From the Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology Iowa State University, Ames, Iowa 50011.
From the Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology Iowa State University, Ames, Iowa 50011
J Biol Chem. 2018 Oct 26;293(43):16842-16850. doi: 10.1074/jbc.RA118.004998. Epub 2018 Sep 13.
Fc γ receptors (FcγRs) 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 FcγRs. Three recent reports indicate that the composition of the asparagine-linked carbohydrate chains (-glycans) of FcγRIIIa/CD16a impacted IgG1-binding affinity. Here we determined how -glycan composition affected the affinity of the "low-affinity" FcγRs for six homogeneous IgG1 Fc -glycoforms (G0, G0F, G2, G2F, A2G2, and A2G2F). Surprisingly, CD16a with oligomannose -glycans bound to IgG1 Fc (A2G2) with a = 1.0 ± 0.1 nm This affinity represents a 51-fold increase over the affinity measured for CD16a with complex-type -glycans (51 ± 8 nm) and is comparable with the affinity of FcγRI/CD64, the sole "high-affinity" FcγR. CD16a -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 with an Asn-to-Gln mutation; mutations at the four other -glycosylation sites preserved tighter binding in the Man5 glycoform. None of the other low-affinity FcγRs showed more than a 3.1-fold increase upon modifying the receptor -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 FcγRs, and modifying only the glycan composition of both the IgG1 Fc ligand and receptor provides a 400-fold range in affinities.
Fcγ 受体（FcγRs）在白细胞表面结合循环 IgG（IgG1）。靶向颗粒表面聚集的抗体通过激活 FcγRs 触发保护性免疫反应。最近的三项报告表明，FcγRIIIa/CD16a 的天冬酰胺连接的糖链（-聚糖）组成影响 IgG1 结合亲和力。在这里，我们确定了 -聚糖组成如何影响“低亲和力”FcγRs 对六种同质 IgG1 Fc -糖型（G0、G0F、G2、G2F、A2G2 和 A2G2F）的亲和力。令人惊讶的是，带有寡甘露糖 -聚糖的 CD16a 与 IgG1 Fc（A2G2）的结合 = 1.0 ± 0.1nm。这种亲和力代表与具有复杂型 -聚糖的 CD16a（51 ± 8nm）相比，亲和力增加了 51 倍，与唯一的“高亲和力”FcγRI/CD64 相当。FcγRIIIa/CD16a 的 -聚糖组成解释了对其他测试的 IgG1 Fc 糖型的结合亲和力增加（10-50 倍）。这种显着的敏感性只能通过用天冬酰胺到谷氨酰胺的突变阻止 Asn 糖基化来消除；在其他四个 -糖基化位点的突变保留了在 Man5 糖型中的更紧密结合。在修饰受体 -聚糖组成时，除了 CD16b 之外，没有其他低亲和力 FcγR 显示出超过 3.1 倍的增加，CD16b 与 CD16a 仅相差四个氨基酸残基。这一结果表明，CD16a 在低亲和力 FcγRs 中是独一无二的，并且仅修饰 IgG1 Fc 配体和受体的聚糖组成就可以提供 400 倍的亲和力范围。
