Mechanisms of immune evasion and drug tolerance in Candida albicans
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Mechanisms of immune evasion and drug tolerance in Candida albicans

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Candida albicans is a major opportunistic pathogen of humans. In immunocompromised patients, C. albicans becomes pathogenic and can cause Oropharyngeal candidiasis, Vulvovaginal candidiasis and Invasive candidiasis. The mortality rate is up to 40% even with anti-fungal drug treatment. Thus, it is critical to study how the immune system defends against C. albicans infection, and how C. albicans offsets the growth inhibitory effect of the anti-fungal drugs.C. albicans can grow as yeast, pseudohyphae and hyphae, and the ability to switch reversibly among different forms is critical for its virulence. The innate immune response is critical for host defense. However, the relationship between morphogenesis and host immune recognition is not fully elucidated. Dectin-1 is a major C-type lectin receptor that recognizes β-glucan within the fungal cell wall. C. albicans β-glucan is protected by the outer mannan layer of the cell wall. Whether and how β-glucan protection is differentially regulated during hyphal morphogenesis is not fully understood. In Chapter 3, we show that the endo-1,3-glucanase Eng1 is differentially expressed in yeast, and together with the Yeast Wall Protein Ywp1, controls β-glucan exposure and Dectin-1-dependent immune activation in yeast cells. An eng1 deletion mutant shows enhanced Dectin-1 binding at the septa of yeast cells, while an eng1 ywp1 double mutant, but not an ywp1 single mutant, shows strong overall Dectin-1 binding. Our data suggest that repression of ENG1 and YWP1 expression during hyphal initiation leads to β-glucan exposure. ENG1 expression is also highly regulated and is responsible for β-glucan protection and immune recognition in response to different carbon sources and antifungal drugs. The eng1 mutant modulates virulence in the model of hematogenously disseminated candidiasis in a Dectin-1-dependent manner. Mice infected with the eng1 mutant showed delayed mortality compared to male mice infected with the wild-type strain, but earlier mortality in females correlated with a higher renal immune response and lower fungal burden. Thus, Eng1-regulated β-glucan exposure is important for modulating the balance between immune protection and immunopathogenesis during disseminated candidiasis. Fluconazole is the most widely deployed antifungal drug due to its favorable bioavailability and safety profile. It targets the Lanosterol 14α-demethylase (Erg11) in the ergosterol biosynthesis pathway and promotes the accumulation of toxic ergosterol through the Δ5,6- desaturase Erg3. Since fluconazole is fungistatic rather than fungicidal, C. albicans isolates has frequently shown ultra-MIC growth which was characterized as trailing growth or tolerance. High tolerance isolates have shown to cause persistent candidiasis with increased mortality in comparison to sensitive isolates when treated with fluconazole. Little is known about whether and how C. albicans maintains fluconazole tolerance. Here, we show that C. albicans develops stable fluconazole tolerance after single dose of treatment. Tolerance is stably maintained through cell divisions without the selective pressure of fluconazole; and allows the majority of the population to grow in the presence of even higher concentrations of fluconazole without changing the MIC. Fluconazole tolerance can be destabilized by inducing chromatin remodeling, indicating that it is maintained by epigenetic regulation. Thus, tolerance is maintained by a reversible cell state that is established during fluconazole treatment. How epigenetic regulation modulates gene expression to confer the stable fluconazole tolerance in Persisters remained to be studied.

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This item is under embargo until August 31, 2027.