The female vagina is a dynamic yet relatively stable ecosystem encompassing the VMB, host endocrine system, vaginal anatomy, and local immune defenses (Qing et al., 2024). The vaginal microbiota constitutes the core of this microecosystem. As an open cavity, the vagina is colonized by various microorganisms, such as bacteria, fungi, and viruses (Ye and Qi, 2024). These microorganisms primarily reside in the vaginal mucosal epithelium and form biofilms through hierarchical and structured colonization. The microbial composition within these biofilms undergoes constant succession in response to physiological and environmental changes (Usyk et al., 2020). The vaginal flora forms a symbiotic relationship with the host, supporting normal physiological functions and ensuring reproductive health.

In women of reproductive age, the VMB is both vast and complex. It is estimated that the total bacterial load usually ranges from 10¹⁰ to 10¹¹. Among these, Lactobacillus is the predominant genus, accounting for over 70% of the total bacteria. Lactobacillus plays a crucial role in maintaining vaginal microbiota balance, curbing the reproduction of pathogenic microorganisms, strengthening local immune defenses, and providing anti-tumor protection (Shen et al., 2024). The key mechanisms by which these beneficial bacteria contribute to health include:

① Lactobacilli produce lactic acid to maintain vaginal acidity. They derive energy from the carbohydrates released by vaginal mucosal epithelial cells. Lactobacilli metabolize glycogen into lactic acid, which creates an acidic environment (pH < 4.5) that significantly inhibits the adhesion, colonization, and proliferation of pathogenic bacteria (Shen et al., 2024).② Lactobacilli secrete H₂O₂, bacteriocins, and other compounds with antibacterial efficacy. H₂O₂ increases cell membrane permeability by generating highly reactive hydroxyl radicals, thereby preventing the invasion of pathogens into cervical epithelial cells (Frąszczak et al., 2022). Additionally, Lactobacilli secrete bacteriocins and biological surfactants, which are antimicrobial peptides or proteins that disrupt epithelial cells and form a frontline defense against pathogen adhesion (Borgogna et al., 2020; Nieves-Ramírez et al., 2021). ③ Lactobacilli prevents pathogenic microorganisms from adhering to vaginal epithelium by competitively binding to mucosal epithelial receptors. Furthermore, it secretes peptidoglycans and extracellular polypeptidoglycans (EPS) to form a biofilm with physical barrier functions to reduce pathogen colonization (Kalia et al., 2020). Different Lactobacillus species secrete distinct protective substances (Wang et al., 2019a). For example, Lactobacillus crispatus (L. crispatus), a predominant vaginal species, produces adhesion factors that facilitate mucosal colonization and inhibit Gardnerella vaginalis adhesion. In contrast, Lactobacillus iners(L. iners) lacks this protective effect (Łaniewski et al., 2019). Women with L. iners-dominant microbiota experience a three to five times higher risk of HPV infection and a two to three times greater likelihood of HR-HPV progression, cervical dysplasia, or cancer than women with L. crispatus-dominant microbiota (Palma et al., 2018). This highlights the superior role of L. crispatus in vaginal defense. ④ Lactobacilli enhance immune function by activating T-cell proliferation and differentiation, ameliorating the immunological recognition and proliferation of B cells (Frąszczak et al., 2022). Lactic acid suppresses toll-like receptor agonists, lowering pro-inflammatory cytokines, such as interleukin-6 (IL-6), tumor necrosis factor (TNF), RANTES, IL-8, and macrophage inflammatory protein 3α (MIP3α). This anti-inflammatory effect helps shield genital tract epithelial cells from infections and damage (Hearps et al., 2017).

Currently, most domestic and international scholars agree that the VMB can be classified into five community state types (CSTs), with dominant bacterial groups identified for each type (McClymont et al., 2022; Peremykina et al., 2024). L. crispatus, Lactobacillus gasseri(L. gasseri), L. iners, and Lactobacillus jensenii(L. jensenii) prevail in CSTs I, II, III, and V, respectively, whereas CST IV is characterized by increased microbial diversity, marked by reduced Lactobacillus abundance and a higher prevalence of anaerobic bacteria. CST IV is further subdivided into: CST IV-A (comprising the Anaerococcus, Peptoniphilus, Corynebacterium, Prevotella, Finegoldia, Streptococcus); CST IV-B (including the Atopobium, Fannyhessea, Gardnerella, Sneathia, Mobiluncus, Megasphaera); and CST IV-C, which features other diverse anaerobic species (France et al., 2020). Notably, differences in vaginal microenvironments across CSTs may directly affect HPV susceptibility and persistence.

