Activation of classical pathway complement in chronic inflammation. Elevated levels of circulating C3d and C4d split products in rheumatoid arthritis and Crohn's disease.

Split products of complement component 3 (C3) and complement component 4 (C4) derived from activation of the alternative and classical complement pathways were measured in untreated outpatients, 20 with Crohn's disease and 19 with rheumatoid arthritis. Elevated levels of the d split product of C4 (C4d) were observed in 12 of 19 patients with rheumatoid arthritis and in 9 of 20 patients with Crohn's disease. Levels of the d split product of C3 (C3d) were increased in 14 of 19 patients with rheumatoid arthritis and in 6 of 20 Crohn's disease patients. The median values of C4d and C3d were significantly increased in both groups of patients. C3d concentrations correlated positively with C4d levels (rs = 0.51-0.56, p less than 0.005). The complement activation was not reflected in reduced plasma levels of native C3 and C4. The data indicate activation of the classical complement pathway in both rheumatoid arthritis and Crohn's disease.