Intrathymic FL expression increases donor-derived cell numbers. CD45.1⁺ LSK cells (A and B) or ETPs (C and D) were injected intrathymically into B6 (CD45.2⁺) and FL^−/− (CD45.2⁺) thymi after sublethal irradiation. (A and C) Total donor-derived cell numbers 8 d after intrathymic injection (CD45.1⁺ gated). (B and D) Numbers of donor-derived cells that had progressed to the TN3 stage of development (CD45.1⁺ gated). n = 8 in all experiments. (E–I) B6 and FL^−/− E15 lobes were grafted under opposite kidney capsules in FL ^−/− hosts and allowed to engraft for 3 wk. (E) Gross anatomy of kidneys engrafted with B6 or FL^−/− lobes (four lobes per kidney) 10 d after lethal irradiation and BMT with GFP⁺ BM cells. (F) FCM analysis of FL expression on MTS15⁺ fibroblasts in B6 lobes 4 d after lethal irradiation. FL expression (bolded line) and isotype (continuous line) are shown. Number in the plot indicates MFIR. (G–I) Donor-derived (GFP⁺) cell numbers 10 d after lethal irradiation and BMT in grafted thymic lobes. (G) Total donor-derived cells. (H and I) Donor-derived thymic subpopulation cell numbers. n = 6 in all experiments. Error bars represent SEM. *, P < 0.05; **, P < 0.005; ***, P < 0.0005.