Studies since the start of the pandemic have noted that pigs can be infected by the virus if exposed to high doses, but the infection is self-limited and pigs don’t show clinical signs of disease nor do they transmit the virus to other animals.
What they found could lead to new ways to treat humans who contract COVID-19, the disease that results from infection by the virus SARS-CoV-2, the researchers say.
“When we looked under the microscope there was an interesting phenomenon going on inside the cells,” Nelli says. “The nuclei of the infected pig cells were starting to shred into fragments but not uninfected pig cells.”
That shredding of the nucleus is a telltale sign of apoptosis, which may be a key in helping pigs avoid symptoms after exposure to SARS-CoV-2. Triggering apoptosis early in the infection essentially causes minimal tissue damage and confines viral replication, thus limiting severe illness. Human cells can undergo apoptosis in response to coronavirus infection as well, but the study found human cells do so much less frequently than porcine cells. Pig cells are roughly 100 times more likely to undergo apoptosis than human cells, according to the study.
The researchers surmise that a wide-scale apoptosis response is helpful for avoiding disease because it disposes of infected cells quickly without the immune system overreacting, while wide-scale necrosis and the resulting hyperimmune response is less favorable to host cells.
“We don’t want to over-conclude, but this response is probably something intrinsic to the pig immune system that is innate and not acquired,” Giminez-Lirola says. “The idea is to kill the virus subtly but fast enough so there’s not an excessive immune response triggered.”
The researchers say further study could lead to therapies designed to trigger apoptosis in human cells, allowing people infected with the coronavirus to avoid severe symptoms.
The next step for the researchers is to identify all the genes activated during the infectious process and compare them with other animal species in which those genes are present. That could give them further clues about how and why other animals, such as deer, can carry the virus without suffering symptoms of disease.
Additional coauthors are from Iowa State and Maryland-based Tetracore, Inc.