Opa protein mediated interactions with human neutrophils affect Neisseria gonorrhoeae survival

Neisseria gonorrhoeae (Gc) is the causative agent of the sexually transmitted infection gonorrhea. If left untreated, gonorrhea infections can lead to serious health sequelae including infertility and pelvic inflammatory disease. Gc infection in a human host elicits a potent immune response characterized by a robust recruitment of neutrophils to the site of infection. Despite the immune response, Gc uses a variety of mechanisms to evade killing by neutrophils, allowing for sustained infection, and for viable Gc to be collected from host exudates. One way Gc modulates interactions with human neutrophils is through expression of Opa proteins which interact with CEACAMs. Opa+ Gc are commonly recovered from infected individuals. Previous literature has shown a selection for specific Opa proteins, and that selection seems to be based on their CEACAM binding capability. In this thesis, I sought to determine the capability of Opa proteins to bind to different CEACAMs affects neutrophilic association. I further investigated how those interactions affect neutrophil activation and Gc survival. Understanding these interactions can allow us to better understand the selection for Opa proteins in vivo.
Development of a new flow cytometric assay allowed us to determine the CEACAM binding profile of a number of Opa proteins whose binding capabilities had been previously unknown. We were able to characterize the Opa protein binding of both neutrophilic and epithelial expressed CEACAMs. These data allow us to understand not only how Gc interact with neutrophils, but how they are able to interact with mucosal surfaces in the host.
This thesis utilized a system of primary human neutrophils as well as Gc expressing non-variable Opa proteins in a background in which all the other opa genes have been deleted. I also used Gc in an Opa variable background in which the expressed Opas were tested with each use. I found that CEACAM binding affects the ability of Gc to associate with neutrophils, and that increased association with neutrophils was correlated with more bacterial death. Overall, binding to both CEACAM1 and CEACAM3 was found to lead to more neutrophilic association and bacterial death, while binding to only CEACAM1 or to no CEACAMs allowed the bacteria to avoid neutrophils and killing. Finally, I found neutrophilic pressure selects for a Gc population that is either Opa- or expresses Opa proteins that do not bind to CEACAM3. Overall, these data allow us to better understand what Opa proteins are likely to be selected for in vivo, and further understand how CEACAM binding by these Opa proteins leads to that selection. In the future, quantifying the expression of Opa on Gc as well as CEACAMs on neutrophils, utilizing an inducible Opa system, and developing a biologically relevant binding assay could begin to answer some of the questions raised by this work.