CD8 regulatory T cell therapy in transplantation: a new path to clinical success?

Abstract

Since the first success using modified T cells redirected against B cells to treat acute lymphoblasJc leukemia and the unexpected, yet durable, remission of cancer observed in Emily Whitehead, cell therapy has emerged as a novel drug masterclass (1). In parJcular, the unique capabiliJes of T cells to acJvely migrate into Jssues and the sites of inflammaJon where convenJonal therapies fail have opened novel strategic avenues for treaJng refractory and severe diseases (2). In fact, the number of clinical trials involving cell-based therapies in medicine is in an exponenJal growth phase, commensurate with the expectaJons and anJcipated therapeuJc benefits associated with their outcomes (3).Conceptually, T cells can be harnessed either to selecJvely deplete specific cellular subpopulaJons, parJcularly B cells, or to suppress the licensing of effector T cells. In this context, naturally occurring CD4⁺ regulatory T cells (CD4⁺ Tregs) have a_racted considerable a_enJon. To date, more than 30 clinical trials have been registered in the field of transplantaJon (4). Thus, since their discovery, awarded with the 2025 Nobel Prize in Physiology or Medicine, CD4⁺ Tregs have generated substanJal enthusiasm within the scienJfic community as they are built with dozens of suppressive mechanisms, enabling Tregs to modulate the immune response in a highly controlled and mulJfaceted manner (5,6). In transplantaJon, the ONE study, an invesJgator-led single uncontrolled arm trial performed across eight internaJonal centers, could show early safety and promising results with CD4⁺ Tregs on reducing the immunosuppressive treatment in kidney recipients (7). The TWO study, a phase 2b, is currently ongoing to validate those results.More recently, the Eight-Treg study is a first-in-human phase I clinical trial designed to evaluate the safety, feasibility, and early signals of efficacy of autologous CD8⁺ Treg therapy in kidney transplantaJon. CD8⁺ Tregs are defined by a CD8⁺CD45RC low/-phenotype, a subset previously characterized by potent suppressive acJvity (8)(9)(10)(11). In this protocol, CD8⁺ Tregs are isolated from the peripheral blood of transplant candidates prior to transplantaJon through cell sorJng, and subsequently expanded ex vivo under GMP condiJons using anJ-CD3/anJ-CD28 sJmulaJon in the presence of low-dose IL-2, IL-15, and rapamycin, condiJons known to promote regulatory stability (12). The expansion process spans 21 days, enabling the generaJon of clinically relevant cell numbers while preserving both the phenotypic idenJty and suppressive funcJon of the cells. At the end of the culture, the cell product exhibits a stable regulatory profile, characterized by homogeneous FOXP3 expression, high GITR levels, and low CD127 expression, consistent with a bona fide Treg signature. The expanded cells are reinfused into the recipient the day before transplantaJon, in lieu of convenJonal inducJon therapy, and in combinaJon with standard immunosuppressive treatment that may be adjusted according to clinical evoluJon. The study follows a dose-escalaJon design primarily aimed at assessing safety, in vivo persistence of the infused cells, and immunological effects, while also exploring preliminary efficacy endpoints, including gram funcJon, incidence of rejecJon, and markers of immune regulaJon. Protocol biopsies performed at months 1 and 3 post-transplantaJon enable the evaluaJon of gram inflammaJon, immune cell infiltraJon, and the presence of regulatory signatures within the Jssue. In parallel, longitudinal immunomonitoring is conducted to assess the persistence, phenotype, and funcJonal impact of transferred CD8⁺ Tregs on the recipient immune system.The raJonale for developing CD8⁺ Treg-based therapies is supported by a growing body of experimental and translaJonal evidence demonstraJng their potent immunoregulatory capacity in transplantaJon and autoimmunity (13). In both rodent and humanized models, CD8⁺ Tregs have been shown to contribute to the maintenance of immune tolerance and to prevent allogram rejecJon or autoimmune pathology (8,10,14,15). MechanisJcally, these cells suppress pathogenic immune responses through mulJple, non-redundant pathways, including IL-2 consumpJon, modulaJon of anJgen-presenJng cell funcJon, and secreJon of regulatory cytokines (16). Notably, CD8⁺ Tregs recognize donor-derived anJgens presented by MHC class I molecules, which are ubiquitously expressed on nucleated cells, including gram parenchymal cells (17,18). This feature confers upon CD8⁺ Tregs the unique ability to exert regulatory acJvity directly within transplanted Jssues, thereby enabling local control of alloimmune responses at sites of inflammaJon. In addiJon, preclinical studies have demonstrated that CD8⁺ Tregs can be efficiently expanded ex vivo while maintaining stable suppressive properJes (10,12,15), supporJng their development as a clinically relevant cellular therapeuJc