Characterization of porcine endogenous retrovirus gamma pro-pol nucleotide sequences

Infection in xenotransplantation: opportunities and challenges

PURPOSE OF REVIEW

Posttransplantation infections are common. It is anticipated that infection will be no less common in xenotransplantation recipients. Prolonged xenograft survivals have resulted from advances in immunosuppressive strategies and development of swine that decrease host immune responses via genetic manipulation, notably CRISPR/cas9 manipulation. As prospects for clinical trials improve, consideration of the unique infectious risks posed by xenotransplantation reemerge.

RECENT FINDINGS

Organisms likely to cause infection in human recipients of porcine xenografts are unknown in advance of clinical trials. Microbiological screening of swine intended as xenograft donors can be more intensive than is currently feasible for human allograft donors. Monitoring infection in recipients will also be more intensive. Key opportunities in infectious diseases of xenotransplantation include major technological advances in evaluation of the microbiome by unbiased metagenomic sequencing, assessments of some risks posed by porcine endogenous retroviruses (PERVs) including antiretroviral susceptibilities, availability of swine with deletion of genomic PERVs, and recognition of the rapidly changing epidemiology of infection in swine worldwide.

SUMMARY

Unknown infectious risks in xenotransplantation requires application of advanced microbiological techniques to discern and prevent infection in graft recipients. Clinical trials will provide an opportunity to advance the safety of all of organ transplantation.

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.

Infectious disease risks in xenotransplantation

Hurdles exist to clinical xenotransplantation including potential infectious transmission from nonhuman species to xenograft recipients. In anticipation of clinical trials of xenotransplantation, the associated infectious risks have been investigated. Swine and immunocompromised humans share some potential pathogens. Swine herpesviruses including porcine cytomegalovirus (PCMV) and porcine lymphotropic herpesvirus (PLHV) are largely species-specific and do not, generally, infect human cells. Human cellular receptors exist for porcine endogenous retrovirus (PERV), which infects certain human-derived cell lines in vitro. PERV-inactivated pigs have been produced recently. Human infection due to PERV has not been described. A screening paradigm can be applied to exclude potential human pathogens from "designated pathogen free" breeding colonies. Various microbiological assays have been developed for screening and diagnosis including antibody-based tests and qualitative and quantitative molecular assays for viruses. Additional assays may be required to diagnose pig-specific organisms in human xenograft recipients. Significant progress has been made in the evaluation of the potential infectious risks of clinical xenotransplantation. Infectious risk would be amplified by intensive immunosuppression. The available data suggest that risks of xenotransplant-associated recipient infection are manageable and that clinical trials can be performed safely. Possible infectious risks of xenotransplantation to the community at large are undefined but merit consideration.

Xenotransplantation of Cells, Tissues, Organs and the German Research Foundation Transregio Collaborative Research Centre 127

Human organ transplantation is the therapy of choice for end-stage organ failure. However, the demand for organs far exceeds the donation rate, and many patients die while waiting for a donor. Clinical xenotransplantation using discordant species, particularly pigs, offers a possible solution to this critical shortfall. Xenotransplantation can also increase the availability of cells, such as neurons, and tissues such as cornea, insulin producing pancreatic islets and heart valves. However, the immunological barriers and biochemical disparities between pigs and primates (human) lead to rejection reactions despite the use of common immunosuppressive drugs. These result in graft vessel destruction, haemorrhage, oedema, thrombus formation, and transplant loss. Our consortium is pursuing a broad range of strategies to overcome these obstacles. These include genetic modification of the donor animals to knock out genes responsible for xenoreactive surface epitopes and to express multiple xenoprotective molecules such as the human complement regulators CD46, 55, 59, thrombomodulin and others. We are using (new) drugs including complement inhibitors (e.g. to inhibit C3 binding), anti-CD20, 40, 40L, and also employing physical protection methods such as macro-encapsulation of pancreatic islets. Regarding safety, a major objective is to assure that possible infections are not transmitted to recipients. While the aims are ambitious, recent successes in preclinical studies suggest that xenotransplantation is soon to become a clinical reality.

