Flt3L enhances homeostatic peripheral expansion and immune competence in athymic T cell-depleted hosts undergoing immune reconstitution. (A) Hatched columns represent C57BL/6 hosts that were thymectomized (TXY) and T-cell depleted using anti-CD4 and anti-CD8 monoclonal antibodies as described in “Materials and methods” and then administered 1 × 10⁶ B6/CD45.1 LN cells intravenously. Open columns are TXY, non-T cell-depleted (TCD) controls. Flt3L (5 μg per day) or buffer alone was administered from day 1 to 28. On day 29, CD4⁺ and CD8⁺ splenic T cells were enumerated using flow cytometry (n = 5 to 6 animals per group). Data are representative of 2 separate experiments. The graph shows median values, with bars representing standard error. (B) On day -14, TXY/TCD hosts and TXY non-TCD control mice received 1 × 10⁶ LN cells intravenously that were harvested from female mice approximately 4 weeks following rejection of male skin grafts. Male tail skin grafts were placed on day 0. Flt3L or buffer alone was administered from day 1 to 28. Dotted lines are TXY, non-TCD controls, sham treated, n = 10; dashed lines are TXY, non-TCD controls, flt3L treated, n = 5; thin solid lines are TXY/TCD, sham treated, n = 10; thick solid lines are TXY/TCD, flt3L treated, n = 10. No difference in the rate of graft rejection was observed in control groups treated with flt3L (dotted versus dashed lines, P = NS); however, flt3L significantly enhanced the rate of graft rejection in TXY/TCD hosts (thin solid versus thick solid lines, P < .0001). Similar results were obtained in 2 separate experiments.