Research highlights immune system's role in depression

Hebrew University of JerusalemNov 11 2024

A new study highlights the link between inflammation and depression, challenging traditional neurotransmitter-focused theories. An examination of decades of research suggests that immune system imbalances may trigger and sustain depressive symptoms, particularly in high-risk groups. This research paves the way for personalized treatments targeting inflammation, offering new hope for those unresponsive to conventional therapies.

Depression, recognized as the leading cause of disability worldwide, affects nearly one in six people over their lifetimes. Despite decades of research, much remains unknown about the biological mechanisms underlying this debilitating condition. Professor Raz Yirmiya, a pioneering researcher in the field of inflammation and depression from the Department of Psychology at the Hebrew University of Jerusalem, has recently published a comprehensive review in Brain, Behavior, and Immunity, offering new insights that challenge long-held beliefs and open pathways toward personalized treatment.

Traditional theories of depression have focused on neurotransmitters like serotonin and norepinephrine, suggesting that a deficiency in these brain chemicals may lead to depressive symptoms. While widely accepted, these theories have failed to explain why a significant portion of patients do not respond to conventional antidepressants. Over the last 30 years, Professor Yirmiya's research, along with others', has pointed to a different culprit: chronic inflammation, both in the body and the brain.

"In many individuals, depression results from inflammatory processes," explains Professor Yirmiya, who was one of the first researchers to draw connections between immune system dysfunction and depression in the 1990s. In his latest review, he carefully analyzed the 100 most-cited papers in the field, creating what he calls a "panoramic view" of the complex interactions between inflammation and depressive symptoms.

Research dating back to the 1980s has highlighted that depressed individuals often exhibit compromised immune functions. Surprisingly, certain immune-boosting treatments for cancer and hepatitis, which induce an inflammatory response, have been found to cause severe depressive symptoms in patients, offering a glimpse into the immune system's role in mental health. Yirmiya's own experiments further established a mechanistic link between inflammation and mood, showing that healthy individuals injected with low doses of immune-stimulating agents exhibit a temporary depressive state, which can be prevented by either anti-inflammatory or conventional antidepressant treatments.

Professor Yirmiya and colleagues have also shown that stress—often a major trigger for depression—can prompt inflammatory processes, impacting the brain's microglia cells, which are the representatives of the immune system in the brain. Their recent findings reveal that stress-related inflammatory responses may initially activate microglia, but prolonged stress eventually exhausts and damages them, thereby sustaining or worsening depression. "This dynamic cycle of activation and degeneration of microglia mirrors the progression of depression itself," says Yirmiya.

The review also highlights studies that suggest specific groups, such as elderly individuals, those with physical illnesses, individuals who suffered from early childhood adversity, and patients with treatment-resistant depression, are particularly susceptible to inflammation-linked depression. The findings reveal the necessity of anti-inflammatory treatments for certain patients and for microglia-boosting treatments to other patients, indicating that a personalized approach to treatment may prove more effective than traditional one-size-fits-all antidepressant therapy.

Professor Yirmiya concludes, "The research findings from the past three decades underscore the critical role of the immune system in depression. Moving forward, a personalized medicine approach—tailoring treatment based on the patient's specific inflammatory profile—offers hope to millions of sufferers who find little relief in standard therapies. By embracing these advancements, we're not just treating symptoms; we're addressing the underlying causes."

This study not only sheds light on the origins of depression but also sets the stage for future therapeutic approaches, particularly those that target the immune system. Through further investigation, Professor Yirmiya aims to inspire a new wave of treatments designed to replace despair with hope for those suffering from depression.