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

The hygiene hypothesis proposes that common, harmless microorganisms, present throughout our evolutionary history, have helped to develop immunoregulatory mechanisms that prevent inappropriate immune 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-beta and IFN-alpha 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.