Clinical Candida albicans Vaginal Isolates and a Laboratory Strain Show Divergent Behaviors during Macrophage Interactions

Inflammatory cytokine signalling in vulvovaginal candidiasis: a hot mess driving immunopathology

Protective immunity to opportunistic fungal infections consists of tightly regulated innate and adaptive immune responses that clear the infection. Immune responses to infections of the vaginal mucosa by Candida species are, however, an exception. In the case of vulvovaginal candidiasis (VVC), the inflammatory response is associated with symptomatic disease, rather than that it results in pathogen clearance. As such VVC can be considered an inflammatory disease, which is a significant public health problem due to its predominance as a female-specific fungal infection. Particularly, women with recurrent VVC (RVVC) suffer from a significant negative impact on their quality of life and mental health. Knowledge of the inflammatory pathogenesis of (R)VVC may guide more effective diagnostic and therapeutic options to improve the quality of life of women with (R)VVC. Here, we review the immunopathogenesis of (R)VVC describing several elements that induce an inflammatory arson, starting with the activation threshold established by vaginal epithelial cells that prevent unnecessary ignition of inflammatory responses, epithelial and inflammasome-dependent immune responses. These inflammatory responses will drive neutrophil recruitment and dysfunctional neutrophil-mediated inflammation. We also review the, sometimes controversial, findings on the involvement of adaptive and systemic responses. Finally, we provide future perspectives on the potential of some unexplored cytokine axes and discuss whether VVC needs to be subdivided into subgroups to improve diagnosis and treatment.

The secretory Candida effector Sce1 licenses fungal virulence by masking the immunogenic β-1,3-glucan and promoting apoptosis of the host cells

Candida albicans deploys a variety of mechanisms such as morphological switch and elicitor release to promote virulence. However, the intricate interactions between the fungus and the host remain poorly understood, and a comprehensive inventory of fungal virulence factors has yet to be established. In this study, we identified a C. albicans secretory effector protein Sce1, whose induction and secretion are associated with vagina-simulative conditions and chlamydospore formation. Sequence alignment showed that Sce1 belongs to a Pir family in C. albicans, which is conserved across several fungi and primarily characterized as a β-glucan binding protein in the Saccharomyces cerevisiae. Mechanically, Sce1 is primarily localized to the cell wall in a cleaved form as an alkali-labile β-1,3-glucan binding protein and plays a role in masking β-glucan in acidic environments and chlamydospores, a feature that might underline C. albicans' ability to evade host immunity. Further, a cleaved short form of Sce1 protein could be released into extracellular compartments and presented in bone marrow-derived macrophages infected with chlamydospores. This cleaved short form of Sce1 also demonstrated a unique ability to trigger the caspases-8/9-dependent apoptosis in various host cells. Correspondingly, genetic deletion of SCE1 led to dampened vaginal colonization of C. albicans and diminished fungal virulence during systemic infection. The discovery of Sce1 as a versatile virulence effector that executes at various compartments sheds light on the fungus-host interactions and C. albicans pathogenesis.

The Interplay between Candida albicans, Vaginal Mucosa, Host Immunity and Resident Microbiota in Health and Disease: An Overview and Future Perspectives

Vulvovaginal candidiasis (VVC), which is primarily caused by Candida albicans, is an infection that affects up to 75% of all reproductive-age women worldwide. Recurrent VVC (RVVC) is defined as >3 episodes per year and affects nearly 8% of women globally. At mucosal sites of the vagina, a delicate and complex balance exists between Candida spp., host immunity and local microbial communities. In fact, both immune response and microbiota composition play a central role in counteracting overgrowth of the fungus and maintaining homeostasis in the host. If this balance is perturbed, the conditions may favor C. albicans overgrowth and the yeast-to-hyphal transition, predisposing the host to VVC. To date, the factors that affect the equilibrium between Candida spp. and the host and drive the transition from C. albicans commensalism to pathogenicity are not yet fully understood. Understanding the host- and fungus-related factors that drive VVC pathogenesis is of paramount importance for the development of adequate therapeutic interventions to combat this common genital infection. This review focuses on the latest advances in the pathogenic mechanisms implicated in the onset of VVC and also discusses novel potential strategies, with a special focus on the use of probiotics and vaginal microbiota transplantation in the treatment and/or prevention of recurrent VVC.

