Genetic alterations of the long terminal repeat of an ecotropic porcine endogenous retrovirus during passage in human cells

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.

Analysis of PERV-C superinfection resistance using HA-tagged viruses

BACKGROUND

Using pigs as organ donors has advanced xenotransplantation to the point that it is almost ready for clinical use. However, there is still a zoonotic risk associated with xenotransplantation, and the potential transmission of porcine endogenous retroviruses needs to be surveyed. Despite significant attempts to eliminate this risk, by the selection of PERV-C free pigs with low expression of PERV-A, -B, and by the genome-wide inactivation of PERV using CRISPR/Cas9, the impact of superinfection resistance (SIR) was not investigated. SIR is a viral trait that prevents reinfection (superinfection). For PERV, the underlying mechanism is unclear, whether and how cells, that harbor functional PERV, are protected. Using PERV-C(5683) as a reference virus, we investigated SIR in a newly developed in vitro model to pursue the mechanism and confirm its protective effect.

RESULTS

We developed three PERV-C constructs on the basis of PERV-C(5683), each of which carries a hemagglutinin tag (HA-tag) at a different position of the envelope gene (SP-HA, HA-VRA, and RPep-HA), to distinguish between primary infection and superinfection. The newly generated PERV-C(5683)-HA viruses were characterized while quantifying the viral RNA, reverse transcriptase activity, protein expression analysis, and infection studies. It was demonstrated that SP-HA and RPep-HA were comparable to PERV-C(5683), whereas HA-VRA was not replication competent. SP-HA and RPep-HA were chosen to challenge PERV-C(5683)-positive ST-IOWA cells demonstrating that PERV-C-HA viruses are not able to superinfect those cells. They do not integrate into the genome and are not expressed.

CONCLUSIONS

The mechanism of SIR applies to PERV-C. The production of PERV-C particles serves as a defense mechanism from superinfection with exogenous PERV-C. It was demonstrated by newly generated PERV-C(5683)-HA clones that might be used as a cutting-edge tool. The HA-tagging of PERV-C is novel, providing a blueprint for the tagging of other human tropic PERV viruses. The tagged viruses are suitable for additional in vitro and in vivo infection studies and will contribute, to basic research on viral invasion and pathogenesis. It will maintain the virus safety of XTx.

First virological and pathological study of Göttingen Minipigs with Dippity Pig Syndrome (DPS)

Dippity Pig Syndrome (DPS) is a well-known but rare complex of clinical signs affecting minipigs, which has not been thoroughly investigated yet. Clinically affected animals show acute appearance of red, exudating lesions across the spine. The lesions are painful, evidenced by arching of the back (dipping), and the onset of clinical signs is generally sudden. In order to understand the pathogenesis, histological and virological investigations were performed in affected and unaffected Göttingen Minipigs (GöMPs). The following DNA viruses were screened for using PCR-based methods: Porcine cytomegalovirus (PCMV), which is a porcine roseolovirus (PCMV/PRV), porcine lymphotropic herpesviruses (PLHV-1, PLHV-2, PLHV-3), porcine circoviruses (PCV1, PCV2, PCV3, PCV4), porcine parvovirus 1 (PPV1), and Torque Teno sus viruses (TTSuV1, TTSuV2). Screening was also performed for integrated porcine endogenous retroviruses (PERV-A, PERV-B, PERV-C) and recombinant PERV-A/C and their expression as well as for the RNA viruses hepatitis E virus (HEV) and SARS-CoV-2. Eight clinically affected and one unaffected GöMPs were analyzed. Additional unaffected minipigs had been analyzed in the past. The analyzed GöMPs contained PERV-A and PERV-B integrated in the genome, which are present in all pigs and PERV-C, which is present in most, but not all pigs. In one affected GöMPs recombinant PERV-A/C was detected in blood. In this animal a very high expression of PERV mRNA was observed. PCMV/PRV was found in three affected animals, PCV1 was found in three animals with DPS and in the unaffected minipig, and PCV3 was detected in two animals with DPS and in the unaffected minipig. Most importantly, in one animal only PLHV-3 was detected. It was found in the affected and unaffected skin, and in other organs. Unfortunately, PLHV-3 could not be studied in all other affected minipigs. None of the other viruses were detected and using electron microscopy, no virus particles were found in the affected skin. No porcine virus RNA with exception of PERV and astrovirus RNA were detected in the affected skin by next generation sequencing. This data identified some virus infections in GöMPs with DPS and assign a special role to PLHV-3. Since PCMV/PRV, PCV1, PCV3 and PLHV-3 were also found in unaffected animals, a multifactorial cause of DPS is suggested. However, elimination of the viruses from GöMPs may prevent DPS.

