The production of transgenic pigs for potential use in clinical xenotransplantation: microbiological evaluation

Monitoring swine virus transmission in embryos derived from commercial abattoir oocytes

Pigs are pivotal in agriculture and biomedical research and hold promise for xenotransplantation. Specific-pathogen-free (SPF) herds are essential for commercial swine production and xenotransplantation research facilities. Commercial herds aim to safeguard animal health, welfare, and productivity, and research facilities require SPF status to protect immunocompromised patients. Somatic cell nuclear transfer (SCNT) embryos are the norm for producing cloned and genetically edited animals. Oocytes for embryo reconstruction are most conveniently sourced from commercial abattoirs with unclear disease statuses. However, research on viral clearance from donor oocytes during embryo reconstruction remains limited. SCNT has previously been shown to reduce the transmission of Porcine reproductive and respiratory syndrome virus, Bovine viral diarrhea virus, Porcine Circovirus type 2, and Porcine parvovirus. Still, it is lacking for other pathogens, including endogenous viruses. This project contains two preliminary studies investigating the polymerase chain reaction (PCR) assay detection of common swine viruses through the phases of producing parthenogenic and SCNT embryos. Exogenous pathogens detected in oocyte donor tissue or the oocyte maturation media were not detected in the produced embryos. Porcine endogenous retrovirus type C (PERVC) was not removed by parthenogenic embryo activation and was detected in 1 of the 2 tested SCNT embryos reconstructed using a PERVC-negative cell line. SCNT and parthenogenic embryo construction similarly reduced exogenous virus detection. SCNT embryo construction helped reduce endogenous virus detection. This project demonstrates the importance of screening embryos for endogenous viruses and shows the usefulness of parthenogenic embryos in future exogenous virus clearance studies.

Screening and Identification of the First Non-CRISPR/Cas9-Treated Chinese Miniature Pig With Defective Porcine Endogenous Retrovirus pol Genes

Pig to human xenotransplantation is considered to be a possible approach to alleviate the shortage of human allografts. Porcine endogenous retrovirus (PERV) is the most significant pathogen in xenotransplantation. We screened for pigs that consistently did not transmit human-tropic replication competent PERVs (HTRC PERVs), namely, non-transmitting pigs. Then, we conducted whole-genome resequencing and full-length transcriptome sequencing to further investigate the sequence characteristics of one non-transmitting pig. Using in vitro transmission assays, we found 5 (out of 105) pigs of the Chinese Wuzhishan minipig inbred line that did not transmit PERV to human cells, i.e., non-transmitting pigs. Whole-genome resequencing and full-length transcriptome sequencing of one non-transmitting pig showed that all of the pol genes were defective at both the genome and transcript levels. We speculate that the defective PERV pol genes in this pig might be attributable to the long-term inbreeding process. This discovery is promising for the development of a strain of highly homozygous and genetically stable pigs with defective PERV pol genes as a source animal species for xenotransplantation.

Analysis of Protein Expression in Human Cells Cocultured with Porcine Peripheral Blood Mononuclear Cells

OBJECTIVE

Porcine endogenous retroviruses (PERV) involved in pig to human xenotransplantation have raised great concerns because of their ubiquitous nature in pigs and their ability of infecting human cells in vitro. Although no significant cytopathic effect attributed to PERV was evident on PERV-infected human embryonic kidney 293 (HEK293) cells, we did proteomic analysis to investigate the differences of protein profile in order to further characterize the effect of PERV infection.

METHODS

HEK293 cells were cocultured with porcine peripheral blood mononuclear cells (PBMCs). Protein profiles of PERV-infected and -noninfected HEK293 cells were analyzed by two-dimensional gel electrophoresis (2-DE). Protein spots with at least 1.5-fold alteration were identified by high-definition mass spectrometry (HDMS) analysis. Then real-time RT-PCR and Western blotting were performed to validate the proteomic results.

RESULTS

Differential analysis of PERV-infected and -noninfected HEK293 cells by 2-DE revealed ten differentially regulated proteins. The proteins identified by HDMS were involved in various cellular pathways including signal transduction, cell apoptosis, and protein synthesis.

CONCLUSION

The results of this study revealed differentially expressed proteins in HEK293 cells cocultured with porcine PBMCs and implied that these changes were probably induced by PERV infection. These results provide clues and potential links to understanding the molecular effect of the infection by human-tropic PERV.

Early weaning completely eliminates porcine cytomegalovirus from a newly established pig donor facility for xenotransplantation

For clinical xenotransplantation, transplants must be free of porcine cytomegalovirus (PCMV). Piglets become infected primarily in the perinatal period by the mother sow. While individual donor animals can be protected from infection by isolation husbandry, success is not guaranteed and this strategy poses the risk of undetected infections and raises animal welfare questions. Here, we present the establishment of a completely PCMV-negative pig herd for breeding donor animals for xenotransplantation. Eleven pregnant DanAvl Basic hybrid sows were purchased from a designated pathogen-free (DPF), PCMV-positive colony and transferred to a new pig facility at the Centre for Innovative Medical Models (CiMM) 4 weeks prior to farrowing. At the age of 24 hours, piglets were early-weaned and transferred to a commercially available Rescue Deck system dedicated to motherless rearing of piglets. Sows were removed from the facility. The PCMV status of F1-generation animals was determined at regular intervals over a period of 14 months by a sensitive real-time PCR-based detection method testing blood, nasal swabs and cultured peripheral blood mononuclear cells (PBMCs). F1 sows were used as recipients of genetically modified embryos to generate a xenotransplant donor herd. Offspring were tested for PCMV accordingly. All offspring have remained PCMV negative over the whole observation period of 14 months. A completely PCMV-negative pig herd for xenotransplantation has thus been successfully established.

