No clear answers on safety of pigs as tissue donor source

Infectious disease risks in xenotransplantation

Hurdles exist to clinical xenotransplantation including potential infectious transmission from nonhuman species to xenograft recipients. In anticipation of clinical trials of xenotransplantation, the associated infectious risks have been investigated. Swine and immunocompromised humans share some potential pathogens. Swine herpesviruses including porcine cytomegalovirus (PCMV) and porcine lymphotropic herpesvirus (PLHV) are largely species-specific and do not, generally, infect human cells. Human cellular receptors exist for porcine endogenous retrovirus (PERV), which infects certain human-derived cell lines in vitro. PERV-inactivated pigs have been produced recently. Human infection due to PERV has not been described. A screening paradigm can be applied to exclude potential human pathogens from "designated pathogen free" breeding colonies. Various microbiological assays have been developed for screening and diagnosis including antibody-based tests and qualitative and quantitative molecular assays for viruses. Additional assays may be required to diagnose pig-specific organisms in human xenograft recipients. Significant progress has been made in the evaluation of the potential infectious risks of clinical xenotransplantation. Infectious risk would be amplified by intensive immunosuppression. The available data suggest that risks of xenotransplant-associated recipient infection are manageable and that clinical trials can be performed safely. Possible infectious risks of xenotransplantation to the community at large are undefined but merit consideration.

Islet cell transplantation today

INTRODUCTION

Long-term studies strongly suggest that tight control of blood glucose can prevent the development and retard the progression of chronic complications of type 1 diabetes mellitus. In contrast to conventional insulin treatment, replacement of a patient's islets of Langerhans either by pancreas organ transplantation or by isolated islet transplantation is the only treatment to achieve a constant normoglycemic state and avoiding hypoglycemic episodes, a typical adverse event of multiple daily insulin injections. However, the cost of this benefit is still the need for immunosuppressive treatment of the recipient with all its potential risks.

MATERIALS AND METHODS

Islet cell transplantation offers the advantage of being performed as a minimally invasive procedure in which islets can be perfused percutaneously into the liver via the portal vein. Between January 1990 and December 2004, 458 pancreatic islet transplants worldwide have been reported to the International Islet Transplant Registry (ITR) at our Third Medical Department, University of Giessen/Germany.

RESULTS

Data analysis of islet cell transplants performed in the last 5 years (1999-2004) shows at 1 year after adult islet transplantation a patient survival rate of 97%, a functioning islet graft in 82% of the cases, whereas insulin independence was meanwhile achieved in 43% of the cases. However, using a novel protocol established by the Edmonton Center/Canada, the insulin independence rates have improved significantly reaching meanwhile a 50-80% level.

CONCLUSION

Finally, the concept of islet cell or stem cell transplantation is most attractive, as it offers many perspectives: islet cell availability could become unlimited and islet or stem cells my be transplanted without life-long immunosuppressive treatment of the recipient, just to mention two of them.

Phylogenetic analysis of porcine endogenous retrovirus variation in three Chinese pigs

PCR amplification was performed on genomic DNA extracted from peripheral blood lymphocytes of three species of Chinese pigs (Banna minipig inbreed [BMI], Wu-Zhi-Shan pig [WZSP], and Nei jiang pig [NJP]), using primers corresponding to the highly conserved regions of polymerase (pol) gene. Extracted PCR products were then cloned in a pGEM-T vector. Phylogenetic analysis of the nucleotide sequences of BMI-PERV, NJJP-PERV, and WZSP-PERV revealed them to be a novel category of PERV. In comparison to other type C retrovirus and lentivirus, their amino acid sequenced show about 30% to 57.7% identities. Our previous research demonstrated that PERV in the three pigs was highly expressed. It appears likely that functional loci encoding these novel PERV sequences exist, but this remains to be established. The novel sequences described in this report will allow such investigations to be actively pursued.

