Cytokine profile in <i>in vitro</i> mouse macrophage culture infected with <i>Bacillus anthracis</i> spores with varying plasmid composition

Abstract

Bacillus anthracis, the causative agent of anthrax, is able to exist both in environmental conditions (soil) and in the macroorganism. The manifestation of pathogenic properties of B. anthracis strains is determined by relevant plasmid composition, because the main toxin and the capsule-related virulence factors are located in bacterial plasmid. Modeling anthrax infection in vitro in macrophage culture might reveal an influence of individual B. anthracis strain characteristics on infection and development of infectious process. The aim of this study was to analyze cytokine level during infection of in vitro macrophage cell cultures with spores of anthrax microbe strains bearing varying plasmid composition. The dependence of the macrophage cell cytokine profile on the plasmid composition of B. anthracis strains was revealed while modeling anthrax infection in vitro. The presence of the toxin-producing plasmid pXO1 in anthrax microbe strains has a powerful stimulating effect on the production of macrophages J774A cell line cytokines. B. anthracis strains lacking the pXO1 plasmid virtually stimulated no production of IL-1, caused very low secretion of IL-1, IL-6, MCP-1, MIP-1, MIP-1, IL-12 (p70) and active G-CSF products. The low cytokine response of macrophage cells infected with monoplasmid strains bearing only the capsule-forming plasmid was due not only to the absence of a binary toxin, but also to disturbed regulation of capsule production associated with the absence of the atxA gene. The capsule, along with lethal and edematous toxins, belongs to the main virulence factors of B. anthracis, but strains lacking the pXO1 virulence plasmid, had its production impaired, because the main regulator of capsule synthesis is the atxA gene localized on the pXO1 plasmid being positively regulated by the acpA and acpB genes, so that strains lacking the toxin-forming plasmid, even in the presence of the encapsulation plasmid, elicit a weak cytokine response in infected cells. Diplasmid strains of B. anthracis, due to produced main virulence factors a two-component toxin and a capsule, enforce macrophages (in the experiment) to actively produce IL-1, IL-6, MCP-1, G-CSF, MIP-1; MIP-1, IL-12 (p70). Strains with moderate virulence and capable of capsulation in air virtually did not differ from highly virulent strains in terms of their effect on in vitro macrophage culture.