Xenotransplantation and porcine cytomegalovirus

Strategies of Bladder Reconstruction after Partial or Radical Cystectomy for Bladder Cancer

The standard strategy is to reconstruct bladder by use of bowel segments as material in bladder cancer with radical cystectomy clinically. Both natural derived and non natural derived materials are investigated in bladder reconstruction. Studies on mechanical bladder, bladder transplantation and bladder xenotransplantation are currently limited although heart and kidney transplantation or xenotransplantation are successful to a certain extent, and bone prostheses are applied in clinical contexts. Earlier limited number of studies associated with bladder xenograft from animals to humans were not particular promising in results. Although there have been investigations on pig to human cardiac xenotransplantation with CRISPR Cas9 gene editing, the CRISPR Cas technique is not yet widely researched in porcine bladder related gene editing for the potential of human bladder replacement for bladder cancer. The advancement of technologies such as gene editing, bioprinting and induced pluripotent stem cells allow further research into partial or whole bladder replacement strategies. Porcine bladder is suggested as a potential source material for bladder reconstruction due to its alikeness to human bladder. Challenges that exist with all these approaches need to be overcome. This paper aims to review gene editing technology such as the CRISPR Cas systems as tools in bladder reconstruction, bladder xenotransplantation and hybrid bladder with technologies of induced pluripotent stem cells and genome editing, bioprinting for bladder replacement for bladder reconstruction and to restore normal bladder control function after cystectomy for bladder cancer.

Monitoring Porcine Cytomegalovirus in Both Donors and Recipients is Crucial for Recipient's Survival in Pig-to-Cynomolgus Xenotransplantation

BACKGROUND

Xenotransplantation, particularly when involving pig donors, presents challenges related to the transmission of porcine cytomegalovirus (pCMV) and its potential impact on recipient outcomes. This study aimed to investigate the relationship between pCMV positivity in both donors and recipients and the survival time of cynomolgus monkey recipients after xenogeneic kidney transplantation.

METHODS

We conducted 20 cynomolgus xenotransplants using 18 transgenic pigs. On the surgery day, donor pig blood was sampled, and DNA was extracted from serum and peripheral blood mononuclear cells. Recipient DNA extraction followed the same protocol from pre-transplantation to post-transplantation. Porcine cytomegalovirus detection used real-time polymerase chain reaction (real-time PCR) with the ViroReal kit, achieving a sensitivity of 50 copies/reaction. A Ct value of 37.0 was the pCMV positivity threshold.

RESULTS

Of 20 cynomolgus recipients, when donors tested negative for pCMV, recipients also showed negative results in 9 cases. In 4 cases where donors were negative, recipients tested positive. All 5 cases with pCMV-positive donors resulted in positive assessments for recipients. Detection of donor pCMV correlated with shorter recipient survival. Continuous recipient positivity during observation correlated with shorter survival, whereas transient detection showed no significant change in survival rates. However, donor pig phenotypes and transplantation protocols did not significantly impact survival.

CONCLUSION

The detection of pCMV in both donors and recipients plays a crucial role in xenotransplantation outcomes. These findings suggest the importance of monitoring and managing pCMV in xenotransplantation to enhance long-term outcomes.

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.

American Society of Transplant Surgeons-American Society of Transplantation report of FDA meeting on regulatory expectations for xenotransplantation products

In June 2022, the US Food and Drug Administration Center for Biologics Evaluation and Research held the 73rd meeting of the Cellular, Tissue, and Gene Therapies Advisory Committee for public discussion of regulatory expectations for xenotransplantation products. The members of a joint American Society of Transplant Surgeons/American Society of Transplantation committee on xenotransplantation compiled a meeting summary focusing on 7 topics believed to be key by the committee: (1) preclinical evidence supporting progression to a clinical trial, (2) porcine kidney function, (3) ethical aspects, (4) design of initial clinical trials, (5) infectious disease issues, (6) industry perspectives, and (7) regulatory oversight.

