A transient wave of T-lineage progenitors is found in the spleen after BMT. (A) Characterization of donor-derived cells in the spleen at 2 weeks after BMT. A significant fraction of donor-derived Lin^– cells expressed the T-lineage marker Thy1.2. Concomitant staining of donor-derived Lin^– cells with Thy1.2/CD44, Thy1.2/CD25, and Thy1.2/c-Kit is shown. Lin^–Thy1.2⁺CD44^–/lo cells are referred to as spleen pre-T cells (SpT), as described.³²  Data are representative of 5 experiments. (B) Expression of RAG as an indicator of lymphoid specification. B6.Ly5^SJL recipient mice were given transplants of BM from rag2-GFP reporter mice.³⁸  At 2 weeks, a high percentage of GFP⁺ cells was observed among SpT cells, whereas GFP expression in the Thy1.2^–CD44⁺ population was much lower. Dashed lines are drawn between panels A and B to graphically depict the identity of the SpT and Thy1.2^–CD44⁺ populations, although the data were generated in similar but separate experiments. Data are representative of 2 experiments. Open histograms indicate cells derived from a rag2-gfp donor; shaded histograms, cells derived from a nontransgenic littermate. (C) Absolute number of donor-derived SpT cells in the spleen at various time points after BMT. Data are shown as mean ± 2 SEM for 3 to 8 mice per time point.