Pathogenic Fungus Microsporum canis Activates the NLRP3 Inflammasome

Microsporum canis is a pathogenic fungus with worldwide distribution that causes tinea capitis in animals and humans. M. canis also causes invasive infection in immunocompromised patients. To defy pathogenic fungal infection, the host innate immune system is the first line of defense. As an important arm of innate immunity, the inflammasomes are intracellular multiprotein complexes that control the activation of caspase-1, which cleaves proinflammatory cytokine pro-interleukin-1 beta (IL-1 beta) into its mature form. To determine whether the inflammasome is involved in the host defense against M. canis infection, we challenged human monocytic THP-1 cells and mouse dendritic cells with a clinical strain of M. canis isolated from patients with tinea capitis. We found that M. canis infection triggered rapid secretion of IL-1 beta from both THP-1 cells and mouse dendritic cells. Moreover, by using gene-specific shRNA and competitive inhibitors, we determined that M. canis-induced IL-1 beta secretion was dependent on NLRP3. The pathways proposed for NLRP3 inflammasome activation, namely, cathepsin B activity, K+ efflux, and reactive oxygen species production, were all required for the inflammasome activation triggered by M. canis. Meanwhile, Syk, Dectin-1, and Card9 were found to be involved in M. canis-induced IL-1 beta secretion via regulation of pro-IL-1 beta transcription. More importantly, our data revealed that M. canis-induced production of IL-1 beta was dependent on the NLRP3 inflammasome in vivo. Together, this study unveils that the NLRP3 inflammasome exerts a critical role in host innate immune responses against M. canis infection, and our data suggest that diseases that result from M. canis infection might be controlled by regulating the activation of inflammasomes.