EXPRESSION OF AN AVIAN CD6 CANDIDATE IS RESTRICTED TO ALPHA-BETA T-CELLS, SPLENIC CD8(-DELTA T-CELLS AND EMBRYONIC NATURAL-KILLER-CELLS() GAMMA)

A candidate avian CD6 homolog is identified by the S3 monoclonal antib ody. The S3 antigen exists in a phosphorylated glycoprotein form of 13 0 kDa and a nonphosphorylated form of 110 kDa. Removal of phosphate gr oups and N-linked carbohydrates indicates a 78-kDa protein core. Durin g thymocyte differentiation, the gamma delta T cells do not express S3 , whereas mature CD4(+) and CD8(+) cells of alpha beta lineage acquire S3 antigen. All alpha beta T cells in the blood and spleen express th e S3 antigen at relatively high levels. In contrast, only the CD8(+) s ubpopulation of gamma delta T cells in the spleen expresses the antige n and neither alpha beta nor gamma delta T cells in the intestinal epi thelium express the S3 antigen. The S3 antigen is also found on embryo nic splenocytes with a phenotypic profile characteristic of avian natu ral killer cells. The biochemical characteristics and this cellular ex pression pattern imply that the S3 antigen is the chicken CD6 homolog.