Lymphoid developmental potential of post-BMT T-lineage progenitor populations in OP9 and OP9-DL1 cultures. BM LSK cells, as well as thymic ETP, DN2, and DN3 cells, were compared witih post-BMT donor-derived Lin^–Thy1.2^–CD44⁺ and Lin^–Thy1.2⁺CD44^–/lo (SpT) cells, as defined in Figure 2. Sorted progenitor populations were cultured with OP9-MigR1 or OP9-DL1 BM stromal cell lines, as described.¹⁶  Flow cytometric analysis was performed at various time points after seeding. The data are shown after gating on CD45⁺GFP^– live cells. Numbers indicate the percentage of cells in the boxed region. (A) CD44/CD25 and CD4/CD8α profile of the cells after 10 days of coculture on OP9-DL1 cells. (B) Similar analysis at day 22 of coculture. Differentiation of SpT cells paralleled DN3 thymocyte differentiation, peaking early at day 10 and then decreasing. The differentiation of Thy1.2^–CD44⁺ cells was similar to BM LSK cells. (C) CD44/CD25 and CD19/B220 staining at day 22 of culture on OP9-MigR1 stromal cells. Both BM LSK and post-BMT Thy1.2^–CD44⁺ cells gave rise to CD19⁺B220⁺ B cells, whereas other progenitors had no in vitro B-lineage potential.