The Effect of Lethal Toxin on the Respiratory Epithelium.

Inhalational anthrax is an acute infectious disease caused by
exposure of the lungs to B. anthracis spores. Alveolar macrophages
engulf spores causing them to germinate to the vegetative form of
B. anthracis, which secretes edema toxin (ET)and lethal toxin (LT).
The pathogenesis of inhalational anthrax is characterized by
flu-like symptoms, respiratory distress, meningitis and shock,
which is fatal in almost all cases. The mechanism behind the
respiratory distress is not well understood. Therefore, our goal
was to determine the effects of lethal toxin in the human lung
epithelium. To study alterations in a more physiological setting,
we developed a differentiated, polarized lung epithelial system.
Lethal toxin exposure disrupted the lung barrier function and wound
healing. Assembly defects of junction proteins and additional
multicellular junction sites resulted in a higher permeability.
Pretreatment with keratinocyte growth factor (KGF) and
dexamethasone increased the viability, resulting in the rescue of
the permeability changes. Upon LT treatment, a more rigid
cytoskeleton was observed, evidenced by enhanced actin stress fiber
formations and tubulin stabilization. Cytoskeleton and adhesion
alterations prevented the epithelial cells from polarization,
directed migration, and wound healing. The MAPK pathway and Cdc42
activity might be partially responsible for these motility defects.
Lethal toxin is known to induce rapid cell death in murine
macrophages. In contrast, human epithelial cells are more resistant
to the cytotoxic effect of LT. By following the growth of
epithelial cells after LT treatment, we observed inhibited cell
proliferation due to a cell cycle arrest in the G1 phase.
Surprisingly, biotinylated lethal factor did not induce
cytotoxicity in murine macrophages. This is not due to an
internalization or proteolytic activity defect; instead changes in
the mitochondrial potential and proteasome activity were observed.
Biotinylated LT did not reduce proteasome activity as seen in LT
treated cells and caused hypopolarization of the mitochondria.
However, it is possible that biotinylation of lethal toxin could
prevent interaction of LT with proteins that induce cell death. The
major challenge for anthrax treatment is to find a treatment, which
can act faster, is easy to use and can bring patient out of the
dangerous physiological state in late pathogenesis. Our study has
implications in saving the viability and barrier function of lung
epithelial cells. One can devise better dosage and delivery of KGF
and dexamethasone as treatment modality for post anthrax exposure
to reduce respiratory distress. Furthermore, overcoming the cell
cycle arrest by the development of a drug would reduce the damage
of lung epithelial cells and induce proliferation. The discovery
that biotinylated LT is non-toxic to murine macrophages could
revolutionize treatment of anthrax infection. Exploring the types
of posttranslational modifications of LT that decrease toxicity and
finding the mechanism behind it might, lead to therapies that
directly counteract the effects of the lethal toxin in vivo.