PROJECT SUMMARY
Neisseria gonorrhoeae (Gc) is a Gram-negative bacterium that causes the sexually transmitted disease
gonorrhea. With an estimated 78 million cases of gonorrhea annually worldwide, increasing frequency of
resistance to all recommended antibiotics, and the lack of a protective vaccine, N. gonorrhoeae is a prominent
and growing threat to human health. A hallmark of Gc infection is the influx of neutrophils, but this inflammatory
response is unsuccessful in clearing infection. Phase-variable opacity-associated (Opa) proteins on the Gc
surface mediate non-opsonic phagocytosis by neutrophils. Expression of Opa proteins like OpaD of strain
FA1090, which activates neutrophils by binding to the phagocytic receptors carcinoembryonic antigen-related
cell adhesion molecule (CEACAM)-1 and CEACAM-3, significantly decreases Gc survival in the presence of
human neutrophils in vitro. Neutrophils respond to OpaD+ Gc by generating reactive oxygen species, releasing
toxic granule contents extracellularly, and internalizing the bacteria into a phagolysosome where they are killed
by proteases and antimicrobial peptides. Despite being killed by neutrophils in vitro, Opa-expressing Gc
survive and even predominate among Gc in neutrophil-rich gonorrheal exudates. As one explanation for these
discordant findings, we have found that opsonization with human serum, which is found in inflammatory
secretions, enhances Opa+ Gc survival from primary human neutrophils and suppresses the neutrophil
oxidative burst. Through ion-exchange chromatography and mass spectrometry, we identified C4-binding
protein (C4BP) as the serum component that binds to Opa+ Gc to limit neutrophil activation and increase Gc
survival from neutrophils. Remarkably, these effects are independent of complement deposition or complement
lytic activity, suggesting a novel mechanism by which binding of C4BP modulates neutrophil functions. In this
proposal, we will define how C4BP mediates these effects on Opa+ Gc, by defining the molecular interactions
between C4BP and the Gc surface and the cellular mechanisms that limit the response of human neutrophils
to Opa+ Gc. Completion of this work will reveal a previously unappreciated way in which Opa+ Gc survives in
neutrophil-rich secretions to enable its continued colonization as well as transmission to new hosts. With this
knowledge, we predict that the development of therapies that interfere with C4BP binding to Gc will increase
the efficacy of neutrophil antigonococcal activity, in addition to enhancing complement lytic activity, thereby
improving outcomes of gonorrhea.
