Anti-tau VHH therapy against PHF6: a safe approach to slowing the phenotype of tau pathology.

Background: Tauopathies share common features, including tau aggregation, which plays a central role in neurodegeneration. However, these disorders are highly heterogeneous, particularly in the spread of pathological tau species between cells. In Alzheimer's disease, intracellular tau aggregation is followed by a propagation between cells leading to a hierarchical pathway of neurodegeneration, whereas in other tauopathies, such as progressive supranuclear palsy (PSP), pathological tau remains largely confined within neurons and exhibits more limited spread. This variability raises the question of whether tailored treatments for each tauopathy might offer more therapeutic benefit. Hence, we designed two different immunological approaches using single domain antibody fragments, also called VHHs, to target intracellular and extracellular tau. This study aims to first evaluate the safety of these immunological tools on physiological tau and then their potential to slow disease progression.

Methods: We selected the pro-aggregative tau hexapeptide PHF6 as a common target for the VHHs. These VHHs were cloned in viral vectors allowing to compare two different expression systems: 1) intracytosolic expression to prevent tau accumulation (intraVHH) and 2) secretion into the interstitial fluid, to prevent tau spreading (extraVHH). By stereotactic injection of viral vectors, these VHHs were expressed in the brain of transgenic or wild-type mice and three readouts were studied: behavior, brain imaging and tau lesions.

Results: We validated the correct addressing of intra- and extraVHHs. These two constructs were not associated with adverse effects, even in the absence of tau overexpression, in wild-type mice. Their efficacy was demonstrated in transgenic mouse tau models, either chronic long-term or in acute seeding with injections of human brain homogenates from Alzheimer's disease patients. They both can slow down several pathological effects (i.e. cognitive deficits, cerebral atrophy and neuronal hyperphosphorylation of tau).

Conclusions: This study is a proof of concept demonstrating that VHHs can be engineered to reduce both intra- and extracellular tau pathologies without major adverse effects, making them of interest for therapeutic applications.