Cervical lesions include CIN and SCC. A previous study revealed that cervical lesion is an infectious disease caused by HPV [
1]. Persistent high-risk HPV infection is a major risk factor to induce cervical lesions and promote its progression [
6]. However, the vast majority of HPV infections and CIN I, half of CIN II, and 30% of CIN III cases, can be spontaneously reversed [
7], which ultimately does not cause cervical cancer [
8]. The cervix is exposed in the vagina, and the local microecology and immunity of the vagina constitute the cervical microenvironment [
9]. It remains to be determined whether changes in the cervical microenvironment would affect the infection, pathogenicity and pathological progress of HPV. Stephen Paget has put forward the theory of “seed and soil” as early as 1889, which predicts that as a “seed”, the tumor cell can settle in the “soil” suitable for its growth; that is, tumor cells must cooperate with the surrounding environment to induce the occurrence and development of tumors.
Vaginal microecology, HPV infection and cervical lesions
Vaginal microecology is an important component of local cervical immunity. Under normal conditions, the dynamic balance system of the vaginal microecology is composed of the microbial flora dominated by dominant bacteria such as Lactobacilli [
10,
11]. When this balance is broken, the number of Lactobacilli are reduced or the function of lactobacilli is impaired [
12,
13]. This increases the chance of infection of other pathogens, causing the inherent protective mechanism of the vaginal microenvironment to be destroyed [
14,
15]. Jinghui Song et al. [
16] reported that the decrease in vaginal Lactobacilli and its H
2O
2 production in childbearing-age women were related to genital infections.
Compared with the control group, the results of this study revealed that in the research groups, Lactobacilli decreased, H
2O
2 function decreased, the density, diversity and normal proportion of flora decreased, the proportions of abnormal flora and the rates of flora imbalance increased, and trichomonad, BV and Chlamydia infections increased; while microecology became imbalanced. So the imbalance was more serious in patients with higher degree of cervical lesions. Lihong Lu [
17] reported that the plantation density of vaginal mucosal Lactobacilli decreased in patients with cervical cancer. Roeters [
18] reported that the imbalance of vaginal flora could enhance the infection and expression of HPV, and change cervical cytology. This was consistent with the results of this study. It can be inferred that with the increase in the severity of the imbalance of microecology, HPV more easily invaded and pathogenicity was enhanced, inducing the occurrence and progression of cervical lesions. The detection of preformed enzymes revealed that the positive rate of H
2O
2 gradually decreased with the worsening of the lesions. The positive rates of SNa, LE, GUS and GADP increased in the CSCC group. This suggests that the vaginal microecological imbalance of CSCC patients was the most serious, and that the imbalance of vaginal microecology and pathogen infections are correlated to the occurrence and development of cervical diseases. The positive rate of H
2O
2 in the CSCC group increased, and the reason might be that the number of Lactobacilli significantly decreased in the late stage of the disease, hence H
2O
2 production function of the vaginal microbia might compensatorily increase to reduce the local pH. These results suggest that the number of Lactobacilli and H
2O
2 function are correlated to high-risk HPV infection and cervical lesions.
This study revealed that no cervical lesions occurred and the evaluation indexes of vaginal microecology were normal in the 26 patients with HPV (+) in the control group. Compared with patients in the research groups and patients with HPV (−) in the control group, these patients had a more normal flora structure, more strong H
2O
2 function, the highest rates of density, diversity and normal proportion of flora, and fewer abnormal bacteria and other pathogen infections. It can be speculated that HPV is the main pathogenic factor for cervical lesions, but the predominance of vaginal Lactobacilli, strong H
2O
2 function, and vaginal microecological balance induces a very strong inhibition effect on the pathogenicity of HPV infection. Megan A Clarke [
19] reported that flora imbalance, bacterial infections and high-risk HPV infection co-promoted the occurrence of lesions and carcinogenesis before cervical cancer. Kriek [
20] revealed that chronic inflammatory conditions increased the risk of persistent HPV infection, which was consistent with the results of this study.
Therefore, the vaginal microecological status has an important influence on HPV infection and cervical lesions. Maintaining the vaginal microecological balance and the timely treatment of vaginal infectious diseases can prevent cervical lesions after HPV infection. Increasing the amount of vaginal Lactobacillus and enhancing H2O2 function can probably improve the outcome of HPV, and slow down and inhibit the occurrence and progression of cervical lesions. The detection of vaginal Lactobacilli, as well as the density and diversity of flora, H2O2, SNa, LE, GUS and GADP, can be potentially used to monitor and determine the prognosis of cervical lesions and therapeutic effects.