Promoter activity analysis and methylation characterization of LTR elements of PERVs in NIH miniature pig

The potential risk of porcine endogenous retrovirus (PERV) transmission is an important issue in xenotransplantation (pig-to-human transplantation). Long terminal repeats (LTRs) in PERV elements show promoter activity that could affect neighboring functional genes. The methylation status and promoter activities of 3 LTR structures (PERV-LTR1, LTR2, and LTR3 elements) belonging to the PERV-A family were examined using luciferase reporter genes in human liver cell lines (HepG2 and Hep3B). The PERV LTR3 element exhibited hypomethylation and stronger promoter activity than the other LTR elements in human liver cells. We also performed comparative sequences analysis of the PERV LTR elements by using bioinformatics tools. Our findings showed that several transcription factors such as Nkx2-2 and Elk-1 positively influenced the high transcriptional activity of the PERV LTR3 element.

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 and classification of feline endogenous retroviruses in the cat genome using degenerate PCR and in silico data analysis

The purpose of this study was to identify and classify endogenous retroviruses (ERVs) in the cat genome. Pooled DNA from five domestic cats was subjected to degenerate PCR with primers specific to the conserved retroviral pro/pol region. The 59 amplified retroviral sequences were used for in silico analysis of the cat genome (Felis_catus-6.2). We identified 219 ERV γ and β elements from cat genome contigs, which were classified into 42 ERV γ and 4 β families and further analysed. Among them, 99 γ and 5 β ERV elements contained the complete retroviral structure. Furthermore, we identified 757 spuma-like ERV elements based on the sequence homology to murine (Mu)ERV-L and human (H)ERV-L. To the best of our knowledge, this is the first detailed genome-scale analysis examining Felis catus endogenous retroviruses (FcERV) and providing advanced insights into their structural characteristics, localization in the genome, and diversity.

Polymorphic integrations of an endogenous gammaretrovirus in the mule deer genome

Endogenous retroviruses constitute a significant genomic fraction in all mammalian species. Typically they are evolutionarily old and fixed in the host species population. Here we report on a novel endogenous gammaretrovirus (CrERVγ; for cervid endogenous gammaretrovirus) in the mule deer (Odocoileus hemionus) that is insertionally polymorphic among individuals from the same geographical location, suggesting that it has a more recent evolutionary origin. Using PCR-based methods, we identified seven CrERVγ proviruses and demonstrated that they show various levels of insertional polymorphism in mule deer individuals. One CrERVγ provirus was detected in all mule deer sampled but was absent from white-tailed deer, indicating that this virus originally integrated after the split of the two species, which occurred approximately one million years ago. There are, on average, 100 CrERVγ copies in the mule deer genome based on quantitative PCR analysis. A CrERVγ provirus was sequenced and contained intact open reading frames (ORFs) for three virus genes. Transcripts were identified covering the entire provirus. CrERVγ forms a distinct branch of the gammaretrovirus phylogeny, with the closest relatives of CrERVγ being endogenous gammaretroviruses from sheep and pig. We demonstrated that white-tailed deer (Odocoileus virginianus) and elk (Cervus canadensis) DNA contain proviruses that are closely related to mule deer CrERVγ in a conserved region of pol; more distantly related sequences can be identified in the genome of another member of the Cervidae, the muntjac (Muntiacus muntjak). The discovery of a novel transcriptionally active and insertionally polymorphic retrovirus in mammals could provide a useful model system to study the dynamic interaction between the host genome and an invading retrovirus.

Evolution of endogenous retroviruses in the Suidae: evidence for different viral subpopulations in African and Eurasian host species

Background

Porcine endogenous retroviruses (PERVs) represent remnants of an exogenous form that have become integrated in the domestic pig (Sus scrofa) genome. Although they are usually inactive, the capacity of γ1 ERVs to infect human cells in vitro has raised concerns about xenotransplantation because the viruses could cross the species barrier to humans. Here we have analyzed the evolution of γ1 ERVs in ten species of Suidae (suids, pigs and hogs) from Eurasia and Africa using DNA sequences for their coding domains (gag, pro/pol and env genes). For comparison with γ1 PERVs, we have also analysed γ2 ERVs which in domestic pigs are known to be inactive and do not pose a risk to xenotransplantation.

