Lack of evidence for PERV expression after apoptosis‐mediated horizontal gene transfer between porcine and human cells

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Abstract
Evidence for porcine endogenous retrovirus (PERV) infection of human cells has provoked a public health debate over the proposed use of porcine xenografts to alleviate the worldwide shortage of human allografts. Nevertheless, the potential relevance of PERV transmission by apoptosis-mediated horizontal DNA transfer, a documented means of infection-independent retrovirus delivery, appears to have been overlooked in this discussion. To examine the hypothesis that apoptotic cell death during porcine xenograft rejection is capable of fostering horizontal DNA transfer, we have now assessed in vitro cocultures, consisting of phagocytic human fibroblasts and apoptotic or necrotic porcine B-lymphoblastoid cells, for evidence of cross-species PERV exchange and eventual replication. Using real-time polymerase chain reaction (PCR) assays, designed to differentiate nuclear and cytoplasmic DNA derived from either porcine or human cells, we now report evidence for the presence of porcine DNA, including PERV, in the nucleus of human fibroblasts exposed to apoptotic porcine cells. This novel demonstration of apoptosis-mediated horizontal PERV transfer is characterized by a low efficiency of transfer and a transient nature, being present in only 0.22% of the cocultured human cells and disappearing to undetectable levels within 4 weeks of exposure to apoptotic porcine cells. In contrast, using PERV-specific real-time reverse-transcriptase PCR (RT-PCR) and ultra-sensitive product-enhanced reverse transcriptase (PERT) assays, we find no evidence for human fibroblast-derived cellular PERV RNA or coculture supernatant-based RT-activity, indicating a lack of subsequent PERV replication. Together, these results suggest that apoptosis-mediated horizontal PERV transfer does not present an overt hazard within the framework of porcine xenotransplantation. However, we also present arguments against extrapolation of these in vitro observations directly to clinical circumstances