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.