Enhancement of monocyte complement component synthesis by antigen--antibody complexes

Antigen--antibody complexes were found to enhance the synthesis of the complement components C2, C4, C3, C5, factor B, properdin, C3b inactivator and beta 1H by human monocytes in tissue culture. The synthesis of all components was increased by complexes in a dose-dependent fashion. Insoluble complexes formed at equivalence (antigen--antibody ratio 2:1) were more effective than complexes formed at eight times antigen excess (antigen--antibody ratio 16:1), two times antigen excess (antigen--antibody ratio 4:1) or four times antibody excess (antigen--antibody ratio 1:2). The latter three species of complexes each consist of a mixture of soluble and insoluble complexes. It was shown that total complexes (soluble and insoluble) were more potent than soluble complexes at stimulating complement component synthesis. Soluble complexes of different molecular sizes were prepared by gel-filtration chromatography; larger complexes enhance C2 synthesis to a greater extent than small complexes. The enhanced synthesis of the functionally active complement components by mononuclear phagocytes induced by antigen--antibody complexes probably facilitates the handling of complexes by promoting their solubilization and degradation.