Fig. 2

Brain metastasis and cancer immunotherapies. (A) In addition to metastasized tumor cells, the tumor microenvironment (TME) of brain metastasis consists of unique cell type(s), including astrocytes/activated astrocytes, microglia cells, myeloid-derived suppressor cells (MDSCs), and neurons. The neurological secretions from astrocytes/microglial cells support the growth of metastasized cancer cells in the brain microenvironment. There are multiple mechanisms for brain metastatic cancer cells to evade immune cells; it could be through upregulating the PD-L1/PD-1 axis in cancer/immune cells, overexpression of surface receptors such as CD44, or by secreting exosomes or other metabolites that enhance the recruitment of immunosuppressive regulatory T cells (Treg) cells. (B) Various types of immunotherapies, including anti-PD-L1/anti-PD1 and anti-CTLA4, are currently being evaluated for the treatment of brain metastasis. The ITs enhance the activity of T effector cells (CD8+) or induce tumor antigen presentation to cause immune activation, which promotes cancer cell death.