Comparison of the binding of C3S and C3F to complement receptors types 1, 2, and 3

The two common polymorphic variants of C3 are C3S/HAV4.1- and C3F/HAV4.1+. It was reported previously that erythrocytes coated with C3F rosetted more strongly with mononuclear cells than erythrocytes coated with C3S. We examined the binding of C3S/HAV4.1- and C3F/HAV4.1+ to complement receptors CR1, CR2, and CR3. The binding of 125I-labeled C3b dimers to erythrocyte CR1 was measured. Scatchard analysis showed a two-binding constant model with very similar binding constants for dimers prepared with C3S and C3F: for C3S Kd1 = 18.3 +/- 2 nM; Kd2 = 6.2 +/- 1 nM; for C3F Kd1 = 21.5 +/- 4 nM; Kd2 = 7.2 +/- 3 nM (mean +/- SEM). One-third of the binding sites were of the higher affinity. The rosetting of erythrocytes with different densities of iC3b, prepared from C3S or C3F, to CR2 on Raji cells was analyzed. The percentage of Raji cells rosetted was related to the coating dose of EC3bi: 400 molecules/cell = 9% rosettes; 5000 molecules/cell = 75%. The dose-response curves were very similar for C3S- and C3F-coated erythrocytes. CR3-dependent rosetting was studied in a similar manner by using neutrophils activated with f-met-leu-phe. CR3-dependent rosette formation with the indicator erythrocytes (600 to 6000 iC3b/cell) increased from 10% to 60% in a dose-dependent manner and was closely similar for C3S and C3F. Inhibition of CR2 and CR3 rosetting by fluid phase ligand was also studied. iC3b dimers (0.4 to 50 micrograms/ml) inhibited CR2-dependent rosetting in a dose-dependent manner but had no inhibitory effect on CR3-dependent rosetting. When the dimers were absorbed to fluorescent microspheres, they mediated phagocytic uptake of the microspheres in a CR3-dependent manner by neutrophils. CR3-dependent binding by activated neutrophils required surface-bound ligand.