March 27, 2023

Why do fevers, colds and flu (but also Covid-19 and other respiratory diseases) strike more in winter despite the pathogens that trigger these reactions and the diseases circulating throughout the year? The answer is perhaps what you imagine, or perhaps slightly different in how it was presented by a group of scientists from Northeastern University in Boston and Harvard Medical School in a study just published in the specialized journal The Journal of Allergy and Clinical Immunology.

The investigation may have identified the biological foundation of the seasonality of these respiratory pathologies: it could be the cold air itself that compromises the immune response that takes place on the front line, i.e. at the level of the nasal mucous membranes. “This is the first time we have a biological and molecular explanation for a factor in our innate immune response that appears to be limited by colder temperatures,” rhinologist Zara Patel, professor of otolaryngology and head and neck surgery at Stanford , told CNN . not involved in the study. I mean, if maybe we could have imagined it, now we have proof.

Reducing the temperature inside the nose by just 5 degrees Celsius knocks out almost 50% of the billions of cells that fight viruses and bacteria in the nostrils , according to the survey . The front line defenses are lower and the enemy therefore transits with fewer problems towards the depths of the organism. “Cold air is associated with increased viral infection because you essentially lose half of your immunity just for that small drop in temperature,” added rhinologist Benjamin Bleier , director of otolaryngology at Massachusetts Eye and Ear and associate professor at the University of California. Harvard Medical School in Boston and the most famous of the authors of the study, which bears the researcher Di Huang as the first signature.

These are in vitro investigations, and therefore although they use human tissue they are still laboratory tests: it is probable that they can be confirmed by in vivo studies even if sometimes it may not go as one would expect. Having said that, why do defenses drop in the cold? Bleier, Huang and their team, together with co-author Mansoor Amiji, have tried to reconstruct the dynamics of what happens when a virus or bacterium lands in our nose, the first point of entry into the body as we well know after years of COVID-19.

As they had already discovered in another 2018 study dedicated exclusively to bacterial infections, the front of the nose immediately detects the germ on first contact or inhalation, well before the back of the nose notices the intruder. Not only that: at that point the cells lining the nasal mucosa immediately begin to generate billions of copies of themselves in a simplified key, called extracellular vesicles , or EVs, small reservoirs of antimicrobial proteins. As if they were small versions of the cells specifically designed to attack the pathogen and traps at the same timefor the enemy: «The EVs act as baits, so when a virus is inhaled it attaches itself to these baits instead of hooking up to the cells» and then replicates and infects deeper and deeper tissues. Once their work is done, they are carried by the cells through the nasal mucus, which will then in turn be expelled. A battle that actually takes place outside the tissues of the target organism.

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The new investigation, which also tackles viral infections by testing immune reactions on a coronavirus and two rhinoviruses, also explains that when the nose is under attack, it increases extracellular vesicle production by 160% . Not only that: these small specialized cells first of all have many more receptors on their surface than the originals, about twenty times more, as if they were many sticky hooks ready to make a clean sweep of the virus or bacterium at the very moment we are inhaling it. And then these bags contain 13 times more microRna sequences , also specialized in counteracting the attack of germs. A very important piece of equipment which however, when the temperatures drop, risks coming out of itdamaged .

In the study, Blaier and team exposed four participants to a 4.4-degree Celsius environment for a quarter of an hour , then tested the condition and activity inside their nasal passages. The result? “What we found is that when you’re exposed to cold air, the temperature in your nose can drop by about 5 degrees . And that’s enough to essentially eliminate all three of those immune benefits that the nose has,” explained the author. The tissues taken from operated patients, also brought to 4.4 degrees Celsius for that period of time with an abrupt transition from an ambient temperature of about 25 degrees, confirmed how the nasal “flak” is partly inhibited. That short but intense jump in temperature would be enough to lower that inside the nose and reduce the quantity of vesicles secreted by the nasal cells by 42%, also compromising the quantity of antiviral proteins present in the bait-cell stars and 70% less receptors on each vesicle.

The consequent consideration, seen in the light of almost three years of pandemic, is that not only does wearing a ffp2 mask in certain conditions protect us from direct inhalation of the virus but, by protecting the nose and helping to keep the internal temperature high, it keeps the the innate mechanism against viruses and bacteria. The discovery obviously opens up to therapeutic solutions that hide this weak point, such as nasal sprays that strengthen weakened immune defenses.

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