S100A8/S100A9 through PAD4 activation of neutrophil extracellular traps promotes granulomatous lobular mastitis

Background Granulomatous lobular mastitis (GLM) is a nonspecific chronic inflammatory breast disorder with an obscure etiology and pathogenesis. Neutrophil extracellular traps (NETs), which are extracellular web-like structures composed of decondensed chromatin and granular proteins released by activated neutrophils, disrupt normal tissue architecture and perpetuate inflammatory responses. The aim of the present study was to explore the role of NETs in GLM and the underlying regulatory mechanisms. Methods Neutrophils were isolated from the blood of GLM patients and healthy controls (HCs) to assess NET formation. The presence of NETs in GLM tissues was detected using Western blot, immunohistochemistry, and immunofluorescence analyses. A mouse model of GLM was established to determine whether the inhibition of NET production, which is dependent on S100A8/S100A9, alleviates mammary gland inflammation. The potential mechanisms and therapeutic implications were further explored through in vitro and in vivo assays. Results NETs were significantly increased in GLM tissues, as characterized by elevated levels of citrullinated histone H3 (CitH3) and myeloperoxidase (MPO). S100A8/S100A9 was highly expressed in GLM and demonstrated significant diagnostic value alongside NET markers. Mechanistically, S100A8/S100A9 promoted NETosis through interactions with peptidylarginine deiminase 4 (PAD4). Both paquinimod (an S100A8/S100A9 inhibitor) and Cl-amidine (a PAD4 inhibitor) effectively suppressed NET formation in vitro . In the GLM mouse model, both inhibitors reduced mammary gland inflammation, NET accumulation, and tissue damage. Conclusions The present findings indicated that NETs contribute to the pathogenesis of GLM. S100A8/S100A9 plays a critical role in promoting NET formation via PAD4 activation. Targeting this axis with paquinimod effectively inhibits NETosis and alleviates GLM, suggesting a promising therapeutic strategy for GLM and other inflammatory diseases.