Concurrent and independent binding of Fc gamma receptors IIa and IIIb to surface-bound IgG

Fc receptor-antibody interactions are key mechanisms through which antibody effector functions are mediated. Neutrophils coexpress two low-affinity Fc gamma receptors, CD16b (Fc gamma RIIIb) and CD32a (Fc gamma RIIa), possess ing overlapping ligand binding specificities but distinct membrane anchor a nd signaling capacities. Using K562 cell transfectants as a model, the kine tics of both separate and concurrent binding of CD16b and CD32a to surface- bound IgG ligands were studied. CD16b bound human IgG with 2-3 times higher affinity than did CD32a (A(c)K(a) = 4.1 and 1.6 x 10(-7) mu m(4), respecti vely) and both Fc gamma Rs had similar reverse kinetic rates (k(r) = 0.5 an d 0.4 s(-1), respectively). Because CD16b is expressed on neutrophils at a 4-5 times higher density than CD32a, our results suggest that CD16b plays t he dominant role in binding of neutrophils to immobilized IgG. The question of possible cross-regulation of binding affinity between CD16b and CD32a w as investigated using our multispecies concurrent binding model (Zhu and Wi lliams, Biophys. J. 79:1850-1857, 2000). Because the model assumes independ ent binding (no cooperation among different species), the excellent agreeme nt between the model predictions and the experimental data suggests that, w hen coexpressed on K562 cells, these two Fc gamma Rs do not interact in a m anner that alters the kinetic rates of either molecule.