IDENTIFICATION AND ANALYSIS OF THE EXPRESSION OF CD8-ALPHA-BETA AND CD8-ALPHA-ALPHA ISOFORMS IN CHICKENS REVEALS A MAJOR TCR-GAMMA-DELTA CD8-ALPHA-BETA SUBSET OF INTESTINAL INTRAEPITHELIAL LYMPHOCYTES

Expression screening has been used to clone cDNAs encoding the alpha- and beta-chains of chicken CD8. Amino acid sequence similarities with the mammalian sequences were about 30%. Many amino acid residues of st ructural or functional importance were more highly conserved, as were the overall structures of both chains. Like human CD8 alpha, the chick en alpha-chain lacked sites for N-linked glycosylation, but the beta-c hain contained three such sites. In COS cells transfected with CD8 bet a cDNA, surface expression of the beta-chain was dependent on co-trans fection of the alpha-chain cDNA, indicating that, as in mammals, chick en CD8 can be expressed as a CD8 alpha alpha homodimer or as a CD8 alp ha beta heterodimer. Immunofluorescence analysis with mAbs that were s hown to identify the CD8 alpha- and CD8 beta-chains revealed that the vast majority of the CD8(+) cells in the thymus, spleen, and blood of adult chickens express both CD8 alpha- and CD8 beta-chains. However, a relatively large proportion of the CD8(+) TCR-gamma delta cells in th e spleens of embryos and young chicks express only the alpha-chain of CD8. Among intestinal epithelial lymphocytes the major CD8(+) T cell p opulations present in mice are conserved, but there is a population of TCR-gamma delta CD8 alpha beta cells that is not found in rodents. Th is observation is important in interpretation of experiments examining the pathways of development of intestinal intraepithelial lymphocytes in chickens.