7. PROJECT SUMMARY/ABSTRACT The primary goal of the proposed research is to build an optimized intervention to promote sustained viral suppression among a group of Latinx people who inject drugs and who are living with HIV on the U.S.-Mexico Border. We are using the multiphase optimization strategy (MOST). MOST is an engineering-inspired framework for designing and building optimized interventions. The primary aim is to conduct a 2-to-4 factorial design where participants are randomized to zero or up to four intervention conditions (each condition comprises a different set of intervention components). The four components are receiving (or not receiving) peer support services for medication-assisted treatment (MAT) uptake and persistence, behavioral activation therapy for depression (BAT), Life-Steps medication adherence program, and patient navigation for HIV care. The intervention is being implemented at Programa Compañeros, a community-based social service organization in Cuidad, Juarez, Chihuahua, Mexico, a city nestled alongside El Paso, Texas. Participants will complete a baseline survey assessment, complete their assigned intervention programs, and then return for 6- ,9-, and 12-month follow-up assessments. The primary outcome is sustained viral suppression defined as viral load test results of <400 copies per mL on all 6-,9-, and 12-month follow-up visits. Results of Aim 1 will yield estimates of the unique impact of each intervention component, as well as each combination of components, on sustained viral suppression. The research team and partners will make decisions about what constitutes the optimized intervention by judging the observed effect sizes and statistical significance against real-world constraints that go into the delivery of this intervention. The secondary aims are to test theoretical mediators to explain the influence of the intervention components on viral suppression outcome and moderators of this relationship. We will first test the effect each component exerted at the first, 6-month, follow-up assessment, followed by testing characteristics that may modify this relationship.