Brain T cell interactions in microbial experienced mice

T cells in the brain have long been associated with pathogenic outcomes, with the healthy brain considered an immune privileged organ. This concept has recently been overturned with the description of meningeal lymphatics, roles for immune cells in normal brain homeostasis, and detection of T cells in the brains of healthy subjects and in aging subjects with neurological disease. Why these T cells are in the healthy brain and what role they play in this critical tissue remain major knowledge gaps. Due to their abnormally hygenic housing conditions, young adult SPF mice display peripheral immune systems that resemble infant cord blood. In contrast, signatures of repeated microbial exposure are already evident in children and young adult humans and these signatures intensify with age. Specific to this proposal, young adult SPF mice, which are the subjects in many if not most animal models of neuroscience research, have low numbers of T cells in their brains at homeostasis. Several models have been implemented to “normalize” the peripheral immune system of inbred SPF mice to better mimic humans. One informative approach, to generate such “dirty mice” leverages the co-housing (CoH) of SPF mice with pathogen-infested “pet store” mice to permit natural transfer of mouse pathogens. Alternatively, repeated exposure of SPF mice to known laboratory pathogens (Specific Pathogen Exposed, SPExp mice) has emerged as a tractable approach that can be used at lower biocontainment than the CoH approach. Studies with these models have largely focused on understanding how “normalization” of the immune system influences peripheral immune responses to infection or tumors within lymphoid organs and barrier tissues. Thus, the impact of repeated microbial exposure on the generation of brain-surveilling T cells is currently unknown. Our preliminary studies reveal that not only does repeated microbial exposure normalize the peripheral immune system, it also results in substantial and specific increases in memory CD4 and CD8 T cell numbers in the brain. In parallel, our preliminary data show here that brain-residing T cells interact with other immune cells in the CNS, including CD11c+ dendritic cells (DCs), that perform essential functions during brain homeostasis and in response to brain trauma, infections, tumors, and neurodegeneration. Due to the heavy reliance on SPF mice for studies of brain function, the consequences having “normalized” populations of brain-residing memory T cells during homeostasis or neurological disease remain unknown. Our long-term goal is to exploit models of repeated microbial exposure to “normalize” the immune system and understand the roles of brain-residing memory T cells in shaping brain function and neuroimmune responses. Specific Aim 1. Determine how “normalized” numbers of brain-residing CD8 and CD4 T cells impact the functional status and cellular interactions of brain microglia and CD11c+ DCs at homeostasis. 1.1. Determine the epigenetic and transcriptomic landscape of myeloid cells in SPF and SPExp mice. 1.2. Visualize the spatiotemporal dynamics of direct T cell-myeloid interactions in vivo in SPF and SPExp mice.