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

A genome-wide study of ruminants uncovers two endogenous retrovirus families recently active in goats

BACKGROUND

Endogenous retroviruses (ERV) are traces of ancestral retroviral germline infections that constitute a significant portion of mammalian genomes and are classified as LTR-retrotransposons. The exploration of their dynamics and evolutionary history in ruminants remains limited, highlighting the need for a comprehensive and thorough investigation of the ERV landscape in the genomes of cattle, sheep and goat.

RESULTS

Through a de novo bioinformatic analysis, we characterized 24 Class I and II ERV families across four reference assemblies of domestic and wild sheep and goats, and one assembly of cattle. Among these families, 13 are represented by consensus sequences identified in the five analyzed species, while eight are exclusive to small ruminants and three to cattle. The similarity-based approach used to search for the presence of these families in other ruminant species revealed multiple endogenization events over the last 40 million years and distinct evolutionary dynamics among species. The ERV annotation resulted in a high-resolution dataset of 100,534 ERV insertions across the five genomes, representing between 0.5 and 1% of their genomes. Solo-LTRs account for 83.2% of the annotated insertions demonstrating that most of the ERVs are relics of past events. Two Class II families showed higher abundance and copy conservation in small ruminants. One of them is closely related to circulating exogenous retroviruses and is represented by 22 copies sharing identical LTRs and 12 with complete coding capacities in the domestic goat.

CONCLUSIONS

Our results suggest the presence of two ERV families with recent transpositional activity in ruminant genomes, particularly in the domestic goat, illustrating distinct evolutionary dynamics among the analyzed species. This work highlights the ongoing influence of ERVs on genomic landscapes and call for further investigation of their evolutionary trajectories in these genomes.

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

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

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

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

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

Sensitive detection systems for infectious agents in xenotransplantation

Xenotransplantation of pig cells, tissues, or organs may be associated with transmission of porcine microorganisms, first of all of viruses, to the transplant recipient, potentially inducing a disease (zoonosis). I would like to define detection systems as the complex of sample generation, sample preparation, sample origin, time of sampling, and the necessary negative and positive controls along with the specific detection methods, either PCR-based, cell-based, or immunological methods. Some xenotransplantation-relevant viruses have already been defined; others are still unknown. The PCR-based methods include PCR and real-time PCR for DNA viruses, and RT-PCR and real-time RT-PCR for RNA viruses as well as for virus expression studies at the RNA level. Furthermore, droplet digital PCR (ddPCR) can be used for the determination of virus and provirus copies. To detect expression at the protein level, immunofluorescence, immunohistochemistry, and Western blot analyses can be used. To detect virus production and to detect infectious viruses, electron microscopy and infection assays can be used. Furthermore, immunological methods such as Western blot analysis or ELISA can be used to detect virus-specific antibodies. Detection of antiviral antibodies is a reliable and sensitive indirect detection method. For these immunological methods, purified viruses, recombinant viral proteins, or synthetic peptides are used as antigens and control sera and control antigens are needed. All these methods have been used in the past for the characterization of different pig breeds including genetically modified pigs generated for xenotransplantation and for the screening of recipients in preclinical and clinical xenotransplantations. Whereas in preclinical trials a few porcine viruses have been transmitted to the non-human primate recipients, in first clinical trials no such transmissions to humans were observed. Further improvement of the detection systems and their application in virus elimination programs will lead to clean donor animals and a safe xenotransplantation.

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

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

Comparative gene expression profiling of pig-derived iPSC-like cells: Effects of induced pluripotency on expression of porcine endogenous retrovirus (PERV)

BACKGROUND

Porcine induced pluripotent stem cells (piPSCs) offer an alternative strategy in xenotransplantation (XTx). As human endogenous retroviruses (HERV), particularly HERV-K, are highly expressed in natural human stem cells, we compared the expression of porcine endogenous retroviruses (PERV) and retrotransposon LINE-1 (L1) open reading frames 1 and 2 (pORF1 and pORF2) in different piPSC-like cell lines with their progenitors (porcine fetal fibroblasts, pFF).

METHODS

Cells reprogrammed via Sleeping Beauty-transposed transcription factors were cultured and analyzed on a custom-designed microarray representing the reference pig genome. Data were complemented by qRT-PCR and reverse transcriptase (RT) assay.

RESULTS

The expression profiles revealed that 8515 of 26 967 targets were differentially expressed. A total of 4443 targets showed log₂ expression ratio >1, and 4072 targets showed log₂ expression ratio less than -1 with 0.05 P-value threshold. Approximately ten percent of the targets showed highly significant expression ratios with log₂ ≥4 or ≤-4. Besides this general switch in cellular gene expression that was accompanied by an altered morphology, expression of both PERV and L1 pORF1/pORF2 was significantly enhanced. piPSC-like cells revealed a 10-fold to 100-fold higher transcription of the viral PERV-A and PERV-B envelope genes (env), viral protease/polymerase (prt/pol), and L1 elements. No functional retrovirus could be detected under these conditions.

CONCLUSION

Epigenetic reprogramming has functional impact on retrotransposons. Thus, the induction of pig-derived pluripotent cells influences their PERV expression profile. Data emphasize the necessity to focus on animals, which show non-functional endogenous viral background to ensure virological safety.

