Abstract
Inflammasome is an intracellular protein complex that serves as cytosolic pattern recognition receptor (PRR) to engage with pathogens and to process cytokines of the interleukin-1 (IL-1) family into bioactive molecules. It has been established that interleukin-1beta (IL-1beta) is important to host defense against Histoplasma capsulatum infection. However, the detailed mechanism of how H. capsulatum induces inflammasome activation leading to IL-1beta production has not been studied. Here, we showed in dendritic cells (DCs) that H. capsulatum triggers caspase-1 activation and IL-1beta production through NLRP3 inflammasome. By reciprocal blocking of Dectin-1 or Dectin-2 in single receptor-deficient DCs and cells from Clec4n-/-, Clec7a-/-, and Clec7a-/-Clec4n-/- mice, we discovered that while Dectin-2 operates as a primary receptor, Dectin-1 serves as a secondary one for NLRP3 inflammasome. In addition, both receptors trigger Syk-JNK signal pathway to activate signal 1 (pro-IL-1beta synthesis) and signal 2 (activation of caspase-1). Results of pulmonary infection with H. capsulatum showed that CD103+ DCs are one of the major producers of IL-1beta and Dectin-2 and Dectin-1 double deficiency abolishes their IL-1beta response to the fungus. While K+ efflux and cathepsin B (but not ROS) function as signal 2, viable but not heat-killed H. capsulatum triggers profound lysosomal rupture leading to cathepsin B release. Interestingly, cathepsin B release is regulated by ERK/JNK downstream of Dectin-2 and Dectin-1. Our study demonstrates for the first time the unique roles of Dectin-2 and Dectin-1 in triggering Syk-JNK to activate signal 1 and 2 for H. capsulatum-induced NLRP3 inflammasome activation.
