No evidence of infection with porcine endogenous retrovirus in recipients of porcine islet-cell xenografts

Successes and disappointments with clinical islet transplantation

Transplantation of pancreatic islets is considered a therapeutic option for patients with type 1 diabetes mellitus who have life-threatening hypoglycaemic episodes. After the procedure, a decrease in the frequency and severity of hypoglycaemic episodes and sustained graft function as indicated by detectable levels of C-peptide can be seen in the majority of patients. However, true insulin independence, if achieved, usually lasts for at most a few years. Apart from the low insulin independence rates, reasons for concern regarding this procedure are the side effects of the immunosuppressive therapy, allo-immunization, and the high costs. Moreover, whether islet transplantation prevents the progression of diabetic micro- and macrovascular complications is largely unknown. Areas of current research include the development of less toxic immunosuppressive regimens, the control of the inflammatory reaction immediately after transplantation, the identification of the optimal anatomical site for islet infusion, and the possibility to encapsulate transplanted islets to protect them from the allo-immune response. At present, pancreatic islet transplantation is still an experimental procedure, which is only indicated for a highly selected group of type 1 diabetic patients with life-threatening hypoglycaemic episodes.

Porcine endogenous retrovirus released by a bioartificial liver infects primary human cells

BACKGROUND

Porcine endogenous retrovirus (PERV) remains a safety risk in pig-to-human xenotransplantation. There is no evidence of in vivo productive infection in humans because PERV is inactivated by human serum. However, PERV can infect human cell lines and human primary cells in vitro and inhibit human immune functions.

AIMS

We investigated the potential of primary porcine liver cells to transmit PERV to primary human cells in a bioreactor-based bioartificial liver (BAL).

METHODS

Primary human hepatocytes, endothelial cells and the human cell line HEK 293 were exposed to supernatants from BAL or from the porcine cell line PK-15. PERV polymerase-specific reverse-transcriptase polymerase chain reaction (RT-PCR) and PCR were used to investigate PERV transmission to human cells. An assay of RT activity was used to detect the presence of retrovirus in the supernatants of BAL, primary human hepatocytes and endothelial cells.

RESULTS

Primary human hepatocytes (hHep), endothelial cells and HEK 293 cells were reproducibly infected by PERV, originating from primary porcine liver cells within the BAL and from PK-15 cells. Infected cells were positive for PERV-specific DNA and RNA after 8-10 days on an average, and RT activity was detectable in the supernatants of infected hHep and HEK 293 cells.

CONCLUSION

A risk of PERV infection in human cells is documented in this study, indicating that short-term contact of primary porcine liver cell supernatants with primary human cells could result in PERV transmission.

Xenotransplantation, xenogeneic infections, biotechnology, and public health

Xenotransplantation is the attempt to use living biological material from nonhuman animal species in humans for therapeutic purposes. Clinical trials and preclinical studies have suggested that living cells and tissue from other species have the potential to be used in humans to ameliorate disease. However, the potential for successful xenotransplantation to cure human disease is coupled with the risk that therapeutic use of living nonhuman cells in humans may also serve to introduce xenogeneic infections of unpredictable significance. Animal husbandry practices and xenotransplantation product preparation may eliminate most exogenous infectious agents prior to transplantation. However, endogenous retroviruses are present in the genomes of all mammalian cells, have an inadequately defined ability to infect human cells, and have generated public health concern. The history of xenotransplantation, the implications for public health, the global consensus on public safeguards necessary to accompany clinical trials, and the future direction of xenotransplantation are discussed in the context of public health. Mt Sinai J Med 76:435-441, 2009. (c) 2009 Mount Sinai School of Medicine.

Toward development and production of human T cells in swine for potential use in adoptive T cell immunotherapy

Immunotherapy and vaccination for cancer or infection are generally approached by administration of antigen or stimulation of antigen-presenting cells or both. These measures may fail if the treated individual lacks T cells specific for the immunogen(s). We tested another strategy-the generation of new T cells from hematopoietic stem cells that might be used for adoptive immunotherapy. To test this concept, we introduced T cell-depleted human bone marrow cells into fetal swine and tested the swine for human T cells at various times after birth. Human T cells were detected in the thymus and blood of the treated swine. These cells were generated de novo as they contained human T cell receptor excision circles not present in the T cell-depleted bone marrow. The human T cells were highly diverse and included novel specificities capable of responding to antigen presented by human antigen-presenting cells. Our findings constitute a first step in a new promising approach to immunotherapy in which tumor- or virus-specific T cell clones lacking in an individual might be generated in a surrogate host from hematopoietic stem cells of the individual to be treated.

Identification of two distinct structural regions in a human porcine endogenous retrovirus receptor, HuPAR2, contributing to function for viral entry

BACKGROUND

Of the three subclasses of Porcine Endogenous Retrovirus (PERV), PERV-A is able to infect human cells via one of two receptors, HuPAR1 or HuPAR2. Characterizing the structure-function relationships of the two HuPAR receptors in PERV-A binding and entry is important in understanding receptor-mediated gammaretroviral entry and contributes to evaluating the risk of zoonosis in xenotransplantation.

RESULTS

Chimeras of the non-permissive murine PAR and the permissive HuPAR2, which scanned the entire molecule, revealed that the first 135 amino acids of HuPAR2 are critical for PERV-A entry. Within this critical region, eighteen single residue differences exist. Site-directed mutagenesis used to map single residues confirmed the previously identified L109 as a binding and infectivity determinant. In addition, we identified seven residues contributing to the efficiency of PERV-A entry without affecting envelope binding, located in multiple predicted structural motifs (intracellular, extracellular and transmembrane). We also show that expression of HuPAR2 in a non-permissive cell line results in an average 11-fold higher infectivity titer for PERV-A compared to equal expression of HuPAR1, although PERV-A envelope binding is similar. Chimeras between HuPAR-1 and -2 revealed that the region spanning amino acids 152-285 is responsible for the increase of HuPAR2. Fine mapping of this region revealed that the increased receptor function required the full sequence rather than one or more specific residues.

