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

Reliable Polymerase Chain Reaction Methods for Screening for Porcine Endogenous Retroviruses-C (PERV-C) in Pigs

Porcine endogenous retrovirus C (PERV-C) is a gammaretrovirus present in the genome of many, but not all, pigs. It is an ecotropic virus, able to infect only pig cells. In contrast, PERV-A and PERV-B, which are present in all pigs, can infect cells of multiple host species, including humans, thereby posing a risk for xenotransplantation when pigs are used as donor animals. Notably, PERV-C can recombine with PERV-A to produce PERV-A/C recombinants that can infect human cells and replicate to higher titers compared to the paternal PERV-A. The objective of this study is to evaluate the reliability of both existing and newly developed polymerase chain reactions (PCR) methods for detecting PERV-C, with the aim of selecting PERV-C-free pigs to be used for xenotransplantation. To detect PERV-C by PCR, specific primers targeting the region of the envelope protein gene, which differs from that of PERV-A and PERV-B due to its unique receptor binding site, must be employed. In this study, new PCR assays were developed to detect PERV-C and a total of ten PCR assays and one real-time PCR assay were evaluated for their reliability in detecting PERV-C. These assays were used to screen indigenous Greek black pigs, Auckland Island pigs, and German slaughterhouse pigs. Two of the PCR assays consistently yielded reliable results, whereas the other PCRs and the real-time PCR gave false positive results. Using the reliable assays, it was shown that one out of four indigenous Greek black pigs (using the same method in a previous publication 11 of 21 pigs were found PERV-C-negative), one out of ten German slaughterhouse pigs, the pig kidney cell line PK15, and all the Auckland Island pigs were PERV-C-negative. The reliable PCR assays will enable the screening of PERV-C-negative donor pigs to be used in xenotransplantation. Most importantly, all the Auckland Island pigs that were genetically modified in Germany for use in clinical trials were PERV-C-negative.

Screening for Viruses in Indigenous Greek Black Pigs

The successful advancement of xenotransplantation has led to the development of highly sensitive detection systems for the screening of potentially zoonotic viruses in donor pigs and preventing their transmission to the recipient. To validate these methods, genetically modified pigs generated for xenotransplantation, numerous minipigs and other pig breeds have been tested, thereby increasing our knowledge concerning the pig virome and the distribution of pig viruses. Of particular importance are the porcine cytomegalovirus, a porcine roseolovirus (PCMV/PRV) and the hepatitis E virus genotype 3 (HEV3). PCMV/PRV has been shown to reduce the survival time of pig transplants in non-human primates and was also transmitted in the first pig heart transplantation to a human patient. The main aim of this study was to determine the sensitivities of our methods to detect PCMV/PRV, HEV3, porcine lymphotropic herpesvirus-1 (PLHV-1), PLHV-2, PLHV-3, porcine circovirus 2 (PCV2), PCV3, PCV4 and porcine parvovirus 1 (PPV1) and to apply the methods to screen indigenous Greek black pigs. The high number of viruses found in these animals allowed for the evaluation of numerous detection methods. Since porcine endogenous retroviruses (PERVs) type A and B are integrated in the genome of all pigs, but PERV-C is not, the animals were screened for PERV-C and PERV-A/C. Our detection methods were sensitive and detected PCMV/PRV, PLHV-1, PLHV-1, PLHV-3, PVC3 and PERV-C in most animals. PPV1, HEV3, PCV4 and PERV-A/C were not detected. These data are of great interest since the animals are healthy and resistant to diseases.

Determination of the Copy Number of Porcine Endogenous Retroviruses (PERV) in Auckland Island Pigs Repeatedly Used for Clinical Xenotransplantation and Elimination of PERV-C

Auckland Island pigs represent an inbred population of feral pigs isolated on the sub-Antarctic island for over 100 years. The animals have been maintained under pathogen-free conditions in New Zealand; they are well characterized virologically and have been used as donor sources in first clinical trials of porcine neonatal islet cell transplantation for the treatment of human diabetes patients. The animals do not carry any of the xenotransplantation-relevant viruses, and in the first clinical trials, no porcine viruses, including porcine endogenous retroviruses (PERVs) were transmitted to the human recipients. PERVs pose a special risk in xenotransplantation, since they are part of the pig genome. When the copy number of PERVs in these animals was analyzed using droplet digital PCR and primers binding to a conserved region of the polymerase gene (PERVpol), a copy number typical for Western pigs was found. This confirms previous phylogenetic analyses of microsatellites as well as mitochondrial analyses showing a closer relationship to European pigs than to Chinese pigs. When kidney cells from very young piglets were analyzed, only around 20 PERVpol copies were detected. Using these cells as donors in somatic cell nuclear transfer (SCNT), animals were born showing PERVpol copy numbers between 35 and 56. These data indicate that Auckland Island pigs have a similar copy number in comparison with other Western pig breeds and that the copy number is higher in adult animals compared with cells from young piglets. Most importantly, PERV-C-free animals were selected and the absence of an additional eight porcine viruses was demonstrated.

