Lyme disease, caused by the pathogenic spirochete Borrelia burgdorferi , is the most common arthropod-borne infection in the United States. B. burgdorferi is maintained in nature by a complex enzootic cycle involving ticks and mammalian hosts. In the past decade, several proteins have been identified that are either up-regulated by B. burgdorferi as it is transmitted from the tick to mammal or appear to be expressed exclusively during mammalian infection. Given the potential role that these differentially expressed proteins may play in host-pathogen interactions and Lyme disease pathogenesis, much attention has recently been placed on identifying additional borrelial proteins up-regulated during transmission and/or mammalian infection. To identify genes important for transmission and infection, we performed a comprehensive gene expression profiling of B. burgdorferi. The microarray data revealed 323 genes differentially expressed by temperature and mammalian specific signals. Strikingly, many of the genes identified encoded putative outer membrane proteins (OMPs) that were found to be specifically down-regulated during infection. However, there also were several genes identified that encode putative OMPs that were specifically up-regulated during the transmission and infection process. Given the extracellular nature of B. burgdorferi, these up-regulated OMPs are at the interface between the host and pathogen during infection and are likely to be important virulence determinants. With the underlying hypothesis that up-regulated genes encoding OMPs are important to the parasitic strategy of B. burgdorferi during infection, we characterized 13 genes identified as up-regulated during transmission and infection that encoded putative OMPs. These 13 genes were selected because they were found to contain putative leader peptide sequences, indicating that they may be translocated to the outer surface of B. burgdorferi during infection. Cellular localization studies of the proteins encoded by these up-regulated genes resulted in the identification of seven novel OMPs in B. burgdorferi (Bb0405, Bb0689, BbA36, BbA64, BbA66, BbA69, and BbI42). Immunoblot analysis using serum from tick-infected baboons revealed that all seven proteins are expressed during a natural infection and are immunogenic. Furthermore, antibodies specific for all seven OMPs were bactericidal to B. burgdorferi cultivated in vitro, suggesting that these antigens could be candidate Lyme disease vaccinogens. Consistent with this notion, mouse protection experiments revealed that at least two of the novel OMPs identified, Bb0689 and BbA36, could protect mice from subsequent B. burgdorferi challenge infection. Given recent advances in borrelial mutagenesis technology, the findings outlined in this dissertation have laid the foundation for examining the role that these novel OMPs play in borrelial virulence and Lyme disease pathogenesis. Finally, these studies also have identified several candidate proteins that may prove useful for developing a second generation Lyme disease vaccine.
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