The nose is the gateway to the various microbes that trigger typical winter respiratory infections, like the flu or the common cold. But why do we get sicker when it’s cold?
“Traditionally, colds and flu were thought to occur in the cold months because people stayed indoors more, where viruses could more easily spread through the air,” points to Benjamin S. Bleier, you Massachussets Eye and Ear, Hearing, Vision, and Smell Research Unit at Boston Hospital.
But a study by his team revealed that this hypothesis wasn’t actually enough to explain the seasonality of viral infections.
Defense extracellular vesicles
First, in 2018, the research team discovered that there was a specific defense mechanism in the nose. “When bacteria are inhaled, cells at the entrance to the nose detect them and release billions of tiny fluid-filled sacs called extracellular vesicles into the mucus to attack the microbes.” describe the authors.
Also called exosomes, these biological elements therefore protect us from bacterial attacks. Especially since they then circulate to the back of the nose and then to the airways, thus protecting other cells from these pathogens before they enter the body.
The cold inhibits the nasal defenses
But these findings don’t explain why winter is more conducive to colds and other flus, mostly caused by viruses, than other seasons.
However, the researchers ended up finding the explanation in a second part of their study. This time, they analyzed how nasal cells and tissues responded to three viruses: a coronavirus and two rhinoviruses.
First, the scientists confirmed that the extracellular vesicle production machinery is also activated for the three pathogens, similar to the reaction against bacteria.
But they mainly tested the effect of cold. After finding that the temperature dropped by about 5°C inside the noses of healthy volunteers when exposed to a 4°C environment, they placed the nasal tissue under the same conditions.
The result provided an immediate insight: “The amount of extracellular vesicles secreted by the nasal cells was therefore reduced by 42% compared to a warmer environment,” say the authors.
“These combined results provide a mechanistic explanation for the very existence of seasonal variation in upper respiratory infections,” they believe. Now it remains to be determined “how to take advantage of this discovery, recreate a defense mechanism in the nose and thus enhance its protection during the cold months”, they conclude.
