Figure 1.

The cellular environment in the healthy brain and BrM. The healthy brain parenchyma consists of resident cell types, including neurons, astrocytes, oligodendrocytes, and cells forming the vasculature (endothelial cells, pericytes). While microglia represent the sole immune cells within the parenchyma, border-associated areas of the brain (e.g., meninges, perivascular areas) harbor every other cell type of the immune system. In contrast, brain metastasis (right) induce the recruitment of all types of myeloid and lymphoid immune cells from the periphery. Tumor-associated macrophages (TAMs) represent a heterogeneous pool of myeloid cells, which consist of brain-resident microglia, as well as monocytes, and monocyte-derived macrophages from the periphery. Recent studies further suggest a partial involvement of recruited CNS/border-associated macrophages (BAMs).