CONCLUSION

HuPAR2 has two distinct structural regions. In one region, a single residue determines binding; however, in both regions, multiple residues influence receptor function for PERV-A entry.

No infection with porcine endogenous retrovirus in recipients of acellular porcine aortic valves: a two-year study

BACKGROUND

Engineered tissue heart valves may become a promising therapeutics for heart valve disease. Compared with synthetic materials, acellular porcine scaffolds are considered as suitable matrices for tissue-engineered heart valves for the mechanical and structural properties of native tissue. Whether acellular porcine scaffolds can cause infection in recipients with porcine endogenous retrovirus (PERV) is critical for evaluating the safety of transplantation of tissue-engineered heart valves based on acellular porcine scaffolds. This study was completed to evaluate the risk of PERV transmission for application of acellular porcine aortic valves (PAVs).

METHODS

Native aortic valves obtained from Chinese pigs of different species were acellularized by our modified detergent-enzymatic acellularization procedure. Polymerase chain reaction and reverse transcriptase polymerase chain reaction for pol sequences were used to detect PERV infection. In vitro, ovine endothelial cells (ECs) were inoculated and cocultured with supernatants of porcine aortic endothelial cells (PAECs) and acellualr PAVs, respectively. On 7th day, DNA and RNA of ovine ECs were isolated and tested for PERV. In vivo, acellular PAVs were implanted in the descending thoracic aorta in 40 sheep. Blood samples from the sheep and implanted acellular PAVs were collected 24 months after operation and tested for PERV.

RESULTS

All cells were removed from the PAVs. Acellularized PAVs were repopulated by autologous cells of the host 24 months after implantation. PERV was detected in all native PAVs, porcine blood samples, acellularized PAVs and ovine ECs inoculated with supernatants of PAECs; no PERV was detected in ovine ECs cocultured with acellularized PAVs, ovine blood samples, and implanted acellular PAVs.

CONCLUSIONS

Acellularized PAVs processed by modified detergent-enzymatic acellularization procedure can be used for cardiovascular tissue-engineered grafts as matrix scaffolds without risk of PERV transmission.

Absence of replication of porcine endogenous retrovirus and porcine lymphotropic herpesvirus type 1 with prolonged pig cell microchimerism after pig-to-baboon xenotransplantation

Porcine endogenous retrovirus (PERV), porcine cytomegalovirus (PCMV), and porcine lymphotropic herpesvirus (PLHV) are common porcine viruses that may be activated with immunosuppression for xenotransplantation. Studies of viral replication or transmission are possible due to prolonged survival of xenografts in baboon recipients from human decay-accelerating factor transgenic or alpha-1,3-galactosyltransferase gene knockout miniature swine. Ten baboons underwent xenotransplantation with transgenic pig organs. Graft survival was 32 to 179 days. Recipient serial samples of peripheral blood mononuclear cells (PBMC) and plasma were analyzed for PCMV, PERV, and PLHV-1 nucleic acids and viral replication using quantitative PCR assays. The PBMC contained PERV proviral DNA in 10 animals, PLHV-1 DNA in 6, and PCMV in 2. PERV RNA was not detected in any PBMC or serum samples. Plasma PLHV-1 DNA was detected in one animal. Pig cell microchimerism (pig major histocompatibility complex class I and pig mitochondrial cytochrome c oxidase subunit II sequences) was present in all recipients with detectable PERV or PLHV-1 (85.5%). Productive infection of PERV or PLHV-1 could not be demonstrated. The PLHV-1 viral load did not increase in serum over time, despite prolonged graft survival and pig cell microchimerism. There was no association of viral loads with the nature of exogenous immune suppression. In conclusion, PERV provirus and PLHV-1 DNA were detected in baboons following porcine xenotransplantation. Viral detection appeared to be due to persistent pig cell microchimerism. There was no evidence of productive infection in recipient baboons for up to 6 months of xenograft function.

Xenotransplantation: benefits, risks and relevance of reproductive technology

Xenotransplantation from pigs provides a possible way around the shortage of human organs for transplantation. The highly inbred Westran line of pigs is genetically well characterised and known to lack endogenous retroviruses able to infect human cells. Like most inbreds, it has poor reproductive performance for which reproductive interventions would be desirable.

Islet transplantation: the quest for an ideal source

The progress of islet transplantation as a new therapy for patients with diabetes mellitus depends directly upon the development of efficient and practical immunoisolation methods for the supply of sufficient quantities of islet cells. Without these methods, large scale clinical application of this therapy would be impossible. Two eras of advances can be identified in the development of islet transplantation. The first was an era of experimental animal and human research that centered on islet isolation procedures and transplantation in different species as evidence that transplanted islets have the capability to reverse diabetes. The second was the era of the Edmonton protocol, when the focus became the standardization of isolation procedures and introduction of new immunosuppressive drugs to maintain human allograft transplantation. The quest for an alternative source for islets (xenographs, stem cells and cell cultures) to overcome the shortage of human islets was an important issue during these eras. This paper reviews the history of islet transplantation and the current procedures in human allotransplantation, as well as different types of immunoisolation methods. It explores novel approaches to enhancing transplantation site vascularity and islet cell function, whereby future immunoisolation technology could offer additional therapeutic advantages to human islet allotransplantation.

