Background Infusion of recipient regulatory T cells (Treg) promotes durable mixed hematopoietic chimerism and allograft tolerance in mice receiving allogeneic BMT with minimal conditioning. We applied this strategy in a Cynomolgus macaque model. Methods CD4+CD25high Treg that were polyclonally expanded in culture were highly suppressive in vitro and maintained high expression of FoxP3. Eight monkeys underwent nonmyeloablative conditioning and MHC-mismatched BMT with or without Treg infusion. Renal transplantation (from the same BMT donor) was performed 4 months post-BMT without immunosuppression to assess for robust donor-specific tolerance. Results Transient mixed chimerism, without significant T cell chimerism, was achieved in the animals that received BMT without Treg (N=3). In contrast, the 2 of 5 recipients of Treg + BMT that were evaluable displayed chimerism in all lineages, including T cells, for up to 335 days post-BMT. Importantly, in the animal that survived long-term, >90% of donor T cells were CD45RA+CD31+, suggesting they were new thymic emigrants. In this animal, the delayed (to 4 months) donor kidney graft was accepted >294 days without immunosuppression, whereas non-Treg BMT recipients rejected delayed donor kidneys within 3-4 weeks. Early cytomegalovirus reactivation and treatment was associated with early failure of chimerism, regardless of Treg administration. Conclusions Our studies provide proof-of-principle that, in the absence of early CMV reactivation (and BM-toxic antiviral therapy), co-transplantation of host Treg can promote prolonged and high levels of multilineage allogeneic chimerism and robust tolerance to the donor.

Core award; Irving Pilot Translational Science award for new investigators Reliable in vitro expansion protocols of regulatory T cells (Tregs) are needed for clinical use. We studied the biology of Mauritian Cynomolgus macaque (MCM) Tregs and developed four in vitro Treg expansion protocols for translational studies. Tregs expanded 3000-fold when artificial antigen presenting cells (aAPCs) expressing human CD80, CD58 and CD32 were used throughout the culture. When donor peripheral blood mononuclear cells (PBMCs) were used as the single source of APCs followed by aAPCs, Tregs expanded 2000-fold. Tregs from all protocols suppressed the proliferation of anti-CD2CD3CD28 bead-stimulated autologous PBMCs albeit with different potencies, varying from 1:2-1:4 Treg:PBMC ratios, up to >1:32. Reculture of cryopreserved Tregs permitted reexpansion with improved suppressive activity. Occasionally, CD8 contamination was observed and resolved by resorting. Specificity studies showed greater suppression of stimulation by anti-CD2CD3CD28 beads of PBMCs from the same donor used for stimulation during the Treg cultures and of autologous cells than of third-party PBMC responders. Similar to humans, the Treg-specific demethylated region (TSDR) within the Foxp3 locus correlated with suppressive activity and expression of Foxp3. Contrary to humans, FoxP3 expression did not correlate with CD45RA or CD127 expression. In summary, we have characterized MCM Tregs and developed four Treg expansion protocols that can be used for preclinical applications. K E Y W O R D S animal models: nonhuman primate, basic (laboratory) research/science, cellular transplantation (nonislet), graft survival, immune regulation, immunobiology, immunosuppression/immune modulation, tolerance: chimerism, translational research/ science | 2187 ALONSO-GUALLART eT AL.

Background. Although short-term outcomes for liver transplantation have improved, patient and graft survival are limited by infection, cancer, and other complications of immunosuppression. Rapid induction of tolerance after liver transplantation would decrease these complications, improving survival and quality of life. Tolerance to kidneys, but not thoracic organs or islets, has been achieved in nonhuman primates and humans through the induction of transient donor chimerism. Since the liver is considered to be tolerogenic, we tested the hypothesis that the renal transplant transient chimerism protocol would induce liver tolerance. Methods. Seven cynomolgus macaques received immune conditioning followed by simultaneous donor bone marrow and liver transplantation. The more extensive liver surgery required minor adaptations of the kidney protocol to decrease complications. All immunosuppression was discontinued on postoperative day (POD) 28. Peripheral blood chimerism, recipient immune reconstitution, liver function tests, and graft survival were determined. Results. The level and duration of chimerism in liver recipients were comparable to those previously reported in renal transplant recipients. However, unlike in the kidney model, the liver was rejected soon after immunosuppression withdrawal. Rejection was associated with proliferation of recipient CD8 T effector cells in the periphery and liver, increased serum interleukin (IL)-6 and IL-2, but peripheral regulatory T cell (Treg) numbers did not increase. Antidonor antibody was also detected. Conclusions. These data show the transient chimerism protocol does not induce tolerance to livers, likely due to greater CD8 T cell responses than in the kidney model. Successful tolerance induction may depend on greater control or deletion of CD8 T cells in this model.

Anti-CD2 treatment provides targeted immunomodulatory properties that have demonstrated clinical usefulness to condition the immune system and to treat transplant rejection. The treatment is species-specific due to structural CD2 antigen differences between nonhuman primates and humans. Herein, we report the safety profile and efficacy of two modifications of the same anti-CD2 monoclonal antibody in cynomolgus macaques. Twelve subjects received one i.v. anti-CD2 (of rat or rhesus type) dose each, range 1-4 mg/kg, and were followed for 1-7 days. Treatment effects were evaluated with flow cytometry on peripheral blood and histopathological evaluation of secondary lymphoid organs. In vitro inhibitory activity on primary MHC disparate mixed lymphocyte reactions (MLRs) was determined. Upon anti-CD2 treatment, CD4 + , CD8 + memory subsets were substantially depleted. Na€ ıve T cells and Tregs were relatively spared and exhibited lower CD2 expression than memory T cells. Early immune reconstitution was noted for na€ ıve cells, while memory counts had not recovered after one week. Both antibodies displayed a concentration-dependent MLR inhibition. Lymph node examination revealed no significant lymphocyte depletion. None of the animals experienced any significant study drug-related adverse events. This study outlines the safety and pharmacodynamic profile of primate-specific anti-CD2 treatment, relevant for translation of anti-CD2-based animal models into clinical trials. Transplant International 2020; 33: 98-107

We developed an approach for the expansion of Mauritian cynomolgus macaque polyspecific regulatory T cells (Tregs) through the combination of MHC‐mismatched CD40L‐engineered B cells. Expanded Tregs expressed high levels of FoxP3 and Helios, a high percentage of TSDR demethylation, and strong suppression of naïve T cell responses in vitro. This approach has the potential to be translated to clinical studies for deceased‐donor transplants.