First experience with the use of a recombinant CD3 immunotoxin as induction therapy in pig-to-primate xenotransplantation: the effect of T-cell depletion on outcome

Research on gene editing and immunosuppressants in kidney xenotransplantation

Gene-edited pig organ transplantation can solve the serious shortage of human donor organs. Currently, xenotransplantation is rapidly developing and has made significant breakthroughs. The use of GTKO (Gal knockout) pigs is an important step forward. The subsequent knockout of three genes combined with the transfer of immune-related genes effectively prolonged the survival time of non-human primate (NHP) transplantation in xenotransplantation. Due to the success of allogeneic kidney transplantation on NHP, this gene editing protocol was recently applied to clinical patients. Two patients underwent allogeneic kidney transplantation and survived for 51 days and 47 days. Exceeding the hyperacute rejection period proves that appropriate gene editing strategies and the combination of immunosuppressive agents contribute to the success of xenotransplantation. To further enhance the feasibility of pig kidney xenograft, this article mainly explores the effects of the NHP xenograft gene editing scheme and immunosuppressants on prolonging transplant survival time.

The Evolution of Immunosuppressive Therapy in Pig-to-Nonhuman Primate Organ Transplantation

An overview is provided of the evolution of strategies towards xenotransplantation during the past almost 40 years, focusing on advances in gene-editing of the organ-source pigs, pre-transplant treatment of the recipient, immunosuppressive protocols, and adjunctive therapy. Despite initial challenges, including hyperacute rejection resulting from natural (preformed) antibody binding and complement activation, significant progress has been made through gene editing of the organ-source pigs and refinement of immunosuppressive regimens. Major steps were the identification and deletion of expression of the three known glycan xenoantigens on pig vascular endothelial cells, the transgenic expression of human "protective" proteins, e.g., complement-regulatory, coagulation-regulatory, and anti-inflammatory proteins, and the administration of an immunosuppressive regimen based on blockade of the CD40/CD154 T cell co-stimulation pathway. Efforts to address systemic inflammation followed. The synergy between gene editing and judicious immunomodulation appears to largely prevent graft rejection and is associated with a relatively good safety profile. Though there remains an incidence of severe or persistent proteinuria (nephrotic syndrome) in a minority of cases. This progress offers renewed hope for patients in need of life-saving organ transplants.

CD3-immunotoxin mediated depletion of T cells in lymphoid tissues of rhesus macaques

Selective T-cell depletion prior to cell or organ transplantation is considered a preconditioning regimen to induce tolerance and immunosuppression. An immunotoxin consisting of a recombinant anti-CD3 antibody conjugated with diphtheria toxin was used to eliminate T-cells. It showed significant T-cell depletion activity in the peripheral blood and lymph nodes in animal models used in previous studies. To date, a comprehensive evaluation of T-cell depletion and CD3 proliferation for all lymphoid tissues has not been conducted. Here, two rhesus macaques were administered A-dmDT390-SCFBdb (CD3-IT) intravenously at 25 μg/kg twice daily for four days. Samples were collected one day prior to and four days post administration. Flow cytometry and immunofluorescence staining were used to evaluate treatment efficiency accurately. Our preliminary results suggest that CD3-IT treatment may induce higher depletion of CD3 and CD4 T-cells in the lymph nodes and spleen, but is ineffective in the colon and thymus. The data showed a better elimination tendency of CD4 T-cells in the B-cell zone relative to the germinal center in the lymph nodes. Further, CD3-IT treatment may lead to a reduction in germinal center T follicular helper CD4 cells in the lymph nodes compared to healthy controls. The number of proliferating CD3 T-cell indicated that repopulation in different lymphoid tissues may occur four days post treatment. Our results provide insights into the differential efficacy of CD3-IT treatment and T-cell proliferation post treatment in different lymphoid tissues. Overall, CD3-IT treatment shows potential efficacy in depleting T-cells in the periphery, lymph nodes, and spleen, making it a viable preconditioning regimen for cell or organ transplantation. Our pilot study provides critical descriptive statistics and can contribute to the design of larger future studies.

