PORCINE ENDOGENOUS RETROVIRUSES AND THE SAFETY OF XENOTRANSPLANTATION

Abstract

469 Xenotransplantation represents a possible solution to the current shortage of donor organs for human transplantation. However, concerns have been raised about the safety of this potential therapy, especially the possibility of cross species transmission of porcine retroviruses. In order to address this issue we have performed a series of in vitro experiments in which a pig kidney cell line (PK-15) that spontaneously releases porcine endogenous retroviruses, has been co-cultured with a variety of human cell lines. The results from these experiments show that the human B lymphoblastoid (Raji) cell line is susceptible to infection by porcine retroviruses. In light of this in vitro data, we have also characterised the expression pattern of the porcine retroviruses in vivo. Northern blot analysis shows that retroviral mRNA transcripts are produced in the heart, lung, liver, spleen and kidneys of both transgenic and normal pigs. The level of mRNA expression was found to differ both between tissues and between pigs. However, despite the presence of retroviral transcripts, analysis by transmission electron microscopy of freshly explanted bone marrow, heart, spleen and lymph node samples from a transgenic pig did not detect viral release from these tissues in vivo. In addition to the above experiments, and to further assess the safety of xenotransplantation using a particularly relevant model, we have established a series of assays to analyse tissues from immunosuppressed non human primates that have been transplanted with hearts or kidneys from transgenic pigs expressing the human complement regulator, decay accelerating factor. The primate tissues are being tested by PCR, using primers specific for the pol gene, for evidence of infection by the retrovirus. In addition, tissues from the primates are being tested by RT-PCR, using env specific primers, for evidence of expression and by western blot for evidence of antibodies against the porcine retroviruses. The results from these experiments will allow a preliminary evaluation to be made of the risk associated with the xenotransplantation of pig organs into immunosuppressed non human primates.