During the normal menstrual cycle, the human endometrium undergoes
extensive tissue remodelling under the influence of ovarian-derived hormones. The
endometrium has well defined stromal and epithelial compartments with the former
containing both a well-developed vasculature as well as a diverse population of
immune cells. Mast cells (MCs) are long-lived tissue resident immune cells
characterised by the presence of granules containing proteases. Mast cells have been
detected in the human uterus but little is known about their regulation or the impact of
steroids on their differentiation status. Recently MCs have been implicated as key
players in physiological and pathological pain pathways but little is known about their
role in endometrial pathologies. Endometriosis is a chronic incurable condition
characterized by the presence of endometrial tissue outside the uterine cavity: women
with endometriosis can suffer from a debilitating range of symptoms including chronic
pain. Whilst the aetiology of endometriosis is uncertain, close proximity between MCs
and nerves has implicated them in aberrant activation of pain pathways.
The aims of the current project were: 1. To determine the spatial and temporal
location of uterine MCs and to explore their phenotype including expression of steroid
receptors. 2. To explore the activation status of MCs in women with endometriosis
and/or pain, 3. To explore the use of cells and mice as models to investigate the
phenotype of mast cells and their regulation by steroids.
Mast cell proteases tryptase and chymase were detected by RTPCR and
immunohistochemistry in “full thickness” (uterine lumen to endometrial-myometrial
junction) biopsies from women undergoing hysterectomy. In agreement with previous
findings MCs were most abundant in the myometrium. Uterine MCs were
predominantly of the classical MC subtypes: tryptasepos/chymaseneg and
tryptasepos/chymasepos but a rare third subtype was also identified as
tryptaseneg/chymasepos. Mast cell activation/degranulation was cycle stage dependent
and for the first time their steroid receptor phenotype was identified as
ERαneg/ERβpos/GRpos, suggesting potential regulation by the uterine steroid
microenvironment. Studies on tissue samples from women with endometriosis revealed MCs with
an altered activation status in the pelvic peritoneal wall, compared to controls, which
showed an intense diffuse immunoexpression of chymase suggestive of MC activation
and release of this protease during normal physiology of the peritoneum. Surprisingly,
analysis of peritoneal fluids from controls, women with pain but no endometriosis, and
pain with endometriosis did not detect differences in numbers of MCs or
concentrations of tryptase or chymase. Analysis of peritoneal biopsies also provided
the first evidence for a striking increase in immunoexpression of PAR-2, a protease-activated
receptor, in women suffering from chronic pelvic pain and/or endometriosis
which may provide a mechanism by which mast cell derived factors may alter pain
pathways.
Studies in a mouse model of endometriosis identified MCs within endometria-llike
lesions and offer a platform for future studies. In vitro explorations using MCs
derived from peripheral blood precursors and HMC-1, a cell line derived from a patient
with MC leukaemia confirmed expression of ERβ but did not support previous studies
claiming cells were ERαpos.
In summary, this study has provided novel insights into the phenotype of
endometrial mast cells in the normal cycling endometrium and contrasted them with
those in women with endometriosis and pelvic pain. This is the first study to identify
MCs as ERβpos. Further studies are required to determine whether inhibition of PAR-
2 might offer a therapeutic target in women with chronic pelvic pain
