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

Suppression of macrophage-mediated xenogeneic rejection by the ectopic expression of human CD177

Cellular xenogeneic rejection by the innate immune system is a major immunological obstruction that needs to be overcome for the successful clinical use of xenografts. Our focus has been on macrophage-mediated xenogeneic rejection, since suppressing macrophage function has considerable potential for practical applications in the area of xenotransplantation. We report herein on an investigation of the suppressive effect of human CD177 (hCD177) against macrophage-mediated xenogeneic rejection. Wild type swine aortic endothelial cell (SEC) and an SEC transfectant with hCD177 (SEC/hCD177) were co-cultured with macrophages, and the degree of cytotoxicity was evaluated by WST-8 assays, and phagocytosis was examined using Calcein-AM labeling methods. The expression of anti/pro-inflammatory cytokines was evaluated by RT-qPCR and the phosphorylation of SHP-1 on macrophages in co-culture was evaluated by Western blotting. The result of cytotoxicity assays indicated that hCD177 suppressed M1 macrophage-mediated xenogeneic rejection (vs. SEC, p < 0.0001). Similarly, the result of phagocytosis assays indicated that hCD177 suppressed it (vs. SEC, p < 0.05). In addition, hCD177 significantly suppressed the expression of IL-1β, a pro-inflammatory cytokine, in M1 macrophages (vs. SEC, p < 0.01). Luciferase assays using THP1-Lucia NF-kB also showed a significant difference in NF-kB activation (vs. SEC, p < 0.001). In addition, hCD177 was found to induce the phosphorylation of SHP-1 in M1 macrophages (vs. SEC, p < 0.05). These findings indicate that hCD177 suppresses M1 macrophage-mediated xenogeneic rejection, at least in part via in the phosphorylation of SHP-1.

Genetically-engineered pigs as sources for clinical red blood cell transfusion: What pathobiological barriers need to be overcome?

An alternative to human red blood cells (RBCs) for clinical transfusion would be advantageous, particularly in situations of massive acute blood loss (where availability and compatibility are limited) or chronic hematologic diseases requiring frequent transfusions (resulting in alloimmunization). Ideally, any alternative must be neither immunogenic nor pathogenic, but readily available, inexpensive, and physiologically effective. Pig RBCs (pRBCs) provide a promising alternative due to their several similarities with human RBCs, and our increasing ability to genetically-modify pigs to reduce cellular immunogenicity. We briefly summarize the history of xenotransfusion, the progress that has been made in recent years, and the remaining barriers. These barriers include prevention of (i) human natural antibody binding to pRBCs, (ii) their phagocytosis by macrophages, and (iii) the T cell adaptive immune response (in the absence of exogenous immunosuppressive therapy). Although techniques of genetic engineering have advanced in recent years, novel methods to introduce human transgenes into pRBCs (which do not have nuclei) will need to be developed before clinical trials can be initiated.

An approach to induction of tolerance to pig cardiac xenografts in neonates

There is a continuing need for donor hearts for infants with complex congenital heart defects. The transplantation of hearts from neonatal pigs would be an alternative to human organs, particularly if donor-specific immunological tolerance could be achieved. The great majority of infant humans do not make natural (preformed) antibodies against triple-knockout (TKO) pigs (that do not express any of the three known pig antigens against which humans have natural anti-pig antibodies). The transplantation of a heart from a TKO pig into an infant would therefore minimize any risk of early antibody-mediated rejection, and, with adequate immunosuppressive therapy, prolonged graft survival may well be achieved. Total host thymectomy (commonly carried out at the time of orthotopic heart transplantation in this age group) ± residual T-cell depletion and donor-specific pig thymus tissue transplantation might induce T-cell tolerance and allow immunosuppressive therapy to be discontinued (if there is in vitro evidence of T-cell and B-cell nonresponsiveness to donor-specific pig cells). Even if tolerance were not achieved, with continuing immunosuppressive therapy, the graft would likely "bridge" the patient until a suitable allograft became available or be associated with prolonged xenograft function.

Cell Viability and Noninvasive In Vivo MRI Tracking of 3D Cell Encapsulating Self-Assembled Microcontainers

Several molecular therapies require the implantation of cells that secrete biotherapeutic molecules and imaging the location and microenvironment of the cellular implant to ascertain its function. We demonstrate noninvasive in vivo magnetic resonance imaging (MRI) of self-assembled microcontainers that are capable of cell encapsulation. Negative contrast was obtained to discern the microcontainer with MRI; positive contrast was obtained in the complete absence of background signal. MRI on a clinical scanner highlights the translational nature of this research. The microcontainers were loaded with cells that were dispersed in an extracellular matrix, and implanted both subcutaneously and in human tumor xenografts in SCID mice. MRI was performed on the implants, and microcontainers retrieved postimplantation showed cell viability both within and proximal to the implant. The microcontainers are characterized by their small size, three dimensionality, controlled porosity, ease of parallel fabrication, chemical and mechanical stability, and noninvasive traceability in vivo.

