Increased Number of IgG Fc Receptors on Monocyte-Enriched Peripheral Blood Leucocytes from Patients with Rheumatoid Arthritis

The number of free Fc receptors (FcR) per cell and the association constant (K_ass) for the binding of monomeric IgG were determined for monocyte-enriched peripheral blood mononuclear cells, isolated from 16 patients with active classical rheumatoid arthritis (RA) and from 15 normal healthy donors. The assay system was based on binding under equili brium conditions of ¹²⁵I-labelled monomeric rabbit IgG to monocytes purified from peripheral blood on a continuous gradient of Petcoll. Monocytes from 14 untreated RA patients (6 seropositive, 8 seronegative) expressed on the average 4.8±1.3 × 10⁴ FcR/cell. This number was significantly higher (P<0.01) than that found in the control group (3.46±0.7 × 10⁴ FcR/cell). There was also a significant difference between the mean K _ass of the RA group and the control group-2.1±0.7 × 10³1/mol and 2.6±1.0 × 10³ 1/mol, respectively (0.05 >P> 0.01). Two seropositive RA patients receiving systemic treatment with penicillamine expressed the same number of FcR/cell as the mean of the control group (3.6 ± 10⁴). Levels of circulating immune complexes (CIC) and of the complement-factor C3 split product C3d were also measured. No correlation was found between the number of FcR/cell and the concentration of C3d, but there was a weak correlation between the number of FcR/ccll and the level of CIC.