Porcine Islet Graft Function is Affected by Pretreatment with a Caspase-3 Inhibitor

During the isolation procedure and after transplantation islets are subjected to numerous variables associated with the induction of apoptosis. The present study investigated the effect of transient pretreatment with caspase inhibitors on function and survival of transplanted pig islets. Isolated porcine islets (3000 IEQ) were incubated overnight in 200 μM of the caspase-3 inhibitor DEVD-CMK prior to transplantation into diabetic nude mice. Glucose-stimulated insulin release of pretreated islets was assessed during static incubation. DEVD-CMK successfully prevented the expression of capase-3 and DFF as demonstrated in heat-shocked pig islets. Nevertheless, transient pretreatment of freshly isolated pig islets with DEVD-CMK resulted in a significantly decreased final graft function of 50.0% (n = 16) compared to 85.7% (n = 14) in control islets (p < 0.05). Glucose-stimulated insulin release of porcine islets (n = 6) was not significantly effected by overnight culture with DEVD-CMK. Morphological assessment revealed that this caspase-3 inhibitor significantly increased the percentage of necrosis to a small, but nevertheless significant, extent in comparison to control islets (p < 0.05). The study demonstrates that short-time pretreatment with the caspase-3 inhibitor DEVD-CMK reduces the capacity of transplanted porcine islets to restore normoglycemia in diabetic nude mice.

Immunological methods for the detection of porcine lymphotropic herpesviruses (PLHV)

Porcine lymphotropic herpesviruses (PLHV-1, -2, and -3) are widespread in pigs and closely related to the human pathogenic gammaherpesviruses Epstein-Barr virus (human herpesvirus 4, HHV-4) and Kaposi sarcoma herpesvirus (HHV-8). In minipigs, PLHV-1 causes a porcine post-transplantation lymphoproliferative disorder (PTLD) after experimental transplantations. Porcine PTLD comes with clinical symptoms similar to those of human PTLD, a serious complication of solid organ and allogeneic bone marrow transplantation linked to HHV-4. Since PLHVs may be transmitted from donor pigs to the human recipient of xenotransplants (pig cells, tissues or organs), sensitive and specific methods should be developed to detect and eliminate PLHVs. Here we describe an ELISA and a Western blot assay using recombinant glycoprotein B of PLHV-1. Using both assays, the presence of specific antibodies in different pig breeds as well as in German slaughterhouse workers was analysed. Antibodies were detected in some animals, but not in human subjects.

Comparison of porcine endogenous retroviruses infectious potential in supernatants of producer cells and in cocultures

BACKGROUND

Porcine endogenous retroviruses (PERV) pose a zoonotic risk potential in pig-to-human xenotransplantation given that PERV capacity to infect different human cell lines in vitro has been clearly shown in the past. However, PERV infectious potential for human peripheral blood mononuclear cells (huPBMC) has been also demonstrated, albeit with controversial results. As productive PERV infection of huPBMC involves immune suppression that may attract opportunistic pathogens as shown for other retroviruses, it is crucial to ascertain unequivocally huPBMC susceptibility for PERV. To address this question, we first investigated in vitro infectivity of PERV for huPBMC using supernatants containing highly infectious PERV-A/C. Second, huPBMC were cocultivated with PERV-A/C producer cells to come a step closer to the in vivo situation of xenotransplantation. In addition, cocultivation of huPBMC with porcine PBMC (poPBMC) isolated from German landrace pigs was performed to distinguish PERV replication competence when they were constitutively produced by immortalized cells or by primary poPBMC.

METHODS

Supernatants containing recombinant highly infectious PERV-A/C were used to infect PHA-activated huPBMC in the presence or absence of polybrene. Next, PERV-producing cell lines such as human 293/5° and primary mitogenically activated poPBMC of three German landrace pigs were cocultivated with huPBMC as well as with susceptible human and porcine cell lines as controls. PERV infection was monitored by using three test approaches. The presence of provirus DNA in putatively infected cells was detected via sensitive nested PCR. Viral expression was determined by screening for the activity of gammaretroviral reverse transcriptase (RT) in cell-free supernatants of infected cells. Virus release was monitored by counting the number of packaged RNA particles in supernatants via PERV-specific quantitative one-step real-time reverse transcriptase PCR.

RESULTS

Porcine endogenous retroviruses-A/C in supernatants of human producer 293/5° cells was not able to infect huPBMC. Neither RT activity nor PERV copies were detected. Even provirus could not be detected displaying the inability of PERV-A/C to induce a productive infection in huPBMC. In cocultivation experiments only non-productive infection of huPBMC with PERV derived from 293/5° cell line and from PHA-activated poPBMC was observed by detection of provirus DNA in infected cells.

CONCLUSION

Recombinant PERV-A/C in supernatants of producer cells failed to infect huPBMC, whereas coculture experiments with producer cell lines lead to non-productive infection of huPBMC. PERV in supernatants seem to have not sufficient infectious potential for huPBMC. However, extensive PERV exposure to huPBMC via cocultivation enabled at least virus cell entry as provirus was detected by nested PCR. Furthermore, results presented support previous data showing German landrace pigs as low producers with negligible infectious potential due to the absence of replication-competent PERV in the genome. The low PERV expression profile and the lack of significant replication competence of German landrace pigs raise hope for considering these animals as putative donor animals in future pig-to-human xenotransplantation. Nonetheless, data imply that PERV still represent a virological risk in the course of xenotransplantation, as the presence of PERV provirus in host cells may lead to a provirus integration resulting in insertional mutagenesis and chromosomal rearrangements.