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 retrovirus: classification, molecular structure, regulation, function, and potential risk in xenotransplantation

Xenotransplantation with porcine organs has been recognized as a promising solution to alleviate the shortage of organs for human transplantation. Porcine endogenous retrovirus (PERV), whose proviral DNAs are integrated in the genome of all pig breeds, is a main microbiological risk for xenotransplantation. Over the last decades, some advances on PERVs' studies have been achieved. Here, we reviewed the current progress of PERVs including the classification, molecular structure, regulation, function in immune system, and potential risk in xenotransplantation. We also discussed the problem of insufficient study on PERVs as well as the questions need to be answered in the future work.

Cardiac xenotransplantation: from concept to clinic

For many patients with terminal/advanced cardiac failure, heart transplantation is the most effective, durable treatment option, and offers the best prospects for a high quality of life. The number of potentially life-saving donated human organs is far fewer than the population who could benefit from a new heart, resulting in increasing numbers of patients awaiting replacement of their failing heart, high waitlist mortality, and frequent reliance on interim mechanical support for many of those deemed among the best candidates but who are deteriorating as they wait. Currently, mechanical assist devices supporting left ventricular or biventricular heart function are the only alternative to heart transplant that is in clinical use. Unfortunately, the complication rate with mechanical assistance remains high despite advances in device design and patient selection and management, and the quality of life of the patients even with good outcomes is only moderately improved. Cardiac xenotransplantation from genetically multi-modified (GM) organ-source pigs is an emerging new option as demonstrated by the consistent long-term success of heterotopic (non-life-supporting) abdominal and life-supporting orthotopic porcine heart transplantation in baboons, and by a recent 'compassionate use' transplant of the heart from a GM pig with 10 modifications into a terminally ill patient who survived for 2 months. In this review, we discuss pig heart xenotransplantation as a concept, including pathobiological aspects related to immune rejection, coagulation dysregulation, and detrimental overgrowth of the heart, as well as GM strategies in pigs to prevent or minimize these problems. Additional topics discussed include relevant results of heterotopic and orthotopic heart transplantation experiments in the pig-to-baboon model, microbiological and virologic safety concepts, and efficacy requirements for initiating formal clinical trials. An adequate regulatory and ethical framework as well as stringent criteria for the selection of patients will be critical for the safe clinical development of cardiac xenotransplantation, which we expect will be clinically tested during the next few years.

Genetically engineered pigs for xenotransplantation: Hopes and challenges

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

Evolution of Xenotransplantation as an Alternative to Shortage of Donors in Heart Transplantation

This review aims to show and illustrate the history, current, ethical considerations, and limitations concerning xenotransplantation. Due to the current shortage of available donor organs for transplantation, many alternative sources are being examined to solve the donor shortage. One of them is xenotransplantation which refers to the transplantation of organs from one species to another. Compared to other nonhuman primates (NHP), pigs are ideal species for organ harvesting as they rapidly grow to human size in a handful of months. There is much advancement in the genetic engineering of pigs, which have hearts structurally and functionally similar to the human heart. The role of genetic engineering is to overcome the immune barriers in xenotransplantation and can be used in hyperacute rejection and T cell-mediated rejection. It is technically difficult to use large animal models for orthotopic, life-sustaining heart transplantation. Despite the fact that some religious traditions, such as Jewish and Muslim, prohibit the ingestion of pork products, few religious leaders consider that donating porcine organs is ethical because it saves human life. Although recent technologies have lowered the risk of a xenograft producing a novel virus that causes an epidemic, the risk still exists. It has major implications for the informed consent procedure connected with clinical research on heart xenotransplantation.

Rare isolation of human-tropic recombinant porcine endogenous retroviruses PERV-A/C from Göttingen minipigs

Background

Porcine endogenous retroviruses (PERVs) can infect human cells and pose a risk for xenotransplantation when pig cells, tissues or organs are transplanted to human recipients. Xenotransplantation holds great promise to overcome the shortage of human donor organs after solving the problems of rejection, functionality and virus safety. We recently described the transmission of a human-tropic recombinant PERV-A/C, designated PERV-F, from peripheral blood mononuclear cells (PBMCs) of a Göttingen Minipig (GöMP) to human 293 cells (Krüger et al., in Viruses 12(1):38, 2019). The goal of this study was to characterize PERV-F in more detail and to analyze the probability of virus isolation from other animals.

Methods

The recombination site in the envelope (env) gene, the long terminal repeats (LTR), the proteins and the morphology of the recombinant PERV-F were characterized by polymerase chain reaction (PCR), sequencing, Western blot analysis, immunofluorescence, and transmissible electron microscopy. Mitogen-stimulated PBMCs from 47 additional pigs, including 17 new GöMP, were co-cultured with highly susceptible human 293 T cells, and the PERV-A/C prevalence and PERV transmission was analyzed by PCR.

