Pro-T cells in fetal thymus express c-kit and RAG-2 but do not rearrange the gene encoding the T cell receptor β chain

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Abstract
Ten percent of 15‐day fetal thymocytes of mice were Pgp‐1+Thy‐1lo cells. Half were strongly stained with monoclonal antibodies (mAb) recognizing the oncogene product, c‐kit, but were not stained with mAb against non‐T cell markers such as B220, Mac‐1 and Gr‐1. The isolated Pgp‐1+c‐kit+ thymocytes showed no rearranged bands for V‐DJ and D‐J of T cell receptor (TcR) β, but Pgp‐1−‐c‐kit− thymocytes showed D‐J rearranged bands. Both cells expressed the RAG‐2 gene which is required for the V(D)J recombination process. When Pgp‐1+c‐kit+ thymocytes were cultured in 2‐deoxyguanosine‐treated alymphocytic fetal thymus, they became TcR‐expressing mature type T cells, but this differentiation was reduced by the addition of anti c‐kit mAb. These data indicate that Pgp‐1+c‐kit+ thymocytes are pro‐T cells with the potential to differentiate mature T cells in the thymic environment. This study also indicates that c‐kit‐mediated signals promote the differentiation of thymocytes during their early stages.