Up to 9-day survival and control of thrombocytopenia following alpha1,3-galactosyl transferase knockout swine liver xenotransplantation in baboons

BACKGROUND

With standard miniature swine donors, survivals of only 3 days have been achieved in primate liver-transplant recipients. The recent production of alpha1,3-galactosyl transferase knockout (GalT-KO) miniature swine has made it possible to evaluate xenotransplantation of pig organs in clinically relevant pig-to-non-human primate models in the absence of the effects of natural anti-Gal antibodies. We are reporting our results using GalT-KO liver grafts.

METHODS

We performed GalT-KO liver transplants in baboons using an immunosuppressive regimen previously used by our group in xeno heart and kidney transplantation. Post-operative liver function was assessed by laboratory function tests, coagulation parameters and histology.

RESULTS

In two hepatectomized recipients of GalT-KO grafts, post-transplant liver function returned rapidly to normal. Over the first few days, the synthetic products of the donor swine graft appeared to replace those of the baboon. The first recipient survived for 6 days and showed no histopathological evidence of rejection at the time of death from uncontrolled bleeding, probably caused by transfusion-refractory thrombocytopenia. Amicar treatment of the second and third recipients led to maintenance of platelet counts of over 40 000 per μl throughout their 9- and 8-day survivals, which represents the longest reported survival of pig-to-primate liver transplants to date. Both of the last two animals nevertheless succumbed to bleeding and enterococcal infection, without evidence of rejection.

CONCLUSIONS

These observations suggest that thrombocytopenia after liver xenotransplantation may be overcome by Amicar therapy. The coagulopathy and sepsis that nevertheless occurred suggest that additional causes of coagulation disturbance must be addressed, along with better prevention of infection, to achieve long-term survival.

Construction of transgenic swine with induced expression of Cre recombinase

Miniature pigs have been recognized as valuable experimental animals in medical research. However, porcine models related to gene knockout of human diseases are not widely available. The objective of this study was to establish Mx1-Cre pigs using somatic cell nuclear transfer. In this study, we created transgenic pigs using somatic cell nuclear transfer (SCNT). Transfer of 210, 230, 250 and 215 zygotes to four surrogates produced 10 piglets. The Cre recombinase expression in transgenic pigs was studied using reverse transcriptase (RT)-PCR and immunohistochemistry. Mx1-Cre swine were shown to harbor the Cre gene in their genomic DNA using the PCR. In conclusion, Mx1-Cre transgenic piglets were successfully produced by SCNT. These transgenic swine, in conjunction with inducible systems for controlling Cre expression and function, are likely to have a profound impact on the study of human diseases.

Infection barriers to successful xenotransplantation focusing on porcine endogenous retroviruses

Xenotransplantation may be a solution to overcome the shortage of organs for the treatment of patients with organ failure, but it may be associated with the transmission of porcine microorganisms and the development of xenozoonoses. Whereas most microorganisms may be eliminated by pathogen-free breeding of the donor animals, porcine endogenous retroviruses (PERVs) cannot be eliminated, since these are integrated into the genomes of all pigs. Human-tropic PERV-A and -B are present in all pigs and are able to infect human cells. Infection of ecotropic PERV-C is limited to pig cells. PERVs may adapt to host cells by varying the number of LTR-binding transcription factor binding sites. Like all retroviruses, they may induce tumors and/or immunodeficiencies. To date, all experimental, preclinical, and clinical xenotransplantations using pig cells, tissues, and organs have not shown transmission of PERV. Highly sensitive and specific methods have been developed to analyze the PERV status of donor pigs and to monitor recipients for PERV infection. Strategies have been developed to prevent PERV transmission, including selection of PERV-C-negative, low-producer pigs, generation of an effective vaccine, selection of effective antiretrovirals, and generation of animals transgenic for a PERV-specific short hairpin RNA inhibiting PERV expression by RNA interference.

Emerging and re-emerging swine viruses

In the past two decades or so, a number of viruses have emerged in the global swine population. Some, such as porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2), cause economically important diseases in pigs, whereas others such as porcine torque teno virus (TTV), now known as Torque teno sus virus (TTSuV), porcine bocavirus (PBoV) and related novel parvoviruses, porcine kobuvirus, porcine toroviruses (PToV) and porcine lymphotropic herpesviruses (PLHV), are mostly subclinical in swine herds. Although some emerging swine viruses such as swine hepatitis E virus (swine HEV), porcine endogenous retrovirus (PERV) and porcine sapovirus (porcine SaV) may have a limited clinical implication in swine health, they do pose a potential public health concern in humans due to zoonotic (swine HEV) or potential zoonotic (porcine SaV) and xenozoonotic (PERV, PLHV) risks. Other emerging viruses such as Nipah virus, Bungowannah virus and Menangle virus not only cause diseases in pigs but some also pose important zoonotic threat to humans. This article focuses on emerging and re-emerging swine viruses that have a limited or uncertain clinical and economic impact on pig health. The transmission, epidemiology and pathogenic potential of these viruses are discussed. In addition, the two economically important emerging viruses, PRRSV and PCV2, are also briefly discussed to identify important knowledge gaps.

Real-time quantitative polymerase chain reaction with SYBR green i detection for estimating copy numbers of porcine endogenous retrovirus from Chinese miniature pigs

Porcine endogenous retrovirus (PERV) in the pig genome represents a potential infectious risk in xenotransplantation. Chinese miniature pigs have been considered to be potential organ donors in China. However, an adequate level of information on PERV from Chinese miniature pigs has not been available. We established an SYBR Green I-based real-time quantitative polymerase chain reaction (PCR) assay for estimating copy numbers of PERV integrated in the host genome. The assay was 100-fold more sensitive compared with conventional PCR. We also evaluated the specificity and reproducibility of the assay. We statistically analyzed the difference in PERV copy numbers integrated into the genomes of Wuzhishan pigs versus Bama minipigs. This approach will be useful to screen donor pigs as well as to examine clinical samples from human subjects treated with porcine xenotransplantation products for evidence of PERV transmission.

