Developing tolerance for organ transplantation has relied on several approaches that have taken longer than expected to translate into clinical use. The earliest animal studies by Medawar identified the capacity for the body to develop tolerance to skin and organ transplantation.1 Crucial advances in immunology have allowed the identification of specific subsets of cells with regulatory capacities that can be expanded.2,3 Successful clinical tolerance has been achieved via bone marrow transplantation (BMT) leading to mixed chimerism—a procedure pioneered by Sachs and Sykes. Full donor chimerism approaches using BMT and facilitator cells have been explored at Northwestern in Chicago; an approach of BMT with lymphoid irradiation in HLA identical patients has been tested at Stanford.4-7 An alternative strategy is to use regulatory cells expanded ex vivo or to expand presumably antigen-specific regulatory cell populations by in-vivo strategies. These are appealing because they lack the risks associated with bone marrow manipulation. However, data demonstrating their effectiveness in clinical transplant, or autoimmune, settings have been limited or ineffective.
The One Study and its large team of accomplished investigators was the first serious attempt at a systematic evaluation of regulatory cell therapy in a kidney transplant setting.8,9 Given that this was a nonrandomized phase 1/2 study of 6 different cell therapies, it was never designed to show superiority (or even noninferiority) over standard of care therapy. The real strength of the One Study was its study design, standardization of cell products, and the development of a standardized immune profile assay that was used to evaluate patients in all study arms.8,9 The use of cell therapy to induce tolerance without the need for full chimerism is desirable as it avoids the disease burden associated with bone marrow suppression or manipulation. However, establishing robust clinical-grade production, enrolling sufficient numbers of patients, evaluating the immune profile of patients with a regulatory subset are all barriers to clinical implementation of cell therapy. Also, there was the need to have a control group that reflects adequately the current best practice at multiple centers while allowing for comparison with treatment groups.
The One Study did all of these by simultaneously undertaking 6 nonrandomized cell therapy trials in conjunction with a reference group trial (RGT) of standard of care therapy. The RGT enrolled patients across all participating centers where patients received standard of care including basiliximab, tapering doses of prednisolone, mycophenolate mofetil, and tacrolimus. This RGT enrolled 66 patients from 8 hospitals in 7 centers across 5 countries; they used biopsy-confirmed acute rejection as the primary endpoint over a 60-week follow-up.
Six cell therapy trials were undertaken simultaneously using the same conditions but removing basiliximab because it would potentially delete CD25 expressing Treg. Cell-based products included 2 polyclonal Treg products (the UK and Berlin), 2 donor antigen expanded Treg products (Boston and San Francisco), 1 of which was prepared with exposure to costimulation blockade, 1 autologous tolerogenic dendritic cell product, and 1 regulatory macrophage product.
As opposed to standard immunosuppression any cell therapy protocol requires immune monitoring. One of the great strengths of the One Study is the quality of the centralized monitoring of immune function and circulating cell content using flow cytometry (8). This approach lays the groundwork for understanding which patients may be more suitable for immune withdrawal. The difficulties in doing this across multiple sites cannot be overestimated and are one of the triumphs of the One Study that provides a benchmark for future studies, and for tolerance-promoting groups such as the Immune Tolerance Network.
Whilst the study was never designed to show the benefit of one therapy over another, the evidence suggests that they could be given safely, and the products appear to have remained true to type and not reverted to effector cells after significant ex vivo manipulation. Overall biopsy proven acute rejection was similar to that seen in the RGT (16% versus 12% respectively) and 40% of patients receiving cell products were weaned off mycophenolate mofetil. The observation that patients receiving cell therapies had fewer episodes of infection was pleasing.
As one would expect with a proof-of-concept study, several caveats need to be stated. Patients were nonsensitized living-related transplant recipients where one would expect low rates of rejection and excellent outcomes. The fact that 13% (38) of screened patients in total were recruited to 6 cell therapy arms points to the logistic difficulties and ongoing unreliability of clinical-grade cell production.
The centralized and standardized immune monitoring of all patients was a major achievement of the study and the restoration of a more normal and less inflammatory immune system rather than loss of circulating Treg is consistent with overall transplant outcomes.8,9 However, the inability to reliably monitor the fate of the infused cells challenges the ongoing development of these products. The capacity to identify and retrieve donor cells is of crucial importance as stability and survival on transfer are important components for efficacy, allowing investigators to identify any worrying shift to a more pathogenic phenotype.
The One Study provides a novel and innovative platform on which future studies can move forward. Based on those principles, the Two Study at Oxford has been brought on its way, testing the potential for future donor-specific Treg trials such as chimeric antigen receptor T cells targeted at the donor or already proposed donor antigen presentation strategies similar to those used in Boston and San Francisco.9 Next steps require a well-designed comparative trial to identify which therapies should be selected for further studies testing their efficacy, and safety, compared with standard of care. Once these parameters are understood, formal evaluations of cellular therapies may lend themselves to a Bayesian or adaptive clinical trial platform where past results and effect size could be used to “adapt” the clinical trial by eliminating less effective therapies as well as modifying patient populations.10
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4. Sykes M. Mixed chimerism and transplant tolerance. Immunity. 2001; 14:417–424
5. Leventhal J, Abecassis M, Miller J, et al. Chimerism and tolerance without GVHD or engraftment syndrome in HLA-mismatched combined kidney and hematopoietic stem cell transplantation. Sci Transl Med. 2012; 4:124ra28
6. Busque S, Scandling JD, Lowsky R, et al. Mixed chimerism and acceptance of kidney transplants after immunosuppressive drug withdrawal. Sci Transl Med. 2020; 12:eaax8863
7. Kawai T, Cosimi AB, Spitzer TR, et al. HLA-mismatched renal transplantation without maintenance immunosuppression. N Engl J Med. 2008; 358:353–361
8. Streitz M, Miloud T, Kapinsky M, et al. Standardization of whole blood immune phenotype monitoring for clinical trials: panels and methods from the ONE study. Transplant Res. 2013; 2:17
9. Sawitzki B, Harden PN, Reinke P, et al. Regulatory cell therapy in kidney transplantation (The ONE Study): a harmonised design and analysis of seven non-randomised, single-arm, phase 1/2A trials. Lancet. 2020; 395:1627–1639
10. Berry DA. Bayesian clinical trials. Nat Rev Drug Discov. 2006; 5:27–36