Chicken CD4, CD8αβ, and CD8αα T cell co-receptor molecules

New knowledge has recently been obtained about the evolutionary conservation of CD4, CD8 alpha alpha and CD8 alpha beta T cell receptor (TCR) co-receptor molecules between chicken and mammals.. This conservation extends from biochemical structure and tissue distribution to function. Panels of monoclonal antibodies and polyclonal antisera against different epitopes of chicken CD8 and CD4 molecules have proven their value in several recent studies. Chicken CD8 allotypes and homozygous strains carrying these allotypes have been established and these strains provide excellent models for further studies. The extensive polymorphism of CD8 alpha in chickens has not been observed in any other species, suggesting that CD8 alpha and CD8 beta have evolved under different selective pressure in the chicken. A large peripheral blood CD4(+)CD8(+) T cell population in chicken resembles that observed in some human individuals but the inheritance of peripheral blood CD4CD8 alpha alpha T cells in the chicken is a unique observation, which suggests the presence of a single gene responsible for CD8 alpha, but not CD8 beta, specific expression. Despite these unique findings in chicken, the data on CD4, CD8 alpha alpha, and CD8 alpha beta molecules show that they have evolved before the divergence of mammalian and avian branches from their reptilian ancestors.