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.

HERV-Derived Ervpb1 Is Conserved in Simiiformes, Exhibiting Expression in Hematopoietic Cell Lineages Including Macrophages

(1) Background: The ERVPb1 gene in humans is derived from an envelope (Env) gene of a human endogenous retrovirus group, HERV-P(b). The ERVPb1 gene reportedly has a conserved open reading frame (ORF) in Old World monkeys. Although its forced expression led to cell-fusion in an ex vivo cell culture system, like other Env-derived genes such as syncytin-1 and -2, its mRNA expression is not placenta-specific, but almost ubiquitous, albeit being quite low in human tissues and organs, implying a distinct role for ERVPb1. (2) Methods: To elucidate the cell lineage(s) in which the ERVPb1 protein is translated in human development, we developed a novel, highly sensitive system for detecting HERV-derived proteins/peptides expressed in the tissue differentiation process of human induced pluripotent stem cells (iPSCs). (3) Results: We first determined that ERVPb1 is also conserved in New World monkeys. Then, we showed that the ERVPb1 protein is translated from a uniquely spliced ERVPb1 transcript in hematopoietic cell lineages, including a subset of macrophages, and further showed that its mRNA expression is upregulated by lipopolysaccharide (LPS) stimulation in primary human monocytes. (4) Conclusions: ERVPb1 is unique to Simiiformes and actually translated in hematopoietic cell lineages, including a subset of macrophages.

High-Throughput Sequencing is a Crucial Tool to Investigate the Contribution of Human Endogenous Retroviruses (HERVs) to Human Biology and Development

Human Endogenous retroviruses (HERVs) are remnants of ancient retroviral infections that represent a large fraction of our genome. Their transcriptional activity is finely regulated in early developmental stages and their expression is modulated in different cell types and tissues. Such activity has an impact on human physiology and pathology that is only partially understood up to date. Novel high-throughput sequencing tools have recently allowed for a great advancement in elucidating the various HERV expression patterns in different tissues as well as the mechanisms controlling their transcription, and overall, have helped in gaining better insights in an all-inclusive understanding of the impact of HERVs in biology of the host.

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.

Human Endogenous Retroviruses and Type 1 Diabetes

PURPOSE OF THE REVIEW

The aim of this review is to discuss recent data pointing at an involvement of human endogenous retroviruses (HERVs) in type 1 diabetes (T1D) onset and progression.

RECENT FINDINGS

The envelope protein of HERV-W family, named HERV-W-Env, was detected in pancreata from T1D patients and was shown to display pro-inflammatory properties and direct toxicity toward pancreatic beta cells. The etiopathogenesis of T1D remains elusive, even if conventional environmental viral infections have been recurrently involved. Nonetheless, a new category of pathogens may provide the missing link between genetic susceptibility and environmental factors long thought to contribute to T1D onset. A number of studies have now shown that HERV sequences, which are normally inactivated or repressed in the human genome, could be activated by environmental viruses. Thus, if similarly activated by viruses associated with T1D, disregarded HERV genes may underlie T1D genetic susceptibility. Moreover, once expressed, HERV elements may display broad pathogenic properties, which identify them as potential new therapeutic targets.

Distribution of endogenous gammaretroviruses and variants of the Fv1 restriction gene in individual mouse strains and strain subgroups

Inbred laboratory mouse strains carry endogenous retroviruses (ERVs) classed as ecotropic, xenotropic or polytropic mouse leukemia viruses (E-, X- or P-MLVs). Some of these MLV ERVs produce infectious virus and/or contribute to the generation of intersubgroup recombinants. Analyses of selected mouse strains have linked the appearance of MLVs and virus-induced disease to the strain complement of MLV E-ERVs and to host genes that restrict MLVs, particularly Fv1. Here we screened inbred strain DNAs and genome assemblies to describe the distribution patterns of 45 MLV ERVs and Fv1 alleles in 58 classical inbred strains grouped in two ways: by common ancestry to describe ERV inheritance patterns, and by incidence of MLV-associated lymphomagenesis. Each strain carries a unique set of ERVs, and individual ERVs are present in 5–96% of the strains, often showing lineage-specific distributions. Two ERVs are alternatively present as full-length proviruses or solo long terminal repeats. High disease incidence strains carry the permissive Fv1ⁿ allele, tested strains have highly expressed E-ERVs and most have the Bxv1 X-ERV; these three features are not present together in any low-moderate disease strain. The P-ERVs previously implicated in P-MLV generation are not preferentially found in high leukemia strains, but the three Fv1 alleles that restrict inbred strain E-MLVs are found only in low-moderate leukemia strains. This dataset helps define the genetic basis of strain differences in spontaneous lymphomagenesis, describes the distribution of MLV ERVs in strains with shared ancestry, and should help annotate sequenced strain genomes for these insertionally polymorphic and functionally important proviruses.

Distribution, Diversity, and Evolution of Endogenous Retroviruses in Perissodactyl Genomes

The evolution of mammalian genomes has been shaped by interactions with endogenous retroviruses (ERVs). In this study, we investigated the distribution and diversity of ERVs in the mammalian order Perissodactyla, with a view to understanding their impact on the evolution of modern equids (family Equidae). We characterize the major ERV lineages in the horse genome in terms of their genomic distribution, ancestral genome organization, and time of activity. Our results show that subsequent to their ancestral divergence from rhinoceroses and tapirs, equids acquired four novel ERV lineages. We show that two of these ERV lineages proliferated extensively in the lineage leading to modern horses, and one contains loci that are actively transcribed in specific tissues. In addition, we show that the white rhinoceros has resisted germ line colonization by retroviruses for more than 54 million years-longer than any other extant mammalian species. The map of equine ERVs that we provide here will be of great utility to future studies aiming to investigate the potential functional roles of equine ERVs and their impact on equine evolution.IMPORTANCE ERVs in the host genome are highly informative about the long-term interactions of retroviruses and hosts. They are also interesting because they have influenced the evolution of mammalian genomes in various ways. In this study, we derive a calibrated timeline describing the process through which ERV diversity has been generated in the equine germ line. We determined the distribution and diversity of perissodactyl ERV lineages and inferred their retrotranspositional activity during evolution, thereby gaining insight into the long-term coevolutionary history of retroviruses and mammals. Our study provides a platform for future investigations to identify equine ERV loci involved in physiological processes and/or pathological conditions.

