Characterization of Transgenic Pigs That Express Human Decay Accelerating Factor and Cell Membrane-tethered Human Tissue Factor Pathway Inhibitor

Mechanisms and strategies to promote cardiac xenotransplantation

End stage heart failure is a terminal disease, and the only curative therapy is orthotopic heart transplantation. Due to limited organ availability, alternative strategies have received intense interest for treatment of patients with advanced heart failure. Recent studies using gene-edited porcine organs suggest that cardiac xenotransplantation may provide a future source of organs. In this review, we highlight the historical milestones for cardiac xenotransplantation and the gene editing strategies designed to overcome immunological barriers, which have culminated in a recent cardiac pig-to-human xenotransplant. We also discuss recent results of studies on the engineering of human-porcine chimeric organs that may provide an alternative and complementary strategy to overcome some of the major immunological barriers to producing a new source of transplantable organs.

Immunogenetics of xenotransplantation

Xenotransplantation may become the highly desired solution to close the gap between the availability of donated organs and number of patients on the waiting list. In recent years, enormous progress has been made in the development of genetically engineered donor pigs. The introduced genetic modifications showed to be efficient in prolonging xenograft survival. In this review, we focus on the type of immune responses that may target xeno-organs after transplantation and promising immunogenetic modifications that show a beneficial effect in ameliorating or eliminating harmful xenogeneic immune responses. Increasing histocompatibility of xenografts by eliminating genetic discrepancies between species will pave their way into clinical application.

Porcine genome engineering for xenotransplantation

The extreme shortage of human donor organs for treatment of patients with end-stage organ failures is well known. Xenotransplantation, which might provide unlimited organ supply, is a most promising strategy to solve this problem. Domestic pigs are regarded as ideal organ-source animals owing to similarity in anatomy, physiology and organ size to humans as well as high reproductive capacity and low maintenance cost. However, several barriers, which include immune rejection, inflammation and coagulative dysfunctions, as well as the cross-species transmission risk of porcine endogenous retrovirus, blocked the pig-to-human xenotransplantation. With the rapid development of genome engineering technologies and the potent immunosuppressive medications in recent years, these barriers could be eliminated through genetic modification of pig genome together with the administration of effective immunosuppressants. A number of candidate genes involved in the regulation of immune response, inflammation and coagulation have been explored to optimize porcine xenograft survival in non-human primate recipients. PERV inactivation in pigs has also been accomplished to firmly address the safety issue in pig-to-human xenotransplantation. Many encouraging preclinical milestones have been achieved with some organs surviving for years. Therefore, the clinical trials of some promising organs, such as islet, kidney and heart, are aimed to be launched in the near future.

Production and Breeding of Transgenic Cloned Pigs Expressing Human CD73

One of the reasons to causing blood coagulation in the tissue of xenografted organs was known to incompatibility of the blood coagulation and anti-coagulation regulatory system between TG pigs and primates. Thus, overexpression of human CD73 (hCD73) in the pig endothelial cells is considered as a method to reduce coagulopathy after pig-to-non-human-primate xenotransplantation. This study was performed to produce and breed transgenic pigs expressing hCD73 for the studies immune rejection responses and could provide a successful application of xenotransplantation. The transgenic cells were constructed an hCD73 expression vector under control porcine Icam2 promoter (pIcam2-hCD73) and established donor cell lines expressing hCD73. The numbers of transferred reconstructed embryos were 127 ± 18.9. The pregnancy and delivery rate of surrogates were 8/18 (44%) and 3/18 (16%). The total number of delivered cloned pigs were 10 (2 alive, 7 mummy, and 1 died after birth). Among them, three live hCD73-pigs were successfully delivered by Caesarean section, but one was dead after birth. The two hCD73 TG cloned pigs had normal reproductive ability. They mated with wild type (WT) MGH (Massachusetts General Hospital) female sows and produced totally 16 piglets. Among them, 5 piglets were identified as hCD73 TG pigs. In conclusion, we successfully generated the hCD73 transgenic cloned pigs and produced their litters by natural mating. It can be possible to use a mate for the production of multiple transgenic pigs such as α-1,3-galactosyltransferase knock-out /hCD46 for xenotransplantation.

Generation of α-1,3-galactosyltransferase knocked-out transgenic cloned pigs with knocked-in five human genes

Recent progress in genetic manipulation of pigs designated for xenotransplantation ha6s shown considerable promise on xenograft survival in primates. However, genetic modification of multiple genes in donor pigs by knock-out and knock-in technologies, aiming to enhance immunological tolerance against transplanted organs in the recipients, has not been evaluated for health issues of donor pigs. We produced transgenic Massachusetts General Hospital piglets by knocking-out the α-1,3-galactosyltransferase (GT) gene and by simultaneously knocking-in an expression cassette containing five different human genes including, DAF, CD39, TFPI, C1 inhibitor (C1-INH), and TNFAIP3 (A20) [GT^{−(DAF/CD39/TFPI/C1-INH/TNFAIP3)/+}] that are connected by 2A peptide cleavage sequences to release individual proteins from a single translational product. All five individual protein products were successfully produced as determined by western blotting of umbilical cords from the newborn transgenic pigs. Although gross observation and histological examination revealed no significant pathological abnormality in transgenic piglets, hematological examination found that the transgenic piglets had abnormally low numbers of platelets and WBCs, including neutrophils, eosinophils, basophils, and lymphocytes. However, transgenic piglets had similar numbers of RBC and values of parameters related to RBC compared to the control littermate piglets. These data suggest that transgenic expression of those human genes in pigs impaired hematopoiesis except for erythropoiesis. In conclusion, our data suggest that transgenic expression of up to five different genes can be efficiently achieved and provide the basis for determining optimal dosages of transgene expression and combinations of the transgenes to warrant production of transgenic donor pigs without health issues.