The testicular-derived Sertoli cell: cellular immunoscience to enable transplantation

There is a renewed enthusiasm for the potential of cellular transplantation as a therapy for numerous clinical disorders. The revived interest is largely due to the unprecedented success of the "Edmonton protocol," which produced a 100% cure rate for type I diabetics following the transplantation of human islet allografts together with a modified immunosuppressive regimen. While these data provide a clear and unequivocal demonstration that transplantation is a viable treatment strategy, the shortage of suitable donor tissue together with the debilitating consequences of lifelong immunosuppression necessitate a concerted effort to develop novel means to enable transplantation on a widespread basis. This review outlines the use of Sertoli cells to provide local immunoprotection to cografted discordant cells, including those from xenogeneic sources. Sertoli cells are normally found in the testes where one of their functions is to provide local immunologic protection to developing germ cells. Isolated Sertoli cells 1) engraft and self-protect when transplanted into allogeneic and xenogeneic environments, 2) protect cografted allogeneic and xenogeneic cells from immune destruction, 3) protect islet grafts to reverse diabetes in animal models, 4) enable survival and function of cografted foreign dopaminergic neurons in rodent models of Parkinson's disease (PD), and 5) promote regeneration of damaged striatal dopaminergic circuitry in those same PD models. These benefits are discussed in the context of several potential underlying biological mechanisms. While the majority of work to date has focused on Sertoli cells to facilitate transplantation for diabetes and PD, the generalized ability of these unique cells to potently suppress the local immune environment opens additional clinical possibilities.

Xenotransplantation and risks of zoonotic infections

The shortage of human organs and tissues for transplantation and the advances in immunology of rejection and in genetic engineering have renewed interest in xenotransplantation--the transplantation of animal organs, tissues or cells to humans. Clinical trials have involved the use of non-human primate, porcine, and bovine cells/tissues/organs. In recent years, research has focused mainly on pigs as donors (especially, pigs genetically engineered to carry some human genes). One of the major concerns in xenotransplantation is the risk of transmission of animal pathogens, particularly viruses, to recipients and the possible adaptation of such pathogens for human-to-human transmission. Porcine endogenous retroviruses (PERVs) have been of special concern because of their ability to infect human cells and because, at present, they cannot be removed from the source animal's genome. To date, retrospective studies of humans exposed to live porcine cells/tissues have not found evidence of infection with PERV but more extensive research is needed. This article reviews infectious disease risks associated with xenotransplantation, some measures for minimizing that risk, and microbiological diagnostic methods that may be used in the follow-up of xenotransplant recipients.

Xenotransplantation: public health risks--patient vs. society in an emerging field

Xenotransplantation is a public health concern because it has the potential to infect human recipients with zoonotic and other infectious agents that are not endemic in human populations, thereby potentially introducing new infections to the human community. From this perspective, xenotransplantation clinical trials combine a potential benefit for individual recipients with a potential risk to the human community. However, the potential for benefit also extends beyond the individual recipient to society as a whole, a fact infrequently recognized in discussions of this topic. Further, diseases neither endemic in human communities nor recognized as classic zoonoses are introduced into humans periodically through routine interactions between human and nonhuman animals. Thus, xenotransplantation is one of multiple potential routes by which infectious agents of nonhuman origin may enter human ecosystems. The intentional and controlled nature of xenotransplantation exposures enables implementation of measures to minimize associated biohazards. Development of guidelines and implementation of regulatory oversight of xenotransplantation clinical trials in most nations where such research occurs has promoted a standard level of practice in the field and markedly reduced the risk of xenotransplant-introduced infection compared to the situation in 1995.

Islet transplantation for diabetes: current status and future prospects

There has been an explosion of interest in developing transplantation strategies to replace the islets lost during the normal progression of diabetes. In large part, the renewed interest is due to the unprecedented success of the 'Edmonton protocol' for Type I diabetics following islet allografting and a modified immunosuppressive regimen. While these data provide a clear and unequivocal demonstration that islet transplantation is a viable treatment strategy, the shortage of suitable donor tissue, together with the debilitating consequences of life long immunosuppression necessitate a concerted effort to develop novel means to enable islet transplantation on a widespread basis. This review outlines several of these possibilities including the development of novel, less toxic immunosuppressants, tolerising the host immune system to accept islet transplants, developing alternative sources of islet cells via gene therapy, stem cell technologies and xenotransplantation, immunoisolation of islets and providing local immunosuppression to islet grafts using Sertoli cells. These possibilities are highlighted in the context of islet cell transplantation as a success, but with need for further refinements before it is a panacea for most patients.

