SUMMARY (PROJECT 1) The four dengue virus serotypes (DENV1-4) cause the most important mosquito-borne viral disease of humans, with ~100 million cases annually, yet no approved vaccines or antivirals exist. Factors that determine disease outcome following DENV infection are not well understood and are thought to be mediated in part by host immune responses. Viral factors and host innate immune interactions likely influence the quality of the adaptive immune response, which can either protection provide or cause enhancement in a subsequent DENV infection. Project 1 of the Dengue Human Immunology Project Consortium (DHIPC) focuses on characterizing immune signatures associated with infection outcomes and disease severity in natural DENV infections. The overall approach of Project 1 is to take advantage of unique sample sets from long-term ongoing studies of dengue in Nicaragua to enable discovery of immune factors associated with symptomatic DENV infection, severe dengue disease, and specific adaptive immune responses. Project 1 will continue the Nicaraguan Pediatric Dengue Cohort Study (PDCS) to collect samples from well-documented natural repeat DENV infections. It will also leverage an on-going hospital-based study of dengue in Nicaragua to enable investigation of severe vs. mild dengue disease and will conduct an index cluster study to provide samples from asymptomatic viremic individuals. Detailed clinical data will accompany all samples. In conjunction with the world-class DHIPC Cores, a systems biology approach will be used to construct a signature of innate immune responses during natural DENV infection. The constellation of complementary cutting-edge genomic, transcriptomic, proteomic, seromic, and immunophenotyping methods will enable the human immune response to dengue to be dissected with unprecedented detail and sophistication. Aim 1 will characterize immune profiles in DENV infections with different clinical outcomes: asymptomatic and symptomatic DENV infection using the index cluster study and mild vs. severe disease in the PDCS and the hospital-based study. Aim 2 will characterize immune profiles in primary and secondary DENV infections, in specific serotype order of infection. Aim 3 will investigate the association between innate immune signatures and antibody and cell-mediated immune responses. Antibody neutralization, isotype, and repertoire will be evaluated, as well as the magnitude and serotype-specificity or cross-reactivity of the B cell response within Project 1. T cell responses will be evaluated for multi-functionality by intracellular cytokine staining. Overall, Project 1 will enable identification of biomarkers and immune signatures predictive of infection and disease outcome and thus contribute to improving clinical management, vaccine design and development of novel antiviral therapies against dengue.