Doctoral Dissertations

Date of Award

5-2025

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Comparative and Experimental Medicine

Major Professor

Girish Neelakanta

Committee Members

Girish Neelakanta, Oudessa Kerro Dego, Daniel J. Mathew, John J. Schaefer

Abstract

Ticks are blood feeding arthropods that harbor bacterial endosymbiont. Several studies have identified rickettsial endosymbionts in hard ticks. However, little is known about the presence of rickettsial bacteria in soft ticks. Ticks also transmits several rickettsial pathogens to humans including Rickettsia parkeri, the causative agent of R. parkeri rickettsiosis. In these studies, we noted the presence of Occidentia species, designated as Occidentia-like species, in a soft tick O. turicata americanus. In addition, we characterized the interactions of R. parkeri with endothelial cells and macrophages. We used quantitative real-time PCR, sequencing, fluorescent microscopy, live/dead staining, siRNA-mediated silencing, protein array analysis, bioinformatics, and cell culture studies in this study. In the first study, we noted that the Occidentia-like species was present in all developmental stages of O. turicata americanus and in different tick tissues including ovaries, synganglion, guts and salivary gland. In addition, the levels of Occidentia-like species was induced by blood feeding. In the second study, we noted a reduced bacterial burden of R. parkeri in macrophages and tick cells treated with AsmA antibody compared to control-IgG treated cells. Additionally, we noted that the bacterial burden for R. parkeri asmA mutant was significantly lower than the wild type at later time points (day 3, 4, and 5 post-infection). This study showed importance of AsmA in R. parkeri survival in both tick and mammalian cells was proven. In the third study, a protein array for human metabolic pathways showed the upregulation of CD157, a molecule that relies on nicotinamide adenine dinucleotide (NAD+) for its functions in endothelial cells upon R. parkeri infection. Moreover, we observed that R. parkeri infection affected NAD+ metabolism in the host cell. Various strategies of NAD+ treatment revealed the importance of NAD+ in R. parkeri-host interactions. Moreover, siRNA-mediated silencing of CD157 showed its involvement in the regulation of NAD+ metabolism and R. parkeri infection. In summary, these studies not only provide molecular insights on the mechanisms modulated by R. parkeri for its survival in mammalian cells but also lead to the development of strategies to target this and perhaps other rickettsial pathogens.

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