Project Summary: Multiple sclerosis (MS) is a chronic autoimmune central nervous system (CNS) disease and the leading cause of non-traumatic neurological disability in young adults. The cause of MS is complex and cannot be ascribed to any single gene with over 70% risk attributed to environmental factors. Recent studies identified an imbalance in the human gut microbiome within MS patients as one such environmental risk, including depletion of the Lactobacillus genus. Animal models support a causal role for the gut microbiome in MS, though the mechanism remains unclear. Utilizing a mouse model of MS, we have identified disease resistant and susceptible microbiomes, with stark differences in Lactobacillus species abundance and notable differences in their circulating metabolic by-products known to modulate the immune system. Further, we have identified a single commensal species, Lactobacillus reuteri (L. reuteri), which is sufficient to accentuate MS-like symptoms in the mouse with whole genome sequencing indicating the necessary enzymatic machinery to account for the observed differences in circulating metabolites. The focus of this proposal is to 1) determine the cellular mechanisms underlying the effects of L. reuteri on EAE including both impact on infiltrating peripheral immune cells and CNS resident glial cells and 2) determine the impact of L. reuteri-derived tryptophan metabolites on neuroinflammation. In direct support of the proposed studies, the training plan will develop the knowledge, expertise, scientific communication skills and technical abilities in 1) mouse models of multiple sclerosis focused on host interactions with the gut microbiome including directed microbiome manipulation, 2) immunology, with a focus on neuroimmunology, flow cytometry, and functional assays in vitro and in vivo, 3) microbiology, with a focus on commensal gut bacteria, their culture, isolation, genomic and metabolic characterization, and manipulation, 4) neuropathology, with a focus on techniques to investigate CNS pathology to characterize inflammatory demyelinating lesions in CNS autoimmune disease and blood brain barrier integrity analysis and 5) metabolomics with a focus on bacterial metabolites and their effects on host physiology. The training environment at the University of Vermont (UVM) is multidisciplinary with a collegial atmosphere that stresses active mentorship and as such is uniquely appropriate to support this proposal which bridges autoimmunity, CNS neuropathology and commensal microbiota. This is evidenced by co-mentors with appointments in the departments of Biomedical and Health Sciences (BHSC), Microbiology and Molecular Genetics (MMG), and Neurological Sciences which are connected to a hub of core facilities and the Larner College of Medicine offering ample opportunity to share resources and promote in-person communication.