AbstractChitinase-3-like 1 protein (CHI3L1) is a secreted glycoprotein, strongly correlated with fibrosis severity in chronic liver diseases including non-alcoholic steatohepatitis (NASH). However, the mechanisms by which CHI3L1 contributes to fibrogenesis remain undefined. Here, we showed that infiltrating monocyte-derived liver macrophages represent the main source of CHI3L1 in murine NASH. We developed a floxed CHI3L1 knock-out (KO) mouse to further study the cell-specific role of CHI3L1 ablation. Wildtype (WT) and myeloid cell-specific CHI3L1 KO mice (CreLyz) were challenged with a highly inflammatory and fibrotic dietary model of NASH by administering choline-deficient high-fat diet for 10 weeks. Macrophage accumulation and inflammatory cell recruitment were significantly ameliorated in the CreLyzgroup compared to WT (F4/80 IHCp< 0.0001, CD11b IHCp< 0.0001). Additionally, hepatic stellate cell (HSC) activation and fibrosis were strongly decreased in this group (α-SMA IHCp< 0.0001, picrosirius red stainingp< 0.0001). In vitro studies were performed stimulating bone marrow derived macrophages, THP-1 (human monocytes) and LX2 (human HSCs) cells with recombinant CHI3L1 to dissect its relationship with fibrosis development. Results showed an important role of CHI3L1 regulating fibrosis-promoting factors by macrophages (TGFB1 p< 0.05,CTGF p< 0.01) while directly activating HSCs (ACTA2 p< 0.01,COL1A1 p< 0.01), involving IL13Rα2 as the potential mediator. Our findings uncovered a novel role of CHI3L1 derived from liver macrophages in NASH progression and identifies this protein as a potential anti-fibrotic therapeutic target.Key messagesWe showed that CHI3L1 expression is increased in murine CDAA-HFAT diet NASH model, and that infiltrating macrophages are a key source of CHI3L1 production.Myeloid cell-specific CreLyz CHI3L1 knock-out in mice fed with CDAA-HFAT diet improved the NASH phenotype, with significantly reduced accumulation of pro-inflammatory macrophages and neutrophils compared with WT group.DEG and qPCR analysis of genes in CreLyz CHI3L1 knock-out mouse liver showed the mechanistic role of CHI3L1 in cellular chemotaxis.HSC is directly activated by CHI3L1 via receptor IL13Rα2, leading to upregulation of collagen deposition and pro-fibrotic gene, TIMP-1 and TIMP-2 release in whole liver.Direct stimulation of macrophages with CHI3L1 leads to upregulated expression of HSC-activation factors, suggesting its role in modulating macrophage-HSC crosstalk.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00109-023-02325-4.