Summary/Abstract Widespread use of effective antiretroviral therapy (ART) in the past two decades has improved the clinical manifestations and altered the pathology of neurologic disease in people living with HIV (PLWH). However, in the context of this treatment that has successfully prolonged millions of lives, new pressing questions have emerged regarding the etiology of persistent neurological and cognitive impairment that frequently manifests in PLWH on ART. The human brain is a complex and inaccessible organ that, to date, defies comprehensive understanding in the context of homeostasis and disease. Thus, the hallmark neuropathologic finding of HIV associated neurocognitive disorder (HAND), synaptodendritic injury, has thus far only been possible to evaluate in post-mortem human autopsy brains and animal models of HIV. Our group at Yale recently developed and validated a novel positron-emission tomography (PET) radiotracer, [11C]UCB-J, a ligand for the presynaptic vesicular membrane protein synaptic vesicular glycoprotein 2A (SV2A), to image synaptic density in the human brain. The premise of this application is based on our ongoing pilot study which demonstrates that brain SV2A PET successfully identifies regions of reduced synaptic density -- including in a hippocampal- frontostriatal neural circuit that correlates with CNS dysfunction -- in PLWH on ART. In the current proposal, we seek to measure synaptic density longitudinally over 24 months in a larger group of PLWH on ART relative to a matched prospectively enrolled HIV-negative group. We will combine this breakthrough imaging methodology of synaptic density with PET imaging of neuroimmune status (using radioligand [11C]PBR28 targeting the 18-kDa translocator protein, or TSPO, sensitive to microglia) to explore the premise that neuroimmune dysregulation during ART mediates reduced synaptic density. Finally, we aim to longitudinally integrate these measures with neurocognitive and laboratory assessments in parallel processing models that allow us to dissect mechanisms and clinical outcomes of HAND. Our application is based on robust and compelling preliminary data that demonstrates the utility of SV2A PET imaging in identifying regions of reduced synaptic density in PLWH on ART. Our proposed project will generate powerful clinical-translational support for potentially reversible brain alterations in synaptic density, and their relation to microglia levels and clinical and biological measures in PLWH. This program will set the stage for identification of therapeutic targets and provide a biomarker for interventional studies aimed to improve HIV-related injury in the CNS.