Background <p>The transition from metabolic dysfunction-associated steatotic liver disease (MASLD) to steatohepatitis (MASH) is characterized by a chronic low-grade inflammation, involving activation of resident macrophages (Kupffer cells; KC) and recruitment of infiltrating macrophages. Macrophages produce cytokines and, after induction of Cyclooxygenase 2 (COX-2), the key enzyme of prostanoid synthesis, prostaglandin E₂ (PGE₂). PGE₂ modulates cytokine production in an autocrine and paracrine manner, therefore playing a pivotal role in regulating inflammatory processes. Changes in the hepatic macrophage pool during MASLD progression to MASH could influence PGE₂- and cytokine-mediated signaling processes. The aim of this study was to characterize these changes in mice with diet-induced MASH and further elucidate the role of COX-2-dependently formed PGE₂ on the inflammatory response in different macrophage populations of mice with a macrophage-specific COX-2-deletion.</p> Methods <p>Male, 6-7-week-old wildtype mice were fed either a Standard or high-fat, high-cholesterol MASH-inducing diet for 4, 12 and 20 weeks. Liver macrophages were isolated and analyzed by flow cytometry. For in vitro experiments primary KC, peritoneal macrophages (PM) and Bone-marrow-derived macrophages (BMDM) were isolated from macrophage-specific COX-2-deficient and wildtype mice and treated with lipopolysaccharide (LPS) and/or PGE₂.</p> Results <p>During MASH-development, the proportion of KC (Clec4F⁺Tim4⁺) decreased, while the proportion of monocyte-derived macrophages (Clec4F⁻Tim4⁻) and monocyte-derived cells exhibiting a phenotype similar to KC (Clec4F⁺Tim4⁻) significantly increased over time. In vitro experiments showed that exogenous PGE₂ completely abrogated the LPS-induced mRNA expression and secretion of tumor necrosis factor-alpha (TNF-α) in primary KC, PM and BMDM from wildtype mice. PM and BMDM, as in vitro models for infiltrating macrophages, were more sensitive to PGE₂ compared to KC. Deletion of COX-2 in all macrophage populations led to an impaired PGE₂-dependent feedback inhibition of TNF-α production. LPSinduced TNF-α mRNA expression was higher compared to the respective wildtype macrophage population.</p> Conclusion <p>The current study, using a murine MASH model, indicates that PGE₂ may have a protective, anti-inflammatory effect, especially by inhibiting the expression of pro-inflammatory cytokines such as TNFα in infiltrating monocyte-derived macrophages. An inhibition of endogenous PGE₂ synthesis in macrophages by pharmacological inhibition of COX-2 could potentially increase inflammation and promote the progression of MASH.</p>