Numerous studies have demonstrated a strong correlation between BV and HPV infection, with BV recognized as an independent risk factor for HPV acquisition and cervical lesions (Xu et al., 2022; Paul et al., 2023). CST III-B, IV-A, and IV-B are prevalent in patients with BV (Dong et al., 2024). BV is commonly caused by pathogens, including Gardnerella, Prevotella, Campylobacter, Bacteroides, Atopobium vaginae, and Sneathia. A meta-analysis encompassing six studies further confirmed the positive association between BV and cervical HPV infections (Martins et al., 2023). Similarly, another study used 16S rRNA gene sequencing to analyze the association between BV, HPV infections, and cervical lesions (Wei et al., 2020). HR-HPV-positive individuals exhibited decreased levels of Lactobacillus and elevated proportions of BV-associated bacteria, such as Gardnerella, Prevotella, Fusobacterium, Actinomyces, Peptococcus, Anaerococcus, Peptostreptococcus, Streptococcus, and Ureaplasma urealyticum. These results underscore a strong association between BV and HR-HPV infection. Dong et al. conducted a 2-year longitudinal study involving reproductive-aged women, demonstrating that BV-positive individuals showed significantly higher rates of persistent HR-HPV infection than BV-negative individuals (Dong et al., 2022). Through combined 16S rRNA sequencing and quantitative reverse transcription polymerase chain reaction analysis of vaginal secretions and cervical cells, vaginal Prevotella overgrowth was found to activate the NF-κB/C-Myc signaling pathway, facilitating HR-HPV persistence and cervical lesion progression. This effect may be further amplified by sialidase secretion. Microbial infection-induced NF-κB activation stimulates C-Myc expression, which in turn upregulates hTERT to drive malignant transformation (Papanikolaou et al., 2011; Ghareghomi et al., 2021). Lam et al. proposed that intratumoral microbiota may contribute to cervical carcinogenesis through immune modulation. They specifically suggested that Prevotella bivia(P. bivia) upregulate the human cancer driver lysosome-associated membrane protein 3 (LAMP3), which promotes metastasis and may help eliminate episomal HPV. This process can lead to overexpression of the E6 and E7 HPV oncogenes, thereby accelerating cervical disease progression (Lam et al., 2018).

BV may provide a biological rationale for HPV infection and invasion. However, Mao et al. identified a temporal sequence between HPV and BV infections, with HPV infection generally preceding BV. This may be attributed to the imbalance in the vaginal microenvironment caused by HPV infection, which increases the likelihood of BV (Mao et al., 2003). Therefore, the direct association between BV and cervical HPV infection, whether BV infection disrupts vaginal microecology and increases the prevalence of HPV infection and cervical lesions, whether HPV infection induces changes in the vaginal microecology that lead to BV infection, or whether these conditions are interdependent and promote simultaneous infections remains unclear. A substantial number of epidemiological and molecular studies are required to further explore the association between HPV infection and cervical lesions. Additionally, further research on the interaction between HPV infection and BV may facilitate the use of simple vaginal microecology tests, such as pH measurement, Gram staining for Nugent scoring, or molecular assays (e.g., quantitative PCR or 16S rRNA sequencing) targeting key bacteria (e.g., Lactobacillus spp., Gardnerella vaginalis, and Atopobium vaginae). These tests may help assess vaginal dysbiosis and predict HPV susceptibility. For instance, a low Lactobacillus dominance combined with a high anaerobic bacterial load may serve as a practical biomarker for increased HPV risk. Such approaches, if validated, might be integrated into routine gynecological screening to improve early detection and prevention strategies.

VVC is a common infectious disease of the lower genital tract caused by Candida albicans, which is a conditionally pathogenic fungus that causes disease only when the local immune capacity of the body or vagina declines (Sobel and Vempati, 2024).