Risk factors for human disease emergence

A comprehensive literature review identifies 1415 species of infectious organism known to be pathogenic to humans, including 217 viruses and prions, 538 bacteria and rickettsia, 307 fungi, 66 protozoa and 287 helminths. Out of these, 868 (61%) are zoonotic, that is, they can be transmitted between humans and animals, and 175 pathogenic species are associated with diseases considered to be 'emerging'. We test the hypothesis that zoonotic pathogens are more likely to be associated with emerging diseases than non-emerging ones. Out of the emerging pathogens, 132 (75%) are zoonotic, and overall, zoonotic pathogens are twice as likely to be associated with emerging diseases than non-zoonotic pathogens. However, the result varies among taxa, with protozoa and viruses particularly likely to emerge, and helminths particularly unlikely to do so, irrespective of their zoonotic status. No association between transmission route and emergence was found. This study represents the first quantitative analysis identifying risk factors for human disease emergence.

Non-human to human organ transplantation: its biologic basis and a potential role for radiation therapy

There is an inadequate supply of human donor organs for transplantation. Xenotransplantation, the transplantation of organs from non-human animals to humans, is one of the potential solutions to this problem. The pig appears to be the preferred donor. For xenotransplantation to be successful, researchers must deal with three fundamental problems: (1) Hyperacute rejection of porcine organs, related to binding of xenoreactive natural antibodies of the recipient to antigens on the graft's endothelial cells, must be overcome. (2) Transmission of animal pathogens to humans must be prevented. Concern about zoonosis is not only directed to the transplant recipient but also concerns the risk that an infectious agent will be transferred from the recipient to the general population. (3) The xenografted organ must be physiologically compatible with the recipient. The physiological function of a pig organ in a human and its ability to sustain a human are problematic. Total lymphoid irradiation (TLI) and thoraco-abdominal irradiation (TAI) as immunosuppressive modalities have been investigated in rodent-to-rodent, large mammals and non-human primates-to-primates, and pig-to-primate models. In certain clinical situations, TLI and TAI may prove to be important components for the preparation of the xenotransplant recipient. Progress in genetic engineering and cloning may soon lead to clinical trials in xenotransplantation.

Xenografts: are the risks so great that we should not proceed?

Animal organs could save patients needing transplants, but further research is necessary to resolve remaining problems with organ rejection. Furthermore, xenotransplantation risks transmitting animal pathogens to patients and to the general population. It would be unethical to proceed with clinical trials before principles and procedures for dealing with this risk are in place.

Infectious disease issues in xenotransplantation

Xenotransplantation, the transplantation of living organs, tissues, or cells from one species to another, is viewed as a potential solution to the existing shortage of human organs for transplantation. While whole-organ xenotransplantation is still in the preclinical stage, cellular xenotransplantation and extracorporeal perfusion applications are showing promise in early clinical trials. Advances in immunosuppressive therapy, gene engineering, and cloning of animals bring a broader array of xenotransplantation protocols closer to clinical trials. Despite several potential advantages over allotransplantation, xenotransplantation encompasses a number of problems. Immunologic rejection remains the primary hindrance. The potential to introduce infections across species barriers, another major concern, is the main focus of this review. Nonhuman primates are unlikely to be a main source for xenotransplantation products despite their phylogenetic proximity to humans. Genetically engineered pigs, bred under special conditions, are currently envisaged as the major source. Thus far, there has been no evidence for human infections caused by pig xenotransplantation products. However, the existence of xenotropic endogenous retroviruses and the clinical evidence of long-lasting porcine cell microchimerism indicate the potential for xenogeneic infections. Thus, further trials should continue under regulatory oversight, with close clinical and laboratory monitoring for potential xenogeneic infections.

