BACKGROUND & AIMSDespite remarkable advancements in immunotherapy, poor responsiveness in intrahepatic cholangiocarcinoma (iCCA) remains a persistent challenge. Here, we explored the immunosuppressive-related secreted proteins derived from iCCA and the underlying regulatory mechanisms in tumor immune microenvironment (TIME) remodeling, with the aim of developing new targets to inhibit tumor growth and improve the efficacy of immunotherapy.METHODSProteomic analysis, single-cell transcriptomics, CyTOF, RNA sequencing, and mass spectrometry were conducted to identify the key secreted protein involved in immune suppression and elucidate the underlying biological mechanisms.RESULTSWe revealed that tumor-derived soluble CD109 (sCD109) is associated with the immunosuppressive TIME, where elevated sCD109 promotes the enrichment of CD73+ TAMs, hindering T cell immune response. Mechanistically, sCD109 upregulates CD73 mRNA transcription by activating the FcγRI/SYK/NFκB signaling pathway. Meanwhile, sCD109 is internalized into the cytoplasm of macrophages and inhibit the degradation of CD73 protein by binding to the E3 ligase TRIM21, competing with CD73 for its binding site. Dual blockade of CD109 and PD-L1 can improve the infiltration and function of lymphocyte, significantly prolonging the anti-tumor response.CONCLUSIONSOur findings reveal sCD109 as a 'secreted immune checkpoint' that reprograms the TIME and suggest that CD109 inhibition is a valuable strategy to sensitize the effectiveness of iCCA immunotherapy.IMPACT AND IMPLICATIONSPoor response to tumor immunotherapy in patients with intrahepatic cholangiocarcinoma (iCCA) has long been a challenge for clinicians. In this study, we used multiomics approaches to elucidate that tumor cells secrete soluble CD109, which reprograms macrophages, leading to the accumulation of CD73+ macrophages in the tumor immune microenvironment (TIME). This effect significantly inhibits T cell proliferation and the immune response of CD8+ T cells, thereby impairing the efficacy of immunotherapy. In preclinical studies, we demonstrated that targeting CD109 in mice can markedly improve the immunosuppressive TIME, sensitizing iCCA cells to anti-PD-L1 immunotherapy. These findings represent a crucial step toward developing more effective therapies for iCCA and have significant implications for clinicians, scientists, and drug developers in the field.