As host immunological molecules, antibodies are the products of biological evolution that firstly exist in vertebrates. In addition to direct binding and neutralization of target antigens, antibodies also participate in the regulation of both innate and adaptive immune mechanisms via interactions with receptors. IgY is produced by hens to provide their offspring with an effective humoral immunity against the commonest avian pathogens until full maturation of their immune system. Three receptors for chicken IgY have been characterized: Chicken Ig-like Receptor AB1 (CHIR-AB1), FcRY and Gallus Gallus Fc Receptor (ggFcR).
The presence of human FcαRI and bovine Fcγ RII receptor genes in the human leukocyte receptor cluster (LRC) encouraged researchers in their study of possible ligands for chicken Ig-like receptors (CHIR) in the chicken LRC on chromosome 31 to include IgY, IgA and IgM in their assay. CHIR-AB1 was the first chicken IgY immune receptor discovered. It is expressed on monocytes and other chicken innate immune cells and has activatory activity and the potential for inhibitory activity. The kinetics of the single extracellular Ig domain of CHIR-AB1 binding to IgY was measured by surface plasmon resonance analysis and was found to be similar to IgY binding to monocytes. Soluble CHIR-AB1 could inhibit most of the IgY binding to the chicken monocyte cell line MQ-NCSU, suggesting that CHIR-AB1 is the predominant monocyte IgY receptor.
A second Fc receptor for IgY was identified in chicken, FcRY, functionally similar to the mammalian MHC-like protein, FcRn, responsible for the transport of maternal IgG to the fetus. FcRY was purified from yolk sacs and the sequence of the N-terminal peptide used to search an expressed sequence tag database, revealing a protein of the mannose receptor family, a structural homolog of the phospholipase A2 receptor. As for FcRn and IgG, FcRY binds IgY at pH 6 and releases it at pH 8, but the domains that constitute these functionally equivalent proteins are from entirely different gene families.
A further receptor, ggFcR, was unexpectedly located to chromosome 20, a location unrelated to the LRC or FcR loci. The receptor consists of four extracellular Ig-like domains, and its binding site in IgY-Fc, mapped by mutagenesis, was found to involve residues of Cυ3. Interestingly, CHIR-AB1 and ggFcR are found in both myeloid and hematopoietic lineages, whereas FcRγ is only present in myeloid lineages, suggesting that CHIR-AB1 function may be cell-type dependent. In the evolution of the relationship between IgY and its receptors, there is evidence of both convergence and divergence, even working independently with respect to both structure and function. The gene for ggFcR is the only one discovered on chromosome 20 that has the characteristics of an Fc receptor: extracellular Ig-like domains that bind antibodies, a transmembrane domain and the ability to transmit an activating signal.
Fig 1. Comparison of putative IgY-receptor interactions with IgE-, IgG- and IgA-receptor complexes. (Zhang, 2017)
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- Zhang, X., et al. IgY: a key isotype in antibody evolution. Biological reviews of the Cambridge Philosophical Society. 2017, 92(4): 2144–2156.
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