FPR2 deficiency alleviates LPS-induced depressive-like behaviors in mice by suppressing the microglial CSF1/NLRP3 inflammasome pathway

Depression is one of the most prevalent psychiatric disorders worldwide, yet its pathogenesis remains unclear. Here, we aimed to investigate the effects of formyl peptide receptor 2 (FPR2), a key regulator of innate immunity and inflammation, on lipopolysaccharide (LPS)-induced depression-related behaviors in mice after intraperitoneal administration, and to elucidate its regulatory mechanisms in microglia. FPR2 knockout (Fpr2 -/- ) significantly attenuated LPS-induced depressive and anxiety-like behaviors in mice. LPS markedly increased FPR2 expression in microglia of the prefrontal cortex (PFC) and hippocampus, while only a minimal increase was observed in neurons. FPR2 deficiency alleviated LPS-induced microglial activation and reduced neuronal synaptic alterations. RNA sequencing and validation experiments confirmed that FPR2 deletion substantially decreased LPS-induced microglial NLRP3 inflammasome activation and IL-1β levels in the brain. Mechanistically, FPR2 regulated downstream NLRP3 activation by modulating CSF1, and FPR2/CSF1 activation was governed by its upstream ligand, serum amyloid A (SAA). Analysis of public clinical datasets revealed that SAA1 levels were significantly upregulated in the orbital ventral PFC of patients with major depressive disorder (MDD) and in the plasma of patients with late-life depression. These findings demonstrate that the SAA/FPR2/CSF1/NLRP3 pathway mediates LPS-induced depressive-like behaviors by regulating microglial activation and neuroinflammation.