Anthrax Lethal Toxin Impairs Innate Immune Functions of Alveolar Macrophages and Facilitates Bacillus anthracis Survival

Abstract

Alveolar macrophages (AM) are very important for pulmonary innate immune responses against invading inhaled pathogens because they directly kill the organisms and initiate a cascade of innate and adaptive immune responses. Although several factors contribute to inhalational anthrax, we hypothesized that unimpeded infection of Bacillus anthracis is directly linked to disabling the innate immune functions contributed by AM. Here, we investigated the effects of lethal toxin (LT), one of the binary complex virulence factors produced by B. anthracis, on freshly isolated nonhuman primate AM. Exposure of AM to doses of LT that killed susceptible macrophages had no effect on the viability of AM, despite complete MEK1 cleavage. Intoxicated AM remained fully capable of B. anthracis spore phagocytosis. However, pretreatment of AM with LT resulted in a significant decrease in the clearance of both the Sterne strain and the fully virulent Ames strain of B. anthracis, which may have been a result of impaired AM secretion of proinflammatory cytokines. Our data imply that cytolysis does not correlate with MEK1 cleavage, and this is the first report of LT-mediated impairment of nonhuman primate AM bactericidal activity against B. anthracis.

Anthrax toxin-induced rupture of artificial lipid bilayer membranes

“…Capsule from B. anthracis Ames was prepared as described earlier. 61 Purified capsule, capsule-toxin complex, PA, and LF from fractionated plasma were detected by Western blot 55 or by methylene blue staining of capsule. 62 Plasma fractions were also subjected to PA capture ELISA with a purified anti-PA antibody (IgG) produced in goats.…”

“…24,25,54 This harvested complex has adverse effects when exposed to macrophages in vitro and when injected into animals. 55 Interestingly, LF is enzymatically active in a complex with the PA 63 channel 25 and the complex reconstitutes into planar bilayer membranes. 24 The latter result led to the hypothesis that the anthrax toxin complex (LF or EF bound to the channel) might enter the cytoplasm and cause cell intoxication.…”

Anthrax Lethal Toxin Kills Macrophages in a Strain-Specific Manner by Apoptosis or Caspase-1-Mediated Necrosis

“…11,[25][26][27] Immune impairment likely promotes bacterial proliferation in hosts infected with B. anthracis. [28][29][30][31] The susceptibility of murine macrophages to LT-mediated killing is strain-dependent. For example, C3H/HeJ and BALB/c-derived macrophages, the prototypical cells in anthrax research, are highly susceptible to killing by LT. 24 LT treatment of these macrophages triggers rapid induction of necrosis, which normally occurs within 2 to 4 hours of treatment.…”

Human Transferrin Confers Serum Resistance against Bacillus anthracis

“…Subsequent spread into the bloodstream often results in sepsis and death (1). The ability to divide within macrophages without inducing an inflammatory reaction depends on the plasmid-encoded tripartite anthrax toxin (pXO1) that blocks intracellular signaling in immune cells (3)(4)(5). Intradermal inoculation with B. anthracis spores results in cutaneous anthrax, a milder infection that normally remains localized and resolves spontaneously (6).…”