No abstract

Binding of the Fc region of IgG antibodies to low affinity Fc gamma receptors (Fc gammaR) triggers important effector functions in the immune system. The type IIIb Fc gammaR (Fc gammaRIIIb or CD16) is a heavily glycosylated protein anchored to the membrane of neutrophils by a glycosylphosphatidylinositol link. This receptor contributes to cell activation by IgG immune complexes. To better understand the nature of the ligand-receptor association, we have studied the affinity and kinetics of the interaction between human IgG subclasses and two soluble forms of Fc gammaRIIIb (sFc gammaRIIIb or sCD16) corresponding to the 188 N-terminal amino acids of the extracellular region of the receptor, a glycosylated one made in eucaryotic cells (euc.sCD16) and a non-glycosylated one (proc.sCD16) made in Escherichia coli. Experiments using a BIAcore instrument, to measure protein binding in real time, showed that monomeric human IgG1 and IgG3, but not IgG2, IgG4, IgA and divalent antigen-binding fragments (F(ab')2) of IgG1, bound to immobilized euc.sCD16 with an affinity constant (K(A)) of 1.3 +/- 0.6 x 10(6) M(-1) and 2.6 +/- 0.4 x 10(5) M(-1), respectively. The affinity constant of proc.sCD16 for human IgG1 was in the same range (1.1 +/- 0.2 x 10(6) M(-1)), whereas that for human IgG3 was twofold higher (4.2 +/- 0.4 x 10(5) M(-1)). The specificity of the non-glycosylated receptor for human IgG subclasses bound to Sepharose was IgG1 > IgG3 >> IgG4 >>> IgG2. Thus, the extracellular polypeptide of Fc gammaRIIIb dictates the interaction of the receptor with IgG subclasses although glycosylation plays an inhibitory role in the interaction with human IgG3.