Results

Phylogenetic analysis using Bayesian inference showed that γ1 and γ2 ERVs have distinctive evolutionary histories. Firstly, two different viral lineages of γ1 ERVs were found and a coevolutionary analysis demonstrated that they correspond broadly to their host phylogeny, one of Eurasian and another of African species, and show no evidence of horizontal transmission. γ2 ERVs, however, show a bush-like evolution, suggesting a rapid viral radiation from a single common ancestor with no correspondence between host and viral evolutionary trees. Furthermore, though γ1 ERV env genes do not possess frequent stop codons, γ2 env genes do. To understand whether γ1 suid ERVs may be still replicating, we have also evaluated their likely mechanism of proliferation by statistically testing internal to terminal branches using nonsynonymous versus synonymous substitution ratios. Our results suggest that γ1 ERVs are increasing in copy number by reinfection, which requires the translocation of the virus from one cell to another.

Conclusions

Evidence of at least two viral subpopulations was observed in γ1 ERVs from Eurasian and African host species. These results should be taken into account in xenotransplantation since γ1 ERVs appear to be codiverging with their host and maintaining ongoing capacity to infect somatic and germ cells.

Distinctive differences in long terminal repeat sequences between γ1 endogenous retroviruses of African and Eurasian suid species

Diversity of long terminal repeats (LTRs) from γ1 endogenous retroviruses (ERVs) was analysed by DNA sequencing in 10 species of the family Suidae (suids, pigs and hogs). Phylogenetic analysis separated LTR sequences into two groups which correlated approximately with either the previously described cluster I and III, or the clusters II, IV and V. Interestingly, a specific LTR exhibiting a novel molecular rearrangement was identified exclusively within African host species when compared to LTRs previously reported from known ERVs in the domestic pig (Sus scrofa). Furthermore, other sections of LTRs appear to be unique to African suids as suggested by phylogenetic analysis. These differences between African and Eurasian ERV lineages show that these ERVs belong to different viral sub-populations, implying coevolution of endogenous viral sequences with their host species and providing no evidence of transfer of viral sequences between African and Eurasian suids.

Analysis of the molecular and regulatory properties of active porcine endogenous retrovirus gamma-1 long terminal repeats in kidney tissues of the NIH-Miniature pig

The pig genome contains the gamma 1 family of porcine endogenous retroviruses (PERVs), which are a major obstacle to the development of successful xenotransplantation from pig to human. Long terminal repeats (LTRs) found in PERVs are known to be essential elements for the control of the transcriptional activity of single virus by different transcription factors (TFs). To identify transcribed PERV LTR elements, RT-PCR and DNA sequencing analyses were performed. Twenty-nine actively transcribed LTR elements were identified in the kidney tissues of the NIH-Miniature pig. These elements were divided into two major groups (I and II), and four minor groups (I-1, I-2, I-3, and II-1), by the presence of insertion and deletion (INDEL) sequences. Group I elements showed strong transcriptional activity compared to group II elements. Four different LTR elements (PL1, PL2, PL3, and PL4) as representative of the groups were analyzed by using a transient transfection assay. The regulation of their promoter activity was investigated by treatment with M.SssI (CpG DNA methyltransferase) and garcinol (histone acetyltransferase inhibitor). The transcriptional activity of PERV LTR elements was significantly reduced by treatment with M.SssI. These data indicate that transcribed PERV LTR elements harbor sufficient promoter activity to regulate the transcription of a single virus, and the transcriptional activity of PERV LTRs may be controlled by DNA methylation events.

Reappraisal of biosafety risks posed by PERVs in xenotransplantation

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

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.

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

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

Porcine endogenous retrovirus integration sites in the human genome: features in common with those of murine leukemia virus

Porcine endogenous retroviruses (PERV) are a major concern when porcine tissues and organs are used for xenotransplantation. PERV has been shown to infect human cells in vitro, highlighting a potential zoonotic risk. No pathology is associated with PERV in its natural host, but the pathogenic potential might differ in the case of cross-species transmission and can only be inferred from knowledge of related gammaretroviruses. We therefore investigated the integration features of the PERV DNA in the human genome in vitro in order to further characterize the risk associated with PERV transmission. In this study, we characterized 189 PERV integration site sequences from human HEK-293 cells. Data showed that PERV integration was strongly enhanced at transcriptional start sites and CpG islands and that the frequencies of integration events increased with the expression levels of the genes, except for the genes with the highest levels of expression, which were disfavored for integration. Finally, we extracted genomic sequences directly flanking the integration sites and found an original 8-base statistical palindromic consensus sequence [TG(int)GTACCAGC]. All these results show similarities between PERV and murine leukemia virus integration site selection, suggesting that gammaretroviruses have a common pattern of integration and that the mechanisms of target site selection within a retrovirus genus might be similar.

