To explore spatial differences and specificity of brain 3R/4R tau seeding and its relationship with histopathological burden using real time quaking induced conversion (RT-QuIC) and digital histological immunohistochemistry in patients with tauopathies.

Alzheimer’s Disease (AD) is pathologically marked by the accumulation of 3R/4R tau whereas Progressive Supranuclear Palsy (PSP) has accumulations of 4R tau species. Recently 3R/4R isoform selective RT-QuIC assays for brain homogenates have been developed but topographic differences in seeding activity and their correlations with histopathology remain unknown.

34 autopsy-validated subjects with AD (n=11), PSP (n=13), AD/PSP mixed pathologies (n=6), and control cases (n=4) were studied. Frozen samples from the hippocampus (HP) and mid frontal cortex (MF) were assayed using 3R/4R RT-QuIC, generating seeding doses needed to turn 50% of wells positive (SD50). Each section was immunostained with AD-specific tau inclusions (GT38) and total phosphorylated tau (PHF1).  We compared the relationship of HP and MF seeding activity with GT38 and PHF1 inclusion burden as measured by digital image analysis methods.

A range of Braak tau stages were represented (B0=2, B1=14, B2=2, B3=15, missing=1). GT38 signal in HP and MF and PHF1 signal in the HP increased over Braak stages (F=10.6-13.5, p>0.0007) while PHF1 signal in MF did not (p=0.2). GT38 and PHF1 signal was higher in the HP than MF (Z=3.98, 5.32, p<0.001). 3R/4R SD50 from HP and MF increased across Braak stages (F=12.2, 37.0, p=0.0004, <0.0001) and was higher in the HP than MF (z=5.9, p<0.0001). There were significant correlations of GT38 staining and 3R/4R seeding activity (HP: R2=0.47. MF: R2=0.48, P<0.001 for both).  PHF1 signal from MF and HP did not correlate with 3R/4R RT-QuIC seeding in PSP cases (p=0.4, 0.5).

3R/4R selective RT-QuIC seeding activity recapitulate spatial patterns expected in AD progression and correlate with global and regional histopathological burden of AD tau pathology.