Mouse retrovirus mediates porcine endogenous retrovirus transmission into human cells in long-term human-porcine chimeric mice

Abstract

Porcine endogenous retrovirus (PERV) is a potential pathogen in clinical xenotransplantation; transmission of PERV in vivo has been suggested in murine xenotransplantation models. We analyzed the transmission of PERV to human cells in vivo using a model in which immunodeficient NOD/SCID transgenic mice were transplanted with porcine and human lymphohematopoietic tissues. Our results demonstrate, we believe for the first time, that human and pig cells can coexist long-term (up to 25 weeks) without direct PERV infection of human cells. Despite the transplantation of porcine cells that did not produce human-tropic PERV, human cells from the chimeric mice were frequently found to contain PERV sequences. However, this transmission was due to the pseudotyping of PERV-C (a virus without human tropism) by xenotropic murine leukemia virus, rather than to de novo generation of human-tropic PERV. Thus, pseudotyping might account for the PERV transmission previously observed in mice. The absence of direct human cell infection following long-term in vivo coexistence with large numbers of porcine cells provides encouragement regarding the potential safety of using pigs that do not produce human-tropic PERV as source animals for transplantation to humans.

Figure 1

BM chimerism in NOD/SCID-Tg mice. NOD/SCID-Tg mice demonstrate porcine, human and murine cell chimerism in the BM cells following the transplantation of (A) porcine BM cells alone, (B) porcine BM cells plus human thymus/liver, or (C) human thymus/liver alone. Shown are representative staining profiles of the recipient BM cells at week 25 after transplantation. Murine, human, and pig cells are located in the lower left, upper left, and lower right quadrants, respectively. The percentage of cells in each quadrant is shown.

Figure 2

Human 293 cells become transiently positive for PERV-C, and productively infected with MLV, following cocultivation with chimeric bone marrow samples that contain murine cells. Human 293 cells were directly cocultured with chimeric bone marrow containing mouse + pig (M/P), mouse + pig + human (M/P/H), or mouse + human (M/H) cells. To determine the replication competence of the viruses in this primary culture (G1), a cell-free supernatant preparation was used to further challenge secondary uninfected 293 cells (G2). Cells were tested by PCR for the presence of MLV and PERV-C sequences.