Deciphering MMP9‑Driven Mechanisms Underlying Rapid Antidepressant Effects from the Perspective of Neutrophil Homeostasis

Abstract Background: Depression exhibits marked clinical heterogeneity, and peripheral neutrophil functional status is closely associated with disease progression and treatment response; however, its role in rapid antidepressant action remains poorly understood. Methods: Taking neutrophil homeostasis as the entry point, this study integrated single-cell transcriptomic sequencing, primary cell functional assays, a Transwell in vitro blood–brain barrier (BBB) model, and three mouse models of depression with distinct neutrophil functional states to systematically dissect the immune mechanism by which matrix metalloproteinase-9 (MMP9) mediates rapid antidepressant effects. Results: The results showed that MMP9 originates predominantly from peripheral neutrophils, whereas brain-resident cells exhibit extremely low basal expression that cannot be substantially upregulated by drug stimulation. The rapid-acting antidepressants 2,5-dimethoxy-4-methylamphetamine (DOM) and ketamine (KET) activate phosphoinositide 3-kinase-protein kinase B (PI3K-Akt) signaling in neutrophils, driving polarization toward an N2 anti-inflammatory phenotype with a moderate upregulation of MMP9 expression. Under neutrophil-homeostatic conditions, MMP9 crosses the BBB via caveolin-1-mediated transcytosis in endothelial cells, without disrupting tight junctions or relying on neutrophil brain infiltration. Upon entering the brain, MMP9 degrades perineuronal nets (PNNs), restores dendritic–spine plasticity, and recovers gamma-oscillation circuit function, thereby mediating rapid antidepressant effects. Notably, therapeutic efficacy was strictly dependent on neutrophil functional status: suppression of neutrophil function depleted peripheral MMP9 supply and markedly attenuated drug responses, which could be rescued by hippocampal MMP9 replenishment; whereas hyperactivation of neutrophils generated excessive, neurotoxic MMP9, and inhibiting MMP9 activity restored drug sensitivity. Conclusions: These findings indicate that neutrophil-derived MMP9 is a key effector molecule linking peripheral immune status to central rapid antidepressant efficacy, and provide an experimental basis and novel molecular targets for neutrophil-state-based immune subtyping of depression and individualized precision intervention.