The role of the IL-33/ST2/MAPK signalling pathway in macrophage polarisation and endometrial fibrosis

Background: Intrauterine adhesion (IUA) is a common complication following endometrial injury, characterized by endometrial fibrosis and partial or complete obliteration of the uterine cavity, severely affecting menstrual function and fertility in women of reproductive age. Recent studies have demonstrated that aberrant repair processes after endometrial injury involve complex inflammatory responses and immune regulation, among which macrophage polarisation imbalance plays a critical role in fibrosis progression. As a member of the IL-1 family, IL-33 can activate the MAPK signalling pathway through binding to its receptor ST2, participating in the regulation of macrophage polarisation; however, its specific mechanisms in the pathogenesis and progression of IUA remain unclear. This study aimed to investigate the mechanism by which the IL-33/ST2/MAPK signalling pathway contributes to endometrial fibrosis following endometrial injury, and to elucidate the molecular basis through which it promotes intrauterine adhesion (IUA) formation by regulating macrophage polarisation, thereby offering a potential target for clinical intervention.

Methods: Endometrial tissues from both normal and IUA patients were collected for pathological examination, including haematoxylin and eosin (H&E) and Masson staining, as well as immunohistochemistry to assess fibrosis markers such as TGF-β, Collagen I, and α-SMA. A mouse model of IUA and adeno-associated virus(AAV) for IL-33 overexpression or knockdown were administered via intrauterine injection was established, and the expression of IL-33, macrophage polarisation phenotypes, and MAPK signalling pathway activity were evaluated via Western blot and RT-qPCR. An in vitro co-culture system involving endometrial-like organoids and RAW 264.7 macrophages was established, and the regulatory influence of the pathway on macrophage polarisation and fibrotic processes was examined through pharmacological interventions (IL-33, IL-4, and MAPK inhibitor) and lentiviral transfection of MAPK pathway components.

Results: Expression levels of IL-33, IL-1β, and fibrosis markers were significantly elevated in the endometrial tissues of IUA patients, consistent with the expression pattern of M1-type macrophages (CD86). In the mouse model, macrophages in the IL-33 pharmacological intervention group and adeno-associated virus overexpression injection group exhibited significantly elevated expression of M1-type inflammatory factors TNF-α and IL-6, along with markedly increased protein levels of IL-1β and IL-33. RNA Sequencing revealed significant enrichment of the MAPK signalling pathway (KEGG analysis). Co-culture experiments confirmed that IL-33 induces M1-type macrophage polarisation through activation of the ST2/MAPK signalling axis, and this effect could be reversed by MAPK inhibitor treatment or lentiviral transfection of MAPK pathway components.

Conclusion: IL-33 activates the MAPK signalling pathway via the ST2 receptor and promotes macrophage M1 polarisation, which exacerbates inflammatory responses and extracellular matrix deposition, ultimately leading to endometrial fibrosis and intrauterine adhesions. Therapeutic targeting of the IL-33/ST2/MAPK axis may offer a novel strategy for the treatment of intrauterine adhesions.