Immunomodulatory effects of mixed hematopoietic chimerism: immune tolerance in canine model of lung transplantation

Evolution of haematopoietic cell transplantation for canine blood disorders and a platform for solid organ transplantation

Pre-clinical haematopoietic cell transplantation (HCT) studies in canines have proven to be invaluable for establishing HCT as a highly successful clinical option for the treatment of malignant and non-malignant haematological diseases in humans. Additionally, studies in canines have shown that immune tolerance, established following HCT, enabled transplantation of solid organs without the need of lifelong immunosuppression. This progress has been possible due to multiple biological similarities between dog and mankind. In this review, the hurdles that were overcome and the methods that were developed in the dog HCT model which made HCT clinically possible are examined. The results of these studies justify the question whether HCT can be used in the veterinary clinical practice for more wide-spread successful treatment of canine haematologic and non-haematologic disorders and whether it is prudent to do so.

Cellular Immunotherapies in Preclinical Large Animal Models of Transplantation

Hematopoietic stem cell (HSC) transplantation and solid organ transplantation remain the only curative options for many hematologic malignancies and end-stage organ diseases. Unfortunately, the sequelae of long-term immunosuppression, as well as acute and chronic rejection, carry significant morbidities, including infection, malignancy, and graft loss. Numerous murine models have demonstrated the efficacy of adjunctive cellular therapies using HSCs, regulatory T cells, mesenchymal stem cells, and regulatory dendritic cells in modulating the alloimmune response in favor of graft tolerance; however, translation of such murine approaches to other preclinical models and in the clinic has yielded mixed results. Large animals, including nonhuman primates, swine, and canines, provide a more immunologically rigorous model in which to test the clinical translatability of these cellular therapies. Here, we highlight the contributions of large animal models to the development and optimization of HSCs and additional cellular therapies to improve organ transplantation outcomes.

Developments and translational relevance for the canine haematopoietic cell transplantation preclinical model

The development of safe and reliable haematopoietic cell transplantation (HCT) protocols to treat human patients with malignant and non-malignant blood disorders was highly influenced by preclinical studies obtained in random-bred canines. The surmounted barriers included recognizing the crucial importance of histocompatibility matching, establishing long-term donor haematopoietic cell engraftment, preventing graft-vs-host disease and advancing effective conditioning and post-grafting immunosuppression protocols, all of which were evaluated in canines. Recent studies have applied the tolerance inducing potential of HCT to solid organ and vascularized composite tissue transplantation. Several advances in HCT and tolerance induction that were first developed in the canine preclinical model and subsequently applied to human patients are now being recruited into veterinary practice for the treatment of malignant and non-malignant disorders in companion dogs. Here, we review recent HCT advancements attained in the canine model during the past 15 years.

The emerging role of regulatory T cells following lung transplantation

Regulatory T cells (Treg) have proven to be a powerful immunologic force in nearly every organ system and hold therapeutic potential for a wide range of diseases. Insights gained from non-transplant pathologies, such as infection, cancer, and autoimmunity, are now being translated to the field of solid organ transplantation, particularly for livers and kidneys. Recent insights from animal models of lung transplantation have established that Tregs play a vital role in suppressing rejection and facilitating tolerance of lung allografts, and such discoveries are being validated in human studies and preclinical trials. Given that long-term outcomes following lung transplantation remain profoundly limited by chronic rejection, Treg therapy holds the potential to significantly improve patient outcomes and should be aggressively investigated.

Induction of Tolerance Towards Solid Organ Allografts Using Hematopoietic Cell Transplantation in Large Animal Models

Background

The application of hematopoietic cell transplantation for induction of immune tolerance has been limited by toxicities associated with conditioning regimens and to graft-versus-host disease (GVHD). Decades of animal studies have culminated into sufficient control of these two problems, making immune tolerance a viable alternative to life-long application of immunosuppressive drugs to prevent allograft rejection.

Methods

Studies in mice have paved the way for the application of HCT with limited toxicity in large animal models. Resultant studies in the pig, dog, and ultimately the nonhuman primate have led to appropriate methods for achieving nonmyeloablative irradiation protocols, dose, and timing of post-grafting immunosuppressive drugs, monoclonal antibody therapy, and biologicals for costimulatory molecule blockade. The genetics field has been extensively evaluated in appreciation of the ultimate need to obtain organs from MHC-mismatched unrelated donors.