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

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

Identification of full-length proviral DNA of porcine endogenous retrovirus from Chinese Wuzhishan miniature pigs inbred

Existence of porcine endogenous retrovirus (PERV) hinders pigs to be used in clinical xenotransplantation to alleviate the shortage of human transplants. Chinese miniature pigs are potential organ donors for xenotransplantation in China. However, so far, an adequate level of information on the molecular characteristics of PERV from Chinese miniature pigs has not been available. We described here the cloning and characterization of full-length proviral DNA of PERV from Chinese Wuzhishan miniature pigs inbred (WZSP). Full-length nucleotide sequences of PERV-WZSP and other PERVs were aligned and phylogenetic tree was constructed from deduced amino-acid sequences of env. The results demonstrated that the full-length proviral DNA of PERV-WZSP belongs to gammaretrovirus and shares high similarity with other PERVs. Sequence analysis also suggested that different patterns of LTR existed in the same porcine germ line and partial PERV-C sequence may recombine with PERV-A sequence in LTR.

Human fibroblasts are permissive for porcine cytomegalovirus in vitro

BACKGROUND

Xenotransplantation with pig organs is being considered to alleviate donor organ shortages; however, the risk of introducing porcine viruses into humans is heightened in this setting. The goal of this study was to determine the infectious potential of porcine cytomegalovirus (PCMV), a xenozoonotic virus of interest, in human fibroblasts in vitro.

METHODS

Confluent human cells were incubated with live PCMV, heat-killed PCMV, or medium alone. Infection was investigated by testing for viral-induced cytopathic effect, assaying viral transcription by nested RT-PCR and subsequent sequencing, and detecting viral protein expression by Western blotting. Plaque neutralization experiments were also performed.

RESULTS

Cells incubated with PCMV demonstrated significant cytopathic effect by 7 days postinfection, and reverse-transcriptase polymerase chain reaction sequencing identified PCMV DNA polymerase in these infected cells. In Western blots, monoclonal antibodies (mAbs) to human CMV glycoprotein B and pig serum presumed to contain anti-PCMV antibodies detected characteristic proteins in experimentally infected human cells and positive controls but not in negative controls. Furthermore, one of these mAbs and the pig serum neutralized PCMV infection in vitro.

CONCLUSIONS

These results are a first demonstration that PCMV can infect human fibroblasts in vitro.

Recombinant porcine endogenous retroviruses (PERV-A/C): a new risk for xenotransplantation?

PERVs are integrated in the genome of all pigs. Some of them infect human cells and represent therefore a potential risk for xenotransplantation using pig cells or organs. Three replication-competent subtypes have been described, PERV-A, PERV-B and PERV-C. Whereas PERV-A and PERV-B are polytropic viruses and infect, among others, human cells, PERV-C is an ecotropic virus, infecting only pig cells. Recombinant PERV-A/C are able to infect human cells, they are characterised by high-titre replication and their proviruses have been found de novo integrated in the genome of somatic pig cells, but not in the germ line. This review compares recombinant PERVs with other recombinant retroviruses in order to evaluate their potential pathogenicity.

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.

Casuistry and the moral continuum. Evaluating animal biotechnology

While the science of animal biotechnology is advancing at a rapid pace, the ethical discussion about the boundaries the public might want to set is at the most nascent stage. There is a tendency in the public debate for opponents to favor an all-out ban on the science, while proponents want to grant it carte blanche. I argue that a more nuanced position on animal biotechnology considers individual projects to be located on a moral continuum, where some are clearly morally justified, others morally impermissible, and some lie in the ethical gray-zone. To begin to define this continuum, we use the bioethical method of casuistry to analyze one case at the end of moral permissibility, and we contrast it with a case that is located at the opposite end of the moral spectrum. I advocate this approach to assessing the moral merit of biotechnology projects because of its attention to the details of individual cases--the protocols, ends, and methods--on which an accurate moral judgment necessarily rests.

Multiplex PCR for the simultaneous detection of pseudorabies virus, porcine cytomegalovirus, and porcine circovirus in pigs

The use of porcine organs is being developed as a means to alleviate the shortage of human organs for transplantation. Recommendations have been published for the microbiological specifications of organ-source pigs to reduce the possibility of a microorganism from pigs being inadvertently transferred to the recipient of the xenograft. The pseudorabies virus (PRV), porcine cytomegalovirus (PCMV), and porcine circovirus (PCV) are infectious agents in pigs that are considered to be of significance for the microbiological safety of xenotransplantation. A multiplex polymerase chain reaction (mPCR) was developed to detect and differentiate among PRV, PCMV, and PCV. The sensitivities of the multiplex PCR were 10(2.5) TCID(50)/ml for PRV, 10(1.8) TCID(50)/ml for PCMV, and 10(1.8) TCID(50)/ml for PCV. The lowest viral concentrations detected by single PCR were 10(1.5) TCID(50)/ml for PRV, 10(1.0) TCID(50)/ml for PCMV, and 10(1.4) TCID(50)/ml for PCV2. Non-specific reactions were not observed when other viruses, bacteria, and Vero cells were used to assess the multiplex PCR. The multiplex PCR was effective in detecting various combinations of one or more of these viruses in pig specimens collected for xenotransplantation.

