Characteristics of iC3b binding to human polymorphonuclear leucocytes

We determined in binding assays using monomeric fluid-phase iC3b and Scatchard analysis that iC3b binds to human polymorphonuclear leucocyte type 3 complement receptor (CR3), a low-density/high affinity receptor (28,200 binding sites, affinity constant (Ka) = 2.1 ± 0.47 x 10⁶ L/M), and to the C3b receptor (CR1), a high-density/low-affinity receptor (54,700 binding sites, Ka = 1.7 ± 2.04 x10⁵ L/M. Binding of iC3b to CR1 was confirmed by blocking experiments with polyclonal F(ab')₂ antibody against CR1, and competitive binding experiments with C3b. Binding of iC3b to CR3 was demonstrated by blocking experiments with the monoclonal antibody OKM10 against the ligand binding site of CR3. Inhibition of both CR1 and CR3 did not completely reduce iC3b binding, indicating the existence of additional iC3b-binding sites on PMN. Using flow cytometric analysis of receptor expression, no positive or negative co-operativity was observed between CR1 and CR3. Expression of both receptors increased in a dose-dependent manner after incubation with f-met-leu-phe or phorbol myristate acetate; however, only CR3 expression was enhanced at very low concentrations of these stimuli. iC3b/CR3 interactions probably play a central role in host defence against microorganisms.