Results

PERV-F, isolated from a GöMP, is an infectious human-tropic PERV-A/C virus with a novel type of recombination in the env gene. The length of the LTR of PERV-F increased after passaging on human cells. In a few minipigs, but not in German landrace pigs, PERV-A/C were found. There was no transmission of human-tropic PERV-A/C from additional 47 pigs, including 17 GöMP, to human cells.

Conclusion

These data show that human-tropic recombinant PERV-A/C proviruses can only be found in a very small number of minipigs, but not in other pigs, and that their isolation as infectious virus able to replicate on human cells is an extremely rare event, even when using highly susceptible 293 cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12985-022-01742-0.

Xenotransplantation and interspecies organogenesis: current status and issues

Pancreas (and islet) transplantation is the only curative treatment for type 1 diabetes patients whose β-cell functions have been abolished. However, the lack of donor organs has been the major hurdle to save a large number of patients. Therefore, transplantation of animal organs is expected to be an alternative method to solve the serious shortage of donor organs. More recently, a method to generate organs from pluripotent stem cells inside the body of other species has been developed. This interspecies organ generation using blastocyst complementation (BC) is expected to be the next-generation regenerative medicine. Here, we describe the recent advances and future prospects for these two approaches.

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.

Porcine genome engineering for xenotransplantation

The extreme shortage of human donor organs for treatment of patients with end-stage organ failures is well known. Xenotransplantation, which might provide unlimited organ supply, is a most promising strategy to solve this problem. Domestic pigs are regarded as ideal organ-source animals owing to similarity in anatomy, physiology and organ size to humans as well as high reproductive capacity and low maintenance cost. However, several barriers, which include immune rejection, inflammation and coagulative dysfunctions, as well as the cross-species transmission risk of porcine endogenous retrovirus, blocked the pig-to-human xenotransplantation. With the rapid development of genome engineering technologies and the potent immunosuppressive medications in recent years, these barriers could be eliminated through genetic modification of pig genome together with the administration of effective immunosuppressants. A number of candidate genes involved in the regulation of immune response, inflammation and coagulation have been explored to optimize porcine xenograft survival in non-human primate recipients. PERV inactivation in pigs has also been accomplished to firmly address the safety issue in pig-to-human xenotransplantation. Many encouraging preclinical milestones have been achieved with some organs surviving for years. Therefore, the clinical trials of some promising organs, such as islet, kidney and heart, are aimed to be launched in the near future.

Impact of porcine cytomegalovirus on long-term orthotopic cardiac xenotransplant survival

Xenotransplantation using pig organs has achieved survival times up to 195 days in pig orthotopic heart transplantation into baboons. Here we demonstrate that in addition to an improved immunosuppressive regimen, non-ischaemic preservation with continuous perfusion and control of post-transplantation growth of the transplant, prevention of transmission of the porcine cytomegalovirus (PCMV) plays an important role in achieving long survival times. For the first time we demonstrate that PCMV transmission in orthotopic pig heart xenotransplantation was associated with a reduced survival time of the transplant and increased levels of IL-6 and TNFα were found in the transplanted baboon. Furthermore, high levels of tPA-PAI-1 complexes were found, suggesting a complete loss of the pro-fibrinolytic properties of the endothelial cells. These data show that PCMV has an important impact on transplant survival and call for elimination of PCMV from donor pigs.

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.

Xenotransplantation: Current Status in Preclinical Research

The increasing life expectancy of humans has led to a growing numbers of patients with chronic diseases and end-stage organ failure. Transplantation is an effective approach for the treatment of end-stage organ failure; however, the imbalance between organ supply and the demand for human organs is a bottleneck for clinical transplantation. Therefore, xenotransplantation might be a promising alternative approach to bridge the gap between the supply and demand of organs, tissues, and cells; however, immunological barriers are limiting factors in clinical xenotransplantation. Thanks to advances in gene-editing tools and immunosuppressive therapy as well as the prolonged xenograft survival time in pig-to-non-human primate models, clinical xenotransplantation has become more viable. In this review, we focus on the evolution and current status of xenotransplantation research, including our current understanding of the immunological mechanisms involved in xenograft rejection, genetically modified pigs used for xenotransplantation, and progress that has been made in developing pig-to-pig-to-non-human primate models. Three main types of rejection can occur after xenotransplantation, which we discuss in detail: (1) hyperacute xenograft rejection, (2) acute humoral xenograft rejection, and (3) acute cellular rejection. Furthermore, in studies on immunological rejection, genetically modified pigs have been generated to bridge cross-species molecular incompatibilities; in the last decade, most advances made in the field of xenotransplantation have resulted from the production of genetically engineered pigs; accordingly, we summarize the genetically modified pigs that are currently available for xenotransplantation. Next, we summarize the longest survival time of solid organs in preclinical models in recent years, including heart, liver, kidney, and lung xenotransplantation. Overall, we conclude that recent achievements and the accumulation of experience in xenotransplantation mean that the first-in-human clinical trial could be possible in the near future. Furthermore, we hope that xenotransplantation and various approaches will be able to collectively solve the problem of human organ shortage.

