Specific tolerance induction across a xenogeneic barrier: production of mixed rat/mouse lymphohematopoietic chimeras using a nonlethal preparative regimen

Physiological basis for xenotransplantation from genetically modified pigs to humans

The collective efforts of scientists over multiple decades have led to advancements in molecular and cellular biology-based technologies including genetic engineering and animal cloning that are now being harnessed to enhance the suitability of pig organs for xenotransplantation into humans. Using organs sourced from pigs with multiple gene deletions and human transgene insertions, investigators have overcome formidable immunological and physiological barriers in pig-to-nonhuman primate (NHP) xenotransplantation and achieved prolonged pig xenograft survival. These studies informed the design of Revivicor's (Revivicor Inc, Blacksburg, VA) genetically engineered pigs with 10 genetic modifications (10 GE) (including the inactivation of 4 endogenous porcine genes and insertion of 6 human transgenes), whose hearts and kidneys have now been studied in preclinical human xenotransplantation models with brain-dead recipients. Additionally, the first two clinical cases of pig-to-human heart xenotransplantation were recently performed with hearts from this 10 GE pig at the University of Maryland. Although this review focuses on xenotransplantation of hearts and kidneys, multiple organs, tissues, and cell types from genetically engineered pigs will provide much-needed therapeutic interventions in the future.

Engineering Human Pluripotent Stem Cell Lines to Evade Xenogeneic Transplantation Barriers

Allogeneic human pluripotent stem cell (hPSC)-derived cells and tissues for therapeutic transplantation must necessarily overcome immunological rejection by the recipient. To define these barriers and to create cells capable of evading rejection for preclinical testing in immunocompetent mouse models, we genetically ablated β2m, Tap1, Ciita, Cd74, Mica, and Micb to limit expression of HLA-I, HLA-II, and natural killer cell activating ligands in hPSCs. Though these and even unedited hPSCs readily formed teratomas in cord blood-humanized immunodeficient mice, grafts were rapidly rejected by immunocompetent wild-type mice. Transplantation of these cells that also expressed covalent single chain trimers of Qa1 and H2-Kb to inhibit natural killer cells and CD55, Crry, and CD59 to inhibit complement deposition led to persistent teratomas in wild-type mice. Expression of additional inhibitory factors such as CD24, CD47, and/or PD-L1 had no discernible impact on teratoma growth or persistence. Transplantation of HLA-deficient hPSCs into mice genetically deficient in complement and depleted of natural killer cells also led to persistent teratomas. Thus, T cell, NK cell, and complement evasion are necessary to prevent immunological rejection of hPSCs and their progeny. These cells and versions expressing human orthologs of immune evasion factors can be used to refine tissue- and cell type-specific immune barriers, and to conduct preclinical testing in immunocompetent mouse models.

Progress in xenotransplantation: overcoming immune barriers

A major limitation of organ allotransplantation is the insufficient supply of donor organs. Consequently, thousands of patients die every year while waiting for a transplant. Progress in xenotransplantation that has permitted pig organ graft survivals of years in non-human primates has led to renewed excitement about the potential of this approach to alleviate the organ shortage. In 2022, the first pig-to-human heart transplant was performed on a compassionate use basis, and xenotransplantation experiments using pig kidneys in deceased human recipients provided encouraging data. Many advances in xenotransplantation have resulted from improvements in the ability to genetically modify pigs using CRISPR-Cas9 and other methodologies. Gene editing has the capacity to generate pig organs that more closely resemble those of humans and are hence more physiologically compatible and less prone to rejection. Despite such modifications, immune responses to xenografts remain powerful and multi-faceted, involving innate immune components that do not attack allografts. Thus, the induction of innate and adaptive immune tolerance to prevent rejection while preserving the capacity of the immune system to protect the recipient and the graft from infection is desirable to enable clinical xenotransplantation.

Progress in Xenotransplantation: Immunologic Barriers, Advances in Gene Editing, and Successful Tolerance Induction Strategies in Pig-To-Primate Transplantation

Organ transplantation is the most effective treatment for end stage organ failure, but there are not enough organs to meet burgeoning demand. One potential solution to this organ shortage is xenotransplantation using pig tissues. Decades of progress in xenotransplantation, accelerated by the development of rapid genome editing tools, particularly the advent of CRISPR-Cas9 gene editing technologies, have enabled remarkable advances in kidney and heart xenotransplantation in pig-to-nonhuman primates. These breakthroughs in large animal preclinical models laid the foundation for three recent pig-to-human transplants by three different groups: two kidney xenografts in brain dead recipients deemed ineligible for transplant, and one heart xenograft in the first clinical grade study of pig-to-human transplantation. However, despite tremendous progress, recent data including the first clinical case suggest that gene-modification alone will not overcome all xenogeneic immunologic barriers, and thus an active and innovative immunologic strategy is required for successful xenotransplantation. This review highlights xenogeneic immunologic barriers, advances in gene editing, and tolerance-inducing strategies in pig-to-human xenotransplantation.

A Review of Cyclophosphamide-Induced Transplantation Tolerance in Mice and Its Relationship With the HLA-Haploidentical Bone Marrow Transplantation/Post-Transplantation Cyclophosphamide Platform

The bone marrow transplantation (BMT) between haplo-identical combinations (haploBMT) could cause unacceptable bone marrow graft rejection and graft-versus-host disease (GVHD). To cross such barriers, Johns Hopkins platform consisting of haploBMT followed by post-transplantation (PT) cyclophosphamide (Cy) has been used. Although the central mechanism of the Johns Hopkins regimen is Cy-induced tolerance with bone marrow cells (BMC) followed by Cy on days 3 and 4, the mechanisms of Cy-induced tolerance may not be well understood. Here, I review our studies in pursuing skin-tolerance from minor histocompatibility (H) antigen disparity to xenogeneic antigen disparity through fully allogeneic antigen disparity. To overcome fully allogeneic antigen barriers or xenogeneic barriers for skin grafting, pretreatment of the recipients with monoclonal antibodies (mAb) against T cells before cell injection was required. In the cells-followed-by-Cy system providing successful skin tolerance, five mechanisms were identified using the correlation between super-antigens and T-cell receptor (TCR) Vβ segments mainly in the H-2-identical murine combinations. Those consist of: 1) clonal destruction of antigen-stimulated-thus-proliferating mature T cells with Cy; 2) peripheral clonal deletion associated with immediate peripheral chimerism; 3) intrathymic clonal deletion associated with intrathymic chimerism; 4) delayed generation of suppressor T (Ts) cells; and 5) delayed generation of clonal anergy. These five mechanisms are insufficient to induce tolerance when the donor-recipient combinations are disparate in MHC antigens plus minor H antigens as is seen in haploBMT. Clonal destruction is incomplete when the antigenic disparity is too strong to establish intrathymic mixed chimerism. Although this incomplete clonal destruction leaves the less-proliferative, antigen-stimulated T cells behind, these cells may confer graft-versus-leukemia (GVL) effects after haploBMT/PTCy.

Transplanting organs from pigs to humans

The success of organ transplantation is limited by the complications of immunosuppression, by chronic rejection, and by the insufficient organ supply, and thousands of patients die every year while waiting for a transplant. With recent progress in xenotransplantation permitting porcine organ graft survival of months or even years in nonhuman primates, there is renewed interest in its potential to alleviate the organ shortage. Many of these advances are the result of our heightened capacity to modify pigs genetically, particularly with the development of CRISPR-Cas9-based gene editing methodologies. Although this approach allows the engineering of pig organs that are less prone to rejection, the clinical application of xenotransplantation will require the ability to avoid the ravages of a multifaceted attack on the immune system while preserving the capacity to protect both the recipient and the graft from infectious microorganisms. In this review, we will discuss the potential and limitations of these modifications and how the engineering of the graft can be leveraged to alter the host immune response so that all types of immune attack are avoided.

