Periodontal disease is a chronic inflammatory disorder driven by polymicrobial infection. The pathogenesis of this disorder involves activation, and possibly perturbation, of both innate and adaptive immune responses. Over the course of our investigation we have demonstrated that Aggregatibacter actinomycetemcomitans (Aa), a putative pathogen implicated in the pathogenesis of localized aggressive periodontitis (LAP), produces an immunotoxin, the cytolethal distending toxin (Cdt). Cdt is a heterotrimeric holotoxin which functions as an AB2 toxin: the cell binding (B) units are comprised of the CdtA and CdtC subunits and the active (A) subunit, CdtB. We have demonstrated that CdtB functions as a 5'-phosphatidylinositol (PI) triphosphate phosphatase as it degrades the PI-3K signaling lipid, PI-3,4,5-triphosphate (PIP3) to PI-3,4P2; this action leads to blockade of the signaling pathway. Blockade of PI-3K signaling in human macrophages leads to a pro-inflammatory cytokine response involving both canonical and noncanonical inflammasome activation. Cdt also induces production of inflammatory mediators derived from arachidonic acid including PGE2 and thromboxanes. Most recently, we have observed phagocytic defects in Cdt-treated macrophages consistent with altered phago-lysosome maturation. We propose that Cdt perturbs macrophage function thereby contributing to both inflammation and sustained infection. Our overarching hypothesis is that the Aa Cdt contributes to altered local host defense which facilitate Aa survival and enables other microbes to evade host defense. The goal of our study is to extend our investigation and advance our knowledge of the molecular events that link PI-3K signaling blockade to downstream pro- inflammatory responses and altered vesicular trafficking and fusion. Specifically, we propose that CdtB induces GSK3-dependent HSP90 activation and further that HSP90, via its effects on NLRP3, Cox-2 and possibly ESCRT proteins is a critical intermediary in events leading to release of mature cytokines and eicosanoids (specific aim 1). We plan to advance our understanding of the role that Cdt plays activating the noncanonical inflammasome which involve gasdermin D (GSDMD) pore formation and pyroptosis. Our focus will be to determine how CdtB activates caspase-4 and contributes to GSDMD pore formation; the latter studies will focus on PI-3,4P2 and pore repair via the ESCRT system (specific aim2).
In specific aim 3 we will focus on perturbation of vesicular transport and phago-lysosome formation. We propose that altered PI distribution due in part to CdtB-mediated production of PI3,4P2 facilitates CdtB retrograde transport and modulates the formation of phago-lysosomes. The long-term goals of our study are to translate our understanding of the molecular events that govern Cdt toxicity and, in turn, the pathogenicity of Cdt-producing organisms. These studies are of particular significance as Cdt is produced not only by Aa but many other pathogens that contribute to chronic infectious and inflammatory disorders. Insight gained through this investigation will advance our understanding of the molecular events underlying LAP and other diseases caused by Cdt-producing organisms and identify new avenues for therapeutic intervention. .
The fundamental goal of the proposed study is to determine the potential of a bacterial-derived toxin (from the oral pathogen Aggratibacter actinomycetemcomitans), the cytolethal distending toxin (Cdt), to alter host defenses. Specifically, this study focuses on a novel role for this toxin in perturbing human macrophage function including inducing a pro-inflammatory response and impairing phagocytosis. We propose that this response contributes to the pathogenesis of inflammatory disorders such as periodontal disease.