Isolation of an Ecotropic Porcine Endogenous Retrovirus PERV-C from a Yucatan SLAD/D Inbred Miniature Swine

Xenotransplantation may compensate the limited number of human allografts for transplantation using pigs as organ donors. Porcine endogenous retroviruses inherit infectious potential if pig cells, tissues, or organs were transplanted to immunosuppressed human recipients. Particularly, ecotropic PERV-C that could recombine with PERV-A to highly replication-competent human-tropic PERV-A/C should be excluded from pig breeds designed for xenotransplantation. Because of their low proviral background, SLAD/D (SLA, swine leukocyte antigen) haplotype pigs are potential candidates as organ donors as they do not bear replication-competent PERV-A and -B, even if they carry PERV-C. In this work, we characterized their PERV-C background isolating a full-length PERV-C proviral clone number 561 from a SLAD/D haplotype pig genome displayed in a bacteriophage lambda library. The provirus truncated in env due to cloning in lambda was complemented by PCR, and the recombinants were functionally characterized, confirming an increased infectivity in vitro compared to other PERV-C. Recombinant clone PERV-C(561) was chromosomally mapped by its 5'-proviral flanking sequences. Full-length PCR using 5'-and 3'-flanking primers specific to the PERV-C(561) locus verified that this specific SLAD/D haplotype pig harbors at least one full-length PERV-C provirus. The chromosomal location is different from that of the previously described PERV-C(1312) provirus, which was derived from the porcine cell-line MAX-T. The sequence data presented here provide further knowledge about PERV-C infectivity and contribute to targeted knockout in order to generate PERV-C-free founder animals. IMPORTANCE Yucatan SLAD/D haplotype miniature swine are candidates as organ donors for xenotransplantation. A full-length replication-competent PERV-C provirus was characterized. The provirus was chromosomally mapped in the pig genome. In vitro, the virus showed increased infectivity compared to other functional PERV-C isolates. Data may be used for targeted knockout to generate PERV-C free founder animals.

Porcine Endogenous Retroviruses and Xenotransplantation, 2021

Porcine endogenous retroviruses (PERVs) are integrated in the genome of all pigs, and some of them are able to infect human cells. Therefore, PERVs pose a risk for xenotransplantation, the transplantation of pig cells, tissues, or organ to humans in order to alleviate the shortage of human donor organs. Up to 2021, a huge body of knowledge about PERVs has been accumulated regarding their biology, including replication, recombination, origin, host range, and immunosuppressive properties. Until now, no PERV transmission has been observed in clinical trials transplanting pig islet cells into diabetic humans, in preclinical trials transplanting pig cells and organs into nonhuman primates with remarkable long survival times of the transplant, and in infection experiments with several animal species. Nevertheless, in order to prevent virus transmission to the recipient, numerous strategies have been developed, including selection of PERV-C-free animals, RNA interference, antiviral drugs, vaccination, and genome editing. Furthermore, at present there are no more experimental approaches to evaluate the full risk until we move to the clinic.

High Prevalence of Recombinant Porcine Endogenous Retroviruses (PERV-A/Cs) in Minipigs: A Review on Origin and Presence

Minipigs play an important role in biomedical research and they have also been used as donor animals for preclinical xenotransplantations. Since zoonotic microorganisms including viruses can be transmitted when pig cells, tissues or organs are transplanted, virus safety is an important feature in xenotransplantation. Whereas most porcine viruses can be eliminated from pig herds by different strategies, this is not possible for porcine endogenous retroviruses (PERVs). PERVs are integrated in the genome of pigs and some of them release infectious particles able to infect human cells. Whereas PERV-A and PERV-B are present in all pigs and can infect cells from humans and other species, PERV-C is present in most, but not all pigs and infects only pig cells. Recombinant viruses between PERV-A and PERV-C have been found in some pigs; these recombinants infect human cells and are characterized by high replication rates. PERV-A/C recombinants have been found mainly in minipigs of different origin. The possible reasons of this high prevalence of PERV-A/C in minipigs, including inbreeding and higher numbers and expression of replication-competent PERV-C in these animals, are discussed in this review. Based on these data, it is highly recommended to use only pig donors in clinical xenotransplantation that are negative for PERV-C.

Pathways to Clinical Cardiac Xenotransplantation

Heart transplantation is the only long-lasting lifesaving option for patients with terminal cardiac failure. The number of available human organs is however far below the actual need, resulting in substantial mortality of patients while waiting for a human heart. Mechanical assist devices are used to support cardiac function but are associated with a high risk of severe complications and poor quality of life for the patients. Consistent success in orthotopic transplantation of genetically modified pig hearts into baboons indicates that cardiac xenotransplantation may become a clinically applicable option for heart failure patients who cannot get a human heart transplant. In this overview, we project potential paths to clinical cardiac xenotransplantation, including the choice of genetically modified source pigs; associated requirements of microbiological, including virological, safety; optimized matching of source pig and recipient; and specific treatments of the donor heart after explantation and of the recipients. Moreover, selection of patients and the regulatory framework will be discussed.

The origin of porcine endogenous retroviruses (PERVs)

Porcine endogenous retroviruses (PERVs) are integrated in the genome of all pigs, and they produce viral particles that are able to infect human cells and therefore pose a special risk for xenotransplantation. In contrast to other pig microorganisms that also pose a risk, such as porcine cytomegalovirus and hepatitis E virus, PERVs cannot be eliminated from pigs by vaccines, antiviral drugs, early weaning, or embryo transfer. Since PERVs are relevant for xenotransplantation, their biology and origin are of great interest. Recent studies have shown that PERVs are the result of a transspecies transmission of precursor retroviruses from different animals and further evolution in the pig genome. PERVs acquired different long terminal repeats (LTRs), and recombination took place. In parallel, it has been shown that the activity of the LTRs and recombination in the envelope are important for the transmissibility and pathogenesis of PERVs. Transspecies transmission of retroviruses is common, a well-known example being the transmission of precursor retroviruses from non-human primates to humans, resulting in human immunodeficiency virus (HIV). Here, recent findings concerning the origin of PERVs, their LTRs, and recombination events that occurred during evolution are reviewed and compared with other findings regarding transspecies transmission of retroviruses.

