By David N. Leff

Transplant surgeons have great expectations that the common pig (Sus scrofa) can become an organ donor for human recipients.

They point out that hogs are the right size, easy to raise, grow quickly to maturity and are free of infectious disease pathogens compared to subhuman primates, such as baboons. With upwards of 60,000 people on waiting lists for donor organs in the U.S. alone, porcine xenotransplantation could provide a virtually unlimited supply of graft organs, tissues and cells, to alleviate the cruel shortage of human material.

But, in recent years, a large, dark cloud has appeared on this bright horizon. Virologists now warn that these prospective porcine donors harbor retroviruses that may be able to infect graft recipients, and possibly spread through the human population, like a replay of HIV. (See BioWorld Today, Dec. 18, 1997, p. 1.)

This week's Lancet, dated Aug. 29, 1998, carries a cautionary paper in its "Early Reports" section titled, "Expression of pig endogenous retrovirus [PERV] by primary porcine endothelial cells and infection of human cells."

Using sophisticated in vitro detection methods, first author Ulrich Martin and his co-authors found that pigs pose "a serious risk of retrovirus transfer after xenotransplantation." The German group is at the Leibniz Research Laboratories for Biotechnology and Artificial Organs, at Hannover Medical School, in Hannover, Germany.

In three breeds of pig, from 12 sites in Denmark, Russia, Germany and France, they detected PERV in every sample of skin, liver, lung and aortic endothelial cells. Co-cultivation of the aortic cells with human embryonic kidney cells "led to productive infection of the human cells and expression of PERV," they wrote.

Counterbalancing this somber heads-up warning were two other "Early Reports" in the same Lancet, describing in vivo human experience with porcine cells. "No evidence of infection with porcine endogenous retrovirus in recipients of porcine islet-cell xenografts," one article reported.

At Sweden's Karolinska Institute, in Stockholm, between 1990 and 1993, 10 patients with late-stage insulin-dependent diabetes received 400 million to 2 billion insulin-secreting islets of Langerhans from fetal pig tissue. In five of these subjects, the xenografts survived and functioned for up to a year.

That paper's first author is virologist Walid Heneine at the U.S. Centers for Disease Control (CDC), HIV and Retrovirology Branch. A co-author is medical epidemiologist Louisa Chapman, project leader of the CDC's working group on porcine xenotransplantation.

She told BioWorld Today that, although the five Swedish subjects also received immunosuppressive treatment to prevent graft rejection, they "were unable to detect markers of PERV infection in any patient."

Another HIV-Like Pandemic?

Chapman explained that PERV "goes into part of its target cell's DNA. Every mammalian species harbors such endogenous retroviruses. They remain dormant in infected cells until activated to become exogenous retroviruses. The concern over PERV is that if it becomes exogenous — like HIV — rather than endogenous, it might spread as a new infectious agent in the population."

It was this potential hazard that led the FDA last year to put a hold on all clinical trials of porcine donor organs, tissues and cells. "But it's not a global moratorium," Chapman pointed out. "Rather, [it's] a trial-by-trial hold. It imposes three requirements. Planners of a human study must develop or identify a PERV assay, undertake prospective monitoring of recipients, and test all other people exposed to the virus."

Diacrin Inc., of Charlestown, Mass., has four clinical trials of xenotransplanted porcine cells under way. The company's CEO, molecular biologist Thomas Fraser, said, "We have one Phase I trial of fetal pig cells transplanted into 12 Parkinson's disease patients, which is nearing completion. A second Phase II/III study [into Parkinson's disease] is still blinded."

A separate Phase I study is treating patients with Huntington's disease. The company is conducting these three trials in collaboration with Genzyme Tissue Repair, of Cambridge, Mass., a subsidiary of Genzyme Corp., also of Cambridge. On its own, Diacrin has a Phase I trial ongoing to treat epileptic patients with fetal porcine GABA-secreting cells.

"We have tested all our clinical-trial patients," Fraser concluded, "and found no evidence of PERV or any unknown pathogens."

Chapman observed that "whole-organ pig grafts are, ironically, years away. They still require a lot of preclinical work. In the short run, porcine cell preparations are more promising."

A different donor strategy consists of perfusing the blood of patients with kidney or liver failure through living porcine organs. Two such patients are reported in the third Lancet paper. Its title tells the story: "No evidence of pig DNA or retroviral infection in patients with short-term extracorporeal connection to pig kidneys."

Its first author, virologist Clive Patience, at the Institute of Cancer Research, in London, surmised that "the absence of porcine cells in the circulation of both patients, even in samples taken as early as six hours after the perfusion, suggests that any porcine cells dislodged from the kidney became rapidly sequestered from the circulation."

However, he concluded, "These negative findings on just two patients must be interpreted with caution."

Preconditions For PERV Hazard To Come True

British virologist Jonathan Stoye postulated that "for xenotransplanted PERVs to pose a public-health threat, a chain of seven events is required." Infectious, human-targeted retroviruses must exist; they must occur in the germ line of pigs used for xenotransplantation; they must be expressed in transplanted cells, tissues or organs; they must infect the recipient; they must replicate and spread; they must result in disease; and they must be transmitted to others.

In a commentary accompanying the three Lancet reports, Stoye observed that there is now "substantial evidence [from the German paper] for the first three events in this chain." But he cited the diabetes and kidney dialysis reports as questioning whether infection of human beings exposed to porcine cells will inevitably follow.

"Some unpublished studies involving other examples of exposure to pig tissues seem to be reaching the conclusion that PERVs will not show very high levels of transmission," Stoye told BioWorld Today. He foresees that companies in the U.K. that have such studies ongoing "will report by year's end on 200 to 250 recipients exposed to porcine tissues."

British health authorities, like FDA, have no global moratorium on xenotransplantation in place, but require case-by-case consideration. "So far," Stoye said, "we haven't had any applications." *