Project Abstract Infants born to HIV-infected mothers are at high risk for HIV acquisition. Additionally, HIV-exposed infants display reduced vaccine responses, and increased disease susceptibility compared to unexposed infants. The development of certain T cell subsets, both in the mucosa and systemically, is determined by the presence of specific microbes in the gut and may be important in determining adaptive immunity. However, the gut microbiota of HIV-exposed uninfected (HEU) infants differs from that of HIV-unexposed (HU) infants, since their mothers have altered gut microbiota. The gut virome also plays a central role in modulating both the bacterial microbiota and immune response of adults, yet the association between the infant enteric virome and maternal HIV status has not yet been explored. As the enteric virome is expanded in HIV infection, it is likely that infants born to HIV-infected mothers inherit a wider range of viruses than unexposed infants. This study proposes that the enteric virome is one of the factors influencing the morbidity of HIV-exposed infants, either by directly altering mucosal immunity or by altering the composition of enteric bacterial communities, as a consequence of bacteriophage or other viral dynamics. This proposal will utilize an already funded, ongoing cohort to longitudinally characterize the infant fecal virome of 40 HEU and 40 HU infants, as well as the breastmilk and fecal virome of 20 paired HIV-infected and 20 paired HIV-uninfected mothers (Aim 1). Viral metagenome data will be integrated with bacterial community datasets and T cell cytokine responses to BCG vaccination to identify viral and bacterial taxa correlated with BCG responses. Bacterial-phage interactions will then be validated using co-culture experiments (Aim 2). Together, these Aims will identify mechanisms of gut dysbiosis in HEU infants and reveal potential therapeutics to restore health to this group. Collectively, this proposal will reveal how maternal HIV infection affects the maternal gut and breastmilk virome and how these alterations are transmitted to and shape the enteric microbiome of associated infants.