Table 1.

Monocytic populations in the central nervous system and CNS metastases

Population	Markers	Brief description of functions	Role in metastases
Microglia	CD11b, CD68, CD115, CD206, CX3CR1, F4/80, Fcrls++, Iba1+, MerTK, P2ry12++, Siglec-H	In steady state: maintain homeostasis, exhibit a phagocytic activity, have low activity as antigen presenting cells
	CD11b, CD45, CD64, CD68, CD115, CD163+/-, CD206, CX3CR1, F4/80, Fcrls+, Iba1++, P2ry12+	Upon activation: depending on stimuli polarization to an inflammatory of pro-tumorigenic phenotype: morphological changes, phagocytosis and antigen presentation, re-organization of ECM, cytokine and chemokine production	Tumor supporting phenotype degradation of ECM, modulating adaptive immune response, aiding in angiogenesis
CNS border-associated macrophages (BAMs)	CD11b, CD45, CD68, CD115, CD163, CD206, CX3CR1, F4/80	In steady state: maintain integrity of BBB. Upon pathological activation: modulate BBB integrity and vascular permeability, interact with circulating immune cells, produce reactive oxygen species.	Tumor supporting phenotype: facilitation of recruitment of cancer cells and immune cells via BBB
Infiltrating bone marrow-derived macrophages	CD11b, CD45, CD49D, CD64, CD68, CD115, CD163+/-, CD206, CCR2, F4/80, Ly6C	Infiltrate brain parenchyma mainly after breakdown of BBB; immunoregulatory and immunosuppressive functions; cytoprotective activity.	Tumor supporting phenotype: ECM remodeling, immune suppression, enhancement of angiogenesis.