The iNKT-Treg axis in GVHD control Free

In this issue of Blood, DeFilipp et al¹ report the results of a phase 2b clinical trial with RGI-2001, testing a promising new principle for preventing acute graft-versus-host disease (GVHD). The potential of allogeneic hematopoietic cell transplantation (allo-HCT) to cure hematological malignancies via graft-versus-leukemia (GVL) effect is hampered by GVHD. Acute GVHD (aGVHD) is characterized by proliferation of alloreactive donor T cells leading to severe tissue damage of the target organs liver, gut, and skin.

How to achieve optimal balance between GVL and GVHD for different donor types and graft sources is still a matter of debate. Although the backbone of calcineurin inhibitors and mycophenolate or methotrexate has long been used in the United States, it is mostly complemented by antithymocyte globulin (ATG)/anti–T-lymphocyte globulin formulations in Europe.² In 2023, posttransplantation cyclophosphamide (PTCy) was found to be superior to tacrolimus plus mycophenolate mofetil (ie, without ATG)³ for transplantation of unmanipulated grafts of matched related and unrelated donors. Furthermore, ex vivo graft manipulation with selection/depletion of different cell populations has a long history in allo-HCT. However, it is currently not considered standard of care and only available in selected centers.

DeFilipp et al point to a new way for “in vivo graft manipulation.” RGI-2001 is a liposomal formulation of a synthetic derivative of α-galactosylceramide (α-GalCer). α-GalCer–like glycolipids, which are presented via CD1d on antigen-presenting cells (APCs), can selectively activate the semi-invariant T-cell receptor of invariant (iNKT) natural killer T (NKT) cells.⁴ The iNKT cells stimulate the proliferation of regulatory T cells (Tregs) via interleukin-4 (see figure). Both iNKT and Tregs have immunosuppressive potential, prevent GVHD in preclinical models, and were found to be protective in correlative clinical studies (iNKT cells) or clinical trials (Tregs).^5-9 In this clinical study, adult patients with hematological malignancies received myeloablative conditioning and transplants from 7/8 to 8/8 matched related or unrelated donors (∼80% peripheral blood stem cell grafts). RGI-2001 was administered in 6 weekly infusions starting on the day of allo-HCT together with the former GVHD standard prophylaxis of tacrolimus plus methotrexate. The 49 patients receiving RGI-2001 were compared with a nonrandomized control group from the Center for International Blood and Marrow Transplant Research registry. RGI-2001–treated patients had superior clinical outcomes, including a higher day-180 grade 2 to 4 aGVHD-free survival (70.8% vs 50.7%; adjusted hazard ratio 0.45, 95% confidence interval, 0.30-0.68) with comparable relapse rates. However, there was no difference in the incidence of chronic GVHD (cGVHD), although the onset of cGVHD was delayed in the study cohort, suggesting that extended dosing might also inhibit cGVHD development. Repeated dosing of RGI-2001 was well tolerated without significant infusion reactions. The most common grade ≥3 treatment-related adverse effects were those associated with the allo-HCT procedure (decreased appetite, leukopenia, thrombocytopenia, stomatitis, infections).

Exploiting the immunoregulatory potential of Tregs and iNKT cells by transplantation of defined cell populations (CD34⁺, conventional T cells) in conjunction with Tregs alone (as in trials NCT03977103 and NCT04013685) or in combination with iNKT cells (trial NCT03802695) has gained traction for its ability to prevent GVHD. Although these approaches have demonstrated promising results,^8,10 the procedures involved, including cell isolation, expansion, and precise timing of administration, are complex and demanding, posing significant logistical challenges for routine clinical application. As an alternative, pharmacological strategies to augment the activity of Tregs and iNKT cells, such as RGI-2001, could offer a more feasible and broadly accessible approach, potentially simplifying GVHD prevention and improving patient outcomes. Because RGI-2001 works through the activation of iNKT cells, it should not be immunosuppressive in contrast to other drugs used to prevent or treat aGVHD. However, more studies are needed to better understand its potential side effects.

Supporting the proposed mechanism of RGI-2001, the authors also observed higher Treg numbers at day 42 in patients who did not develop grade 2 to 4 GVHD (median onset of aGVHD was day 46 [range, 21-92]). The use of steroids in patients with GVHD is a confounding factor that may have influenced lymphocyte counts. In addition, there is considerable variability in Treg counts, particularly among non-GVHD patients. Although NKT cells were quantified, this study did not assess circulating iNKT cell levels.

In summary, DeFilipp et al present RGI-2001 as a promising adjunct to GVHD prophylaxis. Because the study did not evaluate combinations with standard agents such as ATG or PTCy, which may affect the efficacy of RGI-2001 by altering APC function, the optimal combination partner still needs to be determined. Future randomized trials will be essential to fully evaluate and confirm its role as a component of GVHD prevention in patients undergoing allo-HCT.

Conflict-of-interest disclosure: The authors declare no competing financial interests.