Intra-bone Bone Marrow Transplantation in Pig-to-Nonhuman Primates for the Induction of Tolerance Across Xenogeneic Barriers

Mixed chimerism and thymic tissue transplantation strategies have achieved xenogeneic tolerance in pig-to-mouse models, and both have been extended to pig-to-baboon models. A mixed chimerism strategy has shown promise toward inducing tolerance in allogeneic models in mice, pigs, nonhuman primates (NHP), humans, and a rat-to-mouse small animal xeno-model. However, even though α-1,3-galactosyltransferase gene knockout (GalTKO) pigs have been used as bone marrow (BM) donors, direct intravenous injection of porcine BM cells was detected for only up to 4 days (peripheral macro-chimerism) in one case, and the rest lost chimerism within 2 days.Recent data in allogeneic models demonstrated that direct injection of donor BM cells into recipient BM spaces (intra-bone bone marrow transplantation: IBBMTx) produces rapid reconstitution and a higher survival rate compared to i.v. injection. In order to minimize the loss of injected porcine BM peripherally before reaching the BM space, Yamada developed a xeno-specific regimen including IBBMTx coated with a collagen gel matrix in a preclinical pig-to-baboon model (Yamada IBBMTx). This strategy aims to achieve improved, persistent macro-chimerism as well as engraftment of BM across a xenogeneic barrier. The initial study published in 2015 demonstrated that this IBBMTx strategy leads to markedly prolonged peripheral macro-chimerism detectable for up to 23 days. Furthermore, a more recent study using human CD47-transgenic (Tg) GalTKO pigs as xeno-donors achieved long-lasting macro-chimerism >60 days with evidence of reduction of anti-pig natural antibodies (nAb). This is the longest macro-chimerism that has ever been achieved in a preclinical large animal xenotransplant model to date. In this chapter, we introduce a brief summary of our achievements in regard to successful tolerance induction by utilizing our novel strategy of IBBMTx as well as describe the step-by-step methodology of surgical and in vitro procedures that are required for this project.

Xenotransplantation tolerance: applications for recent advances in modified swine

PURPOSE OF REVIEW

The aim of this study was to review the recent progress in xenotransplantation achieved through genetic engineering and discuss the potential of tolerance induction to overcome remaining barriers to extended xenograft survival.

RECENT FINDINGS

The success of life-saving allotransplantation has created a demand for organ transplantation that cannot be met by the supply of human organs. Xenotransplantation is one possible solution that would allow for a nearly unlimited supply of organs. Recent genetic engineering of swine has decreased the reactivity of preformed antibodies to some, but not all, potential human recipients. Experiments using genetically modified swine organs have now resulted in survival of life-supporting kidneys for over a year. However, the grafts show evidence of antibody-mediated rejection on histology, suggesting additional measures will be required for further extension of graft survival. Tolerance induction through mixed chimerism or thymic transplantation across xenogeneic barriers would be well suited for patients with a positive crossmatch to genetically modified swine or relatively negative crossmatches to genetically modified swine, respectively.

SUMMARY

This review highlights the current understanding of the immunologic processes in xenotransplantation and describes the development and application of strategies designed to overcome them from the genetic modification of the source animal to the induction of tolerance to xenografts.

IXA Honorary Member Lecture, 2017: The long and winding road to tolerance

The last 15 years or so have seen exciting progress in xenotransplantation, with porcine organ grafts surviving months or even years in non-human primates. These advances reflect the application of new scientific knowledge, improved immunosuppressive agents, and genetic engineering. The field has recently enjoyed a renaissance of interest and hope, largely due to the exponential increase in our capacity to genetically engineer porcine source animals. However, immune responses to xenografts are very powerful and widespread clinical application of xenotransplantation will depend on the ability to suppress these immune responses while preserving the capacity to protect both the recipient and the graft from infectious microorganisms. Our work over the last three decades has aimed to engineer the immune system of the recipient in a manner that achieves specific tolerance to the xenogeneic donor while preserving otherwise normal immune function. Important proofs of principle have been obtained, first in rodents, and later in human immune systems in "humanized mice" and finally in non-human primates, demonstrating the capacity and potential synergy of mixed xenogeneic chimerism and xenogeneic thymic transplantation in tolerizing multiple arms of the immune system. Considering the fact that clinical tolerance has recently been achieved for allografts and the even greater importance of avoiding excessive immunosuppression for xenografts, it is my belief that it is both possible and imperative that we likewise achieve xenograft tolerance. I expect this to be accomplished through the availability of targeted approaches to recipient immune conditioning, understanding of immunological mechanisms of tolerance, advanced knowledge of physiological incompatibilities, and the availability of inbred miniature swine with optimized use of genetic engineering.

Transplant Tolerance: Current Insights and Strategies for Long-Term Survival of Xenografts

Xenotransplantation is an attractive solution to the problem of allograft shortage. However, transplants across discordant species barriers are subject to vigorous immunologic and pathobiologic hurdles, some of which might be overcome with the induction of immunologic tolerance. Several strategies have been designed to induce tolerance to a xenograft at both the central (including induction of mixed chimerism and thymic transplantation) and peripheral (including adoptive transfer of regulatory cells and blocking T cell costimulation) levels. Currently, xenograft tolerance has been well-established in rodent models, but these protocols have not yet achieved similar success in nonhuman primates. This review will discuss the major barriers that impede the establishment of immunological tolerance across xenogeneic barriers and the potential solution to these challenges, and provide a perspective on the future of the development of novel tolerance-inducing strategies.

The Role of NK Cells in Pig-to-Human Xenotransplantation

Recruitment of human NK cells to porcine tissues has been demonstrated in pig organs perfused ex vivo with human blood in the early 1990s. Subsequently, the molecular mechanisms leading to adhesion and cytotoxicity in human NK cell-porcine endothelial cell (pEC) interactions have been elucidated in vitro to identify targets for therapeutic interventions. Specific molecular strategies to overcome human anti-pig NK cell responses include (1) blocking of the molecular events leading to recruitment (chemotaxis, adhesion, and transmigration), (2) expression of human MHC class I molecules on pECs that inhibit NK cells, and (3) elimination or blocking of pig ligands for activating human NK receptors. The potential of cell-based strategies including tolerogenic dendritic cells (DC) and regulatory T cells (Treg) and the latest progress using transgenic pigs genetically modified to reduce xenogeneic NK cell responses are discussed. Finally, we present the status of phenotypic and functional characterization of nonhuman primate (NHP) NK cells, essential for studying their role in xenograft rejection using preclinical pig-to-NHP models, and summarize key advances and important perspectives for future research.

Homing and Immunogenicity of Murine Stromal Cells Transfected with Xenogeneic Mhc Class II Genes

Syngeneic (murine) and xenogeneic (canine) marrow-derived stromal cells were injected intravenously into SCID and normal mice to examine the homing pattern and persistence of these cells in vivo. By in situ hybridization, these stromal cells were detectable in the bone marrow cavity and the spleen 21 days after injection. Xenogeneic cells did not persist in normal mice but persisted in SCID mice. Conditioning of the recipients with irradiation or S-fluorouracil (5-FU) treatment did not alter these results. In addition, syngeneic murine stromal cells were transfected with the genes for canine MHC class II (DRA + DRB) and transplanted into murine recipients to investigate their homing pattern and immunogenicity. These transfected syngeneic stromal cells did also home to marrow and spleen even in normal recipients. However, these cells led to sensitization of the host towards canine antigens as shown by accelerated skin graft rejection and delayed type hypersensitivity (DTH). Thus, immunodeficient (SCID) mice allow for the homing of xenogeneic stromal cells to hemopoietic organs and for prolonged persistence. In immunocompetent (normal) mice, no xenogeneic stromal cells were identified in spleen and marrow, either because of their inability to home or more likely because of immunological rejection. In contrast, syngeneic stromal cells expressing xenogeneic MHC class II genes did home to spleen and marrow and persisted even though the recipient had become sensitized. Their survival may be due to a loss of expression of the transfected gene. Alternatively, the presentation of these xenogeneic gene products in the hemopoietic organs was such that a cytotoxic response was not induced. These results also show that stromal cells can serve as a vehicle for gene delivery, conceivably with the possibility of organ targeting.

