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

Application of Methods Detecting Xenotransplantation-Relevant Viruses for Screening German Slaughterhouse Pigs

Detection methods have been developed to prevent transmission of zoonotic or xenozoonotic porcine viruses after transplantation of pig organs or cells to the recipient (xenotransplantation). Eleven xenotransplantation-relevant viruses, including porcine cytomegalovirus, porcine roseolovirus (PCMV/PRV), porcine lymphotropic herpesviruses -1, -2, -3 (PLHV-1, 2, 3), porcine parvovirus (PPV), porcine circovirus 2, 3, 4 (PCV2, 3, 4), hepatitis E virus genotype 3 (HEV3), porcine endogenous retrovirus-C (PERV-C), and recombinant PERV-A/C have been selected. In the past, several pig breeds, minipigs, and genetically modified pigs generated for xenotransplantation had been analyzed using these methods. Here, spleen, liver, and blood samples from 10 German slaughterhouse pigs were screened using both PCR-based and immunological assays. Five viruses: PCMV/PRV, PLHV-1, PLHV-3, and PERV-C, were found in all animals, and PCV3 in one animal. Some animals were latently infected with PCMV/PRV, as only virus-specific antibodies were detected. Others were also PCR positive in the spleen and/or liver, indicative of an ongoing infection. These results provide important information on the viruses that infect German slaughterhouse pigs, and together with the results of previous studies, they reveal that the methods and test strategies efficiently work under field conditions.

Monitoring swine virus transmission in embryos derived from commercial abattoir oocytes

Pigs are pivotal in agriculture and biomedical research and hold promise for xenotransplantation. Specific-pathogen-free (SPF) herds are essential for commercial swine production and xenotransplantation research facilities. Commercial herds aim to safeguard animal health, welfare, and productivity, and research facilities require SPF status to protect immunocompromised patients. Somatic cell nuclear transfer (SCNT) embryos are the norm for producing cloned and genetically edited animals. Oocytes for embryo reconstruction are most conveniently sourced from commercial abattoirs with unclear disease statuses. However, research on viral clearance from donor oocytes during embryo reconstruction remains limited. SCNT has previously been shown to reduce the transmission of Porcine reproductive and respiratory syndrome virus, Bovine viral diarrhea virus, Porcine Circovirus type 2, and Porcine parvovirus. Still, it is lacking for other pathogens, including endogenous viruses. This project contains two preliminary studies investigating the polymerase chain reaction (PCR) assay detection of common swine viruses through the phases of producing parthenogenic and SCNT embryos. Exogenous pathogens detected in oocyte donor tissue or the oocyte maturation media were not detected in the produced embryos. Porcine endogenous retrovirus type C (PERVC) was not removed by parthenogenic embryo activation and was detected in 1 of the 2 tested SCNT embryos reconstructed using a PERVC-negative cell line. SCNT and parthenogenic embryo construction similarly reduced exogenous virus detection. SCNT embryo construction helped reduce endogenous virus detection. This project demonstrates the importance of screening embryos for endogenous viruses and shows the usefulness of parthenogenic embryos in future exogenous virus clearance studies.

Screening for Viruses in Indigenous Greek Black Pigs

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

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

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

Virus Safety of Xenotransplantation

The practice of xenotransplantation using pig islet cells or organs is under development to alleviate the shortage of human donor islet cells or organs for the treatment of diabetes or organ failure. Multiple genetically modified pigs were generated to prevent rejection. Xenotransplantation may be associated with the transmission of potentially zoonotic porcine viruses. In order to prevent this, we developed highly sensitive PCR-based, immunologicals and other methods for the detection of numerous xenotransplantation-relevant viruses. These methods were used for the screening of donor pigs and xenotransplant recipients. Of special interest are the porcine endogenous retroviruses (PERVs) that are integrated in the genome of all pigs, which are able to infect human cells, and that cannot be eliminated by methods that other viruses can. We showed, using droplet digital PCR, that the number of PERV proviruses is different in different pigs (usually around 60). Furthermore, the copy number is different in different organs of a single pig, indicating that PERVs are active in the living animals. We showed that in the first clinical trials treating diabetic patients with pig islet cells, no porcine viruses were transmitted. However, in preclinical trials transplanting pig hearts orthotopically into baboons, porcine cytomegalovirus (PCMV), a porcine roseolovirus (PCMV/PRV), and porcine circovirus 3 (PCV3), but no PERVs, were transmitted. PCMV/PRV transmission resulted in a significant reduction of the survival time of the xenotransplant. PCMV/PRV was also transmitted in the first pig heart transplantation to a human patient and possibly contributed to the death of the patient. Transmission means that the virus was detected in the recipient, however it remains unclear whether it can infect primate cells, including human cells. We showed previously that PCMV/PRV can be eliminated from donor pigs by early weaning. PERVs were also not transmitted by inoculation of human cell-adapted PERV into small animals, rhesus monkey, baboons and cynomolgus monkeys, even when pharmaceutical immunosuppression was applied. Since PERVs were not transmitted in clinical, preclinical, or infection experiments, it remains unclear whether they should be inactivated in the pig genome by CRISPR/Cas. In summary, by using our sensitive methods, the safety of xenotransplantation can be ensured.

