What role for FTY720, a novel immunosuppressive agent, in canine nonmyeloablative hematopoietic stem cell transplantation?

Advances in targeting co-inhibitory and co-stimulatory pathways in transplantation settings: the Yin to the Yang of cancer immunotherapy

In the past decade, the power of harnessing T-cell co-signaling pathways has become increasingly understood to have significant clinical importance. In cancer immunotherapy, the field has concentrated on two related modalities: First, targeting cancer antigens through highly activated chimeric antigen T cells (CAR-Ts) and second, re-animating endogenous quiescent T cells through checkpoint blockade. In each of these strategies, the therapeutic goal is to re-ignite T-cell immunity, in order to eradicate tumors. In transplantation, there is also great interest in targeting T-cell co-signaling, but with the opposite goal: in this field, we seek the Yin to cancer immunotherapy's Yang, and focus on manipulating T-cell co-signaling to induce tolerance rather than activation. In this review, we discuss the major T-cell signaling pathways that are being investigated for tolerance induction, detailing preclinical studies and the path to the clinic for many of these molecules. These include blockade of co-stimulation pathways and agonism of coinhibitory pathways, in order to achieve the delicate state of balance that is transplant tolerance: a state which guarantees lifelong transplant acceptance without ongoing immunosuppression, and with preservation of protective immune responses. In the context of the clinical translation of immune tolerance strategies, we discuss the significant challenge that is embodied by the fact that targeted pathway modulators may have opposing effects on tolerance based on their impact on effector vs regulatory T-cell biology. Achieving this delicate balance holds the key to the major challenge of transplantation: lifelong control of alloreactivity while maintaining an otherwise intact immune system.

Allogeneic hematopoietic cell transplantation following nonmyeloablative conditioning as treatment for hematologic malignancies and inherited blood disorders

Allogeneic hematopoietic cell transplantation (HCT) after myeloablative conditioning regimens has been an effective treatment for many patients with hematologic malignancies or inherited blood disorders. Unfortunately, such regimens have been associated with significant toxicity, limiting their use to otherwise healthy, relatively young patients. In an attempt to extend treatment by allogeneic HCT to older patients and those with comorbid conditions, several groups of investigators have developed reduced-intensity or truly nonmyeloablative conditioning regimens, lacking such toxicity. Analogous to conventional regimens, reduced-intensity regimens both eliminated host-versus-graft (rejection) reactions and produced major anti-tumor effects. In contrast, nonmyeloablative regimens have relied on optimizing both pre-and posttransplant immunosuppression to overcome host-versus-graft reactions, while anti-tumor responses have depended mainly on immune-mediated graft-versus-tumor effects. In this review, we define reduced-intensity and truly nonmyeloablative regimens, describe the preclinical development and clinical application of a very low intensity nonmyeloablative regimen, and review results with reduced-intensity regimens in patients with hematologic malignancies or inherited blood disorders.

What role is there for antithymocyte globulin in allogeneic nonmyeloablative canine hematopoietic cell transplantation?

We investigated whether pretransplantation immunosuppression with canine-specific rabbit antithymocyte globulin (ATG), combined with a suboptimal dose of 1 Gy of total body irradiation (TBI), would permit engraftment of canine dog leukocyte antigen-identical marrow. Cumulative ATG doses of 2 to 5 mg/kg produced a T-cell depletion of 1 log in the peripheral blood and 50% in the lymph nodes. Serum levels of ATG peaked on days 4 to 6 after initiation of therapy and became undetectable by day 13 as a result of canine antibody responses to ATG. ATG prolonged allogeneic skin graft survival to 14 days (n = 5), compared with 8 days in control dogs (P = .0003). Five dogs were given marrow transplants after ATG (3.5-5 mg/kg) and 1 Gy of TBI. Posttransplantation immunosuppression consisted of mycophenolate mofetil and cyclosporine. All dogs showed initial engraftment, with maximum donor chimerism levels of 25%. However, only 1 dog achieved sustained engraftment, and 4 rejected their grafts. The duration of engraftment ranged from 8 to > or = 36 weeks (median, 11 weeks), and this is comparable to that in 6 historical controls not given ATG (range, 3-12 weeks; median, 10 weeks; P = .20). The total nucleated cell doses in the marrow grafts had the highest correlation coefficient with the duration of engraftment: 0.82 (P = .09). We concluded that administering ATG before an otherwise suboptimal conditioning dose of 1 Gy of TBI failed to secure uniform stable hematopoietic engraftment.

