Lymphocyte subset reconstitution and clinical outcomes following haploidentical hematopoietic stem cell transplantation

Background Immune reconstitution is critical for outcomes after allogeneic hematopoietic stem cell transplantation (allo-HSCT), but its prognostic role in haploidentical peripheral blood stem cell transplantation (haplo-PBSCT) remains unclear. Methods We retrospectively analysed 577 patients who underwent T-cell replete haplo-PBSCT between 2016 and 2024. Longitudinal reconstitution of CD4⁺, CD8⁺ T cells and their subsets, CD3⁺CD69⁺, CD3⁺HLA-DR⁺ T cells, NK cells and B cells was assessed from 1 to 12 months post-transplant. Cox regression and causal mediation analyses were performed to identify prognostic associations and mechanisms. Results Early and robust regulatory T cell (Treg) reconstitution significantly reduced transplant-related mortality (TRM) and improved overall survival. Naïve CD8⁺ T cell recovery correlated with reduced TRM and relapse. Higher late CD3⁺CD69⁺ T cells were linked to decreased relapse risk. Sustained B cell reconstitution reduced TRM and was associated with a lower incidence of moderate-to-severe chronic graft-versus-host disease. Early Treg reconstitution was associated with lower cytomegalovirus (CMV) reactivation. Mediation analysis revealed that Treg recovery reduced TRM partially through suppression of CMV reactivation (ACME = –0.22, p = 0.032). Conclusion Distinct lymphocyte subset reconstitution profiles predict transplant outcomes in haplo-PBSCT. Early Treg recovery, partly by limiting CMV reactivation, may serve as a target for immune-guided intervention.