Early weaning of piglets fails to exclude porcine lymphotropic herpesvirus

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.

Infectious Diseases and Clinical Xenotransplantation

Xenotransplantation, transplantation into humans of vascularized organs or viable cells from nonhuman species, is a potential solution to shortages of transplantable human organs. Among challenges to application of clinical xenotransplantation are unknown risks of transmission of animal microbes to immunosuppressed recipients or the community. Experience in allotransplantation and in preclinical models suggests that viral infections are the greatest concern. Worldwide, the distribution of swine pathogens is heterogeneous and cannot be fully controlled by international agricultural regulations. It is possible to screen source animals for potential human pathogens before procuring organs in a manner not possible within the time available for surveillance testing in allotransplantation. Infection control measures require microbiological assays for surveillance of source animals and xenograft recipients and research into zoonotic potential of porcine organisms. Available data suggest that infectious risks of xenotransplantation are manageable and that clinical trials can advance with appropriate protocols for microbiological monitoring of source animals and recipients.

Narrative Review of the Safety of Using Pigs for Xenotransplantation: Characteristics and Diagnostic Methods of Vertical Transmissible Viruses

Amid the deepening imbalance in the supply and demand of allogeneic organs, xenotransplantation can be a practical alternative because it makes an unlimited supply of organs possible. However, to perform xenotransplantation on patients, the source animals to be used must be free from infectious agents. This requires the breeding of animals using assisted reproductive techniques, such as somatic cell nuclear transfer, embryo transfer, and cesarean section, without colostrum derived in designated pathogen-free (DPF) facilities. Most infectious agents can be removed from animals produced via these methods, but several viruses known to pass through the placenta are not easy to remove, even with these methods. Therefore, in this narrative review, we examine the characteristics of several viruses that are important to consider in xenotransplantation due to their ability to cross the placenta, and investigate how these viruses can be detected. This review is intended to help maintain DPF facilities by preventing animals infected with the virus from entering DPF facilities and to help select pigs suitable for xenotransplantation.

Mitigating Cross-Species Viral Infections in Xenotransplantation: Progress, Strategies, and Clinical Outlook

Xenotransplantation holds great promise as a solution to address the critical shortage of organs, but it raises concerns regarding the potential transmission of porcine viruses to recipients, leading to infections and even zoonotic diseases. Data used in this review were mainly from literature of Pubmed database. Keywords included xenotransplantation, infection, virus, and epidemiology. The original articles and critical reviews selected were relevant to this review's theme. We review the major viral infections of concern in xenotransplantation, their risk of transmission, diagnosis, treatment, and ways to prevent infection. Then, we pivot to a comprehensive overview of the current status of xenotransplantation. In addition, we offer our own insights and recommendations for propelling xenotransplantation forward, transitioning from preclinical experiments to the critical phase of clinical trials. Viral infections pose considerable safety concerns within xenotransplantation, particularly with the possibility of emerging or currently unidentified viruses. Clinical trials serve as a crucial platform to progress the safety standards of xenotransplantation. However, further studies and dedicated efforts are required to effectively translate findings into practical applications that can improve safety measures in this field.

The prevention strategies of swine viruses related to xenotransplantation

Xenotransplantation is considered a solution for the shortage of organs, and pigs play an indispensable role as donors in xenotransplantation. The biosecurity of pigs, especially the zoonotic viruses carried by pigs, has attracted attention. This review introduces several viruses, including porcine endogenous retroviruses that are integrated into the pig genome in a DNA form, herpesviruses that have been proven to clearly affect recipient survival time in previous xenotransplant surgeries, the zoonotic hepatitis E virus, and the widely distributed porcine circoviruses. The detail virus information, such as structure, caused diseases, transmission pathways, and epidemiology was introduced in the current review. Diagnostic and control measures for these viruses, including detection sites and methods, vaccines, RNA interference, antiviral pigs, farm biosecurity, and drugs, are discussed. The challenges faced, including those posed by other viruses and newly emerged viruses, and the challenges brought by the modes of transmission of the viruses are also summarized.

