Generation of low m.w., C3-bearing immunoglobulin in human serum

The generation of low m.w. C3-bearing immunoglobulin (lg) in normal human serum by an immune complex (IC) model was investigated in vitro by using discontinuous sucrose density gradient centrifugation (DGC) and an assay that measures C3-bearing Ig. In this method developed to measure circulating IC, all C3 and C3-bearing material is precipitated from serum by using anti-C3 sera in C3d antibody excess, and immune precipitated, C3-bearing Ig is quantitated by the uptake of 125I-5S-anti-IgG. When plasma from patients with clinically active systemic lupus erythematosus was assayed after DGC, most of the reactive material was low m.w. (7S), rather than greater than or equal to 19S as expected for IC, in agreement with a previous report. Low m.w., C3-bearing Ig was found in normal EDTA plasma after extended storage at -29 degrees C but not after storage at -70 degrees C. Such material was also generated in normal human serum during incubation at 37 degrees C and its generation was stimulated by the addition of an IC model, high m.w., heat-aggregated IgG (HMW-HAIgG). In experiments in which the participation of serum IgG was monitored by the addition of 125I-7S-IgG and 131I-HMW-HAIgG was used as an IC model, low m.w., C3-bearing Ig was generated exclusively from serum IgG and the amount generated was proportional to the concentration of 131I-HMW-HAIgG. No significant decrease in sedimentation of 131I-HMW-HAIgG was observed, but the ability of anti-C3 sera to precipitate 131I-HMW-HAIgG decreased 66% 4 hr after initial C activation. These results indicate that generation of nascent C3b in serum results in its interaction with monomeric serum IgG, producing low m.w., C3-bearing IgG. In addition, the data indicate that circulating IC that activate C have a brief time span during which they can be detected by methods that depend upon the binding of C3.