Abstract
IgY is the principal serum antibody in birds and reptiles, and an IgY-like molecule was the evolutionary precursor of both mammalian IgG and IgE. A receptor for IgY on chicken monocytes, chicken leukocyte receptor AB1 (CHIR-AB1), lies in the avian leukocyte receptor cluster rather than the classical Fc receptor cluster where the genes for mammalian IgE and IgG receptors are found. IgG and IgE receptors bind to the lower hinge region of their respective antibodies with 1:1 stoichiometry, whereas the myeloid receptor for IgA, FcalphaRI, and the IgG homeostasis receptor, FcRn, which are found in the mammalian leukocyte receptor cluster, bind with 2:1 stoichiometry between the heavy chain constant domains 2 and 3 of each heavy chain. In this paper, the extracellular domain of CHIR-AB1 was expressed in a soluble form and shown to be a monomer that binds to IgY-Fc with 2:1 stoichiometry. The two binding sites have similar affinities: K(a)(1) = 7.22 +/- 0.22 x 10(5) m(-1) and K(a)(2) = 3.63 +/- 1.03 x 10(6) m(-1) (comparable with the values reported for IgA binding to its receptor). The affinity constants for IgY and IgY-Fc binding to immobilized CHIR-AB1 are 9.07 +/- 0.07 x 10(7) and 6.11 +/- 0.02 x 10(8) m(-1), respectively, in agreement with values obtained for IgY binding to chicken monocyte cells and comparable with reported values for human IgA binding to neutrophils. Although the binding site for CHIR-AB1 on IgY is not known, the data reported here with a monomeric receptor binding to IgY at two sites with low affinity suggest an IgA-like interaction.
Highlights
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
The human leukocyte receptor cluster (LRC) is the site of Fc␣RI, the leukocyte receptor for IgA, the fetal IgG receptor (FcRn, involved in adult IgG homeostasis), and a number of natural killer cell receptors including the HLA-G ligand, KIR2DL4 [10]
The value for serum IgY was 6% and was used to calculate the stoichiometry determined by Surface Plasmon Resonance (SPR)
Summary
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 [7]. The first IgY leukocyte receptor, CHIR-AB1, was found to be a member of the chicken leukocyte receptor cluster (LRC) [5], adjacent to over 100 genes with high intersequence homology [8] This finding, together with phylogenetic analysis of the orthologous Fc receptor gene clusters [7, 9], 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. We report here that it binds to IgY and IgY-Fc with 2:1 stoichiometry
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