In an analysis of the results of 114 prior studies, scientists have found that some people with schizophrenia, bipolar disorder, and major depressive disorder have biomarkers in their blood indicative of a high level of inflammation. The research helps point researchers toward biological pathways that may connect inflammation to mental illness in some cases.

Clinical studies have begun to evaluate whether anti-inflammatory treatments can benefit patients with depression and schizophrenia, but results so far have been mixed. The new analysis, published February 23, 2016 in Molecular Psychiatry and conducted by a research team led by David Goldsmith, M.D., at Emory University, and including 1999 NARSAD Independent Investigator Mark Hyman Rapaport, M.D., also at Emory, could help scientists identify which patients are the best candidates for such treatments.

Using data from past studies, the researchers compared how the levels of inflammation markers in the blood vary among patients with schizophrenia, bipolar disorder, and major depressive disorder. They also assessed how those levels changed following treatment for acute episodes of each illness. The markers included both pro-inflammatory and anti-inflammatory molecules.

Profiles of these molecules indicated more inflammation in people with schizophrenia, bipolar disorder, and major depressive disorder compared to unaffected individuals. This was true among chronically ill individuals, as well as those experiencing acute episodes of illness. Levels of three pro-inflammatory molecules were particularly high during acute episodes of any of the three disorders. In general, inflammation-regulating molecules returned to closer-to-normal levels following treatment.

The researchers identified consistent patterns across disorders suggesting that alterations in the same biological pathways may link inflammation—and the immune system dysfunction that causes it—to schizophrenia, bipolar disorder, and major depressive disorder. However, the average effect among all the patients included in the 114 studies was small. According to the researchers, this suggests inflammation is related to mental illness only in some patients. Identifying those with immune system involvement could be important for future studies evaluating anti-inflammatory treatments for mental illness, they say.