Recipient Treg expansion with TL1A-Ig+IL-2_LD before transplant significantly diminished GVHD severity, promoted colonic microbiome diversity, and improved overall survival. (A-F) BALB/c mice were treated with TL1A-Ig+IL-2_LD before TBI. aHSCT was performed using a B6 (donor) → BALB/c (recipient) major MHC-mismatch model. CD45.1 B6 (H2^b) TCD BM cell (5.5 × 10⁶) and an adjusted spleen cell number containing 0.55 × 10⁶ T cells were transplanted into BALB/c (H2^d) recipients. TBI was 7.25 Gy given on day −1. Presented aHSCT data were pooled from 2 independent experiments representing 16 mice per group. (A) Confirmation of Treg expansion (CD4⁺FoxP3⁺/CD4⁺) in the PB on day −2. (B-F) Panels show survival (n = 8) (B), GVHD clinical score (n = 8) (C), percent weight loss (n = 8) (D), general mouse appearance (E), and colon lengths (F). (B) Overall survival was 38% without recipient TL1A-Ig+IL-2_LD treatment vs 89% with recipient TL1A-Ig+IL-2_LD treatment. There was a significant difference in GVHD clinical scores over 63 days (C) and less percent weight loss early after aHSCT (D). Representative photographs depict animal appearance at day +28 after aHSCT (E). (F) Colon lengths of mice (n = 6, 3 from each group) at day +42 after aHSCT. (G) Relative abundance of bacterial microbiome over time in untreated and TL1A-Ig+IL-2_LD–pretreated recipients. Mice were assessed at day 0, day +7, day +10, day +14, and day +20 (n = 10). (H) TL1A-Ig+IL-2_LD–treated recipients displayed greater microbiome diversity at day +7. Bacterial diversity (Simpson reciprocal) and specific genus-level relative abundance of Escherichia and Enterococcus. Data represent the mean ± SEM, with ∗P < .05; ∗∗P < .01; ∗∗∗∗P < .0001 defining significance levels.