New research appears to provide a clearer picture of why cold and flu cases are more common during the winter. The study found evidence that our nose’s innate immune response weakens in colder temperatures, providing some germs a better opportunity to infect the rest of the body. The findings, the authors say, could provide a biological explanation for the seasonality of many respiratory diseases.
The study comes from scientists at Northeastern University as well as Mass Eye and Ear, a teaching hospital affiliated with Harvard Medical School. In 2018, several of the authors published work suggesting that the body possesses a unique first line of defense against potentially dangerous bacteria that are inhaled through the nose. Cells near the front of the nose, they found, can spew out fluid-filled sacs called extracellular vesicles into our mucus, which then swarm bacteria. These sacs also seem to ferry antimicrobial proteins throughout the rest of the nose, helping protect other cells from harm as they come in contact with the bacteria.
In this new research, published Tuesday in the Journal of Allergy and Clinical Immunology, the scientists wanted to see if the nose had a similar defense mechanism against viruses.
They studied samples in the lab that were taken from healthy people and patients undergoing surgery. They found that nasal cells deployed extracellular vesicles in response to a mock viral infection. And when they exposed the cells to three viruses that usually cause the common cold (two rhinoviruses and a non-covid coronavirus), the vesicles subsequently swarmed them. They also found that this defense was triggered using a different pathway from how extracellular vesicles are deployed against bacteria. And the extracellular vesicles additionally acted as decoys, since they carried receptors that the viruses would latch onto instead of going after cells.
Many respiratory infections tend to become more common during the colder parts of the year. There are thought to be several reasons for this seasonal trend, including the crowding together of people indoors to stay warm. But the team wanted to test whether the cold could directly affect this defense mechanism as well.
They asked healthy volunteers to endure relatively cold weather (39.9 degrees Fahrenheit) for 15 minutes and measured the change in temperature inside the nose, finding that it dropped by about 10 degrees Fahrenheit. They then exposed cells to that temperature. Compared to the normal condition, the nose’s innate immune response against the viruses wasn’t as strong in this new temperature setting, the authors found, with the cells producing fewer extracellular vesicles on average.
The findings should be replicated by other studies before they’re widely accepted, and there are likely multiple factors behind the seasonality of respiratory viruses. Studies of the flu, for instance, have found that humidity plays a major role in its transmission, with either hot and humid or cold and dry conditions being optimal for spreading the virus. And some cold viruses are actually most common during the summer. But the results here suggest that biology plays a leading role in the timing of our vulnerability to these germs, the authors say.
“Conventionally, it was thought that cold and flu season occurred in cooler months because people are stuck indoors more where airborne viruses could spread more easily,” said senior study author Benjamin Bleier, director of Otolaryngology Translational Research at Mass Eye and Ear and senior author of the study, in a statement from Mass Eye and Ear. “Our study however points to a biological root cause for the seasonal variation in upper respiratory viral infections we see each year, most recently demonstrated throughout the COVID-19 pandemic.”
If these findings do hold up, they might also lead to improvements in how we fend off these infections. It’s possible that we could someday create nasal sprays that can boost or strengthen the nose’s supply of extracellular vesicles during the winter, according to the team. In the meantime, they plan to test out if this defense mechanism is triggered against other pathogens.