A New Phenotype in Candida-Epithelial Cell Interaction Distinguishes Colonization- versus Vulvovaginal Candidiasis-Associated Strains

Vulvovaginal candidiasis (VVC) affects nearly 3/4 of women during their lifetime, and its symptoms seriously reduce quality of life. Although Candida albicans is a common commensal, it is unknown if VVC results from a switch from a commensal to pathogenic state, if only some strains can cause VVC, and/or if there is displacement of commensal strains with more pathogenic strains. We studied a set of VVC and colonizing C. albicans strains to identify consistent in vitro phenotypes associated with one group or the other. We find that the strains do not differ in overall genetic profile or behavior in culture media (i.e., multilocus sequence type [MLST] profile, rate of growth, and filamentation), but they show strikingly different behaviors during their interactions with vaginal epithelial cells. Epithelial infections with VVC-derived strains yielded stronger fungal proliferation and shedding of fungi and epithelial cells. Transcriptome sequencing (RNA-seq) analysis of representative epithelial cell infections with selected pathogenic or commensal isolates identified several differentially activated epithelial signaling pathways, including the integrin, ferroptosis, and type I interferon pathways; the latter has been implicated in damage protection. Strikingly, inhibition of type I interferon signaling selectively increases fungal shedding of strains in the colonizing cohort, suggesting that increased shedding correlates with lower interferon pathway activation. These data suggest that VVC strains may intrinsically have enhanced pathogenic potential via differential elicitation of epithelial responses, including the type I interferon pathway. Therefore, it may eventually be possible to evaluate pathogenic potential in vitro to refine VVC diagnosis. IMPORTANCE Despite a high incidence of VVC, we still have a poor understanding of this female-specific disease whose negative impact on women's quality of life has become a public health issue. It is not yet possible to determine by genotype or laboratory phenotype if a given Candida albicans strain is more or less likely to cause VVC. Here, we show that Candida strains causing VVC induce more fungal shedding from epithelial cells than strains from healthy women. This effect is also accompanied by increased epithelial cell detachment and differential activation of the type I interferon pathway. These distinguishing phenotypes suggest it may be possible to evaluate the VVC pathogenic potential of fungal isolates. This would permit more targeted antifungal treatments to spare commensals and could allow for displacement of pathogenic strains with nonpathogenic colonizers. We expect these new assays to provide a more targeted tool for identifying fungal virulence factors and epithelial responses that control fungal vaginitis.

Mucosal Infection with Unmasked Candida albicans Cells Impacts Disease Progression in a Host Niche-Specific Manner

Shielding the immunogenic cell wall epitope β(1, 3)-glucan under an outer layer of mannosylated glycoproteins is an essential virulence factor deployed by Candida albicans during systemic infection. Accordingly, mutants with increased β(1, 3)-glucan exposure (unmasking) display increased immunostimulatory capabilities in vitro and attenuated virulence during systemic infection in mice. However, little work has been done to assess the impact of increased unmasking during the two most common manifestations of candidiasis, namely, oropharyngeal candidiasis (OPC) and vulvovaginal candidiasis (VVC). We have shown previously that the expression of a single hyperactive allele of the MAP3K STE11ΔN467 induces unmasking via the Cek1 MAPK pathway, attenuates fungal burden, and prolongs survival during systemic infection in mice. Here, we expand on these findings and show that infection with an unmasked STE11ΔN467 mutant also impacts disease progression during OPC and VVC murine infection models. Male mice sublingually infected with the STE11ΔN467 mutant showed a significant reduction in tongue fungal burden at 2 days postinfection and a modest reduction at 5 days postinfection. However, we find that selection for STE11ΔN467 suppressor mutants that no longer display increased unmasking occurs within the oral cavity and is likely responsible for the restoration of fungal burden trends to wild-type levels later in the infection. In the VVC infection model, no attenuation in fungal burden was observed. However, polymorphonuclear cell recruitment and interleukin-1β (IL-1β) levels within the vaginal lumen, markers of immunopathogenesis, were increased in mice infected with unmasked STE11ΔN467 cells. Thus, our data suggest a niche-specific impact for unmasking on disease progression.