History of xenotransplantation

The present historical review reports the clinical experiences of transplantations from animal to human. The first transplantation attempts were made without any knowledge of the species barrier. The pioneers of xenotransplantation realized xenotransfusions as early as the 16th century, then cell and tissue xenotransplantations in the 19th century. At the beginning of the 20th century, xenotransplantation of testicles became the latest craze. At the same time, and later in the 1960s, organ xenotransplantations were attempted, with disappointing results. Mathieu Jaboulay, Serge Voronoff, Keith Reemtsma, James Hardy, Denton Cooley, Thomas Starzl, Christiaan Barnard and Leonard Bailey were among the pionneers of xenotransplantation. Recent trials concerned above all tissue and cell xenotransplantations. Nowadays, with encapsulation, transgenesis, and cloning, great advances have been made for controlling xenograft rejection, but ethical questions linked to the risk of infections have become a major pre-occupation within the scientific community and the general population.

Porcine Endogenous Retroviruses PERV-A and PERV-B Infect neither Mouse Cells in vitro nor SCID Mice in vivo

OBJECTIVE

Porcine endogenous retroviruses (PERVs) pose a risk for xenotransplantations using pig materials as they are present in the genome of all pigs and are able to infect human cells in vitro. Until recently, transmission of PERVs in vivo was only described in severe combined immunodeficient (SCID) and nude mice inoculated with PERV-producing cells. However, in this series of experiments microchimerism could not be excluded. To overcome this problem, the risk of PERV infection was addressed in a similar way but using cell-free inoculation of mouse cells in vitro and SCID mice in vivo.

METHODS

Mouse cell lines and primary cells were incubated in vitro with PERV-A, with a recombinant PERV-A/C and with PERV-B. Provirus integration was assessed by PCR. Reverse transcriptase activity was measured in the cell supernatants. SCID mice were inoculated in vivo with cell-free virus at high titers.

RESULTS

None of the mouse cell lines and primary cells could be infected by PERV and no provirus integration was observed in different organs of the inoculated SCID mice.

CONCLUSION

The data indicate that PERV-A, PERV-A/C and PERV-B could not infect different mouse cells. These data correlate with the recent finding that mouse cells lack a functional receptor for PERV-A. Although the receptor for PERV-B is still unknown, these data suggest that previously reported PERV transmissions to SCID and nude mice in vivo might be due to microchimerism or pseudotyping with murine viruses and indicate that normal mice are an inappropriate model for the study of PERV infection and pathogenesis.

Herpesvirus infections in xenotransplantation: pathogenesis and approaches

Infectious risk remains an important consideration in the clinical application of xenotransplantation. Vascularized xenografts create unique immunological niches in which bidirectional transmission of pathogens between donor and recipient may occur. Enhanced replication of many pathogens is stimulated by the immune responses induced by transplantation and by the immune suppression used to prevent graft rejection. Herpesviruses are the prototype viruses that are activated during immunosuppression. Quantitative diagnostic molecular assays have been developed for the known herpesviruses causing infection in pigs. Recent data suggest that some herpesviral infections, such as porcine cytomegalovirus, may be excluded from swine used as source animals by careful breeding, while others will require novel strategies for control. This review focuses on porcine and baboon herpesviruses in pig-to-non-human primate solid organ xenotransplantation including direct effects (tissue damage), indirect effects (coagulopathy, rejection), and possible approaches to these infections.

Porcine cell microchimerism but lack of productive porcine endogenous retrovirus (PERV) infection in naive and humanized SCID-beige mice treated with porcine peripheral blood mononuclear cells

Pigs are considered a suitable source of cells and organs for xenotransplantation. All known strains of pigs contain porcine endogenous retrovirus (PERV) and PERV released by porcine cells may infect human cells in vitro and severe-combined immunodeficient (SCID) mice in vivo. Humanized SCID (hu-SCID) mice develop immune response to porcine antigens. Here we investigated PERV transmission in humanized SCID-beige mice using porcine peripheral blood mononuclear cells (PBMC) as the donor tissue (and the source of PERV). Mice were infused in the peritoneal cavity with 1.5-3.0 x 10(7) unfractionated human PBMC. Unfractionated porcine PBMC (1.5-3.0 x 10(7) cell/mouse) were infused to the mice simultaneously with human PBMC or 3 weeks after human PBMC infusion. The treated mice were monitored for weight and skin changes, donor cell chimerism, anti-pig antibodies and PERV transmission. All humanized mice tested 5-12 weeks after human PBMC transplantation were macrochimeric (up to 40% of cells in blood) for human cells, where 99% of the human cells were T-lymphocytes. Although human B lymphocytes were very rare in the blood of humanized mice at that point, the mice were positive for human anti-pig natural antibodies. The control SCID-beige mice or mice treated with porcine PBMC alone were negative for anti-porcine antibodies. Approximately 70% of the humanized mice treated with porcine PBMC were also microchimeric for porcine cells. Although some tissue samples of these mice were positive for PERV DNA in the absence of porcine DNA indicating PERV infection, the infection was non-productive as PERV transcripts were not detectable in those tissues. PERV infection of human and mouse cells in vitro by co-culturing with porcine PBMC was also non-productive. Humanized SCID-beige mice suffered weight loss and occasional minor skin changes due to graft vs. host disease caused by human PBMC but none of the mice showed observable effect attributable to the apparent PERV infection alone.

Early weaning of piglets fails to exclude porcine lymphotropic herpesvirus

BACKGROUND

Xenotransplantation using pigs as source species carries a risk for the activation of latent herpesviruses from the porcine donor and potential transmission to the recipient. In pig-to-baboon xenotransplantation, activation of porcine cytomegalovirus (PCMV) has been associated with xenograft injury and an increased incidence of consumptive coagulopathy and graft loss. Activation of porcine lymphotropic herpesvirus (PLHV)-1 was not observed in pig-to-baboon solid organ xenotransplantation, but was associated with a syndrome of post-transplantation lymphoproliferative disorder (PTLD) after allogeneic stem cell transplantation in pigs.

