An Homage to the Treg Graft in Allogeneic Transplantation Free

This past April, results from the highly anticipated, multicenter, phase III pivotal trial of Orca-T versus standard of care (SOC) allogeneic hematopoietic cell transplantation (HCT) were presented at the European Society of Blood and Marrow Transplantation meeting in Florence, Italy.¹  A total of 187 adults with acute leukemia in complete remission or myelodysplastic syndrome with human leukocyte antigen (HLA)-matched donors were enrolled and randomized to receive myeloablative conditioning followed by either Orca-T plus single-agent tacrolimus or a standard peripheral blood stem cell graft plus tacrolimus and methotrexate. Orca-T (Orca Bio) — an allogeneic cellular immunotherapy composed of CD34+ hematopoietic stem and progenitor cells, purified regulatory T cells (Tregs), and an approximately equal cellular dose of conventional T cells (Tcons) — outperformed SOC by a wide margin. The primary endpoint, chronic graft-versus-host disease (cGVHD)-free survival (cGFS), significantly favored Orca-T (hazard ratio [HR] = 0.26, 95% CI 0.14-0.47). The estimated one-year cGFS rates were 78% (95% CI 65%-87%) for Orca-T and 38% (95% CI 26%-51%) for SOC. The Orca-T graft also yielded impressive secondary outcomes, with one-year relapse-free survival and overall survival rates of 76% and 94%, respectively, and an astonishingly low incidence of non-relapse mortality of just 4%. These results pave the way for potential regulatory approval of Orca-T as the first Treg-based allogeneic product in the U.S.

The history of the Treg graft employed in this study represents a bench to bedside journey that began decades before the definitive trial was ever conceived. The early saga of Tregs, reviewed in depth elsewhere,^2-4  started with observations that certain T cells could have suppressor functions and thereby inhibit autoimmunity. First coined “regulatory T cells” in 1995 following their identification as a small but distinct subset of CD4+ T cells expressing CD25 (the interleukin-2 receptor alpha chain), Tregs were subsequently shown to be regulated by the FoxP3 transcription factor and to play a critical role in maintaining self-tolerance. For scientists studying allogeneic HCT, the discovery of CD4+CD25+FoxP3+ Treg cells offered a unique opportunity to deepen the understanding of transplant biology and to exploit their suppressive capacity against GVHD.

In 2015, shortly after joining the Stanford faculty, I consulted on the case of a young man with refractory acute myeloid leukemia (AML) harboring a complex karyotype. Desperate to get to transplant, but unable to achieve remission, he enrolled in a single-center, phase I, investigator-initiated trial that commenced in 2011 at Stanford. In this study, HLA-matched donor G-CSF-mobilized grafts were first selected for CD34+ cells and then selected and sorted to isolate Treg (CD4+CD127lowCD25+). The trial marked the first human test of a concept that had shown remarkable success — in mice — where infusion of CD34+ cells and purified donor Tregs, followed by an equal number of Tcons, mitigated GVHD while preserving graft-versus leukemia (GVL). Specifically, in a series of seminal papers published a decade prior, investigators from the laboratories of Robert Negrin, MD, and Samuel Strober, MD, demonstrated that CD4+CD25+ Tregs inhibited alloreactive Tcon proliferation in vitro and expansion in GVHD target tissues in vivo — without diminishing their anti-tumor cytotoxicity.^5,6  Additional work from Dr. Negrin’s team added important insights: 1) infusion of Treg two to three days before infusion of Tcon optimally reduced GVHD in target organs; 2) Treg and Tcon should be administered at approximately equal cellular ratios; 3) when infused with this schedule and dosing schema, the GVL activity of Tcon is maintained; and 4) Tregs support post-HCT immune reconstitution and resultant infection prevention by preserving thymic and lymph node niches.^7,8 

The patient received his Treg graft on trial, avoided GVHD, and remained in remission for many months before ultimately relapsing and succumbing to AML. Although his outcome was not as we had hoped, that first clinical trial of the Treg graft at Stanford proved instrumental. It established the safety and maximum feasible dose of Tregs from an adult donor via large-volume apheresis to be 3x106/kg.⁹  Early on, it became apparent that cryopreservation and subsequent thawing of Tregs prior to infusion may impair their functionality, as evidenced by an uptick in GVHD and poor immune reconstitution in a subset of patients who received previously frozen cells.⁹  The initial phase of the trial also tested whether pharmacologic prophylaxis was necessary to prevent severe acute GVHD (aGVHD) with this graft; indeed, single-agent prophylaxis (tacrolimus or sirolimus) appeared superior to no pharmacologic prophylaxis.¹⁰  The phase II expansion cohort confirmed the promise of this approach, reporting low one-year incidence rates of grade 3 to 4 aGVHD and moderate-to-severe cGVHD of 7% and 11%, respectively, with a notable one-year GVHD-free, relapse-free survival rate of 64%.¹¹ 

In 2016, a group of entrepreneurial scientists with an interest in immune tolerance and Treg biology took note of this work and launched Orca Bio with the goal of bringing precision allogeneic cellular therapies to a broader population of patients. As of 2025, Orca-T, the successor to the Stanford Treg graft, has been administered to hundreds of patients across the U.S. with high-risk hematologic malignancies. Most have been cured of their disease without significant GVHD, affirming the results of the initial clinical trials. We are hopeful for U.S. Food and Drug Administration approval in the coming year. This remarkable achievement — rooted decades ago in the recognition of the regulatory T cell, harnessed by scientists in the field of allogeneic HCT, and translated by clinical investigators and industry partners through early- and late-phase clinical trials — is a celebration of bench to bedside research at its finest. This journey highlights how sustained intellectual and financial investment in scientific research can transform discovery into cures — and ultimately benefit those who matter most: our patients.