NLRP3 inflammasome contributes to host defense against Talaromyces marneffei infection

Abstract

Talaromyces marneffei is an important thermally dimorphic fungus that causes fatal systemic mycosis in immunocompromised hosts and it is primarily endemic in southeast Asia. It is considered as an AIDS-defining disease in affected areas. In recent decades, there is a sharp increase in infection rate in non-HIV patients. NLRP3 inflammasome is an important player for antifungal immunity. However, whether T. marneffei is able to induce NLRP3 inflammasome activation to defend against its infection is still poorly defined.

In this thesis, we studied the mechanism of induction of NLRP3 inflammasome in innate immune cells and the induction of adaptive immunity by T. marneffei. we first isolated human peripheral blood mononuclear cells (PBMCs) to compare the IL- response between T. marneffei yeasts and conidia. Even for yeast cells, we compared the different requirement for IL-1 response in human PBMCs, monocytes, monocyte-derived macrophages (MDMs) and DCs. Secondly, we dissected the signaling pathways of pro-IL-1 (signal 1) by using anti-human Dectin-1 blocking antibodies and Syk inhibitors. We also examined whether T. marneffei induced NLRP3 inflammasome activation (signal 2) by detection of NLRP3 and caspase-1 expression and ASC speck formation, and by using wild-type, Nlrp3-/- and Casp1-/- mice. We also investigated whether the activated NLRP3 inflammasome induced IFN- and IL-17A production in T cells.

Our research revealed that T. marneffei yeasts, but not conidia, were able to induce abundant IL-1 production. LPS priming was dispensable for IL-1 response to microorganisms in human PBMCs and monocytes but was essential in MDMs and DCs. Pro-IL-1 production in response to yeast cells was mediated by Dectin-1/Syk signaling pathway. It was found that NLRP3 expression and caspase-1 activation were elevated, and ASC speck was detectable after fungal stimulation. Murine bone marrow-derived DCs (BMDCs) from Nlrp3-/- and Casp1-/- mice had a significant reduction of IL-1 production compared to those from wild-type mice in vitro. Nlrp3-/- and Casp1-/- mice showed shorter survival time and increased fungal load in spleens and livers at 14 days post infection compared to wild-type mice. Histopathological analysis revealed that more granulomas containing fungal yeasts were observed in livers of Nlrp3-/- and Casp1-/- mice when compared with wild-type mice. Furthermore, it was suggested that the activated NLRP3 inflammasome contributed to the induction of IFN- and IL-17A production to defend against T. marneffei infection.

In conclusion, our report is the first to elucidate that NLRP3 inflammasome contributes to host defense against T. marneffei infection, which is partially dependent on the production of IFN- and IL-17A. Furthermore, this study helps us better understand the interaction between host cells and T. marneffei, and provides implications for future antifungal therapeutic designs.