The correlation between VVC, HPV infection, and cervical lesions remains controversial. Some researchers pose that VVC increases susceptibility to HPV and hinders HPV clearance (Wang et al., 2020b; Wu and Xue, 2020). This may result from pathogen-secreted proteolytic enzymes that activate the complement cascade, generating anaphylatoxins and chemokines. These factors cause local vasodilation, increased permeability, and an inflammatory response, finally inhibiting chemotaxis and the activation of neutrophils and lymphocytes (Ghosh et al., 2016). Additionally, VVC produces invasive enzymes that can damage genital epithelial cells, potentially facilitating HPV adhesion and persistence by creating a favorable microenvironment for viral replication (Wang et al., 2024). However, VVC does not raise the risk of HPV infection, and having both VVC and HPV does not lead to more severe cytological abnormalities (Wang et al., 2020b; Long et al., 2023). Furthermore, most women with VVC have a vaginal pH below 4.5; this acidic environment enhances vaginal defense by suppressing pathogen survival (Kwon and Lee, 2022). The low pH further bolsters immune responses by promoting the production of antimicrobial peptides (e.g., defensins) and lactic acid, which inhibit viral replication and maintain epithelial barrier integrity (Czechowicz et al., 2022). Consequently, VVC may confer a protective effect against persistent HPV infection, potentially reducing the risk of cervical intraepithelial lesions. Smalley et al. found that VVC may lower the risk of infection from non-16/18 HPV subtypes. Moreover, VVC functions as a possible booster for HPV vaccines because it may stimulate T-cell activity and improve immune function (Smalley Rumfield et al., 2020). This presents new avenues for vaccine and immunotherapy development. While numerous clinical studies have investigated the association between VVC and HPV infection/cervical lesions, substantial heterogeneity exists across study populations, including both general and high-risk groups. For instance, some studies enrolled balanced cohorts of premenopausal and postmenopausal women, whereas others specifically focused on HPV-vaccinated individuals. These demographic variations (e.g., age, immune status, and geographic distribution) may account for the inconsistent conclusions regarding the VVC-HPV association. Future investigations should utilize stratified analyses controlling for these covariates to elucidate potential confounding effects.

In 2002, Donders et al. introduced the concept of AV based on its bacteriological, immunological, and clinical characteristics (Donders et al., 2002). Similar to BV, AV is characterized by a reduction in H₂O₂-producing Lactobacillus species or a decrease in Lactobacillus activity within the vaginal microenvironment. However, unlike BV, AV is associated with an overgrowth of aerobic bacteria, primarily Streptococcus, Staphylococcus, and Escherichia coli, which are compositionally aligned with CST IV (Zeng et al., 2023). Because of the relatively recent clinical recognition of AV, studies investigating its association with HPV infection and CC remain limited. Jahic et al. conducted a prospective study and reported that AV was significantly more prevalent in women with cervical intraepithelial lesions than in those with healthy cervical cytology (Jahic et al., 2013). Furthermore, AV treatment appeared to promote the regression of cervical precancerous lesions. The proposed mechanism suggests that AV disrupts vaginal microecology by reducing Lactobacillus populations, thereby increasing the vaginal pH. The loss of Lactobacillus, the dominant protective bacterium, weakens the defense against external pathogens, leading to leukocytosis and enhanced interstitial invasion of cervical tissue by inflammatory cells, particularly through leukocyte esterase activity (Donders, 2007; Wang et al., 2020a). Vieira-Baptista et al. reported that moderate-to-severe AV was independently associated with an increased risk of cervical cellular abnormalities, despite no direct correlation with cervical HPV infection (Vieira-Baptista et al., 2016). Considering the limited national and international research on AV and HPV, further large-scale studies are required to elucidate their association.

