Tumor-derived CD109 orchestrates reprogramming of tumor-associated macrophages to dampen immune response

Despite remarkable advances in immunotherapy, poor responsiveness remains a persistent challenge in intrahepatic cholangiocarcinoma (iCCA). Here, we explored potentially immunosuppressive proteins secreted by iCCA and the underlying regulatory mechanisms involved in tumor immune microenvironment (TIME) remodeling, with the aim of developing new targets to inhibit tumor growth and improve the efficacy of immunotherapy.

Proteomic analysis, single-cell transcriptomics, cytometry by time of flight, RNA sequencing, and mass spectrometry were conducted to identify the key secreted proteins involved in immune suppression and elucidate the underlying biological mechanisms.

We revealed that tumor-derived soluble CD109 (sCD109) is associated with the immunosuppressive TIME, where elevated sCD109 promotes the enrichment of CD73⁺ tumor-associated macrophages, hindering T-cell immune responses. Mechanistically, sCD109 upregulates CD73 mRNA transcription by activating the FcγRI/SYK/NF-κB signaling pathway. Meanwhile, sCD109 is internalized into the cytoplasm of macrophages and inhibits 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 lymphocytes, significantly prolonging the antitumor response.

Our findings reveal sCD109 as a 'secreted immune checkpoint' that reprograms the TIME and indicate that CD109 inhibition could be a valuable strategy to sensitize iCCA to immunotherapy.

Poor 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. 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 tumor immune microenvironment, 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.