LONGTERM CLINICAL RECOVERY IN PARKINSONIAN MONKEY RECIPIENTS OF CTLA4-IG TRANSGENIC PORCINE NEURAL PRECURSORS

Genetically engineered pigs for xenotransplantation: Hopes and challenges

The shortage of donor resources has greatly limited the application of clinical xenotransplantation. As such, genetically engineered pigs are expected to be an ideal organ source for xenotransplantation. Most current studies mainly focus on genetically modifying organs or tissues from donor pigs to reduce or prevent attack by the human immune system. Another potential organ source is interspecies chimeras. In this paper, we reviewed the progress of the genetically engineered pigs from the view of immunologic barriers and strategies, and discussed the possibility and challenges of the interspecies chimeras.

Xenogeneic stem cell transplantation: Research progress and clinical prospects

Organ transplantation is the ultimate treatment for end-stage diseases such as heart and liver failure. However, the severe shortage of donor organs has limited the organ transplantation progress. Xenogeneic stem cell transplantation provides a new strategy to solve this problem. Researchers have shown that xenogeneic stem cell transplantation has significant therapeutic effects and broad application prospects in treating liver failure, myocardial infarction, advanced type 1 diabetes mellitus, myelosuppression, and other end-stage diseases by replacing the dysfunctional cells directly or improving the endogenous regenerative milieu. In this review, the sources, problems and solutions, and potential clinical applications of xenogeneic stem cell transplantation will be discussed.

The need for xenotransplantation as a source of organs and cells for clinical transplantation

The limited availability of deceased human organs and cells for the purposes of clinical transplantation remains critical worldwide. Despite the increasing utilization of 'high-risk', 'marginal', or 'extended criteria' deceased donors, in the U.S. each day 30 patients either die or are removed from the waiting list because they become too sick to undergo organ transplantation. In certain other countries, where there is cultural resistance to deceased donation, e.g., Japan, the increased utilization of living donors, e.g., of a single kidney or partial liver, only very partially addresses the organ shortage. For transplants of tissues and cells, e.g., pancreatic islet transplantation for patients with diabetes, and corneal transplantation for patients with corneal blindness (whose numbers worldwide are potentially in the millions), allotransplantation will never prove a sufficient source. There is an urgent need for an alternative source of organs and cells. The pig could prove to be a satisfactory source, and clinical xenotransplantation using pig organs or cells, particularly with the advantages provided by genetic engineering to provide resistance to the human immune response, may resolve the organ shortage. The physiologic compatibilities and incompatibilities of the pig and the human are briefly reviewed.

Xenotransplantation of Cells, Tissues, Organs and the German Research Foundation Transregio Collaborative Research Centre 127

Human organ transplantation is the therapy of choice for end-stage organ failure. However, the demand for organs far exceeds the donation rate, and many patients die while waiting for a donor. Clinical xenotransplantation using discordant species, particularly pigs, offers a possible solution to this critical shortfall. Xenotransplantation can also increase the availability of cells, such as neurons, and tissues such as cornea, insulin producing pancreatic islets and heart valves. However, the immunological barriers and biochemical disparities between pigs and primates (human) lead to rejection reactions despite the use of common immunosuppressive drugs. These result in graft vessel destruction, haemorrhage, oedema, thrombus formation, and transplant loss. Our consortium is pursuing a broad range of strategies to overcome these obstacles. These include genetic modification of the donor animals to knock out genes responsible for xenoreactive surface epitopes and to express multiple xenoprotective molecules such as the human complement regulators CD46, 55, 59, thrombomodulin and others. We are using (new) drugs including complement inhibitors (e.g. to inhibit C3 binding), anti-CD20, 40, 40L, and also employing physical protection methods such as macro-encapsulation of pancreatic islets. Regarding safety, a major objective is to assure that possible infections are not transmitted to recipients. While the aims are ambitious, recent successes in preclinical studies suggest that xenotransplantation is soon to become a clinical reality.

