Porcine endogenous retroviruses in xenotransplantation

Xenotransplantation using pig cells, tissues or organs is under development to alleviate the shortage of human donor organs. Meanwhile, remarkably long survival times of pig organs in non-human primates have been reported, as well as the functionality of pig kidneys and hearts in brain-dead humans. Most importantly, two transplantations of pig hearts in patients were performed with survival times of the patients of 8 and 6 weeks. Xenotransplantation may be associated with the transmission of porcine microorganisms including viruses to the recipient. Porcine endogenous retroviruses (PERVs) are integrated in the genome of all pigs and cannot be eliminated like other viruses can. PERVs are able to infect certain human cells and therefore pose a risk for xenotransplantation. It is well known that retroviruses are able to induce tumors and immunodeficiencies. However, until now, PERVs were not transmitted in all infection experiments using small animals and non-human primates, in all preclinical xenotransplantation trials in non-human primates and in all clinical trials in humans. In addition, several strategies including antiretrovirals, PERV-specific small interfering RNA, vaccines and genome editing using CRISPR/Cas have been developed to prevent PERV transmission.