In vitro studies on the Fc-receptor function of mononuclear phagocytes in rheumatoid arthritis: relation between the Fc-receptor blockade and the concanavalin A-binding capacity of autologous immunoglobulin G

The Fc-receptor (Fc-R) function of monocytes isolated from 19 control subjects and from 30 patients presenting with a rheumatoid arthritis (RA) was assessed in vitro by a classical rosette assay using IgG-coated sheep red blood cells. In RA patients, the percentage of monocytes forming rosettes was significantly lower than in controls (34.4 +/- 20.4 versus 67.4 +/- 4.5%; P less than 0.001). The blockade observed was reversed by a prior trypsin treatment of RA monocytes, the percentage of recovery being correlated with the IgG plasma levels. Besides, IgG purified from the serum of four RA patients bound a mean of 7.3, 5.2, 1.6, and 1.6 times more than normal IgG did onto concanavalin A (Con A), peanut agglutinin (PNA), phytohemagglutinin (PHA), and pokeweed mitogen (PWM), respectively. Although similar amounts of 125I-labeled normal and RA IgG were bound to normal monocytes, RA IgG inhibited more efficiently than normal IgG the Fc-R function of normal monocytes, for all concentrations tested (10 to 100 micrograms/100 microliters). A prior treatment of RA IgG by alpha-mannosidase, but not by beta-galactosidase, significantly reduced their inhibitory properties. The incubation of monocytes with D-mannose or mannan reduced their capacity to form rosettes. The percentage of monocytes forming rosettes in the presence of both mannan and normal IgG was significantly lower than that measured in the presence of normal IgG only. On the contrary, the rosetting capacity of monocytes in the presence of both RA IgG and mannan was the same as that calculated in the presence of RA IgG only. The inhibitory effect of RA IgG was not related to their abnormal circular dichroism. Our data suggest that the greater ability of RA IgG to block the Fc-R function of monocytes probably depends on the presence of a greater number of accessible mannosyl residues on the glycosidic side chains located in the Fc domain of the molecules