For the first time, scientists have discovered a pig retrovirus that infects human cells. The finding, reported in next month's issue of Nature Medicine, raises new questions about whether people could contract exotic diseases if animal organs become routinely transplanted into human patients. It may also force U.S. regulators to tighten their xenotransplantation guidelines.
Critics have long argued that transplanting animal tissue into even a small number of immune-suppressed patients could allow obscure pathogens to jump from animals to humans. Now a team led by Robin Weiss of London's Institute of Cancer Research reports that the "PK" porcine endogenous retrovirus, which does not appear to harm pigs, can replicate in mink and human cells. Moreover, after a single round of viral replication, the human cells lack a crucial sugar that flags them as compromised cells for the immune system to destroy.
Although there's no evidence that PK causes human disease, retroviruses are particularly worrisome candidates for crossing the species barrier, says virologist Jon Allen of San Antonio's Southwest Foundation for Biomedical Research. They thrive in bodily fluids and tend to get copied several times into host gene sequences, which can wreak genetic havoc and sometimes lead to cancer. "We had assumed that pig tissues were more safe than baboon tissues, but now we see they can infect human cells," Allen says. The study suggests, he says, "that maybe we ought to reconsider what guidelines we have and what species we use."
The United States permits limited xenotransplantation, while the British government last month announced that human clinical trials will not proceed until the procedures are shown to be safe (see ScienceNOW, 17 January ). U.S. health officials now "have reason to pause," Allen says. "I hope they take it." Other experts predict the United States will consider stricter guidelines. "I would not be terribly shocked if something was enacted, nor would I think that's in error," says Fritz Bach, a xenotransplantation researcher at Harvard Medical School. "But it's certainly not the time to say let's forget about xenotransplantation."
U.S. officials declined to speculate whether the findings would lead to tighter guidelines. "This reemphasizes the need for caution, and it adds some concrete data to help us quantify the risks and benefits," says Amy Patterson, acting director of the Food and Drug Administration's Division of Cellular and Gene Therapies. The FDA plans to draft new guidance on xenotransplantation later this year.