Infection and clinical xenotransplantation: Guidance from the Infectious Disease Community of Practice of the American Society of Transplantation

This guidance was developed to summarize current approaches to the potential transmission of swine-derived organisms to xenograft recipients, health care providers, or the public in clinical xenotransplantation. Limited specific data are available on the zoonotic potential of pig pathogens. It is anticipated that the risk of zoonotic infection in xenograft recipients will be determined by organisms present in source animals and relate to the nature and intensity of the immunosuppression used to maintain xenograft function. Based on experience in allotransplantation and with preclinical models, viral infections are of greatest concern, including porcine cytomegalovirus, porcine lymphotropic herpesvirus, and porcine endogenous retroviruses. Sensitive and specific microbiological assays are required for routine microbiological surveillance of source animals and xenograft recipients. Archiving of blood samples from recipients, contacts, and hospital staff may provide a basis for microbiological investigations if infectious syndromes develop. Carefully implemented infection control practices are required to prevent zoonotic pathogen exposures by clinical care providers. Informed consent practices for recipients and their close contacts must convey the lack of specific data for infectious risk assessment. Available data suggest that infectious risks of xenotransplantation are manageable and that clinical trials can advance with carefully developed protocols for pretransplant assessment, syndrome evaluation, and microbiological monitoring.

Porcine cytomegalovirus/porcine roseolovirus (PCMV/PRV): A threat for xenotransplantation?

The potential for a donor-derived transmission of porcine cytomegalovirus/porcine roseolovirus (PCMV/PRV) to the recipient has been recognized since pigs were considered candidate donors for xenotransplantation. This review gives a short description of the viral properties and summarizes the current evidence of the effects of PCMV/PRV transmission in preclinical xenotransplantation. Despite evidence that PCMV/PRV does not infect human and non-human primate cells, activation in the transplanted organ and detrimental systemic complications have been described. As PCMV/PRV is a herpesvirus able to establish latency, the importance of adequate screening of donor pigs is emphasized, as no efficient treatment is available. Furthermore, easy and successful ways of elimination of PCMV/PRV from pig herds are indicated.

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.

Drinking Water Supplemented with Acidifiers Improves the Growth Performance of Weaned Pigs and Potentially Regulates Antioxidant Capacity, Immunity, and Gastrointestinal Microbiota Diversity

This study evaluated the potential effects of adding acidifiers to the drinking water on the growth performance, complete blood count, antioxidant indicators, and diversity of gastrointestinal microbiota for weaned pigs. A total of 400 weaned pigs were randomly divided into four treatments. Pigs were fed the same basal diet and given either water (no acidifier was added, control) or water plus blends of different formulas of acidifiers (acidifier A1, A2, or A3) for 35 days. On d 18 and 35 of the experimental period, 64 pigs (four pigs per pen) were randomly selected to collect blood for a CBC test (n = 128) and an antioxidant indicators test (n = 128); 24 pigs (six pigs per group) were randomly selected to collect fresh feces (n = 48) from the rectum for 16S rRNA gene sequencing. Compared to the control, supplementing the drinking water with acidifiers improved the growth performance and survival rate of weaned pigs. Acidifier groups also increased serum catalase (CAT) and total antioxidant capacity (T-AOC) activities, while also displaying a decreased malondialdehyde (MDA) concentration compared to the control. The relative abundance of Firmicutes in the acidifier A1 group was greater than that in the control group (p < 0.05) on d 35; the relative abundance of Lactobacillus in the acidifier A1 group was greater than that in the control group (p < 0.05) on d 18 and 35. The microbial species Subdoligranulum or Ruminococcaceae_UCG-005 had significantly positive correlations with ADG and ADFI or with serum antioxidant indicators, respectively. These findings suggest that supplementing the drinking water with an acidifier has a potential as an antioxidant, which was reflected in the improvement of growth performance, immunity, antioxidant capacity, and intestinal flora.

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.

Porcine Lymphotropic Herpesviruses (PLHVs) and Xenotranplantation

Porcine lymphotropic herpesviruses -1, -2 and -3 (PLHV-1, PLHV-2 and PLHV-3) are gammaherpesviruses which are widespread in pigs. They are closely related to the Epstein-Barr virus (EBV) and Kaposi sarcoma herpesvirus, both of which cause severe diseases in humans. PLHVs are also related to bovine and ovine gammaherpesviruses, which are apathogenic in the natural host, but cause severe diseases after transmission into other species. Until now, no association between PLHVs and any pig diseases had been described. However, PLHV-1 causes a post-transplantation lymphoproliferative disorder (PTLD) after experimental transplantations in minipigs. This disorder is similar to human PTLD, a serious complication of solid human organ transplantation linked to EBV. Xenotransplantation using pig cells, tissues and organs is under development in order to alleviate the shortage of human transplants. Meanwhile, remarkable survival times of pig xenotransplants in non-human primates have been achieved. In these preclinical trials, another pig herpesvirus, the porcine cytomegalovirus (PCMV), a roseolovirus, was shown to significantly reduce the survival time of pig xenotransplants in baboons and other non-human primates. Although PLHV-1 was found in genetically modified donor pigs used in preclinical xenotransplantation, it was, in contrast to PCMV, not transmitted to the recipient. Nevertheless, it seems important to use PLHV-free donor pigs in order to achieve safe xenotransplantation.

