Bacillus anthracis' lethal toxin induces broad transcriptional responses in human peripheral monocyte

Bacillus anthracis, the causative agent of anthrax, is a gram-positive bacterium that is naturally found in the soil, and rarely affects the human population.

Bacillus anthracis, the causative agent of anthrax, is a gram-positive bacterium that is naturally found in the soil, and rarely affects the human population. However, deliberate dissemination of anthrax spores is capable of delivering a highly potent and lethal air-borne bioterrorist agent. The most virulent strains of B. anthracis contain two plasmids, pXO2 and pXO1, encoding an antiphagocytic poly-D-glutamic acid capsule and three exotoxins: lethal factor, edema factor and protective antigen.  Protective antigen is known to bind to two host cell receptors, TEM-8 and CMG-2, facilitating the entry of edema and/or lethal factor into host cells. Lethal factor is a zinc-dependent metalloprotease that cleaves the N-terminus of mitogen-activated protein kinases (MAPKKs or MEKs) while edema factor increases intracellular cAMP levels. Previous studies using anthrax animal models have documented resistance to anthrax lethal toxin (LT) through depletion of host macrophages, suggesting that these cells play a critical role in anthrax LT induced lethality. LT has also been shown to suppress cytokine responses by peripheral blood mononuclear cells, induce macrophage apoptosis, and prevent monocyte proliferation and differentiation.  Thus LT’s targeting of human monocytes/macrophages could help to explain the rapid onset of fatal symptoms and host demise during an inhalation anthrax infection, but the exact effects LT exerts on human peripheral monocytes, along with the mechanisms underlying the impairment of the host immune cell’s responses, have yet to be fully determined.
 
This study therefore worked to determine direct human monocyte susceptibility via cleavage of MEKs, along with the analysis of the transcriptional responses, to anthrax LT. Using Western Blot analysis the researchers demonstrated cleavage of endogenous MEK1 and MEK3 when human monocytes were treated with LT for four hours, proving their susceptibility to anthrax lethal toxin. Furthermore, staining with annexin V and propidium iodide revealed that LT treatment did not induce human peripheral monocyte apoptosis or necrosis. Using Affymetrix Human Genome U133 Plus 2.0 Arrays, the study then identified over 820 probe sets differentially regulated after LT treatment interrupting the normal transduction of over 60 known pathways. As expected, the MAPKK signaling pathway was most drastically affected by LT, but numerous genes outside the well-recognized pathways were also influenced by LT including the IL-18 signaling pathway, Toll-like receptor pathway and the IFN alpha signaling pathway. Concluding that LT directly targets human peripheral monocytes and causes multiple aberrant gene responses that would be expected to be associated with defects in human monocyte's normal signaling transduction pathways and function.
 
Link to article