product. CollecJvely, these observaJons provide a strong mechanisJc and translaJonal framework for the clinical evaluaJon of CD8⁺ Treg adopJve cell therapy as an innovaJve strategy to promote immune tolerance and improve long-term gram outcomes in organ transplantaJon. Key differences arising from ongoing trials tesJng CD4⁺ and CD8⁺ Tregs may depend on the direct and indirect mechanisms of anJgen presentaJon and alloanJgen recogniJon pathways (19). While some evidence suggests that the allogeneic CD8⁺ T cell repertoire is driven by immunodominant, organ-specific pepJdes rather than conserved regions of non-self major histocompaJbility complex (MHC) molecules, the direct pathway is primarily iniJated by a cellular, T-cell mediated, allorecogniJon (20,21). Accordingly, CD8⁺ Tregs, through their recogniJon of HLA class I molecules, are uniquely posiJoned to interact not only with donor and recipient anJgen-presenJng cells but also directly with the transplanted Jssue itself (Figure 1). In contrast, CD4⁺ Tregs, which primarily target HLA class II molecules, expressed at much lower levels within the gram, are be_er posiJoned to inhibit indirect alloanJgen recogniJon pathways. This pathway depends on the processing and presentaJon of allogeneic pepJdes by recipient dendriJc cells and B cells, ulJmately driving chronic anJbody-mediated rejecJon and progressive gram dysfuncJon (22,23).The iniJaJon of clinical trials evaluaJng CD8⁺ Tregs also echoes the recent enthusiasm surrounding the development of chimeric anJgen receptor (CAR) Tregs targeJng HLA-A2 molecules, which are expressed in approximately 30% of the general populaJon (24,25). By exploiJng HLA-A2 mismatches between donor and recipient, Tregs can be engineered to exert potent, anJgen-specific suppressive acJvity in the presence of HLA-A2, even if the selecJvity of CAR-Tregs for dendriJc cells versus gram Jssue or endothelial cells remains sJll unclear (26) (27). UlJmately, this strategy is mainly aiming at inducing a highly suppressive local microenvironment within the gram consistent with the well-documented role of Tregs in promoJng solid tumor progression and reduced immunotherapeuJc efficacy in cancer (28). It should be noted that peer-reviewed clinical outcomes from HLA-A2 CAR-Treg trials are sJll missing. Early safety data from the STEADFAST study, a Phase I/II clinical trial iniJated by Sangamo TherapeuJcs, and the LIBERATE trial by Quell TherapeuJcs, both evaluaJng HLA-A2 CAR Tregs in kidney and liver transplantaJon respecJvely, have been presented at internaJonal meeJngs and correlated with the migraJon and persistence of Treg in the transplanted organs. Yet, it remains unclear whether these results will be sufficient to maintain these programs open and ulJmately implemented in clinical pracJce considering the cost and the manufacturing complexity of those cells.Nevertheless, the field of Treg therapy is receiving encouraging important posiJve signals from fields outside of SOT. The results from the Phase 3 Precision-T study indicate that donor derived Treg therapies (named Orca-T) can significantly enhances survival free from chronic gram-versus-host disease in paJents with acute myeloid leukemia, acute lymphoblasJc leukemia, and myelodysplasJc syndromes, compared with convenJonal therapies following allogeneic hematopoieJc stem cell transplantaJon. Thus, Orca-T is currently under Priority Review by the U.S. Food and Drug AdministraJon (29). Sonoma BiotherapeuJcs has announced favorable interim results from the ongoing Phase 1 REGULATE-RA study evaluaJng SBT-77-7101 in paJents with refractory rheumatoid arthriJs. The data shows encouraging safety profile alongside preliminary evidence of clinical efficacy in this treatment-resistant populaJon (h_ps://acrabstracts.org/abstract/a-phase-1-study-of-autologous-car-treg-cellsin-refractory-rheumatoid-arthriJs-interim-report-of-safety-and-efficacy/). Thus, the iniJaJon of new clinical trials in the field of SOT is not only encouraging but also criJcally important to maintain scienJfic dynamism and knowledge gain. We are convinced that such efforts will contribute to the development of more targeted immunosuppressive strategies, ulJmately improving long-term paJent survival and quality of life. Even if unmodified CD4⁺ or CD8⁺ Treg therapy remains insufficient to induce durable immune tolerance, the expanding repertoire of cellular engineering technologies is an impressive reservoir for future scienJfic innovaJon. Thus, Treg anJgen-specificity may be easily refined by targeJng both direct and indirect presentaJon of immunodominant allogeneic pepJdes, including through TCR engineering (30), CD4-to-CD8 co-receptor swapping strategies (31), or by the development of anJgen/Jssue-specific CARs with opJmized signaling domains (32,33). The incorporaJon of supraphysiological funcJonal properJes, such as controlled release of Treg pro-survival cytokines (34), is another example that could substanJally augment Treg persistence and therapeuJc efficacy in the coming years.