Andrew Pincetic

Type I and type II Fc receptors regulate innate and adaptive immunity

Antibodies produced in response to a foreign antigen are characterized by polyclonality, not only in the diverse epitopes to which their variable domains bind but also in the various effector molecules to which their constant regions (Fc domains) engage. Thus, the antibodys Fc domain mediates diverse effector activities by engaging two distinct classes of Fc receptors (type I and type II) on the basis of the two dominant conformational states that the Fc domain may adopt. These conformational states are regulated by the differences among antibody subclasses in their amino acid sequence and by the complex, biantennary Fc-associated N-linked glycan. Here we discuss the diverse downstream proinflammatory, anti-inflammatory and immunomodulatory consequences of the engagement of type I and type II Fc receptors in the context of infectious, autoimmune, and neoplastic disorders.

General mechanism for modulating immunoglobulin effector function

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 cell-specific 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” anti-inflammatory 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.

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

Significance IgG molecules are capable of inducing pro- and antiinflammatory responses dependent on their fragment crystallizable domain (Fc) glycan composition. Antiinflammatory responses are specifically triggered upon Fc sialylation, which decreases the binding affinity for type I Fc receptors but enhances binding to type II Fc receptors such as SIGN-R1, CD23, or human DC-SIGN. Structural analyses revealed that sialylation induces conformational changes in the Fc portion, which is a prerequisite for the selective binding to type II Fc receptors. Here we generated an Fc variant, F241A, that mimics the conformational state of sialylated Fc. F241A, even when nonsialylated, mediated protection from autoantibody- and T cell-mediated inflammation in a type II Fc receptor-dependent manner. The antiinflammatory activity of intravenous immunoglobulin (IVIG) is dependent on the presence of sialic acid in the core IgG fragment crystallizable domain (Fc) glycan, resulting in increased conformational flexibility of the CH2 domain with corresponding modulation of Fc receptor (FcR) binding specificity from type I to type II receptors. Sialylated IgG Fc (sFc) increases the activation threshold of innate effector cells to immune complexes by stimulating the up-regulation of the inhibitory receptor FcγRIIB. We have found that the structural alterations induced by sialylation can be mimicked by specific amino acid modifications to the CH2 domain. An IgG Fc variant with a point mutation at position 241 (F→A) exhibits antiinflammatory activity even in the absence of sialylation. F241A and sFc protect mice from arthritis in the K/BxN-induced model and, in the T cell-mediated experimental autoimmune encephalomyelitis (EAE) mouse model, suppress disease by specifically activating regulatory T cells (Treg cells). Protection by these antiinflammatory Fcs in both antibody- and T cell-mediated autoimmune diseases required type II FcRs and the induction of IL-33. These results further clarify the mechanism of action of IVIG in both antibody- and T cell-mediated inflammatory diseases and demonstrate that Fc variants that mimic the structural alterations induced by sialylation, such as F241A, can be promising therapeutic candidates for the treatment of various autoimmune disorders.

Reply to Crispin et al.: Molecular model that accounts for the biological and physical properties of sialylated Fc

We recently published a model (1) that proposes a molecular basis for the anti-inflammatory properties of sialylated IgG Fc (sFc), resulting from its observed shift in receptor specificity from FcγRs to dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN)/CD23, a consequence of changes to the structure of the CH2 domain that we observe upon sialylation. However, Crispin et al. (2) report a crystal structure of a sFc that resembles that of an earlier reported structure, 1H3Y (3), which carries a nonsialylated Fc. The basis for this discrepancy likely results from the fact that both crystal structures of sialylated and nonsialylated Fcs have been obtained under similar but nonphysiological conditions (>2 M sodium chloride, pH <4.5) that may alter or even prevent the noncovalent interactions between the carbohydrate and protein moieties. Moreover, both crystals are of the same space group with identical cell constants, indicating that the same crystal contacts are present. It has been previously described (3) that the aforementioned structure of the nonsialylated Fc obtained from high-salt conditions varies significantly from that of other Fcs crystallized under more physiological conditions and in different space groups (e.g., PBD ID code 1FC1). As a consequence, no significant structural reorientations were observed by Crispin, et al. (2).