Analysis of potential porcine endogenous retrovirus transmission to baboon in vitro and in vivo

Suboptimal porcine endogenous retrovirus infection in non-human primate cells: implication for preclinical xenotransplantation

Background

Porcine endogenous retrovirus (PERV) poses a potential risk of zoonotic infection in xenotransplantation. Preclinical transplantation trials using non-human primates (NHP) as recipients of porcine xenografts present the opportunity to assess the zoonosis risk in vivo. However, PERV poorly infects NHP cells for unclear reasons and therefore NHP may represent a suboptimal animal model to assess the risk of PERV zoonoses. We investigated the mechanism responsible for the low efficiency of PERV-A infection in NHP cells.

Principal Findings

Two steps, cell entry and exit, were inefficient for the replication of high-titer, human-tropic A/C recombinant PERV. A restriction factor, tetherin, is likely to be responsible for the block to matured virion release, supported by the correlation between the levels of inhibition and tetherin expression. In rhesus macaque, cynomolgus macaque and baboon the main receptor for PERV entry, PERV-A receptor 1 (PAR-1), was found to be genetically deficient: PAR-1 genes in these species encode serine at amino acid 109 in place of the leucine in human PAR-1. This genetic defect inevitably impacts in vivo sensitivity to PERV infection of these species. In contrast, African green monkey (AGM) PAR-1 is functional, but PERV infection is still poor. Although the mechanism is unclear, tunicamycin treatment, which removes N-glycosylated sugar chains, increases PERV infection, suggesting a possible role for the glycosylation of the receptors.

Conclusions

Since cynomolgus macaque and baboon, species often used in pig-to-NHP xenotransplantation experiments, have a defective PAR-1, they hardly represent an ideal animal model to assess the risk of PERV transmission in xenotransplantation. Alternatively, NHP species, like AGM, whose both PARs are functional may represent a better model than baboon and cynomolgus macaque for PERV zoonosis in vivo studies.

Xenotransplantation and pig endogenous retroviruses

Xenotransplantation, in particular transplantation of pig cells, tissues and organs into human patients, may alleviate the current shortage of suitable allografts available for human transplantation. This overview addresses the physiological, immunological and virological factors considered with regard to xenotransplantation. Among the issues reviewed are the merits of using pigs as xenograft source species, the compatibility of pig and human organ physiology and the immunological hindrances with regard to the various types of rejection and attempts at abrogating rejection. Advances in the prevention of pig organ rejection by creating genetically modified pigs that are more suited to the human microenvironment are also discussed. Finally, with regard to virology, possible zoonotic infections emanating from pigs are reviewed, with special emphasis on the pig endogenous retrovirus (PERV). An in depth account of PERV studies, comprising their discovery as well as recent knowledge of the virus, is given. To date, all retrospective studies on patients with pig xenografts have shown no evidence of PERV transmission, however, many factors make us interpret these results with caution. Although the lack of PERV infection in xenograft recipients up to now is encouraging, more basic research and controlled animal studies that mimic the pig to human xenotransplantation setting more closely are required for safety assessment.

Porcine endogenous retrovirus infects but does not replicate in nonhuman primate primary cells and cell lines

Porcine endogenous retroviruses (PERV) can infect human cell lines in vitro; hence, there is a presumed risk of viral exposure to a recipient when pig cells are transplanted into humans (xenotransplantation). Nonhuman primates (NHP) are considered a potential permissive animal model to study the risk of in vivo infection of PERV after xenotransplantation. We set out to determine whether PERV can infect and replicate in NHP primary cells or established cell lines from African green monkey, rhesus macaque, and baboon. We confirm that the NHP cell lines under investigation were infected with PERV as measured by detection of viral DNA and RNA by PCR and reverse transcription (RT)-PCR, respectively, indicating that a functional receptor must be present on the cell surface. However, the load of detectable viral DNA in infected NHP cells declined over time, and the cells never had detectable reverse transcriptase activity. Utilizing quantitative real-time TaqMan PCR we found detectable levels of unintegrated DNA intermediates, but the levels were approximately 100-fold lower compared to HEK 293 cells infected with PERV. Virions released from infected NHP cells could productively infect naïve human cell lines, HEK 293 and HeLa, as shown by RT-PCR and RT assay. However, naïve NHP cells remained negative in RT-PCR and RT assay after exposure to virions from infected NHP cells. Together our data demonstrate that NHP cells are not permissive to productive replication by PERV, presumably due to inefficient cell entry and replication. In light of these observations, the appropriateness of NHP as suitable animal models to study PERV infection in vivo needs to be reevaluated.

PCR-based cloning and immunocytological titration of infectious porcine endogenous retrovirus subgroup A and B

Two pig endogenous retroviruses (PERV), PERV-A and -B, productively infect human cells and are therefore considered to constitute a potential risk in pig-to-human xenotransplantation. A PCR-based cloning technique to isolate infectious PERV proviruses was established. Overlapping 3' half and 5' halves of PERV proviral genomes were amplified using DNA extracted from human 293 cells infected with PERV-A or -B. These clones were fused at a unique restriction site in the overlapping region and tested for their infectivity. Representative constructs possessed the same infectious properties as their parent isolates. We also developed a polyclonal anti-PERV serum by using recombinant PERV capsid protein derived from one of the infectious constructs as immunogen and established an immunocytological method for detection and titration of PERV infection. This detection method proved to be more sensitive than the current method of choice (transfer of MLV-lacZ vectors) for infectivity assessment of PERV. These findings should be considered for future characterization of PERV isolates.

Analysis of patients treated with living pig tissue for evidence of infection by porcine endogenous retroviruses

The use of pigs as a source of cells and organs for transplantation has the potential to reduce the current chronic shortage of organs for the treatment of many end-stage diseases. The risk of transmission of infectious agents across the species barrier (zoonoses) has to be assessed. Many such agents can be eliminated from the pig herd. However, porcine endogenous retroviruses, which are carried within the pig genome, are not easily eliminated. They can infect primary and immortalized human cells in vitro, but to date no evidence for in vivo infection has been found in retrospective studies of humans exposed to viable porcine cells. Small-scale clinical trials using porcine cells for the treatment of Parkinson's and Huntington's disease are currently in progress. The prospective monitoring of these patients in conjunction with further research into the biology of this virus will help address safety issues.