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Am J Transplant. 2025 Jul;25(7):1461-1470. doi: 10.1016/j.ajt.2025.01.044. Epub 2025 Feb 6.
Induction of immune tolerance in living related human leukocyte antigen–matched kidney transplantation: A phase 3 randomized clinical trial
Dixon B Kaufman, Sanjeev K Akkina, Mark D Stegall, James B Piper, A Osama Gaber, William S Asch, Stephan Busque, Erik Stites, Michael De Vera, Titte R Srinivas, Diane Alonso, Ashesh Shah, Anup Patel, Martin L Mai, Kenneth D Chavin, Meelie DebRoy, Arksarapuk Jittirat, Nadiesda Costa, Matthew Cooper, Gayle Vranic, Mark R Laftavi, Reza F Saidi, Suzon Collette, Daniel C Brennan; MDR-101-MLK Study Group.
PMID: 39922283
Introduction
In Greek mythology a Chimera has the head of a lion, the body of a goat, and a tail with a snake’s head. In its figurative use a “chimera” refers to anything that is unrealistic, wild or fantastical. However, in transplant medicine, a chimera is the introduction of donor hematopoietic stem cells to induce immune tolerance in a recipient (something quixotic indeed; chimeras may be the G.O.A.T. in transplantation, I ain’t lion (lyin’) to sell you snake oil– I can hear your groans from here). Kidney transplant recipients need lifelong immunosuppression to prevent rejection and the loss of the kidney allograft, with rare exceptions. Use of immunosuppression medication exposes organ recipients to drug toxicities, increased infection and cancer risks, and potentially significant expense. Transplantation, then, is life saving but not without significant long-term risk. Enter the concept of “functional tolerance”—a state in which the recipient’s immune system accepts the graft without additional immunosuppression. Historically, this was observed mainly in identical twin transplants, but with nearly a third of those still requiring chronic therapy (Krishnan et al. Am J Transplant 2008), it’s clear that tolerance depends on more than genetic matching.
One of the most promising strategies to create immune tolerance is by introducing donor bone marrow or hematopoietic stem cells into transplant recipients to establish chimerism. Full donor chimerism describes a state where stem cell transplantation from a donor leads to a state where the entire recipient hematopoietic system is replaced by donor cells. This has been used in kidney transplantation earlier without a traditional myeloablative protocol (Leventhal et al, Sci Transl Med 2012). Seventy percent of patients maintained durable donor chimerism across HLA barriers and remained off immunosuppression after the first year of transplant. However, this was associated with 2 graft losses in the first year and 2 cases of graft versus host disease, one of which was a fatal adverse event.
Lesson learnt, enter mixed chimerism. Mixed chimerism, which is what the current study utilised, is a state where host hematopoietic cells are not completely destroyed but co-exist with donor cells to generate a state where tolerance of the donor organ is achieved with a lower risk of GVHD versus a full chimerism.
Figure showing full donor chimerism versus mixed chimerism- adapted from Podesta et al. Frontiers in Immunology 2022.Destruction of hTEC and thymic structure by GvHR may cause failure of negative selection and precipitate graft-versus-host disease (GVHD) (Panel B). In durable mixed chimerism, donor-derived precursors populate the host thymus and differentiate into DC (dDC) without depletion of their host-derived counterparts (hDC). Donor- and host-reactive T cells from both the donor and the host undergo negative selection, allowing allograft tolerance without GVHD (Panel C)
This is not a new concept, the Sam Strober protocol was attempted at Stanford in 2008 (Scandling et al. NEJM 2008). This single-center study (Busque et al. Sci Transl Med 2020) showed that a majority of patients who maintained mixed chimerism for ≥6 months achieved complete immunosuppression drug withdrawal without rejection for at least 2 years.
Figure 1. Sam Strober Protocol. Created by Natalia Nombera with Bio Render.
