Abrogation of STAT-1 signaling in TECs prevents injury in vivo. (A) Phosphorylation of STAT-1 at residue 701 was detected by Western blot in purified primary adult CD45⁻I-A^b+ TECs and CD45⁺I-A^b− thymocytes from mice with aGVHD (b→bd, day 12). (B) Analysis of T cell-deficient Balb/c.nu/nu mice (H-2^d) carrying a heterotopic thymus derived from E14 fetal STAT1^+/+ and STAT-1^−/− TECs (strain 129; H-2^b), respectively (Figure S1). Panels i-vi depict gross anatomy (left), flow cytometric analysis (middle), and histology (right) of ectopic thymi located under the kidney capsule (circles) of recipients not further treated with allogeneic donor T cells. One representative mouse per group is shown. Images in panels iii, vi, ix, and xii were visualized using a Nikon E600 microscope (Nikon, Zurich, Switzerland) equipped with a 10×/0.30 NA objective. Images were acquired using a Nikon DXM 1200F camera and Nikon ACT-1 software version 2.62. Panels vii-xii depict the analyses of ectopic thymi from mice that have received 15 × 10⁶ Balb/c T cells 3 weeks earlier. For analysis of ectopic T-cell development, relative numbers (in percent) of immature CD4⁺CD8⁺ and SP mature thymocytes, respectively, are shown. A total of 3 mice were analyzed for each transplantation group. The experiment was performed twice, with identical results; *P < .05 versus mice receiving no T cells.