Porcine Gammaherpesviruses in Italian Commercial Swine Population: Frequent but Harmless

Differently from alpha- and betaherpesviruses affecting swine, interest in the recently discovered Suid gammaherpesvirus 3, Suid gammaherpesvirus 4, and Suid gammaherpesvirus 5, also known as porcine lymphotropic herpesviruses (PLHV-1, PLHV-2, and PLHV-3), has largely focused on their role as potential zoonotic agents in cases of xenotransplantation. However, their role as primary pathogens of swine or as co-factors for other lymphotropic infections has essentially been neglected. The present study aims at filling this gap, evaluating the association between PLHVs infection and different clinical conditions and/or porcine circovirus (PCV) co-infection. One hundred seventy-six samples were obtained from different animals located in a high-density pig area of Northern Italy in the period 2017–2020. The presence of PLHVs and PCVs was tested and quantified by specific real-time PCR: PLHVs were widespread among pigs (PLHV-1, PLHV-2, and PLHV-3 prevalence was 28.97%, 10.79%, and 4.54%, respectively) and detected in all considered tissues and clinical conditions. Frequent co-infections were also observed among PLHVs and with PCVs, although a significant association was not detected with the exception of a positive interaction between PLHV-1 and PLHV-3, and a negative one between PLHV-2 and PCV-2. Significantly, no association between PLHVs, alone or in co-infection, emerged with any of the considered clinical signs, their frequency being comparable between healthy and diseased animals. Based on these pieces of evidence and despite their high prevalence, PLHVs’ relevance for the swine industry appears negligible, either as primary pathogens or as predisposing factors for circovirus-induced diseases.

Corneal xenotransplantation: Where are we standing?

The search for alternatives to allotransplants is driven by the shortage of corneal donors and is demanding because of the limitations of the alternatives. Indeed, current progress in genetically engineered (GE) pigs, the introduction of gene-editing technology by clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9, and advanced immunosuppressants have made xenotransplantation a possible option for a human trial. Porcine corneal xenotransplantation is considered applicable because the eye is regarded as an immune-privileged site. Furthermore, recent non-human primate studies have shown long-term survival of porcine xenotransplants in keratoplasty. Herein, corneal immune privilege is briefly introduced, and xenogeneic reactions are compared with allogeneic reactions in corneal transplantation. This review describes the current knowledge on special issues of xenotransplantation, xenogeneic rejection mechanisms, current immunosuppressive regimens of corneal xenotransplantation, preclinical efficacy and safety data of corneal xenotransplantation, and updates of the regulatory framework to conduct a clinical trial on corneal xenotransplantation. We also discuss barriers that might prevent xenotransplantation from becoming common practice, such as ethical dilemmas, public concerns on xenotransplantation, and the possible risk of xenozoonosis. Given that the legal definition of decellularized porcine cornea (DPC) lies somewhere between a medical device and a xenotransplant, the preclinical efficacy and clinical trial data using DPC are included. The review finally provides perspectives on the current standpoint of corneal xenotransplantation in the fields of regenerative medicine.

Organoid based personalized medicine: from bench to bedside

Three-dimensional cultured organoids have become a powerful in vitro research tool that preserves genetic, phenotypic and behavioral trait of in vivo organs, which can be established from both pluripotent stem cells and adult stem cells. Organoids derived from adult stem cells can be established directly from diseased epithelium and matched normal tissues, and organoids can also be genetically manipulated by CRISPR-Cas9 technology. Applications of organoids in basic research involve the modeling of human development and diseases, including genetic, infectious and malignant diseases. Importantly, accumulating evidence suggests that biobanks of patient-derived organoids for many cancers and cystic fibrosis have great value for drug development and personalized medicine. In addition, organoids hold promise for regenerative medicine. In the present review, we discuss the applications of organoids in the basic and translational research.

Porcine lymphotropic herpesvirus DNA detection in multiple organs of pigs in Brazil

We investigated the porcine lymphotropic herpesvirus (PLHV) DNA presence in multiple organs of pigs. Biological samples (n = 136) included tissue fragments of the central nervous system, heart, kidney, liver, lungs, spleen, urinary bladder, and urine. Sixty-eight (50%) organs were PLHV DNA-positive. None of the urine samples were detected with the virus genome. Although the presence of the PLHV DNA in the urinary bladder and kidney has been detected, it was not possible to show whether urine can be considered an effective route of virus shedding. This study warns to the risk of PLHV zoonotic transmission by xenotransplantation of tissues of porcine origin.

