Cell type-specific glycoforms of Fc gamma RIIIa (CD16): differential ligand binding

Fc gamma RIIIa, considered an intermediate affinity receptor, can variably bind monomeric IgG and appears to have a higher affinity for IgG than the lower affinity Fc gamma Rs, Fc gamma RII and Fc gamma RIIIb. We explored this property for both NK cell and monocyte Fc gamma RIIIa and found higher affinity ligand binding by Fc gamma RIIIa expressed on NK cells compared with Fc gamma RIIIa on monocytes. In normal whole blood or plasma (containing 8-11 mg/ml IgG), NK cell Fc gamma RIIIa was fully blocked, but in monocytes Fc gamma RIIIa showed approximately 60% blockade of the binding of mAb 3G8, which binds in or near the ligand binding site. The ligand binding site of NK cell Fc gamma RIIIa was blocked with as little as 2 mg/ml of human IgG, while monocyte Fc gamma RIIIa was only partially (30%) blocked by 2 mg/ml of human IgG. In contrast, plasma containing approximately 26 mg/ml of IgG (obtained from patients receiving therapeutic gamma-globulin) showed complete saturation of monocyte Fc gamma RIIIa with blockade of mAb 3G8 binding. These binding differences are not due to allelic polymorphisms or primary sequence differences between donors. Although NK cell and monocyte Fc gamma RIIIa have identical protein cores, they each undergo differential cell type-specific glycosylation. NK cell Fc gamma RIIIa is glycosylated with high mannose- and complex-type oligosaccharides, while monocyte Fc gamma RIIIa has no high mannose-type oligosaccharides. These results indicate that natural glycoforms of Fc gamma RIIIa (cell type-specific glycosylation variants) bind ligand differently, conferring a lower affinity on monocyte/macrophage Fc gamma RIIIa, which makes the receptor ideal for initial immune complex capture and sensitive to moderate changes in serum IgG levels.