Comparison of CD3e Antibody and CD3e-sZAP Immunotoxin Treatment in Mice Identifies sZAP as the Main Driver of Vascular Leakage

Anti-CD3-epsilon (CD3e) monoclonal antibodies (mAbs) and CD3e immunotoxins (ITs) are promising targeted therapy options for various T-cell disorders. Despite significant advances in mAb and IT engineering, vascular leakage syndrome (VLS) remains a major dose-limiting toxicity for ITs and has been poorly characterized for recent "engineered" mAbs. This study undertakes a direct comparison of non-mitogenic CD3e-mAb (145-2C11 with Fc-silentTM murine IgG1: S-CD3e-mAb) and a new murine-version CD3e-IT (saporin-streptavidin (sZAP) conjugated with S-CD3e-mAb: S-CD3e-IT) and identifies their distinct toxicity profiles in mice. As expected, the two agents showed different modes of action on T cells, with S-CD3e-mAb inducing nearly complete modulation of CD3e on the cell surface, while S-CD3e-IT depleted the cells. S-CD3e-IT significantly increased the infiltration of polymorphonuclear leukocytes (PMNs) into the tissue parenchyma of the spleen and lungs, a sign of increased vascular permeability. By contrast, S-CD3e-mAbs-treated mice showed no notable signs of vascular leakage. Treatment with control ITs (sZAP conjugated with Fc-silent isotype antibodies) induced significant vascular leakage without causing T-cell deaths. These results demonstrate that the toxin portion of S-CD3e-IT, not the CD3e-binding portion (S-CD3e-mAb), is the main driver of vascular leakage, thus clarifying the molecular target for improving safety profiles in CD3e-IT therapy.

Long-term survival of pig-to-rhesus macaque renal xenografts is dependent on CD4 T cell depletion

The shortage of available organs remains the greatest barrier to expanding access to transplant. Despite advances in genetic editing and immunosuppression, survival in experimental models of kidney xenotransplant has generally been limited to <100 days. We found that pretransplant selection of recipients with low titers of anti-pig antibodies significantly improved survival in a pig-to-rhesus macaque kidney transplant model (6 days vs median survival time 235 days). Immunosuppression included transient pan-T cell depletion and an anti-CD154-based maintenance regimen. Selective depletion of CD4⁺ T cells but not CD8⁺ T cells resulted in long-term survival (median survival time >400 days vs 6 days). These studies suggested that CD4⁺ T cells may have a more prominent role in xenograft rejection compared with CD8⁺ T cells. Although animals that received selective depletion of CD8⁺ T cells showed signs of early cellular rejection (marked CD4⁺ infiltrates), animals receiving selective CD4⁺ depletion exhibited normal biopsy results until late, when signs of chronic antibody rejection were present. In vitro study results suggested that rhesus CD4⁺ T cells required the presence of SLA class II to mount an effective proliferative response. The combination of low pretransplant anti-pig antibody and CD4 depletion resulted in consistent, long-term xenograft survival.

The Role of Costimulation Blockade in Solid Organ and Islet Xenotransplantation

Pig-to-human xenotransplantation offers a potential bridge to the growing disparity between patients with end-stage organ failure and graft availability. Early studies attempting to overcome cross-species barriers demonstrated robust humoral immune responses to discordant xenoantigens. Recent advances have led to highly efficient and targeted genomic editing, drastically altering the playing field towards rapid production of less immunogenic porcine tissues and even the discussion of human xenotransplantation trials. However, as these humoral immune barriers to cross-species transplantation are overcome with advanced transgenics, cellular immunity to these novel xenografts remains an outstanding issue. Therefore, understanding and optimizing immunomodulation will be paramount for successful clinical xenotransplantation. Costimulation blockade agents have been introduced in xenotransplantation research in 2000 with anti-CD154mAb. Most recently, prolonged survival has been achieved in solid organ (kidney xenograft survival > 400 days with anti-CD154mAb, heart xenograft survival > 900 days, and liver xenograft survival 29 days with anti-CD40mAb) and islet xenotransplantation (>600 days with anti-CD154mAb) with the use of these potent experimental agents. As the development of novel genetic modifications and costimulation blocking agents converges, we review their impact thus far on preclinical xenotransplantation and the potential for future application.

