Kidney-induced systemic tolerance of heart allografts in mice

Tolerogenic Therapies in Cardiac Transplantation

Heart transplantation remains the gold-standard treatment for end-stage heart failure, yet long-term graft survival is hindered by chronic rejection and the morbidity and mortality caused by lifelong immunosuppression. While advances in medical and device-based therapies have reduced the overall need for transplantation, patients who ultimately require a transplant often present with more advanced disease and comorbidities. Recent advances in tolerance-inducing strategies offer promising avenues to improve allograft acceptance, while minimizing immunosuppressive toxicity. This review explores novel approaches aiming to achieve long-term immunological tolerance, including co-stimulation blockade, mixed chimerism, regulatory T-cell (Treg) therapies, thymic transplantation, and double-organ transplantation. These strategies seek to promote donor-specific unresponsiveness and mitigate chronic rejection. Additionally, expanding the donor pool remains a critical challenge in addressing organ shortages. Innovations such as ABO-incompatible heart transplantation are revolutionizing the field by increasing donor availability and accessibility. In this article, we discuss the mechanistic basis, clinical advancements, and challenges of these approaches, highlighting their potential to transform the future of heart transplantation with emphasis on clinical translation.

Can mouse kidney transplant models inform mechanisms of injury and acceptance in clinical kidney transplantation?

Despite standard-of-care immunosuppression, acute rejection remains commonly observed in kidney transplants and leads to chronic graft injury and failure in many transplanted patients. Mechanisms underlying acute and chronic kidney graft injury are incompletely understood, undermining the development and implementation of therapeutic strategies to improve outcomes. This compels the use of preclinical kidney transplant models to identify components and mechanisms mediating acute and chronic graft injury. Mouse models have been instrumental in establishing basic principles of alloimmune responses and the rejection of heart allografts. There is increasing use of mouse models to extend these studies to kidney transplantation, but the relevance of the findings to clinical kidney transplants remains under scrutiny. Here, we discuss the strengths and weaknesses of mouse models of kidney allograft responses and injury. Although obvious weaknesses arise when considering the relevance to clinical kidney transplants, there are new models that recapitulate many features of kidney graft injury in the clinical scenario and have much to contribute to understanding innate and donor alloantigen-specific mechanisms underlying kidney allograft injury. As in most preclinical studies, the pertinent use of kidney allogeneic transplants in mice comes down to the judicious choice of test questions and the choice of appropriate donors and recipients for the chosen model.

iPSCs engrafted in allogeneic hosts without immunosuppression induce donor-specific tolerance to secondary allografts

Currently, most cell or tissue transplantations using induced pluripotent stem cells (iPSCs) are anticipated to involve allogeneic iPSCs. However, the immunological properties of iPSCs in an allogeneic setting are not well understood. We previously established a mouse transplantation model of MHC-compatible/minor antigen-mismatched combinations, assuming a hypoimmunogenic iPSC-setting. Here, we found that iPSCs subcutaneously inoculated into MHC-compatible allogeneic host mice resisted rejection and formed teratomas without immunosuppressant administration. Notably, when skin grafts were transplanted onto hosts more than 40 d after the initial iPSCs inoculation, only the skin of the same strain as the initial iPSCs was engrafted. Therefore, donor-specific immune tolerance was induced by a single iPSC inoculation. Diverse analyses, including single-cell RNA-sequencing after transplantation, revealed an increase in regulatory T cell (Treg) population, particularly CD25+ CD103+ effector Tregs within the teratoma and skin grafts. The removal of CD25+ or Foxp3+ cells suppressed the increase in effector Tregs and disrupted graft acceptance, indicating the importance of these cells in the establishment of immune tolerance. Within the teratoma, we observed an increase in TGF-β2 levels, suggesting an association with the increase in effector Tregs. Our results provide important insights for future applications of allogeneic iPSC-based cell or tissue transplantation.

Cardiac allograft tolerance can be achieved in nonhuman primates by donor bone marrow and kidney cotransplantation

Long-term, immunosuppression-free allograft survival has been induced in human and nonhuman primate (NHP) kidney recipients after nonmyeloablative conditioning and donor bone marrow transplantation (DBMT), resulting in transient mixed hematopoietic chimerism. However, the same strategy has consistently failed in NHP heart transplant recipients. Here, we investigated whether long-term heart allograft survival could be achieved by cotransplanting kidneys from the same donor. Cynomolgus monkeys were transplanted with heart allografts alone or heart and kidney allografts from the same major histocompatibility complex (MHC)-mismatched donors. All animals except one received DBMT, either at the same time or after a 2- to 4-month delay, plus short-term costimulation blockade and calcineurin inhibitor treatment. Long-term, immunosuppression-free heart allograft survival was consistently achieved in heart/kidney, but not heart-alone, recipients. This was not associated with greater donor/recipient histocompatibility or altered lymphoid cell reconstitution after conditioning. The maintenance of tolerance after heart/kidney transplantation was associated with the presence of forkhead box P3 (Foxp3+) regulatory T cell (Treg)-rich organized lymphoid structures in kidneys but not hearts. Substituting high-dose erythropoietin treatment for kidney transplantation was unsuccessful, suggesting that it was not the sole mechanism of action. RNA sequencing analysis revealed that gene expression in hearts from tolerant recipients closely resembled that in hearts from chronically immunosuppressed recipients but differed markedly from rejecting allografts and naïve hearts. A version of this protocol may be able to induce tolerance in patients with end-stage heart and kidney failure who require combined heart and kidney transplantation.