Porcine Endogenous Retroviruses: Quantification of the Viral Copy Number for the Four Miniature Pig Breeds in China

Domestic pigs has served not only as one of the most important economy livestock but also as ideal organ-source animals owing to similarity in anatomy, physiology, and organ size to humans. Howerer, the barrier of the cross-species transmission risk of porcine endogenous retrovirus (PERVs) blocked the pig-to-human xenotransplantation. PERVs are integrated into pigs’ genomes and cannot be eliminated by designated or specified pathogen-free breeding. PERVs are an important biosafety issue in xenotransplantation because they can be released from normal pig cells and infect human cells in vitro under certain conditions. Screening and analyzing the presence of PERVs in pig genome will provide essential parameters for pig breed sources. In China, four miniature pig breeds, such as Guizhou miniature pig (GZ), Bama miniature pig (BM), Wuzhishan miniature pig (WZS), and Juema miniature pig (JM), were the main experimental miniature pig breeds, which were widely used. In this study, PCR was performed to amplify env-A, env-B, and env-C for all individuals from the four breeds. The results revealed that PERV env-A and env-B were detected in all individuals and the lowest ratios of PERV env-C was 17.6% (3/17) in the GZ breed. Then, PERV pol and GAPDH were detected using the droplet digital PCR (ddPCR) method. As the reference of GAPDH copy number, the copy numbers of PERVs were at the median of 12, 16, 14, and 16 in the four miniature pig breeds (GZ, BM, WZS, and JM), respectively. Furthermore, the copy number of the PERV pol gene in many organs from the GZ breed was analyzed using ddPCR. The copy numbers of PERV pol gene were at the median of 7 copies, 8 copies, 8 copies, 11 copies, 5 copies, 6 copies, and 7 copies in heart, liver, spleen, lung, kidney, muscle, and skin, and the maximum number was 11 copies in the lung. The minimum number was 5 copies in the kidney as the reference of GAPDH. These data suggest that GZ breed has the lower PERVs copy number in the genome, and may be an ideal organ-source miniature pig breed for the study of the pig-to-human xenotransplantation.

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.

The origin of porcine endogenous retroviruses (PERVs)

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

Unexpected low expression of porcine endogenous retroviruses (PERVs) in porcine expanded potential stem cells (EPSCs)

Expanded potential stem cells (EPSCs) have been recently derived from porcine preimplantation embryos (Gao et al., 2019). These cells were shown to express key pluripotency genes, to be genetically stable and differentiate to derivatives of the three germ layers and additionally to trophoblast. Their molecular features and expanded potency to contribute to all embryonic and extra-embryonic cell lineages are generally not seen in the embryo-derived or induced pluripotent stem cells (iPSCs). Therefore porcine EPSCs represent a unique state of cellular potency. In the past it had been shown that human and murine embryonic stem cells (ESCs) show an increased expression of murine and human endogenous retroviruses, respectively, and retroviral expression patterns were used as markers of ESC pluripotency. An increased expression of porcine endogenous retroviruses (PERVs) was also detected in porcine iPSCs. Here we investigated 24 passages of five different clones of porcine EPSCs derived from German landrace pigs and show that they harbour PERV-A, PERV-B and PERV-C, but their expression was very low and did not change during cultivation. No recombinant PERV-A/Cs were found in these cells. The low expression despite the presence of spliced mRNA, and negative infection assay and electron microscopy results indicate that no PERV particles were released. Therefore, the absence of PERV expression seems to be a unique feature of porcine EPSCs. Most importantly, the copy number of PERV proviruses was much lower in EPSCs than in young and older pigs (29.1 copies compared with 35.8), indicating an increase in copy number during life time.

Porcine genome engineering for xenotransplantation

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

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.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

β Cell Replacement Therapy: The Next 10 Years

β cell replacement with either pancreas or islet transplantation has progressed immensely over the last decades with current 1- and 5-year insulin independence rates of approximately 85% and 50%, respectively. Recent advances are largely attributed to improvements in immunosuppressive regimen, donor selection, and surgical technique. However, both strategies are compromised by a scarce donor source. Xenotransplantation offers a potential solution by providing a theoretically unlimited supply of islets, but clinical application has been limited by concerns for a potent immune response against xenogeneic tissue. β cell clusters derived from embryonic or induced pluripotent stem cells represent another promising unlimited source of insulin producing cells, but clinical application is pending further advances in the function of the β cell like clusters. Exciting developments and rapid progress in all areas of β cell replacement prompted a lively debate by members of the young investigator committee of the International Pancreas and Islet Transplant Association at the 15th International Pancreas and Islet Transplant Association Congress in Melbourne and at the 26th international congress of The Transplant Society in Hong Kong. This international group of young investigators debated which modality of β cell replacement would predominate the landscape in 10 years, and their arguments are summarized here.

