The emerging role of T cells in depression.

Depression is increasingly recognized as a disorder involving immune brain interactions beyond classical monoaminergic dysfunction. Among immune components, T cells have emerged as key regulators linking peripheral immune dysregulation to central neuroinflammation and impaired neuroplasticity. Accumulating clinical and preclinical evidence indicates that alterations in T cell subsets, including regulatory T cells, Th1 cells, and Th17 cells, contribute to depressive pathophysiology through coordinated effects on blood-brain barrier permeability, glial activation, cytokine signaling, and neurotrophic support. This review synthesizes current evidence on the mechanisms by which T cells migrate into the central nervous system and modulate depressive behaviors. Particular emphasis is placed on the T cell regulation of brain derived neurotrophic factor signaling, and a role for T cell derived extracellular vesicles as modulators of immune neural communication and neuroplasticity. Finally, we discuss the therapeutic implications of targeting T cells in depression, including modulation of T cell subset balance, cytokine-based interventions, microbiota immune regulation, and inhibition of pathogenic T cell trafficking into the brain. Together, these findings position T cells as central orchestrators of immune neural crosstalk and promising targets for mechanism informed immunotherapies in depression.