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.

Characterization of porcine endogenous retrovirus particles released by the CRISPR/Cas9 inactivated cell line PK15 clone 15

The infection of human transplant recipients by porcine endogenous retrovirus (PERV) is a safety issue for xenotransplantation (XTx). CRISPR/Cas9 technology has enabled the generation of pigs free of functional PERVs, and the susceptibility of these animals to reinfection by PERVs remains unclear. To assess virological safety, we characterized a cell line in which PERVs have been inactivated by CRISPR/Cas9 (PK15 clone 15) for its susceptibility to infectious PERV. First, basal expression of PERV pol, the porcine PERV-A receptor (POPAR), and reverse transcriptase (RT) activity of PERV were determined. PK15 clone 15 cells were inoculated with PERV and monitored post infection for virus expression and RT activity. Particles were visualized by electron microscopy. Our data show that PK15 clone 15 cells still produce viral proteins that assemble to produce impaired viral particles. These virions have an irregular morphology that diverges from that of mature wild type. The particles are no longer infectious when tested in a downstream infection assay using supernatants of PK15 clone 15 cells to infect susceptible swine testis-IOWA (ST-IOWA) cells. The expression of POPAR was quantified to exclude the possibility that lack of susceptibility to reinfection, for PERV-A, is caused by absence of viral host receptor(s). PK15 and PK15 clone 15 cells do, in fact, express POPAR equally. PERV RT inactivation mediated by CRISPR/Cas9 does not compromise virus assembly but affects virion structure and proviral integration. The constitutive virion production seems to maintain cellular resistance to superinfection and possibly indicates a protective side effect of this specific CRISPR/Cas9 mediated RT inactivation.

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.

Human SAMHD1 restricts the xenotransplantation relevant porcine endogenous retrovirus (PERV) in non-dividing cells

The release of porcine endogenous retrovirus (PERV) particles from pig cells is a potential risk factor during xenotransplantation by way of productively infecting the human transplant recipient. Potential countermeasures against PERV replication are restriction factors that block retroviral replication. SAMHD1 is a triphosphohydrolase that depletes the cellular pool of dNTPs in non-cycling cells starving retroviral reverse transcription. We investigated the antiviral activity of human SAMHD1 against PERV and found that SAMHD1 potently restricts its reverse transcription in human monocytes, monocyte-derived dendritic cells (MDDC), or macrophages (MDM) and in monocytic THP-1 cells. Degradation of SAMHD1 by SIVmac Vpx or CRISPR/Cas9 knock-out of SAMHD1 allowed for PERV reverse transcription. Addition of deoxynucleosides alleviated the SAMHD1-mediated restriction suggesting that SAMHD1-mediated degradation of dNTPs restricts PERV replication in these human immune cells. In conclusion, our findings highlight SAMHD1 as a potential barrier to PERV transmission from pig transplants to human recipients during xenotransplantation.

Will Genetic Engineering Carry Xenotransplantation of Pig Islets to the Clinic?

PURPOSE OF REVIEW

Porcine islets represent a potentially attractive beta-cell source for xenotransplantation into patients with type 1 diabetes, who are not eligible to islet allo-transplantation due to a lack of suitable human donor organs. Recent progress in genetic engineering/gene editing of donor pigs provides new opportunities to overcome rejection of xeno-islets, to improve their engraftment and insulin secretion capacity, and to reduce the risk for transmission of porcine endogenous retroviruses. This review summarizes the current issues and progress in islet xenotransplantation with special emphasis on genetically modified/gene edited donor pigs.

