Characterization of the immune response induced by non-pathogenic mycobacteria in macrophages and dendritic cells

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The capacity of the host to mount an effective immune response (IR) is crucial for the protection against invading pathogens. The apoptotic and proinflammatory responses of infected cells are important innate immune mechanisms. Consequently, the ability of persistent intracellular pathogens, such as the human pathogen Mycobacterium tuberculosis (Mtb), to inhibit infection-induced apoptosis of macrophages is important for its virulence in the host. Facultative-pathogenic mycobacterial species, like M. kansasii (Mkan), can cause disseminating disease in individuals with immune deficiencies. In contrast, non-pathogenic mycobacteria, like M. smegmatis (Msme), are not known to cause disseminating disease even in immunocompromised individuals. We hypothesized that this difference in phenotype could be explained by the strong induction of an innate IR by non-pathogenic mycobacteria.

Here we analyze the mechanisms by which non-pathogenic mycobacteria induce a strong IR in their macrophage and dendritic cell (DC) host, specifically the induction of host cell apoptosis and the host inflammasome response via the secretion of IL-1β. The comparison of two non-pathogenic mycobacterial species (Msme and Mkan) with two facultative-pathogenic mycobacterial species (M.kan and M. bovis BCG) demonstrated that only the non-pathogenic mycobacteria induce strong apoptosis in murine bone marrow derived macrophages (BMDM) and dendritic cells (BMDC), which was dependent upon caspase-3 activation and TNF secretion. Consistently, BMDMs responded by secreting relatively large amounts of TNF and by upregulating the expression of Il-12. We also demonstrated that Msme infection of BMDCs strongly induces the secretion of IL-1β. This induction was dependent upon the presence of functional ASC and was partially independent of NLRP3. Interestingly this induction was also partially dependent on AIM2 and IFN-β. This AIM2-dpendent induction was observed in infection with non-pathogenic and opportunistic mycobacteria, and attenuated but not virulent Mtb. Surprisingly, caspase-1/11 deficient BMDCs still secreted substantial but reduced amounts of IL-1β upon Msme infection.

In conclusion, we demonstrate a strong induction of the innate IR by non-pathogenic mycobacteria as measured by host cell apoptosis, and IL-12 / TNF / IL-1β cytokine induction. We also demonstrate the partially caspase-1/11 and NLRP3 -independent, partially AIM2-dependent, but ASC-dependent IL-1β secretion in Msme infected BMDCs. Our findings support the hypothesis that the strong induction of the innate IR is a major reason for the lack of pathogenicity in non-pathogenic mycobacteria.