Clinical xenotransplantation, a closer reality: Literature review

Xenotransplantation could provide an unlimited supply of organs and solve the current shortage of organs for transplantation. To become a reality in clinical practice, the immunological and physiological barriers and the risk of xenozoonosis that they possess should be resolved. From the immunological point of view, in the last 30 years a significant progress in the production of transgenic pigs has prevented the hyperacute rejection. About xenozoonosis, attention has been focused on the risk of transmission of porcine endogenous retroviruses; however, today, it is considered that the risk is very low and the inevitable transmission should not prevent the clinical xenotransplantation. Regarding the physiological barriers, encouraging results have been obtained and it's expected that the barriers that still need to be corrected can be solved in the future through genetic modifications.

Modifying the sugar icing on the transplantation cake

As a transplant surgeon, my interest in glycobiology began through my research into ABO-incompatible allotransplantation, and grew when my goal became overcoming the shortage of organs from deceased human donors by the transplantation of pig organs into patients with terminal organ failure (xenotransplantation/cross-species transplantation). The major target for human "natural" (preformed) anti-pig antibodies is galactose-α(1,3)-galactose (the "Gal" epitope), which is expressed on many pig cells, including the vascular endothelium. The binding of human IgM and IgG antibodies to Gal antigens initiates the process of hyperacute rejection, resulting in destruction of the pig graft within minutes or hours. This major barrier has been overcome by the production of pigs in which the gene for the enzyme α(1,3)-galactosyltransferase (GT) has been deleted by genetic engineering, resulting in GT knockout (GTKO) pigs. The two other known carbohydrate antigenic targets on pig cells for human anti-pig antibodies are (i) the product of the cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) gene, i.e., N-glycolylneuraminic acid, and (ii) the product of the β1,4 N-acetylgalactosaminyltransferase gene, i.e., the Sd(a) antigen. Expression of these two has also been deleted in pigs. These genetic manipulations, together with others directed to overcoming primate complement and coagulation activation (the latter of which also relates to glycobiology) have contributed to the prolongation of pig graft survival in nonhuman primate recipients to many months rather than a few minutes. Clinical trials of the transplantation of pig cells are already underway and transplantation of pig organs may be expected within the relatively near future.

The pathobiology of pig-to-primate xenotransplantation: a historical review

The immunologic barriers to successful xenotransplantation are related to the presence of natural anti-pig antibodies in humans and non-human primates that bind to antigens expressed on the transplanted pig organ (the most important of which is galactose-α1,3-galactose [Gal]), and activate the complement cascade, which results in rapid destruction of the graft, a process known as hyperacute rejection. High levels of elicited anti-pig IgG may develop if the adaptive immune response is not prevented by adequate immunosuppressive therapy, resulting in activation and injury of the vascular endothelium. The transplantation of organs and cells from pigs that do not express the important Gal antigen (α1,3-galactosyltransferase gene-knockout [GTKO] pigs) and express one or more human complement-regulatory proteins (hCRP, e.g., CD46, CD55), when combined with an effective costimulation blockade-based immunosuppressive regimen, prevents early antibody-mediated and cellular rejection. However, low levels of anti-non-Gal antibody and innate immune cells and/or platelets may initiate the development of a thrombotic microangiopathy in the graft that may be associated with a consumptive coagulopathy in the recipient. This pathogenic process is accentuated by the dysregulation of the coagulation-anticoagulation systems between pigs and primates. The expression in GTKO/hCRP pigs of a human coagulation-regulatory protein, for example, thrombomodulin, is increasingly being associated with prolonged pig graft survival in non-human primates. Initial clinical trials of islet and corneal xenotransplantation are already underway, and trials of pig kidney or heart transplantation are anticipated within the next few years.

