Secreted aspartic proteases of Candida albicans activate the NLRP3 inflammasome

Abstract

In a recent report, we demonstrated that distinct members of the secreted aspartic protease (Sap) family of Candida albicans are able to induce secretion of proinflammatory cytokines by human monocytes, independently of their proteolytic activity and specific pH optima. In particular, C. albicans Sap2 and Sap6 potently induced IL-1β, TNF-α, and IL-6 production. Here, we demonstrate that Sap2 and Sap6 proteins trigger IL-1β and IL-18 production through inflammasome activation. This occurs via NLRP3 and caspase-1 activation, which cleaves pro-IL-1β into secreted bioactive IL-1β, a cytokine that was induced by Saps in monocytes, in monocyte-derived macrophages and in dendritic cells. Downregulation of NLRP3 by RNA interference strongly reduced the secretion of bioactive IL-1β. Inflammasome activation required Sap internalization via a clathrin-dependent mechanism, intracellular induction of K + efflux, and ROS production. Inflammasome activation of monocytes induced by Sap2 and Sap6 differed from that induced by LPS-ATP in several aspects. Our data reveal novel immunoregulatory mechanisms of C. albicans and suggest that Saps contribute to the pathogenesis of candidiasis by fostering rather than evading host immunity.Keywords: Aspartic proteases r C. albicans r IL-1β r Inflammasome r Virulence factor IntroductionCandida albicans is a commensal fungus that colonizes human mucosal surfaces such as the vaginal and gastrointestinal tracts without causing harm. However, under conditions of primary or secondary immunodeficiency, this yeast can cause opportunistic infections such as mucosal inflammation and systemic sepsis [1]. The mortality rate associated with invasive candidiasis Correspondence: Dr. Anna Vecchiarelli e-mail: vecchiar@unipg.it has been reported to be as high as 40-50% [2]. Candida species are the fourth most common pathogens isolated from nosocomial bloodstream infections in the USA and Europe [3]. Although the immune status of the host plays a key role in the prevention or pathogenesis of C. albicans infections, a number of virulence attributes of C. albicans, such as factors that mediate adhesion, enzyme secretion, or hyphal formation, contribute to the disease process [4]. Particularly, the secretion of aspartic proteases (Saps), * These authors contributed equally to this work.C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu 680Donatella Pietrella et al. Eur. J. Immunol. 2013. 43: 679-692 which are encoded by a gene family with ten members, has long been recognized as a virulence-associated trait of this pathogenic yeast [5].We recently reported that various members of the Sap family, including Sap1, Sap2, Sap3, and Sap6, have different abilities to induce secretion of pro-inflammatory cytokines by human monocytes via Akt/NF-κB activation. Sap1, Sap2, and Sap6 potently induced IL-1β, TNF-α, and IL-6 production. Importantly, Sapinduced cytokine production was independent of the proteolytic activity and of the optimal pH for the individual Sap activities [6]. These data suggest ...

The role of pattern recognition receptors in the innate recognition ofCandida albicans

“…34,36,37,39,128 C. albicans are able to trigger proinflammatory cytokine IL-1b and IL-18 production via NLRP3 inflammasome in murine monocytes, macrophages and dendritic cells. 129 Consistently, downregulation of NLRP3 by RNA interference strongly reduced the secretion of bioactive IL-1b. 129 Pyroptosis is an inflammasome-mediated programmed cell death.…”

“…129 Consistently, downregulation of NLRP3 by RNA interference strongly reduced the secretion of bioactive IL-1b. 129 Pyroptosis is an inflammasome-mediated programmed cell death. In bone marrow-derived macrophages and murine J774 macrophages, NLRP3 can be triggered by C. albicans, leading to NLRP3-mediated pyroptosis.…”

Molecular mechanisms regulating NLRP3 inflammasome activation

“…Candida albicans and its components, the secreted aspartic proteases, and Aspergillus hyphae activate the NLRP3 inflammasome, and this is mediated through pathways involving both ROS generation and K 1 efflux. [88][89][90] In a study on the pathogenesis of osteoarthritis, hydroxyapatite crystals stimulated NLRP3 inflammasome activation through multiple pathways: ROS generation, lysosomal damage, and K 1 efflux. 91 Additionally, the anthracycline doxorubicin induces systemic inflammation associated with IL-1b release, mediated by NLRP3 inflammasome activation, which is controlled by co-treatment with ROS inhibitors or by cultivation of cells with high levels of extracellular K 1 .…”

Vulvovaginal Candida albicans infections: pathogenesis, immunity and vaccine prospects

“…complement, histatin, antibodies), allowing C. albicans to escape the host immune system. 32,33,39 Some past 40,41 and more recent 42 data suggest that Sap may induce inflammation and thus contribute to fungal pathogenicity by fostering, rather than evading, the host immune systems (see below).…”

“…the Als3 adhesin, formulated with alum as adjuvant, and a dominant member of the aspartyl proteinase enzyme family, the Sap2, in a virosomal formulation. 39,75 Both vaccines show solid evidence of protection in rat and mouse models of vaginal infection by C. albicans although with a likely different mechanism of immunological protection. 30,74 Other anti-Candida vaccines, from attenuated strains of C. albicans 76 to a number of glycoconjugate of cell wall polysaccharides, 77 have also been shown to be immunogenic and protective in experimental animal models, although these, to my knowledge, have not yet been entered into clinical trials in humans.…”