If your parents or grandparents were like mine, you probably heard this as a child heading out to play in the snow: "Put your hat and scarf on. If you don't, you'll catch a cold." Years later, all grown up with a microbiology doctorate hanging on my wall, I know that viruses cause colds, not chilly weather. Their admonitions, while well-intentioned, were based on nothing but folklore and superstition. Right?

Perhaps not. New research published in PNAS shows that colder temperatures may make your immune system more susceptible to rhinoviruses, the most common cause of colds.

It was previously known that rhinoviruses are more likely to infect the nasal cavity than the lungs because of the lower temperature. The nasal cavity, which is more exposed to the environment, has a lower temperature (33-35 degrees C) than the lungs (37 degrees C). Rhinoviruses prefer lower temperatures, so they replicate more efficiently in the nasal cavity. The authors were able to reproduce this in mouse airway epithelial cells. (See figure.)

As shown above, after about 40 hours, a mouse-adapted rhinovirus (RV-1BM) incubated with mouse cells at 33 degrees (blue circles) was 1,000 times more efficient at replicating than a virus incubated at 37 degrees (red circles). (Note that the Y-axis is logarithmic. Also, ignore RV-1B, which is not mouse-adapted.) Thus, the data shows that the authors' model confirms previous observations.

Then, the authors turned to the real question: Why? What is it about rhinoviruses that makes them prefer 33 degrees? Previous research has failed to provide an explanation. There appears to be nothing about the virus itself that makes it favor lower temperatures. So, the authors turned their attention to the host's immune response. And here, they found their answer.

Mouse cells infected with rhinovirus produce a weak innate immune response at 33 degrees C. The authors discovered that secretion of interferon-beta, a molecule important in the body's anti-viral response, is much lower at the colder temperature. (See figure.)

Furthermore, the authors found that other cells are less responsive to the presence of interferon-beta at 33 degrees. In other words, the lower temperature diminishes the host's front-line immune system.

This research hints that keeping your nasal cavity warm might be an effective way to prevent rhinovirus infection. Maybe Mom was right, after all.

Source: Ellen F. Foxman et al. "Temperature-dependent innate defense against the common cold virus limits viral replication at warm temperature in mouse airway cells." PNAS. Published online before print. 5-Jan-2014. doi: 10.1073/pnas.1411030112

(AP photo)