The state of the microbiome is dynamic. The microbiome remains in dynamic balance when the body is healthy, whereas high bacterial diversity and low numbers of lactic acid bacteria indicate a bacterial imbalance or inflammation (Figure 1). Furthermore, many factors are reported to affect the balance of microbiome. On the one hand, epidemiological factors, such as dietary habits, contraception, smoking and sex life, are the contributing factors. On the other hand, social environment factors (sanitary conditions, living area, and socioeconomic) also influence the composition of the microbiome. Most notably, the host’s factors are also a kind of factor that cannot be ignored (Audirac-Chalifour et al., 2016; Torres-Poveda et al., 2016). Lactobacillus does not dominate all vaginal microbiome regions in healthy women, possibly due to geographical and socioeconomic influences. In Hispanic women and women of African descent (30–40%), a non-lactobacillus predominance of the vaginal microbiome is more common (Fettweis et al., 2014; Borgdorff et al., 2017; Laniewski et al., 2018). Meanwhile, in another study of Japanese-ancestry, European-ancestry, and African-ancestry, the vaginal microbiome, dominated by lactic acid bacteria, is more common in Japanese and Caucasians than women of African descent, confirming this phenomenon (Zhou et al., 2010). Interestingly, these areas also tend to have higher cancer prevalence rates than areas where Lactobacillus is the dominant species (Miller et al., 2018), which is the subject of future study. The result may suggest that Lactobacillus is a major defender of the microenvironment protecting the female reproductive tract.

The dynamic balance of the vaginal microbiota could also be affected by the host’s hormone or immune system. Due to the effect of estrogen, the vaginal microbiome becomes more volatile during menstruation, relatively stable and less diverse after menstruation (MacIntyre et al., 2015). After menopause, the lack of estrogen in women caused an increase in anaerobic bacteria in the vaginal flora and a decrease in Lactobacillus. The composition of the vaginal microbiota is affected by estrogen, which affects the release of pro-inflammatory cytokines, chemokines, and vaginal antimicrobial peptides (AMPs) (Torcia, 2019). Evidence suggests that when estrogen is elevated, large amounts of glycogen are detected in vaginal epithelial cells. Furthermore, estrogen helps the vaginal epithelium cells to mature and produce α-amylase. Glycogen is degraded to produce simple sugars such as maltotriose, maltotetraose, and α-dextrins. Consequently, Lactobacillus, which cannot break down glycogen, relies on glycogen products to grow, forming colonies in the vagina that produce substances such as lactic acid to inhibit the growth of other bacteria (Spear et al., 2014).

Lactobacillus plays a role in protecting the body and maintaining function in the female reproductive tract or gastrointestinal tract. The composition of the vaginal microbiota may influence local immunity and be involved in the occurrence of cervical cancer and clearance of the HPV. The vaginal microbiota, dominated by various species of Lactobacillus, may play a protective role against opportunistic infections (Audirac-Chalifour et al., 2016). After utilizing those decomposition products of glycogen, Lactobacillus produce lactic acid that keeps the pH of the vaginal environment below 4.5, and acidizes the mucosal surface (Valenti et al., 2018). Lactic acid could reduce the cytotoxicity of natural killer cells, induce the secretion of anti-inflammatory cytokine interleukin-10 (IL-10), and reduce the production of pro-inflammatory cytokine IL-12 in dendritic cells (Sun et al., 2017). Notably, lactic acid is also produced by cancer cells and identified as a tumor marker (Ilhan et al., 2019). Therefore, the origin of lactic acid and its relationship with cervical cancer remains to be studied. In addition, bacteriocins with antimicrobial properties are also produced from Lactobacillus. Bacteriocins, a class of AMPs, are widely found in living organisms. It cannot only resist the invasion of pathogenic microorganisms through direct bactericidal action, but also mediate the acquired immune response and regulate the inflammatory immune response. Its cytotoxicity and cell lysis activity plays an important role in anti-tumor activity. Furthermore, a healthy vaginal microbiota is also associated with elevated defensins, and a previous study found a significant decrease in AMPs levels in patients with BV compared to healthy women (Al-Nasiry et al., 2020). Apart from vaginal AMPs, other substances can improve the vaginal defense system. IgA and IgG neutralize the effects of pathogenic microorganisms at the site of infection, preventing them from binding to the vaginal epithelium and ingesting nutrients. Mannose-binding lectin (MBL) may also protect against vaginal invasion and infection. These metabolites could resist the adhesion and growth of other bacteria in the vaginal epithelium. Furthermore, Lactobacillus compete with pathogens by acquiring nutrients and occupying territories, thus act as repellents and preventing pathogens from adhering to the vaginal epithelium (Maldonado-Barragan et al., 2016). Another study report that the complement system would be activated by Lactobacillus that sticks to the vaginal epithelium. The consequence could be triggering the body’s defenses. When the bacteria absorb nutrients and attach to the cell’s epithelium, the immune system is activated, further controlling microbial growth (Chase et al., 2015; Laura et al., 2016). As mentioned, Lactobacillus is a significant factor that maintains the health of the female reproductive tract, and the lack of Lactobacillus may promote the overflow of several anaerobic bacteria associated with sexual transmission. Cervical lesions could be caused by these anaerobic bacteria.