The key question is whether this strategy can be validated and safely expanded. The MDR-101 protocol trial addresses this in a multicenter, randomized study, to induce mixed chimerism and functional tolerance in 2-haplotype HLA-matched living donor–recipient kidney pairs.
Methods
Study Design
This was a phase 3, prospective, open-label, 2:1 randomized, multicenter clinical trial conducted in the US and Canada. In addition to contemporary controls, historical control patients that mimicked eligibility criteria from study recipients were chosen from the Scientific Registry of Transplant Recipients database (2014-2015). Contemporary control patients received immunosuppression per institutional standard of care.
The study specifically enrolled recipients of a first, single solid organ transplant (kidney only) from a 2-haplotype HLA-matched, living related donor. This was accomplished by using exclusively sibling donor-recipient pairs. The exclusion criteria included a history of underlying kidney disease with a high risk of recurrence in the transplanted kidney, although patients with IgA nephropathy were included.
Inclusion and exclusion criteria
Table S1A: Study Recipients Key Eligibility Criteria from Kaufman et al, Am J Transplant, 2025.
Table S1B: Scientific Registry of Transplant Recipients (SRTR) Historical Group Criteria from Kaufman et al, Am J Transplant, 2025.
Outcome
The primary objective was functional immune tolerance - defined as remaining off all immunosuppression medications for at least 24 months (36 months after transplant) with no episodes of acute rejection, development of de novo donor-specific antibody to donor HLA antigens, graft versus host disease (GVHD), transplant kidney loss, or death.
Sample size
Sample size was estimated as per primary endpoint evaluated based on a historical cohort group from SRTR. The total number of subjects was 30, 20 in the treatment arm and 10 in the control arm.
Interventions
The Conditioning Regimen: The treatment patients underwent a nonmyeloablative protocol. Patients received their kidney transplant on Day 0. Then, they underwent a lymphocyte-depleting regimen that included a total dose of 7.5 mg/kg of rabbit-antithymocyte globulin (rATG) from Day 0 to Day 4 and a total of 1200 cGy of low-dose total lymphocyte irradiation(TLI) over 10 fractions from Day 1 to Day 11. The use of low-dose TLI is a critical distinction from more toxic myeloablative regimens used in other studies that have been associated with a high incidence of GVHD.
The Cellular Product (MDR-101): On Day 11, after the final dose of TLI, patients received a single intravenous infusion of MDR-101. This product was derived from the same donor who gave the kidney and was enriched for CD34+ hematopoietic stem cells and a fixed dose of CD3+ T cells. This combination is crucial for both establishing chimerism (via CD34+ cells) and modulating the recipient’s immune response (via the T cells).
Immunosuppression Protocol and taper: The IS withdrawal protocol was gradual and tied to the stability of the mixed chimerism. Patients in the intervention arm received methylprednisolone in gradually decreasing doses from day 0 to day 10. Tacrolimus was started on the day of transplantation with a target trough level of 8-11 ng/ml, followed by 4-6 ng/ml later.Tacrolimus was continued until Day 180 and was then tapered to complete withdrawal at one year post-transplant, provided the recipient had stable mixed chimerism of ≥5% donor white blood cells. Mycophenolate mofetil (1-2 g/day) was started on day 11 and discontinued on Day 39.
Figure 2. MDR - 101 trial intervention arm. Created by Akshaya Jayachandran with Bio Render.
Figure 1. MDR 101 trial design from Kaufman et al, Am J Transplant, 2025.
Results
A total of 30 patients were included in the analysis, 20 treatment and 10 contemporary controls.
Figure S1: MDR-101-MLK Study Flow Diagram from Kaufman et al, Am J Transplant, 2025.
The overwhelming majority of patients were between 30 and 60 years of age. The majority of recipients were male in both groups. In the treatment arm, 85% were white versus 90%of the control arm. There were no Black or African American patients in either group. The mean BMI was 27kg/m² in both groups. The majority of patients in both groups were not on dialysis pre-transplant. Details of the native kidney disease are not provided.
