In the work, first author Ayodeji Asuni immunized homozygous P301L mutant tau-expressing mice, a strain that has a rapid and massive buildup of hyperphosphorylated, aggregated tau reminiscent of frontotemporal dementia in humans. Vaccination with a phospho-tau peptide led to the production of antibodies that recognized the immunogen, and pathological tau on brain sections. Immunizing young animals starting at 2 months of age led to reductions in tau histopathology at 5 or 8 months, in some brain regions by as much as 96 percent. The greatest reduction was seen in tau aggregates recognized by the MC1 antibody, an earlier stage of tau pathology compared to the paired helical filaments picked up by the PHF1 antibody, which were also reduced but not as dramatically.

The clearance of tau was associated with behavioral improvements in the mice, who normally lose motor skills as tau accumulates. At 5 months of age, the immunized mice displayed better balance on a rotating rod and an elevated beam. The effects diminished with time: by 8 months of age, the immunized animals still performed better than controls, but their abilities had decreased compared to 5 months. Because of their profound motor deficits, extensive cognitive testing is difficult in these mice, and other models will be needed to assess the effect of tau removal on learning and memory.

How might vaccination clear tau from neurons? The researchers showed that the antibodies generated in immunized animals are capable of entering the brain, possibly because tauopathy compromises the blood-brain barrier. Once there, antibodies can be taken up by cells by one of several endocytic pathways, and the investigators show colocalization of antibodies with pathological tau deposits. In the cells, Sigurdsson speculates, the antibodies serve to promote disassembly and degradation of tau assemblies. “In my opinion, the antibodies are more likely to clear smaller assemblies of tau aggregates than tangles," he told ARF. However, a separate, detailed study on tau forms present in brains of treated mice will be required to establish just which forms are targeted by immunotherapy. Nonetheless, tau makes the second case of successful immunization against intracellular neuronal proteins, the first being vaccination against α-synuclein, a cause of Parkinson disease (see ARF related news story).

The P301L homozygous animals used for the studies have an aggressive tau pathology, where tangles show up by 5 months and the animals are nearly completely paralyzed by 12 months. That the vaccine is effective at slowing disease progression leads Sigurdsson to speculate that it may work even better in animals with a slower, more AD-like accumulation of tau. In such models, immunization may have a better chance to clear small tau aggregates prophylactically, he suggests. The researchers plan to follow up their results by testing multiple immunogens and other models of tauopathy, such as the human tau mice where pathology develops more slowly (Andorfer et al., 2003). Also in the works are studies in AD mouse models that have both amyloid and tau pathology. Ultimately, Sigurdsson says, he wants to test combined immunization for amyloid and tau, lobbing a double whammy against the lesions that appear to act together to cause the symptoms of AD.—Pat McCaffrey.

Reference:
Asuni AA, Boutajangout A, Quartermain D, Sigurdsson EM. Immunotherapy targeting pathological tau conformers in a tangle mouse model reduces brain pathology with associated functional improvements. J Neurosci. 2007 Aug 22;27(34):9115-29. Abstract