Obesity raises the risk of certain tumours, but might also amplify the effects of some cancer therapies.

Obesity can fuel tumour growth by suppressing the immune system’s response to cancer — but it could also render tumours particularly sensitive to certain cancer drugs.

Obesity increases the risk of some cancers, but its impact on immune responses to tumours has been unclear. William Murphy at the University of California, Davis, School of Medicine and his colleagues found that immune cells called T cells are more dysfunctional in obese mice and humans than in their non-obese counterparts.

Compared with T cells in thin mice, T cells in obese mice had higher levels of a protein called PD-1, which suppresses immune responses. The researchers found that one link to PD-1 levels is the hormone leptin, which helps to regulate appetite; treating leptin-deficient mice with leptin sped tumour growth and increased PD-1 levels.

The team also showed that tumours in obese mice and humans might be more vulnerable to therapies that inhibit PD-1. But elevated PD-1 expression in obese people could be one of many factors responsible for this effect, the authors say.