Lyme disease, a tick-borne borreliosis of man, has become the most common vector-borne disease in the US. Passage of the pathogen through its vector, Ixodes ticks, is responsible for transmission in the majority of cases and enhances infectivity and pathogenicity. The goal of the proposed research is to elucidate the factors that enable the causative agent, Borrelia burgdorferi, to invade the tick, colonize its gut lumen and penetrate the gut wall to spread to other organs such as salivary glands and ovaries. A better understanding of the tick- spirochete relationship could lead to methods that block acquisition of infection and transmission by ticks. The specific aims are to examine the interaction of tick cells and borreliae at the cellular, molecular and ultrastructural level. This will be accomplished by cocultivating borreliae with tick cell lines and organ cultures. Adherence to and penetration of cells as well as cytopathogenicity will be monitored using radiolabelled borreliae and light and electron microscopy. Findings will be linked to changes in plasmid profile, outer surface proteins (Osp) A and B, morphotype of colonies on solid medium, and hamster infectivity and pathogenicity. The role of membrane components will be examined using polyclonal antisera, monoclonal antibodies against OspA and B, and lectins in binding inhibition assays and immunochemical/histochemical techniques. Specific inhibitors of borrelial and cellular metabolism as well as live or killed cells and spirochetes will be used to analyze the nature (specific vs non-specific) of spirochete-cell interaction, and to determine whether it is controlled at the transcriptional or translational level. The effect of cultivation in the presence or absence of tick cells will be examined with respect to infectivity, plasmid profile and colony morphotype. Evidence for antigenic variation will be sought in switching (disappearance and reoccurrence) of specific colony types and expression of OspA and B.