Why are Colds and Flu More Prevalent in Winter?

A previously unidentified immune response inside the nose that fights off viruses responsible for upper respiratory infections has been discovered by Researchers at Mass Eye and Ear and Northeastern University. Further testing revealed this protective response becomes inhibited in colder temperatures, making an infection more likely to occur. The new study, published in The Journal of Allergy and Clinical Immunology, offers the first biological mechanism to explain why viruses like the common cold, flu, and COVID-19 are more likely to spike in colder seasons.

This 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.

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First-Line Defense in the Nose

The nose is one of the first points of contact between the outside environment and inside the body and, as such, a likely entry point for disease-causing pathogens.

Pathogens are inhaled or directly deposited (such as by the hands) into the front of the nose where they work their way back through the airway and into the body infecting cells, which can lead to an upper respiratory infection. How the airway protects itself against these pathogens has long been poorly understood.

The previous study also showed that the EVs shuttle protective antibacterial proteins through the mucus from the front of the nose to the back of it along the airway, which then protects other cells against the bacteria before it gets too far into the body.

For the new study, researchers sought to determine if this immune response was also triggered by viruses inhaled through the nose, which are the source of some of the most common upper respiratory infections.

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Virus-Fighting Mechanism Tested in Varying Conditions

Researchers analyzed how cells and nasal tissue samples collected from the noses of patients undergoing surgery and healthy volunteers responded to three viruses: a single coronavirus and two rhinoviruses that cause the common cold.

They found each virus triggered an EV swarm response from nasal cells, albeit using a signaling pathway different from the one used to fight off bacteria. The researchers also discovered a mechanism at play in the response against the viruses: Upon their release, the EVs acted as decoys, carrying receptors that the virus would bind itself to instead of the nasal cells.

They then tested how colder temperatures affected this response, which is especially relevant in nasal immunity given the internal temperature of the nose is highly dependent on the temperature of the outside air it inhales.

Combined, these findings provide a mechanistic explanation for the seasonal variation in upper respiratory infections.

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Future Therapeutic Potential

Future studies will aim to replicate the findings with other pathogens. The studies could take place as challenge studies, where an animal model or human is exposed to a virus and their nasal immune response is measured.

From the recent findings, researchers can also imagine ways in which therapeutics can induce and strengthen the nose𠏋 innate immune response.



Source-Eurekalert