{"paper_id":"a797b834-e326-47bd-a6bd-df7f4b9312cd","body_text":"Chronic endometriosis is a devastating condition affecting up to 10% of women of\nreproductive ages (up to 196 million women globally) and it is characterised by\nextensive growth of endometrial tissue and stoma outside of the uterus. The\npresenting symptoms include unexplained infertility, pelvic pain, excessive bleeding\nand pain upon urination or intercourse or even bowel movements. The current medical\nmanagement of such cases include repeated surgery to remove the ectopic endometrial\ntissue and/or hormonal therapy but these approaches seem ineffective in preventing\nrecurrences.\nThe current knowledge of the aetiology of endometriosis is limited but the principal\ntheory is that endometrial tissue implants at ectopic sites far from the uterus but\nnormally the immune cells at such sites are responsible for the clearance of such\nlesions. In 10% of women, it appears that such immune clearance is ineffective\nleading to the establishment and proliferation of the tissue leading to the lesions.\nChronic inflammation in the peritoneal cavity causes the spread of the lesions.\nWhat then is the link between hormone metabolism, the local microbiome and the\npro-inflammatory profile displayed by most women suffering from this condition?\n\nNumerous recent studies have shown chronic endometritis as a common feature of women\npresenting with endometriosis. Most women present with altered microbiota and the\nfirst reports date back to 2014 when these authors showed significant differences in\nthe uterine ad cervical microbiome communities between women with endometriosis and\nthose without ( Takebayashi  et al .,\n2014 ).  Shan  et al. \n(2021)  subsequently observed lower alpha diversity of gut microbiota and\na higher Firmicutes-to-Bacteroidetes ratio in women with stage 3/4 endometriosis\n(n=12) than healthy controls. Such a scenario promotes a pro-inflammatory response\nand a dysregulated immune profile in the affected women eventually leady to\ninfertility and implantation failures. Endometriotic microbiota of affected women\nhas been consistently associated with diminished  Lactobacillus \ndominance and the overgrowth of inflammatory species such as\n Firmicutes , elevated  Gardnerella, Streptococcus,\nEscherichia, Shigella,  and  Ureoplasma  in their cervix\nand elevated  Shigella/Escherichia  in their stool ( Singh & Sethi, 2022 ).\nLessons learnt form studies conducted in Irritable Bowel Syndrome (IBS) and the\nobvious microbiome dysregulation can be applied directly to the microbiological\nprofile seen in endometriosis and chronic endometritis. The role of the microbiota\nin possibly driving local and systemic inflammation and the relationship to the\npathophysiology of multiple gynaecologic conditions remains under investigation\n( Jiang  et al ., 2021 ). The\npathogenesis of endometriosis and comorbidities is likely related to the interplay\nof multiple environmental factors, genetic factors, inflammation, immune\ndysregulation, hormonal imbalance and possibly the microbiome ( Chadchan  et al ., 2023 ). Chronic activation of\nthe immune response can result in pelvic inflammation and severe endometriosis can\nresult in pelvic adhesive disease.\nThe microbiome relates to the collection of genomes of microorganisms in a particular\nenvironment or mucosal site, such as the GI, cervicovaginal and pulmonary mucosa.\nThe gut microbiome plays a key role in physiological processes, including nutrient\nabsorption, maintaining the integrity of the GI lining, regulation of immune and\nendocrine systems, and protection against pathogenic insults. An ‘optimal\nmicrobiome’ can maintain the well-being and homeostasis of the individual and this\nmicrobiota composition confers health benefits at mucosal sites. In addition, the\ngut microbiota can influence host health through mediating changes in the\nmetabolome. The metabolome is composed of all the metabolites present in a\nparticular environment. A diverse collection of bacteria in the gut ensures a varied\nrepertoire of enzymes and metabolic pathways that contribute to health and\nhomeostasis, and this state is referred to as eubiosis. Dysbiosis, on the other\nhand, refers to a disruption or change in the microbiota composition that may be\nassociated with disease, and in the gut, this is reflected by a reduction in\nmicrobiota diversity. The gut microbiota has the ability to regulate circulating\noestrogen levels via the estrobolome, which is defined as the collection of genes\nencoding oestrogen-metabolizing enzymes, specifically in the gut microbiome ( Salliss  et al ., 2021 ).