First virological and pathological study of Göttingen Minipigs with Dippity Pig Syndrome (DPS)

Dippity Pig Syndrome (DPS) is a well-known but rare complex of clinical signs affecting minipigs, which has not been thoroughly investigated yet. Clinically affected animals show acute appearance of red, exudating lesions across the spine. The lesions are painful, evidenced by arching of the back (dipping), and the onset of clinical signs is generally sudden. In order to understand the pathogenesis, histological and virological investigations were performed in affected and unaffected Göttingen Minipigs (GöMPs). The following DNA viruses were screened for using PCR-based methods: Porcine cytomegalovirus (PCMV), which is a porcine roseolovirus (PCMV/PRV), porcine lymphotropic herpesviruses (PLHV-1, PLHV-2, PLHV-3), porcine circoviruses (PCV1, PCV2, PCV3, PCV4), porcine parvovirus 1 (PPV1), and Torque Teno sus viruses (TTSuV1, TTSuV2). Screening was also performed for integrated porcine endogenous retroviruses (PERV-A, PERV-B, PERV-C) and recombinant PERV-A/C and their expression as well as for the RNA viruses hepatitis E virus (HEV) and SARS-CoV-2. Eight clinically affected and one unaffected GöMPs were analyzed. Additional unaffected minipigs had been analyzed in the past. The analyzed GöMPs contained PERV-A and PERV-B integrated in the genome, which are present in all pigs and PERV-C, which is present in most, but not all pigs. In one affected GöMPs recombinant PERV-A/C was detected in blood. In this animal a very high expression of PERV mRNA was observed. PCMV/PRV was found in three affected animals, PCV1 was found in three animals with DPS and in the unaffected minipig, and PCV3 was detected in two animals with DPS and in the unaffected minipig. Most importantly, in one animal only PLHV-3 was detected. It was found in the affected and unaffected skin, and in other organs. Unfortunately, PLHV-3 could not be studied in all other affected minipigs. None of the other viruses were detected and using electron microscopy, no virus particles were found in the affected skin. No porcine virus RNA with exception of PERV and astrovirus RNA were detected in the affected skin by next generation sequencing. This data identified some virus infections in GöMPs with DPS and assign a special role to PLHV-3. Since PCMV/PRV, PCV1, PCV3 and PLHV-3 were also found in unaffected animals, a multifactorial cause of DPS is suggested. However, elimination of the viruses from GöMPs may prevent DPS.

Detection of porcine cytomegalovirus, a roseolovirus, in pig ovaries and follicular fluid: implications for somatic cells nuclear transfer, cloning and xenotransplantation

BACKGROUND

Porcine cytomegalovirus (PCMV) is a porcine roseolovirus (PCMV/PRV) which is widely distributed in pigs. Transmission of PCMV/PRV in preclinical xenotransplantations was shown to significantly reduce the survival time of the pig transplants in non-human primates. PCMV/PRV was also transmitted in the first transplantation of a pig heart into a human patient. To analyze how PCMV/PRV could be introduced into pig breeds, especially considering cloned transgenic pigs, and subsequently spread in breeding facilities, we screened ovaries and derived materials which are used to perform somatic cell nuclear transfer (SCNT).

METHODS

DNA was isolated from ovarian tissues, follicular fluids, oocytes with cumulus cells, denuded oocytes and parthenotes. A real-time PCR with PCMV/PRV-specific primers and a probe was performed to detect PCMV/PRV. Furthermore, a Western blot assay using a recombinant fragment of the gB protein of PCMV/PRV was performed to screen for virus-specific antibodies in the follicular fluids.

RESULTS

PCMV/PRV was found by real-time PCR in ovarian tissues, in the follicular fluid and in oocytes. In parthenotes the virus could not be detected, most-likely due to the low amount of DNA used. By Western blot assay specific antibodies against PCMV/PRV were found in 19 of 20 analyzed follicular fluids.

