Host response of lung cells to virulent Bacillus anthracis



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Bacillus anthracis, the etiological agent of anthrax, is a Gram-positive, spore-forming bacterium that is classified as a Category A select agent. The current paradigm in anthrax literature labels the alveolar macrophage (MØ) as the primary cell utilized by spores as a safe site for germination and transport to host lymphatics. This study was the first to report that, unlike their murine counterparts, human alveolar MØs mediate the killing of the majority of internalized virulent B. anthracis spores/vegetative cells via a robust oxidative burst. However, ~23% of dormant spores remained viable at 24h post-infection. The “persistence” of some spores may allow them to hide long enough to avoid the first wave of the immune response and germinate later. Although the majority of MØs survived spore infection, a small percentage succumbed to infection via apoptosis. \r\n B. anthracis produces edema toxin (EdTx), an adenylate cyclase that increases cAMP levels in host cells. EdTx significantly suppressed human alveolar MØ phagocytosis of Ames spores, accompanied by cytoskeletal changes such as decreased cell spreading and F-actin content. Further, EdTx altered protein levels/activity of cAMP-dependent PKA and exchange protein activated by cAMP (Epac). Including PKA- and Epac-selective cAMP analogs confirmed the involvement of both pathways in this inhibition. This suggests that EdTx-generated cAMP weakens the host immune response by impairing cytoskeletal functions essential for MØ phagocytosis via signaling by PKA and Epac. \r\n Finally, the transcriptional response of murine lungs to inhalational infection with Ames spores was addressed using GeneChip analysis. Although few transcriptional alterations (15 genes) were detected in the lungs 8h post-infection, important inflammatory genes were both up- and down-regulated, indicating that the lung recognized microbial infection. After 48h, greater transcriptional changes (46 genes) occurred and many of the elevated genes serve protective roles, likely assisting the lungs in combating infection while shielding itself from inflammation-induced damage. \r\n The research presented in this report provides new insight into the interaction between anthrax spores and human alveolar MØs, illustrates a novel suppressive role for EdTx, and identifies affected lung genes during in vivo anthrax infection, potentially yielding new opportunities to develop novel therapeutic strategies against this deadly disease. \r\n



phagocytosis, human alveolar macrophages, edema toxin, biological weapon, anthrax