RECENT FINDINGS

Attempts to overcome acute rejection of xeno-islets, especially after intraportal transplantation into the liver, include the genetic elimination of specific carbohydrate antigens such as αGal, Neu5Gc, and Sd(a) for which humans and-in part-non-human primates have natural antibodies that bind to these targets leading to activation of complement and coagulation. A complementary approach is the expression of one or more human complement regulatory proteins (hCD46, hCD55, hCD59). Transgenic attempts to overcome cellular rejection of islet xenotransplants include the expression of proteins that inhibit co-stimulation of T cells. Expression of glucagon-like peptide-1 and M3 muscarinic receptors has been shown to increase the insulin secretion of virally transduced porcine islets in vitro and it will be interesting to see the effects of these modifications in transgenic pigs and islet products derived from them. Genome-wide inactivation of porcine endogenous retrovirus (PERV) integrants by mutating their pol genes using CRISPR/Cas9 is a recent approach to reduce the risk for PERV transmission by xeno-islets. Genetic engineering/gene editing of xeno-islet donor pigs facilitated major progress towards clinical islet xenotransplantation. The required set of genetic modifications will depend on the source of islets (fetal/neonatal vs. adult), the mode of delivery (encapsulated vs. free), and the transplantation site.

Analysis of frozen strawberries involved in a large norovirus gastroenteritis outbreak using next generation sequencing and digital PCR

Virus-contaminated frozen berries have been frequently identified as cause of foodborne disease outbreaks. To provide new tools for virus detection and characterization in berries, next generation sequencing (NGS) and reverse transcription-digital PCR (RT-dPCR) techniques were tested here with strawberries previously involved in a large-scale norovirus (NoV) gastroenteritis outbreak in Germany. By NGS, about 29 million sequence reads were generated, which mainly showed identities to sequences from the plant matrix and from the bacterial flora. Most abundant virus sequences originated from plant-specific viruses, whereas sequences with high identity to human viruses were rare. Only two sequence reads showed homologies to human NoV. They were identical to GII.P16/GII.13 NoV sequences from patients and a strawberry sample independently analyzed during the outbreak. Quantification of the GII NoV RNA of the berries using RT-dPCR confirmed a low mean virus amount of 185 copies/25 g, which is similar to independently assessed RT-qPCR results (257 copies/25 g). The study shows that identification of human-pathogenic viruses in naturally contaminated frozen berries is possible using NGS technologies. However, the method needs to be further optimized in order to enable convenient and reproducible detection of a low amount of human-pathogenic virus sequences in a background of highly abundant nucleic acids of other sources.

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.

Porcine Endogenous Retrovirus (PERV) - Molecular Structure and Replication Strategy in the Context of Retroviral Infection Risk of Human Cells

The xenotransplantation of porcine tissues may help overcome the shortage of human organs for transplantation. However, there are some concerns about recipient safety because the risk of porcine endogenous retrovirus (PERV) transmission to human cells remains unknown. Although, to date, no PERV infections have been noted in vivo, the possibility of such infections has been confirmed in vitro. Better understanding of the structure and replication cycle of PERVs is a prerequisite for determining the risk of infection and planning PERV-detection strategies. This review presents the current state of knowledge about the structure and replication cycle of PERVs in the context of retroviral infection risk.

Why was PERV not transmitted during preclinical and clinical xenotransplantation trials and after inoculation of animals?

Porcine endogenous retroviruses (PERVs) are present in the genome of all pigs, they infect certain human cells and therefore pose a special risk for xenotransplantation using pig cells, tissues and organs. Xenotransplantation is being developed in order to alleviate the reduced availability of human organs. Despite the fact that PERVs are able to infect certain human cells and cells from other species, transmission of PERVs has not been observed when animals (including non-human primates) were inoculated with PERV preparations or during preclinical xenotransplantations. The data indicate that PERVs were not transmitted because they were not released from the transplant or were inhibited by intracellular restriction factors and innate immunity in the recipient. In a single study in guinea pigs, a transient PERV infection and anti-PERV antibodies were described, indicating that in this case at least, the immune system may also have been involved.

Activity analysis of LTR12C as an effective regulatory element of the RAE1 gene

Ribonucleic acid export 1 (RAE1) plays an important role in the export of mature mRNAs from the nucleus to the cytoplasm. Long terminal repeats (LTRs) became integrated into the human genome during primate evolution. One such repeat element, LTR12C, lies within a predicted regulatory region located upstream of the RAE1 gene. We examined the transcriptional activity of LTR12C by using the luciferase assay, and showed that the tandem repeat region (TRR) located within LTR12C was required for its regulatory function. A bioinformatics analysis revealed that the LTR12C element had multiple transcription factor binding sites specific for nuclear transcription factor Y (NF-Y), and the promoter activity of LTR12C was significantly decreased after NF-Y knockdown. Additionally, we discovered novel data indicating that LTR12C was initially inserted into the gorilla genome. Taken together, our results reveal that the TRR of LTR12C has powerful regulatory activity due to its NF-Y binding sites, and the integration of the LTR12C element into the primate genome during evolution may have affected RAE1 transcription.

