Metastasis-promoting functions of myeloid cells

The tumor metastatic dissemination and colonization is a highly complex, multi-step process that requires cooperation between cancer and host cells. Among these, cells of myeloid lineage mobilized from the bone marrow are crucial in facilitating tumor metastatic outgrowth. Recent studies indicate that myeloid cells of bone marrow origin, including myeloid-derived suppressor cells (MDSCs), macrophages, and progenitor cells, promote distant metastasis through immunological and non-immunological mechanisms. These cells are key contributors to creating an immunosuppressive microenvironment in the invaded tissue and draining lymph nodes, protecting metastatic cells from immunosurveillance, and promoting resistance to immunotherapy. Furthermore, the myeloid cells mediate the remodeling of the extracellular matrix (ECM) in a metastatic niche via enzymes MMP9 and Hyal2, stimulating angiogenesis and establishing a metastasis-permissive microenvironment. This review describes recent findings demonstrating the metastasis-promoting functions of recruited marrow-derived myeloid cells throughout metastatic colonization and suggests new therapeutic avenues.