Introduction
Vulvovaginal candidiasis (VVC) is an exceedingly common infection of female vulvovaginal inflammations[1]. The disease is estimated to affect 75% of women at least once during their lives, and approximately 8% of women develop recurrent VVC (RVVC)[2, 3]. The most common clinical symptoms of VVC are increased vaginal discharge that resembles bean curd residue accompanied by vulval itching and burning, leading to dyspareunia and dysuria in more severe cases[4]. While VVC is non-lethal, the morbidity associated with VVC brings tremendous mental distress, causing pain, low self-esteem, anxiety, and even impacting affective relations and career[5]. Moreover, VVC has been reported to cause adverse conditions such as cervicitis [6], pelvic inflammatory disease[7], genital tract malignancy[8], infertility[9], fetal intrauterine infection[10], HPV[11], and preterm birth[12].
A variety of microorganisms are present in the vagina of healthy women, which form a mutually regulated, coordinated, and dynamically balanced vaginal microenvironment. The balance of the vaginal microenvironment plays an important role in maintaining the self-cleaning function of the vagina and the health of the host. Lactobacillus accounts for the majority of the normal vaginal microbiota and helps defend against pathogens by lowering the vaginal pH to maintain an acidic environment[13]. Furthermore, cytokines and chemokines are regulated to maintain an anti-inflammatory vaginal environment[14], competing for adhesion sites to form a mechanical barrier[15], and secreting bio-surface-active substances to regulate the host immune response[16, 17]. However, dysbiosis of the vaginal microbiota occurs due to a variety of endogenous or exogenous factors that cause an imbalance in the vaginal internal environment. The main pathogens causing vaginal inflammatory diseases include Gardnerella vaginalis, Candida albicans, and Trichomonas vaginalis, which are associated with bacterial vaginitis, vulvovaginal candidiasis, and trichomoniasis, respectively.
Candida albicans is a serious opportunistic fungal pathogen[18] and is the most common infectious yeast strain in women with VVC, showing a prevalence of 90%[19]. It can colonize the mucosal surfaces of the genitourinary and gastrointestinal tracts, as well as the asymptomatic oral cavity and skin of healthy people[2]. Changes in microbial community composition and systemic or local immunosuppression (e.g., pregnancy, diabetes, allergies, broad-spectrum antibiotics, oral steroids, psychosocial stress, estrogen, and sexual activity) may result in increased load and virulence of C. albicans, which may exceed the tolerance threshold of epithelial cells, leading to the production and release of pro-inflammatory factors and inducing an intense inflammatory response[20]. However, C. albicans overload is not the only cause of VVC, as it can invade host cells by inducing endocytosis and active penetration[21]. Endocytosis is a passive process mediated by epithelial cells in which C. albicans transforms from a yeast phase to a more adherent mycelial phase, leading to loss of epithelial integrity and barrier function through the interaction of the fungal invasin Als3 and host E- or N-cadherin on the surface of the mycelium[22]. Meanwhile, C. albicans secreted related enzymes (SAPs) can promote tissue invasion by activating epithelial calpain1, mediating active fungal penetration[23]. In addition, mycelial-phase C. albicans produces a peptide toxin ”candida lysin” that activates the p38/cFos and ERK/MKP1 signaling pathways by recruiting innate immune cells such as neutrophils, macrophages, and innate type 17 cells, thereby leading to cytokine and chemokine secretion[24]. Furthermore, candida lysin promotes Candida invasion by affecting mitochondria and altering immunomodulatory signaling in vaginal epithelial cells [25].
VVC is typically treated with short-term topical and single-dose oral antimycotic agents to restore normal vaginal flora[26]. Nevertheless, due to the high level of resistance to commonly used antifungal drugs, antibiotic efficacy is greatly reduced and a high recurrence rate is observed despite standardized and multiple courses of antifungal therapy[27]. Although VVC has been studied for more than ten years[28], little is known about its pathogenesis. Therefore, exploring the pathogenesis of VVC and identifying new targets for alternative antibiotic therapy has great significance.
Metabolomics is an important part of modern omics research. Potential biomarkers are identified by studying the upstream biological processes, their metabolites, and metabolic pathways[29]. With the development of metabolomics, a growing number of metabolites have been discovered and studied as biomarkers, providing new insights for disease diagnosis, pathogenesis, and drug intervention. Furthermore, the metabolic profile of the cervicovaginal microenvironment can effectively be used to distinguish between HPV infection, cervical dysplasia, and ICC[30]. Several metabolites are significantly associated with the clinical signs and symptoms of bacterial vaginosis (BV), which has obvious metabolic signatures across multiple metabolic pathways[31]. Moreover, the composition of bacterial communities is affected during genital infection due to the changes in vaginal metabolome composition; significantly elevated metabolites such as TMA-NOx (TMAO), taurine, and methanol have been observed in VVC vaginal discharge[32]. Nonetheless, the potential role of vaginal bioactive metabolites in the development of VVC has not been reported.
In the present study, vaginal discharge profiling was performed with non-targeted metabolomics methods, such as liquid chromatography-mass spectrometry (LC–MS), to explore the difference in the metabolite expression profiles between VVC patients and normal controls (CTL). In addition to analyzing the global characteristics between VVC and CTL, active substances affecting C. albicans infection could also be identified, providing insights into the mechanism of VVC infection and the corresponding defense mechanism of the body. The findings highlight a new perspective on the treatment of C. albicans vaginal infection.