Absence of porcine endogenous retrovirus (PERV) production from pig lymphoma cell lines

Porcine endogenous retroviruses (PERVs) -A and -B are integrated in the genome of all pigs, whereas PERV-C is found in many, but not all pigs. Some immortalized pig cell lines, among them lymphoma cells, but also mitogen activated primary lymphocytes have been shown to release virus particles, which were able to infect human cells and some of them were recombinant PERV-A/C. Since retroviruses can induce lymphomas, two newly established pig lymphoma cell lines and an older one (L23) were analysed for PERV expression. All three lines harboured PERV-A, PERV-B and PERV-C proviruses, but PERV-A/C recombinants were found only in the genome of L23 cells. The expression at the RNA level was very low and no protein expression and particle release was observed, suggesting that PERVs were not involved in the pathogenesis of these lymphomas. However, all three cell lines were infected with the porcine lymphotropic herpesvirus-3 (PLHV-3), which may have been involved in lymphoma development.

A highly sensitive method for the detection of recombinant PERV-A/C env RNA using next generation sequencing technologies

Several xenogenic cell-based therapeutic products are currently under development around the world for the treatment of human diseases. Porcine islet cell products for treating human diabetes are a typical example. Since porcine cells possess endogenous retrovirus (PERV), which can replicate in human cells in vitro, the potential transmission of PERV has raised concerns in the development of these products. Four subgroups of infectious PERV have been identified, namely PERV-A, -B, -C, and recombinant PERV-A/C. Among them, PERV-A/C shows a high titre and there was a paper reported that an incidence of PERV-A/C viremia was increased in diseased pigs; thus, it would be important to monitor the emergence of PERV-A/C after transplantation of porcine products. In this study, we developed a highly sensitive method for the detection of PERV-A/C using next generation sequencing (NGS) technologies. A model PERV-C spiked with various doses of PERV-A/C were amplified by RT-PCR and the amplicons were analysed by NGS. We found that the NGS analysis allowed the detection of PERV-A/C at the abundance ratios of 1% and 0.1% with true positive rates of 100% and 57%, respectively, indicating that it would be useful for the rapid detection of PERV-A/C emergence after transplantation of porcine products.

Copy Number and Prevalence of Porcine Endogenous Retroviruses (PERVs) in German Wild Boars

Porcine endogenous retroviruses (PERVs) are integrated in the genome of pigs and are transmitted like cellular genes from parents to the offspring. Whereas PERV-A and PERV-B are present in all pigs, PERV-C was found to be in many, but not all pigs. When PERV-C is present, recombination with PERV-A may happen and the PERV-A/C recombinants are characterized by a high replication rate. Until now, nothing has been known about the copy number of PERVs in wild boars and little is known about the prevalence of the phylogenetically youngest PERV-C in ancient wild boars. Here we investigated for the first time the copy number of PERVs in different populations of wild boars in and around Berlin using droplet digital PCR. Copy numbers between 3 and 69 per genome have been measured. A lower number but a higher variability was found compared to domestic pigs, including minipigs reported earlier (Fiebig et al., Xenotransplantation, 2018). The wild boar populations differed genetically and had been isolated during the existence of the Berlin wall. Despite this, the variations in copy number were larger in a single population compared to the differences between the populations. PERV-C was found in all 92 analyzed animals. Differences in the copy number of PERV in different organs of a single wild boar indicate that PERVs are also active in wild boars, replicating and infecting new cells as has been shown in domestic pigs.

A Comprehensive Strategy for Screening for Xenotransplantation-Relevant Viruses in a Second Isolated Population of Göttingen Minipigs

Xenotransplantation using pig tissues and organs is under development in order to alleviate the increasing shortage of human transplants. Since xenotransplantation may be associated with the transmission of porcine microorganisms to the human recipient, the donor pigs should be carefully analyzed, especially for the presence of potentially zoonotic viruses. Göttingen Minipigs (GöMP) are potential donors of islet cells for the treatment of diabetes. Despite the fact that all animals produced at Ellegaard Göttingen Minipigs A/S carry porcine endogenous retroviruses (PERVs) in their genome and that very few animals were infected with porcine cytomegalovirus (PCMV), hepatitis E virus (HEV) and porcine lymphotropic herpesvirus (PLHV), no transmission of these viruses was observed in a preclinical trial transplanting GöMP islet cells into cynomolgus monkeys. Using a new comprehensive strategy, we then analyzed an isolated subpopulation of Göttingen Minipigs which remained at the University of Göttingen. We concentrated on 11 xenotransplantation-relevant viruses and combined co-incubation assays with susceptible human target cells and molecular biological methods to evaluate the risk posed by PERV. All animals in Göttingen carry PERV-A, PERV-B, and PERV-C in their genome but they are not infected with PCMV, PLHV and HEV. The difference may be explained by selection of negative animals and/or de novo infection. The PERV copy number was established using ddPCR (93 copies) and a human-tropic PERV-A/C was found released from PBMCs of one animal with a high expression of PERV-C.