Combinations of anti-LFA-1, everolimus, anti-CD40 ligand, and allogeneic bone marrow induce central transplantation tolerance through hemopoietic chimerism, including protection from chronic heart allograft rejection

Central transplantation tolerance through hemopoietic chimerism initially requires inhibition of allogeneic stem cell or bone marrow (BM) rejection, as previously achieved in murine models by combinations of T cell costimulation blockade. We have evaluated LFA-1 blockade as part of regimens to support mixed hemopoietic chimerism development upon fully allogeneic BALB/c BM transfer to nonirradiated busulfan-treated B6 recipient mice. Combining anti-LFA-1 with anti-CD40 ligand (CD40L) induced high incidences and levels of stable multilineage hemopoietic chimerism comparable to chimerism achieved with anti-CD40L and everolimus (40-O-(2-hydroxyethyl)-rapamycin) under conditions where neither Ab alone was effective. The combination of anti-LFA-1 with everolimus also resulted in high levels of chimerism, albeit with a lower incidence of stability. Inhibition of acute allograft rejection critically depended on chimerism stability, even if maintained at very low levels around 1%, as was the case for some recipients without busulfan conditioning. Chimerism stability correlated with a significant donor BM-dependent loss of host-derived Vbeta11(+) T cells 3 mo after BM transplantation (Tx). Combinations of anti-CD40L with anti-LFA-1 or everolimus also prevented acute rejection of skin allografts transplanted before established chimerism, albeit not independently of allospecific BMTx. All skin and heart allografts transplanted to stable chimeras 3 and 5 mo after BMTx, respectively, were protected from acute rejection. Moreover, this included prevention of heart allograft vascular intimal thickening ("chronic rejection").

Correction of a large animal model of type I Glanzmann's thrombasthenia by nonmyeloablative bone marrow transplantation

OBJECTIVE

The purpose of this study was to determine if nonmyeloablative bone marrow transplantation would induce stable hematopoietic chimerism that would correct the bleeding diathesis associated with type I Glanzmann's thrombasthenia (GT).

METHODS

Three young dogs (less than 12 weeks of age) with GT were transplanted with DLA-matched bone marrow from littermates. Recipients received a sublethal dose (200 cGy) of total-body irradiation (TBI) prior to infusion with bone marrow (1-4 x 10(8) cells/kg). Recipient dogs were immunosuppressed with cyclosporine (15 mg/kg) and mycophenolate mofetil (10 mg/kg). Chimerism was determined by quantitation of donor microsatellite repeat polymorphisms in peripheral blood DNA and by flow cytometry to detect the presence of glycoproteins IIb and IIIa on platelets. Platelet function was assessed by a clot retraction test.

RESULTS

One dog died one week posttransplant due to hemorrhage. Another dog died four weeks posttransplant from an unrecognized congenital heart defect and complications due to canine distemper virus infection. At the time of death, microsatellite analysis indicated 35 to 50% chimerism. Flow cytometry showed 20% of circulating platelets positive for glycoproteins IIb and IIIa. The third dog is alive and doing well approximately two years posttransplant. Hematopoietic chimerism has been sustained at 35 to 60% with approximately 30% of the platelets positive for glycoproteins IIb and IIIa. Platelet function is normal based on clot retraction. The animal does not have clinical signs of bleeding.

CONCLUSIONS

These observations suggest that GT and perhaps other severe inherited platelet disorders can be corrected using nonmyeloablative bone marrow transplantation to establish partial chimerism with normal platelets in the platelet compartment.

FTY720 does not abrogate acute graft-versus-host disease in the dog leukocyte antigennonidentical unrelated canine model1

BACKGROUND

Acute graft-versus-host disease (GVHD) remains a significant impediment to successful hematopoietic stem-cell transplantation (HSCT). Here, we examined the effectiveness of 2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol hydrochloride (FTY720), an immunosuppressant that retraffics activated lymphocytes to secondary lymphoid organs, for the treatment of acute GVHD in an established dog leukocyte antigen-nonidentical unrelated canine HSCT model.

METHODS

Dogs were given HSCT after conditioning with 920 cGy total body irradiation. The dogs received methotrexate 0.4 mg/kg/day on days 1, 3, 6, and 11 and FTY720 (5 mg/kg/day orally) after developing GVHD.

RESULTS

Five of six dogs achieved engraftment, developed acute GVHD, and were treated with FTY720. FTY720 resulted in a profound decrease in lymphocytes and a temporary mitigation of clinical GVHD; however, GVHD recurred in all dogs. Four of five dogs were euthanized because of severe GVHD and the fifth because of severe inanition associated with moderate GVHD.

CONCLUSIONS

Compared with controls, treatment of GVHD with FTY720 did not control this complication or significantly increase survival.

