Profibrotic impact of PAD4-driven macrophage extracellular traps in ulcerative colitis

The inflammatory polarization of macrophages has been implicated in the pathogenesis of fibrosis in ulcerative colitis (UC). Our previous study found that macrophages in an inflammatory polarization state highly express peptidyl arginine deiminase 4 (PAD4) and are more likely to form macrophage extracellular traps (METs). In this study, we aimed to investigate the role of PAD4-driven METs in promoting intestinal fibrosis in UC. Our results demonstrated that PAD4 and PAD4-driven METs were significantly increased in UC patients and DSS-induced chronic UC mice, closely related to intestinal fibrosis, and that PAD4 knockout inhibits MET formation, effectively alleviating intestinal fibrosis in UC mice. Furthermore, when combined with RNA sequencing and in vitro verification, we determined that Spp1 (encoding osteopontin/OPN) is the key fibrosis gene regulated by PAD4 in MET formation. PAD4 induces the expression of OPN/Spp1, which activates the fibroblast-to-myofibroblast transition (FMT) process. Mechanistically, PAD4 modifies the transcriptional activity of RELA and STAT1 via citrullination, thereby altering Spp1 transcription in macrophages. This study demonstrates the crucial role of PAD4 in macrophages, where it drives MET formation and citrullination-dependent crosstalk with fibroblasts, thereby contributing to UC-associated intestinal fibrosis and highlighting PAD4 inhibition as a promising therapeutic strategy.