Treg depletion in non-human primates using a novel diphtheria toxin-based anti-human CCR4 immunotoxin

Regulatory T cells (Treg) play an important role in modulating the immune response and has attracted increasing attention in diverse fields such as cancer treatment, transplantation and autoimmune diseases. CC chemokine receptor 4 (CCR4) is expressed on the majority of Tregs, especially on effector Tregs. Recently we have developed a diphtheria-toxin based anti-human CCR4 immunotoxin for depleting CCR4(+) cells in vivo. In this study, we demonstrated that the anti-human CCR4 immunotoxin bound and depleted monkey CCR4(+) cells in vitro. We also demonstrated that the immunotoxin bound to the CCR4(+)Foxp3(+) monkey Tregs in vitro. In vivo studies performed in two naive cynomolgus monkeys revealed 78-89% CCR4(+)Foxp3(+) Treg depletion in peripheral blood lasting approximately 10 days. In lymph nodes, 89-96% CCR4(+)Foxp3(+) Tregs were depleted. No effect was observed in other cell populations including CD8(+) T cells, other CD4(+) T cells, B cells and NK cells. To our knowledge, this is the first agent that effectively depleted non-human primate (NHP) Tregs. This immunotoxin has potential to deplete effector Tregs for combined cancer treatment.

Preventing T cell rejection of pig xenografts

Xenotransplantation is a potential solution to the limited supply of donor organs. While early barriers to xenograft acceptance, such as hyperacute rejection, are now largely avoided through genetic engineering, the next frontier in successful xenograft survival will require prevention of T cell-mediated rejection. Most successful immunosuppressive regimens in xenotransplantation utilize T cell depletion with antibody therapy. Additionally, the use of T cell costimulatory blockade - specifically blockade of the CD40-CD154 pathway - shows promise with several reports of long-term xenograft survival. Additional therapies, such as transgenic expression of T cell coinhibitory molecules or transfer of immunomodulatory cells to promote tolerance, may be necessary to achieve reliable long-term xenograft acceptance. Further studies in pre-clinical models are essential in order to optimize these regimens prior to trials in patients.

Progress in pig-to-non-human primate transplantation models (1998-2013): a comprehensive review of the literature

BACKGROUND

The pig-to-non-human primate model is the standard choice for in vivo studies of organ and cell xenotransplantation. In 1998, Lambrigts and his colleagues surveyed the entire world literature and reported all experimental studies in this model. With the increasing number of genetically engineered pigs that have become available during the past few years, this model is being utilized ever more frequently.

METHODS

We have now reviewed the literature again and have compiled the data we have been able to find for the period January 1, 1998 to December 31, 2013, a period of 16 yr.

RESULTS

The data are presented for transplants of the heart (heterotopic and orthotopic), kidney, liver, lung, islets, neuronal cells, hepatocytes, corneas, artery patches, and skin. Heart, kidney, and, particularly, islet xenograft survival have increased significantly since 1998.

DISCUSSION

The reasons for this are briefly discussed. A comment on the limitations of the model has been made, particularly with regard to those that will affect progression of xenotransplantation toward the clinic.

