Interleukin-6 (IL-6) superfamily cytokines play critical roles during human pregnancy by promoting trophoblast differentiation, invasion, and endocrine function, and maintaining embryo immunotolerance and protection. In contrast, the unbalanced activity of pro-inflammatory factors such as interferon gamma (IFNγ) and granulocyte–macrophage colony-stimulating factor (GM-CSF) at the maternal–fetal interface have detrimental effects on trophoblast function and differentiation. This study demonstrates how the IL-6 cytokine family member oncostatin M (OSM) and STAT3 activation regulate trophoblast fusion and endocrine function in response to pro-inflammatory stress induced by IFNγ and GM-CSF. Using human cytotrophoblast-like BeWo (CT/BW) cells, differentiated in villous syncytiotrophoblast (VST/BW) cells, we show that beta-human chorionic gonadotrophin (βhCG) production and cell fusion process are affected in response to IFNγ or GM-CSF. However, those effects are abrogated with OSM by modulating the activation of IFNγ-STAT1 and GM-CSF-STAT5 signaling pathways. OSM stimulation enhances the expression of STAT3, the phosphorylation of STAT3 and SMAD2, and the induction of negative regulators of inflammation (e.g., IL-10 and TGFβ1) and cytokine signaling (e.g., SOCS1 and SOCS3). Using STAT3-deficient VST/BW cells, we show that STAT3 expression is required for OSM to regulate the effects of IFNγ in βhCG and E-cadherin expression. In contrast, OSM retains its modulatory effect on GM-CSF-STAT5 pathway activation even in STAT3-deficient VST/BW cells, suggesting that OSM uses STAT3-dependent and -independent mechanisms to modulate the activation of pro-inflammatory pathways IFNγ-STAT1 and GM-CSF-STAT5. Moreover, STAT3 deficiency in VST/BW cells leads to the production of both a large amount of βhCG and an enhanced expression of activated STAT5 induced by GM-CSF, independently of OSM, suggesting a key role for STAT3 in βhCG production and trophoblast differentiation through STAT5 modulation. In conclusion, our study describes for the first time the critical role played by OSM and STAT3 signaling pathways to preserve and regulate trophoblast biological functions during inflammatory stress.

Acute endometritis, often triggered by Gram-negative bacterial endotoxins such as lipopolysaccharide (LPS), disrupts endometrial integrity, induces robust pro-inflammatory macrophage activation, and compromises uterine function, potentially leading to infertility and adverse pregnancy outcomes. Here, we demonstrate that IL-6 family cytokines, Leukemia Inhibitory Factor (LIF) and Oncostatin M (OSM), exert complementary yet distinct immunoregulatory effects that mitigate LPS-induced acute endometritis in vivo. Both cytokines preserved endometrial architecture, limited neutrophil and macrophage infiltration, suppressed cytotoxic iNOS expression, and promoted macrophage-mediated tissue repair, with OSM providing superior systemic and hepatoprotective effects. Mechanistically, these outcomes were linked to STAT3-dependent Mφ reprogramming toward anti-inflammatory M2 phenotypes, with OSM additionally engaging SMAD2 pathways to favor CD163 M2 macrophages marker, whereas LIF preferentially induced anti-inflammatory M2 subsets engaging STAT3 alone. Our findings highlight the therapeutic potential of LIF and OSM as modulators of uterine immune homeostasis and macrophage plasticity modulating and mitigating LPS-acute endometritis and preserving endometrial integrity.