Results

Nonmyeloablative conditioning regimens have been shown to be successful in inducing immune tolerance across all three animal models. Postgrafting immunosuppression is also important in assuring sustained donor hematopoiesis for tolerance. Donor chimerism need not be permanent to establish stable engraftment of donor organs, thereby essentially eliminating the risk of GVHD. Using nonmyeloablative HCT with monoclonal antibody immunosuppression, the kidney has been successfully transplanted in MHC-mismatched nonhuman primates.

Conclusions

Nonmyeloablative HCT for the establishment of temporary mixed chimerism has led to the establishment of stable tolerance against solid organ allografts in large animal models. The kidney, considered a tolerogenic organ, has been successfully transplanted in the clinic. Other organs such as heart, lung, and vascularized composite allografts (face and hands), remain distant possibilities. Further study in large animal models will be required to improve tolerance against these organs before success can be attained in the clinic.

Mixed chimerism renders residual host dendritic cells incapable of alloimmunization of the marrow donor in the canine model of allogeneic marrow transplantation

This study tested whether an alloimmune response can occur in the marrow donor when infused or injected with leukocytes from their mixed chimeric transplant recipient. Two mixed chimeras were produced after conditioning with three Gray total body irradiation, donor marrow infusion, and post-grafting immunosuppression. The marrow donors were then repeatedly infused and injected with leukocytes from their respective chimeric recipient. A donor lymphocyte infusion (DLI) into their mixed chimeras had no effect, even after the experiments were repeated. The presence of blood dendritic cells (DCs) of recipient origin was confirmed in chimeric recipients, as well as the presence of microchimerism in the marrow donors. Donor sensitization did occur following placement of a recipient skin graft that was confirmed following DLI into recipients that changed the mixed chimeras into full donor chimeras. These observations suggest that mixed chimerism renders recipient peripheral blood DCs incapable of inducing a donor T cell response.

Long-term Tolerance Toward Haploidentical Vascularized Composite Allograft Transplantation in a Canine Model Using Bone Marrow or Mobilized Stem Cells

BACKGROUND

The development of safe and reliable protocols for the transplantation of the face and hands may be accomplished with animal modeling of transplantation of vascularized composite allografts (VCA). Previously, we demonstrated that tolerance to a VCA could be achieved after canine recipients were simultaneously given marrow from a dog leukocyte antigen-identical donor. In the present study, we extend those findings across a dog leukocyte antigen mismatched barrier.

METHODS

Eight recipient dogs received total body irradiation (4.5 cGy), hematopoietic cell transplantation (HCT), either marrow (n = 4) or granulocyte-colony stimulating factor mobilized peripheral blood stem cells (n = 4), and a VCA transplant from the HCT donor. Post grafting immunosuppression consisted of mycophenolate mofetil (28 days) and cyclosporine (35 days).

RESULTS

In 4 dogs receiving bone marrow, 1 accepted both its marrow transplant and demonstrated long-term tolerance to the donor VCA (>52 weeks). Three dogs rejected both their marrow transplants and VCA at 5 to 7 weeks posttransplant. Dogs receiving mobilized stem cells all accepted their stem cell transplant and became tolerant to the VCA. However, 3 dogs developed graft-versus-host disease, whereas 1 dog rejected its stem cell graft by week 15 but exhibited long-term tolerance toward its VCA (>90 weeks).

CONCLUSIONS

The data suggest that simultaneous transplantation of mobilized stem cells and a VCA is feasible and leads to tolerance toward the VCA in a haploidentical setting. However, there is a higher rate of donor stem cell engraftment compared with marrow HCT and an increase in the incidence of graft-versus-host disease.

Bone marrow chimerism as a strategy to produce tolerance in solid organ allotransplantation

PURPOSE OF REVIEW

Clinical transplant tolerance has been most successfully achieved combining hematopoietic chimerism with kidney transplantation. This review outlines this strategy in animal models and human transplantation, and possible clinical challenges.

RECENT FINDINGS

Kidney transplant tolerance has been achieved through chimerism in several centers beginning with Massachusetts General Hospital's success with mixed chimerism in human leukocyte antigen (HLA)-mismatched patients and the Stanford group with HLA-matched patients, and the more recent success of the Northwestern protocol achieving full chimerism. This has challenged the original view that stable mixed chimerism is necessary for organ graft tolerance. However, among the HLA-mismatched kidney transplant-tolerant patients, loss of mixed chimerism does not lead to renal-graft rejection, and the development of host Foxp3+ regulatory T cells has been observed. Recent animal models suggest that graft tolerance through bone marrow chimerism occurs through both clonal deletion and regulatory immune cells. Further, Tregs have been shown to improve chimerism in animal models.