Kidney xenotransplantation

Xenotransplantation using pigs as donors offers the possibility of eliminating the chronic shortage of donor kidneys, but there are several obstacles to be overcome before this goal can be achieved. Preclinical studies have shown that, while porcine renal xenografts are broadly compatible physiologically, they provoke a complex rejection process involving preformed and elicited antibodies, heightened innate immune cell reactivity, dysregulated coagulation, and a strong T cell-mediated adaptive response. Furthermore, the susceptibility of the xenograft to proinflammatory and procoagulant stimuli is probably increased by cross-species molecular defects in regulatory pathways. To balance these disadvantages, xenotransplantation has at its disposal a unique tool to address particular rejection mechanisms and incompatibilities: genetic modification of the donor. This review focuses on the pathophysiology of porcine renal xenograft rejection, and on the significant genetic, pharmacological, and technical progress that has been made to prolong xenograft survival.

Oridonin suppresses transplant rejection by depleting T cells from the periphery

Oridonin is a natural compound purified from Rabdosia rubescens that has remarkable anti-inflammatory and antitumor activities. Although oridonin has been used in traditional Chinese medicine for many years to treat inflammatory diseases, the underlying mechanisms of these activities are not well understood. In this paper, we explored whether oridonin could be used in transplantation and the mechanisms of its immunosuppression. Oridonin efficacy in transplantation is manifested by prolonged graft survival and decreased graft infiltration. Oridonin induces T cell apoptosis in a concentration- and time-dependent manner. In vivo, oridonin depleted large numbers of T cells from the spleen and peripheral blood. Decreased T cell numbers resulted from apoptosis, which was accompanied by increased phagocyte numbers in the periphery. Reduction in the number of thymocytes was observed in mice treated for 8days, and CD4+CD8+ cells were more sensitive to apoptosis induced by oridonin. Additionally, successive treatment with oridonin for 16days resulted in a considerable reduction in the total number of spleen cells and spleen volume. Thus, T cell depletion may play an essential role in prolonged graft survival and immunosuppression induced by oridonin.

Recombinant anti-monkey CD3 immunotoxin depletes peripheral lymph node T lymphocytes more effectively than rabbit anti-thymocyte globulin in naïve baboons

T cell depletion is an important procedure for both experimental and therapeutic immune modulation. Rabbit anti-thymocyte globulin (ATG), which is a commonly used T cell depletion antibody in clinical organ and cell transplantation protocols, is effective in temporarily depleting peripheral blood T lymphocytes but only moderately effective in depleting peripheral lymph node T cells which comprise the majority of T lymphocytes. A recombinant anti-CD3 immunotoxin, A-dmDT390-scfbDb (C207), has been developed and shown in an initial study to retain the lymph node depleting properties of conjugated CD3 immunotoxin. This agent could potentially be used synergistically with or as a replacement for rabbit ATG in preclinical primate models of transplantation. We directly compared the peripheral blood and lymph node depleting abilities of this recombinant anti-CD3 immunotoxin and rabbit ATG in naïve animals at clinically tolerated doses. Baboons were treated with a full course of either rabbit ATG (n=2) or CD3 immunotoxin (n=3). Peripheral blood and lymph node T lymphocytes were measured before and following treatment. Peripheral blood CD3+ cells fell below 100cells/μL in every animal. In the two animals receiving ATG, CD3+ cells represented 53% and 68% of lymph node cells two days following a full course of rabbit ATG. In contrast, CD3+ cells represented 3%, 5%, and 38% in lymph nodes following a full course of CD3-IT. Thus, recombinant anti-monkey CD3 immunotoxin showed improved peripheral lymph node T lymphocyte depletion to rabbit ATG and spared other immune cells.

Lymphodepletional strategies in transplantation

Because lymphocytes were shown to mediate transplant rejection, their depletion has been studied as a mechanism of preventing rejection and perhaps inducing immunologic tolerance. Agents that profoundly deplete lymphocytes have included monoclonal antibodies, cytotoxic drugs, and radiation. We have studied several such agents but focused on antibodies that deplete not only peripheral blood lymphocytes, but also lymph node lymphocytes. Depletion of lymph node T lymphocytes appears to permit peripheral tolerance at least for T cells in animal models. Nevertheless, B-cell responses may be resistant to such approaches, and T memory cells are likewise relatively resistant to depleting antibodies. We review the experimental and clinical approaches to depletion strategies and outline some of the pitfalls of depletion, such as limitations of currently available agents, duration of tolerance, infection, and malignancy. It is notable that most tolerogenic strategies that have been attempted experimentally and clinically include depleting agents even when they are not named as the underlying strategy. Thus, there is an implicitly acknowledged role for reducing the precursor frequency of donor antigen-specific lymphocytes when approaching the daunting goal of transplant tolerance.

