Antigen binding allosterically promotes Fc receptor recognition

Abstract

A key question in immunology is whether antigen recognition and Fc receptor (FcR) binding are allosterically linked. This question is also relevant for therapeutic antibody design. Antibody Fab and Fc domains are connected by flexible unstructured hinge region. Fc chains have conserved glycosylation sites at Asn297, with each conjugated to a core heptasaccharide and forming biantennary Fc glycan. The glycans modulate the Fc conformations and functions. It is well known that the antibody Fab and Fc domains and glycan affect antibody activity, but whether these elements act independently or synergistically is still uncertain. We simulated four antibody complexes: free antibody, antigen-bound antibody, FcR-bound antibody, and an antigen-antibody-FcR complex. We found that, in the antibody's "T/Y" conformation, the glycans, and the Fc domain all respond to antigen binding, with the antibody population shifting to two dominant clusters, both with the Fc-receptor binding site open. The simulations reveal that the Fc-glycan-receptor complexes also segregate into two conformational clusters, one corresponding to the antigen-free antibody-FcR baseline binding, and the other with an antigen-enhanced antibody-FcR interaction. Our study confirmed allosteric communications in antibody-antigen recognition and following FcR activation. Even though we observed allosteric communications through the IgG domains, the most important mechanism that we observed is the communication via population shift, stimulated by antigen binding and propagating to influence FcR recognition.

Role of the IgE variable heavy chain in FcεRIα and superantigen binding in allergy and immunotherapy

“…Because we previously demonstrated allosteric communications between the V-and C-regions on IgAs 30 and such an effect was also reported on IgGs by many other research groups, [26][27][28][29] we next sought to study whether such allosteric effects explained the differences in FcεRIa binding present in our pertuzumab IgE variants. To do this, we computationally modeled the full-length structures of all the pertuzumab IgE variants and quantified the allosteric communications between the V-and C-regions by using the AlloSigMA server (Fig 3, B).…”

“…20 They are reported to interact with specific VH FWRs (eg, protein A [spA] to some VH3 FWRs), [21][22][23][24] but because not all the same VH FWRs bind to such superantigens, there is no reason to believe that VH FWRs are the sole factors in superantigen binding. As a possible mechanism for superantigen activation of FcεRIa-bound IgE, recent investigations on antibodies have demonstrated that antigen binding at antibody V-regions can elicit structural changes that facilitate cell signaling 25 and that even VH families can elicit distal effects on antibody constant regions (C-regions) for IgGs [26][27][28][29] and IgA. 30 This allosteric effect was also shown when the changing of antibody C-regions affected antigen binding.…”

Antibody Structure and Function: The Basis for Engineering Therapeutics

“…We outline several tactics to modulate Fc activity linked to FcγR for immune effector cell function and FcRn for pharmacokinetic properties. Nonetheless, it is critical to keep in mind that the Fab domain antigen binding can affect the Fc region activity via structural allostery [398][399][400]. Hence, evaluations of specific Fc mutations should be confirmed empirically.…”

Considerations of Antibody Geometric Constraints on NK Cell Antibody Dependent Cellular Cytotoxicity

“…Furthermore, crystal structures of intact IgGs also indicate that the hinge-regions are largely responsible for this flexibility, although these regions tend to be poorly resolved and in the confines of a crystal lattice (68). However, detailed dynamics studies of antibody behavior in solution provide computational evidence that antigen binding changes the sampling space of Fc as well as the flexibility, providing a type of "rigidity" that is not found in unbound antibodies, which may lead to greater FcγR affinity in the presence of antigen (254).…”