Proceedings of the 2017 Viral Clearance Symposium, Session 1.1: Upstream Mitigation, Part 1-Cell Bank and Bulk Harvest Testing

Monoclonal antibodies (mAbs) are typically produced using mammalian cell lines, which are known to express endogenous retrovirus-like particles (RVLPs) and also have the potential to be infected by viruses. This session focused on the detection and measurement of these viruses and RVLPs. In the first session, it was shown that next-generation sequencing (NGS) can detect, with a similar sensitivity as polymerase chain reaction (PCR), viruses in cells without a priori knowledge of the specific virus and more importantly that a specific NGS approach can decipher whether the signal corresponds to a replicating virus. The second session provided data showing that the PCR assay for detection of ecotropic recombinant virus (ERV) genome is an alternative to quantification by transmission electron microscopy (qTEM) for quantification of RVLP. In addition, the potential use of a harvest filter for RVLP retention in a perfusion process was discussed. In the third session, RVLP data from 67 different Pfizer programs spanning different conditions were presented. No single factor had a significant impact on the level of RVLPs. It was suggested that a "generic" or "worst-case" RVLP value, derived from a well-characterized platform cell culture process, could be used with confidence to determine a conservative retroviral safety margin for platform processes. In the fourth session, the sensitivity of several cell culture- and PCR-based assays for detection of different MMV strains using several production cells was discussed. It was found that molecular assays were generally superior in the breadth of detection with equivalent sensitivity.LAY ABSTRACT: This session focused on the detection and measurement of viruses and virus-like particles in cell lines and manufacturing processes used for production of therapeutic proteins.

Endogenous retroviruses of non-avian/mammalian vertebrates illuminate diversity and deep history of retroviruses

The deep history and early diversification of retroviruses remains elusive, largely because few retroviruses have been characterized in vertebrates other than mammals and birds. Endogenous retroviruses (ERVs) documented past retroviral infections and thus provide ‘molecular fossils’ for studying the deep history of retroviruses. Here we perform a comprehensive phylogenomic analysis of ERVs within the genomes of 92 non-avian/mammalian vertebrates, including 72 fishes, 4 amphibians, and 16 reptiles. We find that ERVs are present in all the genomes of jawed vertebrates, revealing the ubiquitous presence of ERVs in jawed vertebrates. We identify a total of >8,000 ERVs and reconstruct ~450 complete or partial ERV genomes, which dramatically expands the phylogenetic diversity of retroviruses and suggests that the diversity of exogenous retroviruses might be much underestimated in non-avian/mammalian vertebrates. Phylogenetic analyses show that retroviruses cluster into five major groups with different host distributions, providing important insights into the classification and diversification of retroviruses. Moreover, we find retroviruses mainly underwent frequent host switches in non-avian/mammalian vertebrates, with exception of spumavirus-related viruses that codiverged with their ray-finned fish hosts. Interestingly, ray-finned fishes and turtles appear to serve as unappreciated hubs for the transmission of retroviruses. Finally, we find retroviruses underwent many independent water-land transmissions, indicating the water-land interface is not a strict barrier for retrovirus transmission. Our analyses provide unprecedented insights into and valuable resources for studying the diversification, key evolutionary transitions, and macroevolution of retroviruses.

Retroviruses infect a wide range of vertebrates and cause many diseases, such as AIDS and cancers. To date, retroviruses have been rarely characterized in vertebrates other than mammals and birds, impeding our understanding of the diversity and early evolution of retroviruses. Retroviruses can occasionally integrate into host genomes and become endogenous retroviruses (ERVs), which provide molecular fossils for studying the long-term evolution of retroviruses. Here we performed comparative genomic and evolutionary analyses of ERVs within 92 non-avian/mammalian vertebrates (fishes, amphibians, and reptiles) and uncovered extraordinary diversity of retroviruses in non-avian/mammalian vertebrates. Our analyses reveal an ancient aquatic origin of retroviruses and retroviruses underwent frequent host-switching. Our findings have important implications in understanding the deep history and evolutionary mode of retroviruses.

Viral infections and breast cancer - A current perspective

Sporadic human breast cancer is the most common cancer to afflict women. Since the discovery, decades ago, of the oncogenic mouse mammary tumour virus, there has been significant interest in the potential aetiologic role of infectious agents in sporadic human breast cancer. To address this, many studies have examined the presence of viruses (e.g. papillomaviruses, herpes viruses and retroviruses), endogenous retroviruses and more recently, microbes, as a means of implicating them in the aetiology of human breast cancer. Such studies have generated conflicting experimental and clinical reports of the role of infection in breast cancer. This review evaluates the current evidence for a productive oncogenic viral infection in human breast cancer, with a focus on the integration of sensitive and specific next generation sequencing technologies with pathogen discovery. Collectively, the majority of the recent literature using the more powerful next generation sequencing technologies fail to support an oncogenic viral infection being involved in disease causality in breast cancer. In balance, the weight of the current experimental evidence supports the conclusion that viral infection is unlikely to play a significant role in the aetiology of breast cancer.

Identification and Expression Analyses of Equine Endogenous Retroviruses in Horses

Endogenous retroviruses (ERVs) have been integrated into vertebrate genomes and have momentously affected host organisms. Horses (Equus caballus) have been domesticated and selected for elite racing ability over centuries. ERVs played an important role in the evolutionary diversification of the horse genome. In the present study, we identified six equine ERV families (EqERVs-E1, I1, M2, P1, S1, and Y4), their full-length viral open reading frames (ORFs), and elucidated their phylogenetic relationships. The divergence time of EqERV families assuming an evolutionary rate of 0.2%/Myr indicated that EqERV-S3 (75.4 million years ago; mya) on chromosome 10 is an old EqERV family and EqERV-P5 (1.2 Mya) on chromosome 12 is a young member. During the evolutionary diversification of horses, the EqERV-I family diverged 1.7 Mya to 38.7 Mya. Reverse transcription quantitative real-time PCR (RT-qPCR) amplification of EqERV pol genes showed greater expression in the cerebellum of the Jeju horse than the Thoroughbred horse. These results could contribute further dynamic studies for horse genome in relation to EqERV gene function.

