PROJECT SUMMARY Tuberculosis disease (TB), caused by Mycobacterium tuberculosis (Mtb) is a leading cause of morbidity and mortality in young children <5 yr old. The vulnerability of young children to develop TB following primary infection is not understood; this critical knowledge gap hampers efforts to develop a more effective vaccine and improved diagnostic tests for this high-risk population. HIV-exposed (HEU) children are the majority of children living in the homes of HIV+ adults, where they are more likely than HIV-unexposed-uninfected (HUU) children to be TB exposed. Clinical, epidemiologic, and immunologic findings support that young, HEU children will exhibit distinctive immune responses following Mtb-exposure; however, immune responses to Mtb-infection have not been characterized in this high-risk population. Our proposal will comprehensively define adaptive immune responses to Mtb-exposure and TB in children < 5yr using a pediatric TB household contact (HHC) study based in Kampala, Uganda, where up to 20% of children are HEU. Our approach will identify unique immunologic biosignatures driven by where the child sits on the TB disease spectrum, as well as by HIV-exposure status. We hypothesize that immune biosignatures of Mtb-exposed asymptomatic HEU children will more closely resemble those observed among children with TB, as compared to asymptomatic HUU children. If proven, this would suggest that Mtb-exposed HEU children are more likely to be experiencing a pre-clinical rather than quiescent or latent Mtb-infection. Such findings would have implications for the development of immune-based diagnostics for Mtb-infection and TB disease that may perform differently in HIV-exposed and unexposed children, as well as clinical management and monitoring of HEU-children following TB exposure. Working with a biorepository of samples obtained from Ugandan children < 5 yr, and a proposed prospective cohort of Ugandan children < 5 yr who are TB HHC, we will address three specific aims that: 1) define longitudinal, functional and phenotypic Mtb- specific adaptive immune responses among young children who developed TB or remained asymptomatic; 2) develop a pool of novel Mtb-epitopes and focused flow cytometry-based assay customized to detect Mtb-specific T cell responses in young children; and 3) establish a unique Mtb-specific antibody Fc profile that defines children with TB. Our proposal will advance pediatric global health by: 1) identification of immunologic signatures reflecting successful containment of primary Mtb infection that can serve as correlates of protective immunity for novel vaccine trials; 2) development of novel blood-based assays that discriminate between young children with TB and those whom have been exposed but successfully contained their infection.