Impact of Mixed Xenogeneic Porcine Hematopoietic Chimerism on Human NK Cell Recognition in a Humanized Mouse Model

Mixed chimerism is a promising approach to inducing allograft and xenograft tolerance. Mixed allogeneic and xenogeneic chimerism in mouse models induced specific tolerance and global hyporesponsiveness, respectively, of host mouse natural killer (NK) cells. In this study, we investigated whether pig/human mixed chimerism could tolerize human NK cells in a humanized mouse model. Our results showed no impact of induced human NK cell reconstitution on porcine chimerism. NK cells from most pig/human mixed chimeric mice showed either specifically decreased cytotoxicity to pig cells or global hyporesponsiveness in an in vitro cytotoxicity assay. Mixed xenogeneic chimerism did not hamper the maturation of human NK cells but was associated with an alteration in NK cell subset distribution and interferon gamma (IFN-γ) production in the bone marrow. In summary, we demonstrate that mixed xenogeneic chimerism induces human NK cell hyporesponsiveness to pig cells. Our results support the use of this approach to inducing xenogeneic tolerance in the clinical setting. However, additional approaches are required to improve the efficacy of tolerance induction while ensuring adequate NK cell functions.

Immunosuppression for in vivo research: state-of-the-art protocols and experimental approaches

Almost every experimental treatment strategy using non-autologous cell, tissue or organ transplantation is tested in small and large animal models before clinical translation. Because these strategies require immunosuppression in most cases, immunosuppressive protocols are a key element in transplantation experiments. However, standard immunosuppressive protocols are often applied without detailed knowledge regarding their efficacy within the particular experimental setting and in the chosen model species. Optimization of such protocols is pertinent to the translation of experimental results to human patients and thus warrants further investigation. This review summarizes current knowledge regarding immunosuppressive drug classes as well as their dosages and application regimens with consideration of species-specific drug metabolization and side effects. It also summarizes contemporary knowledge of novel immunomodulatory strategies, such as the use of mesenchymal stem cells or antibodies. Thus, this review is intended to serve as a state-of-the-art compendium for researchers to refine applied experimental immunosuppression and immunomodulation strategies to enhance the predictive value of preclinical transplantation studies.

Progress towards inducing tolerance of pig-to-primate xenografts

Xenotransplantation remains the best near-term solution to the shortage of transplantable organs that currently limits the field of transplantation. However, because the immune response to xenografts is considerably stronger than it is to allografts, the amount of non-specific immunosuppression required to avoid xenograft rejection may limit clinical applicability. For this reason, we consider it likely that the success of clinical xenotransplantation will depend on finding ways of safely inducing tolerance across xenogeneic barriers rather than relying entirely on non-specific immunosuppressive agents. In this laboratory, two approaches are being studied for the induction of pig-to-primate tolerance: a) the simultaneous transplantation of vascularized thymus and solid organs; and b) mixed hematopoietic chimerism. A summary of the development of these two approaches and their current status is the subject of this review.

High incidence of xenogenic bone marrow engraftment in pig-to-baboon intra-bone bone marrow transplantation

Previous attempts of α-1,3-galactocyltransferase knockout (GalTKO) pig bone marrow (BM) transplantation (Tx) into baboons have demonstrated a loss of macro-chimerism within 24 h in most cases. In order to achieve improved engraftment with persistence of peripheral chimerism, we have developed a new strategy of intra-bone BM (IBBM) Tx. Six baboons received GalTKO BM cells, with one-half of the cells transplanted into the bilateral tibiae directly and the remaining cells injected intravenously (IBBM/BM-Tx) with a conditioning immunosuppressive regimen. In order to assess immune responses induced by the combined IBBM/BM-Tx, three recipients received donor SLA-matched GalTKO kidneys in the peri-operative period of IBBM/BM-Tx (Group 1), and the others received kidneys 2 months after IBBM/BM-Tx (Group 2). Peripheral macro-chimerism was continuously detectable for up to 13 days (mean 7.7 days; range 3-13) post-IBBM/BM-Tx and in three animals, macro-chimerism reappeared at days 10, 14 and 21. Pig CFUs, indicating porcine progenitor cell engraftment, were detected in the host BM in four of six recipients on days 14, 15, 19 and 28. In addition, anti-pig unresponsiveness was observed by in vitro assays. GalTKO/pCMV-kidneys survived for extended periods (47 and 60 days). This strategy may provide a potent adjunct for inducing xenogeneic tolerance through BM-Tx.

Xenotransplantation: immunological hurdles and progress toward tolerance

The discrepancy between organ need and organ availability represents one of the major limitations in the field of transplantation. One possible solution to this problem is xenotransplantation. Research in this field has identified several obstacles that have so far prevented the successful development of clinical xenotransplantation protocols. The main immunologic barriers include strong T-cell and B-cell responses to solid organ and cellular xenografts. In addition, components of the innate immune system can mediate xenograft rejection. Here, we review these immunologic and physiologic barriers and describe some of the strategies that we and others have developed to overcome them. We also describe the development of two strategies to induce tolerance across the xenogeneic barrier, namely thymus transplantation and mixed chimerism, from their inception in rodent models through their current progress in preclinical large animal models. We believe that the addition of further beneficial transgenes to Gal knockout swine, combined with new therapies such as Treg administration, will allow for successful clinical application of xenotransplantation.

Transgenic expression of human CD47 markedly increases engraftment in a murine model of pig-to-human hematopoietic cell transplantation

Mixed chimerism approaches for induction of tolerance of solid organ transplants have been applied successfully in animal models and in the clinic. However, in xenogeneic models (pig-to-primate), host macrophages participate in the rapid clearance of porcine hematopoietic progenitor cells, hindering the ability to achieve mixed chimerism. CD47 is a cell-surface molecule that interacts in a species-specific manner with SIRPα receptors on macrophages to inhibit phagocytosis and expression of human CD47 (hCD47) on porcine cells has been shown to inhibit phagocytosis by primate macrophages. We report here the generation of hCD47 transgenic GalT-KO miniature swine that express hCD47 in all blood cell lineages. The effect of hCD47 expression on xenogeneic hematopoietic engraftment was tested in an in vivo mouse model of human hematopoietic cell engraftment. High-level porcine chimerism was observed in the bone marrow of hCD47 progenitor cell recipients and smaller but readily measurable chimerism levels were observed in the peripheral blood of these recipients. In contrast, transplantation of WT progenitor cells resulted in little or no bone marrow engraftment and no detectable peripheral chimerism. These results demonstrate a substantial protective effect of hCD47 expression on engraftment and persistence of porcine cells in this model, presumably by modulation of macrophage phagocytosis.

Increased levels of anti-non-Gal IgG following pig-to-baboon bone marrow transplantation correlate with failure of engraftment

BACKGROUND

The development of genetically modified pigs, which lack the expression of alpha 1-3 galactosyl transferase, (GalT-KO pigs) has facilitated the xenogeneic transplantation of porcine organs and tissues into primates by avoiding hyperacute rejection due to pre-existing antibodies against the Gal epitope. However, antibodies against other antigens (anti-non-Gal antibodies), are found at varying levels in the pre-transplant sera of most primates. We have previously found that baboons with high levels of pre-transplant anti-non-Gal IgG, conditioned with a non-myeloablative conditioning regimen, failed to engraft following pig-to-baboon bone marrow transplantation (Xenotransplantation, 17, 2010 and 300). Two baboons with low levels of pre-transplant anti-non-Gal IgG, conditioned with the same regimen, showed porcine bone marrow progenitors at 28 days following transplantation, suggesting engraftment. These baboons also showed evidence of donor-specific hyporesponsiveness. This observation led us to investigate the hypothesis that selecting for baboon recipients with low pre-transplant anti-non-Gal IgG levels might improve engraftment levels following GalT-KO pig-to-baboon bone marrow transplantation.

