Role of Bacillus anthracis Spore Structures in Macrophage Cytokine Responses

The innate immune response of macrophages (M{phi}) to spores, the environmentally acquired form of Bacillus anthracis, is poorly characterized. We therefore examined the early M{phi} cytokine response to B. anthracis spores, before germination. M{phi} were exposed to bacilli and spores of Sterne strain 34F2 and its congenic nongerminating mutant ({Delta}gerH), and cytokine expression was measured by real-time PCR and an enzyme-linked immunosorbent assay. The exosporium spore layer was retained (exo+) or removed by sonication (exo–). Spores consistently induced a strong cytokine response, with the exo– spores eliciting a two- to threefold-higher response than exo+ spores. The threshold for interleukin-1s (IL-1s) production by wild-type M{phi} was significantly lower than that required for tumor necrosis factor alpha expression. Cytokine production was largely dependent on MyD88, suggesting Toll-like receptor involvement; however, the expression of beta interferon in MyD88–/– M{phi} suggests involvement of a MyD88-independent pathway. We conclude that (i) the B. anthracis spore is not immunologically inert, (ii) the exosporium masks epitopes recognized by the M{phi}, (iii) the M{phi} cytokine response to B. anthracis involves multiple pattern recognition receptors and signaling pathways, and (iv) compared to other cytokines, IL-1s is expressed at a lower spore concentration.