The mucosal barrier serves as the first line of immune defense against HPV infection, protecting against harmful environmental factors, including pathogens, while permitting symbiosis with mucosal microorganisms. The vaginal mucosal layer consists of a non-keratinized stratified squamous epithelium with numerous transverse folds. This physiological and anatomical structure functions as a natural defense barrier for the female reproductive system. HPV infection may colonize these vaginal wall folds, making viral clearance more challenging. Additionally, the epithelium contains innate immune cells that express Fc receptors, which bind to the Fc region of immunoglobulins, facilitating antibody-dependent protective functions. Particularly, macrophages and neutrophils express Fc receptor common γ-chain and Fc receptor common α-chain, respectively, allowing them to phagocytose pathogens coated with IgG and IgA (Anderson, 2022). However, when the vaginal mucosa is ruptured, particularly in the squamous-columnar junction zone (transformation zone) of the cervix, which is a preferred site for HPV because of its epithelial characteristics, the virus is more likely to invade to basal cells and integrate into their nuclei. This results in host cell genome alterations and the development of cervical lesions (Boda et al., 2018). Furthermore, dysbiosis of vaginal microorganisms may disrupt epithelial cell proteins and increase cell death, thereby facilitating HPV entry into cervical transformation zone epithelial cells, where the virus can replicate and progress to CIN (Barros et al., 2018). Lactobacilli help maintain an acidic environment and preserve the mucosal barrier by producing metabolites, such as lactic acid, bacteriocins, and biosurfactants, thereby protecting vaginal health and preventing pathogenic infections (Shen et al., 2024). Secretory leukocyte protease inhibitor (SLPI) is a low-molecular-weight protein with antimicrobial, anti-inflammatory, and anti-protease properties. It is secreted by keratinocytes-key targets of HPV infection-and contributes to cervical mucosal immunity (Zhang et al., 2023). Patients with BV exhibit decreased SLPI levels in vaginal secretions, thereby diminishing HPV inhibition (Miquel et al., 2022). Additionally, alterations in the vaginal microbial community reduce L. crispatus and D-lactic acid levels, allowing other bacteria to proliferate rapidly. This raises microbial diversity, expands anaerobic populations, and increases L-lactic acid. Consequently, the expression of extracellular matrix metalloproteinase inducer is enhanced, which activates extracellular matrix metalloproteinase-8 (MMP-8). MMP-8 breaks down the extracellular matrix and cytoskeletal proteins, weakening epithelial structure and accelerating cell death and desquamation. Upon HPV infection, the virus binds to heparan sulfate proteoglycans on basal keratinocytes via its L1 protein, entering through endocytosis before reaching the nucleus in vesicles (Rebolj et al., 2019). In patients with BV, elevated anaerobes and their metabolites, such as porotoxins and sialidase, heighten the activity of mucin-degrading enzymes. This enzymatic activity degrades the protective cervical mucus layer, thereby compromising vaginal epithelial integrity and enhancing viral adhesion, invasion, and genome integration. These effects finally increase cervical susceptibility to HPV (Muzny et al., 2020; Liu et al., 2023). Moreover, clinical proteomic and transcriptional studies have demonstrated that vaginal microbiome alterations lead to significant proteomic changes. These include cytoskeletal modifications (elevated actin histamine, reduced keratin, and keratinized envelope proteins), increased pro-inflammatory cytokine expression, enhanced proteolysis, decreased IgG1/2 levels, antimicrobial peptide imbalances, and altered mucous composition (Borgdorff et al., 2016). Upregulation of cytokine expression strongly correlates with reduced levels of neutrophil proteases (MMP-8 and MMP-9), decreased antiprotease levels, and disruptions in cytoskeletal organization, epithelial differentiation, and keratinization pathways (Mohammadi et al., 2022).

Most women are able to clear HPV infections through immune surveillance and defense mechanisms, thus preventing persistent infection. The immune system consists of two major components, namely innate immunity and adaptive immunity, both of which coordinate and function together to defend against and clear HPV (Gu et al., 2024).

The relationship between vaginal microecology and HPV infection involves intricate biological processes, particularly viral gene integration and transcription. Shifts in vaginal microecology may affect HPV infection development, particularly by playing a key role in viral gene integration and transcriptional regulation (Tian et al., 2022a).

Upon HPV entry into the host cell, its gene integration and transcription processes begin silently. The HPV genome consists of early (E) and late (L) gene regions. During gene integration, HPV DNA fragments are randomly inserted into the host genome, and their location often determines subsequent cellular transformation. When key oncogenes, such as p53 and Rb, serve as integration sites, HPV-derived transcripts may impair their normal activities, disrupting cell cycle regulation and apoptosis (Doorbar et al., 2012; Templeton and Laimins, 2023). The transcription of the E6 and E7 genes produces the corresponding E6 and E7 proteins, which bind to p53 and Rb, respectively. This binding leads to protein degradation, rendering cells more susceptible to uncontrolled proliferation (Xing et al., 2024). After HPV gene integration, viral gene transcription is regulated by host cell transcription factors. An imbalanced vaginal microecology may cause chronic inflammation, stimulating cytokine production. These cytokines activate signaling pathways that indirectly influence HPV promoter regions. This activation upregulates the transcription of key genes, such as NF-κB and AP-1, increasing viral protein synthesis and the risk of cellular lesions (Cruz-Gregorio and Aranda-Rivera, 2021). Furthermore, vaginal microecological disruption induces high levels of oxidative stress, generating ROS that cause double-stranded breaks in both the host genome and viral DNA. This process facilitates viral integration into host cells for replication and transformation. Through this mechanism, the HPV E6 protein suppresses the expression of E1 and E2 proteins, leading to dysregulated E6 and E7 transcription, unchecked viral proliferation, and a significant reduction in apoptosis (Adnane et al., 2018; Szymonowicz and Chen, 2020; Łaniewski and Herbst-Kralovetz, 2021).

In summary, vaginal microecology and HPV infection are intricately linked at the levels of gene integration and transcription. A deeper understanding of these mechanisms may provide novel avenues for the prevention, diagnosis, and treatment of HPV-related diseases.

