The complement fixing ability of putative circulating immune complexes in rheumatoid arthritis and its relationship to extra-articular disease.

The hypothesis that the pathogenicity of putative circulating immune complexes (CIC) in rheumatoid arthritis (RA) is related to their ability to fix complement was investigated. Three assays for CIC were employed; (a) the 125I C1q binding assay (C1q BA), (b) the C1q solid phase assay (C1q SP) and (c) the Raji cell assay (RCA). Evidence for hypercatabolism of complement was obtained by using a highly sensitive quantitative assay for C3d (a breakdown product of C3) by rocket immunoelectrophoresis. One hundred and fifty-two patients with classical or definite RA were studied; 54 had clinical evidence of extra-articular disease including vasculitis, nodules, scleritis, neuropathy and lung disease; 98 patients had clinical evidence of joint disease alone. Plasma levels of C3d were significantly elevated in the RA group as a whole 16.7 +/- 4.4 mg/l (mean +/- 1 s.d.) compared with 13.1 +/- 3.25 mg/l in a group of 55 normal controls (P less than 0.01). Elevated levels of C3d were found in 26% of all patients but occurred significantly more often in the extra-articular disease group (P less than 0.05). Fifty-four percent of patients had at least one positive assay for CIC although no individual assay was positive in more than 36% of the group as a whole. The prevalence of positive CIC was significantly greater in those patients with extra-articular disease than in those with joint disease alone (P less than 0.005). Of the total of 82 patients with putative CIC, 30 (37%) had a raised C3d level. The coincident finding of positive tests for CIC and an elevated C3d level was very significantly correlated with the presence of extra-articular disease (chi 2 = 12.7 P = 10(-3)). Whilst putative CIC are frequent in RA (54%) these findings in contrast to previous work, suggest that the majority are not associated with abnormal complement activation and may account for the relative infrequency of clinically detectable active extra-articular disease.