Immunobiological barriers to xenotransplantation

Binding of natural anti-pig antibodies in humans and nonhuman primates to carbohydrate antigens expressed on the transplanted pig organ, the most important of which is galactose-α1,3-galactose (Gal), activate the complement cascade, which results in destruction of the graft within minutes or hours, known as hyperacute rejection. Even if antibody is removed from the recipient's blood by plasmapheresis, recovery of antibody is associated with acute humoral xenograft rejection. If immunosuppressive therapy is inadequate, the development of high levels of T cell-dependent elicited anti-pig IgG similarly results in graft destruction, though classical acute cellular rejection is rarely seen. Vascular endothelial activation by low levels of anti-nonGal antibody, coupled with dysregulation of the coagulation-anticoagulation systems between pigs and primates, leads to a thrombotic microangiopathy in the graft that may be associated with a consumptive coagulopathy in the recipient. The most successful approach to overcoming these barriers is by genetically-engineering the pig to provide it with resistance to the human humoral and cellular immune responses and to correct the coagulation discrepancies between the two species. Organs and cells from pigs that (i) do not express the important Gal antigen, (ii) express a human complement-regulatory protein, and (iii) express a human coagulation-regulatory protein, when combined with an effective immunosuppressive regimen, have been associated with prolonged pig graft survival in nonhuman primates.

Differentiation ability of multipotent hematopoietic stem/progenitor cells detected by a porcine specific anti-CD117 monoclonal antibody

CD117 is a cytokine receptor expressed on the surface of hematopoietic stem cells with a likely role in cell survival, proliferation and differentiation. In order to study the differentiation activity of porcine CD117 hematopoietic cells in vitro and in vivo we prepared an anti-swine CD117 Mab (2A1) with high specificity for flow-cytometrical analysis. The 2A1 Mab did not recognize mouse or human mast cells suggesting that 2A1 is species-specific. Swine bone marrow (BM) CD117+ cells differentiated in vitro mainly into erythroid and monocyte lineages in the methylcellulose-based colony assay. When the swine BM CD117+ cells were transplanted in vivo into immunodeficient NOG (NOD/SCID/IL-2gc-null) mice, a significant amount of swine CD45+ leukocytes, including CD3 positive T cells, were developed in the mice. These results revealed that the swine BM CD117+ cells possess hematopoietic stem/progenitor activity and when monitored in immunodeficient mice or in vitro they can develop into lymphoid, erythroid, and myeloid cells efficiently with the new monoclonal antibody.

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.

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.

Immune modulation in xenotransplantation

The use of animals as donors of tissues and organs for xenotransplantations may help in meeting the increasing demand for organs for human transplantations. Clinical studies indicate that the domestic pig best satisfies the criteria of organ suitability for xenotransplantation. However, the considerable phylogenetic distance between humans and the pig causes tremendous immunological problems after transplantation, thus genetic modifications need to be introduced to the porcine genome, with the aim of reducing xenotransplant immunogenicity. Advances in genetic engineering have facilitated the incorporation of human genes regulating the complement into the porcine genome, knockout of the gene encoding the formation of the Gal antigen (α1,3-galactosyltransferase) or modification of surface proteins in donor cells. The next step is two-fold. Firstly, to inhibit processes of cell-mediated xenograft rejection, involving natural killer cells and macrophages. Secondly, to inhibit rejection caused by the incompatibility of proteins participating in the regulation of the coagulation system, which leads to a disruption of the equilibrium in pro- and anti-coagulant activity. Only a simultaneous incorporation of several gene constructs will make it possible to produce multitransgenic animals whose organs, when transplanted to human recipients, would be resistant to hyperacute and delayed xenograft rejection.

New concepts of immune modulation in xenotransplantation

The shortage of human organs for transplantation has focused research on the possibility of transplanting pig organs into humans. Many factors contribute to the failure of a pig organ graft in a primate. A rapid innate immune response (natural anti-pig antibody, complement activation, and an innate cellular response; e.g., neutrophils, monocytes, macrophages, and natural killer cells) is followed by an adaptive immune response, although T-cell infiltration of the graft has rarely been reported. Other factors (e.g., coagulation dysregulation and inflammation) appear to play a significantly greater role than in allotransplantation. The immune responses to a pig xenograft cannot therefore be controlled simply by suppression of T-cell activity. Before xenotransplantation can be introduced successfully into the clinic, the problems of the innate, coagulopathic, and inflammatory responses will have to be overcome, most likely by the transplantation of organs from genetically engineered pigs. Many of the genetic manipulations aimed at protecting against these responses also reduce the adaptive response. The T-cell and elicited antibody responses can be prevented by the biological and/or pharmacologic agents currently available, in particular, by costimulation blockade-based regimens. The exogenous immunosuppressive regimen may be significantly reduced by the presence of a graft from a pig transgenic for a mutant (human) class II transactivator gene, resulting in down-regulation of swine leukocyte antigen class II expression, or from a pig with "local" vascular endothelial cell expression of an immunosuppressive gene (e.g., CTLA4-Ig). The immunomodulatory efficacy of regulatory T cells or mesenchymal stromal cells has been demonstrated in vitro but not yet in vivo.