CONCLUSION

PCMV/PRV was found in ovarian tissues, in the follicular fluids and also in denuded oocytes, indicating that the virus is present in the animals of which the oocytes were taken from. Despite several washing steps of the denuded oocytes, which are subsequently used for microinjection or SCNT, the virus could still be detected. Therefore, the virus could infect oocytes during genetic modifications or stay attached to the surface of the oocytes, potentially infecting SCNT recipient animals.

Presence of porcine cytomegalovirus, a porcine roseolovirus, in wild boars in Italy and Germany

Porcine cytomegalovirus (PCMV), a porcine roseolovirus (PRV) that is closely related to human herpesviruses 6 and 7, is commonly found in commercial pigs. PCMV/PRV is important in xenotransplantation, because in preclinical trials in which pig organs were transplanted into non-human primates, transmission of PCMV/PRV was shown to be associated with significantly reduced survival of the xenotransplants. PCMV/PRV was also transmitted in the first transplantation of a pig heart into a human patient worldwide and apparently contributed to the death of the patient. The prevalence of PCMV/PRV in wild boars is largely unknown. In this study, we screened wild boars from several areas of northern Italy and Germany to test for the presence of PCMV/PRV using PCR-based and Western blot assays. By Western blot analysis, 54% and 82% of Italian and German wild boars, respectively, were found to be PCMV/PRV positive, while 36% and 60%, respectively, tested positive by real-time polymerase chain reaction (PCR). These data indicate that the virus is common in German and Italian wild boars and that the Western blot assay detected a PCMV/PRV infection more often than did real-time PCR. The data also indicate that pigs raised for xenotransplantation should be protected from contact with materials from wild boars and commercial pigs.

How, where and when to screen for porcine cytomegalovirus (PCMV) in donor pigs for xenotransplantation

Porcine cytomegalovirus (PCMV), that is actually a porcine roseolovirus (PRV), is a common herpesvirus in domestic pigs and wild boars. In xenotransplantation, PCMV/PRV has been shown to significantly reduce the survival time of pig kidneys and hearts in preclinical trials with different non-human primates. Furthermore, PCMV/PRV has been transmitted in the first pig to human heart xenotransplantation and contributed to the death of the patient. Although transmitted to the recipient, there is no evidence that PCMV/PRV can infect primate cells including human cells. PCMV/PRV is closely related to the human herpesviruses 6 and 7, and only distantly related to the human CMV (HCMV). Antiviral drugs used for the treatment of HCMV are less effective against PCMV/PRV. However, there are well described strategies to eliminate the virus from pig facilities. In order to detect the virus and to eliminate it, highly sensitive detection methods and the knowledge of how, where and when to screen the donor pigs is required. Here, a comparative testing of organs from pigs of different ages using polymerase chain reaction (PCR)-based and immunological methods was performed. Testing young piglets, PCMV/PRV was detected effectively by PCR in blood, bronchoalveolar lavage fluid, tonsils and heart. In adult animals, detection by PCR was not successful in most cases, because the virus load was below the detection limit or the virus was in its latent stage. Therefore, detection of antibodies against selected recombinant proteins corresponding to epitopes detected by nearly all infected animals in a Western blot assay is advantageous. By contrast, immunological testing is not beneficial in young animals as piglets might have PCMV/PRV-specific antibodies obtained from their infected mother via the colostrum. Using a thoughtful combination of PCR-based and immunological methods, detection of PCMV/PRV in donor pigs for xenotransplantation is feasible and a controlled elimination of the virus by early weaning or other methods is possible.

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.

A Review of Murine Cytomegalovirus as a Model for Human Cytomegalovirus Disease-Do Mice Lie?

