Mobilization of the innate immune response by a specific immunostimulant β-glucan confers resistance to chronic stress-induced depression-like behavior by preventing neuroinflammatory responses

Pre-stimulation of the innate immune response is an effective strategy to prevent depression-like phenotypes in animals. However, the use of conventional immunostimulants may cause adverse effects. Therefore, the search for agents that stimulate the innate immune response but do not induce a pro-inflammatory response could be a new research direction for the prevention of depression. β-glucan is a polysaccharide from Saccharomyces cerevisiae with unique immunomodulatory activity in microglia without eliciting a pro-inflammatory response that could lead to tissue damage. This suggests that β-glucan may be a suitable drug that can be used to prevent depression-like phenotypes. Our results showed that a single injection of β-glucan 1 day before stress exposure at a dose of 10 or 20 mg/kg, but notat a dose of 5 mg/kg, prevented depression-like behavior in mice treated with chronic unpredictable stress (CUS). This effect of β-glucan disappeared when the time interval between β-glucan and stress was extended from 1 day or 5 days to 10 days, which was rescued by a second injection 10 days after the first injection or by a repeated injection (4×, once daily) 10 days before stress exposure. A single β-glucan injection (20 mg/kg) 1 day before stress exposure prevented the CUS-induced increase in brain pro-inflammatory cytokines, and inhibition of the innate immune response by minocycline (40 mg/kg) abolished the preventive effect of β-glucan on CUS-induced depression-like behaviors and neuroinflammatory responses. These results suggest that β-glucan may prevent chronic stress-induced depression-like phenotypes and neuroinflammatory responses by stimulating the innate immune response.