Anthrax Lethal Toxin-Mediated Killing of Human and Murine Dendritic Cells Impairs the Adaptive Immune Response

Abstract

Many pathogens have acquired strategies to combat the immune response. Bacillus anthracis interferes with host defenses by releasing anthrax lethal toxin (LT), which inactivates mitogen-activated protein kinase pathways, rendering dendritic cells (DCs) and T lymphocytes nonresponsive to immune stimulation. However, these cell types are considered resistant to killing by LT. Here we show that LT kills primary human DCs in vitro, and murine DCs in vitro and in vivo. Kinetics of LT-mediated killing of murine DCs, as well as cell death pathways induced, were dependent upon genetic background: LT triggered rapid necrosis in BALB/c-derived DCs, and slow apoptosis in C57BL/6-derived DCs. This is consistent with rapid and slow killing of LT-injected BALB/c and C57BL/6 mice, respectively. We present evidence that anthrax LT impairs adaptive immunity by specifically targeting DCs. This may represent an immune-evasion strategy of the bacterium, and contribute to anthrax disease progression. We also established that genetic background determines whether apoptosis or necrosis is induced by LT. Finally, killing of C57BL/6-derived DCs by LT mirrors that of human DCs, suggesting that C57BL/6 DCs represent a better model system for human anthrax than the prototypical BALB/c macrophages.

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

“…32 However, the theory that Nalp1b and caspase-1 control LT-induced necrosis in murine macrophages is challenged by studies using caspase-1 inhibitors, which failed to block LT killing. 18,36,37 Here we show that macrophages derived from C57BL/6, DBA/2 and AKR strains, previously described as "resistant" to LT-mediated cell killing, are actually susceptible to LT-induced cell death by apoptosis. This finding appears to be applicable in vivo, as LT triggered depletion of splenic macrophages in C57BL/6 mice.…”

“…17 Despite the broad entry and activity of LT, it selectively kills only a few cell types. 2,[18][19][20][21][22][23][24] As MAPKK cleavage also occurs in cells that are not killed by LT, MAPKK cleavage is not sufficient for LT killing, and possibly not even required for this process. 24 We have previously shown that human and murine dendritic cells (DCs) are killed by LT in vitro.…”

“…24 We have previously shown that human and murine dendritic cells (DCs) are killed by LT in vitro. 18 LT killing of murine dendritic cells is dependent upon their genetic background. DCs derived from the BALB/c strain undergo rapid necrosis following LT treatment, while DCs from the C57BL/6 strain and human PBMCs die by slow apoptosis upon LT exposure.…”

“…DCs derived from the BALB/c strain undergo rapid necrosis following LT treatment, while DCs from the C57BL/6 strain and human PBMCs die by slow apoptosis upon LT exposure. 18 A corresponding depletion of splenic DCs in LT-treated mice indicated in vivo killing of these cells. 18 LT-mediated killing of antigen-presenting cells, including macrophages and dendritic cells, presumably leads to impairment of immunity.…”

“…18 A corresponding depletion of splenic DCs in LT-treated mice indicated in vivo killing of these cells. 18 LT-mediated killing of antigen-presenting cells, including macrophages and dendritic cells, presumably leads to impairment of immunity. LT-mediated MAPKK cleavage, which renders specific cells nonresponsive to stimulation by mitogens, might further diminish the immune response.…”