Rhinologists have long observed that upper respiratory infections spike during the colder months, but the exact mechanisms involved have been somewhat elusive. Now, a new study suggests that at least three distinct immune responses in the nasal mucosa are involved, with even minor fluctuations in temperature impairing those responses and making people far more susceptible to infection.
Explore This IssueMarch 2023
“There’s a huge amount of epidemiologic evidence showing that viral infections spike every winter and that the number of infections track very well with temperature,” said Benjamin S. Bleier, MD, director of endoscopic skull base surgery and the Claire and John Bertucci Chair in Otolaryngology–Head and Neck Surgery at Massachusetts Eye and Ear in Boston. “But nobody had ever really understood mechanistically why that happened. That’s what we were after in this study.”
Dr. Bleier said the foundation for the study traces back to work he did on how nasal mucosa responds to bacterial infection. He and his colleagues found that when the nose is exposed to bacteria, a robust immune response occurs that’s mediated by the production of toll-like receptor 4 (TLR4). These receptors trigger the release of a “swarm” of nasal epithelium-derived extracellular vesicles (EVs) that recognize bacteria as invading microbes (J Allergy Clin Immunol. 2019;143:1525-1535.e1).
These innate processes trigger “what amounts to the body’s first line of defense against infectious agents,” Dr. Bleier told ENTtoday.
But do similar mucosal responses occur in the presence of viruses? In the case of EV formation, “most definitely,” he said. “What we found in the first arm of our current study is that, in the presence of a TLR variant known as TLR3, EV production increased by 160% over baseline” (J Allergy Clin Immunol. 2022;28:S0091-674901423-3).
Kicking the Hornet’s Nest
Dr. Bleier likened the heightened EV release to what happens when you kick a hornet’s nest and a swarm emerges to defend against any outside threats. “That’s what these EVs are essentially doing,” he explained. “They’re in effect leaving the body, going into the mucus, and attacking the virus—in this case before the virus binds to the cells. And that’s one of the more amazing aspects of our findings: It’s an example of your immune system fighting pathogens in the de facto external environment.”
Though important, that swarm response is actually the least powerful component of EV activation, Dr. Bleier noted. He and his colleagues elucidated a second, far more potent process: Each of the EVs produced in the nose is bedecked with up to 20 times more receptors on the surface of the vesicles than at baseline. That’s key, he noted, because instead of binding to epithelial cells as an entry point for infection, the viruses are tricked into binding to receptor-rich EVs in the nasal mucosa.
“They basically act as decoys,” Dr. Bleier said. “When you inhale a virus, the virus has an option of binding either to your mucosal cells or to the vesicles. By increasing the receptor density by 20 times, it increases the likelihood that the virus binds to the vesicle, where it can be inactivated before it ever has a chance to infect the cell.”
But there’s a third soldier in this fight: an antiviral family of agents known as microRNAs (miRNAs) that, when recruited, exert yet another powerful effect on innate mucosal immune function. Dr. Bleier said he looked at miRNA because it has proven antiviral activity (Int Immunol. 2017;29:157-163). The result? “We found a 13-fold increase in the concentration of miRNAs within the body of the EVs after exposure to TLR3,” he said. “Taken together, we’ve elucidated a three-pronged mechanism of innate nasal immunity occurring in the presence of viruses: an increase in EV formation, an increase in ‘decoy receptors’ on the surface of those EVs, and an increase in potent antiviral microRNA particles.”
Dr. Bleier added that these immune responses occurred against both coronaviruses and rhinoviruses. He likened this effect to that of a broad-spectrum antibiotic. “What we’ve found,” he said, “is a broad-spectrum antiviral mechanism that’s salient and powerful against a host of different respiratory viruses that one might inhale in the environment.”
All these responses, he added, start at the front of the nose—a “geographical” aspect to his study that’s critically important, he noted. “It’s really the anterior aspect of the nose that’s the first area that encounters and then responds to inhaled pathogens,” he explained. “And, remember, we inhale up to 10,000 liters of air per day. So that presents a huge load of insults to the immune system, which is particularly susceptible to temperature fluctuations.”
Measuring the Temperature Gradient
To measure those fluctuations, Dr. Bleier and his colleagues endoscopically guided a thermocouple into healthy human subjects’ noses and then exposed them to 40° F external air temperature. They found that after about 15 minutes of exposure, a drop of about 5 degrees centigrade or about 9 degrees Fahrenheit occurred. The researchers then looked at how all the immune responses elucidated in the first part of their study were affected by that temperature change.
What they found was somewhat surprising: Even this nominal change in temperature had a profound blunting effect on nasal immune responses. Specifically, a 5-degree Celsius drop in ambient air temperature resulted in a 41.9% decrease in total EV release, a 23.8% reduction in miRNA expression, a 24.4% reduction of antiviral surface receptor binding activity, and a 77.2% reduction in EV receptor density.
“The numbers were a bit different based on the individual virus models we tested,” Dr. Bleier said. “But the overall effect was striking and boiled down to about a rough doubling of virus density in the presence of cold air. What this means is that if your nasal mucosa is exposed to a 5-degree drop in ambient air temperature, your risk for becoming infected with a virus roughly doubles and your ability to fight off that infection is cut in half.”
