BackgroundA few tau immunotherapies are now in clinical trials with several more likely to be initiated in the near future. A priori, it can be anticipated that an antibody which broadly recognizes various pathological tau aggregates with high affinity would have the ideal therapeutic properties. Tau antibodies 4E6 and 6B2, raised against the same epitope region but of varying specificity and affinity, were tested for acutely improving cognition and reducing tau pathology in transgenic tauopathy mice and neuronal cultures.ResultsSurprisingly, we here show that one antibody, 4E6, which has low affinity for most forms of tau acutely improved cognition and reduced soluble phospho-tau, whereas another antibody, 6B2, which has high affinity for various tau species was ineffective. Concurrently, we confirmed and clarified these efficacy differences in an ex vivo model of tauopathy. Alzheimer’s paired helical filaments (PHF) were toxic to the neurons and increased tau levels in remaining neurons. Both toxicity and tau seeding were prevented by 4E6 but not by 6B2. Furthermore, 4E6 reduced PHF spreading between neurons. Interestingly, 4E6’s efficacy relates to its high affinity binding to solubilized PHF, whereas the ineffective 6B2 binds mainly to aggregated PHF. Blocking 4E6's uptake into neurons prevented its protective effects if the antibody was administered after PHF had been internalized. When 4E6 and PHF were administered at the same time, the antibody was protective extracellularly.ConclusionsOverall, these findings indicate that high antibody affinity for solubilized PHF predicts efficacy, and that acute antibody-mediated improvement in cognition relates to clearance of soluble phospho-tau. Importantly, both intra- and extracellular clearance pathways are in play. Together, these results have major implications for understanding the pathogenesis of tauopathies and for development of immunotherapies.Electronic supplementary materialThe online version of this article (doi:10.1186/s13024-016-0126-z) contains supplementary material, which is available to authorized users.

Antibodies or their derivatives as imaging probes for pathological tau protein have great potential, but have not been well studied. In particular, smaller, single-chain-variable antibody fragments (scFv's) are attractive for detecting tau lesions in live subjects. Here, we generated libraries of scFv's and identified numerous phospho-tau-selective scFv's. Peripheral injection of one of these scFv's consistently resulted in a strong in vivo brain signal in transgenic tauopathy mice, but not in wild-type or amyloid-␤ plaque mice. The parent tau antibody provided similar results, albeit with a weaker signal intensity. The imaging signal correlated very well with colocalization of the probe with intraneuronal tau aggregates. Both were associated with markers of endosomes, autophagosomes, and lysosomes, suggesting their interaction in these degradation pathways. Such specific antibody-derived imaging probes have great potential as diagnostic markers for Alzheimer's disease and related tauopathies.

BackgroundNumerous studies have shown that the extent of pathological tau protein in the brain correlates strongly with the severity of Alzheimer’s dementia at the time of death, and similar association with functional phenotypes have been seen in other tauopathies. In recent years, several dye‐based imaging probes with selectivity for tau aggregates have been developed. However, all are β‐sheet binders with varying affinity for different amyloids, and thereby lack specificity for tau lesions. In contrast, single domain antibodies (sdAbs) are specific for their target and because of their small size have much greater access into the brain than standard whole antibodies.MethodTo examine sdAbs’ suitability as non‐invasive in vivo imaging probes for tauopathies, we immunized a llama with a full‐length recombinant tau protein followed by boosters with human brain‐derived pathological tau proteins. Phage display libraries were generated from its peripheral blood mononuclear cells and screened for binding to tau proteins using various assays, resulting in a prototype sdAb deemed promising for in vivo imaging. Subsequently, this anti‐tau sdAb was labeled with a near‐infrared tag (680 nm), and injected intravenously in tauopathy and control mice, followed by imaging using an In Vivo Imaging System (IVIS). After the imaging, brains were extracted for tissue analysis.ResultIntravenous injection of labeled sdAb resulted in a strong in vivo brain signal detected through the intact head in two different models of transgenic tauopathy mice, but not in wild‐type or transgenic α‐synucleinopathy mice. Importantly, the in vivo brain signal correlated strongly with insoluble (r = 0.9839, p < 0.0001) and soluble (human tau (CP27) and phospho‐tau (PHF1): r = 0.9603 and 0.9459, p < 0.0001) tau protein within the brain. Furthermore, postmortem analysis revealed extensive co‐localization of the sdAb imaging probe with tau aggregates within neurons in the endosomal‐lysosomal system, indicating their interaction in these degradation pathways.ConclusionOverall, this specific sdAb imaging ligand has great potential as an in vivo diagnostic marker for Alzheimer's disease and related tauopathies, and sdAb‐based imaging may be applicable to a variety of protein conformational disorders.

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BackgroundPhage-display panning is an integral part of biomedical research. Regular panning methods are sometimes complicated by inefficient detachment of the captured phages from the antigen-coated solid supports, which prompted us to modify. Here, we produce an efficient antigen-specific single chain fragment variable (scFv) antibody by using a target-related molecule that favored selection ofrecombinant antibodies.ResultsTo produce more selective and specific anti-idiotypic scFv-antibodies from a cDNA library, constructed from HM-1 killer toxin (HM-1)-neutralizing monoclonal antibodies (nmAb-KT), the method was modified by using an elution buffer supplemented with HM-1 that shares structural and functional similarities with the active site of the scFv antibody. Competitive binding of HM-1 to nmAb-KT allowed easy and quick dissociation of scFv-displayed phages from immobilized nmAb-KT to select specific anti-idiotypic scFv antibodies of HM-1. After modified panning, 80% clones (40/50) showed several times higher binding affinity to nmAb-KT than regular panning. The major populations (48%) of these clones (scFv K1) were genotypically same and had strong cytocidal activity against Saccharomyces and Candida species. The scFv K1 (Kd value = 4.62 × 10-8 M) had strong reactivity toward nmAb-KT, like HM-1 (Kd value = 6.74 × 10-9 M) as judged by SPR analysis.ConclusionThe scFv antibodies generated after modified subtractive panning appear to have superior binding properties and cytocidal activity than regular panning. A simple modification of the elution condition in the phage-display panning protocol makes a large difference in determining success. Our method offers an attractive platform to discover potential therapeutic candidates.