Comparative Analysis of Roseoloviruses in Humans, Pigs, Mice, and Other Species

Viruses of the genus Roseolovirus belong to the subfamily Betaherpesvirinae, family Herpesviridae. Roseoloviruses have been studied in humans, mice and pigs, but they are likely also present in other species. This is the first comparative analysis of roseoloviruses in humans and animals. The human roseoloviruses human herpesvirus 6A (HHV-6A), 6B (HHV-6B), and 7 (HHV-7) are relatively well characterized. In contrast, little is known about the murine roseolovirus (MRV), also known as murine thymic virus (MTV) or murine thymic lymphotrophic virus (MTLV), and the porcine roseolovirus (PRV), initially incorrectly named porcine cytomegalovirus (PCMV). Human roseoloviruses have gained attention because they can cause severe diseases including encephalitis in immunocompromised transplant and AIDS patients and febrile seizures in infants. They have been linked to a number of neurological diseases in the immunocompetent including multiple sclerosis (MS) and Alzheimer's. However, to prove the causality in the latter disease associations is challenging due to the high prevalence of these viruses in the human population. PCMV/PRV has attracted attention because it may be transmitted and pose a risk in xenotransplantation, e.g., the transplantation of pig organs into humans. Most importantly, all roseoloviruses are immunosuppressive, the humoral and cellular immune responses against these viruses are not well studied and vaccines as well as effective antivirals are not available.

Human Organoids: Tools for Understanding Biology and Treating Diseases

Organoids are in vitro-cultured three-dimensional structures that recapitulate key aspects of in vivo organs. They can be established from pluripotent stem cells and from adult stem cells, the latter being the subject of this review. Organoids derived from adult stem cells exploit the tissue regeneration process that is driven by these cells, and they can be established directly from the healthy or diseased epithelium of many organs. Organoids are amenable to any experimental approach that has been developed for cell lines. Applications in experimental biology involve the modeling of tissue physiology and disease, including malignant, hereditary, and infectious diseases. Biobanks of patient-derived tumor organoids are used in drug development research, and they hold promise for developing personalized and regenerative medicine. In this review, we discuss the applications of adult stem cell-derived organoids in the laboratory and the clinic.

Donor-derived infections and infectious risk in xenotransplantation and allotransplantation

Post-transplantation infections are common in allograft recipients and should be expected in all immunocompromised hosts. Based on the need for immunosuppression in xenotransplantation, procedures developed to enhance safety in allotransplantation can be applied in future xenotransplantation clinical trials. Standardized approaches can be developed to guide the evaluation of common infectious syndromes in xenograft recipients. The opportunity created by screening of swine intended as xenograft donors has equal applicability to allotransplantation-notably broader screening strategies for allograft donors such as use of advanced sequencing modalities including broad-range molecular probes, microarrays, and high-throughput pyrosequencing. Considerations in management of allotransplant- and xenotransplant-associated infections are largely the same. Experience in xenotransplantation will continue to inform thinking regarding donor-derived infections in allotransplantation. We expect that experience in managing complex allotransplant recipients will similarly inform clinical trials in xenotransplantation.

Reduction of the survival time of pig xenotransplants by porcine cytomegalovirus

Background

Xenotransplantation using pig cells, tissues and organs may help to overcome the shortage of human tissues and organs for the treatment of tissue and organ failure. Progress in the prevention of immunological rejection using genetically modified pigs and new, more effective, immunosuppression regimens will allow clinical application of xenotransplantation in near future. However, xenotransplantation may be associated with the transmission of potentially zoonotic porcine microorganisms. Until now the only xenotransplantation-associated transmission was the transmission of the porcine cytomegalovirus (PCMV) into non-human primates. PCMV caused a significant reduction of the survival time of the pig transplant.

Main body of the abstract

Here the available publications were analysed in order to establish the mechanism how PCMV shortened the survival time of xenotransplants. PCMV is a herpesvirus related to the human cytomegalovirus and the human herpesviruses 6 and 7. These three human herpesviruses can cause serious disease among immunocompromised human individuals, including transplant recipients. It was shown that PCMV predominantly contributes to the reduction of transplant survival in non-human primates by disruption of the coagulation system and by suppression and exhaustion of the immune system.

Conclusion

Although it is still unknown whether PCMV infects primate cells including human cells, indirect mechanism of the virus infection may cause reduction of the xenotransplant survival in future clinical trials and therefore PCMV has to be eliminated from donor pigs.