SUMMARY

Animal studies continue to suggest ways to improve our current clinical strategies. Advances in chimerism protocols suggest that tolerance may be clinically achievable with relative safety for HLA-mismatched kidney transplants.

Simultaneous transplantation of hematopoietic stem cells and a vascularized composite allograft leads to tolerance

BACKGROUND

We have previously demonstrated that tolerance to a vascularized composite allograft (VCA) can be achieved after the establishment of mixed chimerism. We test the hypothesis that tolerance to a VCA in our dog leukocyte antigen-matched canine model is not dependent on the previous establishment of mixed chimerism and can be induced coincident with hematopoietic cell transplantation (HCT).

METHODS

Eight dog leukocyte antigen-matched, minor antigen mismatched dogs received 200 cGy of radiation and a VCA transplant. Four dogs received donor bone marrow at the time of VCA transplantation (group 1), whereas a second group of four dogs did not (group 2). All recipients received a limited course of postgrafting immunosuppression. All dogs that received HCT and VCA were given donor, third-party, and autologous skin grafts.

RESULTS

All group 1 recipients were tolerant to their VCA (>62 weeks). Three of the four dogs in group 2 rejected their VCA transplants after the cessation of immunosuppression. Biopsies obtained from the muscle and skin of VCA from group 1 showed few infiltrating cells compared with extensive infiltrates in biopsies of VCA from group 2. Compared with autologous skin and muscle, elevated levels of CD3+ FoxP3+ T-regulatory cells were found in the skin and muscle obtained from the VCA of HCT recipients. All group 1 animals were tolerant to their donor skin graft and promptly rejected the third-party skin grafts.

CONCLUSION

These data demonstrated that donor-specific tolerance to all components of the VCA can be established through simultaneous nonmyeloablative allogeneic HCT and VCA transplantation protocol.

Consideration of dual characters of exosomes in the tumour immune response

Efforts to get a strong and sustained anti-tumour immune response induced by a tumour specific antigen have failed, but sipuleucel-T has been approved by the US Food and Drug Administration (FDA). We noticed that exosomes secreted by tumour cells or immune cells may be crucially involved in the tumour immune response, whereas others have had inconsistent findings on exosome involvement. Based on immune network theory, we summarise research advances of exosomes and speculate that in the tumour immune response exosomes follow the immune response curve hypothesis. Exosomes activate simultabeously both immune activation and immune tolerance, but at different intensities. To obtain a desired anti-immune response, the initial point of immunity should be determined to achieve the strongest anti-tumour response, and repeated in vitro to extend and enhance this response. As a result, our hypothesis proposes that studies should now be directed at determining the exact time of exosome activity in maintaining a viable anti-tumour immune response in vivo.

Late infusion of cloned marrow fibroblasts stimulates endogenous recovery from radiation-induced lung injury

In the current study, we used a canine model of radiation-induced lung injury to test the effect of a single i.v. infusion of 10×10⁶/kg of marrow fibroblasts on the progression of damage following 15 Gy exposure to the right lung. The fibroblasts, designated DS1 cells, are a cloned population of immortalized cells isolated from a primary culture of marrow stromal cells. DS1 cells were infused at week 5 post-irradiation when lung damage was evident by imaging with high-resolution computed tomography (CT). At 13 weeks post-irradiation we found that 4 out of 5 dogs receiving DS1 cells had significantly improved pulmonary function compared to 0 out of 5 control dogs (p = 0.047, Fisher’s Exact). Pulmonary function was measured as the single breath diffusion capacity-hematocrit (DLCO-Hct), the total inspiratory capacity (IC), and the total lung capacity (TLC), which differed significantly between control and DS1-treated dogs; p = 0.002, p = 0.005, and p = 0.004, respectively. The DS1-treated dogs also had less pneumonitis detected by CT imaging and an increased number of TTF-1 (thyroid transcription factor 1, NKX2-1) positive cells in the bronchioli and alveoli compared to control dogs. Endothelial-like progenitor cells (ELC) of host origin, detected by colony assays, were found in peripheral blood after DS1 cell infusion. ELC numbers peaked one day after infusion, and were not detectable by 7 days. These data suggest that infusion of marrow fibroblasts stimulates mobilization of ELC, which is associated with a reduction in otherwise progressive radiation-induced lung injury. We hypothesize that these two observations are related, specifically that circulating ELC contribute to increased angiogenesis, which facilitates endogenous lung repair.

