CHARACTERIZATION OF CD34(+) THYMIC STROMAL CELLS LOCATED IN THE SUBCAPSULAR CORTEX OF THE HUMAN THYMUS

In this paper we report that suspensions of human fetal thymocytes con tain cells that express high levels of CD34 and Thy-1. These cells wer e characterized with regard to location within the thymus, phenotype, and function. Confocal laser scan analysis of frozen sections of fetal thymus with anti-CD34 and Thy-1 antibodies revealed that the double-l abeled cells were located in the pericortical area. In addition, it wa s found that the CD34(+)Thy-1(+) cells lacked CD45 and CD50, indicatin g that these cells are not of hematopoietic origin; this was confirmed by the finding that these cells could be cultured as adherent cells i n a medium with cholera toxin and dexamethasone, but failed to grow in mixtures of hematopoietic growth factors. Further analysis indicated that most cultured CD34(+)Thy-1(+) cells expressed cytokeratin (CK) 14 but lacked CK 13, suggesting that these cells are immature epithelial cells. Cultured CD34(+)Thy-1(+) cells were able to induce differentia tion of CD1(-)CD34(+)CD3(-)CD4(-)CD8(-) thymic precursors into CD4(+)C D8(+) cells in a reaggregate culture in the absence of exogenous cytok ines. The CD4(+)CD8(+) cells that developed in these cultures did not express CD3, indicating that CD34(+)Thy-1(+) thymic stromal cells are not capable of completing full T cell differentiation of thymic hemato poietic progenitor cells.