USP18 Inhibition Enhances Type I Interferon Signalling and Immune Activation in the Tumour Microenvironment of Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is one of the most aggressive and treatment-resistant breast cancers. Although immunotherapy has emerged as a promising treatment option, clinical benefit is limited, with only around half of patients responding, even when combined with standard chemotherapeutic agents. This limited efficacy is often attributed to immunologically ″cold ″ tumour microenvironments (TME), which are resistant to current immunotherapies. Addressing this challenge requires approaches that can reprogram ″cold ″ TMEs into ″hot ″ immune-responsive states. USP18, a negative regulator of type I interferon (IFN) signalling, suppresses immune activation by removing ISG15 from target proteins and disrupting IFNAR–STAT2 interactions. Here, we show that both genetic ablation and catalytic inactivation of USP18 enhance type I IFN signalling in TNBC cells, leading to sustained STAT1/STAT2 phosphorylation. This increased IFN responsiveness promotes antigen presentation via MHC–I upregulation and increases expression of pro-apoptotic ligands such as Fas. Proteomic profiling and immunophenotyping revealed that USP18 inhibition in vivo reduces tumour growth and increases immunogenicity, accompanied by cancer-immune infiltration modulation including CD8⁺ T cells, Th1 cells, NK cells, cDC1, and pro-inflammatory M1-like macrophages. These changes reflect a shift in the TME from an immunosuppressive to an immunostimulatory state, driven by heightened and prolonged type I IFN signalling. Our findings highlight the therapeutic potential of USP18 inhibition to convert immunologically ″cold ″ tumours into ″hot ″ tumours, by enhancing IFN–driven immune activation and improving the efficacy of immunotherapy in TNBCs.