Macrophages and Microglia in Brain Malignancies

Tumour and myeloid cell interactions that occur in the brain are exposed to unique microenvironmental conditions. While tumour outgrowth ultimately has a major impact, the intrinsic properties of the brain initially impose the framework on which subsequent cellular interactions must build. The particularities of the brain are partly anatomical, with strictly controlled traffic of cells and molecules from the blood by virtue of the blood brain barrier (BBB). But they are also due to a unique cellular composition not found outside the nervous system; this includes the neurons and glial cells which have potent immunoregulatory properties. Taken together, these factors have been used to attempt to explain the “immune privilege” of the brain, which is often used to describe low level adaptive immune responses. But in addition immune privilege also impacts on, and is influenced by, the brain myeloid cells which are the subject of this chapter.