Table 1 - Demographic and baseline characteristics in kidney transplant recipients from Kaufman et al, Am J Transplant, 2025.
Primary Endpoint: 15 of 20 (75%) of the treated patients achieved functional immune tolerance, successfully staying off all Immunosuppression for two years.
Table 2 - Efficacy outcomes - Primary Safety population from Kaufman et al, Am J Transplant, 2025.
Figure 2- MDR 101 treated patients and time off immunosuppression from Kaufman et al, Am J Transplant, 2025. ( Orange bars indicate treatment failures)
Mixed chimerism was achieved in 19 of the 20 patients who received MDR-101. Chimerism at the one-year mark, when IS was withdrawn, was still present in 89% of treatment patients. By the end of three years, chimerism had been lost in 10 patients, but 6 of those 10 still remained IS-free. While mixed chimerism is a prerequisite for successful IS withdrawal and establishing tolerance, it may not be necessary for maintaining tolerance in the long-term.
Adverse Events & Safety: There were no cases of GVHD, a major concern with other tolerance protocols. Patient and graft survival were 100% in both arms over the three-year period. There were no cases of post-transplant lymphoproliferative disorder (PTLD) or other malignancies. Post transplant diabetes mellitus developed in one patient each in treatment and control arms. The overall rates of adverse events were similar between the two groups.
One patient in the treatment arm developed a SAE of polyarthralgia, possibly related either to MDR-101 or to Thymoglobulin. More patients in the treatment arm developed COVID-19 vs in the control arm, although the authors state this may have been due to a closer follow up of the treatment group vs the control during the COVID 19 pandemic.
Table S3C: Most Common Adverse Events ≥25% of Recipients Safety Population from Kaufman et al, Am J Transplant, 2025.
Kidney Function: There was no significant difference in eGFR between the treatment and control groups at any time point, including at 3 years post-transplant. This is a crucial finding, as it indicates that being off IS did not lead to a decline in kidney function. Furthermore, there was no statistical difference in eGFR between the IS-free patients and those who had to resume IS, which is an important safety signal.
Table 3: Renal and metabolic function from Kaufman et al, Am J Transplant, 2025.
The Burden of Immunosuppression: The most striking secondary outcome was the quality of life data, measured by the KDQOL-SF36. While there was no significant difference at one year (when the treatment group was on tacrolimus monotherapy), there was a clinically and statistically significant improvement in both the Burden of Kidney Disease and Mental Health Composite Scores at two and three years when the treatment patients were completely off all IS. This is a powerful demonstration that even low-dose IS has a substantial negative impact on patients.
Figure 3. Kidney Disease Quality of Life Short Form (KDQOL-SF36) composite scores from Kaufman et al, Am J Transplant, 2025.
IS Restarts: Four of the 20 patients who had IS withdrawn eventually had to resume it. Two of these were due to a recurrence of IgA nephropathy (IgAN). All four of these patients had lost chimerism before resuming IS. A fifth patient, who was a protocol deviation (0-antigen HLA mismatch instead of 2-haplotype), lost chimerism early and was not allowed to proceed with IS withdrawal.
Discussion
Strengths:
Rigorous, Multicenter Design: This is arguably the most significant strength of the paper. By successfully replicating the pioneering single-center work from Stanford across 17 different institutions, the study provides a powerful demonstration that this protocol is not dependent on a specific institutional "magic" but can be safely and effectively managed in a multi-center setting. This is essential for eventual clinical adoption and FDA approval.
Demonstrated Efficacy: The primary endpoint success rate of 75% is clinically meaningful and far surpasses the FDA’s predetermined threshold of 48% (although it is not entirely clear how this “treatment success” was determined). The fact that there were no deaths, graft losses, or cases of GVHD adds significant weight to the protocol's safety profile.
Patient-Centered Outcomes: The inclusion of quality of life data is a huge win. The statistically significant improvements in both the Burden of Kidney Disease and Mental Health Composite Scores at years 2 and 3 highlight that the benefits of Immunosuppression free transplantation extend well beyond graft survival and function.
