Candida albicans Triggers NLRP3-Mediated Pyroptosis in Macrophages

Abstract

Pyroptosis is an inflammasome-mediated programmed cell death pathway triggered in macrophages by a variety of stimuli, including intracellular bacterial pathogens. Activation of pyroptosis leads to the secretion of interleukin-1␤ (IL-1␤) and pore-mediated cell lysis. Although not considered an intracellular pathogen, Candida albicans is able to kill and, thereby, escape from macrophages. Here, we show that C. albicans-infected bone marrow-derived macrophages (BMDM) and murine J774 macrophages undergo pyroptotic cell death that is suppressed by glycine and pharmacologic inhibition of caspase-1. Infection of BMDM harvested from mice lacking components of the inflammasome revealed that pyroptosis was dependent on caspase-1, ASC, and NLRP3 and independent of NLRC4. In contrast to its role during intracellular bacterial infection, pyroptosis does not restrict C. albicans replication. Nonfilamentous Candida spp. did not trigger pyroptosis, while Candida krusei, which forms pseudohyphae in macrophages, triggered much lower levels than did C. albicans. Interestingly, a Saccharomyces cerevisiae strain from the filamentous background ⌺1278 also triggered low, but significant, levels of pyroptosis. We have found that deletion of the transcription factor UPC2 decreases pyroptosis but has little effect on filamentation in the macrophage. In addition, a gainof-function mutant of UPC2 induces higher levels of pyroptosis than does a matched control strain. Taken together, these data are most consistent with a model in which filamentation is necessary but not sufficient to trigger NLRP3 inflammasome-mediated pyroptosis. This is the first example of a fungal pathogen triggering pyroptosis and indicates that C. albicans-mediated macrophage damage is not solely due to hypha-induced physical disruption of cellular integrity.

Estrogen suppresses hepatocellular carcinoma cells through ERβ-mediated upregulation of the NLRP3 inflammasome

“…It is well recognized that NLRP3 inflammasome activation induced caspase 1-dependent cell death, named pyroptosis. 16,17 Caspase 1 activation assay and IL-1β ELISA assay showed that significant caspase 1 activation (Figure 4h and i) and IL-1β secretion (Figure 4j and k) were detected in the NLRP3 inflammasome reconstituted cells. These data indicate that NLRP3 inflammasome reconstitution induced caspase 1-dependent pyroptosis.…”

“…16,17 Recent study showed that pyroptotic cell death of hepatocyte mediated liver damage. 30 Another study showed that NLRP3 inflammasome activation contributed to the progression of non-alcoholic fatty liver disease.…”

Candida and macrophages: a deadly affair

“…Although it is tempting (and dramatic) to speculate that these hyphae kill by exerting pressure on the macrophage membrane and physically breaking through, recent reports from our lab and others suggest that other mechanisms, related to regulation of the host-pathogen 'synapse', mediate host cell death and pathogen escape from macrophages 9,10 .…”

“…The key result that showed that C. albicans does not simply break macrophages, but rather induces a more regulated mechanism of killing, came with the realisation that macrophages derived from mice deficient in a central host immune pathway -the caspase 1 inflammasome -were protected from killing by C. albicans in the first few hours post-phagocytosis 9,10 . This was despite normal formation of fungal hyphae, and therefore it argued that physical disruption of macrophages by fungal filaments is not the only mechanism of host cell death.…”

“…This was despite normal formation of fungal hyphae, and therefore it argued that physical disruption of macrophages by fungal filaments is not the only mechanism of host cell death. Instead, it appears that the initial hyphal formation triggers the macrophage to commit suicide by a programmed cell death pathway called pyroptosis, which is enacted by caspase-1 and occurs before the filament has extended to the surface of the macrophage (Figure 1) 9,10 . In addition to caspase-1, the inflammasome subunits ASC and NLRP3 are also involved in Candida-induced pyroptosis 10 .…”