B-cell XBP-1 modulates T-cell activation during cGVHD. Lethally irradiated BALB/c mice were transplanted with TBCD-BM (5 × 10⁶ per mouse) from B6Ly5.1⁺ mice (n = 10) with 1 × 10⁶ whole splenocytes from XBP-1^flox/floxCD19Cre-⁻ (n = 5) or XBP-1^flox/floxCD19Cre⁺ (n = 5), donors on a B6 background. Mice were monitored for cGVHD clinical scores after allo-HCT (A) (indicated statistical significance is a 2-tailed Student t test performed for the day 30 time point). The recipient mice were killed on day 30, and their spleens were excised and processed into single-cell suspensions for direct flow cytometric analysis of donor H2K^b PD-1 expressing CD8⁺ T cells (B-C) or were stimulated with LPS, PMA, and Ionomycin to detect IFN-γ–producing CD4⁺ (B,D) and CD8⁺ T cells (B,E). Data in panels A-F were collected from 1 cohort of mice (n = 10). P < .05 indicates statistical significance.