Interaction of C3b(2)-IgG complexes with complement proteins properdin, factor B and factor H: implications for amplification

Nascent C3b can form ester bonds with various target molecules on the cell surface and in the fluid phase, Previously, we showed that C3b(2)-IgG compl exes represent the major covalent product of C3 activation in serum [Lutz, Stammler, Jelezarova. Nater and Spath (1996) Blood 88, 184-193]. In the pre sent report, binding of alternative pathway proteins to purified C3b(2)-IgG complexes was studied in the fluid phase by using biotinylated Ige for C3b (2)-IgG generation and avidin-coated plates to capture complexes. Up to sev en moles of properdin 'monomer' bound per mole of C3b(2)-IgG at physiologic al conditions in the absence of any other complement protein, At low propsr din/C3b(2)-IgG ratios bivalent binding was preferred. Neither factor H nor factor B affected properdin binding. On the other hand, properdin strongly stimulated factor B binding. Interactions of all three proteins with C3b(2) -IgG exhibited pH optima. An ionic strength optimum was most pronounced for properdin, while factor B binding was largely independent of the salt conc entration. C3b(2)-IgG complexes were powerful precursors of the alternative pathway C3 convertase. In the presence of properdin, C3 convertase generat ed from C3b(2)-IgG cleaved about sevenfold more C3 than the enzyme generate d on C3b. C3b(2)-IgG complexes could therefore maintain the amplification l oop of complement longer than free C3b.