This project will identify antigen-specific human T cell signatures that allow progression to active TB disease, by prospectively collecting samples from high risk household TB contacts, and comparing M. tuberculosis antigen-specific T cell responses in those that subsequently progress to active TB and in those that do not. We will characterize the breadth (number of antigens) and specific patterns of antigen recognition by T cells, and we will determine the relationship between the subjects' T cell responses, their HLA allotypes, and their bacterial epitope sequences, to test two specific hypotheses: 1) that the ability of an individual's T cells to respond to M. tuberculosis epitopes is determined by the ability of that individual's HLA alleles to present the specific epitope sequences present in the bacteria that infected them; and 2) that the breadth and/or specific nature of antigen recognition is associated with the risk of progression to active TB. To provide more complete characterization of the molecular interactions between host and pathogen required for CD4 T cell responses, we will also characterize the diversity of T cell antigen receptors (TCRs) for selected epitopes, to test the hypothesis that subjects that progress to active TB have lower TCR diversity than do subjects that do not progress to active TB. In studies analogous to those in Project 1, we will use multiparameter mass cytometry to identify T cell phenotypes and/or functional responses characteristic of subsequent progression to active TB. The data from these studies will be used to synthesize T cell signatures associated with a lower or with a higher risk of progression to active TB; signatures associated with progression can be used to identify individuals at highest risk and prioritize them for preventive interventions, while low-risk signatures will guide TB vaccine development. We will also take advantage of our prospective study design to determine the extent to which latent TB infection (LTBI) protects against development of active TB after reexposure to an active TB case. Together, the proposed studies will provide unprecedented insight into the spectrum of antigen-specific T cell responses (T cell signatures) in humans infected with Mtb, and the association of specific T cell signatures with distinct outcomes.