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.

Reciprocal interaction between intestinal microbiota and mucosal lymphocyte in cynomolgus monkeys after alemtuzumab treatment

It has been known that the gut microbiota plays a central role in shaping normal mucosal immunity, however, little information is available whether the variability of mucosal lymphocytes impacts the commensal flora. Here, we applied a cynomolgus monkey model to characterize the structure and composition of the gut microbiota in response to lymphocyte depletion and to determine their potential association. Molecular profiling of 16S rDNA showed that the intestinal microbiota composition was perturbed after the depletion of mucosal lymphocytes and were recovered following the repopulation. Some specific bacteria from the orders Lactobacillales, Enterobacteriales and Clostridiales, and the genus Prevotella and Faecalibacterium, were primarily responsible for the variations of the gut microbiota after lymphocyte depletion. Interestingly, the species richness of the ileal mucosal microbiota was associated the proportions of TCRαβ+ or TCRγδ+ T cells (p<0.01). We demonstrate for the first time the feature of intestinal microbiota composition after lymphocyte depletion and provide novel evidence that the perturbation of gut microbiota is associated with lymphocyte depletion. It may contribute to understand the relationship between gut commensal microbiota and mucosal immune system. Study results provide insight into biological activity of alemtuzumab in intestinal barrier in organ transplantation.

Clinical xenotransplantation: the next medical revolution?

The shortage of organs and cells from deceased individuals continues to restrict allotransplantation. Pigs could provide an alternative source of tissue and cells but the immunological challenges and other barriers associated with xenotransplantation need to be overcome. Transplantation of organs from genetically modified pigs into non-human primates is now not substantially limited by hyperacute, acute antibody-mediated, or cellular rejection, but other issues have become more prominent, such as development of thrombotic microangiopathy in the graft or systemic consumptive coagulopathy in the recipient. To address these problems, pigs that express one or more human thromboregulatory or anti-inflammatory genes are being developed. The results of preclinical transplantation of pig cells--eg, islets, neuronal cells, hepatocytes, or corneas--are much more encouraging than they are for organ transplantation, with survival times greater than 1 year in all cases. Risk of transfer of an infectious microorganism to the recipient is small.

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.

Lymphocyte depletion and repopulation in peripheral blood and small intestine of cynomolgus monkeys after alemtuzumab treatment

BACKGROUND

Alemtuzumab has been used as an induction agent in organ transplantation over 10 years, but the effect of alemtuzumab on lymphocytes in small intestine was not clear. We investigate lymphocyte depletion and repopulation phenomena both in peripheral blood and small intestine of cynomolgus monkeys, to assess the model using in preclinical transplantation.

MATERIALS AND METHODS

Monkeys without CD52 antigen on erythrocytes were selected. Lymphocyte depletion and repopulation was documented by flow cytometry. Sections of ileum were obtained for isolation of intestinal intraepithelial lymphocytes (IEL) and lamina propria lymphocytes (LPL), and also for immunofluorescence examination.

RESULTS

Powerful depletion of lymphocytes (>80%) from blood followed by gradual repopulation was observed. CD20(+) B cells, CD8(+) T cells, CD4(+) T cells returned to pretreatment levels by d 21, 35, 56. IEL, LPL reduced by 70%, 72% on d 9, recovered to 59%, 57% of pretreatment levels by d 35, and were completed by d 56. Depletion and repopulation of IEL and LPL were confirmed by immunofluorescence.

CONCLUSIONS

Depletion of lymphocytes in peripheral blood was less powerful and repopulation occurred faster than in patients. The lymphocyte depletion and repopulation occurred in small intestine. This model can be used in preclinical transplantation.

Overcoming the barriers to xenotransplantation: prospects for the future

Cross-species transplantation (xenotransplantation) has immense potential to solve the critical need for organs, tissues and cells for clinical transplantation. The increasing availability of genetically engineered pigs is enabling progress to be made in pig-to-nonhuman primate experimental models. Potent pharmacologic immunosuppressive regimens have largely prevented T-cell rejection and a T-cell-dependent elicited antibody response. However, coagulation dysfunction between the pig and primate is proving to be a major problem, and this can result in life-threatening consumptive coagulopathy. This complication is unlikely to be overcome until pigs expressing a human 'antithrombotic' or 'anticoagulant' gene, such as thrombomodulin, tissue factor pathway inhibitor or CD39, become available. Progress in islet xenotransplantation has been more encouraging, and diabetes has been controlled in nonhuman primates for periods in excess of 6 months, although this has usually been achieved using immunosuppressive protocols that might not be clinically applicable. Further advances are required to overcome the remaining barriers.