METHODS

Five baboons, with low pre-transplant anti-non-Gal IgG levels, received transplantation of bone marrow cells (1-5 × 10(9) /kg of recipient weight) from GalT-KO pigs. They received a non-myeloablative conditioning regimen consisting of low-dose total body irradiation (TBI) (150 cGy), thymic irradiation (700 cGy), anti-thymocyte globulin (ATG), and tacrolimus. In addition, two baboons received Rituximab and Bortezomib (Velcade) treatment as well as extra-corporeal immunoadsorption using GalT-KO pig livers. Bone marrow engraftment was assessed by porcine-specific PCR on colony forming units (CFU) of day 28 bone marrow aspirates. Anti-non-Gal antibody levels were assessed by serum binding toward GalT-KO PBMC using flow cytometry (FACS). Peripheral macro-chimerism was measured by FACS using pig and baboon-specific antibodies and baboon anti-pig cellular responses were assessed by mixed lymphocyte reactions (MLR).

RESULTS

As previously reported, two of five baboons demonstrated detectable bone marrow engraftment at 4 weeks after transplantation. Engraftment was associated with lack of an increase in anti-non-Gal IgG levels as well as cellular hyporesponsiveness toward pig. Three subsequent baboons with similarly low levels of pre-existing anti-non-Gal IgG showed no engraftment and an increase in anti-non-Gal IgG antibody levels following transplantation. Peripheral macrochimerism was only seen for a few days following transplantation regardless of antibody development.

CONCLUSIONS

Selecting for baboon recipients with low levels of pre-transplant anti-non-Gal IgG did not ensure bone marrow engraftment. Failure to engraft was associated with an increase in anti-non-Gal IgG levels following transplantation. These results suggest that anti-non-Gal-IgG is likely involved in early bone marrow rejection and that successful strategies for combating anti-non-Gal IgG development may allow better engraftment. Since engraftment was only low and transient regardless of antibody development, innate immune, or species compatibility mechanisms will likely also need to be addressed to achieve long term engraftment.

Mouse models in bone marrow transplantation and adoptive cellular therapy

Mouse models of transplantation have been indispensable to the development of bone marrow transplantation (BMT). Their role in the generation of basic science knowledge is invaluable and is subject to discussion below. However, this article focuses on the direct role and relevance of mouse models towards the clinical development and advances in BMT and adoptive T-cell therapy for human diseases. The authors aim to present a thoughtful perspective on the pros and cons of mouse models while noting that despite imperfections these models are obligatory for the development of science-based medicine.

Development and tolerization of hyperacute rejection in a transgenic mouse graft versus host model

BACKGROUND

The hyperacute rejection mediated by preexisting antibodies is a major impediment to the success of transplants across allogeneic and xenogeneic barriers. We report a new mouse model that allows us to not only monitor the sensitization of B cells mediating the hyperacute response but also validate therapeutic strategies for tolerizing them.

MODEL

The new model system uses 5C.C7,RAG2 T-cell receptor transgenic T cells and B10.S(9R),CD3[Latin Small Letter Open E] hosts for adoptive transfer experiments.

RESULTS AND CONCLUSIONS

In the allogeneic hosts, transgenic T cells expanded briefly before being chronically deleted. Once the deletion was initiated, a second graft of donor cells was used to assess a hyperacute response. The rapid rejection of the second cohort correlated with the appearance of donor-specific antibodies in the serum. Interestingly, chronically stimulated T cells were relatively resistant to hyperacute rejection, suggesting an explanation for the slower rejection kinetics of the first cohort even as the second cohort of identical donor cells was being hyperacutely rejected. Finally, we could tolerize the potential for a hyperacute response, by pretreating recipients with a single infusion of naive donor B cells before the first T-cell transfer. This treatment not only abrogated the development of a hyperacute response but also allowed the primary graft to survive in vivo for extended periods.

Innate and adaptive immune responses are tolerized in chimeras prepared with nonmyeloablative conditioning

BACKGROUND

Mixed chimerism is an effective approach for tolerance induction in transplantation. Strategies to achieve mixed chimerism with relatively low toxicity have significantly expanded the clinical use of chimerism.

METHODS

Allogeneic bone marrow transplants were performed between B6 (H2(b)) and BALB/c (H2(d)) mice. Recipient B6 mice were nonmyeloablatively conditioned with anti-αβ-T-cell receptor, anti-CD154, or rapamycin alone or in different combinations. A total of 15 × 10(6) BALB/c bone marrow cells were transplanted after varying doses of cGy of total body irradiation.

RESULTS

Pretreatment of recipients with anti-CD154 and rapamycin with or without T-cell lymphodepletion reduced the total body irradiation requirement to 100 cGy for establishing stable mixed chimerism. The mixed chimeras accepted donor islet allografts long term. Lymphocytes from mixed chimeras did not respond to host or donor antigens, yet were reactive to major histocompatibility complex-disparate third-party alloantigens, demonstrating functional donor-specific T-cell tolerance. No antibodies against donor and host were detected in mixed chimeras, suggesting humoral tolerance. Mixed chimeras showed no cytotoxicity to donor cells, but a similar rapid killing rate for major histocompatibility complex disparate third-party B10.BR cells compared with T-cell-deficient and wild-type controls in in vivo cytotoxicity assays, suggesting donor-specific tolerance in the innate immune cells was achieved in mixed chimeras.

CONCLUSIONS

Mixed chimeras prepared with low-intensity nonmyeloablative conditioning exhibit systemic tolerance in innate immunity and tolerance in adaptive T- and B-cell immune responses.

Xenotransplantation: an overview of the field

Xenotransplantation, the transplantation of cells, tissues, or organs between different species, has the potential to overcome the current shortage of human organs and tissues for transplantation. In the last decade, the progress made in the field is remarkable, suggesting that clinical xenotransplantation procedures, particularly those involving cells, may become a reality in the not-too-distant future. However, several hurdles remain, mainly immunological barriers, physiological discrepancies, and safety issues, making xenotransplantion a complex and multidisciplinary discipline.

Acquisition and presentation of follicular dendritic cell-bound antigen by lymph node-resident dendritic cells

Follicular dendritic cells (DCs [FDCs]) are prominent stromal cell constituents of B cell follicles with the remarkable ability to retain complement-fixed antigens on their cell surface for extended periods of time. These retained immune complexes have long been known to provide the antigenic stimulus that drives antibody affinity maturation, but their role in cellular immunity has remained unclear. In this study, we show that FDC-retained antigens are continually sampled by lymph node-resident DCs for presentation to CD8 T cells. This novel pathway of antigen acquisition was detectable when FDCs were loaded with purified antigens bound into classical antigen-antibody immune complexes, as well as after pregnancy, when they are loaded physiologically with antigens associated with the complement-fixed microparticles released from the placenta into maternal blood. In both cases, ensuing antigen presentation was profoundly tolerogenic, as it induced T cell deletion even under inflammatory conditions. These results significantly broaden the scope of FDC function and suggest new ways that the complement system and persistent antigen presentation might influence T cell activation and the maintenance of peripheral immune tolerance.

Occurrence of specific humoral non-responsiveness to swine antigens following administration of GalT-KO bone marrow to baboons

BACKGROUND

Hematopoietic chimerism induces transplantation tolerance across allogeneic and xenogeneic barriers, but has been difficult to achieve in the pig-to-primate model. We have now utilized swine with knockout of the gene coding for alpha-1,3-galactosyltransferase (GalT-KO pigs) as bone marrow donors in an attempt to achieve chimerism and tolerance by avoiding the effects of natural antibodies to Gal determinants on pig hematopoietic cells.

METHODS

Baboons (n = 4; Baboons 1 to 4 = B156, B158, B167, and B175, respectively) were splenectomized and conditioned with TBI (150 cGy), thymic irradiation (700 cGy), T cell depletion with rabbit anti-thymocyte globulin (rATG) and rat anti-primate CD2 (LoCD2b), and received FK506 and supportive therapy for 28 days. All animals received GalT-KO bone marrow (1 to 2 x 10(9) cells/kg) in two fractions on days 0 and 2, and were thereafter monitored for the presence of pig cells by flow cytometry, for porcine progenitor cells by PCR of BM colony-forming units, and for cellular reactivity to pig cells by mixed lymphocyte reaction (MLR). In vitro antibody formation to LoCD2b and rATG was tested by ELISA; antibody reactivity to GalT-KO pig cells was tested by flow cytometry and cytotoxicity assays. Additionally, Baboons 3 and 4 received orthotopic kidney transplants on days 17 and 2, respectively, to test the potential impact of the protocol on renal transplantation.

