Regulation of NK1.1 expression during lineage commitment of progenitor thymocytes.

We recently identified a stage in fetal ontogeny (NK1.1+/CD117+) that defines committed progenitors for T and NK lymphocytes. These cells are found in the fetal thymus as early as day 13 of gestation, but are absent in the fetal liver. Nonetheless, multipotent precursors derived from both the fetal thymus and fetal liver are capable of rapidly differentiating to the NK1.1+ stage upon transfer into fetal thymic organ culture (FTOC). This suggests that expression of NK1.1 marks a thymus-induced lineage commitment event. We now report that a subset of the most immature fetal thymocytes (NK1.1-/CD117+) is capable of up-regulating NK1.1 expression spontaneously upon short-term in vitro culture. Interestingly, fetal liver-derived CD117+ precursors remain NK1.1- upon similar culture. Spontaneous up-regulation of NK1.1 surface expression is minimally affected by transcriptional blockade, mitogen-induced activation, or exposure of these cells to exogenous cytokines or stromal cells. These data suggest that induction of NK1.1 expression on cultured thymocytes may be predetermined by exposure to the thymic microenvironment in vivo. Importantly, multipotent CD117+ thymocytes subdivided on the basis of NK1.1 expression after short-term in vitro culture show distinct precursor potential in lymphocyte lineage reconstitution assays. This demonstrates that even the earliest precursor thymocyte population, although phenotypically homogeneous, contains a functionally heterogeneous subset of lineage-committed progenitors. These findings characterize a thymus-induced pathway in the control of lymphocyte lineage commitment to the T and NK cell fates.