Characterization of the chicken CD200 receptor family

The modulation of myeloid cells via inhibitory and activating immunoglobulin superfamily members has been a subject of intense study in mammals. One such example is the inhibitory receptor for CD200, which is shown to regulate the activation threshold of myeloid cells by interaction with the broadly distributed CD200 molecule. By looking at sequence homology and synteny conservation in the chicken genome, we identified two members of the CD200 receptor family in chicken on chromosome one. Cloning and further characterization of the protein sequence yielded a potentially inhibitory ggCD200R-B1 with a splice variant lacking a transmembrane region and a potentially soluble ggCD200R-S1. Both showed a typical V/C2-set Ig domain arrangement and we present evidence that these two genes have evolved by gene duplication. The inhibitory receptor displayed an uncharged transmembrane region and a long cytoplasmic tail encoding four tyrosine residues, one of them embedded in a motif similar to the mammalian NPxY motif. Further characterization of ggCD200R-B1 showed that it is expressed as a highly glycosylated protein and that its cytoplasmic tyrosine residues can be phosphorylated. Real-time RT-PCR analysis of various tissues and primary cells showed that ggCD200R-B1 is predominantly expressed in macrophages, whereas ggCD200R-S1 is highly expressed in peripheral blood mononuclear cells, but not macrophages. In summary, we showed that there is a homologue of mammalian CD200R conserved in chicken suggesting a similar function in avian species. Furthermore, the presence of potentially soluble CD200R molecules implies an important role for these in the regulation of myeloid cells in chicken.