Pretreatment with CCL21/CCL25 rescues homing to the thymus after irradiation. (A) BM was incubated with PBS or CCL21 and CCL25 for 30 minutes at 37°C. Pretreated BM cells (10⁷) were injected into unirradiated recipients or mice that had received 600 cGy TBI for the short-term homing assay (n = 3). Representative flow cytometric plots of cocultures are shown. P < .05 using Fisher’s exact test. (B) One hundred sorted Flt3⁺LK cells pretreated with CCL21 and CCL25 for 30 minutes were grown on OP9D4 in the presence of IL7 and Flt3. Cocultures were analyzed by flow cytometry after 10 days. Number of Thy1⁺CD25⁺ cells are shown. (C) BM was incubated with blocking anti-α4 prior to pretreatment or CCL21/CCL25 was washed from the cells following pretreatment and left for 30 minutes at 37°C. After 14 days, thymi were examined for donor reconstitution by flow cytometry (n = 3). (D) After 16 days, BM and thymi from PBS or pretreated groups were examined for total thymocyte numbers and donor reconstitution by flow cytometry (n = 3). Data are represented as mean ± SEM. *P < .05 compared with untreated using the Student t test.