CCL15 Recruits Suppressive Monocytes to Facilitate Immune Escape and Disease Progression in Hepatocellular Carcinoma

Chemokines play a key role in orchestrating the recruitment and positioning of myeloid cells within the tumor microenvironment. However, the tropism regulation and functions of these cells in hepatocellular carcinoma (HCC) are not completely understood. Herein, by scrutinizing the expression of all chemokines in HCC cell lines and tissues, we found that CCL15 was the most abundantly expressed chemokine in human HCC. Further analyses showed that CCL15 expression was regulated by genetic, epigenetic, and microenvironmental factors, and negatively correlated with patient clinical outcome. In addition to promoting tumor invasion in an autocrine manner, CCL15 specifically recruited CCR1⁺ cells toward HCC invasive margin, approximately 80% of which were CD14⁺ monocytes. Clinically, a high density of marginal CCR1⁺ CD14⁺ monocytes positively correlated with CCL15 expression and was an independent index for dismal survival. Functionally, these tumor-educated monocytes directly accelerated tumor invasion and metastasis through bursting various pro-tumor factors and activating signal transducer and activator of transcription 1/3, extracellular signal-regulated kinase 1/2, and v-akt murine thymoma viral oncogene homolog signaling in HCC cells. Meanwhile, tumor-derived CCR1⁺ CD14⁺ monocytes expressed significantly higher levels of programmed cell death-ligand 1, B7-H3, and T-cell immunoglobulin domain and mucin domain-3 that may lead to immune suppression. Transcriptome sequencing confirmed that tumor-infiltrating CCR1⁺ CD14⁺ monocytes were reprogrammed to upregulate immune checkpoints, immune tolerogenic metabolic enzymes (indoleamine and arginase), inflammatory/pro-angiogenic cytokines, matrix remodeling proteases, and inflammatory chemokines. Orthotopic animal models confirmed that CCL15-CCR1 axis forested an inflammatory microenvironment enriched with CCR1⁺ monocytes and led to increased metastatic potential of HCC cells. Conclusion: A complex tumor-promoting inflammatory microenvironment was shaped by CCL15-CCR1 axis in human HCC. Blockade of CCL15-CCR1 axis in HCC could be an effective anticancer therapy.