Mycobacterium vaccae induces a population of pulmonary CD11c+ cells with regulatory potential in allergic mice

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Abstract
The hygiene hypothesis proposes that common, harmless microorganisms, present throughout our evolutionary history, have helped to develop immunoregulatory mechanisms that prevent inappropriateimmune responses by the host. Using a mouse model of allergic pulmonary inflammation, we report that treatment with an ubiquitous saprophytic mycobacterium, Mycobacterium vaccae, significantly reduces allergic inflammation by decreasing type 2 responses such as eosinophilia and IL-4 expression. Rather than observing an increase in type-1 cytokine expression, we found elevated production of IL-10 in the lungs suggesting a role for regulatory T cells. Since induction of these cells may be dependent on APC, we investigated the effects of M. vaccae treatment on pulmonary CD11c+ cells. Increased levels of IL-10, TGF-β and IFN-α mRNA were detected in CD11c+ cells from M. vaccae-treated allergic mice. We propose that M. vaccae-induced CD11c+ cells have a potential regulatory role at the site of inflammation through their secretion of immunomodulatory cytokines.