Differential role of the interleukin-17 axis and neutrophils in resolution of inhalational anthrax.

The roles of interleukin-17 (IL-17) and neutrophils in the lung have been described as those of two intricate but independent players. Here we identify neutrophils as the primary IL-17-secreting subset of cells in a model of inhalation anthrax using A/J and C57BL/6 mice. With IL-17 receptor A knockout (IL-17RA-/-) mice, we confirmed that IL-17A/F signaling is instrumental in the self-recruitment of this population. We also show that the IL-17A/F axis is critical for surviving pulmonary infection, as IL-17RA-/- mice become susceptible to intranasal infection by Bacillus anthracis Sterne spores. Strikingly, infection with a fully virulent strain did not affect IL-17RA-/- mouse survival. Eventually, by depleting neutrophils in wild-type and IL-17RA-/- mice, we demonstrated the crucial role of IL-17-secreting neutrophils in mouse survival of infection by fully virulent B. anthracis. This work demonstrates the important roles of both IL-17 signaling and neutrophils in clearing this pathogen and surviving pulmonary B. anthracis infection.

[Anthrax revisited: Bacillus anthracis toxins as novel actors of immune escape?]. / La maladie du charbon revue: les toxines de Bacillus anthracis comme nouveaux acteurs de l'evasion immunitaire de l'agent pathogène?

The recent bioterrorist attacks have stressed the need of a better knowledge of Bacillus anthracis infection pathophysiology. We present here the increasing interests of B. anthracis studies in term of bio-defense, the main pathogen characteristics, the main clinical features of inhalational anthrax (the pulmonary form of the disease), and recent aspects of its physiopathology. Next, we address the main results concerning the toxin effects on immune system through impairing the dendritic cell functions, and we analyze the singular role of anthrax toxins in immune evasion.

Contribution of toxins to the pathogenesis of inhalational anthrax.

Inhalational anthrax is a life-threatening infectious disease of considerable concern, especially as a potential bioterrorism agent. Progress is gradually being made towards understanding the mechanisms used by Bacillus anthracis to escape the immune system and to induce severe septicaemia associated with toxaemia and leading to death. Recent advances in fundamental research have revealed previously unsuspected roles for toxins in various cell types. We summarize here pathological data for animal models and macroscopic histological examination data from recent clinical records, which we link to the effects of toxins. We describe three major steps in infection: (i) an invasion phase in the lung, during which toxins have short-distance effects on lung phagocytes; (ii) a phase of bacillus proliferation in the mediastinal lymph nodes, with local effects of toxins; and (iii) a terminal, diffusion phase, characterized by a high blood bacterial load and by long-distance effects of toxins, leading to host death. The pathophysiology of inhalational anthrax thus involves interactions between toxins and various cell partners, throughout the course of infection.

Lung dendritic cells rapidly mediate anthrax spore entry through the pulmonary route.

Inhalational anthrax is a life-threatening infectious disease of considerable concern, especially because anthrax is an emerging bioterrorism agent. The exact mechanisms leading to a severe clinical form through the inhalational route are still unclear, particularly how immobile spores are captured in the alveoli and transported to the lymph nodes in the early steps of infection. We investigated the roles of alveolar macrophages and lung dendritic cells (LDC) in spore migration. We demonstrate that alveolar macrophages are the first cells to phagocytose alveolar spores, and do so within 10 min. However, interstitial LDCs capture spores present in the alveoli within 30 min without crossing the epithelial barrier suggesting a specific mechanism for rapid alveolus sampling by transepithelial extension. We show that interstitial LDCs constitute the cell population that transports spores into the thoracic lymph nodes from within 30 min to 72 h after intranasal infection. Our results demonstrate that LDCs are central to spore transport immediately after infection. The rapid kinetics of pathogen transport may contribute to the clinical features of inhalational anthrax.

Resident CD11c+ lung cells are impaired by anthrax toxins after spore infection.

Bacillus anthracis secretes 2 toxins: lethal toxin (LT) and edema toxin (ET). We investigated their role in the physiopathologic mechanisms of inhalational anthrax by evaluating murine lung dendritic cell (LDC) functions after infection with B. anthracis strains secreting LT, ET, or both or with a nontoxinogenic strain. Three lung cell populations gated on CD11c/CD11b expression were obtained after lung digestion: (1) CD11c(high)/CD11b(low) (alveolar macrophages), (2) CD11c(intermediate (int))/CD11b(int) (LDCs), and (3) CD11c(low)/CD11b(high) (interstitial macrophages or monocytes). After infection with LT-secreting strains, a decrease in costimulatory molecule expression on LDCs was observed. All CD11c+ cells infected with a nontoxinogenic strain secreted tumor necrosis factor (TNF)- alpha , interleukin (IL)-10, and IL-6. LT-secreting strains inhibited overall cytokine secretion, whereas the ET-secreting strain inhibited only TNF- alpha secretion and increased IL-6 secretion. Similar results were obtained after preincubation with purified toxins. Our results suggest that anthrax toxins secreted during infection impair LDC function and suppress the innate immune response.

Evidence for adjuvanticity of anthrax edema toxin.

Bacillus anthracis edema factor (EF) is an adenylate cyclase that increases intracellular cAMP concentrations. Since EF is present as a contaminant in the licensed protective antigen(PA)-based vaccines, we investigated its effect on anti-PA humoral immune response in BALB/c mice. We observed a significant increase of anti-PA IgG response in mice immunised with PA in association with EF as compared to PA alone. These results clearly show an adjuvant effect of EF, which is consistent with the data concerning the cellular effects of EF on antigen-presenting cells.