The Pathogen Candida albicans Hijacks Pyroptosis for Escape from Macrophages

Abstract

The fungal pathogen Candida albicans causes macrophage death and escapes, but the molecular mechanisms remained unknown. Here we used live-cell imaging to monitor the interaction of C. albicans with macrophages and show that C. albicans kills macrophages in two temporally and mechanistically distinct phases. Early upon phagocytosis, C. albicans triggers pyroptosis, a proinflammatory macrophage death. Pyroptosis is controlled by the developmental yeast-to-hypha transition of Candida. When pyroptosis is inactivated, wild-type C. albicans hyphae cause significantly less macrophage killing for up to 8 h postphagocytosis. After the first 8 h, a second macrophage-killing phase is initiated. This second phase depends on robust hyphal formation but is mechanistically distinct from pyroptosis. The transcriptional regulator Mediator is necessary for morphogenesis of C. albicans in macrophages and the establishment of the wild-type surface architecture of hyphae that together mediate activation of macrophage cell death. Our data suggest that the defects of the Mediator mutants in causing macrophage death are caused, at least in part, by reduced activation of pyroptosis. A Mediator mutant that forms hyphae of apparently wild-type morphology but is defective in triggering early macrophage death shows a breakdown of cell surface architecture and reduced exposed 1,3 β-glucan in hyphae. Our report shows how Candida uses host and pathogen pathways for macrophage killing. The current model of mechanical piercing of macrophages by C. albicans hyphae should be revised to include activation of pyroptosis by hyphae as an important mechanism mediating macrophage cell death upon C. albicans infection.

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 .…”

“…Pyroptosis per se does not appear to be involved in the antifungal mechanisms exerted by macrophages, as C. albicans can survive in macrophages to the same extent in the presence or absence of pyroptosis 10 . As we have proposed, it appears that some pathogens, like C. albicans, can take advantage of this host response to evade being destroyed by macrophages 9,24 . In light of our studies, Key questions remain about how exactly C. albicans filaments are recognised by the caspase-1 inflammasome to trigger pyroptosis.…”

“…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 .…”

“…Live cell assay of Candida-induced death of macrophages Our lab has recently established powerful live-cell imaging to monitor the interaction of C. albicans with host macrophages using primary bone-derived macrophages from mice 9 . Using this ex vivo assay we could observe functionally distinct events over time in Phase I: C. albicans manipulation of the host macrophage…”