Progenitor homing is profoundly reduced by irradiation conditioning. (A) Short-term homing assay was performed with unirradiated recipients or recipients receiving 600 or 900 cGy total body irradiation (TBI). Representative coculture plots are shown. P < .001 compared with unirradiated using Fisher’s exact test. The number of thymic settling progenitors was estimated using limit dilution analysis. Data are represented as mean ± standard error of the mean (SEM) (n = 3). Limit of detection (dotted line) is calculated from lowest number of donor cells that yielded positive results in short-term homing assay of unirradiated mice. (B) Mice were irradiated at 600 cGy or left untreated. Two hours later, 3 × 10⁵ CD45.1 BM cells or sorted Flt3⁺Lin⁻Kit⁺ cells were intrathymically injected into recipient mice. Thymi were analyzed for donor cells by flow cytometry 22 hours later (n = 3). (C) Intrathymically injected donor BM cells (10⁶) were sorted from irradiated or unirradiated mouse thymi and plated on OP9DL4. Cocultures were analyzed after 10 days (n = 2). Representative plots are shown.