APOBEC3 proteins and porcine endogenous retroviruses

Xenotransplantation of porcine cells, tissues, and organs offers a solution to overcome the shortage of human donor materials. In addition to the immunological and physiological barriers, the existence of numerous porcine microorganisms including viruses poses a risk for xenozoonosis. Three classes of functional gamma-type porcine endogenous retroviruses (PERV) have been identified, whereby functional polytropic PERV-A and PERV-B infect human embryonic kidney (HEK 293) and other cell lines in vitro. In the course of risk assessment for xenotransplantation the capacity of human cells to counteract PERV infections should be analyzed. Primates and other mammals display different means of protection against viral infections. APOBEC3 proteins which are cytidine deaminases and a part of the intrinsic immunity mediate potent activity against a wide range of retroviruses including murine leukemia viruses (MLV). As PERV and MLV belong to the same genus, we raised the question as to whether PERV is affected by APOBEC3 proteins. Initial data indicate that human and porcine cytidine deaminases inhibit PERV replication, thereby possibly reducing the risk for infection of human cells by PERV as a consequence of pig-to-human xenotransplantation.

Reappraisal of biosafety risks posed by PERVs in xenotransplantation

Donor materials of porcine origin could potentially provide an alternative source of cells, tissues or whole organs for transplantation to humans, but is hampered by the health risk posed by infection with porcine viruses. Although pigs can be bred in such a way that all known exogenous microorganisms are eliminated, this is not feasible for all endogenous pathogens, such as the porcine endogenous retroviruses (PERVs) which are present in the germline of pigs as proviruses. Upon transplantation, PERV proviruses would be transferred to the human recipient along with the xenograft. If xenotransplantation stimulates or facilitates replication of PERVs in the new hosts, a risk exists for adaptation of the virus to humans and subsequent spread of these viruses. In a worst-case scenario, this might result in the emergence of a new viral disease. Although the concerns for disease potential of PERVs are easing, only limited pre-clinical and clinical data are available. Small-scale, well-designed and carefully controlled clinical trials would provide more evidence on the safety of this approach and allow a better appreciation of the risks involved. It is therefore important to have a framework of protective measures and monitoring protocols in place to facilitate such initially small scale clinical trials. This framework will raise ethical and social considerations regarding acceptability.

Beta-cell replacement and regeneration: Strategies of cell-based therapy for type 1 diabetes mellitus

Pancreatic islet transplantation has demonstrated that long-term insulin independence may be achieved in patients suffering from diabetes mellitus type 1. However, because of limited availability of islet tissue, new sources of insulin producing cells that are responsive to glucose are required. Development of pancreatic beta-cell lines from rodent or human origin has progressed slowly in recent years. Current experiments for ex vivo expansion of beta cells and in vitro differentiation of embryonic and adult stem cells into insulin producing beta-cell phenotypes led to promising results. Nevertheless, the cells generated to date lack important characteristics of mature beta cells and generally display reduced insulin secretion and loss of proliferative capacity. Therefore, much better understanding of the mechanisms that regulate expansion and differentiation of stem/progenitor cells is necessary. Here, we review recent advances in the identification of potential cellular sources, and the development of strategies to regenerate or fabricate insulin producing and glucose sensing cells that might enable future cell-based therapies of diabetes mellitus type 1.

Engraftment of adult porcine islet xenografts in diabetic nonhuman primates through targeting of costimulation pathways

Recent advances in human allogeneic islet transplantation have established beta-cell replacement therapy as a potentially viable treatment option for individuals afflicted with Type 1 diabetes. Two recent successes, one involving neonatal porcine islet xenografts transplanted into diabetic rhesus macaques treated with a costimulation blockade-based regimen and the other involving diabetic cynomolgus monkeys transplanted with adult porcine islet xenografts treated with an alternative multidrug immunosuppressive regimen have demonstrated the feasibility of porcine islet xenotransplantation in nonhuman primate models. In the current study, we assessed whether transplantation of adult porcine islet xenografts into pancreatectomized macaques, under the cover of a costimulation blockade-based immunosuppressive regimen (CD28 and CD154 blockade), could correct hyperglycemia. Our findings suggest that the adult porcine islets transplanted into rhesus macaques receiving a costimulation blockade-based regimen are not uniformly subject to hyperacute rejection, can engraft (2/5 recipients), and have the potential to provide sustained normoglycemia. These results provide further evidence to suggest that porcine islet xenotransplantation may be an attainable strategy to alleviate the islet supply crisis that is one of the principal obstacles to large-scale application of islet replacement therapy in the treatment of Type 1 diabetes.

[Clinical xenotransplantation]

The growing numerical gap between the number of patients and available human donor organs have led to a revival interest in xenotransplantation. This review will mainly focus on the clinical affairs of xenotransplantation and the project of producing the gene modified pigs. Trials, designed to overcome xenogenic rejection by the expression of human complement regulatory protein (CRP), such as DAF (CD55), on the pig organ and knocking out the alpha-Gal epitope(Galalpha1-3Galbeta1-4GlcNAc-R), which is biosynthesized by the action of alpha1,3 galactosyltransferase (alpha1,3GT), were accomplished in several institutes, such as Harvard University, Pittsburgh University, Mayo Clinic, and BresaGen. We have also produced the [DAF(CD55)+GnT-III+alpha-Gal KO] pigs in last year. On the other hand, the clinical pig islets transplantation was done in many countries, such as Russia, Sweden, Mexico and China, until 2005. In addition, the new clinical trials of pig islets transplantation will be started in USA within three years. In addition, as the current studies in the xenotransplantation field, the strategies for the downregulation of the glycoantigen, complement activation, NK cell, and other immuno responces on the xenografts, are reviewed. The studies for the infectivity of porcine endogenous retrovirus (PERV) to human cells are also introduced.

No evidence of PERV infection in healthcare workers exposed to transgenic porcine liver extracorporeal support

BACKGROUND

Clinical xenotransplantation holds great promise by providing one solution to the shortage of human organs for transplantation, while also posing a potential public health threat by facilitating transmission of infectious disease from source animals to humans. One potential vector for infectious disease transmission is healthcare workers (HCW) who are involved in administering xenotransplantation procedures.

