Quantitative analyses of C3b capture and immune adherence of IgM antibody/dsDNA immune complexes

We isolated the IgM fraction from the plasma of an SLE patient with high titer anti-dsDNA antibodies and prepared soluble IgM/dsDNA immune complexes (IC) that fixed C and captured sufficient C3b to bind to human E via their C3b/C4b receptor, CR1 (immune adherence, IA). We used specific 125I-labeled mAb to IgM, C3b, and IgG to measure the stoichiometries of these C-opsonized IC. They contained 10 to 60 C3b and 10 to 30 IgM per PM2 dsDNA, had no detectable IgG, and the vast majority of the C3b was bound to the IgM, and not to the dsDNA. These stoichiometries are in contrast to those we observed for comparable E-bound IC prepared with IgG anti-dsDNA antibodies (100 to 200 C3b, and 200 to 500 IgG). Our results help explain the greater lability of the IgM IC with respect to IA as evidenced by their plasma-mediated release from human E (presumably due to factor I), and confirm previous predictions of a lower density of "packing" of IgM on dsDNA, compared to IgG. The detailed stoichiometry of C3b capture by the IgM IC (typically 1.5 to 3 C3b per IgM) suggests that individual IgM molecules with multiple C3b facilitate IC binding to clusters of CR1. Finally, comparison of the IgM/dsDNA IC with other IgM IC which have been investigated with respect to C activation, and review of the proposed mechanism by which IgM activates C, suggests that the nature of the Ag plays a fundamental role in determining whether or not an IgM IC can activate C and participate in IA.