No PERV transmission during a clinical trial of pig islet cell transplantation

Xenotransplantation of pig islet cells is a promising alternative for the treatment of diabetes with insulin and may help to prevent numerous late complications such as blindness and amputation. First encouraging results using porcine islets have been reported in preclinical animal models as well in the first clinical trial in New Zealand. The goal of this manuscript is to examine the biological safety of a second trial performed in Argentina, specifically in regards to the transmission of porcine endogenous retroviruses (PERVs) using improved detection methods As in the first trial encapsulated islet cells from the well-characterised Auckland Island pigs were used. The animals were not genetically modified. The islet cells were transplanted in eight human recipients using a modified clinical protocol. Sera taken at different time points after transplantation (up to 55 weeks) were screened for the presence of antibodies against PERV proteins by Western blot analysis using viral antigens from highly purified virus particles. Positive sera obtained by immunization with recombinant PERV proteins were used as control sera. In none of the patients antibodies against PERV were detected, indicating the absence of infection. In parallel at different time points (up to 113 weeks) white blood cells (WBC) have been tested for PERV DNA, and WBC and plasma for PERV RNA by real-time RT-PCR. All tests were negative. In addition, using primers detecting pig mitochondrial cytochrome oxidase (COX) gene, patients were screened for microchimerism. In summary, the data are further evidence for the safety of pig islet cell transplantation.

Detection of koala retrovirus subgroup B (KoRV-B) in animals housed at European zoos

Many koalas carry an endogenous retrovirus, KoRV-A, in their genome. Recently, a second retrovirus, KoRV-B, was detected in koalas in Japanese and U.S. zoos. However, this virus is not endogenous, differs in the receptor binding site of the surface envelope protein, and uses a receptor different from that of KoRV-A. We describe here a KoRV-B found in koalas at zoos in Germany and Belgium that differs slightly from that found in the Los Angeles zoo.

How Active Are Porcine Endogenous Retroviruses (PERVs)?

Porcine endogenous retroviruses (PERVs) represent a risk factor if porcine cells, tissues, or organs were to be transplanted into human recipients to alleviate the shortage of human transplants; a procedure called xenotransplantation. In contrast to human endogenous retroviruses (HERVs), which are mostly defective and not replication-competent, PERVs are released from normal pig cells and are infectious. PERV-A and PERV-B are polytropic viruses infecting cells of several species, among them humans; whereas PERV-C is an ecotropic virus infecting only pig cells. Virus infection was shown in co-culture experiments, but also in vivo, in the pig, leading to de novo integration of proviruses in certain organs. This was shown by measurement of the copy number per cell, finding different numbers in different organs. In addition, recombinations between PERV-A and PERV-C were observed and the recombinant PERV-A/C were found to be integrated in cells of different organs, but not in the germ line of the animals. Here, the evidence for such in vivo activities of PERVs, including expression as mRNA, protein and virus particles, de novo infection and recombination, will be summarised. These activities make screening of pigs for provirus number and PERV expression level difficult, especially when only blood or ear biopsies are available for analysis. Highly sensitive methods to measure the copy number and the expression level will be required when selecting pigs with low copy number and low expression of PERV as well as when inactivating PERVs using the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated nuclease (CRISPR/Cas) technology.

Tolerance and immune response to the porcine endogenous retrovirus in German landrace pigs immunised with viral proteins

Immunisation of goats, mice, rats, rabbits, guinea pigs, and hamsters with the recombinant ectodomain of the porcine endogenous retrovirus (PERV) transmembrane envelope (TM) protein (p15E) induced binding and neutralising immune antibodies in all animals. In contrast, no antibodies were induced when pigs were immunised with p15E, indicating that pigs are tolerant to their endogenous retroviruses, at least to the TM protein. To answer the question of whether pigs are tolerant to other structural proteins of PERV, we immunised German landrace pigs with p15E, this time in conjunction with the surface envelope proteins gp70 and the core capsid Gag protein p27CA. To ensure that the pigs were immunocompetent and that immunisation was successful, all animals also received an injection of an unrelated protein, keyhole limpet hemocyanin (KLH). Whereas all animals produced antibodies against KLH, no animals produced antibodies against the viral envelope proteins, thus confirming previous results for p15E and extending them to the other envelope protein, gp70. However, the pigs did produce antibodies against p27CA, indicating that there is no tolerance to the core capsid protein of PERV.

Preventing transfer of infectious agents

Xenotransplantation using pig cells, tissues and organs may be associated with the transfer of porcine infectious agents, which may infect the human recipient and in the worst case induce a disease (zoonosis). To prevent this, a broad screening program of the donor animals for putative zoonotic microorganisms, including bacteria, viruses, fungi and others, using sensitive and specific detection methods has to be performed. As long as it is still unknown, which microorganism represents a real risk for the recipient, experience from allotransplantation should be brought in. Due to the fact that pigs can be screened long before the date of transplantation, xenotransplantation will become eventually safer compared with allotransplantation. Screening and selection of animals free of potential zoonotic microorganisms, Caesarean section, vaccination and/or treatment with chemotherapeutics are the strategies of choice to obtain donor animals not transmitting microorganisms. In the case of porcine endogenous retroviruses (PERVs) which are integrated in the genome of all pigs and which cannot be eliminated this way, selection of animals with low virus expression and generation of genetically modified pigs suppressing PERV expressions may be performed.

