We have previously shown that immune complexes isolated from plasma and synovial fluids of children with juvenile rheumatoid arthritis vary in size, composition, and phlogistic potential. The studies reported here examined the heterogeneity of immune complexes formed in vitro under a variety of conditions.

We formed BSA-anti-BSA immune complexes at a fixed antigen:antibody ratio, exposed the complexes to serum during or after their formation, and measured complement binding to the complexes using quantitative ELISA assays. We found that performed, solubilized immune precipitates bound significantly more C4 (19.2 ± 7.0 ng/mg total protein) than soluble immune complexes(opsonized complexes) formed in the presence of 20% normal human serum (10.9 ± 3.0 ng/mg total protein; p=0.03). Soluble immune complexes which were added to serum after their formation (preformed complexes) bound an intermediate amount of C4 (11.9 ± 2.5 ng/mg total protein). Preformed complexes, however, bound significantly more C3 (73± 20 ng/mg total protein) than opsonized complexes (33.2 ± 5.8 ng/mg total protein; p=0.01), while solubilized immune precipitates bound an intermediate amount of C3 (41.2 ± 15 ng/mg total protein). The molar ratios of C3:C4 bound to the complexes (expressed in nM/mg total protein) also differed between preformed soluble complexes (1.56) vs. opsonized complexes(0.81) vs. solubilized immune precipitates 0.73).

We conclude that there is considerable variation in the composition of immune complexes which depend upon the means through which such complexes interacted with complement. A fundamental question in our understanding of the role of immune complexes in human disease is the degree to which these differences affect the phlogistic capacity of immune complexes.