NK and T cells constitute two major, functionally distinct intestinal epithelial lymphocyte subsets in the chicken

Non-mammalian NK cells have not been characterized in detail; however, thei r analysis is essential for the understanding of the NK cell receptor phylo geny, As a first step towards defining chicken NK cells, several tissues we re screened for the presence of NK cells, phenotypically defined as CD8(+) cells lacking T- or B-lineage specific markers. By this criteria, similar t o 30% of CD8(+) intestinal intraepithelial lymphocytes (IEL), but <1% of sp lenocytes or peripheral blood lymphocytes were defined as NK cells. These C D8(+)D3(-) IEL were used for the generation of the 28-4 mAb, immunoprecipit ating a 35-kDa glycoprotein with a 28-kDa protein core. The CD3 and 28-4 mA b were used to separate IEL into CD3(+) IEL T cells and 28-4(+) cells, both co-expressing the CD8 antigen. During ontogeny, 28-4(+) cells were abundan t in the IEL and in the embryonic spleen, where two subsets could be distin guished according to their CD8 and c-kit expression. Most importantly, 28-4 (+) IEL lysed NK-sensitive targets, whereas intestinal T cells did not have any spontaneous cytolytic activity. These results define two major, phenot ypically and functionally distinct IEL subpopulations, and imply an importa nt role of NK cells in the mucosal immune system.