Are there better assays to evaluate the risk of transmission of porcine endogenous retroviruses (PERVs) to human cells?

Monitoring for PERV Following Xenotransplantation

Porcine endogenous retroviruses (PERVs) are integrated in the genome of all pigs. PERV-A, PERV-B and PERV-C can be released as infectious virus particles and PERV-A and PERV-B can infect human cells in culture. PERV-C does not infect human cells, but high-titer recombinant PERV-A/C can infect them. Retroviruses are able to induce immunosuppression and/or tumors in the infected host. Numerous methods have been developed to study PERV in donor pigs. No PERV infections were observed in infection experiments as well as in preclinical and clinical xenotransplantation trials. Despite this, several strategies have been developed to prevent PERV infection of the recipient. PCR-based and immunological methods are required to screen xenotransplant recipients. Since the proviruses are integrated into the pig genome, PERV infection has to be distinguished from microchimerism, e.g., the presence of pig cells in the recipient, which is common in xenotransplantation. Sensitive PCR methods using pig short interspersed nuclear elements (SINE) sequences allow to detect pig cells easily. Virus infection can also be detected by an increase of viral genomic or mRNA in human cells. The method of choice, however, is to screen for specific antibodies against PERV using different recombinant PERV proteins, purified viruses or peptides.

Is a Bioengineered Heart From Recipient Tissues the Answer to the Shortage of Donors in Heart Transplantation?

With the increase in life expectancy worldwide, end-organ failure is becoming more prevalent. In addition, improving post-transplant outcomes has contributed to soaring demand for organs. Unfortunately, thousands have died waiting on the transplant list due to the critical shortage of organs. The success of bioengineered hearts may eventually lead to the production of limitless organs using the patient’s own cells that can be transplanted into them without the need for immunosuppressive medications. Despite being in its infancy, scientists are making tremendous strides in “growing” an artificial heart in the lab. We discuss these processes involved in bioengineering a human-compatible heart in this review. The components of a functional heart must be replicated in a bioengineered heart to make it viable. This review aims to discuss the advances that have already been made and the future challenges of bioengineering a human heart suitable for transplantation.

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.

Pathways to Clinical Cardiac Xenotransplantation

Heart transplantation is the only long-lasting lifesaving option for patients with terminal cardiac failure. The number of available human organs is however far below the actual need, resulting in substantial mortality of patients while waiting for a human heart. Mechanical assist devices are used to support cardiac function but are associated with a high risk of severe complications and poor quality of life for the patients. Consistent success in orthotopic transplantation of genetically modified pig hearts into baboons indicates that cardiac xenotransplantation may become a clinically applicable option for heart failure patients who cannot get a human heart transplant. In this overview, we project potential paths to clinical cardiac xenotransplantation, including the choice of genetically modified source pigs; associated requirements of microbiological, including virological, safety; optimized matching of source pig and recipient; and specific treatments of the donor heart after explantation and of the recipients. Moreover, selection of patients and the regulatory framework will be discussed.

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.

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.

A Comprehensive Strategy for Screening for Xenotransplantation-Relevant Viruses in a Second Isolated Population of Göttingen Minipigs

Xenotransplantation using pig tissues and organs is under development in order to alleviate the increasing shortage of human transplants. Since xenotransplantation may be associated with the transmission of porcine microorganisms to the human recipient, the donor pigs should be carefully analyzed, especially for the presence of potentially zoonotic viruses. Göttingen Minipigs (GöMP) are potential donors of islet cells for the treatment of diabetes. Despite the fact that all animals produced at Ellegaard Göttingen Minipigs A/S carry porcine endogenous retroviruses (PERVs) in their genome and that very few animals were infected with porcine cytomegalovirus (PCMV), hepatitis E virus (HEV) and porcine lymphotropic herpesvirus (PLHV), no transmission of these viruses was observed in a preclinical trial transplanting GöMP islet cells into cynomolgus monkeys. Using a new comprehensive strategy, we then analyzed an isolated subpopulation of Göttingen Minipigs which remained at the University of Göttingen. We concentrated on 11 xenotransplantation-relevant viruses and combined co-incubation assays with susceptible human target cells and molecular biological methods to evaluate the risk posed by PERV. All animals in Göttingen carry PERV-A, PERV-B, and PERV-C in their genome but they are not infected with PCMV, PLHV and HEV. The difference may be explained by selection of negative animals and/or de novo infection. The PERV copy number was established using ddPCR (93 copies) and a human-tropic PERV-A/C was found released from PBMCs of one animal with a high expression of PERV-C.