XBP-1 decreases B-cell recovery and promotes the T_FH cell subset after allo-HCT. B10.BR recipients were treated twice with cyclophosphamide (120 mg/kg) 1 time per day on days −3, −2, and sublethally irradiated on day −1 before transplant. On day 0, mice were transplanted with TCD-BM (5 × 10⁶ per mouse) from XBP-^1flox/floxCD19Cre⁻ (n = 10) or XBP-1^flox/floxCD19Cre⁺ donors (n = 9) on a B6 background with (n = 19) or without (n = 4) whole splenocytes at 0.15 × 10⁶ per mouse. On day 28 after allo-HCT, recipient mice were killed and spleens excised and processed into single-cell suspensions for flow cytometry analysis of B220⁺ B cells (A-B) and donor CD4⁺ T_FH cells (PD-1^hiCXCR5⁺) (A,C). Lungs were excised, and lymphocytes were isolated and stained for B220⁺ B cells (D-E) and apoptotic B cells (Annexin V⁺7AAD⁺) (D-F). Data shown in panels A and D are individual flow cytometry plots representative of each group. Data shown in panels B, C, E, and F are pooled from 2 replicate experiments. P < .05 indicates statistical significance. SSC-A, side scatter area.