No evidence for infection of human cells with porcine endogenous retrovirus (PERV) after exposure to porcine fetal neuronal cells

BACKGROUND

Recent demonstration of human cell infection in vitro with porcine endogenous retrovirus (PERV) has raised safety concerns for new therapies that involve transplantation of pig cells or organs to humans. To assess better the specific risk that may be associated with the transplantation of fetal pig neuronal cells to the central nervous system of patients suffering from intractable neurologic disorders (Parkinson's disease, Huntington's disease, and epilepsy), we have performed studies to determine whether there is evidence for in vivo or in vitro transmission of PERV from fetal pig neuronal cells to human cells.

METHODS

Ventral mesencephalon (VM) and lateral ganglionic eminence cells were isolated from fetal pigs and transplanted into patients with neurological conditions as part of clinical studies. Blood samples taken from patients at various time points posttransplant were tested for evidence of PERV. In vitro studies to test for PERV infection of human cells after cocultivation with either fetal porcine ventral mesencephalon or porcine fetal lateral ganglionic eminence cells were also performed.

RESULTS

We found no evidence of PERV provirus integration in the DNA from PBMC of 24 neuronal transplant recipients. In addition, no PERV was released from cultured fetal porcine neuronal cultures, and there was no transfer of PERV from fetal pig neuronal cells to human cells in vitro.

CONCLUSIONS

Our results demonstrate by both examination of transplant patient blood samples and in vitro studies that there is no evidence for transmission of PERV from porcine fetal neural cells to human cells.

Infection of nonhuman primate cells by pig endogenous retrovirus

The ongoing shortage of human donor organs for transplantation has catalyzed new interest in the application of pig organs (xenotransplantation). One of the biggest concerns about the transplantation of porcine grafts into humans is the transmission of pig endogenous retroviruses (PERV) to the recipients or even to other members of the community. Although nonhuman primate models are excellently suited to mimic clinical xenotransplantation settings, their value for risk assessment of PERV transmission at xenotransplantation is questionable since all of the primate cell lines tested so far have been found to be nonpermissive for PERV infection. Here we demonstrate that human, gorilla, and Papio hamadryas primary skin fibroblasts and also baboon B-cell lines are permissive for PERV infection. This suggests that a reevaluation of the suitability of the baboon model for risk assessment in xenotransplantation is critical at this point.

Establishment and characterization of molecular clones of porcine endogenous retroviruses replicating on human cells

The use of pig xenografts is being considered to alleviate the shortage of allogeneic organs for transplantation. In addition to the problems overcoming immunological and physiological barriers, the existence of numerous porcine microorganisms poses the risk of initiating a xenozoonosis. Recently, different classes of type C porcine endogenous retoviruses (PERV) which are infectious for human cells in vitro have been partially described. We therefore examined whether completely intact proviruses exist that produce infectious and replication-competent virions. Several proviral PERV sequences were cloned and characterized. One molecular PERV class B clone, PERV-B(43), generated infectious particles after transfection into human 293 cells. A second clone, PERV-B(33), which was highly homologous to PERV-B(43), showed a G-to-A mutation in the first start codon (Met to Ile) of the env gene, preventing this provirus from replicating. However, a genetic recombinant, PERV-B(33)/ATG, carrying a restored env start codon, became infectious and could be serially passaged on 293 cells similar to virus clone PERV-B(43). PERV protein expression was detected 24 to 48 h posttransfection (p. t.) using cross-reacting antiserum, and reverse transcriptase activity was found at 12 to 14 days p.t. The transcriptional start and stop sites as well as the splice donor and splice acceptor sites of PERV mRNA were mapped, yielding a subgenomic env transcript of 3. 1 kb. PERV-B(33) and PERV-B(43) differ in the number of copies of a 39-bp segment in the U3 region of the long terminal repeat. Strategies to identify and to specifically suppress or eliminate those proviruses from the pig genome might help in the production of PERV-free animals.

Neural tissue xenotransplantation: what is needed prior to clinical trials in Parkinson's disease? Neural Tissue Xenografting Project

Embryonic allografted human tissue in patients with Parkinson's disease has been shown to survive and ameliorate many of the symptoms of this disease. Despite this success, the practical problems of using this tissue coupled to the ethical restrictions of using aborted human fetal tissue have lead to an exploration for alternative sources of suitable material for grafting, including xenogeneic embryonic dopaminergic-rich neural tissue. Nevertheless, xenografted neural tissue itself generates a number of practical, ethical, safety, and immunological issues that have to be addressed prior to any clinical xenotransplant program. In this article we review these critical issues and set out the criteria that we consider need to be met in the development of our clinical xenotransplantation research programs. We advocate that these, or similar, criteria should be adopted and made explicit by other centers contemplating similar clinical trials.

