ABSTRACT The advent of universal maternal antiretroviral therapy (test and treat and Option B Plus) has vastly improved health, development, and survival prospects for children exposed to HIV but uninfected (HEU), a population that grows by 1.5 million each year. However, poor outcomes in these domains have persisted, and underlying mechanisms remain unclear, particularly for neurodevelopment. Recent prospective studies of birth and infant cohorts suggest mild to moderate neurodevelopmental compromise in motor, language and cognitive outcomes. This study will examine neurodevelopment in a unique longitudinal cohort of children with HEU versus HIV unexposed uninfected (HUU) in sub-Saharan Africa (SSA). Additionally, we will examine plausible and potentially modifiable mechanisms, including alterations in the gut microbiome and breastmilk composition. Proper establishment of a healthy gut microbiome in infancy is increasingly recognized as influential for brain development. The infant gut microbiome could be compromised by numerous factors relevant for HEU children, including maternal drug exposure and poor health, infant antibiotic exposure, feeding practice, and breastmilk composition. Recent studies suggest differences in composition and profile of the infant gut microbiome in HEU children vs. their age-matched HUU counterparts. Profile and concentration of certain human milk oligosaccharides, which are nonnutritive glycans that can function as prebiotics, as pathogen blockers, or as immune modulators, may also differ in lactating women infected with HIV. Strategic emphasis of certain complementary foods as prebiotics and supplementation of human breastmilk using specific HMOs offer two promising avenues for intervention for HEU children and are strategies that may also benefit other subsets of the general population at risk for poor neurodevelopmental outcomes. This novel project will leverage an ongoing cohort with detailed serial specimen collection for gut microbiome and breastmilk HMO profile in both HEU and HUU children. In Aim 1, we will compare motor, language, cognition, self-regulation and executive function skills in HEU vs HUU children at 24 and 36 months to inform on the impact of HIV exposure on neurodevelopment in early childhood. In Aim 2, we will examine the relation between these outcomes and early infant gut microbiome diversity and composition, and will add to still nascent literature on early infant gut microbiome changes and neurodevelopment both in and outside the context of HIV. In Aim 3, we will examine the relation between concentration of specific HMOs and neurodevelopmental outcomes, and will directly inform efforts to identify and test candidate HMO supplements for promoting infant health and growth. This project will generate critical evidence to inform future clinical trials for use in SSA and other low-resource settings to improve development in HEU children and strategies to complement existing efforts to promote...