Since murine cytomegalovirus (MCMV) was first described in 1954, it has been used to model human cytomegalovirus (HCMV) diseases. MCMV is a natural pathogen of mice that is present in wild mice populations and has been associated with diseases such as myocarditis. The species-specific nature of HCMV restricts most research to cell culture-based studies or to the investigation of non-invasive clinical samples, which may not be ideal for the study of disseminated disease. Initial MCMV research used a salivary gland-propagated virus administered via different routes of inoculation into a variety of mouse strains. This revealed that the genetic background of the laboratory mice affected the severity of disease and altered the extent of subsequent pathology. The advent of genetically modified mice and viruses has allowed new aspects of disease to be modeled and the opportunistic nature of HCMV infection to be confirmed. This review describes the different ways that MCMV has been used to model HCMV diseases and explores the continuing difficulty faced by researchers attempting to model HCMV congenital cytomegalovirus disease using the mouse model.

Impact of porcine cytomegalovirus on long-term orthotopic cardiac xenotransplant survival

Xenotransplantation using pig organs has achieved survival times up to 195 days in pig orthotopic heart transplantation into baboons. Here we demonstrate that in addition to an improved immunosuppressive regimen, non-ischaemic preservation with continuous perfusion and control of post-transplantation growth of the transplant, prevention of transmission of the porcine cytomegalovirus (PCMV) plays an important role in achieving long survival times. For the first time we demonstrate that PCMV transmission in orthotopic pig heart xenotransplantation was associated with a reduced survival time of the transplant and increased levels of IL-6 and TNFα were found in the transplanted baboon. Furthermore, high levels of tPA-PAI-1 complexes were found, suggesting a complete loss of the pro-fibrinolytic properties of the endothelial cells. These data show that PCMV has an important impact on transplant survival and call for elimination of PCMV from donor pigs.

Epidemiology of bacteria and viruses in the respiratory tract of humans and domestic pigs

Bacteria and viruses were analysed in the upper respiratory tract of symptomatic pig farmers and their domestic pigs. Eighty six human nasal and 495 (50 pools) porcine snout swabs were collected in Schleswig-Holstein, Germany. Staphylococcus (S.) aureus (62.8%, 54/86), human rhino- and coronaviruses (HRV, 29.1%, 25/86; HCoV, 16.3%, 14/86) were frequently detected in humans, while Haemophilus parasuis (90.0%, 45/50), Mycoplasma hyorhinis (78.6%, 11/14), Enterovirus G (EV-G, 56.0%, 28/50) and S. aureus (36.0%, 18/50), respectively, were highly prevalent in pigs. The detection of S. aureus in human follow-up samples indicates a carrier status. The methicillin-resistant phenotype (MRSA) was identified in 33.3% (18/54) of nasal swabs and in one of 18 (5.6%) pooled snout swabs that were tested positive for S. aureus. Strains were indicative of the livestock-associated clonal complex CC398, with t011 being the most common staphylococcal protein A type. Enterobacterales and non-fermenters were frequently isolated from swabs. Their detection in follow-up samples suggests a carrier status. All were classified as being non-multiresistant. There was no example for cross-species transmission of viruses. In contrast, transmission of S. aureus through occupational contact to pigs seems possible. The study contributes to the 'One Health' approach.

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.

Development of a TaqMan Based Real-Time Fluorescent Quantitative PCR Assay for Detection of Porcine Cytomegalovirus in Semen

This study described a TaqMan based real-time fluorescent quantitative PCR (qPCR) method to detect porcine cytomegalovirus (PCMV) infection, targeting the conserved region of the DNA polymerase (DPOL) gene. The standard curve showed a linear regression relationship with a coefficient of 0.999 and a slope of y = -3.249x + 38.958 corresponding to the amplification efficiency at 99.8%. The limit of the qPCR method was 51.9 copies/μl. The established qPCR method showed excellent specificity, with no cross-reaction observed with common porcine pathogens. The coefficient of variation for intra-assay and interassay variability ranged up to 1.51% and 2.24%, respectively. PCMV positive signals can be found in semen using this qPCR method, which suggested that we should pay more attention to PCMV contamination in semen in order to eliminate PCMV infection in artificial insemination and xenotransplantation.

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.

