Abstract Chronic immune activation is a major factor of HIV disease progression. Bacterial products of microbial translocation from the gut are likely to be a cause of systemic immune activation in chronic HIV infection. However, the mechanism(s) responsible for chronic immune activation remain to be determined. Given that drugs of abuse contribute greatly to HIV infection, it is crucial to study whether use of methamphetamine (METH), one of the most commonly abused drugs among HIV-infected individuals, is associated with HIV infection-related systemic inflammation and chronic immune activation. We hypothesize that METH use result in inflammation and immune activation through induction of the inflammatory and neurotoxic miRNAs, which facilitate HIV replication and the BBB/CNS injury. We propose three specific aims to address this hypothesis. In Aim 1, we will determine the plasma levels of several key microbial components and immune activation markers in METH users with or without HIV infection. We will also measure the plasma levels of inflammatory/neurotoxic miRNAs in these subjects; In Aim 2, we will study whether monocytes and T lymphocytes from METH users express higher levels of the inflammatory/neurotoxic miRNAs than those from non-METH users. We will also investigate whether METH has synergetic effect with the microbial products on enhancing HIV infection of macrophages and whether the cells from METH users are more susceptible to HIV infection than those from control subjects; In Aim 3, we will examine the effects of METH and/or HIV on the expression of inflammatory and neurotoxic miRNAs in the brain tissues and CSF from subjects with or without HIV-associated neurocognitive disorder (HAND). In addition, we will mechanistically investigate the role of inflammatory/neurotoxic miRNAs (let-7, miR-17, -20a, -21, -132, -146a) in the BBB and neuronal injury. These proposed studies with combinational in vitro (Aim 3), ex vivo (Aim 2), and in vivo (Aims 1, 2, 3) approaches are clinically relevant and significant, and will determine previously unrecognized biomarkers and mechanisms for METH use-mediated systemic inflammation/immune activation and impairment of the BBB/CNS.