Candida albicans is a major fungal pathogen of humans. It exists as a commensal in the oral cavity, gut or genital tract of most individuals, constrained by the local microbiota, epithelial barriers and immune defences. Their perturbation can lead to fungal outgrowth and the development of mucosal infections such as oropharyngeal or vulvovaginal candidiasis, and patients with compromised immunity are susceptible to life-threatening systemic infections. The importance of the interplay between fungus, host and microbiota in driving the transition from C. albicans commensalism to pathogenicity is widely appreciated. However, the complexity of these interactions, and the significant impact of fungal, host and microbiota variability upon disease severity and outcome, are less well understood. Therefore, we summarise the features of the fungus that promote infection, and how genetic variation between clinical isolates influences pathogenicity. We discuss antifungal immunity, how this differs between mucosae, and how individual variation influences a person's susceptibility to infection. Also, we describe factors that influence the composition of gut, oral and vaginal microbiotas, and how these affect fungal colonisation and antifungal immunity. We argue that a detailed understanding of these variables, which underlie fungal-host-microbiota interactions, will present opportunities for directed antifungal therapies that benefit vulnerable patients.

Vulvovaginal candidiasis (VVC) is a widespread vaginal infection primarily caused by Candida albicans. VVC affects up to 75% of women of childbearing age once in their life, and up to 9% of women in different populations experience more than three episodes per year, which is defined as recurrent vulvovaginal candidiasis (RVVC). RVVC results in diminished quality of life as well as increased associated healthcare costs. For a long time, VVC has been considered the outcome of inadequate host defenses against Candida colonization, as in the case of primary immunodeficiencies associated with persistent fungal infections and insufficient clearance. Intensive research in recent decades has led to a new hypothesis that points toward a local mucosal overreaction of the immune system rather than a defective host response to Candida colonization. This review provides an overview of the current understanding of the host immune response in VVC pathogenesis and suggests that a tightly regulated fungus–host–microbiota interplay might exert a protective role against recurrent Candida infections.

Comparative LDH secretion, Ext_data_figure2.ep a, Assessment of Candida induced cell death of PBMCs after 24 hours Extended Data Fig. 2.Extended Data Fig. 3 Relative C. auris induced ROS production and heatsensitivity of the cell wall components responsible for the C. auris induced cytokine production. Ext_data_figure3.ep sa, Neutrophil ROS release after 1-hour stimulation without (RPMI; negative control) or with heat-killed C. albicans, C. auris strains or zymosan (positive control), depicted in relative light units (RLU) either as time-course (left) or as area under the curve (AUC, right), n=9. b, PBMC ROS release after 1-hour stimulation without (RPMI; negative control) or with heat-killed C. albicans, C. auris strains or zymosan (positive control), depicted in RLU either as time-course (left) or as AUC (right), n=6. c, TNF-α, IL-6, IL-1β, and IL-1Ra levels in the supernatant of PBMCs after stimulation without (RPMI; negative control) or with heat-killed C. albicans and C. auris from all five geographical clades for 24 hours, n=8. d, PBMC production of cytokines IFN-γ (n=10; n=7 for C. auris 10051895), IL-10 (n=6), IL-17 (n=6), and IL-22 (n=14; n=6 for C. auris 10051893; n=11 for C. auris 10051895) after stimulation without (RPMI; negative control) or with heat-killed C. albicans and C. auris for 7 days. Graphs represent mean ± SEM, data are pooled from at least two independent experiments. * p < 0.05, ** p < 0.01, *** p < 0.001, **** p = 0.001, a-b Time curves (left panels) were assessed for statistical differences between C. auris strains and C. albicans by a two-way ANOVA, Area Under curve (AUC) means (right panels) were compared using the two-sided Wilcoxon signed rank test, c-d twosided Wilcoxon matched pairs signed-rank test comparing respective C. auris strains with C. albicans as control or reference species. Data used to make this figure can be found in Source Data Extended Data Fig. 3.Extended Data Fig. 4 Transcriptional changes induced by purified cell wall components and their respective exposure on C. albicans and C. auris Ext_data_figure4.ep s . a, Heatmap displaying the Log 2 Fold change (color scale) of the top 50 DEG of C. albicans live, for both Candida species and their cell wall components, β-glucan and mannan, at 4 hour (left panel) and 24 hours (right panel). b, Flow cytometry plot based on forward scatter component (FSC) and side scatter component (SSC), demonstrating C. surface. auris strains are slightly smaller and of higher complexity than C. albicans. c, Flow cytometry-based comparison of cell wall components of C. albicans and C. auris strains. Mean fluorescent intensity (MFI) of thimerosal-fixed Candida cells stained for Fc-Dectin-1, a marker for β-glucan (left), and ConA, a marker for mannans (right). Graphs represent mean ± SEM of the 3 means, each performed with three replicates in three independent measurements, * p < 0.05, Kruskall Wallis test with two-sided Dunn's multiple comparison test was performed comparing the respective C. auris strains with the two C. albicans refere...

