Abstract: Overall Component Leptospirosis is a widespread and frequently fatal human health problem that disproportionately impacts low resource settings. Research on host-pathogen dynamics in leptospirosis are significant because little is known about leptospiral virulence factors or host response to leptospirosis. The proposed studies will involve highly synergistic collaborations between program project investigators who are leaders in studies of leptospiral virulence genes (Haake and Picardeau), endothelial interactions (Coburn), inflammasome pathways (Sutterwala), and field studies of acute febrile illness (Reller and Woods). With the description of many new leptospiral genomes, a striking pattern of massive species diversity has emerged leading to central hypothesis #1, which is that a core set of leptospiral virulence factors have evolved with roles in survival in mammalian host phagocytes, translocation, and dissemination, which are upregulated in response to the host microenvironment. Central hypothesis #1 will be tested by correlating genome-scale leptospiral evolutionary changes with virulence phenotypes, examining the roles of transcriptional regulators and non-coding small RNAs in adaptation to and survival within host phagocytes, and translocation across endothelial barriers. The correlation of inflammatory markers such as IL-1β levels with disease severity leads to central hypothesis #2, which is that human inflammatory response pathways drive disease outcomes. Central hypothesis #2 will be tested in vitro (interactions with macrophages and endothelial cells), in animal models (infections in hamsters and mice), and in human field studies in Tanzania, Nicaragua, and Sri Lanka. Specifically, we will follow up on our innovative discovery of a striking dichotomy between the high level of inflammasome activation in human macrophages and the low level in macrophages from mice, which are reservoir hosts and do not exhibit disease. We will also follow up on our innovative discovery of dramatic disruption of endothelial VE-cadherins by pathogenic leptospires in terms of the role of intercellular invasion in dissemination. Animal model studies will provide longitudinal host response data in support of human studies that will have a positive impact through development and validation of rapid biomarker diagnostic and triage tools to identify serious infections at an early stage when antibiotics and other interventions can prevent and/or treat critical illness including fatal hepatorenal failure.