The Effects of Neisserial Surface Molecules and Initial Bacterial Dose on Interactions with Human Mononuclear Phagocytes

Abstract

Neisseria gonorrhoeae is an important human pathogen which causes significant worldwide morbidity. The effects of two important neisserial surface molecules, LOS and H.8, as well as initial bacterial challenge dose, upon the interactions of N. gonorrhoeae with human monocytes and macrophages, were examined in this study. The data presented here demonstrate that alterations in the carbohydrate moiety of LOS do not directly impact production of proinflammatory cytokines. However, when specific LOS mutants were sialylated, upregulation of the chemokine MCP-2 was observed. The H.8 antigen was demonstrated to have little effect on monocytic cytokine production, but does appear to play a significant role during physical interactions with macrophages. In comparison with the parent strain, a greater percentage of the H.8- bacterial challenge population was shown to form large aggregates upon contact with human macrophages, and appeared to resist phagocytosis. I propose that host recognition of gonococcal surface structures, such as H.8 and sialylated LOS, may promote elicitation of monocytes and phagocytosis of the bacteria. This may contribute to gonococcal intracellular growth, and result in chronic and disseminated infection.

Finally, the experiments described here demonstrate the significance of bacterial dose during gonococcal challenge. While a high MOI was shown to cause elicitation of many proinflammatory cytokines and chemokines, a low MOI elicited a strong chemokine response in the absence of significant TNF-alpha production. As the cytokine TNF-alpha is thought to lead to tissue damage caused during gonococcal infection, these data may help to explain asymptomatic gonorrhea. Induction of IL-8 expression in low MOI challenges was produced independent of TNF-alpha or IL-1beta, and required NF-kappaB activation, though this activation was limited by the inoculating dose. Analysis of various MAP kinases indicated that low MOI challenges were able to efficiently activate both the ERK and p38 pathways, but failed to activate the JNK pathway. A lack of phosphorylated JNK leads to decreased production of AP-1 dimers, transcription factors that are critical for efficient transcription of TNF-alpha. I therefore propose a mechanism where a low MOI gonococcal challenge results in diminished AP-1 activity and TNF-alpha production while IL-8 levels remain constant.