Notably, numerous data point out that not all Lactobacillus can protect the health of the female reproductive tract and the stability of the vaginal microbial environment. Unlike other Lactobacillus, L. iners are often found in communities where non-Lactobacillus are predominant (Gajer et al., 2012). L. crispatus appears to be the most protective lactic acid bacterium for host health, but several evidence suggest that L. iners is associated with disease (Oh et al., 2015; Kyrgiou et al., 2017), and L. iners are also found in women with vaginal microbial environment disorders caused by HIV, HPV, and HSV-2 (Borgdorff et al., 2014; van de Wijgert et al., 2014). Previous studies have found that the possible reason for L. iners association with diseases is related to H₂O₂ (Mitra et al., 2015; Laura et al., 2016). H₂O₂ is considered to be produced by L. crispatus rather than L. iners. The vaginal microbiota with L. crispatus that is the dominant bacteria is the most stable. This microbiome rarely causes infections. L. crispatus produces both D-lactic acid and L-lactic acid, but L. iners produces only L-lactic acid (Amabebe and Anumba, 2018).

D-lactic acid is only a bacterial product, while L-lactic acid is a bacterial product also produced by vaginal epithelial cells and bacteria (Witkin et al., 2013). High concentrations of D-lactic acid inhibit chlamydia infection due to the pH-dependent effects of the vaginal microenvironment (Witkin et al., 2013). Chlamydia infection can promote the persistence of HPV infection, which will be discussed later. D-lactic acid also prevents upper reproductive tract infections by regulating the production of extracellular matrix metalloproteinase inducer (EMMPRIN) in vaginal epithelial cells induced by L-lactic acid and inhibiting the production of matrix metalloproteinase-8 (MMP-8) (Alvarez-Olmos et al., 2004). In addition, vaginal α-amylase is associated with levels of D-lactic acid and other vaginal epithelial AMPs, but not L-lactic acid. The importance of vaginal α-amylase was also demonstrated because vaginal α-amylase is highest in women without vulvovaginal disease and lowest in women with BV (Nasioudis et al., 2015).

Cervicovaginal microbiological disorders have become a key factor in inflammation, HPV infection and cervical cancer. As mentioned earlier, Lactobacillus competes with pathogens for territory in the vaginal epithelium and inhibits the growth and development of pathogens by secreting lactic acid, bacteriocins and H₂O₂. An additional benefit of Lactobacillus is that it could also activate the complement system, trigger a local immune response, and further control the pathogen against the organism (Kovachev, 2020). However, in addition to the most beneficial Lactobacillus, other microbes, such as anaerobic bacteria, can regulate the body’s immune response when they lose balance. This can illustrate the complex relationship between the female genital tract and cervicovaginal microorganisms. The progression of cervical cancer is related to changes in the composition of the microorganism in the cervix vagina, especially the failure of Lactobacillus and the overgrowth of anaerobic bacteria (Mitra et al., 2015; Audirac-Chalifour et al., 2016). Microbial imbalance is often associated with the decreased abundance of Lactobacillus and the increased quantity of anaerobic bacteria (Mitra et al., 2015; Sodhani et al., 2017). This is a typical characteristic of CST-IV in the microbial spectrum. It is interesting to note that the characteristics of bacterial vaginitis are analogous to the trait of CST-IV (Kovachev, 2020). The conclusion could suggest that bacterial vaginitis and CST-IV are closely linked.

