Bacillus anthracis, the aetiological agent of anthrax, is a highly interesting Gram-positive pathogen as it gives rise to a complex pathology, associating an overwhelming infection and a toxemia. It may be considered as a propotype model for the study of toxiinfection by cutaneous, inhalatory and digestive routes.
Historically, Louis Pasteur and Robert Koch independently characterised this pathogen, proving the infectious nature of anthrax, leading to the establishment of the famous Koch postulates and to the description of the macrophages and their role and function by the Nobel prize and pasteurien Elie Metchnikoff. In order to control this zoonose, Louis Pasteur designed the first bacterial vaccine ever used (Pouilly Le Fort in 18xx). Till the 1950’s, anthrax was thus considered as an infection. However, in 1954, Harry Smith discovered the pathophysological role of the toxins and suggested the presence of two toxins, switching the balance towards a toxin-dominated conception of anthrax.
Bacillus anthracis has the peculiarity to exist in two different forms, a spore (the form of persistence for a prolonged period of time in the environment, and representing the infecting form) and the vegetative bacillus that expresses its virulence factors (two toxins – a metalloprotease cleaving the host cell MAPKKs, and an adenylate cyclase-, and a specific poly-D-glutamic acid non immunogenic and antiphagocytic capsule, encoded by two plasmids) aimed at subverting the defenses of the infected host.
B. anthracis belongs to the Bacillus cereus group. B. cereus is encountered in the everyday life as it exists naturally in our environment. Due to its ubiquitous presence, it is an opportunistic pathogen, as it may contaminate medical implanted devices or food. Recent emergence has been observed, in the USA and in Africa, of B. cereus strains having acquired virulence plasmids closely resembling those of B. anthracis.
A critical gap of knowledge exists as the global model currently depicting anthrax infection mainly relies on in vitro data and interpretation of fragmentary in vivo data, raising the issue of the physiological relevance of the current model. While having led to a wealth of new approaches and hypotheses, translation to in vivo approaches are now urgently needed to elaborate an innovative vision of this toxi-infection. New paradigms are now emerging, due to the availability of innovative imaging technologies, bioluminescence and fluorescence imaging, molecular and cellular tools, and image analysis software development.
The research approaches developped in the research entity Pathogénie des Toxi-Infections Bactériennes aim at a renewed understanding of the crosstalk between the bacterium and its host, the defenses mechanisms, innate and adaptive, triggered in the host upon infection depending on the route of infection, and the strategies exploited by the bacterium to subvert these defense mechanims. As anthrax is a toxi-infection, we favor a global and complementary approach putting together what resorts from the infection arm per se and the toxin arm of the infection.
In recent in vivo studies, we have elucidated novel steps of B. anthracis infection. Analysis of the kinetics and dissemination pattern of B. anthracis during murine cutaneous, inhalational and gastro-intestinal infections has modified the current perception of the interactions between B. anthracis and its host. In vivo real time analysis of bioluminescent B. anthracis dissemination during cutaneous, inhalational and gastrointestinal infection in mice has shown the respective role of capsule and toxins. Previously undescribed portals of entry and target organs were identified. Most recent studies have exemplified striking different patterns of dissemination in inhalational anthrax compared to cutaneous infection and the previously under-estimated pivotal role of edema toxin and a temporal differential expression of the lesions caused by each toxin. We have also shown a previously undescribed role for the capsule, as an adhesin mediating close interaction of the bacteria with vascular endothelium in vivo. Edema toxin has also shown its ability to provoke “macropertures” in endothelial cells, i.e. small transitory openings through the endothelial cell body, the cell retaining its integrity.
We have characterised how B. anthracis toxins inhibits the production of effectors of the innate immune system, such as chemokines, cytokines and the bactericidal enzyme group IIA phospholipase A2 in key target cells (alveolar macrophages, bronchial epithelial cells, dendritic cells). Detailed analysis of the disruption of the cell signal transduction pathways (inhibition of the MAPKKs and activation of the AMPc/PKA-dependent pathways) have shown that the end-effects involve epigenetic mechanisms leading to impaired interactions of transcription factors on their promoter whithout affecting intranuclear recruitment . To approach the initial host control mechanisms and the bacterial evasion strategies, we have detailed how B. anthracis secretion by Natural Killer (NK) cells. Crosstalk between NK cells and spore-activated macrophages is both cytokine- and contact-dependent, leading to cytokine secretion, cytotoxic activity and recruitment of inflammatory cells in vivo, leading to a delayed systemic dissemination. By altering spore-induced cytokine production, toxin-secreting B. anthracis prevent NK cell activation, leading to successful bacterial colonization and spreading of infection. Edema toxin is central in deactivation of NK cell cytotoxic function.
Applications of the results gathered can be conceptually used for therapeutic control of the infection, as for exmaple sPLA2-IIA as an endogenous antibiotic, synthetic analogs of defensins, NK cells, spore-reactive IFNg-secreting CD4 Tcells, polyglutamate capsule. A novel vaccine is now under development in a Phase I trial.
These global approaches, exploiting B. anthracis as a model of infection by a Gram-positive bacterium, can be extended to other pathogens, as it is able to infect its host by cutaneous, inhalational and digestive routes.