US scientists have found the cellular response that protects pigs from Covid-19, which according to them could lead to new treatments in humans.
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.
Scientists at the Iowa State University introduced the virus to cultured porcine and human respiratory epithelial cells, which line most of the respiratory tract.
The findings, published in the journal Cell Death Discovery, showed that the pig cells underwent apoptosis, or controlled cell death, in response to infection at a higher rate than human epithelial cells.
"When we looked under the microscope there was an interesting phenomenon going on inside the cells," said Rahul Nelli, a research assistant professor of veterinary diagnostic and production animal medicine at the varsity.
"The nuclei of the infected pig cells were starting to shred into fragments but not uninfected pig cells," he added.
Nelli explained that the 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.
Human cells are more likely to go through necrosis, another form of cell death that's less controlled than apoptosis. During necrosis, the contents of a cell release into the surrounding space, provoking a strong hyperimmune response that isn't triggered during apoptosis.
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 favourable 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," said Luis Gimenez-Lirola, Associate Professor of veterinary diagnostic and production animal medicine.
"The idea is to kill the virus subtly but fast enough so there's not an excessive immune response triggered," Gimenez-Lirola said.
The researchers said further study could lead to therapies designed to trigger apoptosis in human cells, allowing people infected with the coronavirus to avoid severe symptoms.