PROJECT SUMMARY Influenza viruses are human respiratory pathogens causing mild to severe illness in 10-49 million individuals and 650,000 deaths annually. The objectives of this proposal are to determine the cellular immune responses that provide protection from influenza disease and to determine the immunologic consequences imprinted within us following first exposure to influenza. Protection against influenza is determined by immune correlates of protection– the immune factors that associate with reduced susceptibility to infection. Antibodies generated following natural influenza infection or vaccination are well- studied correlates; however, recent vaccine efficacy has waned even in patients with elevated antibody titers suggesting that antibodies alone do not provide complete protection. Components of cell-mediated immunity (CMI) including innate immune cells and antigen-specific T cells may also be critical in protecting against influenza. However, distinct CMI correlates of protection to influenza are not fully defined in humans leaving a gap in understanding anti-influenza response and limiting the scope of next generation vaccine design. Aim 1 employs biologic (flow cytometry) and computational (statistical modeling) analyses of samples collected from two established human cohorts to determine the distinct CMI responses that protect individuals from influenza, with the hypothesis that CMI correlates provide protection independent from antibody responses. Interestingly, childhood is a crucial time in developing immunity to influenza as first contact with the virus, often during infancy, can imprint intensity and specificity of lifelong responses. Initial antigen exposure occurs in the context of either natural infection or vaccination; however, the immunologic consequences of first exposure context are unknown. Further, evaluating imprinting is challenging in humans as no influenza-naïve cohort has yet been studied. Aim 2 posits that the context of first influenza exposure has lasting effects on influenza-specific T cell development and function. Utilizing samples collected in a novel study enrolling immunologically naïve infants, Aim 2 will determine how the context (vaccination; infection) of initial influenza antigen exposure impacts anti-influenza immune development through biologic (T cell function and specificity) and computational (T cell repertoire diversity) methods. The unique access to large human cohorts positions this proposal to successfully identify protective CMI correlates in a real-world setting and will be groundbreaking in investigating initial influenza antigen exposure in a naïve birth cohort. The support and guidance of mentor Dr. Paul G. Thomas, a prolific investigator with a decade-long record of high-impact research in immunology, and St. Jude Children’s Research Hospital, a world-class research institution with over 35 core facilities and dedicated postdoctoral development programs, enable the success of the p...