Early weaning completely eliminates porcine cytomegalovirus from a newly established pig donor facility for xenotransplantation

For clinical xenotransplantation, transplants must be free of porcine cytomegalovirus (PCMV). Piglets become infected primarily in the perinatal period by the mother sow. While individual donor animals can be protected from infection by isolation husbandry, success is not guaranteed and this strategy poses the risk of undetected infections and raises animal welfare questions. Here, we present the establishment of a completely PCMV-negative pig herd for breeding donor animals for xenotransplantation. Eleven pregnant DanAvl Basic hybrid sows were purchased from a designated pathogen-free (DPF), PCMV-positive colony and transferred to a new pig facility at the Centre for Innovative Medical Models (CiMM) 4 weeks prior to farrowing. At the age of 24 hours, piglets were early-weaned and transferred to a commercially available Rescue Deck system dedicated to motherless rearing of piglets. Sows were removed from the facility. The PCMV status of F1-generation animals was determined at regular intervals over a period of 14 months by a sensitive real-time PCR-based detection method testing blood, nasal swabs and cultured peripheral blood mononuclear cells (PBMCs). F1 sows were used as recipients of genetically modified embryos to generate a xenotransplant donor herd. Offspring were tested for PCMV accordingly. All offspring have remained PCMV negative over the whole observation period of 14 months. A completely PCMV-negative pig herd for xenotransplantation has thus been successfully established.

Xenotransplantation panel for the detection of infectious agents in pigs

Background

Recent advances in xenotransplantation have produced organs from pigs that are well tolerated in primate models because of genetic changes engineered to delete major antigens from donor animals. To ensure the safety of human transplant recipients, it will be essential to understand both the spectrum of infectious agents in donor pigs and their potential to be transmitted to immunocompromised transplant recipients. Equally important will be the development of new highly sensitive diagnostic methods for use in the detection of these agents in donor animals and for the monitoring of transplant recipients.

Methods

Herein, we report the development of a panel of 30 quantitative polymerase chain reaction (qPCR) assays for infectious agents with the potential to be transmitted to the human host. The reproducibility, sensitivity and specificity of each assay were evaluated and were found to exhibit analytic sensitivity that was similar to that of quantitative assays used to perform viral load testing of human viruses in clinical laboratories.

Results

This analytical approach was used to detect nucleic acids of infectious agents present in specimens from 9 sows and 22 piglets derived by caesarean section. The most commonly detected targets in adult animals were Mycoplasma species and two distinct herpesviruses, porcine lymphotrophic herpesvirus 2 and 3. A total of 14 piglets were derived from three sows infected with either or both herpesviruses, yet none tested positive for the viruses indicating that vertical transmission of these viruses is inefficient.

Conclusions

The data presented demonstrate that procedures in place are highly sensitive and can specifically detect nucleic acids from target organisms in the panel, thus ensuring the safety of organs for transplantation as well as the monitoring of patients potentially receiving them.

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.

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.

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.

Monocyte derived macrophages as an appropriate model for porcine cytomegalovirus immunobiology studies

Porcine cytomegalovirus (PCMV) causes lifelong latent infections in swine. The pathogen is occasionally associated with inclusion body rhinitis and pneumonia in piglets, reproductive disorders in pregnant sows and respiratory disease complex in older pigs. Immunosuppressive potential of PCMV infection is discussed. Macrophages were recognised as one of target cell types where propagation of virus occurs. The aim of present study was to set up model PCMV infection of monocyte derived macrophages (MDMs) in vitro for PCMV immunobiology research. Obtained results showed that PCMV is able to infect and propagate in MDMs. Possible immunosuppressive effect of PCMV on infected macrophages was evaluated by measurement of immune relevant gene expression in MDMs. Infection decreased expression of IL-8 and TNF-α (pro-inflammatory cytokines) and increased expression of IL-10 (anti-inflammatory cytokine) on mRNA transcription level. Obtained data support hypothesis that higher sensitivity of animals to coinfection with other swine pathogens and its more severe clinical manifestations could potentially be the consequence of PCMV infection.

