Increased Fc receptor activity in monocytes from patients with rheumatoid arthritis: a study of monocyte binding and catabolism of soluble aggregates of IgG in vitro.

We studied the binding and catabolic function of adherent monocytes from patients with rheumatoid arthritis (RA) and normal subjects using stable, heat aggregates of IgG125I (A-IgG) as a model for soluble immune complexes. Scatchard plots of 4 degrees C binding data showed that RA monocytes had increased binding avidity and higher maximal binding capacity for A-IgG compared with monocytes from normal subjects. These data suggest that RA monocytes have increased numbers of Fc receptors for IgG, although a concomitant increase in the avidity of individual Fc receptors could not be excluded. At 37 degrees C, RA monocytes; kinetic analysis suggested that increases in catabolized A-IgG were due to increased binding of A-IgG with no change in the fractional rates of catabolism. Latex titers of RA patients correlated with the number of Fc receptors detected on RA monocytes. Mononuclear phagocytes from RA patients are often exposed to endogenous immune complexes that may be present in the blood of such patients; immune complexes may stimulate monocytes and possibly other mononuclear phagocytes to increase their capacity to bind and catabolize soluble immune complexes.