Endometrial inflammasome activation accompanies menstruation and may have implications for systemic inflammatory events of the menstrual cycle
Authors: Azlan Aida , Salamonsen Lois A , Hutchison Jennifer , Evans Jemma 
Published: March 2020

Abstract

STUDY QUESTION
Does NLRP3 (NOD-, LRR- and pyrin domain-containing protein 3) inflammasome activation within decidualized endometrial stromal cells accompany menstruation and is this reflected systemically?

SUMMARY ANSWER
Components of the NLRP3 inflammasome immunolocalize to decidualized endometrial stromal cells immediately prior to menstruation, and are activated in an in vitro model of menstruation, as evidenced by downstream interleukin (IL)-1beta and IL-18 release, this being reflected systemically in vivo.

WHAT IS KNOWN ALREADY
Menstruation is a highly inflammatory event associated with activation of NFκB (nuclear factor kappa-light-chain-enhancer of activated B cells), local release of chemokines and cytokines and inflammatory leukocyte influx. Systemically, chemokines and cytokines fluctuate across the menstrual cycle.

STUDY DESIGN, SIZE, DURATION
This study examined the NLRP3 inflammasome and activation of downstream IL-1beta and IL-18 in endometrial tissues from women of known fertility (≥1 previous parous pregnancy) across the menstrual cycle (n ≥ 8 per cycle phase), serum from women during the proliferative, secretory and menstrual phases (≥9 per cycle phase) of the cycle and menstrual fluid collected on Day 2 of menses (n = 18). Endometrial stromal cells isolated from endometrial tissue biopsies (n = 10 in total) were used for an in vitro model of pre-menstrual hormone withdrawal.

PARTICIPANTS/MATERIALS, SETTING, METHODS
Expression and localization of components of the NLRP3 inflammasome (NLRP3 & apoptosis-associated speck–caspase recruit domain [ASC]) in endometrial tissues was performed by immunohistochemistry. Unbiased digital quantification of immunohistochemical staining allowed determination of different patterns of expression across the menstrual cycle. Serum from women across the menstrual cycle was examined for IL-1beta and IL-18 concentrations by ELISA. An in vitro model of hormone withdrawal from estrogen/progestin decidualized endometrial stromal cells was used to more carefully examine activation of the NLRP3 inflammasome. Endometrial stromal cells isolated from endometrial tissue biopsies (n = 10) were treated with estrogen/medroxyprogesterone acetate for 12 days to induce decidualization (assessed by release of prolactin) followed by withdrawal of steroid hormone support. Activation of NLRP3, & ASC in these cells was examined on Days 0–3 after hormone withdrawal by Western immunoblotting. Release of IL-1beta and IL-18 examined during decidualization and across the same time course of hormone withdrawal by ELISA. Specific involvement of NLRP3 inflammasome activation in IL-1beta and IL-18 release after hormone withdrawal was investigated via application of the NLRP3 inflammasome inhibitor MCC950 at the time of hormone withdrawal.

MAIN RESULTS AND THE ROLE OF CHANCE
Critical components of the NLRP3 inflammasome (NLRP3, ASC) were increased in menstrual phase endometrial tissues versus early secretory phase tissues (P &lt; 0.05, n/s, respectively). NLRP3 and ASC were also elevated in the proliferative versus secretory phase of the cycle (P &lt; 0.01, n/s, respectively) with ASC also significantly increased in the late-secretory versus early-secretory phase (P &lt; 0.05). The pattern of activation was reflected in systemic levels of the inflammasome mediators, with IL-1beta and IL-18 elevated in peripheral blood serum during menstruation (Day 2 of menses) versus secretory phase (P = 0.026, P = 0.0042, respectively) and significantly elevated in menstrual fluid (Day 2 of menses) versus systemic levels across all cycle phases, suggesting that local inflammasome activation within the endometrium during menses is reflected by systemic inflammation. NLRP3 and ASC localized to decidualized cells adjacent to the spiral arterioles in the late secretory phase of the menstrual cycle, where the menstrual cascade is thought to be initiated, and to endometrial leukocytes during the menstrual phase. NLRP3 also localized to glandular epithelial cells during the late-secretory/menstrual phases. Localization of both NLRP3 and ASC switched from predominant epithelial localization during the early-secretory phase to stromal localization during the late-secretory/menstrual phase. Using an in vitro model of hormone withdrawal from decidualized human endometrial stromal cells, we demonstrated progressive activation of NLRP3 and ASC after hormone withdrawal increasing from Day 0 of withdrawal/Day 12 of decidualization to Day 3 of withdrawal. Downstream release of IL-1beta and IL-18 from decidualized stromal cells after hormone withdrawal followed the same pattern with the role of NLRP3 inflammasome activation confirmed via the inhibition of IL-1beta and IL-18 release upon application of MCC950.

LARGE SCALE DATA
N/A.

LIMITATIONS, REASONS FOR CAUTION
This study uses descriptive and semi-quantitative measures of NLRP3 inflammasome activation within endometrial tissues. Further, the in vitro model of pre-menstrual hormone withdrawal may not accurately recapitulate the in vivo environment as only one cell type is present and medroxyprogesterone acetate replaced natural progesterone due to its longer stability.

WIDER IMPLICATIONS OF THE FINDINGS
We provide novel evidence that the NLRP3 inflammasome is activated within decidualized endometrial stromal cells immediately prior to menses and that local activation of the inflammasome within the endometrium appears to be reflected systemically in by activation of downstream IL-1beta and IL-18. Given the prevalence of menstrual disorders associated with inflammation including dysmenorrhoea and aspects of pre-menstrual syndrome, the inflammasome could be a novel target for ameliorating such burdens.

STUDY FUNDING/COMPETING INTEREST(S)
The authors have no competing interests. J.E. was supported by a Fielding Foundation fellowship, NHMRC project grants (#1139489 and #1141946) and The Hudson Institute of Medical Research. L.A.S. was supported by The Hudson Institute of Medical Research and J.H. by an Australian Government Research Training Program Scholarship. We acknowledge the Victorian Government’s Operating Infrastructure funding to the Hudson Institute.

TRIAL REGISTRATION NUMBER
N/A