Caspase-1 and ASC but not NLRP3 mediate antifungal defense in candidiasis sepsis

Introduction

IL-1β plays an important role in antifungal defense. The inflammasome is thought to be required for caspase-1 activation and processing of the inactive precursor pro-IL-1β into its active form. Contradictory data have been reported regarding the role of the inflammasome in Candida sepsis. In order to address these discrepancies, we investigated host defense against disseminated candidiasis in knockout mice defective in the various components of the inflammasome.

Methods

Mice defective in caspase-1, ASC, NLRP3 or P2X7 were infected intravenously with Candida albicans. Survival, fungal outgrowth in the organs, histology, and cytokine production were compared in these mouse strains with the wild-type C57/Bl6 control mice. PBMCs from healthy volunteers with or without reactive oxygen species (ROS) inhibitor and PBMCs from patients with chronic granulomatous disease (CGD) that are deficient in ROS production were stimulated with C. albicans.

Results

Caspase-1^-/- mice and ASC^-/- mice had a decreased survival during disseminated candidiasis (50%) compared with the control mice (100%). Caspase-1^-/- mice had a 100-fold increase in fungal loads in the kidneys of the deficient animals (P < 0.05) and histological assessment revealed preferential growth of hyphae in the pyelum of the caspase-1^-/- mice. In contrast, ASC^-/-mice did not have higher fungal loads, but they showed a significant stronger inflammatory reaction in the kidneys. On days 3 and 7 of infection, the ASC^-/- mice splenocytes that were restimulated with Candida specifically showed a higher TNF production. NLRP3^-/- and P2X7^-/- did not display an increased susceptibility to disseminated candidiasis, as shown by normal survival and fungal loads in the organs. Local IL-1β production was lower in caspase-1^-/- mice, but not in the ASC^-/-, NLRP3^-/- or P2X7^-/- animals. Experiments using the NADPH inhibitor diphenyl-eneiodonium, or in monocytes isolated from CGD patients who have defective capacity to form ROS, demonstrated that ROS did not mediate inflammasome activation and C. albicans induced IL-1β production.

Conclusion

Caspase-1-dependent processing of IL-1β is an important step in antifungal host defense during Candida sepsis. However, this process is not dependent on the inflammasome components NLRP3, the ATP receptor P2X7, or ROS. These data confirm previous studies in human monocytes showing that IL-1β processing during Candida infection did not require pathogen-mediated inflammasome activation, due to the constitutive activation of caspase-1. ASC also plays an important role in Candida sepsis, but unexpectedly seems to have a different function, specifically by regulating TNF production and local inflammation in the organs.