Fungal Zymosan and Mannan Activate the Cryopyrin Inflammasome

Some fungal species are opportunistic pathogens that can cause infection in people with compromised immune systems. Activation of caspase-1 and the subsequent secretion of mature interleukin (IL)-1 beta is a major signaling pathway of the innate immune system, but how yeasts induce caspase-1 activation is unknown. We show here that stimulation of macrophages and dendritic cells with heat-killed Saccharomyces cerevisiae or the purified cell wall components zymosan and mannan induced caspase-1 activation and IL-1 beta secretion when combined with ATP. Macrophages deficient for the inflammasome adaptor ASC were defective in caspase-1 activation and IL-1 beta secretion, suggesting involvement of an ASC-dependent inflammasome. Indeed, caspase-1 activation was abrogated in macrophages lacking the NOD-like (NLR) protein Cryopyrin/Nalp3 and in wild type macrophages pre-treated with the pannexin-1 inhibitor probenecid. IL-1 beta secretion further required the Toll-like receptor (TLR) adaptors MyD88 and TRIF, and partially relied on TLR2. We previously showed that bacterial molecules such as lipopolysaccharide (LPS) and peptidoglycan induce activation of caspase-7 through the Cryopyrin inflammasome. Similarly, Cryopyrin and ASC were required for activation of caspase-7 in macrophages stimulated with zymosan or mannan and ATP. These results demonstrate that the conserved fungal components zymosan and mannan require ASC and Cryopyrin for caspase-1 activation and IL-1 beta secretion and suggest an important role for the Cryopyrin inflammasome during fungal infections.