Researchers at the University of Illinois report that IDO, an enzyme found throughout the body and long suspected of playing a role in depression, is in fact essential to the onset of depressive symptoms sparked by chronic inflammation. Their study, just published online in the Journal of Immunology, is the first to identify IDO (indoleamine 2,3 dioxygenase) as a molecular switch that induces depressive symptoms in some cases of chronic inflammation. Doctors have known for decades that patients with chronic inflammation, such as that linked to coronary heart disease or rheumatoid arthritis, are more likely than others to become depressed. Some pro-inflammatory drugs, such as interferon-alpha, which is used to treat Hepatitis C and a cancer known as malignant melanoma, also induce symptoms of depression in a significant number of patients. In the new study, mice were exposed to Bacille Calmette–Guérin (BCG), a vaccine used in many parts of the world to prevent tuberculosis. BCG produces low-grade, chronic inflammation in mice, which can be detected by measuring levels of certain immune system proteins, called inflammatory cytokines, in the blood and brain.