Genomic fossils reveal adaptation of non-autonomous pararetroviruses driven by concerted evolution of noncoding regulatory sequences

The interplay of different virus species in a host cell after infection can affect the adaptation of each virus. Endogenous viral elements, such as endogenous pararetroviruses (PRVs), have arisen from vertical inheritance of viral sequences integrated into host germline genomes. As viral genomic fossils, these sequences can thus serve as valuable paleogenomic data to study the long-term evolutionary dynamics of virus-virus interactions, but they have rarely been applied for this purpose. All extant PRVs have been considered autonomous species in their parasitic life cycle in host cells. Here, we provide evidence for multiple non-autonomous PRV species with structural defects in viral activity that have frequently infected ancient grass hosts and adapted through interplay between viruses. Our paleogenomic analyses using endogenous PRVs in grass genomes revealed that these non-autonomous PRV species have participated in interplay with autonomous PRVs in a possible commensal partnership, or, alternatively, with one another in a possible mutualistic partnership. These partnerships, which have been established by the sharing of noncoding regulatory sequences (NRSs) in intergenic regions between two partner viruses, have been further maintained and altered by the sequence homogenization of NRSs between partners. Strikingly, we found that frequent region-specific recombination, rather than mutation selection, is the main causative mechanism of NRS homogenization. Our results, obtained from ancient DNA records of viruses, suggest that adaptation of PRVs has occurred by concerted evolution of NRSs between different virus species in the same host. Our findings further imply that evaluation of within-host NRS interactions within and between populations of viral pathogens may be important.

Transmission of Porcine Endogenous Retrovirus Produced from Different Recipient Cells In Vivo

Humanized pigs have been developed to reduce the incidence of immune rejection in xenotransplantation, but significant concerns remain, such as transmission of viral zoonosis. Porcine endogenous retroviruses (PERV), which exist in the genome of pigs, are produced as infectious virions from all porcine cells and cause zoonosis. Here, we examined the possibility of zoonosis of hosts under conditions of immune suppression or xenotransplantation of cells producing host-adapted viruses. Upon transplantation of PERV-producing porcine cells into mice, no transmission of PERV was detected, whereas, transmission of PERV from mice transplanted with mouse-adapted PERV-producing cells was detected. In addition, the frequency of PERV transmission was increased in CsA treated mice transplanted with PERV-producing murine cells, compared with PERV-producing porcine cells. Transmission of PERV to host animals did not affect weight but immune responses, in particular, the number of T cells from PERV-transmitted mice, were notably reduced. The observed risk of PERV zoonosis highlights the requirement for thorough evaluation of viral zoonosis under particular host conditions, such as immunosuppressive treatment and transplantation with host-adapted virus-producing cells.

Multiple sclerosis-associated retrovirus and optic neuritis

One prognostic factor for early multiple sclerosis (MS) patients to develop a definite MS may be the presence of the MS-associated retrovirus (MSRV) in the cerebrospinal fluid (CSF). We designed a specific study on a cohort of optic neuritis (ON) patients to evaluate the MSRV-dependent conversion to MS relative to the prediction conferred by magnetic resonance imaging (MRI) and CSF abnormalities. At follow-up, 33.3% MSRV+ and 0% MSRV− ON patients developed MS ( P=0.03). The prediction value is lower than that given by CSF and MRI abnormalities (42.3%). This intriguing finding is discussed in the light of the abundant discrepancies observed in the MSRV literature.

Multiple sclerosis-associated retrovirus in early multiple sclerosis: a six-year follow-up of a Sardinian cohort

The human endogenous retroviruses (HERV)-W family contains an extracellular particle detected in multiple sclerosis (MS) patients and designated as MS-associated retrovirus (MSRV). Through nested RT-PCR assays specific for pol MSRV gene, we preliminary reported that its presence in the cerebrospinal fluid (CSF) of early MS patients could be indicative of a poor prognosis upon a three-year follow-up. In the present clinical study, we enlarged our blind observation up to six years. At study entry, 10 MS patients were MSRV- and eight were MSRV+ in the CSF, both groups having a similar mean age and Expanded Disability Status Scale (EDSS) score. After six year follow-up, the mean EDSS significantly differed between the MSRV- and MSRV+ cohorts (4.3 versus 2.2; P = 0.004), as did the annual relapse rate (0.5 in the MSRV- versus 0.3 in the MSRV+; P = 0.01). Finally, two MSRV- patients entered the progressive phase, whilst none of the MSRV+ group entered this phase, and 9/10 MSRV- versus 2/8 MSRV+ patients were treated with immunomodulatory or immunosuppressive drugs (P = 0.009). In conclusion, we found that the presence of MSRV virions in the CSF at the onset of MS is associated, not only with disability accumulation, but also with a higher rate of clinical re-exacerbations. With the known potential pathogenic effects of MSRV given in the literature, further investigations on MSRV are warranted.

An Evolutionarily Young Polar Bear (Ursus maritimus) Endogenous Retrovirus Identified from Next Generation Sequence Data

Transcriptome analysis of polar bear (Ursus maritimus) tissues identified sequences with similarity to Porcine Endogenous Retroviruses (PERV). Based on these sequences, four proviral copies and 15 solo long terminal repeats (LTRs) of a newly described endogenous retrovirus were characterized from the polar bear draft genome sequence. Closely related sequences were identified by PCR analysis of brown bear (Ursus arctos) and black bear (Ursus americanus) but were absent in non-Ursinae bear species. The virus was therefore designated UrsusERV. Two distinct groups of LTRs were observed including a recombinant ERV that contained one LTR belonging to each group indicating that genomic invasions by at least two UrsusERV variants have recently occurred. Age estimates based on proviral LTR divergence and conservation of integration sites among ursids suggest the viral group is only a few million years old. The youngest provirus was polar bear specific, had intact open reading frames (ORFs) and could potentially encode functional proteins. Phylogenetic analyses of UrsusERV consensus protein sequences suggest that it is part of a pig, gibbon and koala retrovirus clade. The young age estimates and lineage specificity of the virus suggests UrsusERV is a recent cross species transmission from an unknown reservoir and places the viral group among the youngest of ERVs identified in mammals.

No transmission of porcine endogenous retrovirus in an acute liver failure model treated by a novel hybrid bioartificial liver containing porcine hepatocytes

BACKGROUND

A novel hybrid bioartificial liver (HBAL) was constructed using an anionic resin adsorption column and a multi-layer flat-plate bioreactor containing porcine hepatocytes co-cultured with bone marrow mesenchymal stem cells (MSCs). This study aimed to evaluate the microbiological safety of the HBAL by detecting the transmission of porcine endogenous retroviruses (PERVs) into canines with acute liver failure (ALF) undergoing HBAL.

METHODS

Eight dogs with ALF received a 6-hour HBAL treatment on the first day after the modeling by D-galactosamine administration. The plasma in the HBAL and the whole blood in the dogs were collected for PERV detection at regular intervals until one year later when the dogs were sacrificed to retrieve the tissues of several organs for immunohistochemistry and Western blotting for the investigation of PERV capsid protein gag p30 in the tissue. Furthermore, HEK293 cells were incubated to determine the in vitro infectivity.

