Heterogeneity, polypeptide chain composition and antigenic reactivity of autoantibodies (F-42) that are directed against the classical pathway C3 convertase of complement and isolated from sera of patients with systemic lupus erythematosus.

F-42, an autoantibody found in the sera of some patients with systemic lupus erythematosus, was isolated from sera of seven different patients. By binding of the autoantibodies to cell bound C42, the autoantibodies were purified to homogeneity and radiolabelled with 125I. Purified preparations of 125I-F-42 were bound for at least 96% by 10(9) erythrocytes bearing C4bhuC2hu. Analysis of all 125I-F-42 preparations by SDS-PAGE demonstrated apparent mol. wts of approximately 150,000. After reduction in the presence of 8 M urea each 125I-F-42 preparation yielded heavy and light chains. The heavy chains in some instances were slightly heavier than the heavy chain of normal human IgG. The reaction of 125I-F-42 from the seven patients was positive with Sepharose bound antisera to IgG, kappa, lambda, gamma 1, gamma 2, gamma 3 and in one case to gamma 4. These studies indicate that F-42 is an autoantibody directed against antigens expressed by the classical C3 convertase, C42.