New research seems to provide a clearer picture of why cold and flu cases are more common in winter. The study found evidence that our nose’s innate immune response weakens in colder temperatures, giving certain 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 has a unique first line of defense against potentially harmful bacteria that are inhaled through the nose. Cells near the front of the nose, they found, can cough up fluid-filled sacs called extracellular vesicles into our mucus, which then swarm with bacteria. These sacs also appear to carry antimicrobial proteins to the rest of the nose, helping to protect other cells from damage when they come into contact with bacteria.
In this new research, published Tu esday in the Journal of Allergy and Clinical Immunology, scientists wanted to see if the nose had a similar defense mechanism against viruses.
They studied laboratory samples taken from healthy people and patients undergoing surgery. They discovered that nasal cells deployed extracellular vesicles in response to a fake 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 then invaded them. They also discovered that this defense was triggered using a different pathway to how extracellular vesicles are deployed against bacteria. And the extracellular vesicles additionally acted as decoys, as they carried receptors on which the viruses would latch on instead of attacking the cells.
Many respiratory infections tend to become more common during colder times of the year. There are believed to be several reasons for this seasonal trend, including people congregating indoors to stay warm. But the team wanted to test whether the cold could also directly affect this defense mechanism.
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 about 10 degrees Fahrenheit. They then exposed the cells to this temperature. Compared to the normal state, the nose’s innate immune response against viruses was not as strong in this new temperature setting, the authors found, with the cells producing fewer extracellular vesicles on average.
The results would need to be replicated by further studies before they become widely accepted, and there are likely several factors driving the seasonality of respiratory viruses. Studies on influenza, for example, have found humidity plays a major role in its transmission, with hot and humid or cold and dry conditions being optimal for the spread of the virus. And some cold viruses are actually The most common during summer. But the results here suggest that biology plays a leading role in timing our vulnerability to these germs, the authors say.
“Conventionally, cold and flu season was thought to occur in the colder months because people are more stuck indoors where airborne viruses could spread more easily,” said said study lead author Benjamin Bleier, director of translational research in otolaryngology at Mass Eye and Ear and lead author. of the study, in a statement by Mass Eye and Ear. “Our study, however, points to a biological root cause for the seasonal variation in viral upper respiratory tract infections we see each year, most recently demonstrated throughout the COVID-19 pandemic.”
If these results hold, they could also lead to improvements in how we fight these infections. According to the team, it’s possible we could one day create nasal sprays that can increase or boost the nose’s supply of extracellular vesicles during the winter. In the meantime, they plan to test whether this defense mechanism kicks in against other pathogens.
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