A Logistical Advantage: The protocol's post-transplant application is a unique and important strength. It allows for flexibility, as the cellular product (MDR-101) can be infused days after the kidney transplant, and it could even be applied to patients who have already received their HLA-matched kidney, as has been reported in other studies.
Limitations:
Narrow Patient Population: The most prominent weakness is the highly selective patient population. The study was restricted to recipients of a first-time transplant from a 2-haplotype HLA-matched living sibling, an immunologically privileged donor pool that represents a very small fraction of all kidney transplants. The study's design explicitly excluded patients with underlying kidney diseases prone to recurrence, with the exception of IgA nephropathy.The one protocol deviation—a HLA-mismatched patient who lost chimerism and failed IS withdrawal serves as a reminder that this protocol is not a one-size-fits-all solution.
Lack of Diversity: The study's generalizability is severely limited by the demographic profile of the enrolled patients. The authors explicitly state that there were no Black patients or children in this study, which is a significant limitation and a key area for future research.
Small Sample Size and Open-Label Design: The small sample size (N=20 in the treatment arm) means that while the results were statistically significant, they may be less robust when scaled up. The open-label design, while necessary for this type of intervention, introduces the potential for bias, particularly in subjective outcomes like quality of life. Patients knew they were on the "tolerance protocol," which could have influenced their reported well-being.
The Chimerism-Tolerance Conundrum: The finding that many patients lost chimerism but remained IS-free is fascinating, but also raises fundamental questions. While it suggests that chimerism is a means to an end, the mechanism of long-term tolerance maintenance without a stable chimeric state is not yet understood. This leaves us without a clear answer as to what happens immunologically to these patients in the long term, and whether this tolerance could be broken at any time by any immunological challenge.
Recurrence of Underlying Disease: The protocol failed to prevent disease recurrence in two of the six patients with IgA nephropathy, both of whom had to resume IS. This highlights a key clinical problem: while this protocol may induce tolerance to the donor's foreign antigens, it may not address the underlying immune mechanisms that caused the initial kidney failure in cases of glomerulonephritis. It also could possibly limit the applicability of this protocol in patients in whom a pre-transplant definitive etiological diagnosis of kidney disease is not available. Surprisingly, the details of the pre-transplant etiological cause of kidney disease in either the treatment or intervention group is not available in the manuscript.
The concept of a truly "tolerant" patient, albeit someone who accepts their new kidney as their own without lifelong immunosuppression has been the holy grail of transplant nephrology for decades. This landmark Phase 3 trial shows that this isn't just the stuff of (Greek) legends anymore. It's a reproducible strategy that has real world implications for a subgroup of transplant patients.
Unfortunately, it’s not a universal solution to decrease IS exposure for all transplant recipients. The protocol is highly specific, expensive, and currently limited to a very select patient group. The next great challenge will be to adapt and expand this protocol to a more diverse and broader patient population, including those with different HLA matches, higher-risk diseases, and varied demographics. The MDR-101 trial has laid the groundwork, but the real work in making tolerance a reality for all transplant patients is just beginning.
Conclusion
The study demonstrates that the MDR-101 regimen can safely and reliably induce donor mixed chimerism and functional immune tolerance in HLA-matched living donor kidney transplant recipients. The complete elimination of IS was achieved without deaths, graft loss, de novo donor specific antibodies, or GVHD, and was associated with an improved quality of life.
Summary prepared by
Akshaya Jayachandran
Assistant Professor
Department of Nephrology
Christian Medical College & Hospital
Vellore, Tamilnadu, India
and
Natalia Nombera
Research scholar
Department of Nephrology
University of Alabama at Birmingham
NephJC Interns, Batch of 2025
Reviewed by
Pallavi Prasad, Brian Rifkin, Swapnil Hiremath
Header designed by AI and prompts from Akshaya Jayachandran