In the past two to three decades, there has been a surge in the use, abuse, and misuse of opioids. Among opioid users, Illicit fentanyl use has become widespread in the US. Moreover, synthetic fentanyls rapidly flood the illicit drug market due to their cheap manufacturing, enhanced potency, and chemical composition that evades toxicology screening. Furthermore, black-market drug makers create new fentanyl analogs to avoid classification as illegal, to get around policy restrictions. Although drugs of abuse and HIV are entwined epidemics, it is becoming clear that Fentanyl plays an alarming role in new outbreaks. Recent reports suggest that there's an elevated risk of acquiring HIV-1 infection in those who inject fentanyl. Additionally, fentanyl is commonly detected in urine drug-screen tests in people living with HIV (PLWH). Advances in antiretroviral therapy has extended the lives of PLWH. However, as people live longer, the risk of altered neurological manifestations increases. It is also well known that drugs of abuse compound these effects of HIV. Even though it is known that opioids and their receptors are implicated in the pathogenesis of NeuroHIVs, there's essentially no information available about whether fentanyl has effects on the status of HIV infection in the CNS. In this study, we will examine the overall hypothesis that: fentanyl and novel synthetic fentanyl analogs facilitates HIV-1 leukocyte transendothelial migration, microglial HIV infection/replication and impairs the integrity and function of the BBB. We propose three specific aims to explore the hypothesis. In Aim 1, using our latest tissue-engineered microfluidic model of the human BBB, we will perform analyses of the kinetic changes in BBB permeability, transporter status and immune-endothelial interaction in response to fentanyl and other synthetic fentanyls. Since microglial is the primary refuge for HIV in the brain, Aim 2 will determine the effects of synthetic fentanyls on dynamic HIV-1 infection in the resident immune cell of the brain, the microglial. Comparisons will be made to monocyte derived macrophages. Aim 3 will examine the molecular mechanism responsible for a dysfunctional BBB by fentanyls. This proposal comprehensively addresses the key tenets of RFA-DA-23-012 and features multiple synthetic fentanyls, a translational model of the BBB, and experiments to define the underlying molecular mechanism involved. Our studies will provide important insights on how the interplay of two major pathologic factors (HIV and fentanyl) in the CNS impair the BBB and compromise the intracellular anti-HIV immunity of microglia, the key mechanisms that could significantly accelerate the development NeuroHIV.