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.

Distribution of Porcine Cytomegalovirus in Infected Donor Pigs and in Baboon Recipients of Pig Heart Transplantation

The porcine cytomegalovirus (PCMV) is a herpesvirus that may pose a risk for xenotransplantation using pig cells, tissues, or organs. Here, three orthotopic pig heart transplantations into baboons were studied. To detect PCMV, a real-time PCR and a Western blot assay based on four PCMV protein sequences, including two tegument proteins, were used. The transmission of PCMV from the donor pig to the recipient baboon was found in two cases, despite PCMV not being detected in the blood of the donor pigs by real-time PCR. Although it was not in the blood, PCMV was detected in different organs of the donor pigs, and in sibling animals. Immunohistochemistry using an antiserum that is specific for PCMV detected virus protein-expressing cells in all of the organs of the recipient baboon, most likely representing disseminated pig cells. Therefore, for the first time, the distribution of PCMV in organs of the donor pigs and the recipient baboons was described. In addition, baboon cytomegalovirus (BaCMV) was found activated in the recipient, and a screening for hepatitis E virus (HEV) and porcine lymphotropic herpesviruses (PLHV) was performed. For the first time, a cross-reactivity between antibodies directed against PCMV and BaCMV was found.

The porcine virome and xenotransplantation

The composition of the porcine virome includes viruses that infect pig cells, ancient virus-derived elements including endogenous retroviruses inserted in the pig chromosomes, and bacteriophages that infect a broad array of bacteria that inhabit pigs. Viruses infecting pigs, among them viruses also infecting human cells, as well as porcine endogenous retroviruses (PERVs) are of importance when evaluating the virus safety of xenotransplantation. Bacteriophages associated with bacteria mainly in the gut are not relevant in this context. Xenotransplantation using pig cells, tissues or organs is under development in order to alleviate the shortage of human transplants. Here for the first time published data describing the viromes in different pigs and their relevance for the virus safety of xenotransplantation is analysed. In conclusion, the analysis of the porcine virome has resulted in numerous new viruses being described, although their impact on xenotransplantation is unclear. Most importantly, viruses with known or suspected zoonotic potential were often not detected by next generation sequencing, but were revealed by more sensitive methods.

Effective Detection of Porcine Cytomegalovirus Using Non-Invasively Taken Samples from Piglets

Shortage of human organs forced the development of xenotransplantation using cells, tissues, and organs from pigs. Xenotransplantation may be associated with the transmission of porcine zoonotic microorganisms, among them the porcine cytomegalovirus (PCMV). To prevent virus transmission, pigs have to be screened using sensitive methods. In order to perform regular follow-ups and further breeding of the animals, samples for testing should be collected by low-invasive or non-invasive methods. Sera, ear biopsies, as well as oral and anal swabs were collected from ten 10-day-old Aachen minipigs (AaMP) and tested for PCMV using sensitive nested polymerase chain reaction (PCR) as well as uniplex and duplex real-time PCR. Porcine cytomegalovirus DNA was detected most frequently in oral and anal swabs. Comparison of duplex and uniplex real-time PCR systems for PCMV detection demonstrated a lower sensitivity of duplex real-time PCR when the copy numbers of the target genes were low (less 200). Therefore, to increase the efficacy of PCMV detection in piglets, early testing of oral and anal swabs by uniplex real-time PCR is recommended.

Immunological methods for the detection of porcine lymphotropic herpesviruses (PLHV)

Porcine lymphotropic herpesviruses (PLHV-1, -2, and -3) are widespread in pigs and closely related to the human pathogenic gammaherpesviruses Epstein-Barr virus (human herpesvirus 4, HHV-4) and Kaposi sarcoma herpesvirus (HHV-8). In minipigs, PLHV-1 causes a porcine post-transplantation lymphoproliferative disorder (PTLD) after experimental transplantations. Porcine PTLD comes with clinical symptoms similar to those of human PTLD, a serious complication of solid organ and allogeneic bone marrow transplantation linked to HHV-4. Since PLHVs may be transmitted from donor pigs to the human recipient of xenotransplants (pig cells, tissues or organs), sensitive and specific methods should be developed to detect and eliminate PLHVs. Here we describe an ELISA and a Western blot assay using recombinant glycoprotein B of PLHV-1. Using both assays, the presence of specific antibodies in different pig breeds as well as in German slaughterhouse workers was analysed. Antibodies were detected in some animals, but not in human subjects.

