Editor’s Note: Graft-versus-host disease (GVHD) is one of the major complications following allogeneic hematopoietic stem cell transplantation (allo-HSCT), exhibiting diverse clinical manifestations with prolonged course. Improper diagnosis and treatment can not only affect patients’ quality of life but also have a significant impact on long-term survival. With the continuous improvement of modern transplant technology, patients’ demand for post-transplant quality of life is increasing, emphasizing the importance of GVHD prevention and treatment. From April 14th to 17th, 2024, the 50th Annual Meeting of the European Society for Blood and Marrow Transplantation (EBMT2024) was held in Glasgow, UK. On the second day of the conference, Leslie Kean and colleagues from the Dana-Farber/Boston Children’s Cancer and Blood Disorders Center in the United States orally presented a clinical study (Abstract No. GS02-02), suggesting that the post-transplant cyclophosphamide (PT-Cy) regimen, while effectively controlling unrelated donor GVHD, may increase the risk of grade 2 or higher infections. Through further large-scale T-cell receptor (TCR) sequencing (TCR-seq), researchers found that PT-Cy significantly reduces patients’ TCR diversity, which may be a contributing factor to the higher incidence of infections. We are pleased to invite Professor Depei Wu from The First Affiliated Hospital of Soochow University to provide insightful commentary on this study.
Research Background
BMT CTN 1703 is a phase III clinical trial aimed at comparing the GVHD prophylactic effects of Tac/MTX (n=217) and PT-Cy/Tac/MMF (n=214) in unrelated donor hematopoietic stem cell transplantation (allo-HSCT) after RIC conditioning. The results of this study showed that PT-Cy significantly improved patients’ rates of GVHD-free and relapse-free survival, but there was no significant difference in overall survival between the two regimens, and the PT-Cy pretreatment group experienced more grade 2 infections (Bolanos-Meade et al, NEJM 2023). To explore the biological basis behind these results, researchers conducted mechanistic studies (BMT CTN 1801).
Research Methods
A total of 324 patients (159 CNI/MTX, 165 PT-Cy) were enrolled in the study, and samples were collected on the day of allo-HSCT infusion and on days +7, 14, 28, 63, 98, 180, 270, 1 year, and 2 years thereafter. DNA was extracted from 2,225 samples and subjected to TCR-Seq analysis, which delineated a median of 44,077 TCRs per sample, totaling 215,155,719 T cells analyzed. Various TCR diversity indices were longitudinally assessed, including richness and clonality (TCR-seq equivalents of naïve T cells, etc.). Additionally, flow cytometry analysis of peripheral blood mononuclear cells was conducted to identify lymphocyte subsets during immune reconstitution after allo-HSCT.
Research Results
The study found significant differences between Tac/MTX and PT-Cy in TCR reconstitution, which began early and persisted for 2 years post-HCT. PT-Cy led to a significant reduction in TCR repertoire diversity, which had significant implications for its efficacy and toxicity: (1) PT-Cy resulted in significant early depletion of T cells post-allo-HSCT, with CD3 absolute counts of 308.4 cells/μL vs. 74.6 cells/μL in the Tac/MTX group vs. PT-Cy group at day 28; (2) The remaining donor T cells in the PT-Cy group exhibited significant clonal expansion, leading to restricted TCR diversity, which started from day +14 and persisted for 2 years; (3) The combination of T cell depletion and clonal expansion contributed to the development of TCR diversity deficiency following PT-Cy.
After infection, the diversity of TCR repertoires decreased in both the Tac/MTX and PT-Cy groups, indicating T-cell expansion in both groups following infection. However, in the PT-Cy group, the decline in clonality at all studied time points was significantly correlated with the pre-infection decline in clonality. This clonality defect in PT-Cy patients poses a higher risk of missing the critical TCR threshold required for effective response to infection, consistent with the increased rate of grade 2 infections observed in this group.
Research Conclusion
This study longitudinally analyzed over 200 million T cells from 324 allo-HSCT patients using TCR-Seq. It represents the largest-scale post-transplant TCR-Seq analysis to date, tracking the dynamic changes of TCRs post-transplantation. These data reveal that the TCR diversity deficiency induced by PT-Cy, while reducing the occurrence of GVHD, also increases the risk of infectious complications.
Expert Commentary
The PT-Cy regimen post-hematopoietic stem cell transplantation has been shown to effectively control GVHD, but it may increase the risk of grade 2 or higher infections. Immune reconstitution, particularly the restoration of T-cell immune function, is one of the major factors influencing long-term survival of transplant patients.
Currently, TCR repertoire diversity analysis has been a commonly used method to characterize immune reconstitution post-allo-HSCT. This study dynamically monitored TCR diversity using TCR-seq technology and found a bottleneck in diversity with PT-Cy, which may contribute to the higher incidence of infections. The study analyzed T cells from over 200 million longitudinal samples from 324 hematopoietic stem cell transplant patients, representing the largest-scale dynamic analysis of post-transplant TCR-seq to date, with important clinical implications. However, there are still some unresolved issues:
Factors influencing the dynamic changes of TCR post-allo-HSCT include the source of the graft, donor infection history, recipient infection history post-transplantation, donor-recipient age, etc. Are there other influencing factors between the two groups?
During the acute phase of immune response, T cells with high-affinity TCRs are often preferentially selected. However, in conditions such as latent cytomegalovirus (CMV) infection, low-affinity T cells have been shown to play important roles. Therefore, further clarification is needed on the relationship between CMV infection and the bottleneck in TCR diversity.
Professor Depei Wu
Chief Physician, Doctoral Supervisor
Director of Hematology Department, The First Affiliated Hospital of Soochow University
Executive Deputy Director of National Clinical Research Center for Hematologic Diseases
Vice Director of Jiangsu Institute of Hematology
Director of Hematopoietic Stem Cell Transplantation Research Institute, Soochow University
Member of the 13th and 14th National Committee of the Chinese People’s Political Consultative Conference (CPPCC)
Chairman of the Hematology Branch of Chinese Medical Association and Chairman of the National Group of Experimental Diagnosis
Member of the Global Committee of the European Society for Blood and Marrow Transplantation (EBMT)
Vice President of the Hematology Physicians Branch of the Chinese Medical Doctor Association
Vice Chairman of the Expert Committee of China Hematopoietic Stem Cell Donor Database
Executive Member of the Internal Medicine Branch of Chinese Medical Association
Editor-in-Chief of the Chinese Journal of Hematology
Associate Editor of J Hematol Oncol
