Defense Date
2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Microbiology & Immunology
First Advisor
Richard Marconi
Abstract
The Lyme disease spirochetes, Borreliella spp., are maintained in nature through a complex enzootic cycle involving Ixodes ticks and a diverse set of reservoir hosts. The outer surface protein C (OspC) of the bacterium is an essential virulence factor required for transmission from ticks and early infection of mammals. Deletion of ospC abolishes infectivity, yet the precise molecular mechanisms underlying OspC function remain incompletely defined. In this dissertation, I examined the contribution of OspC type identity to host specificity. OspC is highly diverse, with over 30 genetically stable types described. While intra-type sequences are conserved (>95% amino acid identity), inter-type sequences show substantial divergence (< 65% identity). It has been hypothesized that this diversity contributes to host range determination among Borreliella species. To test this, I employed a genetic type-switch strategy in the B. burgdorferi strain B31-5A4, which naturally produces OspC type A. The ospC type A gene was replaced with ospC genes from diverse species-associated types, including B. afzelii type PKo, B. garinii type Pwa, B. andersonii types MOS-1b and MOD1, and several naturally occurring B. burgdorferi types (A, H, I, K, M, E, F, and Mok3A). All type switch strains expressed OspC on the cell surface and retained native plasmid content.
Mouse infection studies revealed that most type-switch strains established infection, as confirmed by PCR, seroconversion, and culture of tissue biopsies. In contrast, type switch strains expressing either OspC type PKo, I, or MOS-1b did not establish infection. Additionally, six OspC type A/PKo hybrid strains were generated to delineate the region of OspC that was essential for establishing infection. Of the hybrid mutants, strains expressing OspC constructs A1A2P3 or A1P2P3, were unable to infect mice, despite robust in vitro expression and stability. These findings provide direct experimental evidence that OspC type identity is a determinant of host specificity and demonstrate that certain inter-type combinations can abolish essential OspC functions.
Together, this work supports the growing hypothesis that OspC is a multifunctional protein, mediating critical yet distinct roles in host colonization, dissemination, and immune activation. The findings also implicate OspC as a driver of host range diversity among Borreliella species. Future studies should focus on identifying additional host-specific ligand binding partners, defining the mechanistic basis of OspC multifunctionality, and testing type-switch strains across alternative reservoirs to assess ecological relevance. By elucidating how OspC mediates both essential and type-specific functions, this dissertation advances our understanding of Lyme disease pathogenesis.
Rights
© The Author
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
11-28-2025