Abstract
Candida albicans is a commensal yeast of the human gut which is tolerated by the immune system but has the potential to become an opportunistic pathogen. One way in which C. albicans achieves this duality is through concealing or exposing cell wall pathogen-associated molecular patterns (PAMPs) in response to host-derived environment cues (pH, hypoxia, and lactate). This cell wall remodeling allows C. albicans to evade or hyperactivate the host's innate immune responses, leading to disease. Previously, we showed that adaptation of C. albicans to acidic environments, conditions encountered during colonization of the female reproductive tract, induces significant cell wall remodeling resulting in the exposure of two key fungal PAMPs (β-glucan and chitin). Here, we report that this pH-dependent cell wall remodeling is time dependent, with the initial change in pH driving cell wall unmasking, which is then remasked at later time points. Remasking of β-glucan was mediated via the cell density-dependent fungal quorum sensing molecule farnesol, while chitin remasking was mediated via a small, heat-stable, nonproteinaceous secreted molecule(s). Transcript profiling identified a core set of 42 genes significantly regulated by pH over time and identified the transcription factor Efg1 as a regulator of chitin exposure through regulation of CHT2 This dynamic cell wall remodeling influenced innate immune recognition of C. albicans, suggesting that during infection, C. albicans can manipulate the host innate immune responses.IMPORTANCECandida albicans is part of the microbiota of the skin and gastrointestinal and reproductive tracts of humans and has coevolved with us for millennia. During that period, C. albicans has developed strategies to modulate the host's innate immune responses, by regulating the exposure of key epitopes on the fungal cell surface. Here, we report that exposing C. albicans to an acidic environment, similar to the one of the stomach or vagina, increases the detection of the yeast by macrophages. However, this effect is transitory, as C. albicans is able to remask these epitopes (glucan and chitin). We found that glucan remasking is controlled by the production of farnesol, a molecule secreted by C. albicans in response to high cell densities. However, chitin-remasking mechanisms remain to be identified. By understanding the relationship between environmental sensing and modulation of the host-pathogen interaction, new opportunities for the development of innovative antifungal strategies are possible.
Highlights
Candida albicans is a commensal yeast of the human gut which is tolerated by the immune system but has the potential to become an opportunistic pathogen
This cell wall remodeling was a transient process, and the cell wall underwent significant remasking of these pathogen-associated molecular patterns (PAMPs) at later time points, despite the pH of the medium remaining constant throughout the time course
The unmasking and remasking of glucan and chitin did not correlate with significant changes in mannan content, as determined by concanavalin A (ConA) staining (Fig. 1C), suggesting that the total amount of mannan in the cell wall does not regulate the exposure of these important PAMPs
Summary
Candida albicans is a commensal yeast of the human gut which is tolerated by the immune system but has the potential to become an opportunistic pathogen. The human body hosts trillions of viruses, bacteria, and fungi without constant activation of the immune system [2, 3] This is the result of millennia of coevolution between humans and their microbiome, where our immune system developed several receptors and mechanisms to identify and eliminate pathogens and to tolerate the presence of commensal organisms. This fine-tuning of innate immune responses prevents excessive inflammation while protecting the host [4]. During periods of dysbiosis or immune suppression, this commensal organism can proliferate and cause a large variety of diseases, ranging from superficial thrush to life-threatening systemic bloodstream infections associated with high mortality rates [10, 11]
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