Current status of xenotransplantation and prospects for clinical application

Xenotransplantation is one promising approach to bridge the gap between available human cells, tissues, and organs and the needs of patients with diabetes or end-stage organ failure. Based on recent progress using genetically modified source pigs, improving results with conventional and experimental immunosuppression, and expanded understanding of residual physiologic hurdles, xenotransplantation appears likely to be evaluated in clinical trials in the near future for some select applications. This review offers a comprehensive overview of known mechanisms of xenograft injury, a contemporary assessment of preclinical progress and residual barriers, and our opinions regarding where breakthroughs are likely to occur.

Molecular genetics of the swine major histocompatibility complex, the SLA complex

The swine major histocompatibility complex (MHC) or swine leukocyte antigen (SLA) complex is one of the most gene-dense regions in the swine genome. It consists of three major gene clusters, the SLA class I, class III and class II regions, that span approximately 1.1, 0.7 and 0.5Mb, respectively, making the swine MHC the smallest among mammalian MHC so far examined and the only one known to span the centromere. This review summarizes recent updates to the Immuno Polymorphism Database-MHC (IPD-MHC) website (http://www.ebi.ac.uk/ipd/mhc/sla/) which serves as the repository for maintaining a list of all SLA recognized genes and their allelic sequences. It reviews the expression of SLA proteins on cell subsets and their role in antigen presentation and regulating immune responses. It concludes by discussing the role of SLA genes in swine models of transplantation, xenotransplantation, cancer and allergy and in swine production traits and responses to infectious disease and vaccines.

Anatomical particularities of the porcine immune system--a physician's view

In this article the anatomical structure of the porcine immune organs is described. The focus is on their particularities that are related to the use of pigs as an animal model. Key issues of the intrauterine development of the lymphoid organs are presented, such as the specific epithelio-chorial placenta, the appearance of the thymic tissue and the initial development of B cells. The role of the thymus for the development of alpha/beta and gamma/delta T cells and the location of tonsillar tissue in the naso-pharynx, in the oral cavity and at the basis of the tongue are described. The porcine spleen is of interest for surgical techniques to treat splenic trauma adequately. The observation of the inverted lymph node structure of pigs is puzzling and it remains unclear why only few species have this distinct morphological organisation. Based on the functional differences in lymphocyte recirculation observed in pigs, specific lymph cannulation experiments are possible in the porcine immune system. The porcine intestinal lymphoid tissue and the lymphocytes in the mucosal epithelium and lamina propria are of interest for studying the gut immune responses. For use as a model the fact that the pig is a monogastric omnivorous animal represents an advantage, although the porcine ileal Peyer's patch has no obvious anatomical equivalent in man. Based on the detailed knowledge of porcine immune morphology the pig is suitable as model animal for immunology--in addition to the various experimental approaches in physiology, pharmacology, surgery, etc. that are applicable to human medicine.

The role of macrophages in xenograft rejection

Safe and effective xenotransplantation would provide a valuable answer to many of the limitations of allogenic transplantation. Such limitations include scarcity of organ supply and morbidity to donors in cases of living-related donor transplantation. The main hurdle to the efficacious application of xenotransplantation in clinical medicine is the fierce host immune response to xenografts. This immune response is embodied in 3 different types of xenograft rejection. Both hyperacute rejection and delayed xenograft rejection are mediated by natural antibodies and are concerned primarily with whole organ rejection. Cellular xenograft rejection (CXR), on the other hand, is concerned with both whole organ and CXR and is mediated by innate immunity rather than natural antibodies. Macrophages, which are cells of the innate immune system, play a role in all 3 types of xenograft rejection (not just CXR). They impart their effects both directly and through T-cell activation.

Rejection of cardiac xenografts transplanted from alpha1,3-galactosyltransferase gene-knockout (GalT-KO) pigs to baboons

The use of alpha1,3-galactosyltransferase gene-knockout (GalT-KO) swine donors in discordant xenotransplantation has extended the survival of cardiac xenografts in baboons following transplantation. Eight baboons received heterotopic cardiac xenografts from GalT-KO swine and were treated with a chronic immunosuppressive regimen. The pathologic features of acute humoral xenograft rejection (AHXR), acute cellular xenograft rejection (ACXR) and chronic rejection were assessed in the grafts. No hyperacute rejection developed and one graft survived up to 6 months after transplantation. However, all GalT-KO heart grafts underwent graft failure with AHXR, ACXR and/or chronic rejection. AHXR was characterized by interstitial hemorrhage and multiple thrombi in vessels of various sizes. ACXR was characterized by TUNEL(+) graft cell injury with the infiltration of T cells (including CD3 and TIA-1(+) cytotoxic T cells), CD4(+) cells, CD8(+) cells, macrophages and a small number of B and NK cells. Chronic xenograft vasculopathy, a manifestation of chronic rejection, was characterized by arterial intimal thickening with TUNEL(+) dead cells, antibody and complement deposition, and/or cytotoxic T-cell infiltration. In conclusion, despite the absence of the Gal epitope, acute and chronic antibody and cell-mediated rejection developed in grafts, maintained by chronic immunosupression, presumably due to de novo responses to non-Gal antigens.