Enhanced Costimulation Blockade With αCD154, αCD2, and αCD28 to Promote Heart Allograft Tolerance in Nonhuman Primates

BACKGROUND

Long-term renal allograft acceptance has been achieved in macaques using a transient mixed hematopoetic chimerism protocol, but similar regimens have proven unsuccessful in heart allograft recipients unless a kidney transplant was performed simultaneously. Here, we test whether a modified protocol based on targeting CD154, CD2, and CD28 is sufficient to prolong heart allograft acceptance or promote the expansion of regulatory T cells.

METHODS

Eight macaques underwent heterotopic allo-heart transplantation from major histocompatibility complex-mismatched donors. Induction treatment for donor bone marrow transplantation (BMT) was administered after a 4-mo delay period under TNX-1500 monotherapy. The BMT induction regimen comprised 1 (group 1, G1; n = 3) or 2 (group 2, G2; n = 5) doses of total body irradiation, thymic irradiation, and antithymocyte globulin, followed by 2 (G1) or 5 (G2) weekly doses of αCD2 and 5 weekly treatments with αCD28 and TNX-1500.

RESULTS

During the delay period, 1 G1 graft was rejected and 2 (1 in each group) exhibited moderate rejection on protocol biopsy before BMT. Lymphocyte chimerism was seen in 3 of 5 G2 animals and in 1 of 2 G1 recipients. One G1 graft was rejected despite chimerism, whereas the other recipient succumbed to anti-cytomegalovirus treatment. Two G2 monkeys succumbed due to infection (cytomegalovirus, bacteremia) post-BMT and 3 due to posttransplantation lymphoproliferative disease.

CONCLUSIONS

Intensive costimulation pathway blockade with αCD2, αCD154, and αCD28 promotes lymphocyte chimerism at the cost of high incidence of posttransplantation lymphoproliferative disease and opportunistic infections, preventing assessment of the effectiveness of the regimen to promote alloimmune tolerance.

What makes the kidney so tolerant?

Various organ allografts differ in their propensity to be spontaneously accepted without any immunosuppressive treatment. Understanding the mechanisms behind these differences can aid in managing alloimmune responses in general. C57BL/6 mice naturally accept DBA/2J kidney allografts, forming tertiary lymphoid organs containing regulatory T cells (rTLOs), crucial for graft acceptance. In this issue of the JCI, Yokose and colleagues revealed that rTLOs promote conversion of cytotoxic alloreactive CD8+ T cells into exhausted/regulatory ones, through an IFN-γ-mediated mechanism. Their study provides insights into tolerance development that could help promote the acceptance of grafts at higher risk of rejection.

Dysfunction of infiltrating cytotoxic CD8+ T cells within the graft promotes murine kidney allotransplant tolerance

Tolerance of mouse kidney allografts arises in grafts that develop regulatory tertiary lymphoid organs (rTLOs). Single-cell RNA-seq (scRNA-seq) data and adoptive transfer of alloreactive T cells after transplantation showed that cytotoxic CD8+ T cells are reprogrammed within the accepted graft to an exhausted/regulatory-like phenotype mediated by IFN-γ. Establishment of rTLOs was required because adoptive transfer of alloreactive T cells prior to transplantation results in kidney allograft rejection. Despite the presence of intragraft CD8+ cells with a regulatory phenotype, they were not essential for the induction and maintenance of kidney allograft tolerance since renal allotransplantation into CD8-KO recipients resulted in acceptance and not rejection. Analysis of scRNA-seq data from allograft kidneys and malignant tumors identified similar regulatory-like cell types within the T cell clusters and trajectory analysis showed that cytotoxic CD8+ T cells are reprogrammed into an exhausted/regulatory-like phenotype intratumorally. Induction of cytotoxic CD8+ T cell dysfunction of infiltrating cells appears to be a beneficial mechanistic pathway that protects the kidney allotransplant from rejection through a process we call "defensive tolerance." This pathway has implications for our understanding of allotransplant tolerance and tumor resistance to host immunity.