Non-myeloablative conditioning is sufficient to induce mixed chimerism and subsequent acceptance of donor specific cardiac and skin grafts

Organ transplant recipients have elevated cancer and viral infection risks due to immunosuppression and long-term results of organ transplantation remain unsatisfactory, mainly because of chronic rejection. The purpose of the current study is to establish a nonmyeloablative perioperative regimen, able to induce mixed chimerism and tolerance of allografts. To establish a nonmyeloablative perioperative regimen, we used Busulfan, an important component of many bone marrow transplantation preparative regimens for a variety of non-neoplastic diseases as an alternative to total body irradiation (TBI), and FTY720, a unique immunosuppression agent, inhibition lymphocyte homing. We found that creating a lymphohematopoietic chimera in which donor and recipient hematopoiesis coexist resulted in prolongation of the donor specific heart and skin allografts. Consistent with graft survival, pathological analysis indicated that the allografts from tolerant recipients were free of myocardial injury and had only a few interstitial infiltrates, and obliterative vasculopathy was not observed. Furthermore, we found that Treg cells were increased in the long-term graft acceptance recipients. Our data revealed that the therapeutic potential for using hematopoietic chimerism in non-myeloablated recipients hope the advances in rodent models described above in the development of minimal, nontoxic host conditioning regimens for mixed chimerism induction and subsequent acceptance of donor specific grafts.

Mixed chimerism and split tolerance: mechanisms and clinical correlations

Establishing hematopoietic mixed chimerism can lead to donor-specific tolerance to transplanted organs and may eliminate the need for long-term immunosuppressive therapy, while also preventing chronic rejection. In this review, we discuss central and peripheral mechanisms of chimerism induced tolerance. However, even in the long-lasting presence of a donor organ or donor hematopoietic cells, some allogeneic tissues from the same donor can be rejected; a phenomenon known as split tolerance. With the current goal of creating mixed chimeras using clinically feasible amounts of donor bone marrow and with minimal conditioning, split tolerance may become more prevalent and its mechanisms need to be explored. Some predisposing factors that may increase the likelihood of split tolerance are immunogenicity of the graft, certain donor-recipient combinations, prior sensitization, location and type of graft and minimal conditioning chimerism induction protocols. Additionally, split tolerance may occur due to a differential susceptibility of various types of tissues to rejection. The mechanisms involved in a tissue's differential susceptibility to rejection include the presence of polymorphic tissue-specific antigens and variable sensitivity to indirect pathway effector mechanisms. Finally, we review the clinical attempts at allograft tolerance through the induction of chimerism; studies that are revealing the complex relationship between chimerism and tolerance. This relationship often displays split tolerance, and further research into its mechanisms is warranted.

Long-term tolerance to kidney allografts after induced rejection of donor hematopoietic chimerism in a preclinical canine model

BACKGROUND

Allogeneic hematopoietic cell transplantation provides a reliable method for inducing tolerance toward solid organ grafts. However, this procedure can result in graft-versus-host disease, thereby limiting its application. Here, we test the hypothesis that mixed chimerism can be intentionally reverted to host hematopoiesis without rejection of a kidney graft.

METHODS

Recipient dogs were given 2-Gy total-body irradiation (TBI) before and a short course of immunosuppression after marrow infusion from dog leukocyte antigen-identical littermates. All dogs achieved stable mixed chimerism. After a mean of 20 weeks, one cohort of dogs received kidney transplants from their respective marrow donors. Subsequently, recipients were reconditioned with 2-Gy TBI and given autologous granulocyte colony-stimulating factor-mobilized leukocytes (recipient leukocyte infusion [RLI]) that had been collected before marrow transplantation.

RESULTS

Dogs receiving a second TBI and RLI without a kidney transplant rejected their donor hematopoietic graft within 3 weeks. Dogs that received kidney grafts, followed by a second TBI and RLI, rejected their marrow graft without rejecting their transplanted kidneys for periods greater than 1 year.

CONCLUSION

Mixed chimerism may be clinically reverted to 100% recipient without rejection of a kidney allograft. This finding may have application toward minimizing the risk of graft-versus-host disease in solid organ transplantation patients given hematopoietic cell transplantation from human leukocyte antigen-identical donors.

Tolerance to vascularized composite allografts in canine mixed hematopoietic chimeras

BACKGROUND

Mixed donor-host chimerism, established through hematopoietic cell transplantation (HCT), is a reproducible strategy for the induction of tolerance toward solid organs. Here, we ask whether a nonmyeloablative conditioning regimen establishing mixed donor-host chimerism leads to tolerance of antigenic vascularized composite allografts.

METHODS

Stable mixed chimerism was established in dogs given a sublethal dose (1-2 Gy) total body irradiation before and a short course of immunosuppression after dog leukocyte antigen-identical marrow transplantation. Vascularized composite allografts from marrow donors were performed after a median of 36 months (range, 4-54 months) after HCT.

