Anthrax toxin targeting of myeloid cells through the CMG2 receptor is essential for establishment of Bacillus anthracis infections in mice

Abstract

Bacillus anthracis kills through a combination of bacterial infection and toxemia. Anthrax toxin working via the CMG2 receptor mediates lethality late in infection, but its roles early in infection remain unclear. We generated myeloid-lineage specific CMG2-deficient mice to examine the roles of macrophages, neutrophils, and other myeloid cells in anthrax pathogenesis. Macrophages and neutrophils isolated from these mice were resistant to anthrax toxin. However, the myeloid-specific CMG2-deficient mice remained fully sensitive to both anthrax lethal and edema toxins, demonstrating that targeting of myeloid cells is not responsible for anthrax toxin-induced lethality. Surprisingly, the myeloid-specific CMG2-deficient mice were completely resistant to B. anthracis infection. Neutrophil depletion experiments suggest that B. anthracis relies on anthrax toxin secretion to evade the scavenging functions of neutrophils to successfully establish infection. This work demonstrates that anthrax toxin uptake through CMG2 and the resulting impairment of myeloid cells are essential to anthrax infection.

Figure 1. CMG2-null mouse macrophages are completely resistant to PA + FP59 while macrophages from TEM8-null mouse are sensitive

BMDMs from various genotypes as indicated were treated with varying concentrations of PA in the presence of FP59 (100 ng/ml) for 48 h. Cell viability was then evaluated by MTT assay. Data are reported as mean viability ± S.D.

Figure 2. Generation and anthrax toxin sensitivity of the myeloid-specific CMG2-null mice

(A) Schematic targeting strategy. Diagram of the CMG2^flox allele having exon 12 flanked by LoxP sites (“Floxed”), and the myeloid-specific CMG2-null allele (CMG2^flox/LysM-Cre). The homozygous myeloid-specific CMG2-null mice (CMG2^flox/flox/LysM-Cre) were obtained by the intercrossing of CMG2^+/flox/LysM-Cre mice. The red arrowheads indicate LoxP sites. (B) RT-PCR analyses of the CMG2 TM domain deletion in various tissues, macrophages, and neutrophils in a CMG2^flox/flox/LysM-Cre mouse. Macrophages and neutrophils from a WT mouse were also included as controls. See Figure S1 for neutrophil isolation. (C) Toxin susceptibility of BMDMs from WT and myeloid-specific CMG2-null mice. BMDMs were treated with various concentrations of PA plus FP59 (100 ng/ml) for 48 h. Cell viability was evaluated by MTT assay. Data are reported as mean viability ± S.D. (D) Binding and processing of PA on mouse neutrophils. Neutrophils isolated from mouse bone marrow were incubated with 1 μg/ml PA and 0.5 μg/ml LF at 37 ^oC for 0–3 h. Cells were washed, lysed, and cell lysates subjected to Western blotting using either an anti-PA antiserum or a MEK2 antibody. A non-specific cross-reactive band indicated by the arrow head at the lower left of the blot serves as protein loading control. (E-F) Sensitivity of myeloid-specific CMG2-null mice to the anthrax toxins. CMG2^flox/flox/LysM-Cre mice and their control mice were treated with 100 μg PA plus 100 μg LF (intraperitoneally) (E) or with 50 μg PA plus 50 μg EF (intravenously) (F) and survival monitored for 2 weeks. Whole body CMG2-null mice (CMG2^−/−) in the same genetic background (C57BL/6) were included as additional controls.

Figure 3. Myeloid-specific CMG2-null mice are resistant to B. anthracis infection

(A) B. anthracis bacteria are efficiently cleared in whole body CMG2-null mice. CMG2^−/− mice and their control mice were injected subcutaneously with 1×10⁸ B. anthracis spores. At 8, 24, or 48 h after the infection, B. anthracis bacteria were counted in the pooled tissue lysates (which include heart, lung, kidney, spleen and liver) by dilution plating on LB agar plates. (B) Myeloid-specific CMG2-null mice are resistant to B. anthracis spore infection. CMG2^flox/flox/LysM-Cre mice and their littermate control mice were subcutaneously injected with 2×10⁷ B. anthracis A35 spores and monitored for signs of malaise and survival for 2 weeks. See also Figure S2. (C) B. anthracis bacteria are efficiently cleared in myeloid-specific CMG2-null mice. CMG2^flox/flox/LysM-Cre mice and their littermate control mice were subcutaneously injected with 1×10⁸ B. anthracis A35 spores. At 48 h after infection, B. anthracis bacteria were counted as described in A. (D) Myeloid-specific CMG2-null mice are resistant to vegetative B. anthracis A35 infection. Mice were intravenously injected with 1×10⁶ B. anthracis A35 vegetative bacteria and monitored for survival for 2 weeks. (E and F) The anthrax toxins, in particular LT, are crucial for B. anthracis to overcome innate immune defense. C57BL/6 mice (WT) were challenged with 1×10⁵ (E) or 5×10⁶ (F) vegetative B. anthracis A35 or mutant strains lacking LF (ΔLF ) or EF (ΔEF) subcutaneously and monitored for signs of malaise and survival for 2 weeks. See also Figure S3.

Figure 4. Depletion of neutrophils sensitizes myeloid-specific CMG2-null mice to B. anthracis infection

(A and B) Mice were depleted of neutrophils by either cyclophosphamide (A) or Ly-6G 1A8 antibody (B), and then challenged with 5 ×10⁷ B. anthracis A35 spores subcutaneously and monitored for signs of malaise and mortality for 2 weeks. See also Figure S4 and Table S1.