Robust, Realistic, Relevant Rejuvenation with Tau-Targeting Immunotherapy

As we’ve noted previously,

Neurofibrillary tangles (NFT — cytoplasmic inclusions composed of phosphorylated and abnormally-cleaved species of  tau protein) accumulate in the aging brain, and at higher levels in Alzheimer’s disease and in vulnerable regions in a range of other neurodegenerative diseases; they are closely associated with neuronal death and with onset of clinical dementing disease. The clearance of neurofibrillary tangles and other intracellular aggregates is a key rejuvenation biotechnology to restore aging brain function.

The priority of a distinct therapy for the removal of tau pathology has become especially clear in light of followup studies in persons receiving  the original, active beta-amyloid [Aβ] vaccine AN1792. On the one hand, vaccine responders’ brains exhibited a nearly complete absence of  Aß pathology at autopsy, along with reduced neuronal loss, and in long-term (4.6 y) followup, a decline on the Disability Assessment for Dementia and Dependence Scale, stabilized hippocampal volume while adjuvant-only controls suffered ongoing declines, and extensive clearance of tau-containing neurites.[references] Yet narrowly cognitive benefits were limited, and more mature tau pathology (NFT and neuropil threads) appeared to be unaffected ([references], and see previous postings).This last finding, combined with the stronger association of NFT burden with clinical disease, recommend NFT clearance as a high-priority (and, ideally, complementary) immunotherapeutic approaches.

In three previous posts,  we’ve discussed progress by on this front by Dr. Hanna Rosenmann’s group at Hadassah University Hospital, Israel, and Dr. Einar M Sigurdsson  and colleagues at New York University, who have tested immunotherapies targeting tau aggregates in preclinical models of neurodegeneration caused by pathological tau species. Heretofore, they have worked with transgenic models express either wild-type human tau or highly aggregation-prone mutant tau species in addition to the native murine tau gene; in such models, they have demonstrated that, somewhat surprisingly, active vaccination with tau species generates tau-targeting antibodies that not only cross the blood-brain barrier, but clear out intraneuronal pathological tau species, via a presumed lysosomal mechanism; in so doing, they have achieved significant reductions in tau pathology and the associated functional deficits. These findings not only support the importance of tau as a therapeutic target, but offer grounds for optimism for the development of tau-targeting rejuvenation biotechnology for human beings.

Amongst the limitations of these findings, however, has been the nature of the model organisms themselves. Some of these models express mutant tau species such as P301S and K257T, which (respectively) cause the human dementing disorder  frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) and a murine tauopathy similar to frontotemporal dementia (FTD). However, these mutated tau species are not present either in Alzheimer’s disease, or in “normal” brain aging (mistakenly so-called). While other models have instead shown similar pathology through transgenic expression of wild-type human tau (htau) superimposed on the native murine tau (and similar benefits from removing it), all such models have to varying degrees shared the significant disadvantage of causing such severe, widespread, and early-onset tau pathology that motion disorders have significantly confounded the evaluation of cognitive function, further emphasizing the limited translatability of the models and introducing uncertainty over the degree to which true cognitive deficits are being induced or alleviated.

Now, Sigurdsson’s group have made a report further advancing the case for the therapeutic importance and  tractability  of pathological tau, by demonstrating similar robust benefits to immunotherapy murine model of tau-based cognitive deficits that are not confounded by brainstem involvement or movement disorders.

The main model in the new study(1) was developed by crossing mice expressing transgenic wild-type htau with a model carrying the human presenilin 1 (PS1) mutation  M146L, on a murine tau knockout background.  “These mice have an earlier onset, at or before 2 months of age, and more rapid progression of tau pathology than the htau mice, while the distribution is similar, with extensive involvement of hippocampal and cortical regions”(1) rather than being concentrated in the brainstem and spinal cord as in previous models. PS1 mutations accelerate tau pathology, the authors suspect, by impairing lysosomal function.(2) These animals develop robust tau pathology consistent with that seen in human Alzheimer’s disease, and exhibit cognitive deficits on several established tests (radial arm maz, object recognition tes, and closed field symmetrical maze) but no impairments of motor function (as evaluated by rotarod, radial arm maze, or traverse beam) — features that make this model especially promising for the screening of potential tau-targeting therapies.

Sigurdsson’s group developed a vaccine based on Tau379–408[P-Ser_{396, 404}], “selected based on its overall immunogenicity and its AD phospho-epitope” and administered “intraperitoneally in 100 μl of alum adjuvant (Adju-Phos, Brenntag Biosector)”.(1) The htau/PS1/mtauKO mice were first immunized at 3-4 mo of age, significantly after the 2 mo disease onset time, followed by 3 biweekly injections and then further booster immunizations monthly thereafter. These animals were compared not only to adjuvant-only htau/PS1/mtauKO mice, but also to adjuvant-only htau/PS1/mtau and htau/mtauKO mice “as additional controls because our preliminary analysis indicated that these models had less pathology than the htau/PS1 [+ mtau knockout] model. … At 7–8 months, the mice went through extensive behavioral testing and were subsequently killed for tissue analyses at 8–9 months of age.”(1)

Vaccinated mice developed strong IgG antibody titers targeting both the phosphorylated tau immunogen and unphosphorylated htau, “As expected, because of the overall high immunogenicity of the immunogen … although a better response is initially generated against the phosphorylated immunogen. Recombinant tau is recognized as well in both controls and immunized mice but to a much lesser degree than the immunogen epitope … Some autoantibodies are detected in controls and are likely also present in the immunized mice. … IgM response was less pronounced, as expected, but was of a similar pattern as the IgG response. Plasma from the immunized mice recognized tau pathology in AD and mouse tissue (data not shown), as we observed previously with this immunogen in [P301L tangle model mice]”(1)

Vaccination with human phosphorylated tau led to the clearance of tau pathology from the brains of immunized mice, as revealed using both immunohistochemical staining with antibodies to the phosphorylated tau species PHF1 and AT8, and Western blot analysis of total and phosphorylated tau species (normalized with actin and total tau levels), evaluated using  (respectively) polyclonal B19 antibody, and with monoclonal antibodies to PHF1 and CP13.