Quantification by ELISA of erythrocyte-bound C3 fragments expressing C3d and/or C3c epitopes in patients with factor I deficiency and with autoimmune diseases.

A sensitive ELISA assay for quantifying erythrocyte (E) bound C3 fragments was developed. The assay employs a double-antibody sandwich technique, using polyclonal anti-C3d or anti-C3c antibodies to quantify C3 fragments, expressing C3d and/or C3c epitopes in washed, detergent-solubilized E. The assay detected 50-120 molecules of C3d per E in healthy individuals. Antigens reacting with anti-C3c antibodies were also detected on E from normal individuals, but the density of C3c-epitopes was 0.9-2.4 times lower than that of C3d-epitopes. In 2 patients with congenital factor I deficiency significantly increased density of E-bound C3c- as well as C3d-antigen was observed. Plasma infusion in one of the patients induced a loss of E-bound C3c-antigens, indicating cleavage of E-bound C3b to iC3b and further to C3c and E-bound C3d. Loss of C3c-antigens also occurred following in vitro treatment with normal human serum of E from one of the patients. Two thirds of 22 patients with systemic lupus erythematosus (SLE) and of 18 patients with rheumatoid arthritis had significantly increased density of E-bound C3d, the highest density being 490 C3d molecules/E in an SLE patient. The density of E-bound C3d correlated with the plasma-C3d concentration, indicating that the coating of E with C3d reflects the degree of complement activation.