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

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

Microbiological characterization of a newly established pig breed, Aachen Minipigs

BACKGROUND

To alleviate the shortage of human donor organs or tissues for the treatment of organ and tissue failure including diabetes, pigs are considered suitable donor animals. As organs from conventional pigs are usually too large, those from minipigs may be better suited. We recently characterized the Göttingen Minipigs, a breed well characterized concerning the presence of zoonotic microorganisms and found hepatitis E virus (HEV) and porcine cytomegalovirus (PCMV) in some animals. Here, we characterize another minipig, the Aachen Minipig (AaMP), a pig breed recently established close to the town Aachen in Germany.

METHODS

The animals were tested for the prevalence and expression of porcine endogenous retroviruses (PERVs) and the presence of some selected microorganisms, among them HEV, PCMV, and porcine lymphotropic herpesviruses (PLHVs) using highly sensitive and specific PCR and RT-PCR methods. In addition, we screened for antibodies against HEV and PLHV.

RESULTS

PERV-A, PERV-B, and PERV-C sequences were found in the genome of all Aachen Minipigs. HEV RNA was found by real-time RT-PCR in most, and DNA of PCMV, PLHV-2, and PLHV-3 was found by PCR in some animals. The animals were free of eight other microorganisms tested, but some were seropositive for porcine circovirus 2 (PCV2), porcine reproductive and respiratory syndrome virus (PRRSV), and porcine epidemic diarrhea virus (PEDV).

CONCLUSION

Based on medical examinations by veterinarians, the AaMP are in a good health status and seem to harbor only few microorganisms. To improve their status for use as donor pigs in xenotransplantation, the viruses detected might be eliminated by selection of negative animals, Cesarean section, and vaccination.

Virus safety of islet cell transplantation from transgenic pigs to marmosets

Transplantation of pig islet cells for the treatment of diabetes may be a more effective approach compared with the application of insulin. However, before introduction into the clinic, efficacy and safety of this treatment have to be shown. Non-human primate models may be used for this, despite the fact that they are characterised by several limitations. Here we investigate the prevalence of porcine endogenous retroviruses (PERVs), which are present in the genome of all pigs and which may infect human cells, as well as of porcine herpes viruses in donor pigs and their potential transmission to non-human primate recipients. Despite the fact that all three subtypes of PERV were present in all and porcine cytomegalovirus (PCMV) was found in some of the pigs, neither PERVs nor PCMV were found in the recipient animals under the experimental conditions applied. Porcine lymphotropic herpes viruses (PLHV) were not found in the donor pigs, hepatitis E virus (HEV) was not found in the recipients.

Detection of porcine endogenous retrovirus in xenotransplantation

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

Quantitative analysis of porcine endogenous retroviruses in different organs of transgenic pigs generated for xenotransplantation

The pig appears to be the most promising animal donor of organs for use in human recipients. Among several types of pathogens found in pigs, one of the greatest problems is presented by porcine endogenous retroviruses (PERVs). Screening of the source pig herd for PERVs should include analysis of both PERV DNA and RNA. Therefore, the present study focuses on quantitative analysis of PERVs in different organs such as the skin, heart, muscle, and liver and blood of transgenic pigs generated for xenotransplantation. Transgenic pigs were developed to express the human α-galactosidase, the human α-1,2-fucosyltransferase gene, or both genetic modifications of the genome (Lipinski et al., Medycyna Wet 66:316-322, 2010; Lipinski et al., Ann Anim Sci 12:349-356, 2012; Wieczorek et al., Medycyna Wet 67:462-466, 2011). The copy numbers of PERV DNA and RNA were evaluated using real-time Q-PCR and QRT-PCR, respectively. Comparative analysis of all PERV subtypes revealed the following relationships: PERV A > PERV B > PERV C. PERV A and B were found in all samples, whereas PERV C was detected in 47 % of the tested animals. The lowest level of PERV DNA was shown in the muscles for PERV A and B and in blood samples for PERV C. The lowest level of PERV A RNA was found in the skin, whereas those of PERV B and C RNA were found in liver specimens. Quantitative analysis revealed differences in the copy number of PERV subtypes between various organs of transgenic pigs generated for xenotransplantation. Our data support the idea that careful pig selection for organ donation with low PERV copy number may limit the risk of retrovirus transmission to the human recipients.