Comparing Immune Reconstitution in Post-Transplant Cy Myeloablative Vs Reduced Intensity Transplants: Notable Trends in Monocyte Population
Authors: Shah Manan , Zhu Mandi , Kumar Rathan , Karunasiri Malith , Neidemire-Colley Lotus , Yuhasz Nicholas , Miller Elizabeth , de Lima Marcos , Ranganathan Parvathi , Choe Hannah 
Published: December 2024

Introduction
Allogeneic hematopoietic stem cell transplant (HSCT) plays a crucial role in the treatment of hematological malignancies. HSCT is associated with severe post-transplant complications such as infections and graft versus host disease (GVHD). GVHD remains the leading case of non-relapse mortality. While tacrolimus and methotrexate (Tac/MTX) has been the most common GVHD prophylaxis in the US for myeloablative conditioning (MAC), reduced intensity conditioning (RIC) regimens are incorporating post-transplant cyclophosphamide (PTCy) in combination with Tac and mycophenolate mofetil (MMF) following the results of BMT CTN 1703. Currently, PTCy is being increasingly used for GVHD prophylaxis in MAC transplants. Recent data with RIC PTCy shows lower rates of acute and chronic GVHD with comparable relapse and survival outcomes with MAC Tac/MTX. The immune reconstitution with PTCy in MAC versus RIC transplants is not yet well understood. Conditioning intensity with PTCy may directly affect immune reconstitution and treatment related complications. Monocyte recovery post-HSCT has been studied as a prognostic marker for acute and chronic GVHD, where faster monocyte recovery predicts lower rates of acute GVHD but higher rates of chronic GVHD. Here, we aim to compare the immune reconstitution of patients treated with MAC PTCy vs RIC PTCy with Tac/MTX controls.
Methods
We reviewed and identified patients that received Tac/MTX and PTCy for MAC and RIC transplants. Mononuclear cells (MNC) collected from patients on Day 28 (w4) stored in the OSU GVHD biorepository were obtained for analysis. Samples were stained with antibodies as per the OMIP-042 protocol (21-color) for flow cytometric analysis of major lymphocyte and myeloid subsets. We analyzed all available patients with longitudinal samples; 5 Tac/MTX, 3 MAC PTCy, 4 RIC PTCy, and 3 healthy donors (HD) for comparisons, all collected on day 28. Chimerism was &gt; 95% for all patients analyzed. The conditioning regimen predominantly used for RIC was Flu/Mel 140, while for MAC, it was Flu/Bu 20.5. Analyses were performed using GraphPad Prism 10.0, with differences between groups identified using ANOVA and students-t test. Cell type compositions were determined as percentages, and a p value of &lt;0.05 was considered significant.
Results
The flow cytometry analysis on day 28 noted distinct patterns in cell dynamics. The immune reconstitution in MAC Tac/MTX vs MAC PTCy showed lower CD4+T cells (mean 9.7% vs 40.7%) while CD8+T cell percentages (mean 21% vs 15%) were alike across both cohorts compared to HD. Similar trends across the CD4/CD8+T cells and non-B/T cells populations were observed in our RIC PTCy vs RIC Tac/MTX and RIC PTCy vs MAC PTCy cohorts, with lower CD3+T cell counts in PTCy receiving regimens. However, a significant difference was noted in the CD14+ monocyte populations with MAC PTCy vs RIC PTCy (77.8% vs 22.20 % p=0.001). Monocytes were also higher in the MAC PTCy vs MAC Tac/MTX (77.8% vs 38.9%, p=0.10) cohort. Similar patterns were noted in the RIC PTCy vs RIC Tac/MTX with higher CD14+ monocyte counts in Tac/MTX cohort (22.2% vs 60%, p= 0.50). On further analysis of those patients that developed GVHD on MAC PTCy vs RIC PTCy, no significant differences were noted in their immune reconstitution. Available samples from Day 56 were further obtained. Of these, only RIC PTCy and RIC Tac/MTX were available for analysis. No significant difference was noted in the monocyte population compared to previous observations.
Conclusion
We observed a distinct difference, despite a small sample size, in monocyte populations between patients treated with PTCy under MAC vs RIC regimens. In the RIC setting, patients receiving Tac/MTX had a higher percentage of monocytes compared to those receiving PTCy. Our data suggests that conditioning regimens may significantly modify monocyte recovery. The variation in dosing and intensity of conditioning regimes could confound these observations, thereby necessitating further evaluation at subsequent time points and close clinical correlation to optimize patient outcomes.