Human Neutrophils Kill Bacillus anthracis

Abstract

Bacillus anthracis spores cause natural infections and are used as biological weapons. Inhalation infection with B. anthracis, the etiological agent of anthrax, is almost always lethal, yet cutaneous infections usually remain localized and resolve spontaneously. Neutrophils are typically recruited to cutaneous but seldom to other forms of anthrax infections, raising the possibility that neutrophils kill B. anthracis. In this study we infected human neutrophils with either spores or vegetative bacteria of a wild-type strain, or strains, expressing only one of the two major virulence factors. The human neutrophils engulfed B. anthracis spores, which germinated intracellularly and were then efficiently killed. Interestingly, neutrophil killing was independent of reactive oxygen species production. We fractionated a human neutrophil granule extract by high-performance liquid chromatography and identified α-defensins as the component responsible for B. anthracis killing. These data suggest that the timely recruitment of neutrophils can control cutaneous infections and possibly other forms of B. anthracis infections, and that α-defensins play an important role in the potent anti-B. anthracis activity of neutrophils.

Poly-γ-Glutamate Capsule-Degrading Enzyme Treatment Enhances Phagocytosis and Killing of EncapsulatedBacillus anthracis

“…A recent study by Mayer-Scholl et al reported that human neutrophils kill encapsulated as well as unencapsulated B. anthracis (32). At a neutrophil-to-bacterium ratio of 0.1:1, there was ϳ45% and ϳ70% killing of the encapsulated wild-type bacilli compared to ϳ90 and ϳ95% killing of the unencapsulated strain after 2 h and 3 h, respectively, showing a minimal effect of the capsule.…”

“…Alveolar phagocytes are thought to be involved in transporting inhaled spores to draining lymph nodes, where the spores are thought to germinate (42). Macrophages (60) and neutrophils (23,32) are reported to phagocytize and kill B. anthracis, and macrophages are required for resistance to infection in mice (7). Recent work suggests that antibody to the B. anthracis capsule enhances phagocytosis and killing of encapsulated bacilli (6,46,59) and that active (6) and passive (27) vaccination with capsule protects against experimental infection.…”

Treatment of Experimental Anthrax with Recombinant Capsule Depolymerase

“…We report here that treatment targeting B. anthracis bacilli to phagocytes by enzymatically removing the capsule can protect animals from infection, further supporting the role of capsule in evading host innate immunity. Although dormant spores may not be susceptible to phagocytic killing until they germinate (17,20,32), the vegetative form of the organism is susceptible to phagocytic killing, particularly when devoid of capsule (20,28,39,42). As mentioned above, enzyme treatment to remove microbial capsules has been successfully used to treat existing infections with pneumococci, Cryptococcus, and E. coli (2,15,31) in mouse models of infection.…”

“…While neutrophils are reported to have a variable role in resistance to anthrax spore infection in mice (7), mouse neutrophils have been shown to kill unencapsulated B. anthracis (42). It has recently been demonstrated that human neutrophils kill encapsulated B. anthracis, although the killing was modest and predominantly extracellular (28), and we have previously reported that they can reduce viability of decapsulated bacilli by up to 3 logs while having minimal activity against encapsulated bacilli (39). The B. anthracis capsule (pX02) and toxin (pX01) plasmids encode factors that enhance survival in the host, likely in part by subverting the bactericidal activity of neutrophils.…”

A killed, genetically engineered derivative of a wild-type extraintestinal pathogenic E. coli strain is a vaccine candidate

“…A neutrophil-mediated bactericidal assay was performed essentially as described [44]. In brief, either 5 × 10 5 purified neutrophils [45] in 500 μl of 20% plasma pre-heated at 56°C for 30 minutes (Δ6°C)-1X PBS pH 7.4 (plasma-PBS) or plasma-PBS alone were added to each well of a tissue culture plate (Costar 3472-clear, Corning Inc., Corning, NY).…”