RESULTS

None of the animals showed detectable pig cells by flow cytometry for more than 12 h post-BM infusion. However, porcine progenitor cell engraftment, as evidenced by pig-derived colony forming units in the BM, as well as peripheral microchimerism in the thymus, lymph node, and peripheral blood was detected by PCR in baboons 1 and 2 for at least 28 days post-transplant. ELISA results confirmed humoral immunocompetence at time of transplantation as antibody titers to rat (LoCD2b) and rabbit (ATG) increased within 2 weeks. However, no induced antibodies to GalT-KO pig cells or increased donor specific cytotoxicity was detectable by flow cytometry. In contrast, baboons 3 and 4 developed serum antibodies to pig cells as well as to rat and rabbit immunoglobulin by day 14. Retrospective analysis revealed that although all four baboons possessed low levels of antibody-mediated cytotoxicity to GalT-KO cells prior to transplantation, the two baboons (3 and 4) that became sensitized to pig cells (and rejected pig kidneys) had relatively high pre-transplantation titers of anti-non-Gal IgG detectable by flow cytometry, whereas baboons 1 and 2 had undetectable titers.

CONCLUSIONS

Engraftment and specific non-responsiveness to pig cells has been achieved in two of four baboons following GalT-KO pig-to-baboon BMT. Engraftment correlated with absence of preformed anti-non-Gal IgG serum antibodies. These results are encouraging with regard to the possibility of achieving transplantation tolerance across this xenogeneic barrier.

Approaches for immunological tolerance induction to stem cell-derived cell replacement therapies

The shortage of donors for organ transplantation and also to treat degenerative diseases has led to the development of the new field of regenerative medicine. One aim of this field, in addition to in vivo induction of endogenous tissue regeneration, is to utilize stem cells as a supplementary source of cells to repair or replace tissues or organs that have ceased to function owing to ageing or autoimmunity. Embryonic stem cells hold promise in this respect because of their developmental capacity to generate all tissues within the body. More recently, the discovery of induced pluripotent stem cells, somatic cells reprogrammed to a primitive embryonic-like state by the introduction of pluripotency factors, may also act as an important cell source for cell replacement therapy. However, before cell replacement therapy can become a reality, one must consider how to overcome the potential transplant rejection of stem cell-derived products. There are several potential ways to circumvent the hurdles presented by the immune system in this setting, not least the induction of immunological tolerance in the host. In this review, we consider this and other approaches for engendering acceptance of stem cell-derived tissues.

Manipulating the immune system for anti-tumor responses and transplant tolerance via mixed hematopoietic chimerism

SUMMARY

Stem cells (SCs) with varying potentiality have the capacity to repair injured tissues. While promising animal data have been obtained, allogeneic SCs and their progeny are subject to immune-mediated rejection. Here, we review the potential of hematopoietic stem cells (HSCs) to promote immune tolerance to allogeneic and xenogeneic organs and tissues, to reverse autoimmunity, and to be used optimally to cure hematologic malignancies. We also review the mechanisms by which hematopoietic cell transplantation (HCT) can promote anti-tumor responses and establish donor-specific transplantation tolerance. We discuss the barriers to clinical translation of animal studies and describe some recent studies indicating how they can be overcome. The recent achievements of durable mixed chimerism across human leukocyte antigen barriers without graft-versus-host disease and of organ allograft tolerance through combined kidney and bone marrow transplantation suggest that the potential of this approach for use in the treatment of many human diseases may ultimately be realized.

Global unresponsiveness as a mechanism of natural killer cell tolerance in mixed xenogeneic chimeras

Mixed xenogeneic chimerism induces T- and B-cell tolerance in mice receiving T-cell-depleted rat bone marrow cells (BMC) following nonmyeloablative conditioning that includes alphabeta and gammadelta T cell and Natural killer (NK) cell-depleting mAbs. NK-cell depletion is essential to permit marrow engraftment, but NK-cell tolerance has not been previously assessed in mixed xenogeneic chimeras. We assessed NK-cell tolerance in rat --> mouse mixed xenogeneic chimeras using in vivo(125)I-5iodo-2-deoxyuridine assays. Additional rapid marrow rejection mechanisms resulted in a requirement for 10-fold more rat than ss2 microglobulin knockout (ss2M(-/-)) (MHC class I-deficient) mouse BMC to achieve engraftment in NK-cell-depleted mice. Both 12-week mixed xenogeneic chimeras and conditioned controls showed reduced resistance to engraftment of ss2M(-/-) mouse and rat BMC. While conditioned control mice recovered NK-cell-mediated resistance to ss2M(-/-) and rat BMC by 16 weeks, mixed chimeras lacked resistance to either, similar to NK-cell-deficient Ly49A transgenic mice. Thus, global NK-cell unresponsiveness is induced by mixed xenogeneic chimerism. Our data suggest that NK-cell anergy is induced by interactions with xenogeneic hematopoietic cells that express activating but not inhibitory ligands for recipient NK cells.

Immune tolerance: mechanisms and application in clinical transplantation

The achievement of immune tolerance, a state of specific unresponsiveness to the donor graft, has the potential to overcome the current major limitations to progress in organ transplantation, namely late graft loss, organ shortage and the toxicities of chronic nonspecific immumnosuppressive therapy. Advances in our understanding of immunological processes, mechanisms of rejection and tolerance have led to encouraging developments in animal models, which are just beginning to be translated into clinical pilot studies. These advances are reviewed here and the appropriate timing for clinical trials is discussed.

Xenogeneic β2-Microglobulin Substitution Alters NK Cell Function

Recently, it has been shown that human beta(2)-microglobulin (h-beta(2)m) blocks the association between the NK cell inhibitory receptor Ly49C and H-2K(b). Given this finding, we therefore sought to assess the immunobiology of NK cells derived from C57BL/6 (H-2(b)) mice expressing exclusively h-beta(2)m. Initial analysis revealed that the Ly49C expression profile of NK cells from h-beta(2)m(+) mice was modified, despite the fact that H-2K(b) expression was normal in these mice. Moreover, the NK cells were not anergic in that IL-2 treatment of h-beta(2)m(+) NK cells in vitro enabled efficient lysis of prototypic tumor cell lines as well as of syngeneic h-beta(2)m(+) lymphoblasts. This loss of self-tolerance appeared to correlate with the activation status of h-beta(2)m(+) NK cells because quiescent h-beta(2)m(+) transplant recipients maintained h-beta(2)m(+) grafts but polyinosine:polycytidylic acid-treated recipients acutely rejected h-beta(2)m(+) grafts. NK cell reactivity toward h-beta(2)m(+) targets was attributed to defective Ly49C interactions with h-beta(2)m:H-2K(b) molecules. With regard to NK cell regulatory mechanisms, we observed that h-beta(2)m:H-2K(b) complexes in the cis-configuration were inefficient at regulating Ly49C and, furthermore, that receptor-mediated uptake of h-beta(2)m:H-2K(b) by Ly49C was impaired compared with uptake of mouse beta(2)m:H-2K(b). Thus, we conclude that transgenic expression of h-beta(2)m alters self-MHC class I in such a way that it modulates the NK cell phenotype and interferes with regulatory mechanisms, which in turn causes in vitro-expanded and polyinosine:polycytidylic acid-activated NK cells to be partially self-reactive similar to what is seen with NK cells derived from MHC class I-deficient mice.

American society of transplantation symposium on B cells in transplantation: harnessing humoral immunity from rodent models to clinical practice

There is growing awareness that B cells and alloantibodies are important mediators of both acute and chronic allograft injury. Unfortunately, few therapies are clinically available to mitigate the function of B cells or the effects of established alloantibody. As a result, many sensitized people await transplantation without a suitable donor, and several rejection syndromes are emerging that appear to involve B cells either as antibody producers or as antigen-presenting cells. In recognition of this unmet need in transplantation, the American Society of Transplantation organized a Symposium on B cells in Organ Transplantation to foster interest in this topic amongst basic researchers attending the annual meeting of the American Association of Immunologists. This manuscript will give an overview of the presentations from this symposium including the current risks of allosensitization, adaptive accommodation, approaches toward B-cell tolerance for allo- and xenoantigens and clinical application of these concepts in ABO incompatible neonatal cardiac transplantation.

