Rat T lymphocyte progenitors in bone marrow can be induced to express more Thy 1

We used a rat thymic epithelial cell line, IT-45R1, as a differentiation inducer to identify rat bone marrow (BM) pre-T cells. The following observations strongly suggest that BM pre-T cells are relatively small to medium-sized and weakly Thy 1 positive (the amount of Thy 1 is much lower than that of majority of adult thymocytes) lymphoid cells: (1) Fraction (Fr.) 5 cells, separated by bovine serum albumin (BSA) density gradient centrifugation, were induced to express more Thy 1. (2) In a time course study and a study with a beta-stimulator, Fr. 5 cells were almost identical to mouse pre-T cells. (3) Thy 1 expression of Fr. 5 cells appeared to be related to the syntheses of DNA, RNA and protein, being in good agreement with the results of human and mouse pre-T cells. (4) The increase of Thy 1 content by STEL was confirmed in the cell sorter (ABCASS) analysis. (5) ABCASS revealed that Thy 1 content of fetal thymocytes of 18 days' gestation was just between that of BM cells and adult thymocytes, indicating that the increase in Thy 1 content may occur also in vivo.