PROJECT SUMMARY People living with HIV (PLWH) are at the higher risk for impaired cognitive functions such as HIV-Associated Neurocognitive Disorder or HAND. Further, higher use of legal and illegal opioids among PLWH could affect their immune functions and exacerbate CNS impairment. However, little is known about the HIV harboring cell types in brain, effect of ART on specific CNS cell types, and the modification of brain functions by opioid use. The human brain is composed of an enormous diversity of cell types defined by distinct gene expression, morphology, connectivity, and physiology. Single cell sequencing assays enable cell type- specific analysis of the role of these cell types in healthy brain function and disease. The overall objective of this proposal is to reveal the single cell determinants of brain relevant to persistent HIV infection and opioid use disorder using snRNA-seq and snATAC-seq technologies, bioinformatics approaches, and validation studies. Opioids, as with other addictive drugs, affect nucleus accumbens (NAc) and prefrontal cortex (PFC) and both of these regions are also affected by HIV. We put forward an unprecedented concept and experimental system to identify single cell determinants of brain relevant to persistent HIV infection and opioid use disorder as well as potential opportunities to illuminate how genetic variation affects gene expression. Our project has two specific aims: Aim 1: Create a single nucleus transcriptomic atlas of PFC and NAc. snRNA-seq will be performed on these two regions isolated from post mortem brains. Aim 2: Create a single nucleus epigenomic atlas of PFC and NAc. snATAC-seq will be performed on these two regions isolated from post mortem brains. Successful completion of these innovative single-cell studies will generate cellular part list for two NIDA-relevant brain regions, PFC and NAc, and will illuminate novel insights into transcriptomic and epigenetic landscapes of CNS and how they are altered by HIV and opioid use.