Homing molecules on endothelial cells continue to be expressed after irradiation. Thymi were harvested from unirradiated mice and mice that had been irradiated with 600 cGy 24 hours prior. (A) Total thymic cellularity was enumerated. Enumeration of endothelial cells was determined by flow cytometric analysis of the CD45⁻CD31⁺ population (n = 3). (B) P-selectin, (C) ICAM-1, and (D) VCAM-1 expression was determined on CD31⁺ endothelial cells by flow cytometry. Shown are histograms gated on unirradiated CD45⁺ cells (gray), unirradiated CD31⁺ cells (solid black line), and irradiated CD31⁺ (dotted black line) (n = 3). Gating for P-selectin was established previously with P-selectin knockout mice. Data are represented as mean ± SEM. *P < .05 compared with unirradiated using the Student t test. (E) Enumeration of epithelial cells was determined by flow cytometric analysis of the CD45⁻EpCam⁺ TEC population (n = 3). (F) Short-term homing assay of WT cells into congenic irradiated WT or p53-deficient recipients (n = 2; P < .05 using Fisher’s exact test). Representative flow cytometric plots of cocultures are shown.