Development of an anti-porcine CD34 monoclonal antibody that identifies hematopoietic stem cells

OBJECTIVES

The isolation of porcine hematopoietic stem cells (HSC) would be an important step toward development of porcine-to-human chimerism for induction of tolerance in clinical xenotransplantation. CD34 is a common marker of HSC and has not been developed as a marker in pigs. In this study we have generated and characterized a monoclonal antibody (mAb) that identifies porcine CD34 on a subset of porcine bone marrow (BM) stem/progenitor cells.

METHODS

The porcine CD34 gene was cloned and a recombinant protein produced. An anti-porcine CD34 mAb was produced that could detect both the recombinant protein and a subset of porcine BM cells. The CD34(+) cells were phenotyped by lineage and HSC associated markers. Furthermore, the CD34(+) cells were analyzed by colony-forming unit (CFU) assay.

RESULTS

Two splice variants of the porcine CD34 gene were cloned and a recombinant protein produced for mAb production. The mAb developed can detect both the recombinant protein and the native CD34 protein on a range of pig tissues, including BM. This subset of BM cells was negative for hematopoietic lineage makers, including CD3, CD14, and CD21 and positive for other known porcine HSC markers, including CD90, CD172a, histocompatibility complex (MHC) class I, and MHC class II. Moreover, the CD34(+) BM cells were enriched for multilineage progenitor cells as determined by CFU assay.

CONCLUSIONS

Similar to human and mouse CD34, pig CD34 detects a subset of BM progenitor cells. This mAb will now provide a means for isolating porcine CD34(+) cells to be further analyzed for HSC activity and to assess their potential to develop pig-to-human chimeras to induce xenograft tolerance.

Long-term survival of xenogeneic heart grafts achieved by costimulatory blockade and transient mixed chimerism

BACKGROUND

Xenotransplantation holds great promise in clinical medicine, but is limited by the vigorous rejection response elicited against solid organs transplanted across species barriers. In this study, we investigated the role of anti-CD40L monoclonal antibody (mAb) in inducing xenogeneic mixed chimerism and donor-specific heart transplantation tolerance.

METHODS

One day before heart transplantation, mice were injected intraperitoneally with anti-mouse CD8/NK1.1/Thy1.2 mAbs. On day 0, the mice received 3 Gy total body irradiation (TBI), an intravenous injection of unseparated bone marrow (BM) harvested from F344 rats, and an intraperitoneal injection of hamster antimouse CD40L mAb, MR1. Heart grafts from F344 rats were heterotopically transplanted into the abdomen of B6 mouse recipients. Using flow cytometric analysis of peripheral white blood cells, we assessed donor hematopoiesis at various times after bone marrow transplantation (BMT).

RESULTS

Chimerism subsided gradually and disappeared completely 18 weeks after BMT. The cardiac graft survived permanently, even after the mixed chimerism disappeared. To determine if the mice acquired donor-specific tolerance, second rat heart grafts were transplanted 120 days after the first heart transplantation. The second transplanted hearts were also accepted over 60 days. Histological analysis revealed no remarkable vasculopathy in the coronary vessels at any stage.

CONCLUSIONS

These findings clearly show that costimulatory blockade plays an important role in inducing xenochimerism, and that transient mixed chimerism can induce permanent acceptance of rat to mouse cardiac xenografts. Transplantation of xenogeneic bone marrow cells under costimulatory blockade at the time of heart transplantation may induce transplantation tolerance.

Anti-CD40L Monoclonal Antibodies Can Replace Anti-CD4 Monoclonal Antibodies for the Nonmyeloablative Induction of Mixed Xenogeneic Chimerism

BACKGROUND

We have previously demonstrated that xenogeneic bone marrow engraftment and donor-specific tolerance can be induced in mice receiving anti-CD4, -CD8, -Thy-1.2, and -NK1.1 monoclonal antibodies (mAbs) on Days -6 and -1, 3 Gy total body irradiation (TBI), and 7 Gy thymic irradiation on Day 0, followed by injection of T-cell depleted (TCD) rat bone marrow cells. We have recently demonstrated that anti-CD40L mAb treatment is sufficient to completely overcome CD4 cell-mediated resistance to allogeneic marrow engraftment and rapidly induce CD4 cell tolerance in an allogeneic combination.

METHODS

We investigated the ability of anti-CD40L mAb to promote mixed xenogeneic chimerism and donor-specific tolerance in B6 mice receiving anti-CD8, -Thy1.2 and -NK1.1 mAbs and 3 Gy TBI followed by TCD bone marrow transplantation (BMT) from F344 rats.

RESULTS

Administration of anti-CD4 mAb in this model could be completely replaced by one injection of anti-CD40L mAb. Evidence for deletional tolerance was obtained in mixed chimeras prepared with this anti-CD40L-based regimen. However, anti-NK1.1 and anti-Thy1.2 mAb could not be replaced by anti-CD40L mAb.

CONCLUSIONS

These results demonstrate that anti-CD40L in combination with xenogeneic BMT can tolerize preexisting peripheral and intrathymic CD4 cells to xenoantigens. However, anti-CD40L does not prevent NK cell and/or gammaDelta cell-mediated rejection of xenogeneic bone marrow.

Anti-mouse CD154 antibody treatment facilitates generation of mixed xenogeneic rat hematopoietic chimerism, prevents wasting disease and prolongs xenograft survival in mice

BACKGROUND

The induction of xenogeneic hematopoietic chimerism is an attractive approach for overcoming the host response to xenografts, but establishing xenogeneic chimerism requires severe myeloablative conditioning of the recipient. The goal of this study was to determine if co-stimulation blockade would facilitate chimerism and xenograft tolerance in irradiation-conditioned concordant recipients.

METHODS

Wistar Furth rat bone marrow (BM) cells were injected into irradiation-conditioned C57BL/6 mice with or without co-administration of anti-mouse CD154 monoclonal antibody (mAb). Chimerism was quantified by flow cytometry, and mice were transplanted with WF rat skin and islet xenografts.

RESULTS

Blockade of CD40-CD154 interaction facilitated establishment of xenogeneic chimerism in mice conditioned with 600 cGy irradiation. Anti-CD154 mAb was not required for establishment of chimerism in mice treated with 700 cGy. However, mice irradiated with 700 cGy but not treated with anti-CD154 mAb developed a "graft-versus-host disease (GVHD)-like" wasting syndrome and died, irrespective of their development of chimerism. Xenogeneic chimeras established with irradiation and anti-CD154 mAb treatment exhibited prolonged skin and, in many cases, permanent islet xenograft survival. Chimerism was unstable and eventually lost in most recipients. Skin xenografts were rejected even in mice that remained chimeric, whereas most islet xenografts survived to the end of the observation period.

CONCLUSIONS

Blockade of host CD40-CD154 interaction facilitates the establishment of xenogeneic chimerism and prevents wasting disease and death. Chimerism permits prolonged xenograft survival, but chimerism generated in this way is unstable over time. Skin xenografts are eventually rejected, whereas most islet xenografts survive long term and perhaps permanently.

Adhesive functions of both chains of VLA-integrins are not fully conserved across the human-porcine species barrier: implications for xenotransplantation

BACKGROUND

A possible solution to the shortage of organs for transplantation would be the use of swine as source animals. As current immunosuppressive protocols cannot prevent rejection of these organs, super-selective immunosuppression or the induction of donor-specific central tolerance represent two promising approaches. Central tolerance induction involves bone marrow transplantation, and depends on intrathymic deletion of donor reactive host cells by donor antigen-presenting cells. In super-selective immunosuppression, the aim would be to block specific adhesive interactions on one species side only, leaving the other species side unaffected. As both processes depend on the interaction of adhesion molecules with their ligands, we investigated whether the beta1-integrins, which play roles in hematopoiesis as well as in rejection, can successfully interact across the swine-to-human species barrier.

