The Role of Immune Cells in the Brain during Physiological and Pathological Conditions

Abstract

Microglia are the primary resident immune cells in the brain, and because peripheral immune cells cannot access the brain, microglia are responsible for mounting a central response to foreign pathogens. However, these cells also play critical physiological roles in the normal brain, including synaptic pruning during development and synaptic plasticity and remodeling in the adult brain. These effects are mediated by cross-talk between microglia and neurons, including synthesis and release of cytokines that communicate signals to promote homeostatic control of neuronal and microglial function. In addition, environmental and psychological stressors cause activation of microglia as part of an adaptive response, but long-term or extreme stress conditions can lead to inflammation and disruption of neuronal function that contributes to “sickness” and depressive behaviors. Characterization of the signaling pathways that control normal and pathological microglial signaling and elevation of proinflammatory cytokines could lead to improved diagnosis and treatment of stress-related psychiatric illnesses. [ Psychiatr Ann . 2015;45(5):232–239.]