Project Summary Immunoregulation, i.e., balanced expression of regulatory and effector T cells, is thought to be compromised in modern high-income settings due in part to reduced contact with commensal and environmentally derived bacteria, also known as “old friends”. Failed immunoregulation is thought to be one factor contributing to recent increases in stress-related and chronic inflammatory disorders as well stress-related psychiatric disorders in which inflammation is a risk factor, such as depression, anxiety, and posttraumatic stress disorder (PTSD), in high-income countries. One of these “old friends” is Mycobacterium vaccae NCTC 11659, a nonpathogenic, environmental saprophyte with anti-inflammatory and immunoregulatory properties. Immunization in the form of a heat-killed preparation of M. vaccae has been shown to increase stress resilience in mice, as measured by prevention of stress-induced increases in anxiety, prevention of stress-induced exaggeration of spontaneous colitis, and prevention of stress-induced exaggeration of chemically induced colitis in a model of inflammatory bowel disease. Immunization with M. vaccae, when conducted either before or after fear conditioning, has also been shown to enhance fear extinction in a rat model of fear-potentiated startle. In other studies, immunization with M. vaccae has been shown to prevent stress-induced exaggeration of anxiety-like defensive behavioral responses in a rat model of learned helplessness in association with suppression of stress-induced microglial priming and neuroinflammation. However, the extent to which these stress resilience and stress-protective effects generalize to other strains of mycobacteria are not known. In this STTR proposal, we propose to screen, using murine bone marrow-derived dendritic cells (BMDCs), human monocyte-derived macrophages (THP-1 cells), and a microglial cell line (BV2 cells), twenty proprietary strains of environmental mycobacteria for anti-inflammatory and immunoregulatory properties. For screening, we will target strains that meet the following criteria: they can readily be cultivated in isolation; they are not closely related to known mycobacterial pathogens; and they represent distinct lineages spanning the phylogenetic breadth of known mycobacterial diversity (of which there are >150 described mycobacterial strains). We will then screen two lead candidates, using male and female rats, to identify novel strains of mycobacteria with stress-protective effects, as measured by prevention of inescapable stress (IS)-induced increases in anxiety, conditioned fear, and escape deficits, and prevention of stress-induced priming of hippocampal microglia and neuroinflammation. We will also evaluate these strains in models of fear-potentiated startle and cued fear in male and female rats. At the completion of these studies, it is our expectation that we will have identified a single lead strain of mycobacteria that has the best properties for further pr...