Donor antigen-specific regulatory T cell administration to recipients of live donor kidneys: A ONE Study consortium pilot trial

Regulatory T cells (Tregs) can inhibit cellular immunity in diverse experimental models and have entered early phase clinical trials in autoimmunity and transplantation to assess safety and efficacy. As part of the ONE Study consortium, we conducted a phase I-II clinical trial in which purified donor antigen reactive (dar)-Tregs (CD4+CD25+CD127lo) were administered to 3 patients, 7 to 11 days after live donor renal transplant. Recipients received a modified immunosuppression regimen, without induction therapy, consisting of maintenance tacrolimus, mycophenolate mofetil, and steroids. Steroids were weaned off over 14 weeks. No rejection was seen on any protocol biopsy. Therefore, all patients discontinued mycophenolate mofetil 11 to 13 months posttransplant, per protocol. An early for-cause biopsy in 1 patient, 5 days after dar-Treg infusion, revealed absence of rejection and accumulation of Tregs in the kidney allograft. All patients had Treg-containing lymphoid aggregates evident on protocol biopsies performed 8 months posttransplant. The patients are now all >6 years posttransplant on tacrolimus monotherapy with excellent graft function. None experienced rejection episodes. No serious adverse events were attributable to Treg administration. These results support a favorable safety profile of dar-Tregs administered early after renal transplant, suggest early biopsy might be an instructive research endpoint and provide preliminary evidence of potential immunomodulatory activity.

Targeting IL-6 to prevent cardiac allograft rejection

Outcomes following heart transplantation remain suboptimal with acute and chronic rejection being major contributors to poor long-term survival. IL-6 is increasingly recognized as a critical pro-inflammatory cytokine involved in allograft injury and has been shown to play a key role in regulating the inflammatory and alloimmune responses following heart transplantation. Therapies that inhibit IL-6 signaling have emerged as promising strategies to prevent allograft rejection. Here, we review experimental and pre-clinical evidence that supports the potential use of IL-6 signaling blockade to improve outcomes in heart transplant recipients.

Immunosuppressive regimens in porcine transplantation models

Pigs, or Sus scrofa domestica, are commonly used animal models in translational transplantation research due to their anatomical, physiological, and immunological similarities to humans. In solid organ transplantation studies, immunosuppressive medications may be administered to pigs to prevent rejection. We provide an overview of the immunosuppressive regimens used in allogeneic solid organ transplantation in pigs, including heart, lung, kidney, bowel and cotransplanted organs and focus on the use of tacrolimus, mycophenolate mofetil, and corticosteroids.

Novel intragraft regulatory lymphoid structures in kidney allograft tolerance

Intragraft events thought to be relevant to the development of tolerance are here subjected to a comprehensive mechanistic study during long-term spontaneous tolerance that occurs in C57BL/6 mice that receive life sustaining DBA/2 kidneys. These allografts rapidly develop periarterial Treg-rich organized lymphoid structures (TOLS) that form in response to class II but not to class I MHC disparity and form independently of lymphotoxin α and lymphotoxin β receptor pathways. TOLS form in situ in the absence of lymph nodes, spleen, and thymus. Distinctive transcript patterns are maintained over time in TOLS including transcripts associated with Treg differentiation, T cell checkpoint signaling, and Th2 differentiation. Pathway transcripts related to inflammation are expressed in early stages of accepted grafts but diminish with time, while B cell transcripts increase. Intragraft transcript patterns at one week posttransplant distinguish those from kidneys destined to be rejected, that is, C57BL/6 allografts into DBA/2 recipients, from those that will be accepted. In contrast to inflammatory tertiary lymphoid organs (iTLOs) that form in response to chronic viral infection and transgenic Lta expression, TOLS lack high endothelial venules and germinal centers. TOLS represent a novel, pathogenetically important type of TLO that are in situ markers of regulatory tolerance.

Regulation of Alloantibody Responses

The control of alloimmunity is essential to the success of organ transplantation. Upon alloantigen encounter, naïve alloreactive T cells not only differentiate into effector cells that can reject the graft, but also into T follicular helper (Tfh) cells that promote the differentiation of alloreactive B cells that produce donor-specific antibodies (DSA). B cells can exacerbate the rejection process through antibody effector functions and/or B cell antigen-presenting functions. These responses can be limited by immune suppressive mechanisms mediated by T regulatory (Treg) cells, T follicular regulatory (Tfr) cells, B regulatory (Breg) cells and a newly described tolerance-induced B (TIB) cell population that has the ability to suppress de novo B cells in an antigen-specific manner. Transplantation tolerance following costimulation blockade has revealed mechanisms of tolerance that control alloreactive T cells through intrinsic and extrinsic mechanisms, but also inhibit alloreactive B cells. Thus, the control of both arms of adaptive immunity might result in more robust tolerance, one that may withstand more severe inflammatory challenges. Here, we review new findings on the control of B cells and alloantibody production in the context of transplant rejection and tolerance.