AFFINITY OF THE INTERACTION BETWEEN FC-GAMMA RECEPTOR-TYPE-III (FC-GAMMA-RIII) AND MONOMERIC HUMAN-IGG SUBCLASSES - ROLE OF FC-GAMMA-RIII GLYCOSYLATION

Binding of the Fc region of IgG antibodies to low affinity Fc gamma re ceptors (Fc gamma R) triggers important effector functions in the immu ne system. The type IIIb Fc gamma R (Fc gamma RIIIb or CD16) is a heav ily glycosylated protein anchored to the membrane of neutrophils by a glycosylphosphatidylinositol link. This receptor contributes to cell a ctivation by Ige immune complexes. To better understand the nature of the Ligand-receptor association, we have studied the affinity and kine tics of the interaction between human IgG subclasses and two soluble f orms of Fc gamma RIIIb (sFc gamma RIIIb or sCD16) corresponding to the 188 N-terminal amino acids of the extracellular region of the recepto r, a glycosylated one made in eucaryotic cells (euc.sCD16) and a non-g lycosylated one (proc.sCD16) made in Escherichia coli, Experiments usi ng a BIAcore(TM) 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 immo bilized 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 constan t 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 ol the non-glycosylated receptor for hum an IgG subclasses bound to Sepharose was IgG1 > IgG3 >> IgG4 >>> IgG2. Thus, the extracellular polypeptide of Fc gamma RIIIb dictates the in teraction of the receptor with IgG subclasses although glycosylation p lays an inhibitory role in the interaction with human IgG3.