PRO-T CELLS IN FETAL THYMUS EXPRESS C-KIT AND RAG-2 BUT DO NOT REARRANGE THE GENE ENCODING THE T-CELL RECEPTOR-BETA CHAIN

Ten percent of 15-day fetal thymocytes of mice were Pgp-1(+)Thy-1(lo) cells. Half were strongly stained with monoclonal antibodies (mAb) rec ognizing the oncogene product, c-kit, but were not stained with mAb ag ainst non-T cell markers such as B220, Mac-1 and Gr-1. The isolated Pg p-1(+)c-kit(+) thymocytes showed no rearranged bands for V-DJ and D-J of T cell receptor (TcR) beta, but Pgp-1(-)-c-kit(-) thymocytes showed D-J rearranged bands. Both cells expressed the RAG-2 gene which is re quired for the V(D)J recombination process. When Pgp-1(+)c-kit(+) thym ocytes were cultured in 2-deoxyguanosine-treated alymphocytic fetal th ymus, they became TcR-expressing mature type T cells, but this differe ntiation was reduced by the addition of anti c-kit mAb. These data ind icate that Pgp-1(+)c-kit(+) thymocytes are pro-T cells with the potent ial to differentiate mature T cells in the thymic environment. This st udy also indicates that c-kit-mediated signals promote the differentia tion of thymocytes during their early stages.