PRECURSORS OF CD3- DELINEATION OF EARLY EVENTS IN HUMAN THYMIC DEVELOPMENT(CD4+CD8+ CELLS IN THE HUMAN THYMUS ARE DEFINED BY EXPRESSION OF CD34 )

Studies of the most immature T cell progenitors in the human thymus ha ve been hampered by the lack of markers and assays that define these c ells. In this report we used a novel human fetal thymic organ culture system to determine the potential of T cell precursors isolated from h uman postnatal thymus, to differentiate into CD3+ thymocytes, and to i nvestigate early stages of human T cell development. It was found that thymocytes that lack the markers CD3, CD4, and CD8 (triple negative [ TN]) can differentiate in an allogeneic organotypic thymic culture. Th e capacity of TN thymocytes to differentiate was exclusively confined to the CD34+ population. CD34- TN thymocytes failed to differentiate i n this system. In contrast, cloned lines of CD3- thymocytes could only be established from CD34- TN thymocytes. Five subsets of CD3- thymocy tes were found with the following phenotype: CD1-TN, CD1+TN, CD1+CD4+C D8-, CD1+CD4+CD8alpha+beta-, and CD1+CD4+CD8alphabeta+. These subpopul ations expressed decreasing levels of CD34. The CD1-CD3- population ex pressed the highest levels of CD34 supporting the notion that this pop ulation is the most immature T cell precursor in the thymus, whereas t he CD1+CD4+CD8alpha+beta+ which did not express CD34 seems to be the m ost mature of these CD3- populations. This notion is supported by the observations that CD34+ cells isolated from fetal liver, which differe ntiated into T cells in a FTOC, developed into CD3+ cells via CD1- and CD4+CD8- intermediates. Based on these data, we present a model of ea rly stages in human intrathymic development.