The current state of xenotransplantation

Pigs as a source of grafts for xenotransplantation can help to overcome the rapidly growing shortage of human donors. However, in the case of pig-to-human transplantation, the antibody-xenoantigen complexes lead to the complement activation and immediate hyperacute rejection. Methods eliminating hyperacute rejection (HAR) include α1,3-galactosyltransferase (GGTA1) inactivation, regulation of the complement system and modification of the oligosaccharide structure of surface proteins. The humoral immune response control and reduction of the risk of coagulation disorders are the priority tasks in attempts to overcome acute humoral xenograft rejection that may occur after the elimination of HAR. The primary targets for research are connected with the identification of obstacles and development of strategies to tackle them. Because of the magnitude of factors involved in the immune, genetic engineers face a serious problem of producing multitransgenic animals in the shortest possible time.

Clinical lung xenotransplantation--what donor genetic modifications may be necessary?

Barriers to successful lung xenotransplantation appear to be even greater than for other organs. This difficulty may be related to several macro anatomic factors, such as the uniquely fragile lung parenchyma and associated blood supply that results in heightened vulnerability of graft function to segmental or lobar airway flooding caused by loss of vascular integrity (also applicable to allotransplants). There are also micro-anatomic considerations, such as the presence of large numbers of resident inflammatory cells, such as pulmonary intravascular macrophages and natural killer (NK) T cells, and the high levels of von Willebrand factor (vWF) associated with the microvasculature. We have considered what developments would be necessary to allow successful clinical lung xenotransplantation. We suggest this will only be achieved by multiple genetic modifications of the organ-source pig, in particular to render the vasculature resistant to thrombosis. The major problems that require to be overcome are multiple and include (i) the innate immune response (antibody, complement, donor pulmonary and recipient macrophages, monocytes, neutrophils, and NK cells), (ii) the adaptive immune response (T and B cells), (iii) coagulation dysregulation, and (iv) an inflammatory response (e.g., TNF-α, IL-6, HMGB1, C-reactive protein). We propose that the genetic manipulation required to provide normal thromboregulation alone may include the introduction of genes for human thrombomodulin/endothelial protein C-receptor, and/or tissue factor pathway inhibitor, and/or CD39/CD73; the problem of pig vWF may also need to be addressed. It would appear that exploration of every available therapeutic path will be required if lung xenotransplantation is to be successful. To initiate a clinical trial of lung xenotransplantation, even as a bridge to allotransplantation (with a realistic possibility of survival long enough for a human lung allograft to be obtained), significant advances and much experimental work will be required. Nevertheless, with the steadily increasing developments in techniques of genetic engineering of pigs, we are optimistic that the goal of successful clinical lung xenotransplantation can be achieved within the foreseeable future. The optimistic view would be that if experimental pig lung xenotransplantation could be successfully managed, it is likely that clinical application of this and all other forms of xenotransplantation would become more feasible.

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.

A brief history of cross-species organ transplantation

Cross-species transplantation (xenotransplantation) offers the prospect of an unlimited supply of organs and cells for clinical transplantation, thus resolving the critical shortage of human tissues that currently prohibits a majority of patients on the waiting list from receiving transplants. Between the 17th and 20th centuries, blood was transfused from various animal species into patients with a variety of pathological conditions. Skin grafts were carried out in the 19th century from a variety of animals, with frogs being the most popular. In the 1920s, Voronoff advocated the transplantation of slices of chimpanzee testis into aged men whose "zest for life" was deteriorating, believing that the hormones produced by the testis would rejuvenate his patients. Following the pioneering surgical work of Carrel, who developed the technique of blood vessel anastomosis, numerous attempts at nonhuman primate organ transplantation in patients were carried out in the 20th century. In 1963-1964, when human organs were not available and chronic dialysis was not yet in use, Reemtsma transplanted chimpanzee kidneys into 13 patients, one of whom returned to work for almost 9 months before suddenly dying from what was believed to be an electrolyte disturbance. The first heart transplant in a human ever performed was by Hardy in 1964, using a chimpanzee heart, but the patient died within 2 hours. Starzl carried out the first chimpanzee-to-human liver transplantation in 1966; in 1992, he obtained patient survival for 70 days following a baboon liver transplant. With the advent of genetic engineering and cloning technologies, pigs are currently available with a number of different manipulations that protect their tissues from the human immune response, resulting in increasing pig graft survival in nonhuman primate models. Genetically modified pigs offer hope of a limitless supply of organs and cells for those in need of a transplant.