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

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

A computational method for prediction of matrix proteins in endogenous retroviruses

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

Induction and characterization of a replication competent cervid endogenous gammaretrovirus (CrERV) from mule deer cells

Endogenous retroviruses (ERVs) were acquired during evolution of their host organisms after infection and mendelian inheritance in the germline by their exogenous counterparts. The ERVs can spread in the host genome and in some cases they affect the host phenotype. The cervid endogenous gammaretrovirus (CrERV) is one of only a few well-defined examples of evolutionarily recent invasion of mammalian genome by retroviruses. Thousands of insertionally polymorphic CrERV integration sites have been detected in wild ranging mule deer (Odocoileus hemionus) host populations. Here, we describe for the first time induction of replication competent CrERV by cocultivation of deer and human cells. We characterize the physical properties and tropism of the induced virus. The genomic sequence of the induced virus is phylogenetically related to the evolutionarily young endogenous CrERVs described so far. We also describe the level of replication block of CrERV on deer cells and its capacity to establish superinfection interference.

Sequence variation of koala retrovirus transmembrane protein p15E among koalas from different geographic regions

The koala retrovirus (KoRV), which is transitioning from an exogenous to an endogenous form, has been associated with high mortality in koalas. For other retroviruses, the envelope protein p15E has been considered a candidate for vaccine development. We therefore examined proviral sequence variation of KoRV p15E in a captive Queensland and three wild southern Australian koalas. We generated 163 sequences with intact open reading frames, which grouped into 39 distinct haplotypes. Sixteen distinct haplotypes comprising 139 of the sequences (85%) coded for the same polypeptide. Among the remaining 23 haplotypes, 22 were detected only once among the sequences, and each had 1 or 2 non-synonymous differences from the majority sequence. Several analyses suggested that p15E was under purifying selection. Important epitopes and domains were highly conserved across the p15E sequences and in previously reported exogenous KoRVs. Overall, these results support the potential use of p15E for KoRV vaccine development.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Improved pig donor screening including newly identified variants of porcine endogenous retrovirus-C (PERV-C)

Porcine endogenous retroviruses (PERV) are widely distributed in the genomes of pigs. PERV-A and PERV-B are present in all pigs. They infect human cells in vitro and therefore represent a risk for xenotransplantation when pig cells, tissues or organs are used. PERV-C infects only pig cells and is not present in the genomes of all pigs. However, PERV-A/C recombinants infecting human cells and characterized by high replication titers were found in pigs. To select PERV-C-free animals that cannot generate such recombinants, PCR-based assays were developed (Kaulitz et al., J Virol Methods, 175:60, 2011). When screening for PERV-C in German wild boars (Sus scrofa scrofa), applying these methods, a new variant of PERV-C was identified. Whereas in all 125 wild boar only the new variant of PERV-C was found, different variants were present in some landrace pigs, and most importantly, some pigs were totally free of PERV-C.

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.

Development of sensitive methods for detection of porcine endogenous retrovirus-C (PERV-C) in the genome of pigs

Porcine endogenous retroviruses (PERV) represent a risk for xenotransplantation using pig cells, tissues or organs. PERV-A and PERV-B are present in the genome of all pigs and both infect human cells in vitro. PERV-C infects only pig cells and it is integrated in the genome of most, but not all pigs. Recombinants between PERV-A and PERV-C were described that infect human cells and replicate at high titres. To avoid such recombinations, PERV-C positive animals should not be used for breeding animals suited for xenotransplantation. In order to detect PERV-C positive pigs, different methods were developed such as specific PCRs using different primers, a highly sensitive nested PCR and a real-time PCR allowing measurement of proviral copy numbers. The real-time PCR was found to be useful to discriminate between contamination and actual provirus copies. The PCRs were optimized and their sensitivity was determined. Screening can be started with PCR1, if the result is negative, PCR2 to PCR5 or the nested PCR should be used, if the result is positive, the real-time PCR should be used to exclude contaminations. All methods were used to evaluate the prevalence of PERV-C and to identify PERV-C free animals. Due to the risk of contamination with cells from other animals testing should be performed with blood cells, not with ear biopsies.

Beneficial and detrimental effects of human endogenous retroviruses

In this mini review, we aim to evaluate the structure and function of Human Endogenous Retroviruses (HERVs) with respect to the benefit they may have for humans or the damage they may cause. Emphasis is laid on their putative roles, if any, in pregnancy, in gene regulation and in cancer. As a basis for this discussion it will first be necessary to briefly describe the structure and function of retroelements, including HERVs, before addressing their positive or negative effects at the cellular and organismal level. Finally, we will give an outlook in which we will attempt to define priorities for future research.

Identification and molecular characterization of PERV gamma1 long terminal repeats

Porcine endogenous retroviruses (PERVs) gamma1 in the pig genome have the potential to act as harmful factors in xenotransplantation (pig-to-human). Long terminal repeats (LTRs) are known to be strong promoter elements that could control the transcription activity of PERV elements and the adjacent functional genes. To investigate the transcribed PERV gamma1 LTR elements in pig tissues, bioin-formatic and experimental approaches were conducted. Using RT-PCR amplification and sequencing approaches, 69 different transcribed LTR elements were identified. And 69 LTR elements could be divided into six groups (15 subgroups) by internal variation including tandem repeated sequences, insertion and deletion (INDEL). Remarkably, all internal variations were indentified in U3 region of LTR elements. Taken together, the identification and characterization of various PERV LTR transcripts allow us to extend our knowledge of PERV and its transcriptional study.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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

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

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.