Human papillomavirus is known to be a pivotal factor in the development of cervical cancer. HPV causes various diseases, including cervical cancer and precancerous lesions (Li et al., 2020). HPV is divided into hr-HPV and lr-HPV. Most of the HPV entering the female reproductive tract is cleared by the host’s immune system (Audirac-Chalifour et al., 2016). Toll-like receptors (TLRs), natural killer cells and other mechanisms may be activated by the host to eliminate HPV (Stanley, 2012). A local innate immune response may mediate the initial host immunity to HPV infection, and acquired immunity mediated by antigen-presenting cells after HPV vaccination can also reduce the infection rate of HPV. However, when the host is subjected to persistent hr-HPV infection, it can develop cervical squamous intraepithelial lesions, and even cervical cancer (Astride et al., 2016). These cancer-related viruses may cause cancer directly by affecting the cellular structure, or indirectly contribute to cancer through immune escape or chronic inflammation (Curty et al., 2019). E6/E7 may disrupt the regulation of gene expression in host cells, leading to abnormal cell proliferation by influencing the control mechanisms of cell cycle and antigen expression, leaving the body unresponsive (Wang et al., 2014; Zhang et al., 2015). In addition, the expression of E6/E7 also overexpressed PD-1, thereby inhibiting the activation of T cells (Allouch et al., 2020). The microenvironment of the female reproductive tract changes with the stage of cervical cancer when a woman has cervical cancer. Vaginal acidity and cytokines are also affected by the development of cervical cancer, leading to further local immunosuppression. For instance, the presence of immunosuppressant agents, such as transforming growth factor-β1 (TGF-β1) and IL-10, may contribute to HPV persistence. The cervicovaginal microflora can increase the expression of immunosuppressive factors (Kyrgiou et al., 2017). A previous study examines that serum FMS-like tyrosine kinase and tumor necrosis factor-α (TNF-α) expression are significantly elevated in patients with CST-IV disease compared to the CST-I group (Anahtar et al., 2015).

Human papillomavirus is a major contributor to cervical cancer and its precancerous lesions; however, HPV is not a sufficient condition for cervical cancer (Muñoz et al., 2003). The persistence of HPV infection can be affected in many ways. Imbalanced cervicovaginal microbiota and inflammation contribute to HPV persistence. Various cytokines are produced and damaged by the epithelial intimal barrier that creates optimal conditions for HPV infection (Astride et al., 2016). Persistent infection with HPV in turn impact the dysregulation and inflammation of cervical and vaginal microbiology, because the host’s immune defenses could be negatively affected by persistent HPV infection (Klein et al., 2019). A previous study has found that HPV may also affect the mucosal metabolism, and further affect the cervix vaginal microenvironment. When HPV infects the mucosal surface, a series of inflammation-related mechanisms are initiated by HPV, such as the activation of overexpressed macrophages and NK cells, and the activation of local mucosal immunity by pro-inflammatory cytokines (Garcea et al., 2005). HPV infection, bacterial vaginitis and microbiota dysregulation are capable of acting on or influencing each other mutually.

Chronic or persistent infection with HPV is necessary, but HPV alone is not enough to induce cervical cancer and require additional endogenous or exogenous cues. At the low abundance of Lactobacillus, there is an increased likelihood of dysregulating the cervix vaginal microbiota and an increased risk of bacterial vaginitis, leading to increased production of mucin-degrading enzymes. Meanwhile, the production of H₂O₂, lactic acid and other products are reduced because of fewer producers, and the barrier of cervical and vaginal epithelium mucosa becomes more fragile (So et al., 2020). The microflora stimulates the production of pro-inflammatory cytokines that further disrupt the epithelial intimal barrier. A previous survey reported 32 women (ages 38–55) diagnosed with cervical cancer (Kovachev, 2020), to determine whether or not they had suffered from bacterial vaginitis and observed their cervix vaginal microflora. The results showed that 23 (71.9%) patients are affected by a vaginal microbiome disorder. Another study supports the hypothesis that inflammation could contribute to cervical cancer. Elevated inflammatory cytokines are found in the vaginas of patients with cervical cancer or precancerous lesions (Mhatre et al., 2012; Caselli et al., 2020). In 85 patients (32%), the prevalence of CST-IV is significantly higher than the general population (10.3%). Meanwhile, compared with patients with CIN2, CST-IV is more common in patients with CIN3 stage. These data illustrate the correlation between the severity of CST-IV and cervical cancer. The study also noted that high concentrations of pro-inflammatory cytokines are found in the vaginal environment of CIN patients. Compared with healthy people, IL-1α, IL-1β, IL-6, IL-8, and TNF-α in the vagina of CIN patients are higher. Interestingly, the CST-IV ratio is also higher in these patients with the cervicovaginal microbial spectrum (Di Paola et al., 2017; Gosmann et al., 2017). In Mitra’s study, by examining the microbiota of 169 women, they found that the rate of a CST IV vaginal microbiome doubled in women with low-grade squamous intraepithelial lesions (LSIL), tripled in women with highly squamous intraepithelial lesions (HSIL), and quadrupled in women with invasive cancer (Mitra et al., 2015).