Analysis of swine leukocyte antigen class I gene profiles and porcine endogenous retrovirus viremia level in a transgenic porcine herd inbred for xenotransplantation research

Molecular characterization of swine leukocyte antigen (SLA) genes is important for elucidating the immune responses between swine-donor and human-recipient in xenotransplantation. Examination of associations between alleles of SLA class I genes, type of pig genetic modification, porcine endogenous retrovirus (PERV) viral titer, and PERV subtypes may shed light on the nature of xenograft acceptance or rejection and the safety of xenotransplantation. No significant difference in PERV gag RNA level between transgenic and non-transgenic pigs was noted; likewise, the type of applied transgene had no impact on PERV viremia. SLA-1 gene profile type may correspond with PERV level in blood and thereby influence infectiveness. Screening of pigs should provide selection of animals with low PERV expression and exclusion of specimens with PERV-C in the genome due to possible recombination between A and C subtypes, which may lead to autoinfection. Presence of PERV-C integrated in the genome was detected in 31.25% of specimens, but statistically significant increased viremia in specimens with PERV-C was not observed. There is a need for multidirectional molecular characterization (SLA typing, viremia estimation, and PERV subtype screening) of animals intended for xenotransplantation research in the interest of xeno-recipient safety.

A computational method for prediction of matrix proteins in endogenous retroviruses

Human endogenous retroviruses (HERVs) encode active retroviral proteins, which may be involved in the progression of cancer and other diseases. Matrix protein (MA), in group-specific antigen genes (gag) of retroviruses, is associated with the virus envelope glycoproteins in most mammalian retroviruses and may be involved in virus particle assembly, transport and budding. However, the amount of annotated MAs in ERVs is still at a low level so far. No computational method to predict the exact start and end coordinates of MAs in gags has been proposed yet. In this paper, a computational method to identify MAs in ERVs is proposed. A divide and conquer technique was designed and applied to the conventional prediction model to acquire better results when dealing with gene sequences with various lengths. Initiation sites and termination sites were predicted separately and then combined according to their intervals. Three different algorithms were applied and compared: weighted support vector machine (WSVM), weighted extreme learning machine (WELM) and random forest (RF). G − mean (geometric mean of sensitivity and specificity) values of initiation sites and termination sites under 5-fold cross validation generated by random forest models are 0.9869 and 0.9755 respectively, highest among the algorithms applied. Our prediction models combine RF & WSVM algorithms to achieve the best prediction results. 98.4% of all the collected ERV sequences with complete MAs (125 in total) could be predicted exactly correct by the models. 94,671 HERV sequences from 118 families were scanned by the model, 104 new putative MAs were predicted in human chromosomes. Distributions of the putative MAs and optimizations of model parameters were also analyzed. The usage of our predicting method was also expanded to other retroviruses and satisfying results were acquired.

Porcine endogenous retroviruses in xenotransplantation--molecular aspects

In the context of the shortage of organs and other tissues for use in human transplantation, xenotransplantation procedures with material taken from pigs have come under increased consideration. However, there are unclear consequences of the potential transmission of porcine pathogens to humans. Of particular concern are porcine endogenous retroviruses (PERVs). Three subtypes of PERV have been identified, of which PERV-A and PERV-B have the ability to infect human cells in vitro. The PERV-C subtype does not show this ability but recombinant PERV-A/C forms have demonstrated infectivity in human cells. In view of the risk presented by these observations, the International Xenotransplantation Association recently indicated the existence of four strategies to prevent transmission of PERVs. This article focuses on the molecular aspects of PERV infection in xenotransplantation and reviews the techniques available for the detection of PERV DNA, RNA, reverse transcriptase activity and proteins, and anti-PERV antibodies to enable carrying out these recommendations. These methods could be used to evaluate the risk of PERV transmission in human recipients, enhance the effectiveness and reliability of monitoring procedures, and stimulate discussion on the development of improved, more sensitive methods for the detection of PERVs in the future.

Long-term effects of PERV-specific RNA interference in transgenic pigs

BACKGROUND

Porcine endogenous retroviruses (PERVs) represent a risk of xenotransplantation using porcine cells, tissues, or organs, as they are integrated in the porcine genome and have been shown to be able to infect human cells in vitro. To increase viral safety by RNA interference, transgenic pigs expressing a PERV-specific small hairpin (sh)RNA targeted to a highly conserved sequence in the pol gene (pol2) were generated in which expression of PERVs was reduced (Xenotransplantation, 15, 2008, 38). However, it remains to be shown how long expression of the shRNA and the RNA interference is effective in reducing PERV expression.

METHODS

To analyze the long-term duration of RNA interference, expression of the PERV-specific pol2 shRNA and inhibition of PERV expression was studied repeatedly in fibroblasts and peripheral blood mononuclear cells (PBMCs) of transgenic pigs over a period of 3 yr, when animals were sacrificed and expression was studied in different organs. Expression of the PERV-specific shRNA was measured using a newly developed real-time PCR, and expression of PERV was measured using a PERV-specific real-time PCR.

RESULTS

Over a period of 3 yr, PERV-specific shRNA and green fluorescent protein (GFP) as reporter of the vector system were consistently expressed in transgenic animals. PERV expression was significantly reduced during the entire period. Levels of PERV and shRNA expression were different in the various organs. PERV expression was highest in the spleen and the lungs and lowest in liver and heart. However, in all organs of the transgenic pigs, PERV expression was inhibited compared with the vector control animals.

CONCLUSIONS

Transgenic pigs expressing PERV-specific shRNA maintained their specific RNA interference long term, suggesting that PERV expression in the xenotransplants will be suppressed over extended periods of time.

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.

