Characterization of the Interaction Between R. Conorii and Human Host Vitronectin in Rickettsial Pathogenesis

Spotted Fever Group Rickettsia are inoculated into the mammalian host during hematophagous arthropod feeding. Once in the bloodstream and during dissemination, the survival of these pathogens is dependent upon their ability to evade innate host defenses until a proper cellular target is reached. The establishment of a successful infection also relies on the ability of the bacteria to attach and invade target cells, as failure to do so results in destruction of the bacterium. Rickettsia conorii expresses an outer membrane protein, Adr1, which binds the multifunctional human glycoprotein, vitronectin, to promote resistance to complement mediated killing. Homologs of Adr1 are present in all sequenced pathogenic Rickettsia species to date, suggesting that the ability to sequester vitronectin is a conserved attribute for this genus of bacteria. The ultimate goal of my dissertation research was to define the roles of vitronectin acquisition by R. conorii in rickettsial pathogenesis. Herein, I characterize the Adr1/vitronectin interaction, demonstrating that this beneficial protein-protein interaction is a heparin-independent, electrostatic interaction that is mediated by the C-terminal region of human vitronectin. By utilizing site-directed mutagenesis, I was able to identify specific amino acids within the Adr1 protein that form interaction with the multimeric form of vitronectin for protection from complement mediated killing. I subsequently employed exogenous vitronectin to examine the role of vitronectin in adherence and invasion of R. conorii into phagocytic and endothelial cells. Interestingly, the presence of vitronectin increased the ability of the bacteria to invade monocytes but decreased the ability of the bacteria to invade endothelial cells. In summary, my findings demonstrate a dual role for vitronectin acquisition in rickettsial pathogenesis. This knowledge will be useful to specifically target the R. conorii/vitronectin/host cell interaction for the development of potential prophylactic or therapeutic interventions