Circulating complement breakdown products in patients with rheumatoid arthritis

C3 breakdown product, C3d, was used to investi-gate complement activation in 45 plasma samples from 30 patients with rheumatoid arthritis (RA). The mean plasma C3d level in these samples (3.0 ± 1.3 mg/100 ml) was significantly increased (P < 0.001) as compared to patients with degenerative joint disease (0.9±0.4 mg/100 ml) and healthy blood donors (0.8±0.5 mg/100 ml). C3d levels were increased by more than 2 SD in 79 % of RA samples. Plasma C3d levels were compared with C3d concentrations in synovial fluid. In most RA patients, the C3d levels were higher in synovial fluid than in plasma. A very significant correlation between plasma C3d levels and circulating immune complexes, as measured by determination of Clq binding activity (Clq BA), was observed (P < 0.001). C3d levels were more ele-vated in RA patients with extra-articular disease mani-festations (3.8±1.2 mg/100 ml) as compared to patients with joint disease alone (2.2±1.0 mg/100 ml). C3d levels and Clq BA were also significantly correlated(P < 0.001) with the RA disease activity expressed by an index derived from sedimentation rate, joint score, and duration of morning stiffness. A close relationship between C3d levels, Clq BA, and the clinical activity further appeared during follow-up studies. The present observations suggest that a parallel but rather independent activation of the complemen