A new study offers a clear picture of why cold and flu cases are more common during the winter months. The study found evidence that our nose’s innate immune response weakens in colder temperatures, providing a better opportunity for certain microbes to infect the rest of the body. The authors say the findings may provide a biological explanation for the seasonality of many respiratory diseases.
This work comes from scientists at Northeastern University, as well as Mass Eye and Ear, a teaching hospital affiliated with Harvard Medical School. In 2018, some of the authors published a study that suggested the body has a unique first line of defense against potentially dangerous nose-breathing bacteria. They found that cells near the front of the nose add fluid-filled sacs called extracellular vesicles to our mucus, which then surround the bacteria. These sacs also seem to carry antimicrobial proteins throughout the rest of the nose and help prevent damage to other cells when they come into contact with bacteria.
In this new study, published Tuesday in the Journal of Allergy and Clinical Immunology, scientists tried to see if the nose has always had a similar defense mechanism against viruses.
In the laboratory, they examined samples from healthy people and patients who had undergone surgery. They found that nasal cells form extracellular vesicles in response to a mock viral infection. And when they exposed cells to the three viruses that usually cause colds—two rhinoviruses and one non-covid coronavirus—vesicles enveloped those cells. They also found that this defense could also be triggered using a different pathway than the secretion of extracellular vesicles against bacteria. And the extracellular vesicles acted as an additional trap, carrying receptors that viruses could target rather than chase after cells.
Why do we get sick more in cold weather?
Many respiratory infections tend to become more common during the colder times of the year. This seasonal trend is thought to have several causes, including the fact that people gather indoors to stay warm. However, the team wanted to test whether the cold directly affects this defense mechanism as well.
They asked healthy volunteers to withstand relatively cold weather (4 degrees Celsius) for 15 minutes, and by measuring the change in temperature inside the nose, they noticed that it had dropped by about five degrees. They then exposed the cells to this temperature. The authors found that compared with the normal situation, the nose’s innate immune response to viruses was not as strong at this new temperature setting, and the cells produced fewer extracellular vesicles on average.
Of course, the findings need to be confirmed by more research by different groups before they can be accepted. If the findings are confirmed, it could also lead to improvements in the way we fight infections. In the future, it may be possible to develop nasal sprays that can increase or strengthen the nose’s extracellular vesicle production during the winter season, the team said.
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