Molecular cloning and functional characterization of infectious PERV and development of diagnostic tests

Pigs are the donor animals of choice for xenotransplantation (XTx) and xenogeneic cell therapy measurements. Most known porcine pathogens can be controlled by conventional means like vaccination, medication or specific pathogen-free breeding conditions. As pigs have co-evolved very closely with humans for a few millennia it is not very likely that even asymptomatic pathogens have escaped attention. Porcine endogenous retroviruses (PERV) are different from conventional pathogens as they are chromosomally fixed in every cell of the animal, hence PERV cannot be easily controlled. While PERV show no phenotype in the porcine host, recent data demonstrate that some polytropic proviruses can be activated by external stimuli and that those can productively infect human cells in vitro. In evaluation of the retrovirological safety of XTx, we determined the number of replication-competent PERV to be limited and to exhibit a heterogeneous distribution, therefore suggesting that they could be removed by conventional breeding. The transcriptional regulation of some PERV due to repetitive elements in their long terminal repeats enables their adaptation to new host cells. The diagnostic tools available, based on immunological and polymerase chain reaction techniques, were shown to be sensitive in both the animal and in vitro, but must still show their potential in human XTx recipients, where they are confronted with very low antigen expression and the phenomenon of microchimerism.