Multiple cytokines are involved in developing various tumors, starting with high-risk HPV infected cells, through maladjustment of many pathways (Barbisan et al., 2012). In the innate immune system, TLRs are used to activate inflammatory defense mechanisms and regulate the clearance of immune complexes, but can also lead to chronic inflammation of unhealthy tissues. Therefore, imbalance or disturbance in one part can promote the formation of tumors (Jouhi et al., 2014). Mutated TNF-α can directly lead to cell canceration (Yang et al., 2018). Meanwhile, IL-6, IL-8, IL-1β, and other inflammatory cytokines can promote tumor proliferation and the development of cervical cancer (Schäfer et al., 2013; Liu et al., 2020). For example, IL-1β regulates the expression of CCL-2 by activating nuclear factor-κ-gene binding (NF-κB). It was also found that TGF-β mediated immune escape by promoting the overexpression of FBP1 gene in NK cells, making NK cells in a state of low activity in tumor microenvironment (Cong et al., 2018).

Abnormal activation of several different signaling pathways is also a factor contributing to the occurrence of cervical cancer. The upstream regulatory regions of some transcription factors, such as NF-κB and signal transducers and activators of transcription 3 (STAT3), contain homeopathic elements associated with carcinogenesis (Gupta et al., 2018). These transcription factors may influence the regulation of the HPV genome (Shukla et al., 2019). Activation of the janus kinase (JAK)/STAT pathway plays an important role in immune escape. STAT3 can not only increase the expression of inhibitory cytokines such as TGF-β, IL-6, and IL-10, but also promote the aggregation of regulatory T cells and induce the immunosuppressive microenvironment by inhibiting the maturation of dendritic cells (Yu et al., 2007). E6/E7 oncoprotein expression has been found to promote the expression of STAT3, which in turn inhibits the production of PRb and p53 (Arany et al., 2002; Basukala et al., 2019). In addition, STAT5 is also activated by oncoprotein E7. Interestingly, this is also required for HPV31 genome amplification (Hong and Laimins, 2013). NF-κB plays an essential role in immune response, inflammatory effects, viral replication and tumorigenesis (Bonab et al., 2021). Mutations in HPV E6/E7 oncoprotein IκB down-regulate NF-κB, thereby blocking the immune response. After developing cancer, NF-κB is reactivated by cytokines released by M2 macrophages. Dysregulation of NF-κB promotes inflammatory responses, abnormal cell proliferation and differentiation, persistent angiogenesis, avoidance of immune destruction, and even tissue infiltration and metastasis. Interestingly, while E6 induces IL-6 expression through NF-κB, IL-6 also promotes STAT3 production and activation. This also demonstrates the phenomenon of pathways in series with each other (Pique et al., 2019).

Of note, oxidative and nitrifying stress is also one of the main mechanisms of inflammation-induced cancer. Some oxygen and nitrogen-related compounds have been linked to inflammation-related cancer caused by microbes (Kipanyula et al., 2013; Nelson et al., 2015). They can cause abnormal oxidative stress in the microenvironment and inhibit the function of immune cells. A previous study has found that unlike CST with Lactobacillus as the dominant species, CST-IV with anaerobes as the dominant species has a higher concentration of amines (Nelson et al., 2015). These anaerobic bacteria can increase the level of biogenic amine production. Biogenic amines not only lead to nitrosamine production but also improve the resistance of pathogens to host-mediated defense systems. Additionally, biogenic amines have been found to promote biofilm formation in some pathogens. Certain species of Lactobacillus prevent the colonization of bacteria that produce high levels of amines. Lactobacillus could also remove the carcinogens of these amines and provide an extra layer of protection. This is because Lactobacillus is the dominant species in the cervicovaginal microbiological community type of amines less reason. In addition, Lactobacillus has a cytotoxic effect on cervical cancer cells, preventing the development of cervical cancer (Kyrgiou et al., 2017). Interestingly, inhibition of glucose metabolism could significantly reduce the clearance capacity of helper T cells (Choi et al., 2018). Cancer cells can also drive metabolic changes that suppress immune responses (Bader et al., 2020). These suggest that immune response is related to metabolic pathways, and immune metabolism may be a new way to treat tumors.

