It has been proposed that the CD7 molecule is the first antigen expressed on the membrane of cells committed to the T-cell lineage during human fetal T-cell ontogeny. To further identify the pre-T cell subpopulation that migrates to the thymus early in ontogeny, we analyzed the phenotypic and functional characteristics of the fetal liver populations separated on the basis of CD7 expression. Three populations expressing different levels of CD7 were observed: CD7bright, CD7dull, and CD7-. A CD7bright population depleted of mature T, B, and myeloid cells (lineage negative, lin-) and mostly composed of CD56+ CD34- natural killer cells did not mature into T cells in a fetal thymic organ culture (FTOC) assay and was devoid of myeloid progenitors in a clonal colony-forming cell assay. In contrast, the CD7-/dull CD34+ lin- populations were capable of differentiating into phenotypically mature T cells after injection into FTOC and contained early myeloid progenitors. Here we phenotypically compared the fetal liver CD7 populations with the most immature fetal thymic subset that differentiated in the FTOC assay, namely the triple negative (TN, CD3- CD4-CD8-) thymocytes. Fetal TN lin- expressed high levels of CD34 marker and were further subdivided by their expression of CD1 antigen, because CD1- TN thymocytes express higher levels of CD34 antigen compared with CD1+ TN cells. CD1- lin -TN thymocytes are characterized by expressing high levels of CD2, CD7, and CD34 markers and dull levels of CD5, CD10, and CD28 molecules. We could not find fetal liver pre-T cells with a phenotype equivalent to that of TN thymocytes. Our data show that CD7 does not necessarily identify T-cell precursors during fetal T-cell development and strongly support the hypothesis that the acquisition of early T-cell markers as CD2, CD28, and CD5 molecules on the cell surface of T-cell progenitors takes place intrathymically.

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