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Background Recurrent vulvovaginal candidiasis (RVVC) affects up to 8% of women. The immunopathogenesis is still poorly known, but it has been suggested that RVVC might be due to a dysregulated innate immune response. The aim of this study was to compare cytokine profiles in stimulated primary mononuclear cells (PBMCs) from RVVC and healthy individuals. Methods PBMCs isolated from RVVC patients (n=24) and healthy volunteers (n=30) were stimulated with both unspecific and pathogen-specific antigens. Cytokine production was assessed after 24 hours, 48 hours, and 7 days using ELISA. Results No significant differences in cytokine production were found in Th1, Th2, and Th17 immunity in response to both unspecific and pathogen-specific stimulations. TNF-α production in response to C. albicans hyphae was significantly higher in patients than in controls and within the patient group, a significant positive correlation was found between IL-1β and both TNF-α and IL-6. Both IL-1β/IL-1Ra and TNF-α/IL-10 ratio in Candida hyphae-stimulated PBMCs were significantly higher in patients than controls. Conclusions Women affected by RVVC showed an increased monocytes-derived cytokine production which might contribute to an exaggerated vaginal immune response to Candida hyphae. RVVC patients show no defective Th-dependent adaptive immune response upon Candida stimulation.

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Circulatory inflammatory proteins play a significant role in anti-Candida host immune defence. However, little is known about the genetic variation that contributes to the variability of inflammatory responses in response to C. albicans. To systematically characterize inflammatory responses in Candida infection, we profiled 91 circulatory inflammatory proteins in peripheral blood mononuclear cells (PBMCs) stimulated with C. albicans yeast isolated from 378 individuals of European origin from the 500 Functional Genomics (500FG) cohort of the Human Functional Genomics Project (HFGP) and Lifelines Deep cohort. To identify the genetic factors that determine variation in inflammatory protein responses, we correlated genome-wide single nucleotide polymorphism (SNP) genotypes with protein abundance (protein quantitative trait loci, pQTLs) produced by the Candida-stimulated PBMCs. Furthermore, we investigated whether differences in survival of candidaemia patients can be explained by modulating levels of inflammatory proteins. We identified five genome-wide significant pQTLs that modulate IL-8, MCP-2, MMP-1, and CCL3 in response to C. albicans. In addition, our genetic analysis suggested that GADD45G from rs10114707 locus that reached genome-wide significance could be a potential core gene that regulates a cytokine network upon Candida infection. Last but not least, we observed that a trans-pQTL marked from SNP rs7651677 at chromosome 3 that influences urokinase plasminogen activator (uPA) is strongly associated with patient survival (Psurvival = 3.52 x 10-5, OR 3). Overall, our genetic analysis showed that genetic variation determines the abundance of circulatory proteins in response to Candida infection.