Early Bacillus anthracis-macrophage interactions: intracellular survival and escape

Abstract

This study describes early intracellular events occurring during the establishment phase of Bacillus anthracis infections. Anthrax infections are initiated by dormant endospores gaining access to the mammalian host and becoming engulfed by regional macrophages (Mφ). During systemic anthrax, late stage events include vegetative growth in the blood to very high titres and the synthesis of the anthrax exotoxin complex, which causes disease symptoms and death. Experiments focus on the early events occurring during the first few hours of the B. anthracis infectious cycle, from endospore germination up to and including release of the vegetative cell from phagocytes. We found that newly vegetative bacilli escape from the phagocytic vesicles of cultured Mφ and replicate within the cytoplasm of these cells. Release from the Mφ occurs 4–6 h after endospore phagocytosis, timing that correlates with anthrax infection of test animals. Genetic analysis from this study indicates that the toxin plasmid pXO1 is required for release from the Mφ, whereas the capsule plasmid pXO2 is not. The transactivator atxA, located on pXO1, is also found to be essential for release, but the toxin genes themselves are not required. This suggests that Mφ release of anthrax bacilli is atxA regulated. The putative ‘escape’ genes may be located on the chromosome and/or on pXO1.

In-vitro characterisation of the phagocytosis and fate of anthrax spores in macrophages and the effects of anti-PA antibody

“…The results of the present study are also consistent with older studies showing a decrease in spores detectable in the lungs early after exposure without an increase in spores in local lymph nodes [21,28]. They are in contrast to those reported by Hanna and co-workers who observed that germination of B. anthracis strain Sterne within RAW264.7 cells was accompanied by outgrowth and replication of the germinated spores, lysis of the infected MQ, and release of the vegetative bacilli from the MQs [22,23,25]. Guidi-Rontani and co-workers reported that Sterne strain 7702 germinated but failed to multiply within MQ; however, the organisms survived and appeared to be cytotoxic for MQs [12].…”

“…As reported recently in experiments with attenuated Sterne-like pX01 þ , pX02 À strains [22][23][24][25], spores of the fully virulent pX01 þ , pX02 þ B. anthracis Ames strain were readily phagocytosed by and germinated within the RAW 264.7 MQ-derived cell line (Fig. 1, Table 1, data not shown).…”

“…However, several steps in the process have been delineated or hypothesised based on observations in animals and in in-vitro model systems [21][22][23][24][25][26][27][28]. Upon inhalation of a lethal dose of B. anthracis spores by a susceptible host, such as a guinea-pig, the spores appear to be taken up by cells lining the alveoli and by free phagocytes [26,28].…”

Human α-defensins neutralize anthrax lethal toxin and protect against its fatal consequences

“…It has been shown that a toxin-producing B. anthracis strain Sterne kills murine macrophages (19,20). To determine whether HNP-1 protects macrophages from toxin-producing B. anthracis, we examined the cytotoxicity of anthrax spores in HNP-1-treated RAW 264.7 cells.…”

mAbs toBacillus anthraciscapsular antigen for immunoprotection in anthrax and detection of antigenemia

“…Endospores of Bacillus anthracis are taken up by macrophages at the site of initial infection and may be transported to regional lymph nodes (2). The spores germinate inside the phagolysosome to become vegetative bacteria that escape from the phagolysosome and replicate within the cytoplasm (3,4). Vegetative cells are released into the extracellular milieu and enter the circulation, where the vegetative cells grow extracellularly to levels as high as 10 8 bacteria per ml of blood.…”