Tumor microenvironment (TME) is a complex and continuously evolving structure around tumor cells consisting of innate and adaptive immune cells, stromal cells and new microvessels. Macrophages, differentiating from monocytes are attracted by chemokines and cytokines secreted by tumor cells and naturally migrate to solid tumors. Macrophage infiltration is particularly high in solid tumors with aggressive behavior and poor prognosis. Their number is several times higher than T cells in glioblastoma, ovarian cancer and lung cancer.
In cancer patients, monocytes extravasate and migrate to hypoxic vascular tumor areas and differentiate into macrophages. By releasing multiple immunosuppressive cytokines, growth factors, and metalloproteinases macrophages facilitate tumor angiogenesis, invasion, and metastasis. Due to the complexity of the TME and dense extracellular matrix, currently available therapies are inefficient.