METHODS

In this study, we studied 49 healthcare workers involved in the care of two subjects who participated in a study of porcine liver perfusion as treatment of fulminant hepatic failure. We looked for serologic and virologic evidence of transmission of porcine endogenous retrovirus, and found that HCW had no evidence of infection.

CONCLUSIONS

Results of our survey demonstrate that application of standard precautions may be sufficient to prevent transmission of porcine endogenous retrovirus, an agent of concern in ex vivo xenotransplantation products.

Selective inhibition of porcine endogenous retrovirus replication in human cells by acyclic nucleoside phosphonates

Several anti-human immunodeficiency virus type 1 reverse transcriptase inhibitors were evaluated for their antiviral activities against porcine endogenous retrovirus in human cells. Among the test compounds, zidovudine was found to be the most active. The order of potency was zidovudine > phosphonylmethoxyethoxydiaminopyrimidine = phosphonylmethoxypropyldiaminopurine > tenofovir > or = adefovir > stavudine.

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.

Transient transmission of porcine endogenous retrovirus to fetal lambs after pig islet tissue xenotransplantation

Evidence for the in vivo transmission of porcine endogenous retrovirus (PERV) from porcine xenografts to various recipient animals has been inconsistent. To characterize the contribution of the host immune system to the potential for PERV transmission from pig islet tissue xenografts to host tissues, we examined two immunoincompetent animal models, thymectomizsed fetal lambs and NODscid mice. Pig proislets were grafted into fetal lambs or adult NODscid mice. Conventional, nested and real-time PCR/RT-PCR tests were used to search for PERV and pig cell-specific sequences (porcine mitochondrial cytochrome oxidase II (COII) or mitochondrial ribosomal 12S) in pig proislets, host liver and spleen at 5-84 days (lambs) or 96 days (mice) after transplantation. Xenografts were harvested at the same time points. The copy number of PERV sequences and host cell-specific nuclear (palmitoylcarnitine transferase) sequences was assessed by real-time PCR to estimate the proportion of PERV-infected host cells. Pig proislets were shown to be PERV+ve by PCR and immunohistochemistry (PERV B env protein p15E). PERV transmission (PERV A, B or C DNA in the absence of porcine COII or 12S sequences) was detected by nested PCR and real-time PCR in 4/12 fetal lamb liver samples 5-23 days after transplantation; the maximum copy number of PERV B env sequences was found at day 5 (700 copies/1 x 10(6) lamb cells). A total of 4/12 fetal lambs demonstrated both PERV and 12S porcine sequences in liver samples (days 5-84) by real-time PCR, suggesting that pig cells had migrated to those tissues and established microchimerism; nested PCR showed evidence for microchimerism (porcine COII sequences alone) in 2/12 lambs (day 5). The incidence of PERV transmission and frequency of microchimerism was similar in host spleen analysed by real-time PCR. Histological examination showed complete xenograft rejection by 23 days after transplantation to fetal lambs. In contrast, pig proislet xenografts survived long term (> or =day 96) in NODscid mice but no PERV transmission was found. Both nested and real-time PCR assays revealed that 2/3 mice had become microchimeric. Long-term expression of PERV A, B and C as well as porcine 12S or COII RNAs was found at the graft site (day 96) only, indicating that PERV transcription and possibly replication, continued in the donor pig islet tissue after transplantation. Overall, detection of PERV transmission and microchimerism was limited by the sensitivity of the PCR assay and the primers chosen. The absence of stable PERV transmission and microchimerism in fetal lambs and the rejection of pig proislet xenografts correlated in time with the establishment of host immunocompetence. We therefore suggest that the frequent failure to identify PERV transmission late after transplantation could be due to the immunological destruction of PERV-infected host cells. Recipient NODscid mice demonstrated long-term microchimerism and intragraft PERV expression, which was consistent with their stable immunoincompetence.

Bioartificial liver: its pros and cons

Both the large variety of liver functions for maintaining body homeostasis and the proven effectivity of whole liver transplantation in the therapy of acute liver failure (ALF), are important reasons to presume that cell-free liver support systems will not be able to adequately support the failing liver. Accordingly, bioartificial liver (BAL) systems have shown their efficacy in experimental ALF models in small and large animals, and have shown to be suitable and safe in phase 1 studies in humans with ALF. However, the optimal BAL system is still under development. Important issues are the source of the cellular component and the configuration of the BAL system with regard to cell attachment, mass transfer characteristics and oxygenation at site. The deficiency of all BAL systems to excrete bile effectively is another important topic for improvement. The great challenge for the future is to develop a well-functioning and safe human hepatic cell line which can replace the widely used porcine (xenogeneic) hepatocytes. Theoretically, a combination of a cell-free liver support system and a BAL system might be optimal for the treatment of ALF patients in the near future.

Islet xenotransplantation: are we really ready for clinical trials?

Four clinical trials of porcine islet transplantation have been reported, and there are verbal reports that clinical trials on much larger scales are continuing in centers in China and Russia. The four reported trials are briefly reviewed and, in the light of the present status of experimental islet xenotransplantation, consideration is given to whether such trials are currently justified. The Ethics Committee of the International Xenotransplantation Association has (1) emphasized the need for encouraging studies in non-human primates before clinical trials should be undertaken, (2) mandatory monitoring for the transfer of porcine microorganisms, and (3) careful regulation and oversight by recognized bodies. Other aspects of the topic, such as the need for informed consent, are briefly discussed. We conclude that, at the present time, more data documenting convincing efficacy, focused on clinically applicable immunosuppressive regimens, are needed to justify the initiation of closely monitored clinical trials. A clinical trial may then be justified even though the potential risk to the patients, and possibly for society, will not be zero.

