Immunotherapy for targeting tau pathology in Alzheimer's disease and tauopathies

The drawbacks of amyloid immunotherapy, including the development of encephalitis, the lack of clinical improvement and of any effect on neurofibrillary tangles (NFTs), coupled with the central role of NFTs in dementia, may point that clearance of amyloid pathology is not sufficient for improving the dementia symptoms in Alzheimer's disease (AD) patients. This further supported the concept that immunotherapy targeting the NFT proteinous aggregates may be preferential. Yet, the encephalitogenicity of full-length tau protein under a proinflammatory CNS milieu, reported by us in immunized mice, demands to carefully and selectively target pathological tau, while not the normal functional tau, and assuring both efficacy (anti-NFT effect) as well as safety (free of encephalitis) of a potential vaccine. Accumulating evidence from animal studies shows that tau-immunotherapy, targeting selectively pathological tau, particularly the phosphorylated-tau isoforms, reduces the tau-pathology and improves the symptoms of dementia. These findings are based on studies from different research groups, including our laboratory, conducted in different animal models and using various immunization protocols. There is also evidence that the decrease in NFTs is antibody-mediated involving the endosomal/ lysosomal pathway. No adverse effects were reported by the research groups, including also our study in which mice were immunized with a single injection of phosphorylated-tau peptide under a CNS proinflammatory milieu. In this review, I discuss the studies reported in this field, focusing on different approaches, different immunization protocols and mechanistic aspects, with a focus on the promising efficacy of the tau-immunotherapy, while addressing the safety issues already in the preclinical stage, before progressing to clinical trials.