Introduction of Chicken Ig-like Receptor AB1 (CHIR-AB1)
Antibodies are critical components of the adaptive immune system that allow specific recognition of a remarkable repertoire of pathogens. In addition to direct neutralization of target antigens, antibodies also participate in the regulation of both adaptive and innate immune mechanisms via interactions with Fc receptors (FcRs). In light of the multiple roles of FcRs and their involvement in various pathological disorders, a detailed understanding of the structure and function of FcRs has become a subject of increasing interest. In birds and reptiles, IgY is the principal serum antibody, and both mammalian IgG and IgE have evolved from an IgY-like ancestor, so studies of IgY offer insights into their origins. The historical contribution of chicken immunology to a wider understanding of the subject has been considerable, and three receptors that interact with chicken IgY have been described to date. A third receptor, a member of the CHIR family designated CHIR-AB1, is encoded by the chicken leukocyte receptor cluster. It has only a single Ig domain, binds IgY with high affinity, and has features of activating as well as inhibitory Ig-like receptors. In an individual chicken, up to 18 CHIR-AB1 homologs could be isolated, including several versions that bound IgY with different affinities. Sequence comparisons of these different CHIR-AB homologs together with site-directed mutagenesis defined the putative IgY binding site on CHIR-AB1. Further analysis of the CHIR-AB1 to IgY interaction revealed a 2:1 receptor-ligand binding stoichiometry and demonstrated that CHIR-AB1 exists as a mixture of monomers and dimers.
CHIR-AB1 - Member of Chicken Leukocyte Receptor Cluster
IgY is shown to bind to a chicken leukocyte receptor, CHIR-AB1, in a different manner from that of its mammalian orthologues, IgG and IgE, to their respective Fc receptors. Phagocytosis, mediated in mammals by IgG, and passive cutaneous anaphylaxis, mediated by both IgG and IgE in mammals, have been observed in chickens, presumably both affected by IgY. In vitro, IgY binds to monocyte cell lines, and CHIR-AB1 has been identified that it is able to mediate the influx of calcium into cells. The genes for the mammalian high-affinity IgE receptor, and several IgG receptors, are located in the classical Fc receptor cluster, whereas in chickens, this cluster is represented by a single gene, the product of which has been expressed and found not to bind IgY. Intriguingly, the first IgY leukocyte receptor, CHIR-AB1, was found to be a member of the chicken leukocyte receptor cluster, adjacent to over 100 genes with high intersequence homology. This finding, together with a phylogenetic analysis of the orthologous Fc receptor gene clusters, implies that during the evolution of the IgY-like ancestor of both IgG and IgE, antibody-Fc binding function migrated from proteins expressed in the LRC to those in the classical Fc receptor cluster.
Fig 1. Models for the CHIR-AB1/FcY interaction. (Arnon, 2008)
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Reference
- Arnon, T. I., et al. The crystal structure of CHIR-AB1: a primordial avian classical Fc receptor. Journal of molecular biology. 2008, 381(4): 1012–1024.
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