ObjectiveMonocyte chemotactic protein-1-induced protein 1 (MCPIP1) is highly expressed in inflamed mucosa of inflammatory bowel disease (IBD) and negatively regulates immune response, while the underlying mechanisms regulating mucosal macrophage functions remain unknown. Here, we investigated the roles of MCPIP1 in modulating the differentiation and functions of intestinal macrophages in the pathogenesis of IBD.DesignScRNA-seq was used to cluster the monocyte/macrophage lineage from macrophage-specificMcpip1-deficient (Mcpip1^∆Mye) mice andMcpip1^fl/fllittermates. The differentially expressed genes were confirmed by RNA-seq, luciferase assay, CUT&Tag assay and Western blotting. Effects of MCPIP1 and the activating transcription factor 3 (ATF3)-AP1S2 axis were assessed in patients with IBD.ResultsMcpip1^∆Myemice developed more severe dextran sulfate sodium (DSS)-induced colitis characterised by an increase in macrophage migratory capacity and M1 macrophage polarisation but a decrease in the monocyte-to-macrophage maturation in gut mucosa compared with their littermates. ScRNA-seq unravelled a proinflammatory population (Ccr2⁺Il-1β⁺Tlr2⁺Cx3cr1⁻Cd163⁻Mrc1⁻Ly6c⁺) of the monocyte/macrophage lineage from lamina propria CD11b⁺cells and an arrest ofMcpip1^∆Myemonocyte-to-macrophage maturation in an Atf3-Ap1s2 axis-dependent manner. Silencing of Ap1s2 or Atf3 markedly suppressedMcpip1^∆Myemacrophage migration, M1-like polarisation, and production of proinflammatory cytokines and chemokines. Notably, in vivo blockage of Ap1s2 ameliorated DSS-induced colitis inMcpip1^ΔMyemice through enhancing intestinal macrophage maturation. Furthermore, MCPIP1, ATF3 and AP1S2 were highly expressed in inflamed mucosa of active patients with IBD and blockage of ATF3 or AP1S2 significantly suppressed IBD CD14⁺-derived M1-like macrophage polarisation and proinflammatory cytokine production.ConclusionsMacrophage-specificMcpip1deficiency polarises macrophages towards M1-like phenotype, arrests macrophage maturation and exacerbates intestinal inflammation in an Atf3-Ap1s2-dependent manner, thus providing novel mechanistic insight into intestinal macrophage functions during IBD.