Biological and Biomaterial Approaches for Improved Islet Transplantation

Islet transplantation may be used to treat type I diabetes. Despite tremendous progress in islet isolation, culture, and preservation, the clinical use of this modality of treatment is limited due to post-transplantation challenges to the islets such as the failure to revascularize and immune destruction of the islet graft. In addition, the need for lifelong strong immunosuppressing agents restricts the use of this option to a limited subset of patients, which is further restricted by the unmet need for large numbers of islets. Inadequate islet supply issues are being addressed by regeneration therapy and xenotransplantation. Various strategies are being tried to prevent beta-cell death, including immunoisolation using semipermeable biocompatible polymeric capsules and induction of immune tolerance. Genetic modification of islets promises to complement all these strategies toward the success of islet transplantation. Furthermore, synergistic application of more than one strategy is required for improving the success of islet transplantation. This review will critically address various insights developed in each individual strategy and for multipronged approaches, which will be helpful in achieving better outcomes.

Pancreatic islet transplantation for treating diabetes

Pancreatic islet transplantation is one of the options for treating diabetes and has been shown to improve the quality of life of severe diabetic patients. Since the Edmonton protocol was announced, islet transplantation have advanced considerably, including islet after kidney transplantation, utilisation of non-heart-beating donors, single-donor islet transplantation and living-donor islet transplantation. These advances were based on revised immunosuppression protocols, improved pancreas procurement and islet isolation methods, and enhanced islet engraftment. Further improvements are necessary to make islet transplantation a routine clinical treatment. To synergise efforts towards a cure for type 1 diabetes, a Diabetes Research Institute (DRI) Federation is currently being established to include leading diabetes research centres worldwide, including DRIs in Miami, Edmonton and Kyoto among others.

Mice transgenic for a human porcine endogenous retrovirus receptor are susceptible to productive viral infection

Porcine endogenous retrovirus (PERV) is considered one of the major risks in xenotransplantation. No valid animal model has been established to evaluate the risks associated with PERV transmission to human patients by pig tissue xenotransplantation or to study the potential pathogenesis associated with PERV infection. In previous work we isolated two genes encoding functional human PERV receptors and proved that introduction of these into mouse fibroblasts allowed the normally nonpermissive mouse cells to become productively infected (T. A. Ericsson, Y. Takeuchi, C. Templin, G. Quinn, S. F. Farhadian, J. C. Wood, B. A. Oldmixon, K. M. Suling, J. K. Ishii, Y. Kitagawa, T. Miyazawa, D. R. Salomon, R. A. Weiss, and C. Patience, Proc. Natl. Acad. Sci. USA 100:6759-6764, 2003). In the present study we created mice transgenic for human PERV-A receptor 2 (HuPAR-2). After inoculation of transgenic animals with infectious PERV supernatants, viral DNA and RNA were detected at multiple time points, indicating productive replication. This establishes the role of HuPAR-2 in PERV infection in vivo; in addition, these transgenic mice represent a new model for determining the risk of PERV transmission and potential pathogenesis. These mice also create a unique opportunity to study the immune response to PERV infection and test potential therapeutic or preventative modalities.

Variation of host cell tropism of porcine endogenous retroviruses expressed in chinese Banna minipig inbred

A serious donor-organ shortage urges the use of animal donors to treat a wide appropriate variety of major health problems including organ failure and diabetes. However, the promise of clinical xenotransplantation is offset at the present time by the potential of a public health risk due to the cross-species transmission of pathogens from animal donors to human patients. In particular, the transmission of porcine endogenous retrovirus (PERV) is a major concern. In this study, cell tropism of PERV was tested by in vitro infection of human primary cells and cell lines. Coculture of PERV supernatant derived from PK15 with human primary cells and cell lines resulted in the transfer and expression of PERV-specific sequences and the establishment of a productive infection. In the detection of tropism variation of PERV in pigs, 293 cells were cocultured with mitogenic-activated and lethally irradiated PBMC from 12 Banna minipig inbred (BMI). The results were that six coculture groups were PERV-positive. However, infectious virus was not detected when activated PBMC from the other 7 pigs were cocultivated with human cells known to be permissive for PERV, which indicated a tropism variation among the tested individuals. All these findings demonstrate that the presence of endogenous viruses in source animals needs to be carefully considered when the infectious disease potential of xenotransplantation is being assessed.

The bioartificial pancreas: progress and challenges

Diabetes remains a devastating disease, with tremendous cost in terms of human suffering and healthcare expenditures. A bioartificial pancreas has the potential as a promising approach to preventing or reversing complications associated with this disease. Bioartificial pancreatic constructs are based on encapsulation of islet cells with a semipermeable membrane so that cells can be protected from the host's immune system. Encapsulation of islet cells eliminates the requirement of immunosuppressive drugs, and offers a possible solution to the shortage of donors as it may allow the use of animal islets or insulin-producing cells engineered from stem cells. During the past 2 decades, several major approaches for immunoprotection of islets have been studied. The microencapsulation approach is quite promising because of its improved diffusion capacity, and technical ease of transplantation. It has the potential for providing an effective long-term treatment or cure of Type 1 diabetes.

Xenotransplantation of neonatal porcine liver cells

Xenotransplantation of porcine liver cell types may provide a means of overcoming the shortage of suitable donor tissues to treat hepatic diseases characterized by inherited inborn errors of metabolism or protein production. Here we report the successful isolation, culture, and xenotransplantation of liver cells harvested from 7- to 10-day-old piglets. Liver cells were isolated and cultured immediately after harvesting. Cell viability was excellent (>90%) over the duration of the in vitro studies (3 weeks) and the cultured cells continued to significantly proliferate. These cells also retained their normal secretory and metabolic capabilities as determined by continued release of albumin, factor 8, and indocyanin green (ICG) uptake. After 3 weeks in culture, porcine liver cells were loaded into immunoisolatory macro devices (Theracyte devices) and placed into the intraperitoneal cavity of immunocompetant CD1 mice. Eight weeks later, the devices were retrieved and the cells analyzed for posttransplant determinations of survival and function. Post mortem analysis confirmed that the cell-loaded devices were biocompatible, and were well-tolerated without inducing any notable inflammatory reaction in the tissues immediately surrounding the encapsulated cells. Finally, the encapsulated liver cells remained viable and functional as determined by histologic analyses and ICG uptake/release. The successful harvesting, culturing, and xenotransplantation of functional neonatal pig liver cells support the continued development of this approach for treating a range of currently undertreated or intractable hepatic diseases.

