Immune Evasion in Cancer Metastasis: An Unappreciated Role of Monocytes

Metastasis is the leading cause of cancer-related deaths. During the metastatic cascade, cancer cells tightly interact with immune cells influencing each other in the tumor microenvironment and systemically. Monocytes are important components of immune evasion and critical regulators of cancer progression. They circulate through the bloodstream and contribute to the formation of a pro-tumor microenvironment both in the tumor and pre-metastatic niche. Whereas monocyte participation in cancer development and response to therapy has been described extensively, its impact on metastasis remains a completely uncovered area. This review first summarizes data concerning the influence of monocytes on metastasis formation during their presence in the circulation, primary tumor, and pre-metastatic niche. We also highlight the latest examinations into the clinical relevance of targeting monocytes to prevent metastasis.