EXPERIMENTS on mice are widely used to help determine which new cancer therapies stand a good chance of working in human patients. Such studies are not perfect and, all too often, what works in a rodent produces little or no benefit in people. This has led researchers to explore the ways in which mice and men are dissimilar, in order to pick apart why the responses are different. A new study now proposes that the temperature in which lab mice are kept is one thing that does matter.

Mice, if left to their own devices, will seek places with a temperature of around 30°C to minimise heat loss from their small bodies. But lab mice rarely enjoy such toasty climes. Researchers tend to keep them at 20-26°C so that their cages stay cleaner for longer (mice then drink and urinate less) and so that lab technicians do not have to endure sweltering conditions. It has not been seen as a problem because mice are perfectly capable of maintaining their body temperatures in cooler environments. However, Elizabeth Repasky, an immunologist at Roswell Park Cancer Institute in Buffalo, New York, wondered if there might be more to it than that and designed an experiment to find out.

Dr Repasky and her colleagues put mice into two different types of enclosures: one sort kept at 22-23°C and the other at 30-31°C. The mice were left for two weeks to acclimatise before being injected with tumour cells. The results, just published in the Proceedings of the National Academy of Sciences, revealed that the tumours in the mice caged in the cooler environment showed a typical rapid rate of growth whereas those in the warmer mice grew 40-60% slower, depending on the cancer type. Was this, Dr Repasky wondered, because their immune systems were functioning differently?

To investigate this, blood samples were collected from the mice and tested for the presence of anti-tumour immune cells, known as cytotoxic T lymphocytes. A striking increase in these cells was found in the tumours of the mice in the warmer housing. The researchers also found an increase in the types of cell that promote tumour growth in the mice kept in cooler conditions. This suggested that warmer temperatures were in some way helping the mice mount a more effective defence against cancer.

Another experiment was arranged to see if, given the choice, mice with cancer would move to warmer places. Both healthy and tumour-carrying mice were put in an apparatus with five chambers maintained at either 22, 28, 30, 34 or 38°C. As expected, the healthy mice spent the most time in the 30°C chamber, but those with cancer stayed mainly in the 38°C one.

Dr Repasky argues that these effects are likely to be due to the immunosuppressive response of cold “stress”. The mice in cool laboratories maintain stable body temperatures, she suspects, by diverting resources away from their immune functions and this, in turn, makes them less capable of fighting cancer. Because of the implications for cancer research she proposes that it would be better if the ambient temperatures in which laboratory mice are housed is raised so that their immune systems can function more normally.

Is there also a lesson to be learned from laboratory mice for human cancer patients? Dr Repasky explains that, unlike mice kept in cages, most humans are able to manipulate their environment by putting on extra clothes or turning up the heating, so should not experience chronic cold stress. Still, she adds, some cancer patients do indeed report feeling chills, although cold is only one of many forms of stress that the body has to cope with. Others, such as psychological stress, might also have an immunological effect on the progress of the disease.