No evidence of in vitro and in vivo porcine endogenous retrovirus infection after plasmapheresis through the AMC-bioartificial liver

BACKGROUND

Currently a number of bioartificial livers (BAL) based on porcine liver cells have been developed as a treatment to bridge acute liver failure patients to orthotopic liver transplantation or liver regeneration. These xenotransplantation related treatments hold the risk of infection of treated patients by porcine endogenous retrovirus (PERV) released from the porcine cells, as in vitro infection experiments and transplantations in immunocompromised mice have shown that PERV is able to infect human cells. The Academic Medical Center (AMC)-BAL, unlike other BALs, is characterized by direct contact between porcine liver cells and human plasma, and might therefore be permissive for PERV transfer.

METHODS

Prior to a clinical phase I trial, human plasma perfused through the AMC-BAL was investigated for PERV DNA and RNA. Moreover productive infectivity was analyzed by exposing the plasma to HEK-293 cells that were subsequently tested for PERV DNA, PERV RNA and reverse transcriptase activity.

RESULTS

Although PERV DNA was detected in the perfused plasma, no productive infectivity was detected. Consequently fourteen patients were treated with the AMC-BAL and monitored for PERV transmission. Immediately after treatment the plasma of the patients was positive for PERV DNA, most probably due to porcine liver cell lysis. The PERV DNA was cleared within 2 weeks post-treatment and no PERV RNA was detected. No productive infectivity in human embryonic kidney (HEK)-293 cells exposed to plasma of treated patients was detectable.

CONCLUSION

To conclude, no release of infective PERV particles from the AMC-BAL was observed. Therefore we consider the AMC-BAL as safe, however careful surveillance of patients will be continued.

Porcine endogenous retrovirus encodes xenoantigens involved in porcine cellular xenograft rejection by mice

BACKGROUND

Identification of the antigens that stimulate transplant rejection can help develop graft-specific antirejection strategies. The xenoantigens recognized during rejection of porcine cellular xenografts have not been clearly defined, but it has been assumed that major histocompatibility complex (MHC) xenoantigens are involved.

METHODS

The role of porcine endogenous retrovirus (PERV) as a source of xenoantigens was examined. The authors used morphometry to compare the kinetics of swine leukocyte antigen (SLA) pig thyroid xenograft rejection in control mice and mice immunized with PERV PK15 cells (porcine kidney epithelial cells), PERV SLA pig peripheral blood lymphocytes (PBL), PERV virions purified from PK15 cells, and PERV or PERV A pseudotypes produced from infected human 293 cells. The tempo of rejection for cellular xenografts of PERV A pseudotype-producing human 293 cells, uninfected human 293 cells, and PK15 cells in PERV-preimmunized and control mice was also compared.

RESULTS

Mice immunized with PK15 cells rejected pig thyroid xenografts significantly faster at day 5 than control mice and mice immunized with pig PBL. This correlated with the amount of PERV RNA and virions produced, but not with the amount of SLA class I MHC expressed by PK15 cells. Immunization of mice with PERV virions purified from porcine PK15 cells and with PERV virions or PERV A pseudotypes produced by human 293 cells also induced accelerated xenograft rejection by 5 days. Accelerated rejection induced by virus pretreatment was CD4 T-cell dependent and restricted to PERV-expressing cellular xenografts of porcine or nonporcine origin.

CONCLUSIONS

PERV acts as a significant source of xenoantigens that target porcine cellular xenografts for rejection.

Animal models to study adult stem cell-derived, in vitro-generated islet implantation

Type 1 diabetes (T1D) is an autoimmune disease characterized by hyperglycemia following the destruction of the insulin-producing beta cells of the pancreatic islets of Langerhans by the body's own immune system. Although routine insulin injections can provide diabetic patients with their daily insulin requirements, this treatment is not always effective in maintaining normal glucose levels. A true "cure" is considered possible only through replacement of the beta cell mass, by pancreas transplantation, islet implantation, or implantation of nonendocrine cells modified to secrete insulin. With the recent success of islet implantation to reverse T1D, this procedure has become a welcome therapy for T1D patients. Unfortunately, this procedure is hampered by the limited number of transplantation quality pancreata available for the harvesting of islets. This shortage has sparked great interest in finding a replacement for organ donation, primarily the possible use of stem cell-derived islets starting with stem cells, or alternatively the harvesting of nonhuman islets. This review focuses on progress with growing islets in the laboratory from stem cells and a comparison between this developing technology and the current use of islets harvested from nonhuman sources.

Progress in xenotransplantation

Organ transplantation is considered the most effective treatment for end-stage organ failure; currently it is limited by a severe worldwide shortage of human donor organs. This has led to investigation of the potential use of animals as organ donors. For a number of reasons, the pig represents the most likely organ donor candidate. Transplantation of a vascularised porcine organ into a human or non-human primate results in an immediate and dramatic rejection process, known as hyperacute rejection, which is mediated by the binding of pre-existing antibody to the porcine graft and the subsequent activation of host complement. Strategies aimed at preventing this initial rejection have been largely successful in experimental models. This has allowed attention to turn towards an understanding of the immunological barriers comprising the next phase of xenograft rejection, termed acute vascular rejection. This delayed rejection process appears to be a humoral event, and it is likely that the control of antibody synthesis will play a pivotal role in overcoming the current barrier to successful xenotransplantation.

