Exposure to acute and chronic stressors sensitizes the proinflammatory response of microglia to a subsequent immune challenge. However, the proximal signal by which stressors prime microglia remains unclear. Here, high mobility group box-1 (HMGB-1) protein was explored as a potential mediator of stress-induced microglial priming and whether HMGB-1 does so via the nucleotide-binding domain, leucine-rich repeat, pyrin domain containing protein 3 (NLRP3) inflammasome. Exposure to 100 inescapable tail shocks (ISs) increased HMGB-1 and NLRP3 protein in the hippocampus and led isolated microglia to release HMGB-1ex vivo. To determine whether HMGB-1 signaling is necessary for stress-induced sensitization of microglia, the HMGB-1 antagonist BoxA was injected into the cisterna magna before IS. Hippocampal microglia were isolated 24 h later and stimulated with LPSex vivoto probe for stress-induced sensitization of proinflammatory responses. Previous IS potentiated gene expression of NLRP3 and proinflammatory cytokines to LPS, that is, microglia were sensitized. Treatment with BoxA abolished this effect. To determine whether HMGB-1 is sufficient to prime microglia, IS was replaced with intracerebral administration of disulfide or fully reduced HMGB-1. Intracerebral disulfide HMGB-1 mimicked the effect of the stressor, because microglia isolated from HMGB-1-treated rats expressed exaggerated NLRP3 and proinflammatory cytokine expression after LPS treatment, whereas fully reduced HMGB-1 had no effect. The present results suggest that the CNS innate immune system can respond to an acute stressor as if it were cellular damage, thereby releasing the danger signal HMGB-1 in the brain to prime microglia by acting on the NLRP3 inflammasome, in preparation for a later immune challenge.