METHODS

We employed static cell-to-extracellular protein and cell-to-cell adhesion assays, using different cell types and monoclonal antibody as well as peptide-fragments to analyze conservation of cross-species adhesive interactions.

RESULTS

We found that porcine and human cells interact differently with their cross-species ligands than their own and that the adhesive function of the beta1-chain does not seem to be fully conserved across the species barrier.

CONCLUSIONS

Integrin functions are not fully conserved across the pig-to-human species barrier. While the development of multi-transgenic pigs, whose integrins interact with human ligands in a more ''human-like'' manner may be necessary to facilitate tolerance induction, these facts give rise to new possibilities concerning super-selective immunosuppression.

Roles of Deletion and Regulation in Creating Mixed Chimerism and Allograft Tolerance Using a Nonlymphoablative Irradiation-Free Protocol

The induction of mixed chimerism (MC) is a powerful and effective means to achieve transplantation tolerance in rodent models. Host conditioning with irradiation or cytotoxic drugs has been used in many protocols for chimeric induction across allogeneic barriers. The deletion of alloreactive T cell clones has been described as the main mechanism responsible for the induction of a stable MC. In this study, we demonstrate that a stable MC and skin allograft tolerance can be established across MHC barriers by a noncytotoxic, irradiation-free approach using costimulation blockade plus rapamycin treatment. By using an adoptive transfer model of skin allograft and using specific Vbeta TCR probes, we demonstrated that deletion of donor-reactive cytopathic T cell clones is indeed profound in tolerant hosts. Nonetheless, the challenge of tolerant mixed chimeras with 5 million mononuclear leukocytes (MNL) from naive syngeneic mice was neither able to abolish the stable MC nor to trigger skin allograft rejection, a hallmark of peripheral, not central tolerance. Furthermore, in an adoptive transfer model, MNLs harvested from tolerant hosts significantly inhibited the capacity of naive MNLs to reject same donor, but not third-party, skin allografts. Moreover, when we transplanted skin allografts from stable tolerant chimeras onto syngeneic immune-incompetent mice, graft-infiltrating T cells migrated from the graft site, expanded in the new host, and protected allografts from acute rejection by naive syngeneic MNLs. In this model, both deletional and immunoregulatory mechanisms are active during the induction and/or maintenance of allograft tolerance through creation of MC using a potentially clinically applicable regimen.

Role of VLA-4 and VLA-5 in ex vivo maintenance of human and pig hematopoiesis in human stroma-supported long-term cultures

OBJECTIVE

The advantage of recipient hematopoiesis over that of xenogeneic donors poses a fundamental obstacle to the induction of xenograft tolerance through mixed hematopoietic chimerism. Here we explore the role of beta1 integrins in maintenance of human vs porcine hematopoiesis within a human hematopoietic environment.

METHODS

Porcine and human c-kit+ bone marrow cells were purified and cultured on human bone marrow stroma for 6 weeks. The role of VLA-4 and VLA-5 in the maintenance of porcine vs human hematopoiesis in this human stroma-supported long-term bone marrow culture (LTBMC) system was evaluated by using blocking mAbs that bind to both species.

RESULTS

Blocking VLA-4 with HP2/1 inhibited both human and porcine hematopoiesis, whereas anti-VLA-5 (SAM-1) suppressed the function of human, but not porcine, hematopoietic cells. In mixed LTBMC of porcine and human cells on a human stroma, porcine hematopoietic cells were at a competitive disadvantage, as seen by a rapid decline in cellularity, including clonogenic progenitors. This disadvantage was substantially overcome by the addition of SAM-1. Furthermore, human, but not porcine, cell adhesion to human fibronectin was inhibited by arginine-glycine-aspartic acid (RGD) peptides.

CONCLUSION

Taken together, these results indicate that VLA-4 plays critical role for porcine hematopoiesis in a human hematopoietic environment, and raise the possibility that porcine VLA-5 might be unable to bind the respective human ligand and/or to initiate adequate post-ligand-binding signaling. Thus, VLA-5 may provide a potential target for developing approaches to improve porcine hematopoiesis in human recipients.

Mixed hematopoietic chimerism for the simultaneous induction of T and B cell tolerance

Nonmyeloablative induction of mixed hematopoietic chimerism provides a strategy for inducing T cell tolerance across allogeneic and xenogeneic barriers. We have utilized alpha1-3Gal transferase (GalT) knockout mice, which, like humans, produce anti-Gal natural antibodies, to investigate the ability of mixed chimerism to tolerize B cells producing antibodies of this important specificity, which limits xenotransplantation by causing hyperacute and delayed xenograft rejection. Mixed allogeneic or xenogeneic chimerism indeed tolerizes both preexisting anti-Gal-producing B cells and those developing de novo after establishment of mixed chimerism, even in presensitized mice. We present evidence that different mechanisms are involved in the tolerization of the preexisting and newly-developing antibody-secreting cells.

Induction and Maintenance of T-dependent or T-independent Xenotolerance by Nonprimarily-Vascularized Skin or Thymus Grafts

BACKGROUND

The success of clinical xenotransplantation will depend on induction of xenotolerance. We have previously shown that combined xenothymus and vascularized xenoheart transplantation under the coverage of a tolerizing regimen (TR) can induce and maintain full xenotolerance. Here, induction/maintenance of xenotolerance using nonprimarily-vascularized thymus and/or skin grafts was investigated.

MATERIALS AND METHODS

Hamster skin or thymus or combined skin and thymus transplantation was performed in nude rat recipients with or without administering a TR (NK cell depletion, day -14; xenoantigen infusion, day -14; Leflunomide, day -14 through +14). Xenotolerance was confirmed by subsequent transplantation of a vascularized hamster heart, measurement of xenoantibody formation, or mixed lymphocyte reaction (MLR).

RESULTS

Skin grafts were as effective as vascularized heart grafts to induce/maintain T-independent xenotolerance. Even without TR and despite being rejected themselves, xenoskin grafts lead to progressively developing xenononreactivity. Xenothymus transplantation induced xenotolerance in the T-dependent but not in the T-independent immune compartment, leading to rejection of subsequently transplanted hamster hearts by T-independent mechanisms (production of IgM but not IgG xenoantibodies (Xabs), presence of antihamster MLR nonresponsiveness). Combined skin and thymus xenotransplantation sensitized the T-cell compartment, leading to hyperacute rejection of subsequently transplanted hamster hearts. This was not the case when the skin grafts were transplanted late (2 months) after the thymus grafts.

CONCLUSIONS

Xenogeneic skin and xenogeneic thymus grafts have opposite xenotolerance inducing capacities in the T-independent as compared to the T-dependent immune compartment. Thymus grafts induce and maintain T-dependent but not T-independent xenotolerance. Skin grafts alone induce T-independent xenotolerance but sensitize the T-cell compartment when transplanted concomitantly with thymus grafts.

Bone marrow transplantation from alpha1,3-galactosyltransferase gene-knockout pigs in baboons

BACKGROUND

Successful hematopoietic cell allotransplantation results in donor-specific tolerance, but this approach has been unsuccessful in the wild-type pig-to-baboon xenotransplantation model, as pig cells were lost from the circulation within 5 days. However, after cessation of immunosuppressive therapy on day 28, all baboons demonstrated non-specific unresponsiveness on mixed leukocyte reaction (MLR) for at least 30 days. We have now investigated the transplantation of bone marrow (BM) cells from miniature swine homozygous for alpha1,3-galactosyltransferase gene-knockout (GalT-KO).

METHODS

Baboons (n = 3) were pre-treated with whole body and thymic irradiation, anti-thymocyte globulin, and splenectomy, and received immunosuppressive and supportive therapy for 28 days. BM was harvested from GalT-KO swine (n = 3). The baboons were monitored for the presence of pig cells by flow cytometry and colony-forming units (CFUs), and for cellular reactivity by MLR.

