Our application will address the need for diagnostics to identify infection with a virulent hemorrhagic fever virus, Lassa fever virus (LASV). Within hours of entering the body, pathogens elicit responses from circulating white blood cells. The pathogen's capacity to alter cellular gene expression can be monitored by analysis of cellular RNA. Our approach has been to expose a standardized culture of white blood cells to a pathogen and to monitor changes in cellular mRNA expression on gene microarrays. Preliminary studies with select agents revealed several pathogen-specific changes in gene expression. Here we propose to expand our analyses to a hemorrhagic fever-causing arenavirus, Lassa Fever virus. We will test the hypothesis that we can discriminate between a dangerous hemorrhagic fever-causing virus and closely related viruses that do not cause disease. By monitoring gene expression responses at different timepoints we will determine the earliest time at which pathogen-specific gene expression signals arise. These gene-expression changes can be validated by the analysis of RNA from human and monkey peripheral blood mononuclear cells (PBMCs), and will serve as diagnostic markers of exposure to dangerous pathogens. Sets of genes will be identified that discriminate infection with Lassa Fever virus from infection with a related avirulent arenavirus, and these genes will be used to make smaller microarrays to further streamline diagnosis. Such diagnostics would eliminate the need to wait for symptoms or for pathogen replication before identifying the pathogen. This would enable more appropriate and rapid responses to a bioterrorist attack.