Allogeneic stem cell transplantation (aHSCT) remains one of the most curative treatments for blood cancers, but its success is constrained by graft-versus-host disease (GVHD), a condition in which donor immune cells attack the patient’s organs. Standard prevention relies on broad immunosuppression, which reduces GVHD but compromises immunity, increases infection risk, and can blunt the graft-versus-leukemia (GVL) effect essential for preventing cancer relapse. A study now published in Blood describes a fundamentally different strategy: priming the recipient’s own immune system before transplant to create a more tolerant environment without dampening anti-tumor activity.
The team, led by senior author Robert Levy, PhD, at Sylvester Comprehensive Cancer Center, tested a protocol that expands regulatory T cells (Tregs) in vivo prior to transplantation. Tregs are central to immune tolerance, and their abundance and activation state strongly correlate with GVHD outcomes. In contrast to approaches that require ex vivo expansion of donor Tregs, this strategy amplifies the patient’s remaining Tregs after conditioning chemotherapy and radiation, capitalizing on their relative resistance to cytotoxic damage. In the preclinical model described in the study, six days of treatment with a TL1A-Ig fusion protein and low-dose IL-2 induced robust expansion of endogenous Tregs in tissues normally targeted by GVHD, including the colon, liver, skin, lungs and ocular surfaces.
Mechanistic analysis confirmed that this expansion depends on TNFRSF25 signaling, a receptor expressed at high levels on Tregs. Mice lacking TNFRSF25 did not respond to TL1A-Ig and failed to accumulate tissue-protective Tregs, highlighting the receptor’s necessity for the treatment effect. Expanded Tregs in TNFRSF25-competent animals showed increased Ki-67 expression and activation signatures, consistent with a transcriptional program supporting enhanced suppressive function. These Tregs persisted in large numbers even after radiation and were already embedded within vulnerable organs by the time donor immune cells were introduced.
This pre-transplant immune conditioning produced marked improvements in survival and clinical GVHD severity. Treated mice had reduced tissue injury, less weight loss, improved epithelial integrity, and substantially lower GVHD scores compared with controls. The colon, a major site of GVHD pathology, displayed preserved crypt architecture and reduced inflammatory infiltration. The liver and lungs showed similar protection, suggesting that the tissue-resident or tissue-trafficking Tregs induced by the protocol can modulate alloreactive immune responses across diverse organ sites.
The study also highlights an increasingly recognized predictor of transplant outcomes: the gut microbiome. Mice given the Treg-expansion therapy maintained microbial diversity and avoided outgrowth of Enterococcus and other pathobionts associated with severe GVHD. Instead, they preserved beneficial commensals and butyrate-producing species, which have been linked to lower inflammatory tone and improved epithelial repair. These findings suggest that pre-transplant immune conditioning shapes not only immune responses but also the microbial ecosystem, which in turn reinforces tissue protection after transplant.
One of the most clinically important observations is that the protocol did not impair the GVL response. In a leukemia challenge model, treated animals cleared malignant cells as effectively as controls, with no evidence of tumor escape. Leukemia cells were eliminated from the bone marrow, spleen, liver and reproductive organs, demonstrating that pre-transplant Treg expansion protects tissues from donor-mediated injury without shutting down donor-derived anti-leukemia immunity. This distinction has been difficult to achieve with pharmacologic immunosuppression, which broadly inhibits immune responses rather than targeting pathways involved specifically in GVHD.
Levy emphasized that the strategy does not aim to suppress the immune system indiscriminately. “Our approach is about helping the patient’s own immune system create a safer environment for the stem cell transplant,” he said. “We’re not just suppressing the immune response—we’re guiding it in key tissues involved following the transplant to promote success.” The approach also avoids the need to manipulate donor cells ex vivo, a labor-intensive process that limits scalability. Instead, the protocol uses targeted biologics to recondition the recipient immune landscape directly, a model that could integrate more easily into clinical workflows.
The study provides a foundation for clinical translation, particularly given the accessibility of the targets involved: TNFRSF25 and CD25 are cell-surface receptors amenable to biologic therapies, and low-dose IL-2 has been used safely in multiple clinical contexts. The authors note that expanding endogenous Tregs before transplant could represent a shift toward more precise immune modulation, supporting tolerance where needed while preserving beneficial immune functions such as GVL and antimicrobial defense.
Further work will be required to determine optimal timing, dosing, and compatibility with various conditioning regimens, but the concept is compelling: rather than attempting to extinguish GVHD after it starts, reshape the immunologic environment beforehand so that donor cells encounter tissues already defended by regulatory mechanisms. As Levy put it, “We’re working toward therapies that are both effective and practical for real-world use.”
If validated in clinical trials, this pre-transplant Treg-boosting approach may offer patients a safer path through transplantation—protecting organs, stabilizing the microbiome, and maintaining the anti-cancer immunity that makes the procedure curative.