Minor histocompatibility antigen-specific cytotoxic T lymphocytes generated with dendritic cells from DLA-identical littermates

Donor cytotoxic T lymphocytes (CTL) specific for minor histocompatibility antigens (mHA) mediate the graft-versus-host effect whereas host mHA-specific CTL mediate graft rejection in the setting of major histocompatibility complex identical allogeneic hematopoietic stem cell transplantation. Development of a large animal model from which mHA-specific CTL can be isolated would accelerate translation in clinical studies to improve control of the graft-versus-host effect as well as prevention of graft rejection in sensitized hosts. The aims of the current study were to isolate mHA-specific CTL from dog leukocyte antigen-identical littermate nonsensitized recipients before transplantation, from stable mixed hematopoietic chimeras, and from dogs sensitized to mHA after graft rejection. Donor dendritic cells (DCs) were cultured from bone marrow-derived CD34(+) cells and were used to stimulate recipient T lymphocytes on days 1, 10, and 20 of CTL culture. We reliably generated and expanded mHA-specific CTL ex vivo from sensitized dogs that were given a donor-specific blood transfusion to boost immune recall after graft rejection after a nonmyeloablative transplantation. The mHA-specific cytotoxicity measured by (51)Cr release assay was enriched from less than 5% in the starting population of sensitized peripheral blood mononuclear cells to a median of 63% after 4 weeks in CTL culture. The expanded mHA-specific CTLs were not tissue-specific: hematopoietic cells, fibroblast, and stromal cell lines were lysed in an mHA-specific manner. Allogeneic DCs, but not peripheral blood mononuclear cells, were necessary for stimulating ex vivo expansion of mHA-specific CTL. We were unable to generate mHA-specific CTL from nonsensitized dogs before transplantation, from previously sensitized dogs but without recent recall immunization, or from stable mixed hematopoietic chimeras. We conclude that after recent in vivo sensitization, large-scale ex vivo expansion of mHA-specific CTL was feasible using allogeneic DCs.

The influence of immunosuppressive drugs on tolerance induction through bone marrow transplantation with costimulation blockade

We recently developed a murine protocol for the induction of allogeneic mixed chimerism and tolerance employing nonmyeloablative total body irradiation (TBI), standard-dose bone marrow transplantation (BMT), and costimulation blockade (cobl) with an anti-CD154 monoclonal antibody (mAb) plus CTLA4Ig. We now evaluated whether a short course (1 month) of immunosuppressive drugs, which would be ethically required in the clinical setting of organ transplantation to prevent graft loss in case tolerance is not achieved, interferes with tolerance induced with this regimen. Our results show that calcineurin inhibitors (cyclosporin A [CyA] or tacrolimus [FK]) inhibit development of long-term chimerism and abrogate tolerance induction in this model. Rapamycin (rapa), methylprednisolone (MP), FTY720, and mycophenolate mofetil (MMF), in contrast, have no negative effect on chimerism or tolerance development. Peripheral deletion of donor-reactive T cells, which usually occurs in the weeks following BMT in this model, is blocked by CyA and FK, but not by the other drugs tested. Furthermore, we found that the additional use of compatible immunosuppressive drugs (rapa plus MMF plus MP) allows the dose of TBI to be reduced, so that mixed chimerism and donor skin-graft acceptance can be achieved with 1 Gy using clinically feasible cell numbers. Thus, this protocol of BMT with costimulation blockade can be safely combined with a clinically tested immunosuppressive regimen to permit success with a lower dose of irradiation. These results should facilitate clinical application of this tolerance strategy.

Graft-versus-host disease can be separated from graft-versus-lymphoma effects by control of lymphocyte trafficking with FTY720

Graft-versus-host disease (GvHD) mediated by donor T cells recognizing host alloantigens is associated with beneficial graft-versus-tumor effects in recipients of allogeneic hematopoietic cell transplants. Since leukemias and lymphomas reside largely within the lymphohematopoietic system, we have proposed that the desired graft-versus-leukemia or graft-versus-lymphoma effect can be separated from the complication of GvHD by confinement of the graft-versus-host alloresponse to the lymphohematopoietic tissues. Since the new sphingosine-1-phosphate receptor agonist immunosuppressive drug FTY720 leads to trapping of T cells in secondary lymphoid tissues, we evaluated the possibility that this drug could diminish GvHD, a disease involving epithelial target tissues, while permitting a beneficial alloresponse to take place within the lymphohematopoietic system, leading to graft-versus-lymphoma effects. We demonstrate here that FTY720 markedly reduces GvHD in a clinically relevant, haploidentical strain combination, while permitting antitumor effects against a T cell lymphoma of unshared host MHC haplotype to proceed unhindered. These results establish a potential new immunotherapeutic approach to separating graft-versus-leukemia effects from GvHD.