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.

Xenotransplantation: back to the future?

The field of xenotransplantation has fluctuated between great optimism and doubts over the last 50 years. The initial clinical attempts were extremely ambitious but faced technical and ethical issues that prompted the research community to go back to preclinical studies. Important players left the field due to perceived xenozoonotic risks and the lack of progress in pig-to-nonhuman-primate transplant models. Initial apparently unsurmountable issues appear now to be possible to overcome due to progress of genetic engineering, allowing the generation of multiple-xenoantigen knockout pigs that express human transgenes and the genomewide inactivation of porcine endogenous retroviruses. These important steps forward were made possible by new genome editing technologies, such as CRISPR/Cas9, allowing researchers to precisely remove or insert genes anywhere in the genome. An additional emerging perspective is the possibility of growing humanized organs in pigs using blastocyst complementation. This article summarizes the current advances in xenotransplantation research in nonhuman primates, and it describes the newly developed genome editing technology tools and interspecific organ generation.

Antibody Cross-Reactivity between Porcine Cytomegalovirus (PCMV) and Human Herpesvirus-6 (HHV-6)

Porcine cytomegalovirus (PCMV) infection is widely prevalent among pigs, and PCMV is one of the viruses which may be transmitted during xenotransplantation using pig cells, tissues, or organs. While human cytomegalovirus (HCMV) is a major risk factor for allotransplantation, it is still unclear whether PCMV is able to infect human cells or pose a risk for xenotransplantation. Previously, it was shown that transmission of PCMV after pig kidney to non-human primate transplantations resulted in a significantly reduced survival time of the transplanted organ. To detect PCMV, PCR-based and immunological methods were used. Screening of pigs by Western blot analyses using recombinant viral proteins revealed up to 100% of the tested animals to be infected. When the same method was applied to screen human sera for PCMV-reactive antibodies, positive Western blot results were obtained in butchers and workers in the meat industry as well as in normal blood donors. To exclude an infection of humans with PCMV, the sera were further investigated. PCMV is closely related to human herpesvirus-6 (HHV-6) and human herpesvirus-7 (HHV-7), and a sequence alignment of glycoprotein B suggests that the antibodies may cross-react with identical epitope sequences. HCMV is not related with PCMV, and no correlation between antibody reactivity against PCMV and HCMV was detected. These data indicate that antibodies against PCMV found in humans are cross-reactive antibodies against HHV-6.

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.

Porcine Circoviruses and Xenotransplantation

Allotransplantation and xenotransplantation may be associated with the transmission of pathogens from the donor to the recipient. Whereas in the case of allotransplantation the transmitted microorganisms and their pathogenic effect are well characterized, the possible influence of porcine microorganisms on humans is mostly unknown. Porcine circoviruses (PCVs) are common in pig breeds and they belong to porcine microorganisms that still have not been fully addressed in terms of evaluating the potential risk of xenotransplantation using pig cells, tissues, and organs. Two types of PCVs are known: porcine circovirus (PCV) 1 and PCV2. Whereas PCV1 is apathogenic in pigs, PCV2 may induce severe pig diseases. Although most pigs are subclinically infected, we do not know whether this infection impairs pig transplant functionality, particularly because PCV2 is immunosuppressive. In addition, vaccination against PCV2 is able to prevent diseases, but in most cases not transmission of the virus. Therefore, PCV2 has to be eliminated to obtain xenotransplants from uninfected healthy animals. Although there is evidence that PCV2 does not infect—at least immunocompetent—humans, animals should be screened using sensitive methods to ensure virus elimination by selection, Cesarean delivery, vaccination, or embryo transfer.

