The role of peripheral immune cells in the CNS in steady state and disease

Historically, the CNS has been considered immunologically privileged and separated from the peripheral immune system. In this Review, the authors highlight recent advances in our understanding of how the CNS interacts with peripheral immune cells in the context of health and disease. The CNS is protected by the immune system, including cells that reside directly within the CNS and help to ensure proper neural function, as well as cells that traffic into the CNS with disease. The CNS-resident immune system is comprised mainly of innate immune cells and operates under homeostatic conditions. These myeloid cells in the CNS parenchyma and at CNS–periphery interfaces are highly specialized but also extremely plastic cells that immediately react to any changes in CNS homeostasis and become reactive in the context of neurodegenerative disorders such as Alzheimer's disease or Parkinson's disease. However, when the blood–brain barrier is impaired during CNS diseases such as multiple sclerosis or altered with cerebral ischemia, peripheral adaptive and innate immune cells, including monocytes, neutrophils, T cells and B cells, can enter the CNS, where they execute distinct cell-mediated effects. On the basis of these observations, we assess strategies for targeting peripheral immune cells to reduce CNS disease burden.