Sensitizing Anthrax Lethal Toxin-resistant Macrophages to Lethal Toxin-induced Killing by Tumor Necrosis Factor-α

Abstract

Macrophages from different inbred mouse strains exhibit striking differences in their sensitivity to anthrax lethal toxin (LeTx)-induced cytolysis. Although LeTxinduced cytolysis of macrophages plays an important role in the outcome of anthrax infection, the sensitivity of macrophages in vitro does not correlate with in vivo susceptibility to infection of Bacillus anthracis. This divergence suggests that additional factors other than LeTx are involved in the cytolysis of LeTx-resistant macrophages in vivo. We found that LeTx-resistant macrophages became sensitive to LeTx-induced cytolysis when these cells were activated by bacterial components. Tumor necrosis factor-␣ induced by bacterial components was a key factor that cooperated with LeTx in inducing LeTx-resistant macrophage death. Tumor necrosis factor-␣/LeTx-induced death of LeTx-resistant macrophages was dependent on mTor (mammalian target of rapamycin), but independent of caspases. Our data indicate that host responses to anthrax infection contribute to cytolysis of LeTx-resistant macrophages.

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

“…Macrophage susceptibility to LT killing appears to be dependent upon maturation state, and certain external stimuli, like lipopolysaccharide (LPS), can confer susceptibility to C57BL/6 macrophages. 24,42,43 Moreover, we recently showed significant LT killing of another myeloid lineage cell type, dendritic cells (DCs), derived from BALB/c and C57BL/6 mice, as well as human PBMCs. 18 Due to the similar responses of macrophages and DCs from BALB/ c mice to LT treatment, we hypothesized that, like C57BL/6 DCs, C57BL/6-derived macrophages may be susceptible to LT-mediated cell killing.…”

“…24 In contrast, macrophages from C57BL/6 mice have largely served as the prototype for LT-resistant macrophages. 24,[32][33][34] A recent report suggested that a highly polymorphic gene, Nalp1b, controls LT susceptibility in murine macrophages. 32 Nalp1b is a critical immune protein that functions in assembling the inflammasome, a multimeric complex, which contains and activates caspase-1.…”

“…[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. 24 In contrast, macrophages from C57BL/6 mice have largely served as the prototype for LT-resistant macrophages.…”

“…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. 24 In contrast, macrophages from C57BL/6 mice have largely served as the prototype for LT-resistant macrophages. 24,[32][33][34] A recent report suggested that a highly polymorphic gene, Nalp1b, controls LT susceptibility in murine macrophages.…”

“…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.…”