Scientists predict that many coronaviruses similar to the strain responsible for the COVID-19 pandemic, such as Middle East respiratory syndrome (MERS) and severe acute respiratory syndrome (SARS), have their origins in bats. While several coronavirus strains can infect and sicken humans, bats appear to remain unaffected while carrying the virus.
In a recent study, two researchers at the University of Saskatchewan (USask) demonstrated that the MERS coronavirus can persist in the kidney cells of a species of brown bat for months at a time. The unique ability of bat cells to carry viral particles is due to adaptations both in their immune system and within the virus itself.
“Very quickly after we put the virus in bat cells, the virus adapts, or there is selection or mutations in one particular gene,” Vikram Misra, co-author of the study, said in an interview with The McGill Tribune. “We don’t know much about that protein, but what seems to be is that that particular protein is responsible for regulating antiviral responses and cell death.”
The study added more knowledge to what scientists already understand about how viruses interact with a bat’s immune system. Previous studies have suggested that physiological stress can cause an imbalance in the replication of viral RNA, leading to increased viral shedding, the process in which viruses exit the body and are released into the environment. This mechanism, as proposed by recent studies, provides a possible explanation for how coronaviruses can infect human populations.
Since researchers do not observe the symptoms of coronavirus in bats, they instead compare a bat’s immune response to the virus to that of a human.
When the human body encounters a virus, there is both an antiviral and inflammatory response. With most viral diseases, the virus blocks the body’s natural antiviral response and instead causes a strong inflammatory response, which can damage surrounding organs, such as the lungs in the case of respiratory viruses. Arinjay Banerjee, co-author of the USask study, highlights that bats can somehow suppress the inflammatory response induced by viruses.
“Bats have evolved to do absolutely the opposite,” Banerjee said in an interview with the Tribune. “When bat cells are infected, we see a good antiviral response, but we do not see the strong overdrive of inflammation in bats.”
This “alternative response” on the part of the bat immune system is thought to contribute to the virus’ ability to persist within members of the bat family.
Selena Sagan, Associate Professor in the Department of Microbiology & Immunology at McGill, further explained why bats are a common reservoir for viruses.
“Bats account for about 25 per cent of all mammalian species […] and they have a long evolutionary history, [meaning that] they have a long period of potential virus coevolution,” Sagan said in an interview with the Tribune. “[Bats] also have worldwide distribution […] and, in fact, their migratory patterns actually match the geographic distribution of several viruses.”
Sagan also noted that bats can live anywhere from 10 to 40 years, allowing them ample time to cohabitate and transmit viruses to other species. The plethora of new research into how the coronavirus made its way into humans is further shedding light on how diseases can jump from one species to another.
Scientists are just beginning to understand how a bat’s unique immune response allows viruses to persist within bat cells. However, with over 1400 known species of bats, it is unlikely that all will respond in the same way to different viruses.
“If we can figure out why bat cells respond to viruses by turning off their inflammatory responses, we may be able to figure out how certain chemicals or drugs might be able to do the same in [human] immune cells,” Misra said.