The pathobiology of pig-to-primate xenotransplantation: a historical review

The immunologic barriers to successful xenotransplantation are related to the presence of natural anti-pig antibodies in humans and non-human primates that bind to antigens expressed on the transplanted pig organ (the most important of which is galactose-α1,3-galactose [Gal]), and activate the complement cascade, which results in rapid destruction of the graft, a process known as hyperacute rejection. High levels of elicited anti-pig IgG may develop if the adaptive immune response is not prevented by adequate immunosuppressive therapy, resulting in activation and injury of the vascular endothelium. The transplantation of organs and cells from pigs that do not express the important Gal antigen (α1,3-galactosyltransferase gene-knockout [GTKO] pigs) and express one or more human complement-regulatory proteins (hCRP, e.g., CD46, CD55), when combined with an effective costimulation blockade-based immunosuppressive regimen, prevents early antibody-mediated and cellular rejection. However, low levels of anti-non-Gal antibody and innate immune cells and/or platelets may initiate the development of a thrombotic microangiopathy in the graft that may be associated with a consumptive coagulopathy in the recipient. This pathogenic process is accentuated by the dysregulation of the coagulation-anticoagulation systems between pigs and primates. The expression in GTKO/hCRP pigs of a human coagulation-regulatory protein, for example, thrombomodulin, is increasingly being associated with prolonged pig graft survival in non-human primates. Initial clinical trials of islet and corneal xenotransplantation are already underway, and trials of pig kidney or heart transplantation are anticipated within the next few years.

Preventing transfer of infectious agents

Xenotransplantation using pig cells, tissues and organs may be associated with the transfer of porcine infectious agents, which may infect the human recipient and in the worst case induce a disease (zoonosis). To prevent this, a broad screening program of the donor animals for putative zoonotic microorganisms, including bacteria, viruses, fungi and others, using sensitive and specific detection methods has to be performed. As long as it is still unknown, which microorganism represents a real risk for the recipient, experience from allotransplantation should be brought in. Due to the fact that pigs can be screened long before the date of transplantation, xenotransplantation will become eventually safer compared with allotransplantation. Screening and selection of animals free of potential zoonotic microorganisms, Caesarean section, vaccination and/or treatment with chemotherapeutics are the strategies of choice to obtain donor animals not transmitting microorganisms. In the case of porcine endogenous retroviruses (PERVs) which are integrated in the genome of all pigs and which cannot be eliminated this way, selection of animals with low virus expression and generation of genetically modified pigs suppressing PERV expressions may be performed.

Developing a Vaccine against Congenital Cytomegalovirus (CMV) Infection: What Have We Learned from Animal Models? Where Should We Go Next?

Congenital human cytomegalovirus (HCMV) infection can lead to long-term neurodevelopmental sequelae, including mental retardation and sensorineural hearing loss. Unfortunately, CMVs are highly adapted to their specific species, precluding the evaluation of HCMV vaccines in animal models prior to clinical trials. Several species-specific CMVs have been characterized and developed in models of pathogenesis and vaccine-mediated protection against disease. These include the murine CMV (MCMV), the porcine CMV (PCMV), the rhesus macaque CMV (RhCMV), the rat CMV (RCMV), and the guinea pig CMV (GPCMV). Because of the propensity of the GPCMV to cross the placenta, infecting the fetus in utero, it has emerged as a model of particular interest in studying vaccine-mediated protection of the fetus. In this paper, a review of these various models, with particular emphasis on the value of the model in the testing and evaluation of vaccines against congenital CMV, is provided. Recent exciting developments and advances in these various models are summarized, and recommendations offered for high-priority areas for future study.

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.

Emerging and re-emerging swine viruses

In the past two decades or so, a number of viruses have emerged in the global swine population. Some, such as porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2), cause economically important diseases in pigs, whereas others such as porcine torque teno virus (TTV), now known as Torque teno sus virus (TTSuV), porcine bocavirus (PBoV) and related novel parvoviruses, porcine kobuvirus, porcine toroviruses (PToV) and porcine lymphotropic herpesviruses (PLHV), are mostly subclinical in swine herds. Although some emerging swine viruses such as swine hepatitis E virus (swine HEV), porcine endogenous retrovirus (PERV) and porcine sapovirus (porcine SaV) may have a limited clinical implication in swine health, they do pose a potential public health concern in humans due to zoonotic (swine HEV) or potential zoonotic (porcine SaV) and xenozoonotic (PERV, PLHV) risks. Other emerging viruses such as Nipah virus, Bungowannah virus and Menangle virus not only cause diseases in pigs but some also pose important zoonotic threat to humans. This article focuses on emerging and re-emerging swine viruses that have a limited or uncertain clinical and economic impact on pig health. The transmission, epidemiology and pathogenic potential of these viruses are discussed. In addition, the two economically important emerging viruses, PRRSV and PCV2, are also briefly discussed to identify important knowledge gaps.

