The Immune Microenvironment in Human Papilloma Virus-Induced Cervical Lesions-Evidence for Estrogen as an Immunomodulator

Abstract

Globally, human papilloma virus (HPV) infection is a common sexually transmitted disease. However, most of the HPV infections eventually resolve aided by the body's efficient cell-mediated immune responses. In the vast majority of the small group of patients who develop overt disease too, it is the immune response that culminates in regression of lesions. It is therefore a rarity that persistent infection by high-risk genotypes of HPV compounded by other risk factors progresses through precancer (various grades of cervical intraepithelial neoplasia-CIN) to cervical cancer (CxCa). Hence, although CxCa is a rare culmination of HPV infection, the latter is nevertheless causally linked to >90% of cancer. The three 'Es' of cancer immunoediting viz. elimination, equilibrium, and escape come into vogue during the gradual evolution of CIN 1 to CxCa. Both cell-intrinsic and extrinsic mechanisms operate to eliminate virally infected cells: cell-extrinsic players are anti-tumor/antiviral effectors like Th1 subset of CD4+ T cells, CD8+ cytotoxic T cells, Natural Killer cells, etc. and pro-tumorigenic/immunosuppressive cells like regulatory T cells (Tregs), Myeloid-Derived Suppressor Cells (MDSCs), type 2 macrophages, etc. And accordingly, when immunosuppressive cells overpower the effectors e.g., in high-grade lesions like CIN 2 or 3, the scale is tilted towards immune escape and the disease progresses to cancer. Estradiol has long been considered as a co-factor in cervical carcinogenesis. In addition to the gonads, the Peyer's patches in the gut synthesize estradiol. Over and above local production of the hormone in the tissues, estradiol metabolism by the gut microbiome: estrobolome versus tryptophan non-metabolizing microbiome, regulates free estradiol levels in the intestine and extraintestinal mucosal sites. Elevated tissue levels of the hormone serve more than one purpose: besides a direct growth-promoting action on cervical epithelial cells, estradiol acting genomically via Estrogen Receptor-α also boosts the function of the stromal and infiltrating immunosuppressive cells viz. Tregs, MDSCs, and carcinoma-associated fibroblasts. Hence as a corollary, therapeutic repurposing of Selective Estrogen Receptor Disruptors or aromatase inhibitors could be useful for modulating immune function in cervical precancer/cancer. The immunomodulatory role of estradiol in HPV-mediated cervical lesions is reviewed.

Keywords: carcinoma-associated fibroblasts; cervical cancer microenvironment; cervical intraepithelial neoplasia (CIN); estrogen; human papilloma virus; myeloid-derived suppressor cells; regulatory T cells; selective estrogen receptor disruptors.

Figure 1

Cervical tumors are enriched in estradiol (E2) and express estrogen receptor α. (A) (i) Concentrations of 17β-estradiol as determined by ELISA in blood plasma from healthy donors (Pl HD) or patients with CxCa (Pl CxCa) as well as in (ii) tissue samples of cervical tumors (CxCa), areas adjacent to the tumors (CxCa adj), and healthy cervices (Normal Cx). Graph shows mean values ± SEM of n = 30 per group. (B) Staining distribution of 17β-estradiol, estrogen receptor α, and aromatase in a representative tissue section of SCC cervix. Upper left image (i) shows haematoxylin and eosin staining of a tumour section; upper right image (ii) shows estradiol (E2) staining which was predominantly cytoplasmic in the tumor and both nuclear and cytoplasmic in the stroma and infiltrating cells; lower left image (iii) shows the nuclear staining of ERα in the stromal cells only; lower right image (iv) shows aromatase expression detected in the cytoplasm of the tumor, stroma and infiltrating cells. Inset: normal rabbit serum negative control. Symbol T indicates tumor location in each picture; * indicates stroma. Images are representative of n = 30. (reproduced from: Adurthi et al, Sci Rep. 2017 Dec 11;7(1):17289. doi: 10.1038/s41598-017- 17102-w. https://pubmed.ncbi.nlm.nih.gov/29229929/#&gid=article-figures&pid=figure-1-uid-0). **P < 0.01, ***P < 0.001.

Figure 2

Diagrammatic representation of biosynthesis of estradiol in the peripheral tissues. High levels of circulatory E1S (estrone sulfate) has been reported especially in post menopausal women. Estrone sulfatase (STS) converts E1S into estrone (E1) in the peripheral tissues, which subsequently gets reduced to estradiol (E2) by type I 17β-Hydroxysteroid Dehydrogenase (17, β-HSD). Estradiol in the tissues binds to intracytoplasmic or membrane Estrogen Receptors (ERs) and carries out its function. Development of STS inhibitors and 17, β-HSD inhibitors are in the preclinical stage. E1, Estrone; E2, Estradiol; E1S, Estrone sulphate; E1 STS, Estrone sulfatase; E1 ST, (Estrone sulfotransferase). (Reproduced from: Jayshree R.S., et al., Med J Obstet Gynecol 2020; 8(2): 1136. DOI: 10.1007/978-981-13-3438-2_31).

Figure 3

Estrogen metabolism by the gut microbiome. At one end of the spectrum is the “estrobolome”—the overall gut microbial genes which can metabolize estrogens: e.g., β-glucuronidases and sulfatases. These enzymes deconjugate estrogens (both parent and metabolites) and release free estrogens in the gut lumen which can get absorbed into the circulation. At the other end is tryptophan non-metabolizing microbiome (e.g., Klebsiella spp.)—which doesn’t compete with the host cells for tryptophan and thereby promotes the synthesis of melatonin in the gut. Melatonin antagonizes estrogens by acting as a selective estrogen receptor modulator (SERM) and a selective estrogen enzyme modulator (SEEM) by inhibiting enzymes which synthesize estradiol, viz., 17β-hydroxysteroid dehydrogenase 1 (17β-HSD1), sulfatases, and aromatase (reproduced from: Jayshree R. S. and Kumar R. V. (2019) Contribution of the Gut and Vaginal Microbiomes to Gynecological Cancers. In: Mehta S., Singla A. (eds) Preventive Oncology for the Gynecologist. Springer, Singapore. https://doi.org/10.1007/978-981-13-3438-2_31).