A factor activating complement via the alternative pathway in the supernatants of B cell lines transformed by Epstein-Barr virus and in sera obtained from patients with systemic lupus erythematosus.

Serum factors activating the alternative pathway of the complement (APC) were detected in 5 of 14 patients with systemic lupus erythematosus (SLE). Epstein-Barr virus (EBV)-transformed B cell lines were subsequently established from these patients and 6 of these produced factors capable of activating the APC. Using a limiting dilution technique, we obtained a clone which was producing a factor activating the APC (AF); by affinity column fractionation and polyacrylamide gel electrophoresis, the AF was found to have heavy and light chains comparable to those of normal human IgG. Normal human serum exhibited C3 split products (demonstrated by immunoelectrophoresis) in the presence of AF and under conditions permitting activation of the APC. Sera devoid of factor B, but not of C2 and C4, failed to catabolize C3 in the presence of AF. The AF failed to stabilize erythrocyte-bound C3bBb or C4b2a convertases, indicating that it was not a nephritic-factor-like molecule. We conclude that IgG molecules present in the sera and produced by EBV-transformed B cell lines from patients with SLE are apparently responsible, at least partially, for complement consumption in these patients.