Microbiological safety of porcine islets: comparison with source pig

BACKGROUND

Pig islet donors intended for clinical xenotransplantation for the treatment of diabetes must meet stringent conditions. Among others, viruses with the potential to cross the species barrier should be excluded from the herd: this list includes encephalomyocarditis virus (EMCV), hepatitis E virus (HEV), porcine cytomegalovirus (PCMV) and porcine γ-lymphotropic herpesvirus (PLHV). As an islet product is isolated from the pancreas and then subjected to culture before implantation, the question is raised whether islets could be negative even if the animal itself is positive for a distinct pathogen.

METHODS

To answer this question, sensitive quantitative real-time PCR assays were established for EMCV, HEV, PCMV and PLHV. Twelve adult animals from a high-hygienic herd were then evaluated; testing tissues, where the virus is expected to reside in latent infection, testing islets immediately after isolation, and then isolated islets after a 7-day culture.

RESULTS

None of the tissues tested positive for EMCV, HEV or PLHV. PCMV was observed in spleen tissue from six animals: three of these six animals were positive for isolated islets, and two of these three cases were also positive for islets after culture. Older animals in particular showed positivity, and within a given litter not all animals were PCMV positive.

CONCLUSIONS

These data fit with spread through the herd by horizontal transmission, not in utero infection. PCMV has to be excluded from the herd to ensure that islets for transplantation are negative for PCMV.

Retroviral restriction factors and infectious risk in xenotransplantation

The clinical application of xenotransplantation poses immunologic, ethical, and microbiologic challenges. Significant progress has been made in the investigation of each of these areas. Among concerns regarding infectious risks for human xenograft recipients is the identification in swine of infectious agents including porcine endogenous retroviruses (PERV) that are capable of replication in some human cell lines. PERV replication has, however, been difficult to demonstrate in primate-derived cell lines and in preclinical studies of non-human primates receiving porcine xenografts. Endogenous 'retroviral restriction factors' are intracellular proteins and components of the innate immune system that act at various steps in retroviral replication. Recent studies suggest that some of these factors may have applications in the management of endogenous retroviruses in xenotransplantation. The risks of PERV infection and the potential role of retroviral restriction factors in xenotransplantation are reviewed in detail.

Absence of replication of porcine endogenous retrovirus and porcine lymphotropic herpesvirus type 1 with prolonged pig cell microchimerism after pig-to-baboon xenotransplantation

Porcine endogenous retrovirus (PERV), porcine cytomegalovirus (PCMV), and porcine lymphotropic herpesvirus (PLHV) are common porcine viruses that may be activated with immunosuppression for xenotransplantation. Studies of viral replication or transmission are possible due to prolonged survival of xenografts in baboon recipients from human decay-accelerating factor transgenic or alpha-1,3-galactosyltransferase gene knockout miniature swine. Ten baboons underwent xenotransplantation with transgenic pig organs. Graft survival was 32 to 179 days. Recipient serial samples of peripheral blood mononuclear cells (PBMC) and plasma were analyzed for PCMV, PERV, and PLHV-1 nucleic acids and viral replication using quantitative PCR assays. The PBMC contained PERV proviral DNA in 10 animals, PLHV-1 DNA in 6, and PCMV in 2. PERV RNA was not detected in any PBMC or serum samples. Plasma PLHV-1 DNA was detected in one animal. Pig cell microchimerism (pig major histocompatibility complex class I and pig mitochondrial cytochrome c oxidase subunit II sequences) was present in all recipients with detectable PERV or PLHV-1 (85.5%). Productive infection of PERV or PLHV-1 could not be demonstrated. The PLHV-1 viral load did not increase in serum over time, despite prolonged graft survival and pig cell microchimerism. There was no association of viral loads with the nature of exogenous immune suppression. In conclusion, PERV provirus and PLHV-1 DNA were detected in baboons following porcine xenotransplantation. Viral detection appeared to be due to persistent pig cell microchimerism. There was no evidence of productive infection in recipient baboons for up to 6 months of xenograft function.

The pig-to-primate immune response: relevance for xenotransplantation

BACKGROUND

The allotransplantation of some solid organs can be associated with a graft-vs.-host (GVH) response from the activity of donor B or T cells. We have investigated whether there is a risk of a GVH response following pig-to-primate organ xenotransplantation.

