Avian IgY Binds to a Monocyte Receptor with IgG-like Kinetics Despite an IgE-like Structure

Abstract

An ancestor of avian IgY was the evolutionary precursor of mammalian IgG and IgE, and present day chicken IgY performs the function of human IgG despite having the domain structure of human IgE. The kinetics of IgY binding to its receptor on a chicken monocyte cell line, MQ-NCSU, were measured, the first time that the binding of a non-mammalian antibody to a nonmammalian cell has been investigated (k ؉1 ‫؍‬ 1.14 ؎ 0. 46 ). IgE has an extra pair of immunoglobulin domains when compared with IgG. Their presence reduces the dissociation rate of IgE from its receptor 20-fold, thus contributing to the high affinity of IgE. To assess the effect of the equivalent domains on the kinetics of IgY binding, IgY-Fc fragments with and without this domain were cloned and expressed in mammalian cells. In contrast to IgE, their presence in IgY has little effect on the association rate and no effect on dissociation. Whatever the function of this extra domain pair in avian IgY, it has persisted for at least 310 million years and has been co-opted in mammalian IgE to generate a uniquely slow dissociation rate and high affinity.IgY is the principal serum antibody of amphibians, reptiles, and birds and shares a common ancestor with both mammalian IgG and IgE (1, 2). The cDNA sequence of the chicken upsilon () heavy chain (3) reveals that, like more basal amphibian IgY (4, 5), avian IgY contains a domain pair (C2) 4 that has been conserved in mammalian IgE (as C⑀2) but truncated to form the "hinge" region in mammalian IgG (3). Thus, an orthologous domain pair must have existed in the common (IgY-like) ancestor prior to its duplication and subsequent divergence in the mammalian lineage. Avian IgY is the closest extant protein to this ancestor and therefore the most logical choice for study of the evolution of modern IgG and IgE from an ancient IgY-like ancestor.IgG, the principal mammalian serum antibody, aggregates and facilitates opsonization of antigens, activates complement, and provides protection for the fetus following transport across the placenta. IgE does not activate complement nor cross the placenta but can sensitize effector cells (principally mast cells and basophils) and typically mediates anaphylactic reactions (6). It is well known for its involvement in allergic disease but probably also provides defense against parasitic infections (7).Avian IgY appears to combine the functions of mammalian IgG and IgE. As the major serum antibody, it provides defense against infection (8) but also mediates anaphylaxis (9). Chicken mast cells activated by IgY are responsible for local anaphylactic reactions in the gut, which play a role in defense against protozoan infections (10, 11). Antibody-dependent hypersensitivity and fatal systemic anaphylactic shock have also been demonstrated in chickens (9, 12, 13) and shown to be mediated by basophils (14), which are much more numerous than mast cells in birds (in contrast to mammals in which the reverse is true) (15). These phenomena constitute indirect evidence for the presence of IgY...

A Monomeric Chicken IgY Receptor Binds IgY with 2:1 Stoichiometry

“…Preparation and Characterization of Proteins-Fc2-4 was prepared as described previously (6). sfpCHIR-AB1 (5) was expressed in HEK293E cells and purified on a protein A-Sepharose column.…”

“…MQ-NCSU monocyte cells were grown and prepared as described previously (6). Incubations and washes used PBS with 1% bovine serum albumin, and 10 nM sIgY and 33 nM anti-IgY FITC (Promega) were used for labeling.…”

“…Phagocytosis, mediated in mammals by IgG, and passive cutaneous anaphylaxis, mediated by both IgG and IgE in mammals, have been observed in chickens (3,4), presumably both effected by IgY. In vitro, IgY binds to monocyte cell lines (5,6), and an IgY receptor (CHIR-AB1) has been identified that is able to mediate the influx of calcium into cells (5).…”

“…An x-ray structure for the IgY-smCHIR-AB1 complex is clearly needed. Our previous finding that the C2 domains play little or no role in the kinetics of IgY binding to receptor (6), unlike the homologous C⑀2 domains in IgE (30) (29). Dephosphorylation of Fc␣RI by protein phosphatase 2A has now been identified as the activation switch for IgA bound to its receptor on neutrophils (31).…”

The evolution of the Th2 immune responses and its relationships with parasitic diseases and allergy

“…Si bien en la IgE el dominio cε2 es responsable de la alta afinidad que tiene con su receptor FcεRI, no es así con el dominio homólogo en la IgY (cυ2), pues su eliminación no altera la afinidad con el receptor Fc. Además, la afinidad de la IgY con su receptor está en el mismo orden que el de la IgG, 10 veces menor que el de la IgE (30).…”