New Ways to Cut Acute GVHD Risk After Stem Cell Transplants

Acute graft‑versus‑host disease (aGVHD) still threatens patients who receive allogeneic hematopoietic stem cell transplants (allo‑HSCT). Recent data show that newer drug combos, the growing use of frozen grafts, and an off‑the‑shelf cell therapy are each reshaping how doctors prevent and treat this complication.

Key takeaways

• Post‑transplant cyclophosphamide (PTCy) alone lowered severe aGVHD compared with antithymocyte globulin (ATG) in reduced‑intensity matched‑donor transplants [1].
• Adding ATG to PTCy did not further improve survival, but both PTCy and ATG reduced chronic GVHD versus no in‑vivo T‑cell depletion [1].
• Freezing donor stem cells delayed early CD3⁺ (T‑cell) chimerism and engraftment, yet did not change relapse or overall survival in unrelated donor transplants using ATG‑based prophylaxis [2].
• The bone‑marrow‑derived mesenchymal stromal cell product tomostrocel showed safety and promising response signals in patients with multi‑refractory severe aGVHD [3].
• Early response to tomostrocel on day 28 predicted overall survival, regardless of prior ruxolitinib exposure [3].

PTCy and ATG: New Prophylaxis Options

A French registry looked at adults with acute myeloid leukemia (AML) or myelodysplastic syndromes (MDS) who received reduced‑intensity conditioning (RIC) and a peripheral‑blood stem cell (PBSC) graft from a matched related or unrelated donor. Patients were grouped by GVHD prophylaxis: PTCy alone, ATG alone, PTCy + ATG, or no in‑vivo T‑cell depletion. The study included 28 patients who received the combined PTCy + ATG regimen.

Multivariable analysis showed that PTCy alone cut the risk of grade III–IV aGVHD by more than half compared with ATG (hazard ratio 0.44, p = 0.026) [1]. Both PTCy and ATG also lowered the rate of extensive chronic GVHD versus no T‑cell depletion, while not increasing non‑relapse mortality (NRM) or relapse risk. One‑year overall survival (OS) was 69.92%, progression‑free survival (PFS) 62.46%, and GVHD‑free, relapse‑free survival (GRFS) 48.39%, with no clear differences across donor type or prophylaxis group [1].

These findings suggest that PTCy can serve as an effective alternative to ATG for preventing severe aGVHD in RIC matched‑donor transplants. The data also provide a benchmark for ongoing randomized trials that are still maturing.

Frozen Grafts: Does Cryopreservation Matter?

The COVID‑19 pandemic pushed many transplant centers to freeze donor stem cell products before infusion. A separate analysis examined unrelated donor allo‑HSCTs that used RIC and ATG‑based GVHD prophylaxis, comparing fresh versus cryopreserved grafts.

Cryopreservation was linked to a lower chance of achieving full donor chimerism (≥95% of both myeloid and lymphoid cells) on day +60—33.8% for frozen grafts versus 54.4% for fresh grafts (p = 0.03) [2]. Multivariate modeling confirmed that freezing independently reduced the odds of full chimerism (odds ratio 2.39; 95% CI 1.08‑5.30). The frozen group also showed modestly lower early donor CD3⁺ (T‑cell) chimerism and a slight delay in engraftment.

Importantly, these chimerism differences did not translate into higher relapse rates, lower OS, PFS, or GRFS. The authors concluded that cryopreserved grafts appear safe in this setting, though larger studies are needed to confirm the impact on early immune reconstitution [2].

Mesenchymal Stromal Cells for Refractory aGVHD

When aGVHD does not respond to first‑line steroids and multiple subsequent agents, outcomes are grim. An off‑the‑shelf bone‑marrow‑derived mesenchymal stromal cell (MSC) product called tomostrocel was evaluated in a real‑world cohort of 317 adults with severe, multi‑refractory aGVHD.

Patients had already failed several lines of therapy, including ruxolitinib in many cases. Response rates measured on day 28 after MSC infusion were linked to overall survival, regardless of prior ruxolitinib exposure [3]. The safety profile was favorable, with no new safety signals beyond what earlier MSC studies reported.

Although the analysis is limited by its observational nature, the data suggest efficacy signals and a reassuring safety record for tomostrocel in this high‑risk population. Ongoing randomized controlled trials will be needed to confirm these early observations [3].

What Remains Uncertain

The PTCy versus ATG comparison comes from a retrospective registry, not a randomized trial, so unmeasured factors could influence outcomes. The benefit of adding ATG to PTCy remains unclear, as the study did not show a clear survival advantage for the combination [1].

Cryopreservation’s effect on early immune recovery (lower CD3⁺ chimerism) raises questions about infection risk, but the study did not report infection‑related outcomes. Larger prospective studies are required to determine whether the modest delay in engraftment matters clinically.

For MSC therapy, the real‑world cohort lacks a control arm, making it difficult to separate the drug’s effect from natural disease course. The predictive value of day‑28 response needs prospective validation, and the optimal timing and dosing of MSCs remain to be defined.

Bottom line

New prophylactic strategies like PTCy, either alone or with ATG, are showing promise in lowering severe aGVHD without harming survival. Freezing donor grafts appears safe, though it may slow early immune reconstitution. For patients whose aGVHD does not respond to standard drugs, the MSC product tomostrocel offers a potential, well‑tolerated option, with early response predicting better survival. Ongoing trials will clarify how these approaches can be best combined to improve outcomes for transplant recipients.