Potential dissemination of Bacillus anthracis utilizing human lung epithelial cells

Abstract

SummaryDissemination of Bacillus anthracis spores from the lung is a critical early event in the establishment of inhalational anthrax. We recently reported that B. anthracis could adhere to and be internalized by cultured intestinal epithelial and fibroblast cells. Here, using gentamicin protection assays and/or electron microscopy, we found that Sterne strain 7702 spores were able to adhere to and subsequently be internalized by polarized A549 cells and primary human small airway epithelial cells. We showed for the first time that internalized spores were able to survive and that spores could translocate across an A549 cell barrier from the apical side to the basolateral side without disrupting the barrier integrity, suggesting a transcellular route. In addition, dormant spores of fully virulent Ames and UT500 strains were able to adhere to A549 cells at a frequency similar to that of 7702, whereas the capsule in germinated Ames and UT500 spores prevented adherence. Fluorescence microscopy also revealed that dormant Ames spores were internalized at a frequency similar to that of 7702. These findings highlight the possibility of a novel route of dissemination in which B. anthracis utilizes epithelial cells of the lung. The implications of these results to B. anthracis pathogenesis are discussed.

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

“…An alternative model for phagocyte independent dissemination is that the spores and vegetative bacilli can be internalized by epithelial cells directly [62,271,[278][279][280][281]. These studies are supported by in vitro and in vivo studies that demonstrate that the spores are able to adhere, invade, and transcytose through the epithelial layer [271,279,280].…”

“…These studies are supported by in vitro and in vivo studies that demonstrate that the spores are able to adhere, invade, and transcytose through the epithelial layer [271,279,280]. Additionally, B. anthracis spores are unique from related Bacillus spores in that they can reside in the lung for long periods of time [278].…”

“…In vitro assays with lung and colonic epithelial cell lines found that spores were capable of adherence and internalization into epithelial cells, as well as transcytosis through epithelial layers [271,280]. Spores were found to bind to epithelial cells in a concentration dependent manner, suggesting a receptor mediated adherence [60].Furthermore, in vivo analysis of mice lungs after intranasal instillation of spores found that within 2 hours of infection spores were both adhered to and internalized into lung epithelial cells.…”

“…However, future studies will need to be performed in order to determine if and to what degree the mouse lung epithelium becomes permeabilized. Epithelial layers have also been identified as being capable of spore uptake and to allow transcytosis of spores [271]. Therefore, another possibility is that spores are directly internalized by the lung epithelium to then transit directly to the circulatory system and thereby bypass the lymph nodes.…”

“…A more recent model of dissemination has been suggested whereby the spores are internalize or transcytose through lung epithelial cells [271,280]. In vitro assays with lung and colonic epithelial cell lines found that spores were capable of adherence and internalization into epithelial cells, as well as transcytosis through epithelial layers [271,280].…”