Tissue-specific immunosuppressive and proliferating macrophages fuel early metastatic progression of human colorectal cancer to liver

Early synchronous colorectal liver metastasis (CRLM) represents a clinical condition characterized by the simultaneous presence of primary colorectal cancer (CRC) and metastatic liver lesions (CRLM). The present study characterizes the tissues-specific transcriptomes, phenotypes, and functional relevance of tumor associated macrophages (TAMs) within the tumor microenvironment (TME) of CRC and CRLM specimens in patients who underwent simultaneous surgical removals of these malignancies. The high-throughput single-cell transcriptional analysis revealed an inverse ratio of inflammatory and immunoregulatory TAMs in CRC and CRLM TMEs, along with significant heterogeneity in both tumoral tissues. Importantly, we found that inflammatory TAMs in CRC express inhibitory ligands that might support the tumor immune escape, thus favoring liver metastatic progression. In contrast, CRLM lesions possess a highly immunosuppressive milieu characterized by large proliferative CTLA4pos immunoregulatory TAMs and by the presence of IL7Rpos cytotoxic TAMs. Higher frequencies of these specific TAM subsets in CRLM are associated with a shorter disease-free survival and worse patient prognosis. The identification and characterization of immunoregulatory TAMs preferentially enriched in CRLM is key for the development of novel immune-therapeutic strategies aimed at boosting anti-cancer immune responses within TME. Single-cell RNA sequencing (scRNA-seq) was performed on samples collected from three synchronous colorectal-liver metastasis patients who underwent simultaneous surgical resection of the primary tumor and liver metastasis. For each patient, matched samples were obtained from core-tumor adjacent tissue regions (peritumor) of the colon (n = 3) and liver (n = 3), as well as from peripheral blood (n = 2). scRNA-seq was performed on FACS-sorted viable/CD45+ immune-enriched populations in the colon and liver, and on peripheral blood mononuclear cells (PBMCs) isolated from peripheral blood.