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Porcine cytomegalovirus/porcine roseolovirus (PCMV/PRV), a porcine herpesvirus, has been shown to significantly reduce the survival time of porcine xenotransplants in non-human primates. The virus was detected in all the examined organs of baboons transplanted with PCMV/PRV-positive organs and it was also transmitted to the first human recipient of a pig heart, contributing to the patient’s death. PCMV/PRV induces consumptive coagulopathy and thrombocytopenia in xenotransplant recipients. Initial studies in baboons revealed that the virus triggered increased release of tumor necrosis factor α (TNFα) and interleukin 6 (IL-6), along with elevated levels of tissue plasminogen activator (tPA) and plasminogen activator inhibitor 1 (PAI-1) complexes. Since there is no evidence that PCMV/PRV infects primate cells, including human cells, the virus appears to directly interact with immune and endothelial cells, disrupting cytokine signaling and coagulation pathways. The highest viral load was detected in the explanted pig heart, suggesting active replication at this site. Additionally, cells expressing PCMV/PRV proteins were identified in all the examined baboon organs, where pig cells were also found. Since PCMV/PRV affects only xenotransplant recipients and not healthy humans, this condition should be classified as a xenozoonosis. Interestingly, antibodies against human herpesvirus 6 (HHV-6) cross-react with PCMV/PRV and may contribute to protection against infection in humans. Further research is needed to uncover the molecular mechanisms underlying this xenozoonotic disease.

Porcine cytomegalovirus, more accurately classified as porcine roseolovirus (PCMV/PRV), was shown to be pathogenic in the context of xenotransplantation. Transmission of PCMV/PRV to non-human primates receiving hearts or kidneys from virus-positive pigs significantly reduced the survival time of the recipients. PCMV/PRV was also transmitted to the first human recipient of a pig heart transplant and contributed to the patient’s death. Although PCMV/PRV is highly prevalent in all pig breeds and wild boars, including slaughterhouse pigs, no infections or diseases have been reported in healthy, ill, or immunocompromised humans, suggesting that this virus is not zoonotic and should therefore be classified as xenozoonotic. This indicates that this virus is not zoonotic and must be classified as xenozoonotic. Moreover, it remains unclear whether PCMV/PRV is capable of infecting human cells in vitro. To address this question, human 293T cells resistant to hygromycin were co-cultured with porcine fallopian tube (PFT) cells producing PCMV/PRV. After hygromycin selection, the remaining human cells showed no evidence of infection. Because herpesviruses are generally considered to be species-specific—a notion that has been shown to be not entirely correct—it was also investigated whether PCMV/PRV can infect mouse cells using the same approach. Similarly, no infection was observed. Since the target cells employed in both assays had a reduced capacity to resist viral infection, the findings strongly suggest that PCMV/PRV is unable to infect human or mouse cells, which are equipped with functional antiviral mechanisms. This is supported by findings from the patient who received the first pig heart transplantation.

Porcine lymphotropic herpesviruses -1, -2, and -3 (PLHV-1, PLHV-2, and PLHV-3) are gammaherpesviruses that are widespread in pigs. These viruses are closely related to the human pathogens Epstein–Barr virus (EBV) and Kaposi sarcoma-associated herpesvirus (KSHV), both of which are known to cause severe diseases in humans. To date, however, no definitive association has been established between PLHVs and any disease in pigs. With the growing interest in xenotransplantation as a means to address the shortage of human organs for transplantation, the safety of using pig-derived cells, tissues, and organs is under intense investigation. In preclinical trials involving pig-to-nonhuman primate xenotransplantation, another porcine herpesvirus—porcine cytomegalovirus, a porcine roseolovirus (PCMV/PRV)—was shown to be transmissible and significantly reduced the survival time of the xenotransplants. In the present study, we examined donor pigs and their respective baboon recipients, all of which were part of preclinical pig heart xenotransplantation studies, for the presence of PLHV. PLHV-1, PLHV-2, and PLHV-3 were detected in nearly all donor pigs; however, no evidence of PLHV transmission to the baboon recipients was observed.

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