Contributions of Histamine, Prostanoids, and Neurokinins to Edema Elicited by Edema Toxin fromBacillus anthracis

Abstract

Bacillus anthracis edema toxin (ET), composed of protective antigen and an adenylate cyclase edema factor (EF), elicits edema in host tissues, but the target cells and events leading from EF-mediated cyclic-AMP production to edema are unknown. We evaluated the direct effect of ET on several cell types in vitro and tested the possibility that mediators of vascular leakage, such as histamine, contribute to edema in rabbits given intradermal ET. ET increased the transendothelial electrical resistance of endothelial monolayers, a response that is mechanistically inconsistent with the in vivo vascular leakage induced by ET. Screening of several drugs by intradermal treatment prior to toxin injection demonstrated reduced ET-induced vascular leakage with a cyclo-oxygenase inhibitor (indomethacin), agents that interfere with histamine (pyrilamine or cromolyn), or a neurokinin antagonist (spantide). Systemic administration of indomethacin or celecoxib (cyclo-oxygenase inhibitors), pyrilamine, aprepitant (a neurokinin 1 receptor antagonist), or indomethacin with pyrilamine significantly reduced vascular leakage associated with ET. Although the effects of pyrilamine, cromolyn, or aprepitant on ET-induced vascular leakage suggest a possible role for mast cells (MC) and sensory neurons in ET-induced edema, ET did not elicit degranulation of human skin MC or substance P release from NT2N cells in vitro. Our results indicate that ET, acting indirectly or directly on a target yet to be identified, stimulates the production/release of multiple inflammatory mediators, specifically neurokinins, prostanoids, and histamine. These mediators, individually and through complex interactions, increase vascular permeability, and interventions directed at these mediators may benefit hosts infected with B. anthracis.Bacillus anthracis is a gram-positive, spore-forming bacillus that causes the disease known as anthrax. The vegetative form of B. anthracis results from spore inoculation of the host animal via cutaneous, gastrointestinal, or inhalational routes (57). Subsequently, spores germinate in a complex process that involves phagocytosis of spores by macrophages and transport of germinating spores within macrophages to regional lymphatics. The vegetative bacilli may then gain entry to the systemic circulation and spread to other sites in the host. This process is facilitated by virulence factors expressed by vegetative bacilli: (i) an antiphagocytic capsule and (ii) lethal toxin (LT) and edema toxin (ET). LT consists of protective antigen (PA) and lethal factor (LF), while ET consists of PA and edema factor (EF) (57).PA undergoes proteolytic cleavage to a 63-kDa monomer, with subsequent formation of a heptamer capable of binding up to three molecules of EF or LF (15). PA heptamers bind to two different widely expressed cell surface receptors, anthrax toxin receptor/tumor endothelial marker 8 and/or capillary morphogenesis protein 2, and undergo receptor-mediated endocytosis with trafficking to an acidic endosomal compartment (5,6,54). Aci...

Updating Perspectives on the Initiation of Bacillus anthracis Growth and Dissemination through Its Host

“…This may reflect the exotoxins' ability to break down epithelial and endothelial cell barriers and cause hemorrhaging in animal models (22,57,74). The ability to affect endothelial barrier function can be seen even with low exotoxin concentrations (100 ng ET and 1 g LT) in vitro, where ET and LT alter the transendothelial electrical resistance of endothelial cell monolayers (69,74). Likewise, capsule plays an important role in dissemination.…”

Opening moves: early events in anthrax pathogenesis and their consequences for dissemination

“…This presumably would reduce diapedesis through capillaries or epithelial layers to the site of infection; however, this has not been tested [188]. These studies are in contrast to earlier work from Wade et al which found ET increases PMN chemotaxis and an in vivo observation that greater amounts of neutrophils were found near injection sites of ET [189,190].…”

“…Sec15/Rab11 are also inhibited by ET and LT in human brain microendothelial cell lines and suggested that ET induced edema indirectly by increasing inflammatory lipid mediators in vivo [189]. Furthermore, pharmacological inhibition of prostanoids, histamines, and neurokinins, which were postulated to induce histamine production, reduced edema formation via vascular leakage.…”

Defensive strategies of Bacillus anthracis that promote a fatal disease

“…This increase in vascular permeability does not appear to occur through a direct effect of edema toxin on endothelial cell function, however, but through the stimulation of multiple inflammatory mediators [18]. Inhibition of neurokinins, prostanoids and histamine reduce the vascular leakage caused by edema toxin, although the cell types that release these mediators have not been identified.…”