RESULTS

A mean of 11 x 10(8) BM cells/kg was infused into each baboon. The mean absolute numbers and percentages of pig cells detected in the blood at 2 h and on days 1, 2 and 4, respectively, were 641/microl (9.5%), 132/microl (3.4%), 242/microl (3.9%), and 156/microl (2.9%). One baboon died (from accidental hemorrhage) on day 6, at which time chimerism was present in the blood (2.0%) and BM (6.4%); pig cell engraftment in the BM was confirmed by polymerase chain reaction (PCR) of CFUs. In the two other baboons, blood chimerism was lost after day 5 but returned at low levels (<1%) between days 9 to 16 and 7 to 17, respectively, indicating transient BM engraftment. Both surviving baboons showed non-specific unresponsiveness on MLR until they were euthanized on days 85 and 110, respectively.

CONCLUSIONS

By using BM cells from GalT-KO pigs, chimerism was detected at levels comparable with previous studies when 30-fold more growth factor-mobilized peripheral blood progenitor cells had been transplanted. In addition, cellular hyporesponsiveness was prolonged. However, long-term engraftment and chimerism were not achieved.

Porcine Hematopoietic Progenitor Cell Transplantation in Nonhuman Primates: A Review of Progress

The critical shortage of human donor organs for transplantation would be overcome if a suitable animal, e.g., the pig, could be used as an organ source. There are, however, several immune barriers that have to date resulted in limited function of pig organs transplanted into nonhuman primates. It would be beneficial, and indeed may be essential, to induce a state of tolerance in the primate recipient to the pig organ. In allotransplantation, the successful transplantation of hematopoietic progenitor cells with the development of mixed chimerism is associated with the induction of tolerance toward a donor-specific organ. For some years, this approach has been explored in the pig-to-nonhuman primate model. This experience is briefly reviewed. The problems of natural and elicited anti-pig antibodies, recipient platelet adhesion to pig hematopietic progenitor cells, and the rapid removal of these cells by the host macrophage-phagocytic system are highlighted. Recent experience with the use of hematopoietic cells from pigs homozygous for alpha1,3-galactosyltransferase gene-knockout is reported.

Baboon bone-marrow xenotransplant in a patient with advanced HIV disease: case report and 8-year follow-up

BACKGROUND

Xenotransplantation offers a solution to the shortage of organ donors and may offer resistance to human-specific pathogens. Baboons are resistant to productive infection with HIV-1. A baboon bone-marrow transplant (BMT) was performed in an attempt to reconstitute the immune system of a patient with advanced AIDS. The aims of this pilot study were to evaluate the safety of the procedure and develop an approach to prevent and monitor for xenozoonoses.

METHODS

A source animal was selected on the basis of infectious disease surveillance protocols. Baboon bone marrow, engineered to remove graft-versus-host-disease-producing mature lineages, but to retain hematopoietic stem cells and facilitating cells, was infused into the patient after nonmyeloablative conditioning. Serial clinical, virologic, immunologic, and hematologic evaluations were performed.

RESULTS

A 38-year-old male with advanced AIDS, who had failed to respond to triple-drug antiretroviral therapy, underwent baboon BMT in 1995. The patient tolerated the procedure without complication. Baboon cells were detected in the peripheral blood on days 5 and 13 after transplantation. Baboon endogenous virus (BaEV) was detected on day 5 but not subsequently. Antibody to BaEV was not detected. HIV-1 viral load declined 1.5 log and remained low until 11 months. The patient improved clinically, and no adverse events occurred. The patient is alive 8 years after the procedure.

CONCLUSIONS

Baboon BMT to treat AIDS was attempted using nonmyeloablative conditioning and resulted in transient microchimerism and clinical and virologic improvements. Long-term improvement was not achieved; however, no adverse events occurred, and no evidence of transmission of xenogeneic infections was found.

T Cells from Presensitized Donors Fail to Cause Graft-versus-Host Disease in a Pig-to-Mouse Xenotransplantation Model

BACKGROUND

The ability of T cells from pigs, the most suitable donors for clinical xenotransplantation, to induce graft-versus-host disease (GVHD) and to facilitate hematopoietic cell engraftment in highly disparate xenogeneic recipients remains unclear. In this article, the authors address these questions in a presensitized pig-to-mouse transplantation model using porcine cytokine-transgenic mice.

METHODS

Swine donors were presensitized by mouse skin grafting and boosted by the injection of mouse cells after the skin graft was rejected. Porcine peripheral blood mononuclear cells (PBMC) and splenocytes were collected at various times after mouse skin grafting, and their potential to induce GVHD and to facilitate donor hematopoietic cell engraftment in conditioned murine recipients was evaluated.

RESULTS

Presensitization of donor pigs resulted in marked enhancement of anti-mouse responses, as reflected in augmented anti-mouse mixed lymphocyte responses, cell-mediated cytotoxicity, and antibody production. However, injection of high numbers of PBMC and splenocytes (>1 x 10(8)) with bone marrow cells from the presensitized pigs failed to induce detectable GVHD or long-term chimerism in mice that were treated with depleting anti-T-cell and natural killer cell antibodies, cobra venom factor, medronate-liposomes, and 4 Gy of whole-body and 7 Gy of thymic irradiation. Histologic analysis revealed no mononuclear cell infiltration or GVHD-associated lesions in the liver, intestine, lungs, or skin of the mouse recipients. Furthermore, the recipient mice had no detectable T cells or anti-pig immunoglobulin G antibodies in the blood by 6 weeks after injection of porcine cells.

CONCLUSION

These results demonstrate that porcine T-cell function is severely impaired in the xenogeneic murine microenvironment.

The balance of deletion and regulation in allograft tolerance

Although the precise mechanisms involved in the establishment and maintenance of peripheral allograft tolerance are still not fully understood, it is now clear that acquisition of transplantation tolerance is an active, highly regulated, multistep process. According to the pool size model of allograft tolerance, the allograft outcome, rejection, or tolerance, often depends on the balance between cytopathic and regulatory T cells (Tregs). Although both deletion and regulation play important roles in allograft tolerance, our recent studies showed that the quantitative details for each mechanism differ from model to model. Therefore, we hypothesize that there is a delicate balance between deletion and regulation in allograft tolerance. In a model of allograft tolerance in which the deletional mechanism plays a dominant role, e.g. tolerance produced via creation of mixed chimeras, the regulatory mechanism, albeit sometimes present, is far less important. Whilst in a model in which the regulation mechanism plays a critical role, e.g. donor-specific transfusion plus MR1-induced allograft tolerance, a deletional mechanism lowers the threshold for effective Treg action.

The Rat RT1-A1cMHC Molecule Is a Xenogeneic Ligand Recognized by the Mouse Activating Ly-49W and Inhibitory Ly-49G Receptors

Mouse Ly-49 receptors are known to recognize xenogeneic ligands from hamster and rat. However, until now, there has been no description of a specific rat xenogeneic ligand for any mouse Ly-49 receptor. In this report, we identify RT1-A1c, a rat classical class I MHC molecule, as a ligand for the Ly-49G(BALB/c) inhibitory receptor and the closely related activating receptor, Ly-49W. Xenogeneic class I recognition of targets from PVG but not DA strain rats was mapped to the classical region of the RT1c haplotype by using Con A blasts from RT1c/RT1av1 intra-MHC recombinant rats as targets for RNK-16 cells expressing either Ly-49W or Ly-49G(BALB/c) receptors. Individual expression of class I molecules from PVG and DA rat strains in YB2/0 target cells demonstrate the xenogeneic recognition to be allele specific, because other class I molecules of the RT1c haplotype, RT1-A2c and RT1-U2c, and a classical class I molecule encoded by the RT1av1 haplotype, RT1-Aa, are not recognized by Ly-49W and -G(BALB/c). Furthermore, specificity for RT1-Ac can be transferred from Ly-49W to Ly-49P, which is normally unable to recognize RT1-Ac, by substitution of three residues shared by Ly-49W and -G(BALB/c) but not Ly-49P. These residues are located in the Ly-49 beta4-beta5 loop, which can determine class I allele specificity in mouse Ly-49 receptor interactions with mouse class I ligands, suggesting that mouse Ly-49 recognition of rat class I molecules follows similar principles of interaction. These findings have implications for xenotransplantation studies and for discerning Ly-49 recognition motifs present in MHC molecules.