Xenotransplantation: where are we in 2008?

Xenotransplantation holds promise to solve the ever increasing shortage of donor organs for allotransplantation. In the last 2 decades, major progress has been made in understanding the immunobiology of pig-into-(non)human primate transplantation and today we are on the threshold of the first clinical trials. Hyperacute rejection, which is mediated by pre-existing anti-alpha Gal xenoreactive antibodies, can in non-human primates be overcome by complement- and/or antibody-modifying interventions. A major step forward was the development of genetically engineered pigs, either transgenic for human complement regulatory proteins or deficient in the alpha1,3-galactosyltranferase enzyme. However, several other immunologic and nonimmunologic hurdles remain. Acute vascular xenograft rejection is mediated by humoral and cellular mechanisms. Elicited xenoreactive antibodies play a key role. In addition to providing B cell help, xenoreactive T cells may directly contribute to xenograft rejection. Long-term survival of porcine kidney- and heart xenografts in non-human primates has been obtained but required severe T and B cell immunosuppression. Induction of xenotolerance, e.g. through mixed hematopoietic chimerism, may represent the preferred approach, but although proof of principle has been delivered in rodents, induction of pig-to-non-human primate chimerism remains problematic. Finally, it is now clear that innate immune cells, in particular macrophages and natural killer cells, can mediate xenograft destruction, the determinants of which are being elucidated. Chronic xenograft rejection is not well understood, but recent studies indicate that non-immunological problems, such as incompatibilities between human procoagulant and pig anticoagulant components may play an important role. Here, we give a comprehensive overview of the currently known obstacles to xenografting: immune and non-immune problems are discussed, as well as the possible strategies that are under development to overcome these hurdles.

Frankenswine, or bringing home the bacon: How close are we to clinical trials in xenotransplantation?

Xenotransplantation-specifically from pig into human-could resolve the critical shortage of organs, tissues and cells for clinical transplantation. Genetic engineering techniques in pigs are relatively well-developed and to date have largely been aimed at producing pigs that either (1) express high levels of one or more human complement-regulatory protein(s), such as decay-accelerating factor or membrane cofactor protein, or (2) have deletion of the gene responsible for the expression of the oligosaccharide, Galalpha1,3Gal (Gal), the major target for human anti-pig antibodies, or (3) have both manipulations. Currently the transplantation of pig organs in adequately-immunosuppressed baboons results in graft function for periods of 2-6 months (auxiliary hearts) and 2-3 months (life-supporting kidneys). Pig islets have maintained normoglycemia in diabetic monkeys for >6 months. The remaining immunologic barriers to successful xenotransplantation are discussed, and brief reviews made of (1) the potential risk of the transmission of an infectious microorganism from pig to patient and possibly to the public at large, (2) the potential physiologic incompatibilities between a pig organ and its human counterpart, (3) the major ethical considerations of clinical xenotransplantation, and (4) the possible alternatives that compete with xenotransplantation in the field of organ or cell replacement, such as mechanical devices, tissue engineering, stem cell biology and organogenesis. Finally, the proximity of clinical trials is discussed. Islet xenotransplantation is already at the stage where clinical trials are actively being considered, but the transplantation of pig organs will probably require further genetic modifications to be made to the organ-source pigs to protect their tissues from the coagulation/anticoagulation dysfunction that plays a significant role in pig graft failure after transplantation in primates.

Rapidly induced, T-cell–independent xenoantibody production is mediated by marginal zone B cells and requires help from NK cells

Xenoantibody production directed at a wide variety of T lymphocyte–dependent and T lymphocyte–independent xenoantigens remains the major immunologic obstacle for successful xenotransplantation. The B lymphocyte subpopulations and their helper factors, involved in T-cell–independent xenoantibody production are only partially understood, and their identification will contribute to the clinical applicability of xenotransplantation. Here we show, using models involving T-cell–deficient athymic recipient mice, that rapidly induced, T-cell–independent xenoantibody production is mediated by marginal zone B lymphocytes and requires help from natural killer (NK) cells. This collaboration neither required NK-cell–mediated IFN-γ production, nor NK-cell–mediated cytolytic killing of xenogeneic target cells. The T-cell–independent IgM xenoantibody response could be partially suppressed by CD40L blockade.

