The current opioid injection incidence in the United States has risen to epidemic levels and is associated with increased risk of HIV-1 transmission and disease progression as well as with other infections in several US states. Given that most HIV+ people who inject drugs (PWID) are now likely to receive anti-retroviral therapy, traditional bio-markers such as HIV viral load and CD4 counts are unlikely to provide a comprehensive assessment of disease progression especially as it affects neurocognitive performance. New biomarkers are needed to track the effects of ongoing HIV infection as it impacts neurocognitive functions. Candidate biomarkers can be found in the interaction between HIV infection, dysbiosis, microbial translocation and systematic inflammation. HIV-1 infection is associated with lymphoid depletion in tissues underlying the gut epithelium, allowing microbial products to transduce into the parenchyma where they act as potent inducers of systemic inflammation. Even when HIV-1 viral load is suppressed by anti-retroviral treatment, chronic immune activation resulting from dysbiosis and translocation can persist. Opioid use also induces gut dysbiosis and supports bacterial translocation. The resulting inappropriate immune activation in PWID may enhance HIV-1 replication, lead to premature aging of T cells, and promote HIV disease progression, including exacerbated HIV-associated neurological disorders (HAND). A more comprehensive set of markers is therefore needed to better understand the course of HIV infection among people who inject drugs (PWID)—biomarkers that reflect the combined physiologic impact of ongoing injection drug use and a range of biomic changes that can exaggerate and reinforce the cumulative impact of physiological effects on mental health. The overall objective of our proposed study is to advance the understanding of the effects of HIV infection and injection drug use on the microbiome, with the goal of discovering biomarkers for neurological disease progression among PWID that are related to chronic inflammation and dysbiosis coupled inflammation. Our highly experienced research team will make use of a well-established cohort of PWID in Puerto Rico—a region with historically high levels of injection drug use and an HIV incidence rate that is disproportionately associated with drug use. A prospective, multi-cohort longitudinal study will allow repeated measures from the study population and compare them with similar measures from control groups of HIV negative PWID and HIV-1 infected non-drug users from the same location. This longitudinal evaluation will allow our team to test the hypothesis that HIV-1 infection in PWID intensifies dysbiosis and inflammation which, in turn, exacerbates HIV replication, HAND and HIV treatment failures. We expect that this hypothesis, if true, will have important implications for the emerging drug epidemic in remainder of the United States. In addition to meeting the goals of RFA 18-023, this project will contribute to NIDA and OBSSR's strategic plans for research that bridges pre-clinical and socio-behavioral research, and is responsive to the NIH-wide mandate of improving minority health and reducing health disparities in the United States.