Neuroinflammation in major depressive disorder: Mechanisms, biomarkers, and emerging therapeutic targets
Major depressive disorder (MDD) affects an estimated 280 million people worldwide and remains a leading cause of disability. The monoamine hypothesis, and later the neuroplasticity hypothesis, transformed treatment but leave a substantial share of patients with delayed response, partial remission, or outright treatment resistance. Over the past two decades, evidence has accumulated that a subset of MDD is driven by chronic, low-grade neuroinflammation rather than, or in addition to, monoaminergic deficits. This review synthesizes current evidence on the neuroimmune mechanisms involved in MDD microglial activation, pro-inflammatory cytokine signaling, blood–brain barrier disruption, oxidative stress, kynurenine pathway dysregulation, and impaired neuroplasticity and examines how these mechanisms are captured by peripheral, central, neuroimaging, and emerging molecular biomarkers. It then reviews therapeutic strategies that target inflammation directly (minocycline, celecoxib, TNF-α antagonists), indirectly (selective serotonin reuptake inhibitors (SSRIs), serotonin-norepinephrine reuptake inhibitors (SNRIs), ketamine), or through lifestyle modification (exercise, diet, sleep), with attention to why inflammation-stratified trials have outperformed unstratified ones. Across mechanisms, biomarkers, and treatment response, a consistent pattern emerges: neuroinflammation does not simply co-occur with depression, it may mark a biologically distinct, identifiable subtype roughly a quarter to a third of patients with elevated high-sensitivity C-reactive protein (hsCRP) or IL-6 for whom anti-inflammatory or immune-modulating strategies produce disproportionate benefit. Overall, the findings suggest that neuroinflammation represents a promising and increasingly actionable target for biomarker-guided, personalized psychiatric care.