Xenotransplantation-associated infectious risk: a WHO consultation

Xenotransplantation carries the potential risk of the transmission of infection with the cells or tissues of the graft. The degree of risk is unknown in the absence of clinical trials. The clinical application of xenotransplantation has important implications for infectious disease surveillance, both at the national and international levels. Preclinical data indicate that infectious disease events associated with clinical xenotransplantation from swine, should they occur, will be rare; data in human trials are limited but have demonstrated no transmission of porcine microorganisms including porcine endogenous retrovirus. Xenotransplantation will necessitate the development of surveillance programs to detect known infectious agents and, potentially, previously unknown or unexpected pathogens. The development of surveillance and safety programs for clinical trials in xenotransplantation is guided by a "Precautionary Principle," with the deployment of appropriate screening procedures and assays for source animals and xenograft recipients even in the absence of data suggesting infectious risk. All assays require training, standardization and validation, and sharing of laboratory methods and expertise to optimize the quality of the surveillance and diagnostic testing. Investigation of suspected xenogeneic infection events (xenosis, xenozoonosis) should be performed in collaboration with an expert data safety review panel and the appropriate public health and competent authorities. It should be considered an obligation of performance of xenotransplantation trials to report outcomes, including any infectious disease transmissions, in the scientific literature. Repositories of samples from source animals and from recipients prior to, and following xenograft transplantation are essential to the investigation of possible infectious disease events. Concerns over any potential hazards associated with xenotransplantation may overshadow potential benefits. Careful microbiological screening of source animals used as xenotransplant donors may enhance the safety of transplantation beyond that of allotransplant procedures. Xenogeneic tissues may be relatively resistant to infection by some human pathogens. Moreover, xenotransplantation may be made available at the time when patients require organ replacement on a clinical basis. Insights gained in studies of the microbiology and immunology of xenotransplantation will benefit transplant recipients in the future. This document summarizes approaches to disease surveillance in individual recipients of nonhuman tissues.

Potential zoonotic infection of porcine endogenous retrovirus in xenotransplantation

Porcine endogenous retrovirus (PERV) is considered the major biosafety issue in xenotransplantation. Several techniques have been employed for the analysis of the PERV status in the animal donor and for the assessment of PERV transmission/infection in the xenograft recipient. In this chapter, methods to assess the expression of PERV and the potential for PERV transmission from a donor animal are described in addition to the identification of relevant loci within the porcine genome.PERV detection can be carried out using several techniques of which quantitative polymerase chain reaction (PCR) and RT-PCR are the most sensitive. However, other procedures can be employed such as detection of reverse transcriptase activity (i.e. viral replication) in the sample or immunostaining of the infected cells using an anti-PERV antibody. The PERV transmission assay has been described to identify the transmission phenotype of the pig donor, and subsequent risk from a donor. This assay can, therefore, direct the selection of the most suitable animal. Finally, it is important to determine the presence of critical PERV loci involved in transmission in the pig genome and compare between different animals. One of the methods for the analysis of these PERV integration sites is described.

Detection of porcine endogenous retrovirus (PERV) viremia in diseased versus healthy US pigs by qualitative and quantitative real-time RT-PCR

Previous studies have linked levels of porcine endogenous retroviruses (PERV) with poor health and disease in pigs. To determine the levels of expression of PERVs and their potential association with disease expression, real-time reverse transcriptase (RT) PCR assays were used to assess PERV-ABC, PERV-C and PERV-A/C levels in three commercial swine operations in the United States. Pigs (n = 204) aged 3-25 weeks were screened, and all 369 serum samples collected were found to be positive for PERV-ABC RNA as expected. PERV-C and PERV-A/C RNA were detected in 24.1% (89/369) and 18.7% (69/369) of the samples, respectively. When divided into age groups, PERV-A/C RNA was identified in 20.0% (43/215) of the nursery pig samples (3-9 weeks of age) compared to 16.9% (26/154) finisher pig samples (12-25 weeks of age). On two of the farms, serum was collected from healthy pigs (n = 60) and from pen-mates with various clinical conditions including diarrhoea, wasting and respiratory disease (n = 60). Overall, 25% (15/60) of the samples from clinically affected pigs were found to be positive for PERV-A/C RNA, whereas in clinically healthy pigs, only 8.3% (5/60) of the samples were found to be PERV-A/C positive (P = 0.026). It was possible to identify PERV-A/C in the same pigs on more than one consecutive bleeding, indicating variable length of PERV-A/C viremia. The results indicate that there is an increased incidence of PERV-A/C viremia in diseased pigs and that PERV-A/C can be detected over time in selected pigs within commercial pig production systems in the United States.

Porcine endogenous retrovirus (PERV) and its transmission characteristics: a study of the New Zealand designated pathogen-free herd

Previously a strategy for monitoring of pigs intended for cell transplantation was developed and successfully applied to several representative herds in New Zealand. A designated pathogen-free (DPF) herd has been chosen as a good candidate for xenotransplantation. This herd has previously tested free of infectious agents relevant to xenotransplantation and we present here an in depth study of porcine endogenous retrovirus (PERV) transmission. A panel of assays that describes the constraints for the transmission of PERV has been suggested. It includes a) infectivity test in coculture of DPF pig primary cells with both human and pig target cell lines; b) RT activity in supernatant of stimulated primary cells from DPF pigs; c) viral load in donor's blood plasma; d) PERV proviral copy number in DPF pig genome; e) PERV class C prevalence in the herd and its recombination potential. There was no evidence of PERV transmission from DPF pig tissue to either pig or human cells. Additionally, there was no evidence of PERV RNA present in pig blood plasma. PERV copy number differs in individual pigs from as low as 3 copies to 30 copies and the presence of PERV-C varied between animals and breeds. In all DPF pigs tested, a specific locus for PERV-C potentially associated with the recombination of PERV in miniature swine was absent. Presented data on the PERV transmission allows us to classify the DPF potential donors as "null" or noninfectious pigs.

