[Characterization of human immature T cell precursors in fetal liver and postnatal thymus]

The sequence of events that accompany development of immature T cell precursors in man is not precisely known. Recently, we identified subpopulations of thymocytes that may represent intermediate stages between CD3-CD4-CD8- (TN) and CD3+CD4+CD8+ (DP) thymocytes. Multicolor analysis provided evidence for the existence of CD3-CD4+CD8- and CD3-CD4+CD8 alpha+beta- populations. In vitro culture of TN thymocytes with IL-7 gave rise to cells of these phenotypes, suggesting that the novel subpopulations we found are the progeny of TN thymocytes. Expression of CD34 was detected on most of TN thymocytes and progressively reduced as cells differentiate. The novel subpopulations were shown to express CD34. 6-22-week-old human fetal liver cells were also analyzed in search for the possible prethymic T cell precursors. It was revealed that most of CD3-CD7+ cells which had been considered as T cell precursors were CD34-CD56+ and exhibited non-MHC-restricted cytotoxicity. Thus, although they expressed cytoplasmic CD3 antigens, these cells seemed to represent NK cells rather than T cell precursors. Existence of the latter is still to be demonstrated.