Local vaginal immunity, HPV infection, and cervical lesions
Local vaginal immune includes the cellular immunity represented by IL-2, IL-10 and humoral immunity mainly mediated by SIgA and IgG [
21]. IL-2 is the representative factor of Th1, which can strengthen cellular immunity. IL-10 is the representative factor of Th2, inhibiting immune response. The expression levels of IL-2 and IL-10 and the ratio between the two indirectly reflect the levels of Th1 and Th2 [
22]. Under normal conditions, Th1/Th2 keeps in a dynamic balance, and the function of Th1 cells is dominant, to maintain normal immune function of the body. Once the balance is broken, Th1 drifts to Th2, then immunosuppression occurs.
The results of this study revealed that differences in immune factors between the research groups and the control group were statistically significant. With the increase in the degree of cervical lesions, IL-2 increased slightly in LSIL cases, decreased significantly in HSHL cases and increased again in CSCC cases, IL-10 increased with the progression of the disease, and IL-2/IL-10 ratio decreased. Toshiyuki Sasagawa [
23] and Peghini [
24] reported that after HPV infection, IL-2 decreased and IL-10 increased, which was consistent with part of the results of this study. It is speculated that with the progression of the disease, IL-10 gradually increases and Th1 gradually drifts to Th2, which might be the manifestation of an immune inhibitory state and a further progress of lesions. This may be one of the mechanisms of the immune escape of cervical cancer cells. In this study, although IL-2 increased in CSCC cases, the Th1/Th2 ratio was lowest in CSCC cases, therefore the IL-2/IL-10 drift could better reflect the immune state of the body, and it indicates that the imbalance of T cell immune response was probably more and more obvious with the progression of the disease, causing Th1 to drift to Th2 in the body, and decrease cellular immune function. Hence, the body might gradually fall into an immune inhibitory state, inducing the occurrence and development of cervical lesions after HPV infection. Further studies are needed in the future to support this hypothesis.
SIgA plays an anti-infection role by binding with the microorganisms in the mucosal surface, and neutralizing viruses. This study revealed that SIgA was lower in the research groups than in the control group, but it increased with the progress of cervical lesions. Lilin Yang [
25] reported that the secretion of SIgA increased in mild vaginal infection, and decreased in severe infections. Through this study, it was speculated that in early lesions, SIgA participates in local mucosal immunity by binding with corresponding pathogenic microorganisms and neutralizing viruses to prevent pathogens from adhering to the cell surface, allowing it to have a local anti-infection role, and reducing the concentration. In the late stage of the disease, HPV infection persisted, vaginal flora became severely imbalanced, H
2O
2-producing Lactobacilli disappeared, and IgA protease secretion decreased; preventing the disulfide bond in hinge region of SIgA from being dissociated [
25], and increasing SIgA concentration. Therefore, the duration for SIgA production was short, and the duration of the neutralizing virus was positively correlated with the production of antibodies. Therefore, SIgA level has the potential to be used to immediately evaluate the local vaginal immune and protection function, and predict the direction of the development of lesions.
In this study, IgG was higher in the research groups than in the control group, and significantly increases with the progression of cervical lesions. Shirong Li [
26] reported that IgG increased with the progression of lesions. This indicates that with the persistent infection of HPV, a strong and long-lasting humoral immune response is induced in local cervical tissues, producing a large amount of IgG antibodies. This is the reason of the continuous progress of cervical lesions. Therefore, IgG can be used as a monitoring index for determining the condition of the HPV infection and cervical lesions.
Local cervical immune levels significantly changes in patients with cervical lesions. With the increase in the degree of lesions, the levels of IL-10 and IgG increase, and the IL-2/IL-10 ratio seriously shifts, revealing that the body is in an immune inhibitory state that promotes the development of cervical lesions. IL-2 and SIgA levels vary in different stages, and its immediate levels can be used as indexes to predict the progress of the disease and evaluate the prognosis after HPV infection or cervical lesions. Relatively high IL-2 and SIgA levels suggest the normal immune function of the body, but as the disease slowly progresses, IL-2 and SIgA levels are becoming lower, which might reflect that the immunity is weaker and its protective function decreases, therefore promoting disease progression.