MONOMERIC HOMOLOG OF MAMMALIAN CD28 IS EXPRESSED ON CHICKEN T-CELLS

A mAb recognizing a 40- to 44-kDa monomeric molecule on the surface of chicken T cells was used to screen a cDNA expression library made fro m Con A-stimulated chicken spleen cells. The sequence of the cDNA obta ined encoded a molecule having 50% amino acid sequence identity with m ammalian CD28, but the cysteine residue involved in the inter-chain br idge of the mammalian CD28 homodimer was not conserved in the chicken sequence. The molecule produced in transfected COS-7 cells was also re cognized by another mAb that had previously been thought to recognize an avian homologue of CD2. The sequence data establish that this molec ule is a homologue of mammalian CD28 in the strict evolutionary sense.