Tacrolimus has better performance in B cell reconstitution after allo-HSCT compared to cyclosporine A: a real-world study

B-cell reconstitution is closely associated with prognosis, infection risk, and graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (allo-HSCT). However, the impact of different calcineurin inhibitors (CNIs) on B-cell recovery remains unclear. We retrospectively analyzed 312 allo-HSCT recipients receiving cyclosporine (CsA, n = 211) or tacrolimus (FK506, n = 101). B-cell proportions in bone marrow and peripheral lymphocyte subsets were compared, and propensity score matching was applied to balance confounders. Multivariate analysis demonstrated that FK506 was independently associated with superior B-cell reconstitution at 3 months (OR 2.93, 95% CI 1.49-5.76, p = 0.010) and 4 months post-transplant (OR 3.53, 95% CI 1.11-11.15, p = 0.024). Higher B-cell proportions at 3-5 months were associated with a significantly lower risk of viral infection. Mediation analysis further revealed that improved B-cell reconstitution at day + 90 partially mediated the protective effect of FK506 against viral infection. These findings suggest that FK506 promotes improved B-cell recovery and may reduce viral infection risk after allo-HSCT.