PROJECT SUMMARY/ABSTRACT Over one million infants born to women living with HIV are spared from vertical HIV acquisition. This public health success is attributable to scaling up of maternal antiretroviral treatment (ART), with current World Health Organization (WHO) policies recommending lifetime ART for persons living with HIV, including pregnant women. Yet, HIV-exposed uninfected (HEU) infants in resource-constrained settings experience 2- to 4-fold higher infectious cause morbidity and mortality compared with HIV-unexposed (HU) infants in the same settings. This is primarily due to diarrheal disease and respiratory illness, including tuberculosis (TB). HEU children also experience 20-fold increased incidence of latent TB infection (LTBI) compared with HIV- unexposed (HU) children. Several studies have described immune abnormalities among HEU children, which might explain a portion of HEU infant vulnerability. In both Botswana and South Africa, also settings with high TB and HIV prevalence, bacille Calmette-Guérin (BCG) immunization rates at birth exceed 98%. While some studies have shown that BCG vaccination protects against TB meningitis and disseminated TB in HU infants and may have heterologous effects that extend beyond TB protection, the reported alterations in HEU infant immunity may impact the efficacy of the BCG vaccine both in directly protecting against TB and in providing the non-specific immune protection. Leveraging two existing prospective observational maternal-child health studies underway in Botswana and South Africa, respectively, each of which is following both HEU and HU infants from birth and beyond infancy, we will use data and specimens from 900 infants (600 HEU infants) to evaluate BCG vaccine-induced specific and non-specific immune responses while using immunological studies to assess correlates of BCG vaccine protection. Specifically, we will 1) Define TB-related and TB- independent causes of morbidity and mortality in BCG vaccinated HEU and HU infants; 2) Define immune correlates of risk for tuberculosis infection in HEU infants using polychromatic flow cytometry to compare conventional and unconventional T-cell responses to BCG vaccination; 3) Explore the effect of HIV exposure on BCG-induced peripheral blood monocytes. We anticipate that study findings will inform TB vaccine development for HEU infants,and might identify further potential interventions to protect against infectious morbidity and mortality more frequently experienced by this vulnerable population of infants.