Molecular Basis of Immunity to Tick-borne Rickettsioses

ABSTRACT A recent study from the Centers for Disease Control and Prevention revealed a pressing health crisis for the United States: the number of reported cases of tick-borne diseases has increased significantly during the past two decades. Importantly, the reported annual incidence captures only a small fraction of the real number of individuals infected with tick-borne pathogens. The broad spectrum of clinically important tick-borne diseases includes Lyme disease, anaplasmosis, ehrlichiosis, tularemia, babesiosis, and Spotted Fever rickettsiosis. Spotted Fever group rickettsiae include R. rickettsii (Rocky Mountain Spotted Fever, RMSF), R. conorii (Mediterranean Spotted Fever), and R. parkeri (Rickettsia parkeri rickettsiosis) as well as many newly discovered Rickettsia species with unknown pathogenicity. Doxycycline is considered as the antibiotic of choice for the treatment of tick-borne rickettsiosis; however, delay in diagnosis and antibiotic treatment can lead to severe disease and death. The search for long-term immune protection against invasive rickettsial diseases (RMSF and epidemic typhus caused by R. prowazekii) has been a goal since the discovery of the causative microorganisms by Dr. Howard T. Ricketts. However, whole cell live-attenuated or formalin/phenol- inactivated vaccines generate limited protective immune responses in humans and, because of safety concerns, are no longer considered for rickettsial vaccine development. We have developed kkaebi transposon mutagenesis technology and studied the genetic requirements of the rickettsial intracellular life-cycle (bacterial attachment to and invasion into host cells, escape from endo-lysosome, intracellular replication, and release from host cells). This work determined that the polysaccharide synthesis operon (pso) is responsible for O- antigen biosynthesis, contributes to pathogenesis, and is essential for the development of bactericidal Weil– Felix antibodies. Immunization with carbohydrate conjugate vaccines, including the capsular polysaccharide or the O-antigen of lipopolysaccharide, generated serotype-specific protective immunity that correlated with the induction of bactericidal antibodies. This proposal aims to understand the adaptive immune responses to invasive rickettsial infections and to determine the contribution of rickettsial O-antigen conjugate vaccine and Weil–Felix antibodies toward protective immunity against tick-borne rickettsial infections. !