COMPLEMENT ACTIVATION BY ANTIBODIES TO SM IN SYSTEMIC LUPUS-ERYTHEMATOSUS

Abstract

An enzyme linked immunosorbent assay was developed to quantitate antibodies to Sm (anti-Sm) and to measure complement activation by anti-Sm in vitro. Anti-Sm in plasma of patients with systemic lupus erythematosus (SLE) were bound to purified Sm bound to polyvinyl chloride microtiter plates and assayed for bound IgG or IgM using enzyme linked anti-.gamma. or anti-.mu.. The activation of C4 [complement component 4] by anti-Sm was measured by adding diluted normal human serum (complement) to the wells and quantitating the amount of C4 bound to the well surface using (Fab'')2 goat anti-C4 followed by enzyme linked rabbit anti-goat IgG. The plasmas of 12 of 36 patients with SLE contained anti-Sm and all 12 activated complement (complement activating anti-Sm). Plasmas (28) containing anti-Sm from 12 patients with SLE were studied. Ten of the 12 patients had anti-Sm of the IgG class whereas 2 had anti-Sm of both IgG and IgM classes. The amount of C4 activating anti-Sm correlated significantly with the in vivo activation of C4 measured by rocket immunoelectrophoresis for C4d and C4, suggesting that complement activation by anti-Sm is important in vivo.