Xenotransplantation - A special case of One Health

The chronic shortage of human transplants to treat tissue and organ failure has led to the development of xenotransplantation, the transplantation of cells, tissues and organs from another species to human recipients. For a number of reasons, pigs are best suited as donor animals. Successful, routine xenotransplantation would have an enormous impact on the health of the human population, including the young, who sometimes require a replacement organ or islet cells, but especially the elderly, who more often suffer the consequences of organ failure. The first form of xenotransplantation applied to humans is the use of pig islet cells to treat insulin-dependent diabetes, a procedure that will have a significant economic impact. However, although xenotransplantation using pig cells, tissues and organs may save and prolong the lives of patients, it may also be associated with the transmission of porcine microorganisms to the recipient, eventually resulting in emerging infectious diseases. For this reason, the health of both the donor animals and the human recipients represents a special and sensitive case of the One Health concept. Basic research leading to strategies how to prevent transmission of porcine microorganisms by selection of virus-free animals, treatment of donor pigs by antiviral drugs, vaccines, colostrum deprivation, early weaning, Caesarean delivery, embryo transfer and/or gene editing should be undertaken to supply an increasing number of potential recipients with urgently required transplants. The methods developed for the detection and elimination of porcine microorganisms in the context of xenotransplantation will also contribute to an improvement in the health of pig populations in general and an increase in the quality of meat products. At present, there is evidence for transmission of porcine viruses to humans eating pork and having contact with pigs, however the impact of these viruses on public health is still unknown.

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.

A new Western blot assay for the detection of porcine cytomegalovirus (PCMV)

Porcine cytomegalovirus (PCMV) may be harmful for human recipients if xenotransplantation using pig cell, tissue or organ will be performed transmitting the virus from donor pigs to human recipients. PCMV is widespread in pigs and closely related to human pathogenic herpesviruses, however there are no data concerning infection of humans. In contrast, recently it had been shown that transplantation of organs from pigs infected with PCMV into non-human primate recipients resulted in a significant reduction of the survival time compared with the transplantation of organs from uninfected pigs. To prevent transmission of PCMV in future pig to human xenotransplantations, sensitive and specific detection methods should be used. Here a new Western blot assay using recombinant proteins corresponding to two domains of the glycoprotein gB of PCMV is described. With this assay, the presence of PCMV-specific antibodies in different pig breeds was analysed. Antibodies were detected in a high percentage of animals, in one breed up to 85%.

Islet cell transplantation from Göttingen minipigs to cynomolgus monkeys: analysis of virus safety

BACKGROUND

Xenotransplantation using pig cells, tissues or organs may be associated with the transmission of porcine zoonotic micro-organisms. Hepatitis E virus (HEV), porcine cytomegalovirus (PCMV) and porcine endogenous retroviruses (PERVs) are potentially zoonotic micro-organisms which do not show clinical symptoms in pigs and which are due to the low expression level difficult to detect. Göttingen Minipigs (GöMP) are often used for biomedical investigations and they are well characterized concerning the presence of numerous bacteria, fungi, viruses and parasites and therefore may be used for islet cell transplantation.

METHODS

Islet cells derived from three GöMP were transplanted into four healthy, non-diabetic cynomolgus monkeys using a macroencapsulation device. PCR, nested PCR, real-time PCR, real-time RT-PCR and Western blot analyses were used to estimate the presence of PERV, PCMV and HEV in the donors and recipients.

RESULTS

Using sensitive detection methods, no HEV was found in the donor pigs and in the pig islet cell preparations. Antibodies against PERV, PCMV and HEV were not found in all cynomolgus monkeys with exception of one monkey showing an immune response against HEV. Using real-time PCR, no PCMV and HEV were found in the sera of all monkeys.

CONCLUSION

Although the donor islet cells and the recipients were negative for HEV using PCR and Western blot analysis, in one recipient, antibodies against HEV were found, indicating infection in a single case. All recipients were negative for antibodies against PERV, and all were negative for PCMV, indicating absence of infection. As HEV was not detected in the donor pig before transplantation, a more complex and regular screening of the animals using highly sensitive methods is required to avoid virus transmission.

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.

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.

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.