Protective Autoimmunity: A Unifying Model for the Immune Network Involved in CNS Repair

Immune activity in the CNS parenchyma under various acute and chronic neurodegenerative conditions has been often interpreted as a sign of pathological inflammation. The apparent resemblance of the local neuroinflammatory processes to autoimmune diseases, such as multiple sclerosis (MS), generated the view that, despite differences in etiology and pathology, neurodegenerative disorders with a local inflammatory component can benefit from systemic anti-inflammatory therapy. In addition, as CNS self-reactive T cells are associated with the etiology of MS, autoimmunity was assumed to solely reflect pathology, and therefore, was universally linked to autoimmune disease. Yet, it is becoming increasingly clear that CNS-specific T cells, along with circulating and local innate immune cells, can enhance CNS healing processes following non-infectious injuries, or any deviation from homeostasis, including chronic pathological conditions. Here, we discuss the theory of "protective autoimmunity," which describes the activity of an immune cell network encompassing effector and regulatory T cells with specificity for CNS antigens, in CNS maintenance and repair. Such an immune network, evoked in response to external and internal threats, functions in a tightly regulated way, ensuring restoration of the brain's equilibrium and return to homeostasis.