RESULTS

PERV RNA and reverse transcriptase activity were observed in the plasma of circuit 3, suggesting that PERV particles released in circuit 3. No positive PERV RNA and reverse transcriptase activity were detected in other plasma. No HEK293 cells were infected by the plasma in vitro. In addition, all PERV-related analyses in peripheral blood mononuclear cells and tissues were negative.

CONCLUSION

No transmission of PERVs into ALF canines suggested a reliable microbiological safety of HBAL based on porcine hepatocytes.

Characterization of a novel full-length bovine endogenous retrovirus, BERV-β1

Recent studies have suggested that certain classes of endogenous retroviruses (ERVs) may be present in cattle. The aim of this study was increase the scope of knowledge regarding bovine ERVs. The ovine ERV β1 pro/pol sequence was used to design a primer set for polymerase chain reaction (PCR) amplification of a similar sequence in the bovine genome. Through phylogenetic and bioinformatic analysis of the PCR product sequence together with its flanking region, a sequence 8107 bp in length was characterized. This sequence had a typical 5'-LTR-gag-pro-pol-env-LTR-3' organization, and phylogenetic investigation defined it as a bovine ERV β1. Thus, we were able to identify a novel bovine endogenous retrovirus element.

Infectivity and insertional mutagenesis of endogenous retrovirus in autoimmune NZB and B/W mice

Murine leukaemia virus has been suggested to contribute to both autoimmune disease and leukaemia in the NZB mouse and in the (NZB × NZW) F1 (abbreviated B/W) mouse. However, with apparently only xenotropic but no ecotropic virus constitutively expressed in these mice, few mechanisms could explain the aetiology of either disease in either mouse strain. Because pseudotyped and/or inducible ecotropic virus may play a role, we surveyed the ability of murine leukaemia virus in NZB, NZW and B/W mice to infect and form a provirus. From the spleen of NZB mice, we isolated circular cDNA of xenotropic and polytropic virus, which indicates ongoing infection by these viruses. From a B/W lymphoma, we isolated and determined the complete sequence of a putative ecotropic NZW virus. From B/W mice, we recovered de novo endogenous retroviral integration sites (tags) from the hyperproliferating cells of the spleen and the peritoneum. The tagged genes seemed to be selected to aid cellular proliferation, as several of them are known cancer genes. The insertions are consistent with the idea that endogenous retrovirus contributes to B-cell hyperproliferation and progression to lymphoma in B/W mice.

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.

Ultrasensitive Detection and Subtyping of Porcine Endogenous Retrovirus Provirus Based on Magnetic Nanoparticles and Chemiluminescence

Porcine endogenous retrovirus (PERV) is commonly integrated in pig genomes, and could cause a cross-species infection by xenotransplantation. In this study, we developed a rapid and ultrasensitive approach for detection and subtyping of PERV provirus based on magnetic nanoparticles (MNPs) and chemiluminescence (CL). The carboxylated MNPs (CMNPs) were covalently coupled with aminated probes for capturing biotinylated target fragments of PERV, the product of polymerase chain reaction (PCR). Agarose gel electrophoresis analysis approved the reliability of biotinylated fragments. The MNPs composites were incubated with streptavidin-alkaline phosphatase (SA-ALP) and CL signal intensities were determined by subsequently adding 3-(2'-spiroadamantane)-4-methoxy-4-(3"-phosphoryloxy) phenyl-1,2-dioxetane (AMPPD). The optimal assay conditions of this approach were 1 mM for SA modification, 10 µM for probe modification, 55 (PERV), 54 (PERV-A), 50 (PERV-B), and 56 °C (PERV-C) for hybridization temperatures respectively, and 30 min for hybridization time. This approach was specific and highly sensitive, and the limit of detection (LOD) was 100 amol, which has the potential for screening out safe pig donors for xenotransplantation as well as to examine clinical samples from human patients treated with porcine xenotranplantation.

Characterization and comparative analysis of promoters from three plant pararetroviruses associated with Dahlia (Dahlia variabilis)

Two distinct caulimoviruses, Dahlia mosaic virus (DMV) and Dahlia common mosaic virus (DCMV), and an endogenous plant pararetroviral sequence (DvEPRS, formerly known as DMV-D10) were reported from dahlia (Dahlia spp). Promoter elements from these dahlia-associated pararetroviruses were identified and characterized. The TATA box, the CAAT box, the transcription start site, the polyadenylation signal, and regulation factors, characteristic of caulimovirus promoters, were present in each of these promoter regions. Each of the promoter regions was separately cloned into a binary vector containing β-glucuronidase (GUS) reporter gene and delivered into Agrobacterium tumefaciens by electroporation followed by agroinfiltration into Nicotiana benthamiana. The activity of the 35S promoter homologs was determined by transient expression of the GUS gene both in qualitative and quantitative assays. The length of the promoter regions in DMV, DCMV, and DvEPRS corresponded to 438, 439, and 259 bp, respectively. Quantitative GUS assays showed that the promoters from DMV and DCMV resulted in higher levels of gene expression compared to that of DvEPRS in N. benthamiana leaf tissue. Significant differences were observed among the three promoters (p < 0.001). Qualitative GUS assays were consistent with quantitative GUS results. This study provides important information on new promoters for prospect applications as novel promoters for their potential use in foreign gene expression in plants.

Frequent infection of human cancer xenografts with murine endogenous retroviruses in vivo

Infection of human cancer xenografts in mice with murine leukemia viruses (MLVs) is a long-standing observation, but the likelihood of infection in vivo and its biological consequences are poorly understood. We therefore conducted a prospective study in commonly used xenograft recipient strains. From BALB/c nude mice engrafted with MCF7 human mammary carcinoma cells, we isolated a virus that was virtually identical to Bxv1, a locus encoding replication-competent xenotropic MLV (XMLV). XMLV was detected in 9/17 (53%) independently isolated explants. XMLV was not found in primary leukemias or in THP1 leukemia cells grown in Bxv1-negative NSG (NOD/SCID/γCnull) mice, although MCF7 explants harbored replication-defective MLV proviruses. To assess the significance of infection for xenograft behavior in vivo, we examined changes in growth and global transcription in MCF7 and the highly susceptible Raji Burkitt lymphoma cell line chronically infected with XMLV. Raji cells showed a stronger transcriptional response that included up-regulation of chemokines and effectors of innate antiviral immunity. In conclusion, the risk of de novo XMLV infection of xenografts is high in Bxv1 positive mice, while infection can have positive or negative effects on xenograft growth potential with significant consequences for interpretation of many xenograft studies.