MATERIAL AND METHODS

Early weaning of piglets was used to try to reduce the viral burden of xenograft donors. This consisted of separating the piglets of a litter from the sow within the first 2 weeks after birth and raising them in isolation from the remaining herd.

RESULTS

We have previously demonstrated that PCMV could be excluded from source animals by early weaning of piglets. However, early weaning failed to exclude PLHV-1 from source pigs.

CONCLUSIONS

This disparity between PCMV and PLHV-1 reflects differing pathogenesis of infection of these herpesviruses. New approaches will be needed to exclude PLHV-1 from pig colonies.

Determinants of high titer in recombinant porcine endogenous retroviruses

Porcine endogenous retroviruses (PERVs) pose a potential stumbling block for therapeutic xenotransplantation, with the greatest threat coming from viruses generated by recombination between members of the PERV subgroup A (PERV-A) and PERV-C families (PERV-A/C recombinants). PERV-A and PERV-B have been shown to infect human cells in culture, albeit with low titers. PERV-C has a more restricted host range and cannot infect human cells. A recombinant PERV-A/C virus (PERV-A14/220) contains the PERV-A sequence between the end of pol and the middle of the SU region in env. The remaining sequence is derived from PERV-C. PERV-A14/220 is approximately 500-fold more infectious than PERV-A. To determine the molecular basis for the increased infectivity of PERV-A14/220, we have made a series of vector constructs. The primary determinant for the enhanced replicative potential of the recombinant virus appeared to be the env gene. Using a series of chimeric env genes, we could identify two determinants of high infectivity; one was an isoleucine to valine substitution at position 140 between variable regions A and B, and the other lies within the proline rich region. Taken together, these results show that the novel juxtaposition of env gene sequences enhanced the infectivity of PERV-A14/220 for human cells, perhaps by stabilization of the envelope glycoprotein or increased receptor binding.

Challenges facing islet transplantation for the treatment of type 1 diabetes mellitus

Islet transplantation represents a most impressive recent advance in the search for a type 1 diabetes mellitus cure. While several hundred patients have achieved at least temporary insulin independence after receiving the islet "mini-organs" (containing insulin-producing beta cells), very few patients remain insulin independent beyond 4 years after transplantation. In this review, we describe historic as well as technical details about the procedure and provide insight into clinical and basic research efforts to overcome existing hurdles for this promising therapy.

Challenges facing islet transplantation for the treatment of type 1 diabetes mellitus

Islet transplantation represents a most impressive recent advance in the search for a type 1 diabetes mellitus cure. While several hundred patients have achieved at least temporary insulin independence after receiving the islet "mini-organs" (containing insulin-producing beta cells), very few patients remain insulin independent beyond 4 years after transplantation. In this review, we describe historic as well as technical details about the procedure and provide insight into clinical and basic research efforts to overcome existing hurdles for this promising therapy.

Novel herpesviruses of Suidae: indicators for a second genogroup of artiodactyl gammaherpesviruses

Five novel herpesviruses were identified in suid species from Africa (common warthog, Phacochoerus africanus) and South-East Asia (bearded pig, Sus barbatus; babirusa, Babyrousa babyrussa) by detection and analysis of their DNA polymerase genes. Three of the novel species, P. africanus cytomegalovirus 1, P. africanus lymphotropic herpesvirus 1 (PafrLHV-1) and S. barbatus lymphotropic herpesvirus 1 (SbarLHV-1), were closely related to known beta- (porcine cytomegalovirus) and gammaherpesviruses [porcine lymphotropic herpesvirus (PLHV) 1 and 3] of domestic pigs. In contrast, two novel species, S. barbatus rhadinovirus 1 (SbarRHV-1) and Babyrousa babyrussa rhadinovirus 1 (BbabRHV-1), were more closely related to a ruminant gammaherpesvirus, bovine herpesvirus 4 (BoHV-4), than to the porcine gammaherpesviruses PLHV-1, -2, -3, PafrLHV-1 and SbarLHV-1. SbarRHV-1, BbabRHV-1 and BoHV-4 were therefore tentatively assigned to a novel genogroup of artiodactyl gammaherpesviruses. This latter genogroup may also contain an as yet undiscovered gammaherpesvirus of domestic pigs, thereby adding a concern to their use in xenotransplantation.

Absence of replication-competent human-tropic porcine endogenous retroviruses in the germ line DNA of inbred miniature Swine

The potential transmission of porcine endogenous retroviruses (PERVs) has raised concern in the development of porcine xenotransplantation products. Our previous studies have resulted in the identification of animals within a research herd of inbred miniature swine that lack the capacity to transmit PERV to human cells in vitro. In contrast, other animals were capable of PERV transmission. The PERVs that were transmitted to human cells are recombinants between PERV-A and PERV-C in the post-VRA region of the envelope (B. A. Oldmixon, J. C. Wood, T. A. Ericsson, C. A. Wilson, M. E. White-Scharf, G. Andersson, J. L. Greenstein, H. J. Schuurman, and C. Patience, J. Virol. 76:3045-3048, 2002); these viruses we term PERV-A/C. This observation prompted us to determine whether these human-tropic replication-competent (HTRC) PERV-A/C recombinants were present in the genomic DNA of these miniature swine. Genomic DNA libraries were generated from one miniature swine that transmitted HTRC PERV as well as from one miniature swine that did not transmit HTRC PERV. HTRC PERV-A/C proviruses were not identified in the germ line DNAs of these pigs by using genomic mapping. Similarly, although PERV-A loci were identified in both libraries that possessed long env open reading frames, the Env proteins encoded by these loci were nonfunctional according to pseudotype assays. In the absence of a germ line source for HTRC PERV, further studies are warranted to assess the mechanisms by which HTRC PERV can be generated. Once identified, it may prove possible to generate animals with further reduced potential to produce HTRC PERV.

