A novel soil-derived Mycolicibacterium decreases anxiety-like defensive behavioral responses in association with decreases in biomarkers of neuroinflammation and hippocampal microglial priming in adult male rats.

Major traumatic life events are risk factors for stress-related neuropsychiatric disorders, often accompanied by systemic inflammation, neuroinflammation, and microglial priming. As systemic inflammation, neuroinflammation, and microglial priming are considered risk factors for developing stress-related psychiatric disorders, one novel therapeutic strategy is to identify interventions that mitigate these responses. In this study, we investigated the effects of a novel soil-derived Mycolicibacterium, Mycolicibacterium sp. strain KGA-10, on in vitro immunoregulatory potential in murine bone marrow-derived dendritic cells (BMDCs) and on biomarkers of systemic inflammation, biomarkers of hippocampal neuroinflammation and microglial priming, and anxiety-like defensive behavioral responses in adult male rats exposed to inescapable tail-shock stress (IS). In Experiments 1 and 2, BMDCs were exposed to the type strain, Mycolicibacterium vaccae ATCC 15483 (0, 10, 30, 100, 300 µg/mL; Experiment 1) or M. sp. strain KGA-10 (100 µg/mL; Experiment 2) or sterile borate-buffered saline (BBS) vehicle followed, 24 h later, by exposure to lipopolysaccharide (LPS; 250 ng/mL) or a cell culture media vehicle, then, 24 h later, assessed for Il10, Il12a, and Il12b mRNA expression. Exposure of murine BMDCs to M. vaccae ATCC 15483 or M. sp. strain KGA-10 induced an immunoregulatory phenotype, characterized by increased ratios of Il10:Il12a and Il10:Il12b mRNA expression in both naïve and lipopolysaccharide- (LPS; 250 ng/mL) challenged conditions. In Experiment 3, adult male rats received weekly injections of heat-killed M. sp. strain KGA-10 (0.1 mg/0.1 mL, s.c.) or sterile BBS vehicle over three weeks prior to IS. Anxiety-like defensive behavioral responses were assessed 24 h following IS or home cage control conditions using the juvenile social exploration (JSE) test, while biomarkers of hippocampal neuroinflammation and microglial priming were assessed using real-time reverse transcription - polymerase chain reaction (real-time RT-PCR). M. sp. strain KGA-10 treatment promoted an anti-inflammatory immunophenotype, evidenced by decreased hippocampal Il12a, and decreased biomarkers of microglial priming, Nfkbia and Nlrp3 mRNA expression among rats exposed to IS, in association with prevention of IS-induced increases in anxiety-like defensive responses in the JSE test. These findings suggest that M. sp. strain KGA-10 is a promising candidate for a novel intervention for promotion of stress resilience and prevention of stress-related psychiatric disorders.