Phylogeny, recombination and expression of porcine endogenous retrovirus gamma2 nucleotide sequences

Endogenous retroviral sequences in the pig genome represent a potential infectious risk in xenotransplantation. Porcine endogenous retrovirus (PERV) gamma sequences described to date have been classified into several families. The known infectious, human-tropic PERVs have been assigned to the PERV gamma1 subfamilies A, B and C. High copy numbers and full-length clones have also been observed for an additional family, designated PERV gamma2. The aim of this study was to examine the PERV gamma2 family by analysis of retroviral pro/pol gene sequences. The proviral load was observed to be similar among various pig breeds. Although clones harbouring an open reading frame in the examined region were found, analysis of published large PERV gamma2 clones revealed multiple deleterious mutations in each of the retroviral genes. Various recombination events between gamma2 genomes were revealed. In contrast to PERV gamma1, phylogenetic analyses did not distinguish defined subfamilies, but indicated the independent evolution of the proviruses after a single event of retroviral amplification. Expression analysis showed large PERV gamma2 transcripts and variable transcription in several tissues. Analysis of the two published gamma2 env gene sequences observed the partial lack of the receptor-binding domain. Overall, this study indicated the low infectious potential for PERV gamma2.

Reliable Classification and Recombination Analysis of Porcine Endogenous Retroviruses

Prevention of cross-species infection with porcine endogenous retroviruses (PERV) is crucial for xenotransplantation. Previous studies described the potential risk of infection for the PERV gamma1 subfamilies A, B and C. Replication competent PERV gamma1 proviruses designated to a particular subfamily and hybrid viruses originating from retroviral recombination events between the subfamilies were observed. Future pig genome sequencing projects will reveal multiple novel PERV proviruses from additional breeds and animals. Evaluation of these viral genomes has to be carried out to assess the potential risk of retroviral cross-species infection. In this study, we tested common sequence comparison methods for the classification of PERV sequences and the detection of hybrid clones. The examination of the polymorphic nucleotide positions was found to be the most suitable procedure. We describe a fast and simple method using bioinformatic software tools which can also be applied to analogous analyses of other viral genomes.

Determinants of high titer in recombinant porcine endogenous retroviruses

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

Evidence and consequence of porcine endogenous retrovirus recombination

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

Xenotransplantation: infectious risk revisited

Xenotransplantation is a possible solution for the shortage of tissues for human transplantation. Multiple hurdles exist to clinical xenotransplantation, including immunologic barriers, metabolic differences between pigs--the source species most commonly considered--and humans, and ethical concerns. Since clinical trials were first proposed almost 10 years ago, the degree of risk for infection transmitted from the xenograft donor to the recipient has been extensively investigated. A number of potential viral pathogens have been identified including porcine endogenous retrovirus (PERV), porcine cytomegalovirus (PCMV), and porcine lymphotropic herpesvirus (PLHV). Sensitive diagnostic assays have been developed for each virus. Human-tropic PERV are exogenous recombinants between PERV-A and PERV-C sequences and are present in only a subset of swine. Porcine cytomegalovirus can be excluded from herds of source animals by early weaning of piglets. In contrast, the risks associated with PLHV remain undefined. Microbiologic studies and assays for potential xenogeneic pathogens have furthered understanding of risks associated with xenotransplantation. Thus far, clinical xenotransplantation of pig tissues has not resulted in transmission of viral infection to humans; significant risks for disease transmission from swine to humans have not been confirmed. If immunologic hurdles can be overcome, it is reasonable to initiate carefully monitored clinical trials.