\nAn immense array of metabolic reactions occurs in the intestinal lumen, one being the\ndeconjugation of oestrogen from its conjugate glucuronic acid ( Salliss  et al ., 2021 ). This reaction requires\nthe bacterial enzyme β-glucuronidase, found in specific gut bacteria such as,\n Escherichia coli, Bacteroides fragilis  and\n Streptococcus agalactiae  which can deconjugate glucuronide. The\nliver conjugates oestrogen, including 17β-oestradiol (E2) the predominant\noestrogen in humans, with glucuronic acid (glucuronide) and secretes the glucuronide\ninto bile salts, where the glucuronides are later released into the intestinal tract\nfor excretion of the unused conjugated toxins and hormones ( Baker  et al ., 2017 ). Glucuronide cannot be\nreabsorbed into the circulatory system. However, oestrogen that has been\ndeconjugated from glucuronic acid, by the bacterial enzymatic action of\nβ-glucuronidase, can be absorbed into the circulatory system as active\noestrogen. Changes in the gut microbial composition, and thereby\nβ-glucuronidase activity, could perturb or dysregulate circulating oestrogen\nlevels and drive oestrogen-mediated conditions by contributing to hyperor\nhypo-estrogenic states ( Salliss  et\nal ., 2021 ).\nIn the cervicovaginal microbiome , Lactobacillus  dominance is\nassociated with optimal gynaecologic and reproductive health.\n Lactobacilli  create a competitive environment for invading\npathogens and dysbiotic bacteria. Lactic acid, produced by\n Lactobacillus , lowers the vaginal pH to less than or equal to\npH 4.5, and this low pH microenvironment is optimal for vaginal health. To survive\nand thrive,  Lactobacillus spp . require glycogen by-products, which\nare provided by an oestrogen-dominant vaginal epithelium and host amylases ( Spear  et al ., 2015 ). In\naddition,  Lactobacillus spp . contribute to homeostasis by occupying\nthis niche (pathogen exclusion) and by production of anti-inflammatory cytokines and\nantimicrobial peptides from epithelial cells, which fortifies the epithelial cell\nbarrier ( Spear  et al .,\n2015 ).\n\nWithin the endometrial environment during this stage, known as ‘the implantation\nwindow’, a very peculiar influx of immune cells occurs and nearly completely\nswitches local immunity from the inflammatory Th1 cell type to the\ntolerance-inducing Th2 cell type. This switch is crucial for implantation. During\nthis period, 65-70% of the immune cells in the endometrium are uterine natural\nkiller (uNK) cells that belong to the innate immunity compartment. Macrophages and\ndendritic cells are also detected, together with adaptive immune T cells, such as T\nregulatory cells (Tregs) ( Mukherjee  et\nal ., 2023 ). Embryo attachment requires active local\nendometrial reactivity on the maternal side. The adhesion step is followed\nimmediately by an anti-inflammatory reaction to enable the induction of the\nmechanisms of local tolerance, required for effective invasion. Early on, the ideal\nenvironment during the implantation window was thought to contain mainly Th2\n(compared with Th1) cytokines, which would selectively allow the development of\nlocal mechanisms that promote immunotrophism and angiogenesis at the same time that\nthey down-regulate inflammation and cytotoxic pathways ( Marron & Harrity, 2019 ). Over time, the concept of pregnancy\nas a Th2 phenomenon has evolved: both the absence and a large excess of Th1\ncytokines are thought to be deleterious for implantation and placentation, as is the\nabsence of Th2 cytokines. This transient immune switch, together with the adequate\nuNK cell activation, appears fundamental in enabling the establishment of local\nmaternal tolerance and survival of the foetus.\nIn endometriosis, the peritoneal environment is in a chronic state of local\ninflammation and contains immune cells with altered functions. This immune\ndysregulation in endometriosis creates an ideal environment for disease progression\n( Jiang  et al.,  2021 ). At\npresent, it is unclear whether immune dysfunction is a pathophysiological feature or\na cause of endometriosis. In either case, there is a strong association demonstrated\nby the findings as reported by  Jiang  et\nal.  (2021) .\nMany authors now believe that chronic endometriosis is in fact a disease of the local\nimmune system in that the peritoneal macrophages of affected women display a\ndecreased ability to phagocytosis but an up-regulated and increased activation of\nNF- Ϗ B pathways leading to the downstream upregulation of\nproinflammatory cytokines (TNF-α, IL-1β, and IL-6), proangiogenic\nfactors (VEGF), growth factors and adhesion molecules ( Fonseca  et al ., 2023 ).