PROJECT SUMMARY Bacteria of the genus Rickettsia are obligate intracellular endosymbionts and the causative agents of many significant human and animal infections. These bacteria are transmitted to humans by hematophagous arthropods. Pathogenic Rickettsia species parasitize endothelial cells of the circulatory system, causing vascular leakage with significant morbidity and mortality. Rickettsia species are amongst the most important pathogens in history, with millions of human fatalities associated with these infections. The complement system is a well-known potion of the mammalian immune system that has classically been considered a secreted patroller of the extracellular spaces of the vasculature. Until recently it was considered unlikely that extracellular immune components like the complement system would encounter intracellular pathogens like Rickettsia; let alone that evolution would drive these systems into an adversarial relationship. Consequently, we are only beginning to appreciate how complement can control intracellular bacterial infections. Emerging data from Rickettsia and other organisms has begun to identify some of the molecular and cellular mechanisms of complement efficacy against intracellular pathogens. As the newly-emerging concept of “intracellular complement” is rapidly gaining traction, the experiments outlined in this proposal will be central to redefining an innate immune component that was discovered over a century ago. Based on both clinical and experimental evidence, we have hypothesized that complement activation is essential for the effective immune response to obligate intracellular Rickettsia infections. However, experimental data suggests that complement efficacy does not result directly from the antibacterial components of complement. Instead, complement efficacy against Rickettsia is derived from secondary interaction with other portions of the immune system. We therefore aim to 1) assess the contribution of the proinflammatory components of complement to the anti-Rickettsia immune response, and 2) define complement-induced components of the immune system that specifically target intracellular pathogens. By studying the interaction between complement and clinically relevant pathogens, the achieved results will advance our collective understanding of basic complement biology, further define the specific immune response to Rickettsia infection, directly influence the design of anti-Rickettsia vaccines, and provide the premise for future studies to analyze complement manipulation as a preventative or therapeutic intervention for intracellular bacterial pathogens.