Immunosuppression and xenotransplantation of cells for cardiac repair

The death of highly vulnerable cardiomyocytes during ischemia leads to cardiac dysfunction, including heart failure. Due to limited proliferation of adult mammalian cardiomyocytes, the dead myocardium is replaced by noncontractile fibrotic tissue. Introducing exogenous cells to participate in the regeneration of infarcted myocardium has thus been proposed as a novel therapeutic approach. In view of the availability of various xenogeneic cells and fewer ethical and political concerns that surround human embryonic stem cells and fetal cardiomyocytes, cellular xenotransplantation may be a potential alternative approach for cardiac repair in humans. However, one of the most daunting challenges of xenotransplantation is immunorejection. This article summarizes the progress in cellular xenotransplantation for cardiac repair in experimental settings and the current understanding of possible immune responses following the engraftment of xenogeneic cells. The public attitude towards xenotransplantation is reportedly more favorable to receiving cells or tissues than a whole organ, but many scientific obstacles need to be overcome before the utilization of xenogeneic cells for cardiac repair in patients with heart disease becomes applicable to clinical practice.

Detection and significance of PERV in primary porcine hepatocytes

AIM: To investigate the significance of laboratory detection for PERV in primary porcine hepatocytes.

METHODS: Porcine hepatocytes was isolated and cultured with a two-stage perfusion method. Polymerase chain reaction (PCR) were used to detect PERV previrus sequences and reverse transcription polymerase chain reaction (RT-PCR) assays were used to detect PERV RNA sequences by using specific primers in primary porcine hepatocytes and culture supernate, controlled with no RT-PCR.

RESULTS: All these PCR assays gave positive results in tissue and serum samples from 2 HCV patients, 1 rabbit and 1 rat. Observation showed the persistent releasing of PERV in the culture supernate in different time points without the stimulation of mitogen by the established method and could last till cell death.

CONCLUSION: The method is rapid, cheap and safe, and it will be helpful to the further study of PERV infection and biosafety. The releasing of PERVs in the culture model demonstrates the existence of PERV security in the bioartificial liver support system.

Control of microbial contamination during surgical harvest of pig renal xenografts

The pig has been identified as the most likely source of xenograft material for clinical use and studies are ongoing to overcome the immunological hurdles of pig-to-human transplantation. Attention is now being focussed on identifying and reducing the potential microbiological hazards associated with this technique. Studies have primarily addressed issues surrounding the production and health monitoring of xenograft source pigs and none have so far specifically evaluated the possible risks of microbial contamination during xenograft harvest. In this report, we evaluate the possible routes for contamination of a pig kidney xenograft during organ harvest and describe approaches to the control of these hazards, including the novel use of a custom designed airtight surgical canopy. A standard procedure for microbiological monitoring during xenograft harvest was devised and evaluated. This allowed the rapid identification and anti-microbial sensitivity testing of any isolated organisms. This would enable an early and appropriate pre-emptive treatment of infection because of transmission of pig micro-organisms.

Relative age of proviral porcine endogenous retrovirus sequences in Sus scrofa based on the molecular clock hypothesis

Porcine endogenous retroviruses (PERV) are discussed as putative infectious agents in xenotransplantation. PERV classes A, B, and C harbor different envelope proteins. Two different types of long terminal repeat (LTR) structures exist, of which both are present only in PERV-A. One type of LTR contains a distinct repeat structure in U3, while the other is repeatless, conferring a lower level of transcriptional activity. Since the different LTR structures are distributed unequally among the proviruses and, apparently, PERV is the only virus harboring two different LTR structures, we were interested in determining which LTR is the ancestor. Replication-competent viruses can still be found today, suggesting an evolutionary recent origin. Our studies revealed that the age of PERV is at most 7.6 x 10(6) years, whereas the repeatless LTR type evolved approximately 3.4 x 10(6) years ago, being the phylogenetically younger structure. The age determined for PERV correlates with the time of separation between pigs (Suidae, Sus scrofa) and their closest relatives, American-born peccaries (Tayassuidae, Pecari tajacu), 7.4 x 10(6) years ago.

Xenogeneic transplantation of porcine islets: an overview

The extreme demand for human organs or tissues for transplantation has driven the search for viable alternatives. Pigs are considered a possible source of tissue for a number of reasons including shared physiology, plentiful supply, short gestation, and, more recently, the generation of transgenic animals. Porcine islets show promise as a source of islets for the treatment of type 1 diabetes mellitus. Porcine islets regulate glucose levels in the same physiologic range as humans, and porcine insulin has been used for years as an exogenous source of insulin for glucose control. In this review, we discuss the advantages and disadvantages of the use of adult or neonatal porcine islets, the immunologic challenges facing transplantation of xenogeneic islets, and the concerns regarding transmission of infectious agents between species. Porcine islets isolated from both adult and neonatal pigs are capable of restoring euglycemia in experimental animal models of diabetes. Adult islets are more difficult to isolate, whereas neonatal islets have great proliferation potential but require several weeks to function posttransplantation. Xenogeneic islets are susceptible to complement-mediated lysis after the binding of preformed natural antibodies and cellular immunity involving both macrophages and CD4+ T cells. In addition, the potential for transmission of porcine endogenous retroviruses, porcine cytomegalovirus, and porcine lymphotropic herpesvirus type 1 are all concerns that must be addressed. Despite the challenges facing xenotransplantation, the extreme need for donor organs and tissues continues to drive progress toward overcoming the unique issues associated with transplantation between species.