The HepatAssist® Bioartificial Liver Support System: clinical study and pig hepatocyte process

Based on perfusion of patient plasma through a circuit incorporating a hollow fibre membrane cartridge containing porcine hepatocytes, a bioartificial support system, or HepatAssist System, has been developed. Thirty-nine patients with acute liver failure (ALF) were treated in a Phase I multicentre clinical study. Thirty-two patients were bridged to orthotopic liver transplantation and six patients recovered without requiring a graft. Patient survival rate at one month was 90%. On the basis of these results, a multicentre Phase II/III, randomised, controlled, parallel group study of the HepatAssist System, compared to standard care in ALF patients has been initiated.

Cytoprotection of PEG-modified adult porcine pancreatic islets for improved xenotransplantation

Functional poly(ethylene glycol) (PEG) derivatives, including monosuccinimidyl PEG (MSPEG) with molecular weight (MW) of 2000 (2 kDa) as well as 5 kDa and disuccinimidyl PEG (DSPEG) with MW of 3 and 6 kDa, were synthesized and characterized. They were used to modify the surface of adult porcine islets for cytoprotection. The islets were isolated, purified and modified with functional PEG. Untreated porcine islets were used as control. An in vitro human antibody/complement-mediated cytotoxicity test based on the release of intracellular lactate dehydrogenase was used to evaluate cytotoxicity of human serum to the modified islets. In vitro cell viability was assessed using membrane-integrity straining and islet metabolism in culture. In vitro islet functionality was evaluated by glucose-stimulated insulin release of islets in static incubation with human serum. In vivo islet functionality was evaluated by monitoring non-fasting blood glucose level in streptozotocin-induced diabetic (SCID) immunocompromized mice after intraportal transplantation of porcine islets. Results show that all the PEG derivatives used in the study showed significant in vitro and in vivo cytoprotections against cytotoxic effects elicited by human serum and diabetic SCID mice, respectively, to porcine islets. DSPEG derivatives combined with human albumin exhibited a better cytoprotection, as compared to MSPEG ones, due to the capacity of the succinimidyl groups to selectively react with amino groups of the albumin under physiological conditions. The effects of both MW and concentration of the PEG derivatives on cytoprotection were significant. It appears that this novel biotechnology will be an attractive approach for improved xenotransplantation of islets.

Monitoring for presence of potentially xenotic viruses in recipients of pig islet xenotransplantation

This study represents a long-term follow-up of human patients receiving pig islet xenotransplantation. Eighteen patients had been monitored for up to 9 years for potentially xenotic pig viruses: pig endogenous retrovirus, pig cytomegalovirus, pig lymphotropic herpesvirus, and pig circovirus type 2. No evidence of viral infection was found.

Evidence and consequence of porcine endogenous retrovirus recombination

The genetic nature and biological effects of recombination between porcine endogenous retroviruses (PERV) were studied. An infectious molecular clone was generated from a high-titer, human-tropic PERV isolate, PERV-A 14/220 (B. A. Oldmixon, et al. J. Virol. 76:3045-3048, 2002; T. A. Ericsson et al. Proc. Natl. Acad. Sci. USA 100:6759-6764, 2003). To analyze this sequence and 15 available full-length PERV nucleotide sequences, we developed a sequence comparison program, LOHA(TM) to calculate local sequence homology between two sequences. This analysis determined that PERV-A 14/220 arose by homologous recombination of a PERV-C genome replacing an 850-bp region around the pol-env junction with that of a PERV-A sequence. This 850-bp PERV-A sequence encompasses the env receptor binding domain, thereby conferring a wide host range including human cells. In addition, we determined that multiple regions derived from PERV-C are responsible for the increased infectious titer of PERV-A 14/220. Thus, a single recombination event may be a fast and effective way to generate high-titer, potentially harmful PERV. Further, local homology and phylogenetic analyses between 16 full-length sequences revealed evidence for other recombination events in the past that give rise to other PERV genomes that possess the PERV-A, but not the PERV-B, env gene. These results indicate that PERV-A env is more prone to recombination with heterogeneous backbone genomes than PERV-B env. Such recombination events that generate more active PERV-A appear to occur in pigs rather frequently, which increases the potential risk of zoonotic PERV transmission. In this context, pigs lacking non-human-tropic PERV-C would be more suitable as donor animals for clinical xenotransplantation.

Transplanting encephalomyocarditis virus-infected porcine islet cells reverses diabetes in recipient mice but also transmits the virus

Previous studies demonstrated that porcine encephalomyocarditis virus (EMCV) caused acute and persistent infection in the myocardium, central nervous system, and spleen of non-human primates (cynomolgus macaques); and it productively infected primary human cardiomyocytes, suggesting that the virus may pose a risk in pig-to-human transplantation. Recently, transplantation of myocardial and pancreatic tissues from acutely infected pigs transmitted the virus to recipient mice, resulting in acute fatal EMCV disease. Here, we examined whether porcine islet cells (PICs), which are under clinical trial for treatment of type I diabetes in humans, are susceptible to porcine EMCV, and whether EMCV-infected PICs could function in vivo to reverse diabetes. PICs were infected with EMCV in vitro for 5 h, and resulting insulin production compared with that produced by uninfected PICs. Subsequently, infected PICs were transplanted intra-abdominally or under the kidney capsule of C57BL/6 mice, and both virus transmission and PIC function analyzed. PICs were highly susceptible to porcine EMCV, resulting in a 1500-fold increase in production of infectious virus within 5 h of inoculation and cytolysis that destroyed up to 50% of cells within 96 h. However, as long as they were viable, infected PICs produced insulin at levels comparable with uninfected PICs. Intra-abdominal transplantation of 2000 PICs, infected with one plaque forming unit (pfu) per cell of porcine EMCV, into C57BL/6 mice transmitted the virus resulting in acute fatal EMCV disease characterized by hind limb paresis and paralysis and acute respiratory distress in 40% of recipient mice. More importantly, transplantation of 2500 EMCV-infected PICs under the kidney capsule of diabetic C57BL/6 mice (glucose level > or =350 mg/dl) reversed diabetes in 83% of recipient mice (glucose level < or =170 mg/dl); however these mice succumbed to acute EMCV disease transmitted by the xenograft 5 days after transplantation. EMCV infection does not appear to affect insulin production by PICs, but infected xenografts can transmit the virus to recipient animals, resulting in severe disease.

