Impact of Transplant-Related Complications on Lymphocyte Subset Reconstitution Following Allogeneic Hematopoietic Stem Cell Transplantation

Introduction: Allogeneic hematopoietic stem cell transplantation (allo-HSCT) remains a cornerstone treatment for hematologic malignancies, though frequently complicated by graft-versus-host disease (GVHD), infections, and immune dysregulation. The timely and effective recovery of lymphocyte subsets post-transplant is critical for long-term prognosis, yet remains poorly understood in the context of these complications. Aim: The aim of this study was to evaluate the impact of transplant-related complications of the quantitative reconstitution of lymphocyte subsets in adult recipients of allogeneic hematopoietic stem cell transplantation. Materials and Methods: This retrospective study analyzed 89 adult patients who underwent allo-HSCT at St. Marina University Hospital, Varna, between 2017 and 2023. Data on lymphocyte subset reconstitution (CD3⁺CD4⁺, CD3⁺CD8⁺CD38⁺, NK, and CD19⁺ cells) were collected at days +100, +180, and +270 post-transplant and correlated with the occurrence of transplant-related complications (GVHD, infections, relapse, and non-infectious complications). Various conditioning regimens and immunosuppressive protocols were applied, with most patients receiving HLA-matched or haploidentical grafts. Results: Patients who experienced complications post-allo-HSCT exhibited significantly impaired immune recovery across all lymphocyte subsets. Graft-versus-host disease was the strongest negative predictor, associated with the lowest counts of CD3⁺CD4⁺ and CD3⁺CD8⁺CD38⁺ cells at all time points (p<0.001). Infectious complications also significantly reduced the reconstitution of CD4⁺, CD8⁺, NK, and B-cell populations (p<0.05). Conclusion: Transplant-related complications, particularly GVHD and infections, substantially impair lymphocyte subset reconstitution following allo-HSCT, which may negatively impact clinical outcomes. Monitoring immune recovery dynamics provides prognostic insight and could inform individualized immunomodulatory strategies. These findings support the integration of immunological biomarkers into post-transplant risk stratification and patient management.