METHODS

The responses of 16 pigs (six farm-housed wild-type and five wild-type housed under high herd health conditions [all designated WT], and 5 alpha1,3-galactosyltransferase gene-knockout [GT-KO] housed under high herd health conditions) to human (n = 6) and baboon (n = 6) peripheral blood mononuclear cells (PBMC) were determined. Assays included flow cytometry, complement-dependent cytotoxicity, and mixed lymphocyte reaction.

RESULTS

Anti-primate cytotoxic IgM antibodies were detected in the sera of all pigs, but anti-primate IgG antibodies were minimal. All pigs demonstrated a cellular proliferative response to primate PBMC that was equivalent to, or greater than, the allo response. The strength of the pig-to-primate GVH responses was proportional to the health status of the pigs, those from a high health status herd, particularly from a specific pathogen-free herd maintained under clean husbandry conditions, where colonization of the gastrointestinal tract may be reduced, having lower responses.

CONCLUSIONS

After pig organ transplantation in a primate, if the organ is from an early-weaned, early-segregated GT-KO pig, the strength of a GVH response is likely to be relatively weak. Although not investigated here, any GVH response is likely to be suppressed by the immunosuppressive therapy administered to the recipient to suppress the anti-donor immune response.

Molecular interactions between porcine and human gammaherpesviruses: implications for xenografts?

BACKGROUND

Reactivation of latent herpesviruses is an important cause of morbidity and mortality in human transplantation. This issue might be further complicated in the case of xenotransplantation. Zoonotic viruses could reactivate and replicate in the transplanted tissue, and interactions with homologous human viruses could take place. Since the pig is a favoured animal as donor of organs for human transplants, we analysed the possibility of interactions between porcine and human herpesviruses. Porcine lymphotropic herpesvirus 1 (PLHV-1) is a gammaherpesvirus homologous to Epstein-Barr virus (EBV) and to human herpesvirus 8 (HHV-8), is highly prevalent in pigs and is associated to lymphoproliferative disease in immunosuppressed and transplanted miniature swine.

METHODS

The main viral transactivators of PLHV-1, ORF50, ORF57, ORFA6/BZLF1(h), were cloned and tested for their transactivating ability on several EBV and HHV-8 promoters using reporter assays. Also the effects of HHV-8 ORF50, ORF57 and ORFK8 and EBV BRLF1/ R-transactivator (Rta) and BZLF1/ Z-transactivator (Zta) on PLHV-1 lytic promoters were analysed.

RESULTS

Porcine lymphotropic herpesvirus 1 ORF50 upregulated all HHV-8 promoters and PLHV-1 ORFA6/BZLF1(h) transactivated EBV promoters. Furthermore, transfection of PLHV-1 ORF50 into BC-3 cells, latently infected with HHV-8, resulted in HHV-8 reactivation. Likewise, HHV-8 ORF50 and EBV BRLF1/Rta had a strong transactivating effect on PLHV-1 promoters. Also EBV BZLF1/Zta and HHV-8 ORF57 induced PLHV-1 transactivation, but at lower levels.

CONCLUSION

The results suggest that reciprocal molecular interactions between human and porcine herpesviruses might occur in vivo, and support the hypothesis that PLHV-1 might have pathogenic relevance in the course of xenotransplantation.

Infection of pigs in Ireland with lymphotropic gamma-herpesviruses and relationship to postweaning multisystemic wasting syndrome

Three species of porcine lymphotropic herpesviruses (PLHVs) have been described but there are few reports on the distribution and prevalence of these viruses in domestic pigs. We aimed to determine the PLHV status of Irish commercial pig herds, and to this end spleens taken from 110 healthy adult pigs sourced from 22 geographically distributed farms in Ireland were analysed for PLHV DNA using novel species-specific polymerase chain reaction assays. We now report that PLHV infection is widespread in the Irish domestic pig population and that PLHV-1 infections are most common (74% of all animals tested), followed by PLHV-3 and PLHV-2 (45% and 21%, respectively) and that infections with multiple PLHV species were frequently detected. As the PLHVs are lymphotrophic agents, we also investigated if co-infection with PLHVs was linked to the development of porcine circovirus-2 (PCV2)-associated postweaning mutlisystemic wasting syndrome (PMWS), a disease characterised in part by histopathological lesions in lymphoid tissues. We examined the PLHV infection status of young animals on two farms that were experiencing outbreaks of PMWS. Overall the findings are further evidence of the widespread prevalence of PLHVs in domestic pigs and are a first indication that co-infection with PCV2 and PLHVs does not lead to the development of PMWS in the absence of other cofactors.