Divergent EGFR/MAPK-Mediated Immune Responses to Clinical Candida Pathogens in Vulvovaginal Candidiasis

Vulvovaginal candidiasis (VVC) is characterized by symptomatic inflammatory responses in the vagina caused by Candida albicans and non-albicans Candida (NAC) species. The epidermal growth factor receptor (EGFR) -mitogen-activated protein kinase (MAPK) signaling pathway has been linked to immune responses of oral mucosa after C. albicans exposure, but whether this pathway plays a similar response in vaginal epithelial cells is not known. Here, we observed that phosphorylation of EGFR and p38 was continuously activated in vaginal epithelial cells by C. albicans strain SC5314. This differs markedly from oral epithelial cells, which respond in a biphasic manner in order to properly discriminate the morphology of C. albicans. When compared with SC5314, a highly azole-resistant C. albicans isolate 1052 can induce a stronger phosphorylated signal of EGFR and p38, while clinically-isolated NAC strains including C. tropicalis, C. glabrata, C. parapsilosis and C. auris trigger higher levels of phosphorylated ERK1/2 and c-Fos than C. albicans. Inhibition of EGFR significantly reduces inflammatory response and epithelial damage induced by C. albicans both in vitro and in vivo, while inhibition of p38 leads to significant repair of epithelial damage triggered by both C. albicans and NAC species. These results confirm the importance of the EGFR-MAPK signaling in VVC pathogenesis and highlight the remarkable immunogenic differences between C. albicans and NAC species in host-microbe interactions.

Vulvovaginal Candida albicans Clinical Isolates’ Resistance to Phagocytosis In-Vitro

Previous studies have revealed that Candida albicans isolates involved in chronic vulvovaginal candidosis (cVVC) phenotypically express less virulent traits than clinical isolates involved in sporadic infections. In this study, we aimed to further explore this finding by studying the behaviour of those same clinical isolates in in-vitro models of infection. Eighteen clinical Candida albicans isolates were collected from women suffering sporadic (eight isolates) or chronic infections (ten isolates). Adhesion to HeLa cells (human cervical cancer epithelial cell line) and resistance to phagocytosis by RAW 264.7 cells (murine macrophages cell line) were tested in-vitro. In addition, phenotypic expression of virulence factors related with either adhesion or resistance to phagocytosis was tested in-vitro. Results indicated that yeast isolates involved in sporadic infection adhered in a higher proportion of HeLa cells than those of chronic infections, which was related with their ability to produce biofilm (p < 0.05). The ability to evade phagocytosis was related to an elevated production of proteases (p < 0.05) by chronic isolates, while sporadic isolates’ resistance to phagocytosis was related to a higher hydrophobicity of cell walls (p < 0.05). We conclude that the evasion of macrophage-mediated phagocytosis related to the production of proteases might be an important factor involved in the recurrence of vulvovaginal candidosis infection.

Rapid Hypothesis Testing in Candida albicans Clinical Isolates Using a Cloning-Free, Modular, and Recyclable System for CRISPR-Cas9 Mediated Mutant and Revertant Construction

As increasing evidence emerges that interstrain genetic diversity among Candida albicans clinical isolates underpins phenotypic variation compared to the reference isolate SC5314, new genetic tools are required to interrogate gene function across strain backgrounds. Here, the SAT1-flipper plasmid was reengineered to contain a C. albicans codon optimized hygromycin B resistance gene (CaHygB). Cassettes were PCR-amplified from both SAT1-flipper and CaHygB-flipper plasmids using primers with homologous sequences flanking target genes of interest to serve as repair templates. Ribonucleoprotein (RNP) complexes containing proprietary CRISPR RNAs (crRNAs), universal transactivating CRISPR RNA (tracrRNA), and Cas9 protein were assembled in vitro and transformed, along with both repair templates, by electroporation into C. albicans. Homozygous deletion of the ADE2 gene results in red-pigmented colonies and this gene was used to validate our approach. Both in SC5314 and a variety of clinical isolates (529L, JS15, SJCA1, TW1), homozygous gene targeting was nearly 100% when plating on media containing nourseothricin and hygromycin B with transformation efficiencies exceeding 10⁴ homozygous deletion mutants per μg of DNA. A gene reversion system was also employed with plasmids pDUP3 and pDIS3 engineered to contain the ADH1 terminator and an overlap extension PCR-mediated approach combined with CRISPR-Cas9 targeting at the NEUT5 neutral locus. A variety of single or compound mutants (Δ/Δals3, Δ/Δcph1 Δ/Δefg1, Δ/Δece1) and their revertant strains were constructed and phenotypically validated by a variety of assays, including biofilm formation, hyphal growth, and macrophage IL-1β response. Thus, we have established a cloning-free, modular system for highly efficient homozygous gene deletion and reversion in diverse isolates.