Some morphological, growth, and genomic properties of human cells chronically infected with porcine endogenous retrovirus (PERV)

A major concern in using porcine organs for transplantation is the potential of transmission of porcine endogenous retrovirus (PERV). To investigate the long-term effects of PERV infection on human cells, human embryonic kidney cell line HEK-293 infected with PERV PK-15 was maintained for up to 72 passages and samples were harvested at intervals for use in morphological, growth, and genomic analyses. Morphology, DNA content/cell, and doubling time of uninfected and infected cells were similar. Restriction fragment length polymorphism (RFLP) analysis of PCR-amplified nearly full-length PERV genome showed no alterations in band pattern. RFLP analysis of the long terminal repeats (LTR) showed some changes in band pattern, but not in length. Southern blot analysis of genomic DNA of infected cells indicated random integration of PERV without structural alterations in proviral genome. Semi-quantitative PCR demonstrated a gradual increase of proviral load in the infected cells. Sequence analysis of the LTR region of PERV from infected cells indicated a relatively low rate (6.0 × 10–4/bp or about 2 × 10–6/bp/generation) of mutation. There were also indications of recombination of PERV strains A and B. Finally, PERV infection had no effect on transcription of human endogenous retrovirus-K (HERV-K) genes. Together, no significant effect attributable to PERV infection was evident on chronically PERV-infected HEK-293 cells.Key words: porcine endogenous retrovirus (PERV), human endogenous retrovirus-K (HERV-K), xenotransplantation, zoonosis.

Reduced efficacy of ganciclovir against porcine and baboon cytomegalovirus in pig-to-baboon xenotransplantation

In pig-to-baboon xenotransplantation, porcine cytomegalovirus (PCMV) causes viremia, consumptive coagulopathy, and tissue-invasive disease. Baboon cytomegalovirus (BCMV) is associated with invasive disease in xenograft recipients. The efficacy of prophylaxis with intravenous ganciclovir (GCV) was studied for prevention of PCMV and BCMV infections in pig-to baboon xenotransplantation. GCV prophylaxis did not alter the incidence of BCMV activation in recipients, but reduced the amount of virus in tissues (mean 8.38 x 10(2) vs. 3.24 x 10(5) copies/ micro g DNA without treatment) and prevented tissue-invasive infections. PCMV viral loads were unaltered by GCV prophylaxis (8.36 x 10(8) copies/ micro g DNA without prophylaxis vs. 1.20 x 10(9) copies/ micro g DNA with prophylaxis). In vitro, PCMV was relatively resistant to GCV [90% inhibitory concentration (IC90) of 10 micro m, IC50 = 3 micro m], acyclovir (100 micro m), and leflunomide (not achievable). Only cidofovir (IC90 1 micro m) and foscarnet (IC90 100 micro m) might have therapeutic efficacy for PCMV in vivo in achievable concentrations, although these agents often carry significant toxicity in transplant recipients. GCV has limited activity against BCMV and no therapeutic efficacy against PCMV at standard doses in vivo. GCV and other antiviral agents have limited activities against PCMV in vitro. Breeding of PCMV-free xenograft donors may be necessary to prevent PCMV infections in clinical trials.

Packaging of human endogenous retrovirus sequences is undetectable in porcine endogenous retrovirus particles produced from human cells

The chronic shortage of human donor organs and tissues for allotransplantation could be relieved if clinical xenotransplantation were to become a viable clinical therapy. Balanced against the benefits of xenotransplantation are the possible consequences of zoonotic infections, and in particular, infection by porcine endogenous retrovirus (PERV). An often-proclaimed risk of PERV infection is the possible recombination of PERV with human endogenous retroviruses (HERV). To address this issue, we examined the potential for HERV sequences to be cross-packaged into PERV particles produced from infected human 293 cells. Although HERV-K, W, E, R, and ERV-9 RNA transcripts are expressed in 293 cells, we did not detect cross-packaging of any of these HERV groups. Quantitative analysis indicated that less than approximately 1 in 10(4)-10(7) PERV particles might contain HERV sequences. In comparison, we found that murine leukemia virus (MLV)-based vector transcripts were cross-packaged at a rate of approximately one copy in 10(4) PERV particles. Our results indicate that the potential for recombination of PERV and HERV sequences is low and that novel viruses generated by this mechanism are unlikely to represent a significant risk for xenotransplantation.

Methods for the exclusion of circoviruses and gammaherpesviruses from pigs

The use of porcine tissues is being developed as a means to alleviate the shortage of allogeneic tissues and organs available for transplantation. To reduce the possibility of a microorganism of pigs being inadvertently transferred to the recipient of the xenograft, recommendations have been published on the microbiological specifications for organ source pigs. The porcine circoviruses (PCV1 and PCV2) and porcine lymphotropic herpesviruses (PLHV1 and PLHV2) are two infectious agents of pigs which are considered to be of significance for the microbiological safety of xenotransplantation. To ensure the exclusion of these microorganisms from animals destined for use under clinical conditions, reliable breeding methodologies are required. We investigated the efficiency of established derivation procedures for the removal of PCV and PLHV. In comparison with conventionally reared pigs, caesarian and barrier derived animals showed a markedly reduced prevalence of PCVs and PLHVs. Our results indicate that the derivation of animals free of both of these microorganisms is achievable and will enhance the microbiological safety of xenotransplantation.