Host-Dependent Expression, Transcriptional Regulation and Protection of Ehrlichia chaffeensis Tandem Repeat Proteins

Ehrlichia chaffeensis is an obligately intracellular bacterium that is the causative agent of human monocytotropic ehrlichiosis, an emerging life-threatening zoonosis. E. chaffeensis is transmitted by the lone star tick, Amblyomma americanum, and replicates in mononuclear phagocytes in mammalian hosts. The major immunoreactive proteins of E. chaffeensis include a group of tandem repeat proteins (TRPs) that are involved in various molecular strategies to reprogram the host cell during infection. The objectives of this dissertation were to examine host-specific expression of E. chaffeensis TRPs, the role of CtrA as a gene expression regulon and the protection mediated by antibodies directed at the molecularly characterized epitopes within the TRs of TRP32, 47 and 120. Our results revealed three novel characteristics of these ehrlichial proteins. First, we demonstrated differential expression of TRP transcripts and host-specific post-transcriptional regulation of TRP32 and TRP47 mRNA. Second, the transcriptional binding motif of the two-component system (TCS), response regulator, CtrA was identified, and using a histidine kinase inhibitor, we demonstrated transcriptional regulation of TRP by TCSs. Finally, using in vitro and in vivo models, we demonstrated protection during E. chaffeensis infection with antibodies directed against linear species-specific epitopes of TRP32, 47 and 120 of the IgG1 isotype through an extracellular and intracellular antibody-mediated mechanism. Collectively, these studies have improved our understanding of the pathogenesis of ehrlichiosis and have identified novel targets and mechanism for development of vaccines and therapies for ehrlichial diseases.