POSITIVE SELECTION OF MOUSE NK1(-CELLS BY CD1-EXPRESSING CORTICAL THYMOCYTES() T)

Mouse NK1(+) T cells constitute a subset of alpha/beta TCR(+) T cells that specialize in the rapid production of cytokines, in particular IL -4, and may promote the differentiation of Th2-type CD4 T cells. Their TCRs, like those of a homologous subset of human T cells, use an inva riant TCR alpha chain and were recently shown to be specific for the b eta 2-microglobulin-associated, MHC class I-like CD1 molecules, which are encoded outside the MHC. In contrast to mainstream thymocytes, whi ch recognize their positively selecting MHC ligand on thymic epithelia l cells, positive selection of NK1(+) T cells requires their CD1 ligan d to be expressed on bone marrow-derived cells. To investigate the nat ure of the bone marrow-derived cell involved, chimeric mice were const ructed with tissues from normal, SCID, and MHC-deficient mice, so that CD1 could be selectively expressed by different subsets of bone marro w-derived cells in the thymus. CD1 expression was also directly assess ed using an anti-CD1 mAb, and a CD1-specific T cell hybridoma. The res ults suggest that immature (CD4(+)8(+) double-positive) cortical thymo cytes are the source of CD1 presentation for positive selection of NK1 (+) T cells.