Author ORCID Identifier

Defense Date


Document Type


Degree Name

Doctor of Philosophy


Microbiology & Immunology

First Advisor

Cynthia Cornelissen

Second Advisor

Kimberly Jefferson

Third Advisor

Joyce Lloyd

Fourth Advisor

Phillip Hylemon

Fifth Advisor

Todd Kitten


Neisseria gonorrhoeae, the causative agent of the sexually-transmitted infection gonorrhea, has recently been labeled a superbug. With dwindling treatment options, there is urgent need for a gonococcal vaccine. TonB-dependent transporters (TDTs) are important gonococcal virulence factors that allow the gonococcus to pirate metals directly from host proteins. The TDTs, TbpA and TbpB, are promising vaccine targets because both proteins are expressed and highly conserved in gonococcal strains, and they are not subject to high-frequency antigenic variation. Because the Tbps bind to host protein as their natural function, our immune system does not recognize them as foreign antigens; thus, immunogenicity is weak. Recent work with Haemophilus parasuis TbpB has demonstrated that pigs vaccinated with a nonbinding TbpB mutant elicit enhanced immune-cell responses and superior protection against challenge with H. parasuis compared to pigs vaccinated with WT TbpB. The extracellular loop 3 helix (L3H) in TbpA is essential for binding to human transferrin (hTf). Neutral or charge mutations in the L3H are insufficient for abrogating hTf binding; therefore, this study inserted proline mutations in the L3H to further disrupt the helical structure and abrogate hTf binding. This study generated several single point mutations in TbpA that result in significantly decreased hTf binding. Both TbpA D355P and A356P mutants show significantly reduced hTf-TbpA binding and iron uptake from Fe-loaded hTf. Both TbpA D355P and A356P vaccine candidates will need to undergo structural studies and testing in a hTf transgenic mouse model to characterize their true potential as vaccine antigens.


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