plasma they show reduced phagocytosis of Candida guilliermondii targets. Under the pre-incubation conditions used, 2% of offered ICs remain cell associated and their binding to monocytes is energy-independent and complement-independent. At these low concentrations of ICs, only C3-dependent phagocytosis is inhibited, although Fc-dependent phagocytosis is inhibited at high concentrations of AHG. Inhibition of C3-dependent phagocytosis requires a critical concentration of plasma (greater than or equal to 2%), complement C3, and functionally intact classical and alternative complement pathways. The observations could be explained by blockade or modulation of C3 receptors by IC associated C3b or by an effect on ingestion. The abnormality induced in normal monocytes in vitro closely resembles that found in monocytes from patients with active IC disease.