CD16a with oligomannose-type N-glycans is the only “low-affinity” Fc γ receptor that binds the IgG crystallizable fragment with high affinity in vitro
Journal of Biological Chemistry(2018)
摘要
Fc receptors (FcRs) bind circulating IgG (IgG1) at the surface of leukocytes. Antibodies clustered at the surface of a targeted particle trigger a protective immune response through activating FcRs. Three recent reports indicate that the composition of the asparagine-linked carbohydrate chains (N-glycans) of FcRIIIa/CD16a impacted IgG1-binding affinity. Here we determined how N-glycan composition affected the affinity of the low-affinity FcRs for six homogeneous IgG1 Fc N-glycoforms (G0, G0F, G2, G2F, A2G2, and A2G2F). Surprisingly, CD16a with oligomannose N-glycans bound to IgG1 Fc (A2G2) with a K-D = 1.0 +/- 0.1 nm. This affinity represents a 51-fold increase over the affinity measured for CD16a with complex-type N-glycans (51 +/- 8 nm) and is comparable with the affinity of FcRI/CD64, the sole high-affinity FcR. CD16a N-glycan composition accounted for increases in binding affinity for the other IgG1 Fc glycoforms tested (10-50-fold). This remarkable sensitivity could only be eliminated by preventing glycosylation at Asn(162) with an Asn-to-Gln mutation; mutations at the four other N-glycosylation sites preserved tighter binding in the Man5 glycoform. None of the other low-affinity FcRs showed more than a 3.1-fold increase upon modifying the receptor N-glycan composition, including CD16b, which differs from CD16a by only four amino acid residues. This result indicates that CD16a is unique among the low-affinity FcRs, and modifying only the glycan composition of both the IgG1 Fc ligand and receptor provides a 400-fold range in affinities.
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关键词
Fc receptor,Fc-gamma receptor,antibody,N-linked glycosylation,post-translational modification (PTM)
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