The CNS mononuclear phagocyte system in health and disease.

CNS-resident macrophages-including parenchymal microglia and border-associated macrophages (BAMs)-contribute to neuronal development and health, vascularization, and tissue integrity at steady state. Border-patrolling mononuclear phagocytes such as dendritic cells and monocytes confer important immune functions to the CNS, protecting it from pathogenic threats including aberrant cell growth and brain malignancies. Even though we have learned much about the contribution of lymphocytes to CNS pathologies, a better understanding of differential roles of tissue-resident and -invading phagocytes is slowly emerging. In this perspective, we propose that in CNS neuroinflammatory diseases, tissue-resident macrophages (TRMs) contribute to the clearing of debris and resolution of inflammation, whereas blood-borne phagocytes are drivers of immunopathology. We discuss the remaining challenges to resolve which specialized mononuclear phagocyte populations are driving or suppressing immune effector function, thereby potentially dictating the outcome of autoimmunity or brain cancer.