Fig. 2

The recruitment of myeloid-derived cells is a crucial step in metastasis. In the early phase of the primary tumor, it is not ready for myeloid-derived cells (inflammatory monocytes, MDSCs) to recruit primary tumor sites and distant organs (in this case, the lungs; do not walk). Once tumors develop to a certain size and acquire invasiveness, the primary tumor microenvironment secretes cytokines/chemokines (i.e., VEGF, TNFα, and TGF-β) to produce S100A8 in the lungs. These chemokines, such as Ccl2, S100A8/S100A9, can send a signal to myeloid-derived cells in bone marrow as “walk”