Researchers may have sniffed out why colds are more likely in wintertime: The answer may lie within the nose.
A previously unidentified immune response inside the nose is responsible for fighting off the viruses that cause upper respiratory infections, according to researchers at Massachusetts Eye and Ear and Northeastern University in Boston.
Unfortunately, cold weather inhibits this protective response, making it more likely that a person will come down with anything from a cold to COVID-19.
The new study offers the first biological explanation why respiratory virus infections are more likely to spike in colder seasons, researchers said.
"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 researcher Dr. Benjamin Bleier, director of Otolaryngology Translational Research at 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," he said in a hospital news release.
The nose is a prime entry point for viruses or bacteria, which can either be inhaled or directly deposited if a person does something like rub their nose.
Once inside the nose, germs work their way backward up the airway and into the body, infecting the cells along the way.
But the nose is capable of fighting this infection. Cells in the front of the nose can detect inhaled germs, and respond by releasing billions of tiny fluid-filled sacs into mucus to surround and attack the pathogen.
These sacs, called extracellular vesicles (EVs), shuttle antibacterial proteins throughout the airway and form a protective blockade intended to keep germs from spreading up the airway.
To see how this response works in varying conditions, researchers collected samples from the noses of patients and volunteers. They then exposed those samples to three viruses -- a coronavirus and two rhinoviruses that cause the common cold.
The viruses each triggered an EV response from nasal cells. The researchers also found that the EVs respond in a different way to viruses -- they carry receptors that snag the viruses before they can infect cells.
However, the researchers found that colder temperatures affect this response.
Healthy volunteers placed in cold conditions -- about 40 degrees Fahrenheit -- experienced a temperature drop inside their nose.
Nose cells in the lab exposed to such a temperature decline delivered a blunted immune response, researchers found, secreting about 42% fewer EVs. Further, the antiviral receptors on the EVs were hampered by the cold.
"Combined, these findings provide a mechanistic explanation for the seasonal variation in upper respiratory infections," said lead author Di Huang, a research fellow at Mass Eye and Ear and Northeastern.
The researchers plan to explore in more detail how the nose's immune response protects against all sorts of pathogens.
They also think new drugs could be created that would induce and strengthen the nose's immune response.
"The question now changes to, 'How can we exploit this natural phenomenon and recreate a defensive mechanism in the nose and boost this protection, especially in colder months?'" said co-author Mansoor Amiji, professor of pharmaceutical sciences at Northeastern.
The findings were published Dec. 6 in The Journal of Allergy and Clinical Immunology.
Children's Hospital of Philadelphia has more about upper respiratory infections.
SOURCE: Massachusetts Eye and Ear, news release, Dec. 6, 2022