Post-Transplant Cyclophosphamide May Protect Against GVHD but Ups Infection Risk

Post-allogeneic hematopoietic stem-cell transplantation (HSCT) cyclophosphamide (PT-Cy) is associated with a pan-T-cell depletion that appears both to protect patients against graft-versus-host disease (GVHD) and to place them at higher risk of infectious complications, according to findings from the BMT CTN 1801 companion study of the randomized phase 3 BMT CTN 1703 study. These findings were presented at the 2023 ASH Annual Meeting in San Diego, California.

Tacrolimus/methotrexate (Tac/MTX) is standard prophylaxis against GVHD in patients undergoing HSCT. The BMT CTN 1703 trial (ClinicalTrials.gov Identifier: NCT03959241) compared outcomes associated with PT-Cy, tacrolimus, and mycophenolate mofetil vs standard Tac/MTX prophylaxis. The study showed improved outcomes for PT-Cy using a composite study endpoint of GVHD-free and relapse-free survival, but the regimens had similar overall survival and relapse-free survival outcomes.

In the companion BMT CTN 1801 biological mechanisms study, researchers analyzed post-HSCT T-cell reconstitution to determine the immunobiology of the 2 GVHD-prophylactic regimens, explained lead study author Leslie Kean, MD, PhD, of Dana-Farber/Boston Children’s Cancer and Blood Disorders Center in Massachusetts. 

The team extracted DNA from 2225 samples from 159 Tac/MTX study arm patients and 165 PT-Cy arm patients and performed Adaptive Biotech β-strand T Cell Receptor (TCR) Immunosequencing (TCR-Seq) to identify immunologic trade-offs associated with PT-Cy vs Tac/MTX. 

“One of the key study questions that we were able to answer with this deep analysis was what is the comparative impact of PT-Cy and Tac/Methotrexate on T-cell reconstitution and repertoire diversity, post-transplant,” Dr Kean said. “Central to answering these questions are two measures of a diversity: richness and the number of TCR singletons.” 

Richness is a measure of the breadth of high- and middle-frequency TCR clones. Singletons are “the TCR-Seq equivalent to naive T-cells.” 

Decreased TCR richness in PT-Cy persisted long-term after transplant, Dr Kean noted. The research team also found a much-reduced number of TCR singletons in these patients.

That suggests that PT-Cy might have induced a T-cell diversity “bottleneck” followed by clonal expansion. “A population bottleneck is a sharp reduction in the size of population due to external events. Populations that have undergone a bottleneck can exhibit a significant reduction in that population’s genetic variation. … The second phase of a bottleneck is the release of the obstruction in subsequent clonal expansion from a smaller founder population,” she explained.

“Very importantly, the [BMT CTN] 1801 study has allowed us an unprecedented possibility to test in a human system with a control arm, which is very important, whether this depletion by PT-Cy was T-cell-subset specific. We find that there was not subset specificity and that rather, PT-Cy resulted in early pan-T-cell depletion including, of note, no sparing of depletion of TREGs, gamma delta T-cells, [and] no sparing of TCMs.”

To determine the clinical outcomes of this bottleneck, the researchers looked at chronic GVHD and infection, examining singletons before and after the dates of these events.  

In analyzing the Tax/MTX cohort the researchers identified a decreased proportion of singletons when comparing before and after chronic GVHD and Tax/MTX, which suggested clonal expansion during chronic GVHD. But they did not find a decreased proportion of singletons following PT-Cy compared to TAC/MTX, suggesting that a drop in singletons in Pt-Cy administered patients protects against GVHD development – implying that chronic GVHD involves different mechanisms in these patients than among those who received Tac/MTX. 

That was consistent with an increased rate of infection observed in the PT-Cy vs Tac/MTX arms in the BMT CTN 1703 study, she noted. 

“By performing pre-specified, state-of-the-art immune studies, we have been able to derive major insights into the mechanisms driving clinical outcomes,” Dr Kean concluded. “We find that PT-Cy results in significant in vivo pan-T-cell depletion early after transplant, and following depletion, the remaining donor T-cells and PT-Cy undergoes substantial clonal expansion, resulting in a significantly constrained TCR diversity in PT-Cy versus TAC methotrexate.”

This pattern of early T-cell depletion followed by clonal expansion was “consistent with an evolutionary bottleneck effect for TCR diversity after PT-Cy — and because of this bottleneck, while the proportion of transplanted T-cell present a year and two years post-transplant are similar between the two [regimens], there are significantly fewer unique TCRs that contribute to the T-cell compartment and T-cell immunity in PT-Cy,” Dr Kean concluded. “We believe that this may have clinical consequences with respect to both chronic graft versus host disease and to infections.” 

Disclosures: The study was supported by the Blood & Marrow Transplant Clinical Trials Network, US National Heart, Lung, and Blood Institute (NHLBI), and US National Cancer Institute. Some study authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of disclosures.

Reference
Kean L, Siegel SJ, Schmalz JT, et al. Large-scale post-transplant TCR deep sequencing reveals a major T cell diversity bottleneck with post-transplant cyclophosphamide with implications for both efficacy and toxicity: results of the BMT CTN 1801 study. Presented at ASH23. December 9-12, 2023. San Diego, CA. Abstract #475.