It’s common knowledge that colds and flu are more common in the colder months, but why this is has long been a matter of scientific debate.
Now researchers claim to have finally solved the riddle – and it has to do with a previously unidentified immune response in the nose.
According to the experts, this evolved immune response fights viruses responsible for infections, but is suppressed by colder temperatures.
Their research challenges the theory that winter illnesses are more common simply because people are trapped indoors.
Scientists have discovered the biological reason why colds are more common when there are cold temperatures in winter
The study was led by researchers at Mass Eye and Ear Hospital and Northeastern University in Boston, Massachusetts.
Cold weather and flu: past theories
– People are indoors more often, making it easier for viruses to pass from one person to another
– Cold, dry air can weaken resistance to pathogens
– Lower temperatures affect the defense against infection
Source: Johns Hopkins University
“There’s never been a compelling reason why you have this very marked increase in viral infectivity in the cold months,” said study author Dr. Benjamin Bleier of Mass Eye and Ear.
“It was conventionally thought that the cold and flu season occurred in the colder months because people are more confined indoors where airborne viruses can spread more easily.
“However, our study points to a biological cause for the seasonal variation in upper respiratory tract viral infections we see each year, most recently demonstrated during the Covid-19 pandemic.”
Dr. Bleier and colleagues previously discovered an innate immune response that is activated when bacteria are inhaled through the nose.
Cells at the front of the nose detect the bacteria and then release billions of tiny fluid-filled sacs called extracellular vesicles (or EVs, formerly known as exosomes) into the mucus to surround and attack the bacteria.
“It’s like kicking a wasp’s nest and all the hornets come out and attack,” Dr. Bleier said.
The sacs also transport protective antibacterial proteins through the mucus from the front of the nose to the back of it along the airways, which then protect other cells from the bacteria before it gets too far into the body.
A 2018 study led by Dr. Bleier revealed an innate immune response that is activated when bacteria are inhaled through the nose. It found that cells at the front of the nose sensed the bacteria and then released billions of tiny fluid-filled sacs called extracellular vesicles (or EVs, formerly known as exosomes) into the mucus to surround and attack the bacteria.
For the new study, the researchers wanted to see if this immune response was also triggered by viruses that were inhaled through the nose.
Viruses are the source of many common upper respiratory infections, including sinusitis, pharyngitis, and the common cold.
Common upper respiratory infections
Upper respiratory infections (RTIs) affect the sinuses and throat.
– Sinusitis (sinusitis)
Researchers also wanted to see if the temperature of the air reduces the antiviral immune response, trying to explain why we become particularly susceptible to colds in the winter.
The team analyzed how cells and 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, typical of the winter flu season.
They found that each virus triggered an EV swarm response from nasal cells, albeit using a different signaling pathway than the one used to fight bacteria.
Researchers also discovered a mechanism that plays a role in the response to the viruses, under normal conditions of body heat.
Upon release, the EVs acted as decoys to which the virus would bind instead of the nasal cells.
Next, the team tested how colder temperatures affected this response, to simulate the drop to icy winter conditions.
Healthy people from a room temperature environment were exposed to temperatures of 39.9°F (4.4°C) for 15 minutes, so the temperature in the nose dropped about 9°F (5°C).
Researchers then applied this temperature reduction to the nasal tissue samples and saw that the immune response was not as strong.
The amount of EVs emitted from the nasal cells dropped by nearly 42 percent, and the antiviral proteins in the EVs were also affected.
“Combined, these findings provide a mechanistic explanation for the seasonal variation in upper respiratory tract infections,” said study author Dr. Di Huang of Harvard Medical School and Mass Eye and Ear.
The findings, which will be published in The Journal of Allergy and Clinical Immunology, could lead to treatments based on the body’s own defense mechanism.
For example, a nasal spray could be designed to increase the number of EVs in the nose, which could help fight pathogens ranging from Covid to the common cold.
The researchers hope to replicate the findings with other diseases in the future, including SARS-CoV-2, the virus that causes Covid.
It was right under our noses all along! Study finds never-before-seen nasal defense system against germs releasing billions of tiny insecticidal sacs
In 2018, scientists discovered an unprecedented defense system against germs that enter the nose.
Cells in the nostrils release billions of tiny insecticidal sacs into mucus when they sense dangerous bacteria have been inhaled.
Not only do these sacs, called exosomes, kill bacteria, they also warn surrounding cells to protect themselves from invading pathogens.
Researchers at the Massachusetts Eye and Ear Teaching Hospital made the discovery.
To determine the exact role of exosomes, they analyzed nasal tissue in the lab, as well as in patients undergoing surgery on their noses.
The results showed that within five minutes of cells in the nose being exposed to potentially dangerous bacteria, the number of exosomes released into the mucus doubled.
Speaking of exosomes, sinus surgeon and lead author Dr. Benjamin Bleier told me New scientist: ‘They are just as potent at killing bacteria as an antibiotic.’
But not all exosomes destroy bacteria. Many move to the back of the nose where they fuse with other cells to alert them to invading pathogens.
They then deposit proteins and part of their genetic material in these cells, so that they too are armed against dangerous bacteria, the study found.
This could explain another discovery made by the scientists, who also found that tiny hairs in the nostrils sweep pathogens higher up the nose instead of pushing them out.
Once the cells at the back of the nose are made aware of this dangerous bacteria, the pathogens can be swallowed and destroyed by the gut, Dr. Bleier said.
“We have shown in a living patient that the immune system reaches outside the body and actually attacks pathogens before they enter the body,” he said.
“It’s the only example of this I know of.”
The researchers plan to study how exosomes fuse with cells in hopes of using this process to deliver drugs more effectively.
Discovered in 1983, exosomes have previously been linked to a person’s overall health, including cancer risk.
This comes after research published earlier this month suggested llamas could hold the key to a long-lasting flu vaccine.
Laboratory tests conducted by pharmaceutical giant Janssen, Beerse, showed that a protein produced by the fluffy animals, as well as camels, fought off the virus in mice.