Rocky Mountain spotted fever (RMSF), caused by Rickettsia rickettsii, is considered the most severe of the human rickettsioses. Usually transmitted by the bite of a tick, the organisms are introduced during a blood meal directly beneath the skin where they invade and destroy the endothelial cells of small blood vessels. Our laboratory has established an in vitro model to study different parameters of rickettsiae-host interaction including mechanisms of injury to endothelial cells infected by this organism. An understanding of the specific mechanism(s) of cell injury caused by R. rickettsii should significantly enhance existing knowledge of the pathogenesis of RMSF, and could provide for more specific therapeutic management of severe forms of the disease. The proposed study will be carried out primarily in cultured human endothelial cells derived from the umbilical vein. The cells will be infected as monolayers by strains of R. rickettsii of high, intermediate, or low virulence. Current evidence strongly suggests that R. rickettsii-induced endothelial cell injury is mediated by free radicals that cause peroxidation of cell membrane lipids. This hypothesis will be pursued in the context of : (a) identification of the specific lipid hydroperoxides formed in infected endothelial cells using high-performance liquid chromatography; (b) an analysis of strains of R. rickettsii of varying degrees of virulence and their capacity to infect, replicate, and injure endothelial cells as determined by microscopic and biochemical techniques; (c) determination of comparative intracellular levels of reduced glutathione and the enzymes, catalase and glutathione peroxidase as a measure of inherent cellular defense mechanisms against oxidative stress; and (d) the capacity of the glutathione precursor, gamma-glutamylcysteine, to reduce intracellular peroxide levels and abrogate endothelial cell injury.