International and US guidelines for the housing of laboratory mice dictate that temperatures be kept constant and in the range of 20–26 °C. When given a choice, mice instead prefer temperatures of ∼30 °C; this is the temperature at which they can maintain homeostasis without expending extra energy to combat cold stress. So why are lab mice housed at lower temperatures? One reason is that laboratory staff are more comfortable at room temperature, given the heavy protective clothing they are required to wear. Other reasons for lower temperatures are also practical, such as improved breeding efficiency and reduced need for frequent cage cleaning.

But Elizabeth Repasky of the Roswell Park Cancer Institute in Buffalo, NY, and others have suggested that housing mice at these low temperatures does more than just cause the mice to experience cold stress: it influences research results as well. Testing four different mouse cancer models, scientists in Repasky's lab found that mice had slower tumor formation and reduced metastasis when housed at 30–31 °C than when housed at ∼22–23 °C (Proc. Natl. Acad. Sci. USA published online 18 November 2013; doi:10.1073/pnas.1304291110).

Furthermore, their immune responses were also more robust. The warmer mice showed higher numbers of CD8+ lymphocytes, activated CD8+ T cells and white blood cells when compared with the chillier mice. The warmer mice also had lower counts of myeloid-derived suppressor cells that often suppress immune responses to cancer. The authors therefore attributed the difference in tumor growth to the increased antitumor immune response seen in the warmer mice. This idea is further supported by their finding that mice with compromised immune systems showed no temperature-dependent difference in tumor growth. “I don't think any one of us predicted how many fundamental aspects of the antitumor immune response would be affected by simply feeling cold,” Repasky told Nature News.

The findings have important implications, particularly for studies of cancer drugs that target the immune system. Furthermore, Repasky says her research sheds light on “why people who seem to be a lot more chronically stressed than others do more poorly in response to not only their cancer risk but also their overall outcome after therapy.” But the study's findings also indicate how the way in which research animals are housed can have an important effect on research outcomes. “I think we're really undermining the ability of these models to be accurate,” Repasky told The Scientist.