Neuroinflammatory Mechanisms in Depression: From Biomarkers to Anti-Inflammatory Therapy

Major depressive disorder (MDD) is a complex and heterogeneous psychiatric disorder with a high prevalence. Neuroinflammation may define biologically distinct patient subgroups with different mechanisms, clinical phenotypes, and treatment responses. This narrative review integrates current evidence around three linked questions: how neuroinflammatory processes contribute to depression, how biomarkers can identify clinically relevant inflammatory phenotypes, and how these findings can inform anti-inflammatory treatment strategies. The major mechanisms discussed include microglial activation and neuroimmune signaling, hypothalamic–pituitary–adrenal axis dysregulation and glucocorticoid receptor resistance, kynurenine pathway alterations, and cytokine-driven impairment of neurogenesis and synaptic plasticity. These pathways interact with stress responses, neurotransmitter systems, and neuronal function, while their expression may vary according to sex, age, hormonal status, disease stage, and treatment exposure. These interconnected pathways may contribute to depressive symptoms by disrupting neurotransmitter systems and impairing neural plasticity. In addition, this review discusses several candidate biomarkers, including C-reactive protein (CRP), interleukin-1β (IL-1β), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), brain-derived neurotrophic factor (BDNF) and transforming growth factor-β1 (TGF-β), which may support patient stratification, treatment prediction, and assessment of target engagement. Clinical trials of anti-inflammatory agents have shown inconsistent and generally modest effects in unselected MDD populations. By integrating mechanistic evidence with biomarker-guided therapeutic implications, this review aims to clarify how neuroinflammatory research may inform more precise and individualized treatment strategies for depression.