Risk assessment of porcine reproductive and respiratory syndrome virus (PRRSV) transmission via somatic cell nuclear transfer (SCNT) embryo production using oocytes from commercial abattoirs

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.

Risk of pathogenic virus transmission by somatic cell nuclear transfer (SCNT): implications for xenotransplantation

Using somatic cell nuclear transfer (SCNT) for the generation of cloned and transgenic animals bears the risk of transmission of viruses, either by the oocyte or by the introduced donor cell. There is evidence that the zona pellucida (ZP) surrounding the oocyte prevents virus infection, however, virus infections despite intact ZP were reported. Furthermore, the protective ZP has to be penetrated in order to place the somatic cell in the oocyte's perivitelline space during SCNT. Transmission of viruses represents also a severe problem during in vitro fertilization (IVF). Genetically modified and IVF-produced pigs serve as an important biomedical model for numerous diseases and it is important to evaluate whether infections of the model animals can falsify the research data. Of special significance is this topic in the case of xenotransplantation using genetically modified pigs as donor animals, because transmission of porcine viruses may be harmful for the human recipient. This was repeatedly demonstrated in preclinical pig to non-human primate trials. Therefore, donor pigs, oocytes used for SCNT and genetically modified donor cells should be screened for potentially zoonotic viruses when creating genetically modified pigs designed for xenotransplantation.

Regulation and safety considerations of somatic cell nuclear transfer-cloned farm animals and their offspring used for food production

This document discusses recent developments in cloning of husbandry animals through somatic cell nuclear transfer, particularly with a view on improvements in their efficacy. Commercial developments in North and South America, Australia-New Zealand, and China are noted. The regulations and safety aspects surrounding the use of clones and their offspring for the purpose of food production are discussed. It is generally considered that foods from offspring of clones are no different than similar foods from conventional animals, yet besides safety, also ethical and animal welfare considerations come into play at the policy level. The related topic of detection and traceability of clones is discussed, which covers both molecular and documentary methods.

Evaluation of porcine circovirus type 2 infection in in vitro embryo production using naturally infected oocytes

In vitro fertilization (IVF) and somatic cell nuclear transfer (SCNT) are important breeding techniques for livestock. High-quality MII oocytes produced from in vitro maturation (IVM) are required for the two techniques listed above. The ovaries used for IVM operations are primarily acquired from commercial abattoirs, and the pathogen status of slaughtered animals becomes an unavoidable issue. Our previous monitoring data showed that porcine circovirus type 2 (PCV-2) is the main pathogen present in ovaries from abattoirs. However, the characteristics and effects of PCV-2 infection in oocyte maturation and in vitro production (IVP) of embryos are unclear, and currently there are no relevant studies. Therefore, the aim of this study was to determine the PCV-2 infection pattern and determine whether it affects oocyte in vitro maturation and IVP embryo development. More than five hundred ovaries and five thousand oocytes were utilized in the present study. Polymerase chain reaction (PCR) was used to detect PCV-2 DNA in ovaries, follicular fluid (FF), oocytes, cumulus cells and IVP embryos. The effects of viral infections on the rate of oocyte maturation and IVP embryo development were evaluated. We also analyzed the number of copies of the virus in the IVM and IVP process by absolute quantitative fluorescence PCR. Our study showed that the prevalent virus subgenotype in ovaries was PCV-2a. PCV-2a infection did not significantly affect ovarian/oocyte morphology and maturation. Moreover, virus infection did not have a significant effect on the development of the IVP embryos except for a reduction in IVF blastocyst cell numbers. Further tests showed that the viral copy numbers fluctuated at different stages between the IVP embryos and culture medium. For the first time, this study identified the infection pattern of naturally sourced PCV-2 in the course of oocyte maturation and embryo development.

Advances on in vitro production and cryopreservation of porcine embryos

There have been intensive attempts to establish reliable in vitro production (IVP) and cryopreservation methods of embryos in pigs. Although a great deal of progress has been made, current IVP systems and cryopreservation still suffer from insufficient cytoplasmic abilities of in vitro matured oocytes, polyspermic fertilization, poor quality of in vitro produced embryos and low efficiency of embryo cryopreservation. Compared to other mammalian species, pig oocytes and embryos are characterized by large amounts of lipid content stored mainly in the form of lipid droplets in the cytoplasm. This fact has a negative influence on biotechnological applications on porcine oocytes and embryos. In this review, we will discuss recent studies about methods and techniques for modifying porcine embryo IVP system and embryo cryopreservation that produces high quality of pig blastocysts using in vitro maturation, in vitro fertilization, in vitro culture, microsurgical manipulation, addition of protein, the use of cytoskeleton stabilizing agents and various physical methods. The presented methods and techniques make it possible to modify the characteristics of oocytes and embryos and thus may become major tools in mammalian gamete and embryo agricultural or biotechnological applications in the future.