Enhanced de novo alloantibody and antibody-mediated injury in rhesus macaques

Chronic allograft rejection is a major impediment to long-term transplant success. Humoral immune responses to alloantigens are a growing clinical problem in transplantation, with mounting evidence associating alloantibodies with the development of chronic rejection. Nearly a third of transplant recipients develop de novo antibodies, for which no established therapies are effective at preventing or eliminating, highlighting the need for a nonhuman primate model of antibody-mediated rejection. In this report, we demonstrate that depletion using anti-CD3 immunotoxin (IT) combined with maintenance immunosuppression that included tacrolimus with or without alefacept reliably prolonged renal allograft survival in rhesus monkeys. In these animals, a preferential skewing toward CD4 repopulation and proliferation was observed, particularly with the addition of alefacept. Furthermore, alefacept-treated animals demonstrated increased alloantibody production (100%) and morphologic features of antibody-mediated injury. In vitro, alefacept was found to enhance CD4 effector memory T cell proliferation. In conclusion, alefacept administration after depletion and with tacrolimus promotes a CD4+memory T cell and alloantibody response, with morphologic changes reflecting antibody-mediated allograft injury. Early and consistent de novo alloantibody production with associated histological changes makes this nonhuman primate model an attractive candidate for evaluating targeted therapeutics.

T-cell-mediated immunological barriers to xenotransplantation

Xenotransplantion remains the most viable option for significant expansion of the donor organ pool in clinical transplantation. With the advent of nuclear transfer technologies, the production of transgenic swine has become a possibility. These animals have allowed transplant investigators to overcome humoral mechanisms of hyperacute xenograft rejection in experimental pig-to-non-human primate models. However, other immunologic barriers preclude long-term acceptance of xenografts. This review article focuses on a major feature of xenogeneic rejection: xenogeneic T cell responses. Evidence obtained from both small and large animal models, particularly those using either islet cells or kidneys, have demonstrated that T cell responses play a major role in xenogeneic rejection, and that immunosuppression alone is likely incapable of completely suppressing these responses. Additionally, both the direct and indirect pathway of antigen presentation appear to be involved in these anti donor processes. Enhanced understanding of (i) CD47 and its role in transduced xeno-bone marrow (ii) CD39 and its role in coagulation dysregulation and (iii) thymic transplantation have provided us with encouraging results. Presently, experiments evaluating the possibility of xenogeneic tolerance are underway.

Current progress in xenogeneic tolerance

PURPOSE OF REVIEW

The present review updates the current status of research regarding the immunologic responses of the recipient following xenotransplantation. Additionally, we present the recent progress with attempts to induce xenogeneic tolerance induction.

RECENT FINDINGS

There continues to be great interest in xenotransplantation. Recently, descriptions of the mechanisms responsible for attempted T-cell xenogeneic tolerance in both large and small animal models have improved xenogeneic graft survivals. Additionally, the cellular signaling mechanisms, such as those involving CD39, CD44, and CD47, are proving to be highly important. Using the mixed chimerism approach to tolerance in xenogeneic model may be encouraging, especially given the recent clarification of the role for macrophage-induced phagocytosis of xenogeneic donor cells.

SUMMARY

Induction of tolerance to xenogeneic antigens has been accomplished only in small animals; however, graft survivals in large animal models continue to improve. Further clarification of both the adaptive and innate immune responses to xenogeneic antigens is required for success to continue.