Kidney xenotransplantation

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

Genetically modified pigs for biomedical research

During the last two decades, pigs have been used to develop some of the most important large animal models for biomedical research. Advances in pig genome research, genetic modification (GM) of primary pig cells and pig cloning by nuclear transfer, have facilitated the generation of GM pigs for xenotransplantation and various human diseases. This review summarizes the key technologies used for generating GM pigs, including pronuclear microinjection, sperm-mediated gene transfer, somatic cell nuclear transfer by traditional cloning, and somatic cell nuclear transfer by handmade cloning. Broadly used genetic engineering tools for porcine cells are also discussed. We also summarize the GM pig models that have been generated for xenotransplantation and human disease processes, including neurodegenerative diseases, cardiovascular diseases, eye diseases, bone diseases, cancers and epidermal skin diseases, diabetes mellitus, cystic fibrosis, and inherited metabolic diseases. Thus, this review provides an overview of the progress in GM pig research over the last two decades and perspectives for future development.

Protective effects of different combinations of human MCP, DAF, and CD59 on complement-dependent cytolysis in NIH 3T3 cells

OBJECTIVES

To analyze the protective effects against complement-mediated cytolysis of the MCP, DAF, and CD59 human complement regulatory proteins, alone and in combination, on NIH 3T3 mouse fibroblast cells.

MATERIALS AND METHODS

We constructed 3 double and 3 single-human complement regulatory protein plasmids (pIRES-hMCP-hDAF, pIRES-hMCP-hCD59, pIRES-hDAF-hCD59, pIRES-A-hMCP, pIRES-B-hDAF, and pIRES-B-hCD59). The plasmids were transfected into NIH 3T3 cells, and stable transfectants were obtained by treatment with 200 kg/m3 G418 for 2 weeks. Normal human serum (50%) as a source of complement was added to the culture medium of stable transfectants. The 3-(4,5-dimethylthiazol-2-yl)- 2,5-diphenyltetrazolium bromide assay was used to analyze the protective ability of different human complement regulatory protein plasmids on complement-dependent cytolysis.

RESULTS

The viability of double-human complement regulatory protein stable transfectants was significantly higher than that of single-human complement regulatory protein stable transfectants (P < .05). Among the double-transfectants, cells expressing pIRES-hMCP-hDAF and pIRES-hMCPhCD59 survived better than cells expressing pIREShDAF- hCD59 (91.75% ± 3.30% and 84.88% ± 2.36% vs 66.19% ± 6.52%; P < .05). Among the single transfectants, cells expressing pIRES-A-hMCP or pIRES-B-hDAF survived better than cells expressing pIRES-B-hCD59 or pIRES empty vector (53.76% ± 3.84% and 56.32% ± 2.83% vs 43.28% ± 0.96% and 40.27% ± 1.11%; P < .05).

CONCLUSIONS

These results suggest that the MCP+DAF and MCP+CD59 combinations could be more effective than DAF+CD59 in protecting the NIH 3T3 cells from injury caused by complement-dependent cytolysis, whereas MCP or DAF alone is stronger than CD59 alone in inhibiting membrane attack complex formation.

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.

Controlling coagulation dysregulation in xenotransplantation

PURPOSE OF REVIEW

Deletion of the α1,3-galactosyltransferase (GalT) gene in pigs has removed a major xenoantigen but has not eliminated the problem of dysregulated coagulation and vascular injury. Rejecting GalT knockout organ xenografts almost invariably show evidence of thrombosis and platelet sequestration, and primate recipients frequently develop consumptive coagulopathy. This review examines recent findings that illuminate potential mechanisms of this current barrier to successful xenotransplantation.

RECENT FINDINGS

The coagulation response to xenotransplantation differs depending on the type of organ and quite likely the distinct vasculatures. Renal xenografts appear more likely to initiate consumptive coagulopathy than cardiac xenografts, possibly reflecting differential transcriptional responses. Liver xenografts induce rapid and profound thrombocytopenia resulting in recipient death within days due to bleeding; ex-vivo data suggest that liver endothelial cells and hepatocytes are responsible for platelet consumption by a coagulation-independent process.It has been proposed that expression of recipient tissue factor on platelets and monocytes is an important trigger of consumptive coagulopathy. Finally, pigs transgenic for human anticoagulants and antithrombotics are slowly but surely coming on line, but have not yet been rigorously tested to date.

SUMMARY

Successful control of coagulation dysregulation in xenotransplantation may require different combinatorial pharmacological and genetic strategies for different organs.