General mechanism for modulating immunoglobulin effector function

Abstract

Immunoglobulins recognize and clear microbial pathogens and toxins through the coupling of variable region specificity to Fc-triggered cellular activation. These proinflammatory activities are regulated, thus avoiding the pathogenic sequelae of uncontrolled inflammation by modulating the composition of the Fc-linked glycan. Upon sialylation, the affinities for Fcγ receptors are reduced, whereas those for alternative cellular receptors, such as dendritic cellspecific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN)/CD23, are increased. We demonstrate that sialylation induces significant structural alterations in the Cγ2 domain and propose a model that explains the observed changes in ligand specificity and biological activity. By analogy to related complexes formed by IgE and its evolutionarily related Fc receptors, we conclude that this mechanism is general for the modulation of antibody-triggered immune responses, characterized by a shift between an "open" activating conformation and a "closed" antiinflammatory state of antibody Fc fragments. This common mechanism has been targeted by pathogens to avoid host defense and offers targets for therapeutic intervention in allergic and autoimmune disorders.conformational change | sialylated IgG Fc I gG and IgE mediate their proinflammatory properties through the crosslinking of the 1:1 complex of the Fc receptor (FcR) monomer in the Fc dimer cleft (1, 2). By contrast, both IgG and IgE can engage a second class of receptors, the evolutionarily related, C-type lectins dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) (3) and CD23 (4), respectively, resulting in anti-inflammatory and immunosuppressive responses (5, 6). The structural basis for the ability of IgE to interact either with one or the other of these two disparate classes of receptors has recently been defined (7). The intrinsic flexibility of the IgE Ce3 domain results in both open and closed conformations of the IgE Fc, resulting in the binding of either FceRI or CD23, respectively. Binding of either receptor induces an allosteric change in the IgE Fc to the alternative conformation, thus precluding the interaction with the other receptor (7). Binding of IgE to the type II, C-type lectin CD23 is neither carbohydrate-nor calcium-dependent, mediated exclusively through protein-protein interactions, generating a 2:1 complex of CD23 with the Ce3-Ce4 interface (7). DC-SIGN is a structurally homologous, calcium-dependent, carbohydratebinding, type II lectin, tightly linked to CD23 on chromosome 19 (8), displaying ligand specificity for mannose-containing glycoconjugates and fucose-containing Lewis antigens. Binding of DC-SIGN to IgG requires that the complex, biantennary glycan, attached to the evolutionarily conserved glycosylation site Asn-297 and enclosed within the cavity formed by the Cγ2 domains of the A and B chains of the Fc dimer, be processed to the α2,6 sialylated form (9, 10). Importantly, no evidence has been found for DC-SIGN binding to sialylate...

Protection in antibody- and T cell-mediated autoimmune diseases by antiinflammatory IgG Fcs requires type II FcRs

“…In contrast, asialylated Fc structures uniformly result in open Fc conformations, consistent with their binding specificity for type I FcRs (21). Glycan interactions with amino acid residues of the C H 2 domain are disrupted upon sialylation, providing a basis for the observed conformational changes seen in the protein structure and consistent with a model proposed for the binding specificity of sialylated Fc for type II FcRs (11). Based on these observations, we generated a series of Fc variants targeting the amino acids of the C H 2 domain that interact with the glycan, with the goal of determining their impact on type II FcR binding and the resulting antiinflammatory activity.…”

“…Furthermore, both sFc as well as F241A stimulate the production of IL-33 that in turn activates T reg cells through the ST2 receptor, contributing to the suppression of T cell-mediated autoimmune responses. (20) enabling it to bind to type II Fc receptors (11). As shown in Fig.…”

“…Its antiinflammatory activity has been shown to result from the presence of a specific glycan, the α2,6-sialylated, complex biantennary structure present on the C H 2 domain of the fragment crystallizable domain (Fc) and found in a small proportion of heterogeneous antibody preparations in IVIG (4). Sialylation of the Fc glycan on the C H 2 domain results in IgGs that can engage type II Fc receptors (FcRs) such as specific ICAM-3 grabbing non-integrin-related 1 (SIGN-R1), dendritic cell-specific ICAM-3 grabbing non-integrin (DC-SIGN), and CD23 (5)(6)(7)(8), while reducing their binding affinity to type I FcRs (9)(10)(11). Studies in mouse models of serum-induced arthritis, antibody-dependent ITP, nephrotoxic nephritis, and autoimmune blistering diseases confirmed the antiinflammatory activity of the sialylated Fc, whether from IVIG or generated from recombinantly expressed IgG1 (5,9,12,13).…”

“…Sialylation of the complex, biantennary glycan of the IgG Fc results in increased conformational flexibility of the C H 2 domain (20), thereby sampling the closed conformations of the C H 2 domain required for type II FcR binding (11). In contrast, asialylated Fc structures uniformly result in open Fc conformations, consistent with their binding specificity for type I FcRs (21).…”

Designed IgM from glycoengineering

“…By virtue of lacking fucose, this glycan confers significantly enhanced FcγRIII binding and enhanced antibodydependent cellular cytotoxicity (ADCC) activity to IgG (9). Moreover, this core glycan can further serve as the substrate for additional modifications including mammalian galactosylation and sialylation, both of which can have significant functional consequences on IgG antibodies (10,11). We have no such appreciation for the impact of glycan structure on IgM function.…”