Prevalence of porcine endogenous retrovirus in domestic pigs in Japan and its potential infection in dogs xenotransplanted with porcine pancreatic islet cells

The prevalence of porcine endogenous retrovirus (PERV) proviral DNA among various pig breeds raised in Japan was investigated by polymerase chain reaction (PCR). Moreover, potential infection of PERV was investigated by PCR and reverse transcriptase-polymerase chain reaction (RT-PCR) in experimentally induced diabetic dogs (n=5) implanted with the diffusion chamber type bio-artificial endocrine pancreas (Bio-AEP) containing porcine pancreatic endocrine (PE) cells. No immunosuppressant was used after the transplantation. PERV gag, pol, env-A and env-B genes were detected in any pigs examined. In two of three Landrace breeds, env-C gene was absent. PERV proviral DNAs and viral RNAs were also detected from the cultured porcine PE-cells. In the peripheral blood mononuclear cells and the spleen obtained at 6, 30, 32, 36, 79 weeks of xenotransplantation in dogs, however, no evidence of microchimerism, infection and viremia were confirmed. These results suggested that the risk of PERV infection through xenotransplantation of Bio-AEP containing porcine islet cells without immunosuppressants may be quite low.

Identification of exogenous forms of human-tropic porcine endogenous retrovirus in miniature Swine

The replication of porcine endogenous retrovirus subgroup A (PERV-A) and PERV-B in certain human cell lines indicates that PERV may pose an infectious risk in clinical xenotransplantation. We have previously reported that human-tropic PERVs isolated from infected human cells following cocultivation with miniature swine peripheral blood mononuclear cells (PBMC) are recombinants of PERV-A with PERV-C. Here, we report that these recombinants are exogenous viruses in miniature swine; i.e., they are not present in the germ line DNA. These viruses were invariably present in miniature swine that transmitted PERV to human cells and were also identified in some miniature swine that lacked this ability. These data, together with the demonstration of the absence of both replication-competent PERV-A and recombinant PERV-A/C loci in the genome of miniature swine (L. Scobie, S. Taylor, J. C. Wood, K. M. Suling, G. Quinn, C. Patience, H.-J. Schuurman, and D. E. Onions, J. Virol. 78:2502-2509, 2004), indicate that exogenous PERV is the principal source of human-tropic virus in these animals. Interestingly, strong expression of PERV-C in PBMC correlated with an ability of the PBMC to transmit PERV-A/C recombinants in vitro, indicating that PERV-C may be an important factor affecting the production of human-tropic PERV. In light of these observations, the safety of clinical xenotransplantation from miniature swine will be most enhanced by the utilization of source animals that do not transmit PERV to either human or porcine cells. Such animals were identified within the miniature swine herd and may further enhance the safety of clinical xenotransplantation.

Characterization of endogenous retroviruses in sheep

Endogenous retrovirus (ERV) sequences have been found in all mammals. In vitro and in vivo experiments revealed ERV activation and cross-species infection in several species. Sheep (Ovis aries) are used for various biotechnological purposes; however, they have not yet been comprehensively screened for ERV sequences. Therefore, the aim of the study was to classify the ERV sequences in the ovine genome (OERV) by analyzing the retroviral pro-pol sequences. Three OERV beta families and nine OERV gamma families were revealed. Novel open reading frames (ORF) in the amplified proviral fragment were found in one OERV beta family and two OERV gamma families. Hybrid OERV produced by putative recombination events were not detected. Quantitative analysis of the OERV sequences in the ovine genome revealed no relevant variations in the endogenous retroviral loads of different breeds. Expression analysis of different tissues from fetal and pregnant sheep detected mRNA from both gammaretrovirus families, showing ORF fragments. Thus, the release of retroviruses from sheep cells cannot be excluded.

Recombination analysis of human-tropic porcine endogenous retroviruses

Prevention of cross-species infection of porcine endogenous retroviruses (PERV) is crucial for xenotransplantation. The potential risk of infection is caused by replication-competent PERV as well as by hybrid viruses derived from recombination events of distinct PERV genomes. Recently, human-tropic, replication-competent PERV genomes obtaining hybrid sequences have been observed. Here, complete polymorphism pattern analysis was performed on the full-length PERV γ1 clones and on the complete envelope (env) gene sequences published to date. Several recombined full-length clones and a high number of different recombination patterns in the env gene were identified. In addition, recombinations with retroviral genomes not yet known were found. Thus, the potential risk of infection also exists for recombination products, including defective PERV loci.

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

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