The phenotypic similarities between natural killer (NK) and T cells have led to the hypothesis that these distinctive lymphocyte subsets may be developmentally related and thus may share a common progenitor (Lanier, L. L., H., Spits, and J. H. Phillips. 1992. Immunol. Today. 13:392; Rodewald, H.-R., P. Moingeon, J. L. Lurich, C. Dosiou, P. Lopez, and E. L. Reinherz, 1992. Cell. 69:139). In this report, we have investigated the potential of human CD34+ triple negative thymocytes ([TN] CD3-, CD4-, CD8-) to generate both T cells and NK cells in murine fetal thymic organ cultures (mFTOC) and in vitro clonogenic assays. CD34+ TN thymocytes, the majority of which express prominent cytoplasmic CD3ε (cytoCD3ε) protein, can be divided into high (CD34(Bright)) and low (CD34(Dim)) surface expressing populations. CD34(Bright) TN thymocytes were capable of differentiating into T and NK cells when transferred into mFTOC, and demonstrated high NK cell clonogenic capabilities when cultured in interleukin (IL)-2, IL-7, and stem cell factor (SCF). Likewise, CD34(Bright) TN thymocyte clones after 5 d in culture were capable of generating NK and T cells when transferred into mFTOC but demonstrated clonogenic NK cell differentiation capabilities when maintained in culture with IL-2. CD34(Dim) TN thymocytes, however, possessed only T cell differentiation capabilities in mFTOC but were not expandable in clonogenic conditions containing IL-2, IL-7, and SCF. No significant differentiation of other cell lineage was detected in either mFTOC or in clonogenic assays from CD34+ TN thymocytes. These results represent the first definitive evidence of a common T/NK cell progenitor in the human fetal thymus and delineate the point in thymocyte differentiation where T and NK cells diverge.
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