The N-linked oligosaccharide at Fc RIIIa Asn-45: an inhibitory element for high Fc RIIIa binding affinity to IgG glycoforms lacking core fucosylation

Human leukocyte receptor IIIa (Fc gamma RIIIa) plays an important role in mediating therapeutic antibodies' antibody-dependent cellular cytotoxicity (ADCC), which is closely related to the clinical efficacy of anticancer processes in humans in vivo. The removal of the core fucose from oligosaccharides attached to the Fc region of antibodies improves Fc gamma RIIIa binding, allowing the antibodies to enhance dramatically the antibody effector functions of ADCC. In this study, the contribution of Fc gamma RIIIa oligosaccharides to the strength of the Fc gamma RIIIa/antibody complex was analyzed using a serial set of soluble human recombinant Fc gamma RIIIa lacking the oligosaccharides. A nonfucosylated antibody IgG1 appeared to have a significantly higher affinity to the wild-type Fc gamma RIIIa fully glycosylated at its five N-linked oligosaccharide sites than did the fucosylated IgG1, and this increased binding was almost abolished once all of the Fc gamma RIIIa glycosylation was removed. Our gain-of-function analysis in the Fc gamma RIIIa oligosaccharide at Asn-162 (N-162) confirmed that N-162 is the element required for the high binding affinity to nonfucosylated antibodies, as previously revealed by loss-of-function analyses. Interestingly, beyond our expectation, the Fc gamma RIIIa modified by N-162 alone showed a significantly higher binding affinity to nonfucosylated IgG1 than did the wild-type Fc gamma RIIIa. Attachment of the other four oligosaccharides, especially the Fc gamma RIIIa oligosaccharide at Asn-45 (N-45), hindered the high binding affinity of Fc gamma RIIIa to nonfucosylated IgG1. Our data clearly demonstrated that N-45 is an inhibitory element for the high Fc gamma RIIIa binding affinity mediated by N-162 to nonfucosylated antibodies. This information can be exploited for the structural-based functional study of Fc gamma RIIIa.