Common cold actually triggered by chilly weather

People are more likely to catch a cold in cool-weather conditions due to lower immune response to viruses, a new study suggests.
According to the study led by Yale University the common cold virus can reproduce itself more efficiently in the cooler temperatures found inside the nose than at core body temperature.
This finding may confirm the popular yet contested notion that people are more likely to catch a cold in cool-weather conditions, researchers said.
Researchers have long known that the most frequent cause of the common cold, the rhinovirus, replicates more readily in the slightly cooler environment of the nasal cavity than in the warmer lungs.

However, the focus of prior studies has been on how body temperature influenced the virus as opposed to the immune system, said study senior author and Yale professor of immunobiology Akiko Iwasaki.
To investigate the relationship between temperature and immune response, Iwasaki and an interdisciplinary team of Yale researchers spearheaded by Ellen Foxman, a postdoctoral fellow in Iwasaki's lab, examined the cells taken from the airways of mice.

They compared the immune response to rhinovirus when cells were incubated at 37 degrees Celsius, or core body temperature, and at the cooler 33 degrees Celsius.
"We found that the innate immune response to the rhinovirus is impaired at the lower body temperature compared to the core body temperature," Iwasaki said.

The study also strongly suggested that the varying temperatures influenced the immune response rather than the virus itself.
Researchers observed viral replication in airway cells from mice with genetic deficiencies in the immune system sensors that detect virus and in the antiviral response.
They found that with these immune deficiencies, the virus was able to replicate at the higher temperature.
"That proves it's not just virus intrinsic, but it's the host's response that's the major contributor," Iwasaki said.
Although the research was conducted on mouse cells, it offers clues that may benefit people.
"In general, the lower the temperature, it seems the lower the innate immune response to viruses," noted Iwasaki.

Yale researchers also hope to apply this insight into how temperature affects immune response to other conditions, such as childhood asthma.
The study was published in the journal PNAS.