Xenotransplantation as a model of integrated, multidisciplinary research

Xenotransplantation was proposed a long time ago as a possible solution to the world-wide shortage of human organs. For years, researchers in this field have almost exclusively directed their efforts towards combating the immunological barrier that precluded long-term xenograft survival. Studies have been conducted in both small and large animal models and the most relevant results have been obtained in pre-clincal studies, specifically those utilising the pig-to-nonhuman primate combination. In this context, a better understanding of the immunological mechanisms underlying the rejection of a xenograft have allowed the identification of specific targets of intervention that have resulted in considerable improvements in survival of porcine organs or cells in nonhuman primates. However it has also become apparent that if xenotransplantation has to enter the clinical arena, a multidisciplinary approach will be needed to comprehensively tackle the different issues related to the use of a xenograft to cure human disease.In this regard, the safety, ethics and regulatory aspects of xenotransplantation are currently being aggressively addressed to enable the initiation of xenotransplantation with a favourable risk/benefit ratio.

Strategies to enhance the safety profile of xenotransplantation: minimizing the risk of viral zoonoses

PURPOSE OF REVIEW

Pig-to-human xenotransplantation has taken steps closer to reality through advances in animal engineering to address immunological as well as microbial problems. The most highlighted problem in xenotransplantation safety has been the potential risk for zoonotic infection mediated by porcine endogenous retroviruses. Safety issues regarding viral zoonosis, particularly porcine endogenous retroviruses, are summarized and commented upon.

RECENT FINDINGS

Several molecular, transgenic strategies to provide safer transplant source animals with less porcine endogenous retrovirus infectivity have been developed. A genomics approach by selective breeding and porcine endogenous retrovirus loci knockout is at least theoretically possible. For other viruses, advances have been made in technologies for virus discovery and identification.

SUMMARY

The consequences of possible zoonoses in xenotransplantation are largely unknown. Further work to identify and control potential zoonotic agents based on recent progress will improve the safety profile of xenotransplantation. Advances made should be subjected to cautious testing in well controlled, preclinical and clinical experiments.

Xenotransplantation of solid organs in the pig-to-primate model

Xenotransplantation using pig organs could solve the significant increasing shortage of donor organs for allotransplantation. In the last two decades, major progress has been made in understanding the xenoimmunobiology of pig-to-nonhuman primate transplantation, and today we are close to clinical trials. The ability to genetically engineer pigs, such as human decay-accelerating factor (hDAF), CD46 (membrane cofactor protein), or alpha1,3-galactosyltransferase gene-knockout (GT-KO), has been a significant step toward the clinical application of xenotransplantation. Using GT-KO pigs and novel immunosuppressant agents, 2 to 6 months' survival of heterotopic heart xenotransplants has been achieved. In life-supporting kidney xenotransplantation, promising survival of close to 3 months has been achieved. However, liver and lung xenotransplantations do not have such encouraging survival as kidney and heart xenotransplantation. Although the introduction of hDAF and GT-KO pigs largely overcame hyperacute rejection, acute humoral xenograft rejection (AHXR) remains a challenge to be overcome if survival is to be increased. In several studies, when classical AHXR was prevented, thrombotic microangiopathy and coagulation dysregulation became more obvious, which make them another hurdle to be overcome. The initiating cause of failure of pig cardiac and renal xenografts may be antibody-mediated injury to the endothelium, leading to the development of microvascular thrombosis. Potential contributing factors toward the development of the thrombotic microangiopathy include: 1) the presence of preformed anti-non-Gal antibodies, 2) the development of very low levels of elicited antibodies to non-Gal antigens, 3) natural killer cell or macrophage activity, and 4) inherent coagulation dysregulation between pigs and primates. The breeding of pigs transgenic for an 'anticoagulant' or 'anti-thrombotic' gene, such as human tissue factor pathway inhibitor, hirudin, or CD39, or lacking the gene for the prothrombinase, fibrinogen-like protein-2, is anticipated to inhibit the change in the endothelium to a procoagulant state that takes place in the pig organ after transplantation. A further limitation for organ xenotransplantation is the potential for cross-species infection. As far as exogenous viruses are concerned, porcine cytomegalovirus has been detected in the tissues of recipient non-human primates, although no invasive disease was reported. Until today, no formal evidence has been presented from in vivo studies in non-human primates or from humans exposed to pig organs, tissues, or cells that porcine endogenous retroviruses infect primate cells. Xenotransplantation is a potential answer to the current organ shortage. Its future depends on; 1) further genetic modification of pigs, 2) the introduction of novel immunosuppressive agents that target the innate immune system and plasma cells, and 3) the development of clinically-applicable methods to induce donor-specific tolerance.

