Essential roles of IL-4 signaling in the prophylactic effect of gardiquimod on chronic stress-induced depression-like behaviors and neuroinflammatory responses.

Neuroinflammation is a key pathological feature of depression, and strategies to mitigate neuroinflammatory response hold therapeutic potential. Innate immune priming is a physiological process in which prior immune activation enhances defense against subsequent insults. Evidence indicates that pre-stimulation of innate immune response can confer resilience to stress-induced depression. This study examined the prophylactic effect of gardiquimod (GDQ), a selective Toll-like receptor 7 (TLR7) agonist, in a mouse model of chronic unpredictable stress (CUS). Administration of GDQ (0.5-1.5 mg/kg) one day before stress prevented the development of depression-like behaviors in a dose-dependent manner, whereas a lower dose (0.1 mg/kg) was ineffective. The protective window was temporally limited: a single 1.5 mg/kg dose was effective when given one day before stress onset, but its efficacy was lost with a ten-day pretreatment interval. This loss could be overcome by administering a second GDQ injection ten days after the first, or by using a multi-dose regimen ten days prior to stress. At the molecular level, GDQ pretreatment prevented CUS-induced up-regulation of pro-inflammatory cytokine mRNA in the hippocampus and medial prefrontal cortex. The preventive effects of GDQ on CUS-induced depression-like behaviors and neuroinflammatory responses were abolished by pre-treatment with the innate immune inhibitor minocycline, or by central infusion of an interleukin-4 (IL-4) neutralizing antibody before stress onset. In summary, these findings demonstrate that GDQ, via a mechanism dependent on innate immune priming and subsequent IL-4 signaling, can confer resistance to chronic stress-induced neuroinflammation and depression-like behaviors, supporting its investigation as a prophylactic immunomodulatory agent.