Human papillomavirus infection, inflammation and microbiome dysregulation interact or influence each other. We respectively summarized the mechanism of changes in the reproductive tract microenvironment under the conditions of Lactobacillus-dominated vaginal microbial environment and cervical and vaginal microbiota disorders, as shown in Figures 2, 3.

An increase in certain bacterial or microbial diversity may serve as a biomarker for cervical changes to identify the risk of persistent hr-HPV infection, CIN, and even cervical cancer (Norenhag et al., 2020). Recently, the CST-IV microflora spectrum has become a potential risk factor for cervical cancer (Torcia, 2019). One of the dominant anaerobes is carcinogenic, such as Gardnerella, Sneathia, Fusobacterium spp., and Atopobium vaginae (Drago et al., 2016; Brotman et al., 2018). High levels of Gardnerella, A. vaginae, and Sneathia spp. were most common among women who are persistently infected with hr-HPV for 1 year. Moreover, these bacteria are the common anaerobic bacteria in BV. Interestingly, in addition to these anaerobic bacteria, L. iners is also abundant (Di Paola et al., 2017). Large amounts of A. vaginae are inversely associated with HPV (Brotman et al., 2018). In addition, some studies have found that high concentrations of Sneathia spp. and Fusobacterium spp. are found in the genital tract environment of patients with the squamous intraepithelial disease and cervical cancer. In the meantime, mycoplasma and chlamydia have also entered the research field, and increasing numbers of reports have found a link between them and persistent HPV infection.

Probiotics are made up of living bacteria such as Bifidobacterium, Lactobacillus, and Streptococcus. It reduces the risk of certain cancers by regulating the composition of the microbiome and increasing inflammation (Li et al., 2020).

Probiotics containing Lacticaseibacillus Paracasei Shirota or Lacticaseibacillus casei can significantly improve HPV clearance (Verhoeven et al., 2013). In addition, Bacteroides fragilis can also improve the effectiveness of CTLA-4 targeted immunotherapy. The combination of L. gasseri and L. crispatus has shown in vitro cytotoxic effects on HeLa cervical cancer cells (Motevaseli et al., 2013a, b). Interestingly, milk consumption containing Lacticaseibacillus rhamnosus and Bifidobacterium lactate increased NK cell activity, and some Lactobacillus also stimulated the production of cytokines such as TNF-α and γ-IFN, which in turn increased immune cell activity (Bermúdez-Humarán et al., 2011; Dallal et al., 2012; Shin et al., 2016; Pique et al., 2019). Intranasal administration of recombinant Lactococcus lactis expressing E7 antigen can also induce the body to produce antibodies to E7, thus initiating specific immunity (Bermúdez-Humarán et al., 2011; Carse et al., 2021). We summarized the probiotic, in vivo and vitro development and mechanism in Table 1, providing a reference for readers.

Immunity is a kind of self-protection system of the body, which plays an important role in protecting the body against infections. The importance of immune checkpoints is highlighted. PD-1 receptor, the mediator of immune checkpoint, also plays a vital role in developing tumor. Typically, the PD-1 pathway regulates specific immunity, which in turn prevents cells from becoming cancerous (Allouch et al., 2020). When the PD-1/PD-L1 pathway is activated, T cell receptor signal transduction is inhibited and T cell activity is significantly weakened. Therefore, PD-1 mab drugs have been used clinically to specifically block the binding between PD-L1 and PD-1, reactivate the activity of T cells, and then kill tumor cells (Riley, 2009). The use of PD-1 mab drugs has also been approved by the FDA and is already being used to treat advanced cervical cancer. In addition, some STAT3 or NF-κB inhibitors are in pre-clinical development (Pakish and Jazaeri, 2017). However, due to high toxicity, the development of these inhibitors in tumor therapy has been slow. Therefore, many studies have focused on regulating upstream pathway targets to inhibit STAT3 or NF-κB activation. At present, there are clinical treatment options for IL-6 expression (Johnson et al., 2018). Proteasome inhibitors that block IκB have also been found to reduce NF-κB phosphorylation and treat multiple myeloma (Vrabel et al., 2019).