Expansion of a novel endogenous retrovirus throughout the pericentromeres of modern humans

BACKGROUND

Approximately 8% of the human genome consists of sequences of retroviral origin, a result of ancestral infections of the germ line over millions of years of evolution. The most recent of these infections is attributed to members of the human endogenous retrovirus type-K (HERV-K) (HML-2) family. We recently reported that a previously undetected, large group of HERV-K (HML-2) proviruses, which are descendants of the ancestral K111 infection, are spread throughout human centromeres.

RESULTS

Studying the genomes of certain cell lines and the DNA of healthy individuals that seemingly lack K111, we discover new HERV-K (HML-2) members hidden in pericentromeres of several human chromosomes. All are related through a common ancestor, termed K222, which is a virus that infected the germ line approximately 25 million years ago. K222 exists as a single copy in the genomes of baboons and high order primates, but not New World monkeys, suggesting that progenitor K222 infected the primate germ line after the split between New and Old World monkeys. K222 exists in modern humans at multiple loci spread across the pericentromeres of nine chromosomes, indicating it was amplified during the evolution of modern humans.

CONCLUSIONS

Copying of K222 may have occurred through recombination of the pericentromeres of different chromosomes during human evolution. Evidence of recombination between K111 and K222 suggests that these retroviral sequences have been templates for frequent cross-over events during the process of centromere recombination in humans.

Identification and characterization of endogenous viral elements for the three key schistosomes of humans

Endogenous viral elements (EVEs) are widely distributed throughout eukaryotic genomes, and their evolution and potential function have attracted a lot of interest. Draft genome sequences for Schistosoma mansoni, Schistosoma japonicum and Schistosoma haematobium are now available; however, information about EVEs in blood flukes of the genus schistosoma is scanty. Here, genome-wide survey into the putative EVE sequences of the three key schistosome genomes were present. Totally 4, 117 gene sequences were identified, including retrovirus-like gypsy elements, RNA viruses and dsDNA viruses. Compared with S. japonicum and S. haematobium, S. mansoni appeared to greatly out numbered by gypsy members. Phylogenetic analysis revealed one novel endogenous retrovirus element in S. mansoni. This initial characterization of schistosomes showed that schistosomes harbour distinct EVEs that may have played an important evolutionary role. Studies of schistosomes' endogenous viruses helped us to glance at an earlier viral event in the class Trematoda, greatly broadening the field of palaeovirology.

Epigenetic marking and repression of porcine endogenous retroviruses

Endogenous retroviruses (ERVs) are remnants of retroviral germ line infections and have been identified in all mammals investigated so far. Although the majority of ERVs are degenerated, some mammalian species, such as mice and pigs, carry replication-competent ERVs capable of forming infectious viral particles. In mice, ERVs are silenced by DNA methylation and histone modifications and some exogenous retroviruses were shown to be transcriptionally repressed after integration by a primer-binding site (PBS) targeting mechanism. However, epigenetic repression of porcine ERVs (PERVs) has remained largely unexplored so far. In this study, we screened the pig genome for PERVs using LTRharvest, a tool for de novo detection of ERVs, and investigated various aspects of epigenetic repression of three unrelated PERV families. We found that these PERV families are differentially up- or downregulated upon chemical inhibition of DNA methylation and histone deacetylation in cultured porcine cells. Furthermore, chromatin immunoprecipitation analysis revealed repressive histone methylation marks at PERV loci in primary porcine embryonic germ cells and immortalized embryonic kidney cells. PERV elements belonging to the PERV-γ1 family, which is the only known PERV family that has remained active up to the present, were marked by significantly higher levels of histone methylations than PERV-γ2 and PERV-β3 proviruses. Finally, we tested three PERV-associated PBS sequences for repression activity in murine and porcine cells using retroviral transduction experiments and showed that none of these PBS sequences induced immediate transcriptional silencing in the tested primary porcine cells.

Identification of a group of Mus dunni endogenous virus-like endogenous retroviruses from the C57BL/6J mouse genome: proviral genomes, strain distribution, expression characteristics, and genomic integration profile

About 10 % of the mouse genome is occupied by sequences associated with endogenous retroviruses (ERVs). However, a comprehensive profile of the mouse ERVs and related elements has not been established yet. In this study, we identified a group of ERVs from the mouse genome and characterized their biological properties. Using a custom ERV mining protocol, 191 ERVs (159 loci reported previously and 32 new loci), tentatively named Mus dunni endogenous virus (MDEV)-like ERVs (MDL-ERVs), were mapped on the C57BL/6J mouse genome. Seven of them retained putative full coding potentials for three retroviral polypeptides (gag, pol, and env). Among the 57 mouse strains examined, all but the Mus pahari/Ei strain had PCR amplicons corresponding to a conserved MDL-ERV region. Interestingly, the Mus caroli/EiJ's amplicon was somewhat larger than the others, coinciding with a substantial phylogenetic distance between the MDL-ERV populations of M. caroli/EiJ and C57BL/6J strains. MDL-ERVs were highly expressed in the lung, spleen, and thymus of C57BL/6J mice compared to the brain, heart, kidney, and liver. Seven MDL-ERVs were mapped in the introns of six annotated genes. Of interest, some MDL-ERVs were mapped periodically on three clusters in chromosome X. The finding that these MDL-ERVs were one of several types of retroelements, which form mosaic-repeat units of tandem arrays, suggests that the formation of the mosaic-repeat unit preceded the tandem arrangement event. Further studies are warranted to understand the biological roles of MDL-ERVs in both normal and pathologic conditions.

Transmission of zoonoses in xenotransplantation: porcine endogenous retroviruses from an immunological and molecular point of view

BACKGROUND AND OBJECTIVES

Pigs offer an unlimited source of xenografts for humans. The use of transplants from animal origin offers a potential solution to the limited supply of human organs and tissues. However, like many other mammalian species, pigs harbor porcine endogenous retrovirus (PERV), which are encoded in their genomic DNA and are assumed to have been integrated into the porcine germ line more than 7.6 × 106 years ago and showing that the age correlates with the time of separation between pigs and peccaries 7.4 × 106 years ago. The ability of the PERV to infect human cells in vitro has heightened safety concerns regarding the transmission of PERV to pig xenograft recipients. In this study, we detected PERV-AC recombinant virus in porcine genomic DNA that may have resulted from exogenous viral recombination. Infectious risk in xenotransplantation will be defined by the activity of PERV loci in vivo. We identified unique Haemophilus aegyptius III HaeIII gag restriction fragment length polymorphism (RFLP) profiles resulting from single nucleotide polymorphisms; these were found only in animals that produced human tropic PERV.

