PROJECT SUMMARY Stroke is the second leading cause of death and leading cause of disability worldwide, and people with HIV (PWH) are at more than double the risk of stroke compared to HIV-uninfected (HIV-) individuals. Sub-Saharan Africa (SSA) shoulders the greatest burden of stroke and, because it’s also home to ~2/3 of the global population of PWH, HIV likely substantially contributes to this burden. However, the mechanisms leading to excess stroke risk amongst PWH are not well-understood and, therefore, optimal ways to prevent stroke amongst PWH are not established. We hypothesize that vitamin D deficiency (VDD) may be involved in the etiopathogenesis of stroke in PWH. VDD is more common in PWH than HIV- populations, can be exacerbated by antiretroviral therapy (ART), and is highly prevalent in SSA. There is conflicting evidence about whether VDD predisposes to stroke in HIV- populations. However, because both HIV infection and VDD lead to endothelial dysfunction which may then result in stroke, we hypothesize that VDD and HIV infection synergistically interact to increase the risk of stroke among PWH. In this proposal, we seek to demonstrate that VDD is a mechanistic link between HIV infection, endothelial dysfunction, and ischemic stroke in SSA. We will leverage developing neurology clinical and research capacity in Zambia in collaboration with more senior investigators from the United States to rigorously test this hypothesis. In order to do this, we will conduct a case-control study at the University Teaching Hospital (UTH) in Lusaka, Zambia consisting of (1) cases: ART-treated PWH with ischemic strokes; and (2) controls: ART- treated PWH without stroke. Participants will undergo clinical and demographic assessments, and blood will be collected for determination of 25-hydroxyvitamin D level and biomarkers of endothelial dysfunction, including intracellular adhesion molecule-1, vascular cellular adhesion molecule-1, and C reactive protein. If VDD is associated with stroke in PWH, this research will provide the preliminary data necessary for a subsequent large-scale, multinational clinical trial of vitamin D supplementation for primary stroke prevention in PWH. If successful, vitamin D may offer a relatively inexpensive and scalable intervention to prevent strokes in PWH across the world, an increasing concern especially amongst ART-treated people aging with HIV.