Project Summary HIV-exposed uninfected (HEU) infants have higher rates of mortality, infectious morbidity and growth deficits compared to HIV-unexposed uninfected (HUU) infants despite the success of maternal antiretroviral therapy (ART) in reducing vertical transmission. Our group has observed that human milk oligosaccharide (HMO) composition of breastmilk, and its relationships to the infant gut microbiome maturity and composition, may account for some of the increased risks seen in HEU infants. Currently, no specific interventions have been shown to correct for disparities in health outcomes experienced by HEU infants. To address this gap, we propose a proof-of-concept, randomized, placebo-controlled trial of a synbiotic in breast-fed HEU infants. The synbiotic is composed of 2'-fucosyllactose (2'FL) HMO and B. infantis probiotic. 2'FL is associated with reductions in mortality, infectious morbidity and growth deficits. B. infantis is included to support 2'FL fermentation and thereby maximize benefits from the 2'FL intervention. We will randomize 120 breast-fed HEU infants at 4 weeks of age 1:1 to receive synbiotic or placebo through 24 weeks of age, with a follow-up through 48 weeks of age. We will also recruit 60 breast-fed HUU infants as a comparator group and they will be followed for the same duration with no intervention. This study will be conducted in rural South Africa, in a region with high maternal HIV prevalence and high rates of infectious morbidity and growth deficits in infants. In specific aim 1, we will evaluate whether the synbiotic i) reduces infectious morbidity and growth faltering, and ii) influences biological pathways related to infant gut microbiome, metabolism, and inflammation while the intervention is in place during the first 24 weeks of age. In specific aim 2, we will evaluate whether effects persist after the intervention is discontinued through 48 weeks of age. Specific aim 3 will compare HEU and HUU cohorts and investigate biological pathways associated with infectious morbidity and growth. Overall, we hypothesize that the synbiotic will reduce infectious morbidity and improve growth in HEU infants. Our study will provide novel information on whether some of the excess risks in HEU infants can be ameliorated through interventions targeting breastfeeding-mediated microbiome and inflammatory pathways.