Alpha1,3-galactosyltransferase gene-knockout pigs for xenotransplantation: where do we go from here?

The ability to genetically engineer pigs that no longer express the Galalpha1,3Gal (Gal) oligosaccharide has been a significant step toward the clinical applicability of xenotransplantation. Using a chronic immunosuppressive regimen based on costimulatory blockade, hearts from these pigs have survived from 2 to 6 months in baboons. Graft failure was predominantly from the development of a thrombotic microangiopathy. Potential contributing factors include the presence of preformed anti-nonGal antibodies or the development of low levels of elicited antibodies to nonGal antigens, natural killer (NK) cell or macrophage activity, and inherent coagulation dysregulation between pigs and primates. The breeding of pigs transgenic for an "anticoagulant" gene, such as human tissue factor pathway inhibitor, hirudin, or CD39, or lacking the gene for the prothrombinase, fibrinogen-like protein-2, is anticipated to inhibit the change in the endothelium to a procoagulant state that takes place in the pig organ after transplantation. The identification of the targets for anti-nonGal antibodies and/or human macrophages might allow further genetic modification of the pig, and xenogeneic NK cell recognition and activation may be inhibited by the transgenic expression of human leukocyte antigen molecules and/or by blocking the function of activating NK receptors. The ultimate goal of induction of T-cell tolerance may be possible only if these hurdles in the coagulation system and innate immunity can be overcome.

The pig-to-primate immune response: relevance for xenotransplantation

BACKGROUND

The allotransplantation of some solid organs can be associated with a graft-vs.-host (GVH) response from the activity of donor B or T cells. We have investigated whether there is a risk of a GVH response following pig-to-primate organ xenotransplantation.

METHODS

The responses of 16 pigs (six farm-housed wild-type and five wild-type housed under high herd health conditions [all designated WT], and 5 alpha1,3-galactosyltransferase gene-knockout [GT-KO] housed under high herd health conditions) to human (n = 6) and baboon (n = 6) peripheral blood mononuclear cells (PBMC) were determined. Assays included flow cytometry, complement-dependent cytotoxicity, and mixed lymphocyte reaction.

RESULTS

Anti-primate cytotoxic IgM antibodies were detected in the sera of all pigs, but anti-primate IgG antibodies were minimal. All pigs demonstrated a cellular proliferative response to primate PBMC that was equivalent to, or greater than, the allo response. The strength of the pig-to-primate GVH responses was proportional to the health status of the pigs, those from a high health status herd, particularly from a specific pathogen-free herd maintained under clean husbandry conditions, where colonization of the gastrointestinal tract may be reduced, having lower responses.

CONCLUSIONS

After pig organ transplantation in a primate, if the organ is from an early-weaned, early-segregated GT-KO pig, the strength of a GVH response is likely to be relatively weak. Although not investigated here, any GVH response is likely to be suppressed by the immunosuppressive therapy administered to the recipient to suppress the anti-donor immune response.

Pig-to-non-human primate heart transplantation: immunologic progress over 20 years

The major developments in pig-to-non-human primate heart xenotransplantation during the past 20 years are summarized, largely through the experience of one investigator. Genetic modifications to organ-source pigs have been important steps in increasing heart xenograft survival from a few minutes in 1986 to 2 to 6 months in 2005.

Role for CD47-SIRPalpha signaling in xenograft rejection by macrophages

We have previously proven that human macrophages can phagocytose porcine cells even in the absence of Ab or complement opsonization, indicating that macrophages present a pivotal immunological obstacle to xenotransplantation. A recent report indicates that the signal regulatory protein (SIRP)alpha is a critical immune inhibitory receptor on macrophages, and its interaction with CD47, a ligand for SIRPalpha, prevents autologous phagocytosis. Considering the limited compatibility (73%) in amino acid sequences between pig and human CD47, we hypothesized that the interspecies incompatibility of CD47 may contribute to the rejection of xenogeneic cells by macrophages. In the present study, we have demonstrated that porcine CD47 does not induce SIRPalpha tyrosine phosphorylation in human macrophage-like cell line, and soluble human CD47-Fc fusion protein inhibits the phagocytic activity of human macrophages toward porcine cells. In addition, we have verified that manipulation of porcine cells for expression of human CD47 radically reduces the susceptibility of the cells to phagocytosis by human macrophages. These results indicate that the interspecies incompatibility of CD47 significantly contributes to the rejection of xenogeneic cells by macrophages. Genetic induction of human CD47 on porcine cells could provide inhibitory signaling to SIRPalpha on human macrophages, providing a novel approach to preventing macrophage-mediated xenograft rejection.

