Quantitation of (C1INH)2 C1r-C1s complexes in glomerulonephritis as an indicator of C1 activation.

C1 activation was assessed in several forms of glomerulonephritis by radioimmunoassay quantitation of circulating (C1INH)2 C1r-C1s complexes (INC). Eight patients with active systemic lupus erythematosus (SLE) and nephritis had elevated serum INC (mean = 15.3 vs control = 5.8, P less than 0.01). Their INC levels were normal during remission. Serum INC had a weak inverse correlation with serum C1q greater than 3 mg/dl (r = 0.42, P = 0.02). In longitudinal studies, serum INC also had a weak inverse correlation with serum C3 and C4. Only 1 of 10 patients with type I and 1 of 15 with type III membrano-proliferative glomerulonephritis (MPGN) had elevated serum INC. No patient with type II MPGN had elevated levels. Two of 10 patients with poststreptococcal glomerulonephritis (P-SGN) had elevated serum INC, but all normalized with convalescence. Patients with IgA nephropathy had normal serum INC. The data demonstrate the importance of C1 activation in SLE and P-SGN. The mechanism of complement activation in types I and III MPGN remains unclear; the data suggest, but do not prove, that C1-independent complement activation may occur in these patients.