Non-canonical activation of IRE1α during Candida albicans infection enhances macrophage fungicidal activity.

Infection by intracellular pathogens can trigger activation of the IRE1α branch of the unfolded protein response (UPR), which then modulates innate immunity and infection outcomes during bacterial or viral infection. However, the mechanisms by which infection activates IRE1α have not been fully elucidated. While recognition of microbe-associated molecular patterns can activate IRE1α, it is unclear whether this depends on the canonical role of IRE1α in detecting misfolded proteins. Here, we report that Candida albicans infection of macrophages results in IRE1α activation through C-type lectin receptor signaling, reinforcing a role for IRE1α as a central regulator of host responses to infection by a broad range of pathogens. However, IRE1α activation was not preceded by protein misfolding in response to either C. albicans infection or lipopolysaccharide treatment, implicating a non-canonical mode of IRE1α activation after recognition of microbial patterns. Investigation of the phenotypic consequences of IRE1α activation in macrophage antimicrobial responses revealed that IRE1α activity enhances the fungicidal activity of macrophages. Macrophages lacking IRE1α activity displayed inefficient phagolysosomal fusion, enabling C. albicans to evade fungal killing and escape the phagosome. Together, these data provide mechanistic insight for the non-canonical activation of IRE1α during infection, and reveal central roles for IRE1α in macrophage antifungal responses.

Candida albicans selection for human commensalism results in substantial within-host diversity without decreasing fitness for invasive disease.

Candida albicans is a frequent colonizer of human mucosal surfaces as well as an opportunistic pathogen. C. albicans is remarkably versatile in its ability to colonize diverse host sites with differences in oxygen and nutrient availability, pH, immune responses, and resident microbes, among other cues. It is unclear how the genetic background of a commensal colonizing population can influence the shift to pathogenicity. Therefore, we examined 910 commensal isolates from 35 healthy donors to identify host niche-specific adaptations. We demonstrate that healthy people are reservoirs for genotypically and phenotypically diverse C. albicans strains. Using limited diversity exploitation, we identified a single nucleotide change in the uncharacterized ZMS1 transcription factor that was sufficient to drive hyper invasion into agar. We found that SC5314 was significantly different from the majority of both commensal and bloodstream isolates in its ability to induce host cell death. However, our commensal strains retained the capacity to cause disease in the Galleria model of systemic infection, including outcompeting the SC5314 reference strain during systemic competition assays. This study provides a global view of commensal strain variation and within-host strain diversity of C. albicans and suggests that selection for commensalism in humans does not result in a fitness cost for invasive disease.

Monitoring Inflammasome Priming and Activation in Response to Candida albicans.

Candida albicans is a common mucosal colonizer, as well as a cause of lethal invasive fungal infections. The major predisposing factor for invasive fungal disease is a compromised immune system. One component of the host immune response to fungal infection is the activation of the inflammasome, a multimeric protein complex that is critical for regulating host pro-inflammatory responses. Here, we describe methods for investigating the interactions between C. albicans and host macrophages, with a focus on the inflammasome. C. albicans isolates differ in the degree to which they activate the inflammasome due to differences in internalization, morphogenic switching, and inflammasome priming. Therefore, we include protocols for identifying these factors. This simple in vitro model can be used to elucidate the contributions of specific C. albicans strains or mutants to different aspects of interactions with macrophages. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Measuring inflammasome priming in response to Candida albicans Basic Protocol 2: Measuring inflammasome activation in response to Candida albicans Support Protocol: Controlling for phagocytosis.