MATERIALS AND METHODS

Porcine tissues were analyzed using validated assays specific for PERV: polymerase chain reaction (PCR) (for PERV DNA), RT-PCR (for PERV RNA), cell culture, RFLP analysis, and sequence analysis.

CONCLUSIONS AND INTERPRETATION

These findings have demonstrated that the presence of both DNA and RNA forms of porcine endogenous retrovirus in porcine tissues needs to be carefully considered when the infectious disease potential of xenotransplantation is being assessed.

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.

Characterization of a full-length endogenous beta-retrovirus, EqERV-beta1, in the genome of the horse (Equus caballus)

Information on endogenous retroviruses fixed in the horse (Equus caballus) genome is scarce. The recent availability of a draft sequence of the horse genome enables the detection of such integrated viruses by similarity search. Using translated nucleotide fragments from gamma-, beta-, and delta-retroviral genera for initial searches, a full-length beta-retrovirus genome was retrieved from a horse chromosome 5 contig. The provirus, tentatively named EqERV-beta1 (for the first equine endogenous beta-retrovirus), was 10434 nucleotide (nt) in length with the usual retroviral genome structure of 5'LTR-gag-pro-pol-env-3'LTR. The LTRs were 1361 nt long, and differed approximately 1% from each other, suggestive of a relatively recent integration. Coding sequences for gag, pro and pol were present in three different reading-frames, as common for beta-retroviruses, and the reading frames were completely open, except that the env gene was interrupted by a single stopcodon. No reading frame was apparent downstream of the env gene, suggesting that EqERV-beta1 does not encode a superantigen like mouse mammary tumor virus (MMTV). A second proviral genome of EqERV-beta1, with no stopcodon in env, is additionally integrated on chromosome 5 downstream of the first virus. Single EqERV-beta1 LTRs were abundantly present on all chromosomes except chromosome 24. Phylogenetically, EqERV-beta1 most closely resembles an unclassified retroviral sequence from cattle (Bos taurus), and the murine beta-retrovirus MMTV.

Evaluation of the potential risk of porcine endogenous retrovirus (PERV) infection in nude mice

Nude mice (BALB/c) were grafted with human 293 cells and PERV (porcine endogenous retrovirus)-IRES-EGFP (a packageable retroviral vector plasmid containing an internal ribosome entry site-enhanced green fluorescent protein)-producing pig PK15 cells in order to determine whether the pig cells could transmit PERV-IRES-EGFP to mice and human 293 cells in vivo. None of the transplanted human 293 cell lines were infected by PERV, but PCR analysis identified PERV-B provirus integration into both the heart and salivary gland of the inoculated nude mice. Our data indicate that hearts and salivary glands can be used to identify PERV-B receptors.

Molecular characterization of long terminal repeat of porcine endogenous retroviruses in Chinese pigs

Pigs offer an unlimited source of xenografts for humans. However, vertically transmitted Porcine endogenous retrovirus (PERV) poses an infectious risk in the course of pig-to-human transplantation. In this study, we characterized PERV long terminal repeat (LTR) sequences from three species of Chinese pigs Banna minipig inbred (BMI), Wu-Zhi-Shan pig (WZSP), and Neijiang pig (NJP-A), and compared them with those of known PERVs (PERV-A, PERV-B, PERV-C, PERV-NIH, and 293-PERV-43). Genomic DNA extracted from peripheral blood mononuclear cells (PBMCs) of the Chinese pigs was used for PCR-amplification, cloning, and sequencing of LTRs. The sequences of BMI and WZSP LTRs were found identical with those of PERV-A and PERV-B, while that of NJP-A LTR was found close to those of PERV-C and PERV-NIH. The gammaretroviral nature of PERV LTRs from Chinese pigs was proved. These LTRs contained also promoter elements including enhancer-like repeats comparable with those of other PERVs. These findings suggested that PERVs from Chinese pigs were similar to PERV-A and PRV-B. Moreover, this study provided new data for the evaluation and selection of pigs to be used in the xenotransplantation.

Isolation of an infectious endogenous retrovirus in a proportion of live attenuated vaccines for pets

The genomes of all animal species are colonized by endogenous retroviruses (ERVs). Although most ERVs have accumulated defects that render them incapable of replication, fully infectious ERVs have been identified in various mammals. In this study, we isolated a feline infectious ERV (RD-114) in a proportion of live attenuated vaccines for pets. Isolation of RD-114 was made in two independent laboratories using different detection strategies and using vaccines for both cats and dogs commercially available in Japan or the United Kingdom. This study shows that the methods currently employed to screen veterinary vaccines for retroviruses should be reevaluated.

OVEX1, a novel chicken endogenous retrovirus with sex-specific and left-right asymmetrical expression in gonads

BACKGROUND

In chickens, as in most birds, female gonad morphogenesis is asymmetrical. Gonads appear first rather similarly, but only the left one undergoes full differentiation and gives rise to a functional ovary. The right gonad, in which the cortex does not develop, remains restricted to the medulla and finally regresses. Opportunity was taken of this left-right asymmetry to perform a suppression subtractive hybridization screening to select for transcripts preferentially expressed in the developing left ovary as compared to the right one, and thus identify genes that are potentially involved in the process of ovarian differentiation.

RESULTS

One of these transcripts, named Ovex1 according to its expression profile, corresponds to an endogenous retrovirus that has not been previously characterized. It is transcribed as full-length and singly spliced mRNAs and contains three uninterrupted open reading frames coding potentially for proteins with homology to Gag and Pro-Pol retroviral polyproteins and a third protein showing only a weak similarity with Env glycoproteins. Ovex1 is severely degenerated; it is devoid of typical long terminal repeats and displays some evidence of recombination. An orthologous Ovex1 locus was identified in the genome of zebra finch, a member of a different bird order, and similar sequences were detected in turkey, guinea fowl, and duck DNA. The relationship between these sequences follows the bird phylogeny, suggesting vertical transmission of the endogenous retrovirus for more than 100 million years. Ovex1 is transcribed in chicken gonads with a sex-dependent and left-right asymmetrical pattern. It is first expressed in the cortex of the left indifferent gonads of both sexes. Expression is transient in the left testis and absent in the right one. In developing ovaries, Ovex1 transcription increases sharply in the left cortex and is weakly detected in the medulla. After folliculogenesis, Ovex1-expressing cells constitute the follicular granulosa cell layer. Ovex1 expression highlights a striking desquamation process that leads to profound cortical remodeling associated with follicle morphogenesis.