The findings have clear public health implications. “I think it’s fair to say that our results offer a mechanistic explanation for what we’ve learned throughout the COVID-19 pandemic: that using masks, at least in the cold months, probably is one of the best things you can do if you want to avoid an infection,” Dr. Bleier said. In addition to blocking viral particles, he explained, “the masks also retain a cushion of warm air in front of the face, thereby preventing the temperature drop in the nose that we have shown can be so damaging to innate mucosal immunity.”
The findings also have drug development implications. Most of these immune processes occur in the anterior nasal passages. “So, all we may need to do is deliver an antiviral spray or a gel to the front of the nose; that could well be the only area that needs to be dosed for a significant antiviral effect to occur,” Dr. Bleier said, adding that he’s currently in the planning stages for such research.
5 Degrees of Separation?
Noam A. Cohen, MD, PhD, the Ralph Butler Endowed Professor for Medical Research and director of rhinology research in the Department of Otorhinolaryngology at the University of Pennsylvania in Philadelphia, has been researching nasal epithelial biology and innate immunology for nearly 20 years. “Because of the COVID-19 pandemic, as well as findings such as Dr. Bleier’s, I’m now realizing that for the last two decades, I may have been doing those experiments at the wrong temperature.”
Dr. Cohen explained that most of his SARS-CoV-2 viral replication research was done at 37° C (Proc Natl Acad Sci U S A. 2021;118:e2022643118). “Now we’re seeing that these respiratory viruses do much better at 32 or 33 degrees; a lot of them have adapted to thrive at these colder temperatures,” he told ENTtoday. “Put that together with Dr. Bleier’s findings, [and] it becomes clear that our innate immune systems actually are downregulated at colder temperatures, where these viruses really have a leg up” in terms of replication and infectivity.
Are such findings groundbreaking? “They could be,” Dr. Cohen said. “Dr. Bleier’s finding that the EV cell secretes parts of itself into the mucus and is transported into other parts of the nose where other cells take it up is really interesting. He’s taken a very elegant approach toward elucidating one of the primary defense mechanisms of the upper respiratory system.
“Having said that, I do have questions about some of his methods,” Dr. Cohen added. “For example, he didn’t actually look at virus viability. He used mRNA to detect viral messaging, but he didn’t actually take the traditional classic virology step of looking for plaque-forming units. So maybe there’s downregulation of the mRNA, but does that necessarily translate to less virus replicating at warmer temperatures? Maybe yes, maybe no.”
Dr. Bleier responded by saying, “Of course, Dr. Cohen is correct that [plaque-forming unit] experiments would further buttress our dataset. However, recall that the gold standard in viral load detection clinically is PCR, which detects RNA [in ways] similar to our approach.”
As for any clinical implications of Dr. Bleier’s work, “One can imagine that warming air in a nasal cannula, which we typically do at ambient room temperature, might make some sense,” Dr. Cohen said, given the extent to which immune defense functions were blunted by colder air in Dr. Bleier’s study.
More on Masking, Drug Development
The implications of Dr. Bleier’s findings on the value of masks during cold and flu outbreaks resonated with several rhinologists interviewed by ENtoday. “I love the idea that moms everywhere can now feel vindicated and justified in their incessant bundling of all our faces and heads in the winter when we were kids!” quipped Zara Patel, MD, a professor of otolaryngology and director of endoscopic skull base surgery at Stanford University School of Medicine in California.
Joshua M Levy, MD, MPH, an associate professor and director of resident research in the department of otolaryngology–head and neck surgery at Emory University School of Medicine in Atlanta, had a more forward-thinking view. “This is an exciting start to research that will hopefully support public health best practices to minimize the transmission of respiratory viral illness.”
Several experts also saw the study’s drug development implications. “Most of the antivirals that we’re dealing with these days are systemic,” Dr. Cohen said. “But there’s a burgeoning area of research stimulated by the COVID-19 pandemic looking into nasal sprays and other topicals.
“Perhaps we should be warming up these antiviral nasal sprays that we’re putting in the nose,” Dr. Cohen added. “It would only have to be by 4 to 5 degrees Celsius, as Dr. Bleier demonstrated.” Taking this approach to the development of nasal sprays and clinical implications for their use, he noted, “we might actually not only increase innate immunity, but also decrease what the virus has already done to nasally adapt.”
As for next steps in research and development on the topic, several experts stressed that Dr. Bleier’s work, although sound, has a “basic science” component that needs to be acknowledged. This duality was expressed well by Dr. Levy, who first praised the study design. “The findings represent a true discovery and identify novel mechanisms for innate mucosal immunity,” he said. “It’s surprising that, in the year 2023, we continue to gain foundational insight into the immune mechanisms that protect our airways. I do think the overall clinical implications of this study are yet to be completely defined, however. It’s important for future research to evaluate the impact of this change on clinical outcomes such as infection susceptibility and symptom severity.”
David Bronstein is a freelance medical writer based in New Jersey.