This K23 award will provide an opportunity for Dr. Horne to develop as an independent investigator in patient-oriented clinical research on genetic variation in innate immunity and susceptibility to tuberculosis (TB) infection. This career development application describes a comprehensive plan to accomplish the following goals: 1) assemble a cohort of patients in order to identify genetic, exposure-, and pathogen-related risk factors for TB infection, 2) develop expertise in host genetic variation and TB infection, 3) develop an understanding of the host innate immune response following an exposure to Mycobacterium tuberculosis (MTb), and 4) develop an independent clinical research career. These goals will be accomplished through mentorship by key personnel, didactic course work, instruction in practical laboratory skills, and participation in scientific meetings. We will assemble a cohort of patients in Seattle, Washington, who have a known TB exposure with the following specific aims: 1) thoroughly characterize clinical and exposure-related factors, identify MTb phylogenetic lineage from the index case, and use these factors to develop a risk factor model for TB infection; and 2) perform a nested case- control study in unrelated subjects to determine whether single nucleotide polymorphisms (SNPs) in candidate genes related to innate immunity are associated with TB infection. In our analysis of genetic associations, we will adjust for significant clinical, exposure and pathogen-related risk factors identified in Aim 1. We will confirm genetic associations in a previously collected family- based cohort. This study is novel for the outcome of TB infection and the inclusion of MTb lineage. The rich academic environment at the University of Washington is ideal for Dr. Horne's training and has allowed him to assemble an advisory committee whose members possess expertise in innate immunology, MTb phylogenetics, genetic epidemiology, biostatistics, risk factor modeling and TB transmission. Dr. Hawn, the primary mentor, is a successful and recognized expert in innate immunity, especially pathogen recognition receptors and associated pathways. He has investigated the role of Toll-like receptors (TLRs) in mycobacterial infection in functional and genetic association studies. Dr. Peter Small, co-mentor, is an internationally recognized expert in MTb phylogenetics. /Relevance TB infection, compared to TB disease, is an understudied area and little is known about host genetics and susceptibility to TB infection. TB is estimated to infect one-third of the global population and findings from this study could be important to TB vaccine development. Findings may also have relevance to other infectious diseases.