\nT cell subset profiles are altered in women with endometriosis. There are higher\nnumbers of Th17 cells in the peritoneal fluid of endometriosis patients, and\nconsequently higher concentrations of IL-17 ( Shi\n et al ., 2022 ). The presence of elevated Th17 cells\nand IL-17 plays a major role in promoting chronic inflammation: IL-17 stimulates\nproduction of cytokines that induce angiogenesis and inflammation, contributing to\nthe progression of endometriosis.\nFemales have a normal functioning immune system that competes with a semi allogenic\nconceptus during pregnancy. Thus, the acceptance and tolerance of the semi allogenic\nconceptus mandates the transformation of the maternal immune system. While the\nprecise mechanisms by which the embryo is protected from the maternal immune assault\nare not fully understood, a picture that has emerged out of numerous studies\nsuggests a dramatic transformation of multiple immune cells in the peripheral blood\nand endometrium. At the implantation stage, the immune cells, mainly the uterine\nnatural killer (uNK) cells, T cells, dendritic cells, and macrophages make up half\nof the total number of endometrial cells. In order for a pregnancy to proceed, the\nrole of the CD4+ CD25+ FOXP3+ regulatory T cells (T-regs) are important in mediating\nmaternal immune tolerance to the allogeneic foetus during embryo implantation and\nearly pregnancy. In general, pregnancy is associated with Th2 dominance, and Th1\nimmune response is associated with pregnancy losses. T helper 17 (Th17) cells\ndifferentiate from naive T-cells and produce the cytokine IL-17 that has an\nimportant role in the feto-maternal interface. During implantation, the Th17 cells\nare present in decidua and their numbers increase in the peripheral blood in the\nfirst trimester. However, it is proposed that lack of the T-regs (numerically as\nwell as functionally) present in the endometrium leads to a subtle but constant\ninflammatory scenario which is not conductive to a successful implantation and live\nbirth outcome. These immunosuppressive T cells work in conjunction with the uNKs\nwhich happen to be tolerance-inducing cells (unlike their peripheral blood\ncounterparts which happen to be highly cytotoxic). In the endometrium, the uNKs are\nphenotypically CD3-CD56+, secreting cytokines such as TGF-β or IL10, thereby\ncreating an immune supressed environment to allow implantation. It is an interesting\nbut important fact that T-regs require the presence of commensals to become\nactivated and when the microbiome is disturbed, the T-regs do not carry out their\nimmune suppressive functions and control local inflammation.\n\nThe immediate answer is a resounding YES! There is abundant literature from IBS cases\nwhich clearly suggest that a state of dysbiosis with overgrowth by inflammatory\nbacteria leads to an immune profile conductive to chronic inflammation. It is\nbelieved that when the gut microbiome is disrupted and replaced by non-physiological\ncommensals, the immune cells are typical of that of a dysregulated immune response\nwith little to no tolerance to ingested foods, especially foreign proteins. The same\napplies to the endometrial surface: when the microbiome is replaced by endometrial\npathogens ( Gardnerella, Ureoplasma, Enterococcus, Staphylococcus, E.\ncoli , and other mixed cultures), these bacteria induce an uncontrolled\ninflammatory response with all its consequences. To add to the injury, the\npredominant estrogenic milieu (due to lack of oestrogen-metabolizing bacteria) feeds\nthe inflammatory cells and overtakes the tolerogenic cells (T-regs and uNKs) which\nare progesterone sensitive!\n\nIt is logical to hypothesize that decreasing the numbers of bacterial pathogens in\nthe reproductive tract and increasing the proportion of beneficial\n Lactobacillus  could improve reproductive outcomes in those\npatients with an abnormal microbiota. The consequence of increasing the beneficial\nbacteria would ultimately lead to immune tolerance due to the presence of the Tregs\nand lower inflammatory cytokines. To address this, several approaches should be\nimplemented: the use of antibiotics to eliminate the bacterial pathogens causing the\ndysbiosis, the use of probiotics specifically designed for reproductive health (high\nLactobacilli probiotics) and hormonal balancing to control the raised oestrogen\nlevels seen in these affected women. It is imperative to determine the immune\nprofile of the cells in the endometrium (menstrual blood profiling) in order to\nmonitor the beneficial changes induced by the therapy of choice and predict a\npossible positive outcome on the  in vitro  reproductive\nprocesses.","source_license":"public-domain-us","license_restricted":false}