Advances in swine biomedical model genomics

This review is a short update on the diversity of swine biomedical models and the importance of genomics in their continued development. The swine has been used as a major mammalian model for human studies because of the similarity in size and physiology, and in organ development and disease progression. The pig model allows for deliberately timed studies, imaging of internal vessels and organs using standard human technologies, and collection of repeated peripheral samples and, at kill, detailed mucosal tissues. The ability to use pigs from the same litter, or cloned or transgenic pigs, facilitates comparative analyses and genetic mapping. The availability of numerous well defined cell lines, representing a broad range of tissues, further facilitates testing of gene expression, drug susceptibility, etc. Thus the pig is an excellent biomedical model for humans. For genomic applications it is an asset that the pig genome has high sequence and chromosome structure homology with humans. With the swine genome sequence now well advanced there are improving genetic and proteomic tools for these comparative analyses. The review will discuss some of the genomic approaches used to probe these models. The review will highlight genomic studies of melanoma and of infectious disease resistance, discussing issues to consider in designing such studies. It will end with a short discussion of the potential for genomic approaches to develop new alternatives for control of the most economically important disease of pigs, porcine reproductive and respiratory syndrome (PRRS), and the potential for applying knowledge gained with this virus for human viral infectious disease studies.

Elicited Antibodies in Baboons Exposed to Tissues from ??1,3-Galactosyltransferase Gene-Knockout Pigs

BACKGROUND

This study investigates anti-nonGal antibodies (Abs) in baboons after alpha1,3-galactosyltransferase gene-knockout (GalT-KO) pig heart transplantation (Tx).

METHODS

Four baboons underwent pig heart Tx under chronic immunosuppression, which was discontinued after graftectomy. During follow-up, one baboon also received a pig splenocyte infusion. Hearts and splenocytes were from GalT-KO pigs (n = 3) or pigs with low Gal expression (Gal-low, n = 2), all of swine leukocyte antigen (SLA) dd haplotype. Several weeks after graftectomy, sera were tested by flow cytometry and cytotoxicity assay on porcine peripheral blood mononuclear cells (PBMC) for elicited anti-nonGal Abs. Sera were adsorbed on a Gal immunoaffinity matrix, and tested for SLA haplotype specificity using PBMC from SLA aa, cc, and dd haplotypes.

RESULTS

Before heart Tx, no baboon had anti-nonGal Abs demonstrable by binding or cytotoxicity to GalT-KO PBMC. All four baboons developed anti-nonGal Abs after Tx, demonstrable by flow cytometry, and three sera from baboons showed cytotoxicity to GalT-KO PBMC of SLA(dd) haplotype. After adsorption of anti-Gal Abs, the elicited anti-nonGal Abs showed similar binding to PBMCs from pigs of all three haplotypes (SLA(dd), SLA(aa), SLA(cc)).

CONCLUSIONS

Anti-nonGal Abs developed after GalT-KO pig heart Tx into baboons. The most potent of these antibodies appeared to detect antigens shared by the three pig haplotypes tested. It remains unclear whether these antibodies are directed towards shared SLA determinants or other pig antigens, and whether antibodies with specificity for allelic SLA determinants are also present, but at lower titer.

Nonhuman primate embryonic stem cells as a preclinical model for hematopoietic and vascular repair

Stem cell-based regenerative medicine therapies have been touted recently as a novel therapeutic approach to treat and cure a wide range of diseases. Both adult and embryonic stem (ES) cells can serve as important sources of precursor cells to derive more mature cells potentially utilized for clinical applications. Nonhuman primates have proven useful as a preclinical model, as demonstrated in studies of hematopoietic cell transplantation, gene therapy, and other areas. The derivation of nonhuman primate ES cells now provides an optimal resource to characterize and test ES cell-based therapies prior to trials with human ES cells. This review describes work to define strategies and mechanisms to derive blood and endothelial cells from nonhuman primate ES cells isolated from various species. Preclinical testing that solely relies on studies of putative therapeutic cells derived from mouse ES cells transplanted into other mice, or analyses of human ES cell-derived cells transplanted into immunodeficient or immunosuppressed rodents may not be predictive of efficacy in subsequent human trials. However, future testing using nonhuman primate ES cell-derived therapeutic cells done as an allogeneic transplant may best predict success for subsequent studies using human ES cells. Therefore, additional research on nonhuman primate ES cells, in addition to work on mouse and human ES cells, is greatly needed to facilitate clinical translation of new stem cell treatments.