Systemic administration of β-glucan induces immune training in microglia

Abstract

BackgroundAn innate immune memory response can manifest in two ways: immune training and immune tolerance, which refers to an enhanced or suppressed immune response to a second challenge, respectively. Exposing monocytes to moderate-to-high amounts of bacterial lipopolysaccharide (LPS) induces immune tolerance, whereas fungal β-glucan (BG) induces immune training. In microglia, it has been shown that different LPS inocula in vivo can induce either immune training or tolerance. Few studies focused on impact of BG on microglia and were only performed in vitro. The aim of the current study was to determine whether BG activates and induces immune memory in microglia upon peripheral administration in vivo.MethodsTwo experimental designs were used. In the acute design, mice received an intraperitoneal (i.p.) injection with PBS, 1 mg/kg LPS or 20 mg/kg BG and were terminated after 3 h, 1 or 2 days. In the preconditioning design, animals were first challenged i.p. with PBS, 1 mg/kg LPS or 20 mg/kg BG. After 2, 7 or 14 days, mice received a second injection with PBS or 1 mg/kg LPS and were sacrificed 3 h later. Microglia were isolated by fluorescence-activated cell sorting, and cytokine gene expression levels were determined. In addition, a self-developed program was used to analyze microglia morphological changes. Cytokine concentrations in serum were determined by a cytokine array.ResultsMicroglia exhibited a classical inflammatory response to LPS, showing significant upregulation of Tnf, Il6, Il1β, Ccl2, Ccl3 and Csf1 expression, three h after injection, and obvious morphological changes 1 and 2 days after injection. With an interval of 2 days between two challenges, both BG and LPS induced immune training in microglia. The training effect of LPS changed into immune tolerance after a 7-day interval between 2 LPS challenges. Preconditioning with BG and LPS resulted in increased morphological changes in microglia in response to a systemic LPS challenge compared to naïve microglia.ConclusionsOur results demonstrate that preconditioning with BG and LPS both induced immune training of microglia at two days after the first challenge. However, with an interval of 7 days between the first and second challenge, LPS-preconditioning resulted in immune tolerance in microglia.

Early-life stress lastingly impacts microglial transcriptome and function under basal and immune-challenged conditions

“…Morphological characterization of hippocampal microglia from ELS-exposed mice under basal and immune-challenged conditions Microglia can adapt a range of morphologies in response to homeostatic disbalance (16,49,50). We determined the effect of ELS on microglia density, coverage and morphology in subregions of the hippocampus of adult mice under basal conditions and in response to LPS (Figure 1A).…”

Early-life stress lastingly impacts microglial transcriptome and function under basal and immune-challenged conditions

“…Microglia can adapt a range of morphologies in response to homeostatic disbalance [16,50,51]. We determined the effect of ELS on microglia density, coverage, and morphology in subregions of the hippocampus of adult mice under basal conditions and in response to LPS (Fig.…”

Early-life stress lastingly impacts microglial transcriptome and function under basal and immune-challenged conditions

“…Morphological characterization of hippocampal microglia from ELS-exposed mice under basal and immunechallenged conditions Microglia can adapt a range of morphologies in response to homeostatic disbalance (16,49,50). We determined the effect of ELS on microglia density, coverage and morphology in subregions of the hippocampus of adult mice under basal conditions and in response to LPS (Figure 1A).…”