IMPORTANCE Recently, phenotypic heterogeneity in Candida albicans isolates has been recognized as an underappreciated factor contributing to gene diversification and broadly impacts strain-to-strain antifungal resistance, fitness, and pathogenicity. We have designed a cloning-free genetic system for rapid gene deletion and reversion in C. albicans clinical isolates that interlaces established recyclable genetic systems with CRISPR-Cas9 technology. The SAT1-flipper was reengineered to contain CaHygB encoding resistance to hygromycin B. Using a modular PCR-mediated approach coupled with in vitro ribonucleoprotein assembly with commercial reagents, both SAT1- and CaHygB-flipper cassettes were simultaneously integrated at loci with high efficiency (10⁴ transformants per μg DNA) and upward of 99% homozygous gene targeting across a collection of diverse isolates of various anatomical origin. Revertant strains were constructed by overlap extension PCR with CRISPR-Cas9 targeted integration at the NEUT5 locus. Thus, this facile system will aid in unraveling the genetic factors contributing to the complexity of intraspecies diversity.

Immunosurveillance of Candida albicans commensalism by the adaptive immune system

The fungal microbiota (mycobiota) is an integral part of the microbial community colonizing the body surfaces and is involved in many key aspects of human physiology, while an imbalance of the fungal communities, termed fungal dysbiosis, has been described in pathologies ranging from infections to inflammatory bowel disease. Commensal organisms, such as the fungus Candida albicans, induce antigen-specific immune responses that maintain immune homeostasis. Adaptive immune mechanisms are vital in this process, while deficiencies in adaptive immunity are linked to fungal infections. We start to understand the mechanisms by which a shift in mycobiota composition, in particular in C. albicans abundance, is linked to immunopathological conditions. This review discusses the mechanisms that ensure continuous immunosurveillance of C. albicans during mucosal colonization, how these protective adaptive immune responses can also promote immunopathology, and highlight therapeutic advances against C. albicans-associated disease.

When Is It Appropriate to Take Off the Mask? Signaling Pathways That Regulate ß(1,3)-Glucan Exposure in Candida albicans

Candida spp. are an important source of systemic and mucosal infections in immune compromised populations. However, drug resistance or toxicity has put limits on the efficacy of current antifungals. The C. albicans cell wall is considered a good therapeutic target due to its roles in viability and fungal pathogenicity. One potential method for improving antifungal strategies could be to enhance the detection of fungal cell wall antigens by host immune cells. ß(1,3)-glucan, which is an important component of fungal cell walls, is a highly immunogenic epitope. Consequently, multiple host pattern recognition receptors, such as dectin-1, complement receptor 3 (CR3), and the ephrin type A receptor A (EphA2) are capable of recognizing exposed (unmasked) ß(1,3)-glucan moieties on the cell surface to initiate an anti-fungal immune response. However, ß(1,3)-glucan is normally covered (masked) by a layer of glycosylated proteins on the outer surface of the cell wall, hiding it from immune detection. In order to better understand possible mechanisms of unmasking ß(1,3)-glucan, we must develop a deeper comprehension of the pathways driving this phenotype. In this review, we describe the medical importance of ß(1,3)-glucan exposure in anti-fungal immunity, and highlight environmental stimuli and stressors encountered within the host that are capable of inducing changes in the levels of surface exposed ß(1,3)-glucan. Furthermore, particular focus is placed on how signal transduction cascades regulate changes in ß(1,3)-glucan exposure, as understanding the role that these pathways have in mediating this phenotype will be critical for future therapeutic development.

