Role of complement in neurodegeneration and neuroinflammation

The complement system provides an innate defence mechanism against pathogenic microorganisms. Although viewed for many years as an immune-privileged organ, the central nervous system contains many components of the immune system, including components of the complement system that are synthesized by astrocytes, microglia, and neurons. During the past two decades, a wide range of inflammatory markers, typically absent in the normal elderly population, have been reported in Alzheimer's disease brains. It is becoming evident that sustained brain inflammation might be an essential cofactor in Alzheimer disease and other neurodegenerative disorders such as Parkinson disease, dementia with Lewy bodies, Huntington's and prion diseases. The complement system may be useful in eliminating aggregated and toxic proteins associated with these neurological disorders and thus have a protective effect. However, an exaggerated or insufficient activation of the complement system can have deleterious effect through the activation of microglia, secretion of many proinflammatory cytokines, and generation of oxidative products. The role of complement-mediated inflammation in Alzheimer disease has drawn greater attention recently in view of new therapeutic advances made in the management of the disease. This review is meant to update the role of complement in Alzheimer's disease and other neurodegenerative disorders in view of recent vaccination and immunotherapeutic approaches.