How colorectal cancer liver metastasis evade the immune system

Many colorectal cancers, especially when they spread to the liver, do not respond to today’s immunotherapies because the tumor builds a strong local immune shield. Our study shows that the hormone TGF-β is a main architect of that shield, and it does it in two coordinated steps. First, TGF-β acts directly on killer T lymphocytes (CD8 T cells) and reduces the entry of their precursors memory-like T cells – from the circulation into the metastatic tumor. When TGFβ signaling is blocked, more CD8 T cells are seen in blood and more T cells reach the tumor. Second, TGF-β “educates” tumor-associated macrophages (a major immune cell type inside these metastases) to become suppressive, especially a SPP1+ macrophages. These macrophage subset promotes a scarlike environment with more collagen and supportive stromal cells, and this environment prevents the newly arrived T cells from multiplying into a large anti-tumor army. Strikingly, removing Spp1 in experimental models that reproduce the human disease makes metastases less able to form and makes many lesions sensitive to immunotherapy by itself, arguing that SPP1 is a key downstream driver. Finally, we identify a macrophage gene program linked to poor outcome in hundreds of patient tumors that is reduced when TGF-β is blocked—suggesting a practical biomarker and pointing to therapies that target both T-cell entry and macrophage-driven immunosuppression.