Identification of residues outside of the receptor binding domain that influence the infectivity and tropism of porcine endogenous retrovirus

Identification of determinants of human tropism of porcine endogenous retrovirus (PERV) is critical to understanding the risk of transmission of PERV to recipients of porcine xenotransplantation products. Previously, we showed that a chimeric envelope cDNA encoding the 360 N-terminal residues of the human-tropic PERV envelope class A (PERV-A) SU and the 130 C-terminal residues of the pig-tropic PERV-C SU and all of TM (PERV-A/C) showed a 100-fold decrease in infectivity titer on human cells (M. Gemeniano, O. Mpanju, D. R. Salomon, M. V. Eiden, and C. A. Wilson, Virology 346:108-117, 2006). To identify residues important for human cell infection, we performed site-directed mutagenesis on each of the nine residues, singly or in combination, that distinguish the C-terminal region of PERV-C from PERV-A. Of the nine amino acids, two single-amino-acid substitutions, Q374R and I412V, restored the infectivity of human cells to the chimeric PERV-A/C to a titer equivalent to that of PERV-A. In contrast, PERV-A/C mutant envelope Q439P resulted in undetectable infection of human cells and an approximately 1,000-fold decrease in control pig cells. Mutation of K441R rescued mutants that carried Q439P, suggesting an incompatibility between the proline residue at this position and the presence of KK in the proteolytic cleavage signal. We confirmed this incompatibility with vectors carrying PERV-A envelope mutant R462K that were also rendered noninfectious. Finally, tropism of vectors carrying PERV-C envelope mutants with only four amino acid changes in the C terminus of PERV-C envelope, NHRQ436YNRP plus K441R, was shifted to one similar to that of PERV-A. Our results show an important and previously unrecognized role for infectivity and tropism for residues at the C terminus of SU.

Pigs taking wing with transposons and recombinases

Swine production has been an important part of our lives since the late Mesolithic or early Neolithic periods, and ranks number one in world meat production. Pig production also contributes to high-value-added medical markets in the form of pharmaceuticals, heart valves, and surgical materials. Genetic engineering, including the addition of exogenous genetic material or manipulation of the endogenous genome, holds great promise for changing pig phenotypes for agricultural and medical applications. Although the first transgenic pigs were described in 1985, poor survival of manipulated embryos; inefficiencies in the integration, transmission, and expression of transgenes; and expensive husbandry costs have impeded the widespread application of pig genetic engineering. Sequencing of the pig genome and advances in reproductive technologies have rejuvenated efforts to apply transgenesis to swine. Pigs provide a compelling new resource for the directed production of pharmaceutical proteins and the provision of cells, vascular grafts, and organs for xenotransplantation. Additionally, given remarkable similarities in the physiology and size of people and pigs, swine will increasingly provide large animal models of human disease where rodent models are insufficient. We review the challenges facing pig transgenesis and discuss the utility of transposases and recombinases for enhancing the success and sophistication of pig genetic engineering. 'The paradise of my fancy is one where pigs have wings.' (GK Chesterton).

Concise classification of the genomic porcine endogenous retroviral gamma1 load to defined lineages

We investigated the infection history of porcine endogenous retroviruses (PERV) gamma1 by analyzing published env and LTR sequences. PERV sequences from various breeds, porcine cell lines and infected human primary cells were included in the study. We identified a considerable number of retroviral lineages indicating multiple independent colonization events of the porcine genome. A recent boost of the proviral load in an isolated pig herd and exclusive occurrence of distinct lineages in single studies indicated the ongoing colonization of the porcine genome with endogenous retroviruses. Retroviral recombination between co-packaged genomes was a general factor for PERV gamma1 diversity which indicated the simultaneous expression of different proviral loci over a period of time. In total, our detailed description of endogenous retroviral lineages is the prerequisite for breeding approaches to minimize the infectious potential of porcine tissues for the subsequent use in xenotransplantation.

An update on xenotransplantation

Xenotransplantation is one of the possible avenues currently being explored to address the shortage problem of human organs. With this in mind, this article will briefly review the current situation with respect to the immunological, physiological and biosafety aspects related to the transplantation of pig organs into primates. Acute humoral xenograft rejection (AHXR) currently remains the central immunological obstacle and the development of strategies for both a better control of the elicited anti-pig humoral immune response or the prevention of the onset of coagulation disorders that accompany AHXR are the two primary focuses of research. To date, porcine xenografts have been shown to sustain the life of nonhuman primates for several months. Such preclinical studies have also demonstrated the absence of insurmountable physiological incompatibilities between pig and primate. In addition, reassuring findings regarding biosafety aspects have been generated and pro-active research aimed at the identification of an organ source with a higher safety profile is also underway. These advancements, in conjunction with ongoing research in pig genetic engineering, immunosuppression and tolerance are expected to further extend the survival of porcine xenografts transplanted into primates. However, until further physiological, efficacy and safety data are generated in relevant primate models, clinical xenotransplantation should not be considered.

Pre-screening of miniature swine may reduce the risk of transmitting human tropic recombinant porcine endogenous retroviruses

BACKGROUND

It has been reported that peripheral blood mononuclear cells from miniature swine are capable of transmitting human tropic porcine endogenous retrovirus (PERV) recombinants to both human and pig cells. It has been suggested that these recombinants are exogenous and/or driven by one or more critical loci present in the pig genome.

METHODS AND RESULTS

Genomic analysis of a miniature swine capable of transmitting human tropic replication competent (HTRC) recombinant PERV-A/C identified a PERV-C provirus in a region with homology to sequences located on chromosome 7. In "null" swine, incapable of in vitro transmission of PERV to human or pig cells, amplification using specific primers revealed that only two of five animals retained this locus in comparison to a total of five out of five transmitters (recombinant PERV-A/C transmission to both human and pig cells) and seven out of seven non-transmitters (replication of non-recombinant PERV in pig cells only).

CONCLUSION

These data suggest that further analysis of these loci may provide a genetic basis for identifying pigs that are less likely to transmit human tropic PERV and would, therefore, be more suitable as source animals for human xenotransplantation.

Clinical xenotransplantation of organs: why aren't we there yet?