Metabolic dysregulation is an emerging hallmark of cancer, and metabolomics is increasingly being explored to identify specific biomarkers. Metabolomic analysis enables the rapid and precise detection of metabolites, making it highly valuable for studying cervical lesions and CC pathogenesis (Yang et al., 2024a).

Lactic acid, a metabolite produced by Lactobacillus plays a crucial role in HPV infection. It enhances cervical mucus’s ability to capture viral particles and inhibits HPV entry into basal cells (Pawar and Aranha, 2022). However, the antibacterial and anticancer effects of lactic acid depend on its type (D-lactic acid vs. L-lactic acid). CST I and II are typically dominated by D-lactic acid-producing L. crispatus or L. gasseri, forming a stable acidic environment. In contrast, CST III is primarily characterized by L-lactic acid-producing L. iners, resulting in an unstable acidic environment prone to dysbiosis. Additionally, the lack of other antimicrobial molecules (such as H₂0₂) further diminishes the defensive function of the vaginal microenvironment. This state is strongly associated with persistent HPV infection and recurrent BV. During CST IV, the microbiota becomes dysregulated, with an increase in anaerobic bacteria and a significant rise in vaginal pH (>4.5). The decrease in D-lactic acid concentration further weakens antiviral capacity (Borgogna et al., 2020; Dong et al., 2024). H₂O₂ impedes the progression of cervical lesions by selectively inducing apoptosis in malignant cells and denaturing bacterial proteins (Krüger and Bauer, 2017; Denys et al., 2019). The vulvovaginal metabolic profiles of HPV-infected women differ significantly from those of healthy controls in terms of lipid metabolism and amino acid metabolism, based on calculated metabolomic scores (Ilhan et al., 2019). Lipid metabolism is strongly related to genital inflammation and cervical lesions, with notably higher lipid accumulation in patients with high-grade CIN and CC because of its role in promoting cell proliferation and membrane synthesis via oncogene activation (Alvarez-Sieiro et al., 2016). Patients with HSIL show significantly elevated levels of acetylated phospholipids, sphingomyelins, phosphatidylcholine, and long-chain polyunsaturated fats. Similarly, 3-hydroxybutyrate, eicosapentaenoic acid esters, and oleic acid esters are markedly increased in patients with CC (Ilhan et al., 2019). Acetylated phospholipids and long-chain polyunsaturated fatty acids act as precursors to inflammatory mediators and may induce abnormal gene expression in cervical cells (Bokulich et al., 2022). Short-chain fatty acids (SCFAs), which are key microbial metabolites in the vaginal environment, regulate local immune responses by modulating vaginal epithelial cell function. SCFA concentrations are elevated in the vaginal tract of patients with BV. High SCFA levels may induce vaginal epithelial cells to secrete pro-inflammatory cytokines, impairing normal antiviral immune function. Additionally, they may disrupt the integrity of the vaginal barrier by affecting tight junction proteins in vaginal epithelial cells, thereby increasing susceptibility to HPV (Mirzaei et al., 2023). Changes in VMB and pH are influenced by amino acid metabolism. HPV infection is associated with reduced levels of key metabolites, including nicotinamide, succinate, and dipeptides (e.g., cysteinylglycine and cysteinyl) (McKenzie et al., 2021) as well as both oxidized and reduced glutathione (Borgogna et al., 2020). The total depletion of glutathione may contribute to oxidative stress, leading to irreversible cervical cell damage and promoting HPV persistence and carcinogenesis (Lebeau et al., 2022). Furthermore, ammonia produced by anaerobic bacterial metabolism and carcinogenic amyl nitrite have been detected in the vaginal environment of patients with BV. These compounds can stimulate the release of inflammatory cytokines, such as IL-1β and IL-8, which may interact with HPV and other factors to induce pathological changes in cervical epithelial cells. This process weakens immune defenses against HPV infection, whereas carcinogenic nitrosamines increase the likelihood of DNA damage (Wang et al., 2020b). Lactic acid produced by Lactobacillus lactis not only regulates vaginal pH but also indirectly affects nucleotide metabolism. The acidic environment can inhibit certain phosphatases involved in nucleotide phosphorylation and modification, affecting deoxynucleotide triphosphate (dNTP) production. This limitation in dNTP availability may restrict HPV replication (Ilhan et al., 2019; Fan et al., 2024).

Future research should further explore the association between vaginal microbial metabolites, HPV infection, and cervical carcinogenesis. Investigating targeted interventions in specific metabolic pathways may highlight novel approaches for disease prevention and management.