After a thorough antivirus scan, de-bugged pigs are a step closer to growing organs for us.

Researchers used the latest gene editing technology to deactivate 25 remnants of ancient viruses, called porcine endogenous retroviruses (PERVs), that had embedded in the DNA of a pig cell line. Pig genomes are rife with lurking PERVs, which threaten to emerge and infect humans. But with a genome wiped of active viruses, the researchers produced 37 piglets that are PERV-free. The creation of those clean little porkers, reported Thursday in Science, is progress toward using pigs as human organ donors, the researchers say.

“Our study highlighted the value of PERV inactivation to prevent cross-species viral transmission and demonstrated the successful production of PERV-inactivated animals to address the safety concern in clinical xenotransplantation,” the authors concluded.

Pigs have long been considered an ideal animal for incubating organs that can be transplanted into humans. Their organs are roughly the same size as ours and function similarly, too. But there are also plenty of barriers to the cross-species sharing. For one thing, tissue from a different human—let alone a different animal—is enough to freak out our immune systems and cause them to attack. Any pig organs fit for human transplant would need to be carefully genetically tweaked to preclude such a severe immune response. Then there are other issues, such as blood clotting problems—and the PERVs.

Researchers have always worried about PERVs in pig-to-human transfers. The retroviruses, which are passed on through hog generations, have never proven to transmit to humans—no human PERV disease cases have ever been reported, even in patients who have received pig tissue transplants. Still, the concern lingers. And in labs, PERVs can jump from pig cells to human cells.

Hog wild genetic engineering

Researchers saw this first hand in the new study, led by Harvard geneticist George Church and Luhan Yang, a bioengineer and president of eGenesis, a biotech start-up she and Church co-founded. Before sweeping away PERVs from a pig cell line, they showed that PERVs from a line of pig cells infected a line of human cells when researchers grew them together. And that infected line of human cells infected another line of human cells when researchers grew them together.

That was enough to convince Church, Yang, and colleagues that PERVs are something they needed to obliterate if they wanted to use pigs as organ donors. So, they turned to the flashy new gene editing tool, CRISPER-Cas9 , to slice up and deactivate all instances of PERV genes in a pig cell line. They used a line of pig cells from connective tissue that contained 25 PERVs. When the researchers initially tried to disable all the PERVs, they found that many cells either still had PERVs and/or didn’t grow normally. They speculated that the genetic slicing and dicing was enough to signal to the cells that something was drastically wrong and that they should commit suicide—a cell process called apoptosis. So, the researchers added a cocktail of chemicals and factors that suppressed such suicidal signals. The result was 100 percent PERV eradication.

By sucking up the virus-cleared genetic material and injecting it into the emptied nuclei of pig egg cells, the researchers created embryos devoid of PERVs. Implanted into sows, those embryos grew up into 37 piglets, 15 of which were still healthy after four months.

There’s still a lot of work ahead to turn the swine into human organ factories. And it’s unclear if researchers will end up needing PERV-free piglets for the feat. But for now, Church, Yang, and their team think their new pigs may “serve as a foundation pig strain, which can be further engineered to provide safe and effective organ and tissue resources for xenotransplantation.”

Science, 2017. DOI: 10.1126/science.aan4187 (About DOIs).