Acute GVHD impairs mTEC^high compartment size. The mTEC^high compartment was analyzed as a function of recipient age in 4 different murine models of allo-HSCT, as detailed in supplemental Figures 1 and 2. (A) Acute GVHD was induced in 8-week-old unconditioned BDF₁ recipients (b→bd, black circles). Untransplanted BDF₁ mice (gray squares) or syngeneically transplanted mice (bd→bd, open circles) served as controls. In 1 group, Fgf7 was administered (b→bd + Fgf7, triangles) as described.¹⁵  A total of 6 experiments were performed, with 3 to 5 mice per group and experiment. (B) Acute GVHD (TCD+T group, black circles) was induced in irradiated 8-week-old BDF₁ recipients (b→bd, left panel), 9-week-old B6 recipients (d→b, middle panel) or 8-week-old B6 recipients (b→b, right panel) as described in supplemental Figures 1 and 2. Untransplanted BDF₁ or B6 mice (gray squares) and recipients of TCD bone marrow (TCD group, open circles) were used as controls without disease in all models. A total of 3 experiments were performed. The line graphs show total numbers (mean ± SD) of mTEC^high cells in thymi from individual mice. *P < .05, analysis of variance (ANOVA), aGVHD versus transplanted mice without aGVHD. TCD, T-cell–depleted; n.d., not done. (C-E) Qualitative and quantitative analysis of thymic Aire expression, as described in supplemental Methods. Immunohistochemistry and confocal microscope analysis (C) was performed on thymic frozen sections taken from unconditioned transplant recipients at ages 10, 12, and 15 weeks (ie, 2, 4, and 7 weeks after transplantation; [i-iii] bd→bd, [iv-vi] b→bd, [vii-ix] b→bd + Fgf7). Cytokeratin-18 (CK18, blue) and CK14-positive cells (red) denote cTEC and mTEC, respectively.^14,17  Aire⁺ cells are yellow in color and localize to the thymus medulla. Syngeneically transplanted mice (bd→bd: 18 ± 4 Aire⁺mTEC/0.04 mm² medulla) were not different from age-matched untransplanted control mice (data not shown⁵ ). The flow cytometry plots in (D) depict Aire and MHC class II expression of DAPI^–CD45^–EpCam⁺Ly51^–UEA1⁺ mTECs. Numbers represent frequencies (%) of cells (mean ± SD). Total numbers of Aire⁺mTEC^high in thymi from individual recipient mice (E) was calculated from flow cytometry data (mean ± SD). Groups are the same as in panel (A). A total of 5 experiments were performed, with 3 to 5 mice per group. *P < .05, ANOVA, aGVHD vs transplanted mice without aGVHD.