Myeloid cells in the microenvironment of brain metastases

Brain metastasis (BrM) from peripheral solid tumors has a high mortality rate and remains a daunting clinical challenge. In addition to the targeting of tumor cells, studies have focused on the regulation of the tumor microenvironment (TME) for BrM treatment. Here, through a review of recent studies, we revealed that myeloid infiltration is a common feature of the TME in BrMs from different primary sites even though the brain is regarded as an immune-privileged site and is always in an immunosuppressive state. Tumor-educated bone marrow progenitors, especially mesenchymal stem cells (MSCs), may impact the brain tropism and and phenotypic switching of myeloid cells. Additionally, chronic inflammation may be key factors regulating the immunosuppressive TME and myeloid cell reprogramming. Here, the role of myeloid cells in the formation of the TME and strategies for targeting these cells before and after BrM are reviewed, emphasizing the potential for the use of myeloid cells in BrM treatment. However, the direct relationship between the neuronal system and myeloid cell filtration is still unclear and worthy of further study.