CONCLUSION

Evidence for a selection pressure at the protein level suggests that this endogenous retrovirus, expressed in the ovarian supporting cell lineage, might play an active role in bird ovarian physiology.

Large-scale survey of porcine endogenous retrovirus in Chinese miniature pigs

We conducted a large-scale survey on the existence and expression status of porcine endogenous retrovirus (PERV) in seven breeds of Chinese miniature pigs. Genotyping of PERV was examined by PCR using type-specific primers according to the env genotyping method. The presence and expression status of viral gag, pol and env genes were further analyzed in Wuzhishan pigs (WZSP) and Bama minipigs (BMP). The results showed that PERV existed in all 348 genomic DNA samples. The genotype distribution was subtype A-74.43%, subtype B-95.40% and subtype C-30.46%. No expression of subtype C was found in WZSP and BMP. This research obtained an adequate level of information on the molecular epidemiology of PERV in China. The results indicated that it is possible to monitor pig herds for individuals with the lowest PERV prevalence, especially lacking PERV-C.

Analysis of natural recombination in porcine endogenous retrovirus envelope genes

Human tropic Porcine Endogenous Retroviruses (PERVs) are the major concern in zoonosis for xenotransplantation because PERVs cannot be eliminated by specific pathogen-free breeding. Recently, a PERV A/C recombinant with PERV-C bearing PERV-A gp70 showed a higher infectivity (approximately 500-fold) to human cells than PERV-A. Additionally, the chance of recombination between PERVs and HERVs is frequently stated as another risk of xenografting. Overcoming zoonotic barriers in xenotransplantation is more complicated by recombination. To achieve successful xenotransplantation, studies on the recombination in PERVs are important. Here, we cloned and sequenced proviral PERV env sequences from pig gDNAs to analyze natural recombination. The envelope is the most important element in retroviruses as a pivotal determinant of host tropisms. As a result, a total of 164 PERV envelope genes were cloned from pigs (four conventional pigs and two miniature pigs). Distribution analysis and recombination analysis of PERVs were performed. Among them, five A/B recombinant clones were identified. Based on our analysis, we determined the minimum natural recombination frequency among PERVs to be 3%. Although a functional recombinant envelope clone was not found, our data evidently show that the recombination event among PERVs may occur naturally in pigs with a rather high possibility.

Human RNA "rumor" viruses: the search for novel human retroviruses in chronic disease

Retroviruses are an important group of pathogens that cause a variety of diseases in humans and animals. Four human retroviruses are currently known, including human immunodeficiency virus type 1, which causes AIDS, and human T-lymphotropic virus type 1, which causes cancer and inflammatory disease. For many years, there have been sporadic reports of additional human retroviral infections, particularly in cancer and other chronic diseases. Unfortunately, many of these putative viruses remain unproven and controversial, and some retrovirologists have dismissed them as merely "human rumor viruses." Work in this field was last reviewed in depth in 1984, and since then, the molecular techniques available for identifying and characterizing retroviruses have improved enormously in sensitivity. The advent of PCR in particular has dramatically enhanced our ability to detect novel viral sequences in human tissues. However, DNA amplification techniques have also increased the potential for false-positive detection due to contamination. In addition, the presence of many families of human endogenous retroviruses (HERVs) within our DNA can obstruct attempts to identify and validate novel human retroviruses. Here, we aim to bring together the data on "novel" retroviral infections in humans by critically examining the evidence for those putative viruses that have been linked with disease and the likelihood that they represent genuine human infections. We provide a background to the field and a discussion of potential confounding factors along with some technical guidelines. In addition, some of the difficulties associated with obtaining formal proof of causation for common or ubiquitous agents such as HERVs are discussed.

Identification and classification of endogenous retroviruses in cattle

The aim of this study was to identify the endogenous retrovirus (ERV) sequences in a bovine genome. We subjected bovine genomic DNA to PCR with degenerate or ovine ERV (OERV) family-specific primers that aimed to amplify the retroviral pro/pol region. Sequence analysis of 113 clones obtained by PCR revealed that 69 were of retroviral origin. On the basis of the OERV classification system, these clones from degenerate PCR could be divided into the beta3, gamma4, and gamma9 families. PCR with OERV family-specific primers revealed an additional ERV that was classified into the bovine endogenous retrovirus (BERV) gamma7 family. In conclusion, here we report the results of a genome scale study of the BERV. Our study shows that the ERV family expansion in cattle may be somewhat limited, while more diverse family members of ERVs have been reported from other artiodactyls, such as pigs and sheep.

Transmission of porcine endogenous retrovirus to human cells in nude mouse

Xenotransplantation is associated with the risk of Porcine endogenous retrovirus (PERV) transmission, since it has been shown that PERV can infect human cells in vitro (Specke et al., Virology 285, 177-180, 2001). We evaluated the possibility of PERV infection of human cells in nude mice model. Porcine kidney cells PK15 carrying PERV and human liver cancer cells SMMC-7721 were injected separately into the right and left axilla of nude mice, respectively. Two months later, pig cytochrome oxidase II (COII) gene, PERV DNA, PERV mRNA, and PERV-Gag protein were detected in the mass formed in both axillas and in several organs of nude mice. The pig COII genes were detected in the right and left axilla, but not in other organs of nude mice implicating that the microchimerism of pig cells occurred in human SMMC-7721 cells and induced the formation of the mass. PERV gene and gag protein were detected in all mouse tissues except liver. These data indicated that (i) PERV may be transmitted from porcine to mouse cells, (ii) PERV genes and proteins were detectable in the mass formed by injection of human cells and consequently (iii) there was a possibility of PERV transmission to human cells after xenotransplantation.

No evidence for PERV release by islet cells from German landrace pigs

BACKGROUND

Islet cells from pig could be used as an alternative to the current treatment of diabetic patients. However, xenotransplantation from pig to humans may be associated with the risk of transmission of porcine endogenous retroviruses (PERVs) that are present in the genome of all pigs and infect human cells in vitro. Although transplantation of pig islet cells for treatment of diabetes may be not accompanied by immunosuppression that may facilitate virus survival, since islets will be used encapsulated, it is nevertheless of importance to study whether islet cells release PERVs able to infect human cells during co-incubation.

