The 3.2-Å crystal structure of the human IgG1 Fc fragment–FcγRIII complex

Abstract

The immune response depends on the binding of opsonized antigens to cellular Fc receptors and the subsequent initiation of various cellular effector functions of the immune system. Here we describe the crystal structures of a soluble Fc gamma receptor (sFc gammaRIII, CD16), an Fc fragment from human IgG1 (hFc1) and their complex. In the 1:1 complex the receptor binds to the two halves of the Fc fragment in contact with residues of the C gamma2 domains and the hinge region. Upon complex formation the angle between the two sFc gammaRIII domains increases significantly and the Fc fragment opens asymmetrically. The high degree of amino acid conservation between sFc gammaRIII and other Fc receptors, and similarly between hFc1 and related immunoglobulins, suggest similar structures and modes of association. Thus the described structure is a model for immune complex recognition and helps to explain the vastly differing affinities of other Fc gammaR-IgG complexes and the Fc epsilonRI alpha-IgE complex.

Rapid screening of IgG quality attributes – effects on Fc receptor binding

“…In the purified monomeric deamidated sample, reduced binding was still measured on the low‐affinity Fcγ receptors, which could only be attributed to D‐N. The main deamidation site of this IgG is present in the CDR at the LC (up to 40% modified), which is positioned relatively far away from the Fc interaction site (lower hinge, upper CH2 domain 37, 38). Deamidation at this position is not likely to change the folding of the protein in such a way that it would have a large impact on Fc receptor binding.…”

The “less-is-more” in therapeutic antibodies: Afucosylated anti-cancer antibodies with enhanced antibody-dependent cellular cytotoxicity

“…The first crystal structure of FcγRIII-IgG1-Fc complex was reported in 2000 9 . The FcγRIII used in the study was a soluble FcγRIIIb (sFcγRIIIb) produced in E. coli and the Fc was isolated from pooled human IgG1.…”

“…As the sFcγRIII preparation used in the study was unglycosylated, it was impossible to evaluate the impact of its N -linked glycan on the FcγRIII-Fc interaction. Nonetheless, the authors did highlight that Asn162 is a potential glycosylation site of FcγRIII that is close to a binding site and a larger carbohydrate moiety attached to this site may influence the affinity to IgG 9 . Indeed, a subsequent study revealed that, compared to the unglycosylated form of FcγRIII (by mutating Asn162 to Gln162), the glycosylated FcγRIII (Asn162) showed reduced affinity for native (fucosylated) IgG antibodies, while antibodies with or without the core fucose showed a similar affinity for unglycosylated FcγRIII 20 .…”

“…demonstrated that the glycosylation at Asn162 of FcγRIII is not essential for the expression of the receptor; however, this glycosylation site is conserved among all FcγRIIIs (or the equivalent) in all mammals studied 19 . Furthermore, in all FcγRs, the regions that interact with the antibody are highly conserved, yet all other receptors lack this glycosylation site 9 . It is tempting to speculate that ADCC may be modulated by IgG core fucosylation because of the presence of the glycan at Asn162 of FcγRIII.…”

“…, 9 This Fc-FcγRIII interaction is significantly affected by the glycan present at the conserved N -glycosylation site Asn297 (N297) in each of the CH2 domains 10 . Mutations in the CH2 domain that destroyed the conserved N -glycosylation motif and hence gave rise to aglycosylated Fc resulted in complete loss of binding to most FcγRs except FcγRI 11 .…”