Prevalent human coxsackie B-5 virus infects porcine islet cells primarily using the coxsackie-adenovirus receptor

BACKGROUND

We have previously demonstrated that transplanting porcine encephalomyocarditis virus (EMCV)-infected porcine islet cells (PICs) results in transmission of the virus to recipient mice, which is manifested by acute fatal infection within 5 to 8 days. Here, we determined PIC susceptibility to a related and highly prevalent human picornavirus, coxsackie B-5 virus (CVB-5).

METHODS

PICs were inoculated with CVB-5 in vitro for up to 96 hours and infectivity, level of virus replication, and cellular function determined. Subsequently, monoclonal and polyclonal antibody blocking experiments were used to investigate the receptor CVB-5 uses to enter PICs, and the ability of CVB-5-infected islets to reverse diabetes analyzed in mice.

RESULTS

Adult pig islets inoculated with CVB-5 in vitro showed a typical picornaviral replication cycle with a 2-h lag phase followed by a 4-h exponential phase during which the virus titer increased by 4 logs. However, CVB-5 was less cytolytic to PICs than EMCV, resulting in a persistent productive infection lasting for up to 96 h, with minimal evidence of cell lysis. Double immunostaining confirmed the presence of CVB-5 antigens in insulin-producing islets. Infection of PICs in the presence of antibodies against human coxsackie-adenovirus receptor (CAR) resulted in near complete blockage in production of infectious virus particles whereas blocking with anti-porcine decay-accelerating factor (DAF, also called CD55) or anti-porcine membrane cofactor protein (MCP, also called CD46) only slightly decreased the number of infectious CVB-5 particles produced. Immunofluoresence staining showed CAR and MCP expression on the islet surface, but not DAF. Transplanting CVB-5-infected PICs into diabetic C57BL/6 mice resulted in reversal of diabetes.

CONCLUSION

Although PICs are susceptible to human CVB-5, the infection does not appear to affect xenograft function in vitro or in vivo in the short term.

Position Paper of the Ethics Committee of the International Xenotransplantation Association

Xenotransplantation (XTx) provides a potential solution to the shortage of human organs and tissues, and has several advantages over other possible solutions to this problem. However, a number of scientific and ethical barriers exist, and need to be addressed in order to advance the field of XTx in a manner that optimizes its potential to benefit society and minimizes its risk. Some of the most pressing ethical issues are discussed, and the position of the Ethics Committee of the International Xenotransplantation Association is presented.

Mouse retrovirus mediates porcine endogenous retrovirus transmission into human cells in long-term human-porcine chimeric mice

Porcine endogenous retrovirus (PERV) is a potential pathogen in clinical xenotransplantation; transmission of PERV in vivo has been suggested in murine xenotransplantation models. We analyzed the transmission of PERV to human cells in vivo using a model in which immunodeficient NOD/SCID transgenic mice were transplanted with porcine and human lymphohematopoietic tissues. Our results demonstrate, we believe for the first time, that human and pig cells can coexist long-term (up to 25 weeks) without direct PERV infection of human cells. Despite the transplantation of porcine cells that did not produce human-tropic PERV, human cells from the chimeric mice were frequently found to contain PERV sequences. However, this transmission was due to the pseudotyping of PERV-C (a virus without human tropism) by xenotropic murine leukemia virus, rather than to de novo generation of human-tropic PERV. Thus, pseudotyping might account for the PERV transmission previously observed in mice. The absence of direct human cell infection following long-term in vivo coexistence with large numbers of porcine cells provides encouragement regarding the potential safety of using pigs that do not produce human-tropic PERV as source animals for transplantation to humans.

In vitro evaluation of cross-circulation system using semipermeable membrane combined with whole liver perfusion

INTRODUCTION

Many types of isolated hepatocytes-based bioartificial liver have been developed. However, to maintain hepatocyte-specific functions for a long period is still a significant challenge. The possibilities of rejection or viral transmission still remain as untackled obstacles. We developed a cross-circulation system, using a semipermeable membrane combined with whole liver perfusion. Detoxifying functions of the extracorporeal porcine liver and molecular movements across the membrane were evaluated in vitro.

METHODS

The hollow-fiber module has a molecular cutoff of 100 kD. A spiked solution containing 500 mL low molecular dextran solution spiked with 12 mg ammonium chloride, 500 mg D-galactose, and 300 mg lidocaine, which mimicked a patient, was recirculated through the inner fiber space. The extracorporeal liver perfusion circuit consisted of an extra-fiber spaces. A reservoir containing 1000 mL healthy pig plasma, a membrane oxygenator, and a porcine whole liver. Both circuits circulated in the opposite direction for 6 hours.

RESULT

In 6 hours, 47.3% +/- 10.2% of ammonia, 89.5% +/- 1.7% of D-galactose, and 95.5% +/- 1.0% of lidocaine were eliminated from the circuits; 66.5 +/- 11.1 mg of urea were produced at the same time. Oxygen consumption was maintained between 0.248 and 0.259 mL/100 g liver/min for 6 hours. Movement of IgM was completely blocked by the 100-kD membrane, whereas albumin was freely transferred from the reservoir to the intrafiber space.

CONCLUSION

The perfusion experiments showed the possibility of using a whole liver with oxygenated plasma perfusion in a bioartificial liver system in vitro.