MATERIAL/METHODS

Isolated islets from German landrace pigs were incubated with highly susceptible human 293 cells for one week. In order to prevent microchimerism 293 cells were made neomycin-resistant (293(neo+)), that allows the elimination of pig cells by a selection medium. The infection of 293(neo+ )target cells was analysed by PCR using PERV-specific primers up to fi ve weeks after co-cultivation. In addition, expression of viral mRNA in pig islet cells was studied by RT-PCR analysis, the expression of viral protein by FACS analysis.

RESULTS

Despite the presence of numerous PERV proviruses in the genome of all pigs, no expression of PERV was observed in German landrace pig islet cells, neither as mRNA, nor as protein, nor as viral particles.

CONCLUSIONS

Islet cells from German landrace pigs do not express PERVs and may therefore be used for breeding genetically modified pigs suitable for xenotransplantation and treatment of diabetes.

Endogenous retrovirus expression in testis and epididymis

ERVs (endogenous retroviruses), which comprise 8–10% of mouse and human genomes, are present in thousands of copies, ranging in size from complete 9 kb virus to truncated partial sequences. Despite well-documented differential expression of ERVs in normal and diseased tissues, their biological significance remains controversial. Work in this laboratory revealed remarkably high ERV expression in mouse epididymis, but not in testis. Similar early studies revealed expression of human ERV-E4.1 in both testis and epididymis, but expression of other HERVs (human ERVs) was not examined. Using degenerate primers to conserved regions of reverse transcriptase specific for each of nine HERV families, we have detected expression of six HERV families in epididymis and three in testis. Differential HERV expression may reflect the fully differentiated state of epididymal epithelium in contrast with the immature germ cell population in the testis. These two tissues may therefore lay the groundwork not only for understanding the influence of cellular differentiation on HERV expression, but also to reveal HERVs that are routinely exposed to sperm.

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.

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

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

In vivo screening of porcine endogenous retrovirus in Chinese Banna minipig inbred

The risk of porcine endogenous retrovirus (PERV) infection is one of the major barriers in clinical trials of pig-to-human xenotransplantation. Previous experiments showed that PERV could infect many types of human and nonhuman primate cells, but there is no reported evidence of in vivo infection. In this study, extracted genomic DNA from tissues of seventeen pigs was analyzed using specific sequence primers for gag, pol, and env. The results suggested that PERV exist in the genomes of all tissues. A subtype analysis indicated that PERV-A and PERV-B were in the tissue genome with no positive PERV-C. A greater understanding of the properties of PERV in different pig tissues is necessary to evaluate the risk posed by PERV.

Phylogenetic analysis of porcine endogenous retroviruses expressed in Chinese pigs based on envelope sequences

The promise of successful clinical xenotransplantation is now offset by the potential risk of transmission of porcine endogenous retrovirus (PERV). PERV consists of three subtypes according to the varieties of env sequences. We analyzed PERV subtypes in two species of Chinese pigs (Banna minipig inbred, BMI, and Wu-Zhi-Shan pig, WZSP). Positive A and B were detected while positive C was absent in the analyzed Chinese pigs. The polymerase chain reaction products were then cloned into a pGEM-T vector system and sequenced. Phylogenetic trees were constructed from the translated amino acids of PERVs and other type C and type D retrovirus, as well as the lentivirus in the GeneBank. The results suggested that PERV-A and PERV-B that exist in Chinese pig genomes share similarities with other PERV from the GeneBank and some type C retroviruses, including lymphotropic, leukemic and endogenous retroviruses.

[Porcine endogenous retrovirus in Daweizi pigs in Hunan]

OBJECTIVE

To detect porcine endogenous retrovirus (PERV) in Daweizi pigs and to provide basic parameters of evaluating the biological safety for xenotransplantation from pigs to humans.

METHODS

Ear tissues from 42 individuals were randomly collected from a Daweizi pig population. PCR and RT-PCR were performed to detect PERV proviral DNA and mRNA respectively. Finally, env-A, env-B, and env-C were amplified, sequenced, and analyzed using the BLAST software in National Center for Biotechnology Information.

RESULTS

PERV proviral DNA and mRNA could be detected in the 42 individuals by PCR and RT-PCR, respectively. env-A, env-B and env-C were detected in all the individuals. Compared with other pig species (AY288779, DQ011794 and AY534304), there was 1 and 8 bp differences in the sequences of env-A and env-C, while no difference in env-B.

CONCLUSION

PERV exists and has transcriptive activity in Daweizi pigs. The predominate subtype is PERV-ABC. Env genes are firstly cloned and sequenced in Daweizi pigs and there are polymorphism in the breed. As to the biological safety, the breed was not suitable as a donor in xenotransplantation.

Molecular characterization of the porcine endogenous retrovirus subclass A and B envelope gene from pigs

Xenotransplantation of porcine organs has the potential to overcome the current critical shortage of allogenic organs for transplantation in humans. However, the existence of porcine endogenous retroviruses (PERVs) presents a problem for the clinical use of xenografts from pigs. In an attempt to understand the molecular characteristics of PERVs, we cloned the PERV env gene from six pig breeds (ie, Berkshire, Duroc, Landrace, Yorkshire, and two types of miniature pigs) in Korea. A total of 141 env clones were isolated and their sequences were analyzed. Phylogenetic analyses of these genes revealed the presence of PERVs, from both classes A and B, in 54% and 46% of the env clones, respectively. Among these clones, 37 isolates had the correct open reading frame (ORF; 27 clones in subclass A and 10 clones in subclass B), while the others had premature termination. These PERV nucleotide sequences can be used in a database for comparisons of PERV distribution among different pig breeds and for monitoring PERV infection using isolates with functional ORFs. Recombinant envelope of subclass A and B with functional ORF was expressed by vaccinia virus systems. Additionally isolated env clones can be used for various experiments, such as PERV control and infectivity tests, and may enhance the understanding of molecular mechanisms through pseudotyped PERV viruses.

Inhibition of porcine endogenous retroviruses (PERVs) in primary porcine cells by RNA interference using lentiviral vectors

A potential risk in pig-to-human xenotransplantation is the transmission of PERVs to human recipients. Here we show for the first time the inhibition of PERV expression in primary porcine cells by RNA interference using lentiviral vectors. Cells were transduced with lentiviral vectors coding for short hairpin (sh) RNAs directed against PERV. In all primary porcine cells studied and in the porcine kidney cell line PK-15, expression of PERV-mRNA was significantly reduced as measured by real-time PCR. Most importantly, expression of PERV proteins was almost completely suppressed, as shown by Western blot analysis. Thus, lentiviral shRNA vectors could be used to knockdown PERV expression and create transgenic pigs with a reduced risk of PERV transmission during xenotransplantation.