Influence of human fulminant hepatic failure sera on endogenous retroviral expression in pig hepatocytes

A porcine endogenous retrovirus (PERV) has been shown to infect human embryonic kidney 293 (HEK293) cells in vitro. The PERV proviral sequence exists in the genome of all porcine cells, including hepatocytes used in a bioartificial liver (BAL). We examined the possibility of PERV infection in HEK293 cells during exposure to supernatant from cultured pig hepatocytes. Pig hepatocytes were cultured in media supplemented with serum from patients in fulminant hepatic failure (FHF) to simulate conditions of an extracorporeal BAL. Pig hepatocytes were cultured in serum-free media for 24 hours and then exposed to fresh medium containing serum from a patient with FHF (22 patients tested). Twenty-four hours later, supernatant was collected and analyzed by polymerase chain reaction (PCR), with and without reverse transcriptase. Primers targeting the pol gene of PERV were used for PCR. Products of amplification were detected by an enzyme-linked immunosorbent assay-based technique using an internal capture probe also targeting the pol gene. Levels of PERV sequences were estimated by serial dilution. All positive samples were tested for infectivity in HEK293 cells. Porcine kidney 15 cell supernatant and fresh culture media were studied as positive and negative controls, respectively. Pig hepatocytes were also studied in the absence of FHF sera and in the presence of mitogenic stimulation with phytohemagglutinin (PHA) and phorbol 12-myristate-13-acetate (PMA). PERV DNA and PERV RNA were detected in all supernatants of cultured pig hepatocytes. The level of PERV RNA in the supernatant of pig hepatocytes was not altered by exposure to human FHF serum or stimulation with PHA and PMA. In addition, PERV RNA was undetectable in the supernatant of HEK293 cells for up to 50 days after exposure to pig hepatocyte supernatant (with or without FHF sera). These findings show that production of PERV by cultured pig hepatocytes was unaffected by exposure to growth factors and cytokines present in human FHF sera.

Fetal porcine dopaminergic cell survival in vitro and its relationship to embryonic age

One of the critical factors in the survival of embryonic neural grafts is the age at which the population of donor neurons is harvested. This is especially the case for the developing dopaminergic neurons in the embryonic ventral mesencephalon, which are used for neural grafts in Parkinson's disease (PD). The donor age for optimal harvesting of these cells has been well characterized in the mouse, rat, and marmoset, and to a lesser extent in humans. However, the best donor age for porcine ventral mesencephalic tissue has not been ascertained, even though the use of this tissue for xenografts has been explored both experimentally and clinically. In this study the effect of donor age on dopaminergic cell survival was assessed in vitro, from a range of fetal pigs aged from E24 to E35. The number of tyrosine hydroxylase (TH)-positive cells per ventral mesencephalon was then calculated after 1 and 7 days in culture. E26-E27 embryos gave the highest yield of such cells at both survival time points, suggesting that this will be the optimal age for harvesting tissues whether for experimental or clinical nigral xenograft programs.

Search for cross-species transmission of porcine endogenous retrovirus in patients treated with living pig tissue. The XEN 111 Study Group

Pig organs may offer a solution to the shortage of human donor organs for transplantation, but concerns remain about possible cross-species transmission of porcine endogenous retrovirus (PERV). Samples were collected from 160 patients who had been treated with various living pig tissues up to 12 years earlier. Reverse transcription-polymerase chain reaction (RT-PCR) and protein immunoblot analyses were performed on serum from all 160 patients. No viremia was detected in any patient. Peripheral blood mononuclear cells from 159 of the patients were analyzed by PCR using PERV-specific primers. No PERV infection was detected in any of the patients from whom sufficient DNA was extracted to allow complete PCR analysis (97 percent of the patients). Persistent microchimerism (presence of donor cells in the recipient) was observed in 23 patients for up to 8.5 years.