Breaking peripheral immune tolerance to CNS antigens in neurodegenerative diseases: Boosting autoimmunity to fight-off chronic neuroinflammation

• Circulating leukocytes are needed for CNS maintenance and repair.
• Breaking peripheral immune tolerance to fight-off neurodegenerative conditions.
• Recruitment of immunoregulatory cells to the CNS is orchestrated by autoimmunity.
• Immunosuppressive drugs are counterproductive in neurodegenerative conditions.

Immune cell infiltration to the brain's territory was considered for decades to reflect a pathological process in which immune cells attack the central nervous system (CNS); such a process is observed in the inflammatory autoimmune disease, multiple sclerosis (MS). As neuroinflammatory processes within the CNS parenchyma are also common to other CNS pathologies, regardless of their etiology, including neurodegenerative disorders such as Alzheimer's disease (AD) and Amyotrophic lateral sclerosis (ALS), these pathologies have often been compared to MS, a disease that benefits from immunosuppressive therapy. Yet, over the last decade, it became clear that autoimmunity has a bright side, and that it plays a pivotal role in CNS repair following damage. Specifically, autoimmune T cells were found to facilitate CNS healing processes, such as in the case of sterile mechanical injuries to the brain or the spinal cord, mental stress, or biochemical insults. Even more intriguingly, autoimmune T cells were found to be involved in supporting fundamental processes of brain functional integrity, such as in the maintenance of life-long brain plasticity, including spatial learning and memory, and neurogenesis. Importantly, autoimmune T cells are part of a cellular network which, to operate efficiently and safely, requires tight regulation by other immune cell populations, such as regulatory T cells, which are indispensable for maintenance of immunological self-tolerance and homeostasis. Here, we suggest that dysregulation of the balance between peripheral immune suppression, on one hand, and protective autoimmunity, on the other, is an underlying mechanism in the emergence and progression of the neuroinflammatory response associated with chronic neurodegenerative diseases and brain aging. Mitigating chronic neuroinflammation under these conditions necessitates activation, rather than suppression, of the peripheral immune response directed against self. Accordingly, we propose that fighting off acute and chronic neurodegenerative conditions requires breaking peripheral immune tolerance to CNS self-antigens, in order to boost protective autoimmunity. Nevertheless, the optimal approach to fine tune such immune response must be individually explored for each condition.