Multiple invasions of an infectious retrovirus in cat genomes

Endogenous retroviruses (ERVs) are remnants of ancient retroviral infections of host germ-line cells. While most ERVs are defective, some are active and express viral proteins. The RD-114 virus is a replication-competent feline ERV, and several feline cell lines produce infectious RD-114 viral particles. All domestic cats are considered to have an ERV locus encoding a replication-competent RD-114 virus in their genomes; however, the locus has not been identified. In this study, we investigated RD-114 virus-related proviral loci in genomes of domestic cats, and found that none were capable of producing infectious viruses. We also found that all domestic cats have an RD-114 virus-related sequence on chromosome C2, termed RDRS C2a, but populations of the other RDRSs are different depending on the regions where cats live or breed. Our results indicate that RDRS C2a, the oldest RD-114-related provirus, entered the host genome before an ancestor of domestic cats started diverging and the other new RDRSs might have integrated into migrating cats in Europe. We also show that infectious RD-114 virus can be resurrected by the recombination between two non-infectious RDRSs. From these data, we conclude that cats do not harbor infectious RD-114 viral loci in their genomes and RD-114-related viruses invaded cat genomes multiple times.

Novel neutralising antibodies targeting the N-terminal helical region of the transmembrane envelope protein p15E of the porcine endogenous retrovirus (PERV)

Previously, immunising different species with the transmembrane envelope protein p15E of the porcine endogenous retrovirus (PERV), neutralising antibodies were induced which recognised epitopes in the fusion peptide proximal region (FPPR) and in the membrane-proximal external region (MPER). Only the MPER-specific antibodies were shown to neutralise and these antibodies targeted epitopes in the MPER similarly localised as the epitopes recognised by antibodies broadly neutralising HIV-1 such as 2F5 and 4E10. To study whether neutralising antibodies could be induced immunising with subunits of p15E, recombinant proteins corresponding to the N-terminal, the C-terminal helical region (NHR, CHR) and a p15E with a mutation in the Cys-Cys loop were produced. Whereas none of these antigens induced MPER-specific neutralising antibodies, the animals immunised with the FPPR/NHR subunit and the mutated p15E produced neutralising antibodies binding to the NHR. Therefore, for the first time, antibodies specific for the NHR and neutralising PERV were described.

Lack of antibody response in pigs immunized with the transmembrane envelope protein of porcine endogenous retroviruses

Recently, we immunized different mammalian species (goats, mice, rats, rabbits, guinea pigs and hamsters) with the recombinant ectodomain of the transmembrane envelope (TM) protein p15E of porcine endogenous retrovirus (PERV). In all cases, neutralizing immune sera were induced, which recognized epitopes in the fusion peptide proximal region and the membrane proximal external region of p15E. In order to analyse whether pigs are also able to produce such antibodies, and whether such antibodies can be used to study the involvement of the TM protein in placental development (as was shown for endogenous retroviruses of other species), German landrace pigs were immunized with PERV p15E. No binding and neutralizing antibodies were produced as shown in three Western blot analyses and in a neutralization assay, indicating that pigs are tolerant to their endogenous retroviruses, at least for the ectodomain of the TM protein.

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.

Microbiological safety of the first clinical pig islet xenotransplantation trial in New Zealand

BACKGROUND

Xenotransplantation using pig cells, tissues, or organs may be associated with the transmission of porcine microorganisms and the development of zoonoses. Among all porcine microorganisms porcine endogenous retroviruses (PERVs) represent a special risk because they are integrated in the genome of all pigs and able to infect human cells. In previous preclinical and retrospective clinical trials of xenotransplantation, no transmission of PERV was observed. The first clinical trial of (alginate-encapsulated) porcine islet cell transplantation in New Zealand, which was approved by the New Zealand Government as an open-label phase I/IIa safety/efficacy trial, offers the possibility to analyze microbiological safety in a prospective clinical study.

METHODS

Before the trial started, a multilevel testing strategy was used to screen for 26 microorganisms in donor pigs of the Auckland Island strain and the islet cell preparations used for treatment. Donor testing was performed using molecular methods including multiplex real-time PCR. Blood samples from 14 pig islet cell recipients were also investigated by molecular biological methods at weeks 1, 4, 8, 12, 24, and 52 post-transplant for the transmission of porcine microorganisms. Sera were also monitored at these time points for antibodies against PERVs.