GNP2 Encodes a High-Specificity Proline Permease in Candida albicans

The tight association of Candida albicans with the human host has driven the evolution of mechanisms that permit metabolic flexibility. Amino acids, present in a free or peptide-bound form, are abundant carbon and nitrogen sources in many host niches. In C. albicans, the capacity to utilize certain amino acids, like proline, is directly connected to fungal morphogenesis and virulence. Yet the precise nature of proline sensing and uptake in this pathogenic fungus has not been investigated. Since C. albicans encodes 10 putative orthologs of the four Saccharomyces cerevisiae proline transporters, we tested deletion strains of the respective genes and identified Gnp2 (CR_09920W) as the main C. albicans proline permease. In addition, we found that this specialization of Gnp2 was reflected in its transcriptional regulation and further assigned distinct substrate specificities for the other orthologs, indicating functional differences of the C. albicans amino acid permeases compared to the model yeast. The physiological relevance of proline uptake is exemplified by the findings that strains lacking GNP2 were unable to filament in response to extracellular proline and had a reduced capacity to damage macrophages and impaired survival following phagocytosis. Furthermore, GNP2 deletion rendered the cells more sensitive to oxidative stress, illustrating new connections between amino acid uptake and stress adaptation in C. albicans. IMPORTANCE The utilization of various nutrients is of paramount importance for the ability of Candida albicans to successfully colonize and infect diverse host niches. In this context, amino acids are of special interest due to their ubiquitous availability, relevance for fungal growth, and direct influence on virulence traits like filamentation. In this study, we identify a specialized proline transporter in C. albicans encoded by GNP2. The corresponding amino acid permease is essential for proline-induced filamentation, oxidative stress resistance, and fungal survival following interaction with macrophages. Altogether, this work highlights the importance of amino acid uptake for metabolic and stress adaptation in this fungus.

A variant ECE1 allele contributes to reduced pathogenicity of Candida albicans during vulvovaginal candidiasis

Vulvovaginal candidiasis (VVC), caused primarily by the human fungal pathogen Candida albicans, results in significant quality-of-life issues for women worldwide. Candidalysin, a toxin derived from a polypeptide (Ece1p) encoded by the ECE1 gene, plays a crucial role in driving immunopathology at the vaginal mucosa. This study aimed to determine if expression and/or processing of Ece1p differs across C. albicans isolates and whether this partly underlies differential pathogenicity observed clinically. Using a targeted sequencing approach, we determined that isolate 529L harbors a similarly expressed, yet distinct Ece1p isoform variant that encodes for a predicted functional candidalysin; this isoform was conserved amongst a collection of clinical isolates. Expression of the ECE1 open reading frame (ORF) from 529L in an SC5314-derived ece1Δ/Δ strain resulted in significantly reduced vaginopathogenicity as compared to an isogenic control expressing a wild-type (WT) ECE1 allele. However, in vitro challenge of vaginal epithelial cells with synthetic candidalysin demonstrated similar toxigenic activity amongst SC5314 and 529L isoforms. Creation of an isogenic panel of chimeric strains harboring swapped Ece1p peptides or HiBiT tags revealed reduced secretion with the ORF from 529L that was associated with reduced virulence. A genetic survey of 78 clinical isolates demonstrated a conserved pattern between Ece1p P2 and P3 sequences, suggesting that substrate specificity around Kex2p-mediated KR cleavage sites involved in protein processing may contribute to differential pathogenicity amongst clinical isolates. Therefore, we present a new mechanism for attenuation of C. albicans virulence at the ECE1 locus.

Prevalence of vulvovaginal candidiasis in Brazil: a systematic review

Vulvovaginal candidiasis (CVV) is a condition whose signs and symptoms are related to inflammation caused by Candida spp infection. It is the second leading cause of vaginitis in the world, representing a public health problem. The present systematic review comes with the proposal of analyze and identify the available evidence on CVV prevalence in Brazil, pointing out its variability by regions. For this, a systematic literature review was carried out with meta-analysis of cross-sectional and cohort studies, following the Preferred Reporting Items for Systematic Reviews and Meta-Analyzes (PRISMA) guide recommendations, and was registered in the International Prospective Register of Systematic Reviews (PROSPERO 2020 CRD42020181695). The databases used for survey were LILACS, Scielo, Scopus, PUBMED, Web of Science and CINAHL. Fifteen studies were selected to estimate CVV prevalence in the Brazilian territory. South and Southeast regions have higher prevalences than the North and Northeast regions, no data were found for the Midwest region. The estimated prevalence for Brazil is 18%, however, it is suggested that this number is higher due to underreporting and the presence of asymptomatic cases. Therefore, new epidemiological studies are recommended throughout Brazil, to elucidate the profile of this disease in the country, in addition to assisting in the elaboration of an appropriate prevention plan by state.

LAY ABSTRACT

Data found in the literature regarding the epidemiological profile of VVC in Brazil are obsolete and incomplete, so the present systematic review has the proposal to analyze and identify the evidence on VVC prevalence in Brazil. The estimated prevalence is 18%, however, this number can be higher.