Neurological diseases are a leading cause of death and disability, and despite diverse etiologies, share a common theme of immunological dysfunction. The immune and nervous systems are finely tuned to both sense and orchestrate responses to external stimuli. There are billions of neurons in the brain, yet we know next to nothing about how brain neurons interface with the immune system in the CNS. Recent advances in neuroimmunology include the discovery of meningeal lymphatic, roles for immune cells in CNS homeostasis and characterization of tissue resident memory T cells (Trm) that persist in the brain after CNS infection. We recently (Urban et al, Nature Immunology, 2020) provided evidence that Trm were also generated uniquely in the brain after peripheral immunizations and infections, suggesting that processes unique to the brain may foster Trm persistence in this organ after peripheral immunization. However, it is unknown if specific neuronal pathways in the brain contribute to Trm generation or persistence in this organ and whether neuronal signaling influences protection by CNS Trm. Several CNS diseases in humans have immunological etiologies, with T cells thought contribute to the pathogenesis of viral encephalitis (VE), cerebral malaria (CM) Thus, a second unknown is the role of brain neuronal signaling pathways in T cell-mediated diseases of the CNS. We will attack these problems using chemogenetics. Specifically, there are highly conserved neurons within the brainstem, such as the dorsal raphe (DR), which project broadly throughout the CNS and activate in response to immunity-relevant signals such as stress, pain, and itch. Transgenic B6 mice expressing SERT- cre target DR neurons. Stereotaxic injection of adeno-associated viruses expressing flox/stop designer receptors exclusively activated by designer drugs (DREADD), results in expression of the DREADD only in cre- expressing neurons. Transduced cre-expressing neurons are detected by mCherry expression and can be manipulated, depending on their nature (activating or inhibitory signals specified by the precise DREADD), by injection of a designer ligand. We have obtained these mice and virus systems and provided proof of our ability to perform the sophisticated stereotaxic surgeries and deliver the DREADD constructs to the targeted neurons. We will use the chemogenetic technology and our capacity to evaluate CNS immunity and immune-mediated pathogenesis to test the central hypothesis that brain neuronal signaling pathways contribute to the generation, maintenance and function of CNS Trm and the outcome of T cell-mediated CNS diseases. Specific Aim 1. Determine if DR neuronal signaling regulates the generation, maintenance or protective function of brain Trm. Specific Aim 2. Determine if DR neuronal signaling regulates CD8 T cell-mediated lymphocytic choriomeningitis virus (LCMV) induced VE or experimental cerebral malaria (ECM).