RESULTS

All marrow recipients maintained mixed donor-host hematopoietic chimerism and accepted vascularized composite allografts for periods ranging between 52 and 90 weeks; in turn, marrow donors rejected vascularized composite allografts from their respective marrow recipients within 18 to 29 days. Biopsies of muscle and skin of vascularized composite allografts from mixed chimeras showed few infiltrating cells compared with extensive infiltrates in biopsies of vascularized composite allografts from marrow donors. Elevated levels of CD3+ FoxP3+ T-regulatory cells were found in skin and muscle of vascularized composite allografts of mixed chimeras compared with normal tissues. In mixed chimeras, increased numbers of T-regulatory cells were found in draining compared with nondraining lymph nodes of vascularized composite allografts.

CONCLUSIONS

These data suggest that nonmyeloablative HCT may form the basis for future clinical applications of solid organ transplantation and that T-regulatory cells may function toward maintenance of the vascularized composite allograft.

Antagonistic and agonistic anti-canine CD28 monoclonal antibodies: tools for allogeneic transplantation

BACKGROUND

It has been presumed that antibody-mediated selective costimulatory molecule blockade of CD28 is superior to cytotoxic T lymphocyte antigen 4 (CTLA4)-Ig. This is based on the premise that specifically blocking CD28 allows inhibitory signals through CTLA-4 to proceed, which furthermore suppresses T-cell function.

METHODS

The extracelluar domain of canine (ca)CD28 was cloned from dog peripheral blood mononuclear cells. Mice were immunized with a caCD28/murine IgG2a fusion protein. Hybridomas were produced by fusing splenocytes with mouse NSO cells and screened for caCD28 binding by ELISA. Agonistic and antagonistic activities of the monoclonal antibodies (mAb) were tested in mixed leukocyte reactions. Canine regulatory T cells were expanded using plate-bound anti-CD3 and an anti-CD28 agonist mAb.

RESULTS

One agonistic and seven antagonistic mAbs to canine (ca)CD28 were cloned. Binding studies indicated that an agonistic (5B8) and an antagonistic (1C6) mAb bound equally well to a caCD28/caIgG1 fusion protein and to CD28 expressed on CD4+ and CD8+ peripheral blood T cells. Antagonistic antibody blocked mixed lymphocyte reactions (MLR) in a dose-dependent manner similar to CTLA4-Ig, whereas the agonistic antibody to caCD28 enhanced MLR. The 5B8 was superior to 1C6 when either was combined with anti-caCD3 to stimulate lymphocyte proliferation. Furthermore, the agonistic mAb, 5B8, together with anti-CD3 mAb induced 100-fold proliferation of canine regulatory T cells. Relative to untreated control cells, anti-caCD28 (1C6) and CTLA4-Ig equivalently inhibited cytotoxic T lymphocyte-mediated killing of alloreactive target cells.

CONCLUSION

These studies demonstrated that mouse anti-caCD28 mAbs can be generated with agonistic or antagonistic function.

A novel monoclonal antibody specific for canine CD25 (P4A10): selection and evaluation of canine Tregs

A monoclonal antibody (mAb), P4A10, was made to the canine interleukin-2 receptor alpha chain (IL-2Ralpha; p55; Tac antigen; CD25) to facilitate studies of canine regulatory T-cells (Treg). By non-reduced Western blot, P4A10 bound to a 55kDa protein, the size of human IL-2Ralpha. In flow cytometry assays, it reacted with a minor population of circulating dog CD3(+)CD4(+) T-cells and the majority (>60%) of in vitro PMA-Ionomycin (PMA-IO)-activated canine CD3(+) T-cells. P4A10 recognized a hematopoietic cell population enriched for FoxP3+ cells as measured by flow cytometry. The P4A10-selected fractions of T-cells had significantly increased copy numbers of CD25, FoxP3, IL-10, and TGFbeta as detected by RT-PCR (reverse transcriptase-PCR) compared to the negative fractions. The P4A10-selected cells inhibited (3)H (tritiated) thymidine incorporation in a mixed leukocyte reaction (MLR) containing responders of the same origin. P4A10-selected T-cells from fresh peripheral blood mononuclear cells had less FoxP3 (p=0.07) by qRT-PCR (quantitative RT-PCR) and were less suppressive (p=0.01) than in vitro alloantigen-activated Treg. The mAb P4A10 is specific for canine CD25 and can be used to facilitate studies of CD25+FoxP3+ Treg in this clinically relevant large animal model.