The most prevalent HPV genotype in the analyzed population was 16, similar to other studies previously reported in Mexican population [30, 31]. Persistent HPV infection, along with environmental and genetic factors such as IL-10, predisposes individuals to SICL and subsequent progression to cancer [2–4, 6, 32].
Previously, Hobbs et al., described a C to A exchange in the IL-10 promoter located 592 bp upstream from the transcription start site, which is located between putative consensus binding sequences for Sp1 (GGGGCGG), and a sequence with similarity to that recognized by members of the ETS family proteins (AGGAA). This −592 polymorphism is present in the promoter in a region with negative enhancer activity and is associated with loss of this activity . Steinke et al., were the first ones that demonstrated that the C to A nucleotide exchange results in increased IL-10 gene promoter activity, supporting its role as a repressor element of the gene C variant . Previous work from our group discovered that HPV-16 E6 and E7 oncoproteins binds to Sp1 transcription factor in the TGFß1 promoter and up regulates gene expression, through a GGGGCGG consensus sequence located at −180 to −172 of the TGFß1 human gene. Taken all together with the results of this paper, support the hypothesis that HPV-16 E6 and E7 oncoproteins may up regulate IL-10 gene expression .
This SNP has been linked with increased severity of a number of autoimmune diseases [36–39], and is associated with changes in tumorigenesis and transplantation tolerance , and rapid progression to AIDS in individuals infected with HIV [41, 42]. The current study has shown that individuals homozygous for the A-allele of the IL-10 -592 polymorphism are at two times greater odds of having SICL [OR 2.07, (95% CI, 1.10-3.89), p = 0.02], as compared to NCL. A previous study carried out in 311 patients with cervical intraepithelial neoplasia (CIN), 695 cervical cancer patients, 115 family-based patients and 586 unrelated controls, in Caucasian population, revealed the same association, an increased risk for CIN (II–III) (OR 1.44 [1.06–1.97]) and squamous cell carcinoma of the cervix (OR 1.35 [1.04–1.75]) for individuals heterozygous for the A-allele of the IL-10 -592 polymorphism .
With respect to IL-10 mRNA expression at the systemic and cervix levels in patients with SICL and NCL, we also found that the level of IL-10 mRNA relative to GAPDH and HPRT1 in SICL was significantly higher than in NCL, at the systemic level. It has been shown that PBMC from patients with both SICL and CC produce higher levels of IL-10 following mitogenic stimulation, compared with control groups [2, 7]. In addition, it has been shown that tumors can induce IL-10 production by PBMC [18, 43]. Women with HPV infection have been found to have higher percentages of IL-10 positive T cells than healthy women, at the systemic level .
We also found that the level of IL-10 mRNA relative to GAPDH and HPRT1 in SICL was significantly higher than in NCL, at the cervix level. However, here we showed that IL-10 mRNA was undetectable in some cases with cervix without HPV infection, suggesting that normal cervical epithelium does not produce IL-10, similar findings had been found previously [4, 5]. Abnormal IL-10 production at the cervix level has already been reported in patients with SICL and CC [6, 45]. In CC, we report a high correlation between IL-10 immunostaining and the level of IL-10 mRNA expression in cervical biopsies, where there is a Th2/Th3 cytokine pattern expression, suggesting that HPV infection induces the transcription of immunosuppressive cytokines, IL-10 and TGF-ß, as a means to evade the host immune system [4, 6, 35].
In women with SICL, significant differences of the levels of IL-10 protein in serum were found between groups, with virtually undetectable serum protein in NCL women. The SICL presence, as well as the progression to CC has been associated with increased serum levels of IL-10 [6, 44]. The level of IL-10 mRNA expression reported here, at the systemic level and in cervix, and protein level in serum are higher in SICL cases; this confirms our previous finding .