RESULTS

Beginning in 2009, fourteen patients with severe unaware hypoglycemia were treated with one of four different dosages of alginate-encapsulated porcine islets ranging from 5000-20,000 islet equivalents delivered in a single dose. No transmission of either PERVs or other porcine microorganisms was detected by PCR and immunological methods.

CONCLUSION

These findings support previous results and strongly indicate the safety of xenotransplantation as performed here.

Detection of porcine endogenous retrovirus in xenotransplantation

Xenotransplantation can provide a virtually limitless supply of cells, tissues and organs for a variety of therapeutic procedures. Cells and tissues for use in human transplantation procedures could be supplied using material taken from pigs. However, there is a potential risk of transmission of porcine infectious agents, including porcine endogenous retroviruses (PERVs), to a novel human host, with as yet unknown consequences. Three subtypes of PERV have been identified, of which both PERV-A and PERV-B have the ability to infect human cells in vitro. The third subtype, PERV-C, does not show this ability. Recombinant PERV-A/C forms have demonstrated infectivity in human cell culture. Monitoring in xenotransplantation should comprise screening of the source pig herd (PERV-A and PERV-B level expression assessment, PERV-C detection) and screening of recipients (differentiation between PERV transmission and chimerism). The detection of PERVs includes analyses of both DNA and RNA (PCR and RT-PCR), quantitative determination of the level of PERV nucleic acids (real-time PCR and real-time RT-PCR), assessment of reverse transcriptase (RT) activity (RT assays) and viral and recipient protein detection (immunological methods). In summary, all available methods should be used in monitoring of PERVs in xenotransplantation, and caution should be exercised at all stages of monitoring. Such monitoring has enormous significance for eliminating the possibility of transmission of PERV infection, thus contributing to higher levels of safety in xenotransplantation.

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.

Koala retroviruses: characterization and impact on the life of koalas

Koala retroviruses (KoRV) have been isolated from wild and captive koalas in Australia as well as from koala populations held in zoos in other countries. They are members of the genus Gammaretrovirus, are most closely related to gibbon ape leukemia virus (GaLV), feline leukemia virus (FeLV) and porcine endogenous retrovirus (PERV) and are likely the result of a relatively recent trans-species transmission from rodents or bats. The first KoRV to be isolated, KoRV-A, is widely distributed in the koala population in both integrated endogenous and infectious exogenous forms with evidence from museum specimens older than 150 years, indicating a relatively long engagement with the koala population. More recently, additional subtypes of KoRV that are not endogenized have been identified based on sequence differences and host cell receptor specificity (KoRV-B and KoRV-J). A specific association with fatal lymphoma and leukemia has been recently suggested for KoRV-B. In addition, it has been proposed that the high viral loads found in many animals may lead to immunomodulation resulting in a higher incidence of diseases such as chlamydiosis. Although the molecular basis of this immunomodulation is still unclear, purified KoRV particles and a peptide corresponding to a highly conserved domain in the envelope protein have been shown to modulate cytokine expression in vitro, similar to that induced by other gammaretroviruses. While much is still to be learned, KoRV induced lymphoma/leukemia and opportunistic disease arising as a consequence of immunomodulation are likely to play an important role in the stability of koala populations both in the wild and in captivity.

Role of DNA methylation in expression and transmission of porcine endogenous retroviruses

Porcine endogenous retroviruses (PERV) represent a major safety concern in pig-to-human xenotransplantation. To date, no PERV infection of a xenograft recipient has been recorded; however, PERVs are transmissible to human cells in vitro. Some recombinants of the A and C PERV subgroups are particularly efficient in infection and replication in human cells. Transcription of PERVs has been described in most pig cells, but their sequence and insertion polymorphism in the pig genome impede identification of transcriptionally active or silenced proviral copies. Furthermore, little is known about the epigenetic regulation of PERV transcription. Here, we report on the transcriptional suppression of PERV by DNA methylation in vitro and describe heavy methylation in the majority of PERV 5' long terminal repeats (LTR) in porcine tissues. In contrast, we have detected sparsely methylated or nonmethylated proviruses in the porcine PK15 cells, which express human cell-tropic PERVs. We also demonstrate the resistance of PERV DNA methylation to inhibitors of methylation and deacetylation. Finally, we show that the high permissiveness of various human cell lines to PERV infection coincides with the inability to efficiently silence the PERV proviruses by 5'LTR methylation. In conclusion, we suggest that DNA methylation is involved in PERV regulation, and that only a minor fraction of proviruses are responsible for the PERV RNA expression and porcine cell infectivity.