Mohiuddin discusses the lessons learned from large animal xenograft models and why the immunological barrier is still the most important hurdle preventing clinical xenotransplantation of organs.

Isolation and characterization of an infectious replication-competent molecular clone of ecotropic porcine endogenous retrovirus class C

Xenotransplantation of pig organs is complicated by the existence of polytropic replication-competent porcine endogenous retroviruses (PERV) capable of infecting human cells. The potential for recombination between ecotropic PERV-C and human-tropic PERV-A and PERV-B adds another level of infectious risk. Proviral PERV-C were characterized in MAX-T cells derived from d/d haplotype miniature swine. Three proviruses were cloned from a genomic library. Clone PERV-C(1312) generated infectious particles after transfection into porcine ST-IOWA cells. Electron microscopy revealed the same morphologies of virions in MAX-T cells and in ST-IOWA cells infected with cell-free PERV-C(1312) particles, indicating that MAX-T cells harbor one functional PERV-C provirus.

Genomic presence of recombinant porcine endogenous retrovirus in transmitting miniature swine

The replication of porcine endogenous retrovirus (PERV) in human cell lines suggests a potential infectious risk in xenotransplantation. PERV isolated from human cells following cocultivation with porcine peripheral blood mononuclear cells is a recombinant of PERV-A and PERV-C. We describe two different recombinant PERV-AC sequences in the cellular DNA of some transmitting miniature swine. This is the first evidence of PERV-AC recombinant virus in porcine genomic DNA that may have resulted from autoinfection following exogenous viral recombination. Infectious risk in xenotransplantation will be defined by the activity of PERV loci in vivo.

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.

Sequence analysis of proviral DNA of porcine endogenous retroviruses

Among all species analyzed, the domestic pig seems to be the most appropriate organ donor for xenotransplantation. Porcine endogenous retroviruses (PERVs) are present in genomes of all pigs and are capable of infecting human cells in vitro thus posing a serious threat for xenotransplantation procedures. Despite the abundant distribution of PERVs integrated with porcine genome, the majority of PERV proviral DNA is not capable of expressing viral proteins unless seriously mutated. The aim of the study was to analyze PERV genome for mutations. The study was performed on blood samples from 146 pigs. Long-range polymerase chain reaction (Long-PCR) was performed with primer sets designed within long terminal repeats (LTRs). Long-PCR products of different molecular weights were obtained: 530 bp (33.1% of individuals), 580 bp (76.7%), 933 bp (100%), and 2900 bp (59.8%). Amplimers of 7200 bp were absent in 12.8% of individuals, indicating the lack of intact proviral DNA. Sequence analysis showed that most PERV proviral DNA was significantly mutated, thus suggesting the inability to express functional viral RNA; however, it cannot be ruled out that compensatory recombination processes could occur enabling replication of defective proviruses.

The infectivity and host range of the ecotropic porcine endogenous retrovirus, PERV-C, is modulated by residues in the C-terminal region of its surface envelope protein

Endogenous retroviral genetic material serves as a reservoir for the generation of retroviral pathogens by recombination between activated endogenous or exogenous infectious agents. Some porcine tissues actively express infectious porcine endogenous retroviruses (PERVs). Of the three classes of PERV characterized to date, two, PERV-A and B, are capable of infecting human cells in vitro, whereas PERV-C cannot. Here, we demonstrate that the PERV-C envelope surface protein (SU) when disassociated from its C-terminus binds human cells. Further, we show that PERV-C binding to human cells is not inhibited in 293 cells productively infected with PERV-A, confirming that the molecule PERV-C interacts with on human cells is distinct from that used by PERV-A. Moreover, we demonstrate that the envelope region encompassing the proline-rich region is required for binding to cells in addition to the putative variable region A (VRA) and B (VRB). The region in the C-terminus of the SU that alters the binding and infectivity properties of PERV-C differs by only nine residues from the analogous region of PERV-A. Caution may be warranted even when a xenotransplantation product is from source pigs that do not express human-tropic viruses, as minimal mutations within PERV-C combined with selection in a human recipient could render PERV-C infectious in humans.

Organ transplantation—how much of the promise has been realized?

Since the introduction of organ transplantation into medical practice, progress and optimism have been abundant. Improvements in immunosuppressive drugs and ancillary care have led to outstanding short-term (1--3-year) patient and graft survival rates. This success is mitigated by several problems, including poor long-term (>5-year) graft survival rates, the need for continual immunosuppressive medication and the discrepancy between the demand for organs and the supply. Developing methods to induce transplant tolerance, as a means to improve graft outcomes and eliminate the requirement for immunosuppression, and expanding the pool of organs for transplantation are the major challenges of the field.

Xenotransplantation-current status and future perspectives

Research efforts have shed light on the immunological obstacles to long-term survival of pig organs transplanted into primates and allowed the identification of targets for specific immune intervention. Accordingly, the development of genetically engineered animals has overcome the hyperacute rejection barrier, with acute humoral xenograft rejection (AHXR) currently remaining the most important immunological obstacle. At this stage, a better control of the elicited anti-pig humoral immune response and avoidance of coagulation disorders are the two primary research fronts being pursued in order to overcome AHXR. Nonetheless, it is encouraging that porcine xenografts can sustain the life of non-human primates for several months. Proactive research aimed at the development of a safer organ source is also underway. It is anticipated that ongoing research in several fields, including accommodation, tolerance, immune suppression and genetic engineering, will result in further improvements in non-human primate survival. However, until convincing efficacy data and a more favourable risk/benefit ratio can be established in relevant animal models, progression to the clinic should not be viewed as an option.

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.