. Decreased amyloid-β and increased neuronal hyperactivity by immunotherapy in Alzheimer's models. Nat Neurosci. 2015 Dec;18(12):1725-7. Epub 2015 Nov 9 PubMed.

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  1. This study examined the effects of chronic administration of two different anti-amyloid antibodies, 3D6 and β1, in two cohorts of two different mouse strains, PDAPP and Tg2576, which both stand as models for the amyloid component of Alzheimer’s disease. Although the study-design appears sub-optimal, the unexpected and interesting outcomes of both sets of experiments are (i) the increased neuronal activity induced by both antibodies 3D6 and b1, as measured by Ca2+imaging, and (ii) the lack of relation to amyloid burden, which was respectively decreased and unchanged by the two antibodies. These observations exclude also a potential gender effect, because the PDAPP cohorts contained only females and the Tg2576 were all male mice.

    The surprise observation was that chronic treatment with either 3D6 or β1 triggered neuronal hyperactivity with higher frequency and in more neurons in both mutant APP mice. Higher synchronous firing in about half of the mice is taken by the authors to reflect the known incidence in AD patients of mild to severe epileptic symptoms. Seizure activity is evident in several amyloid mouse models and is held responsible for their high early death rate, and is, interestingly, strongly dependent on their genetic background. 

    There is no question that the outcome here is the opposite of what one expected, even more so because the exacerbated neuronal activity induced by both antibodies is unrelated to their effect on the amyloid burden in the three cohorts of old amyloid mice analyzed. While the observations are undisputable, much less so is the molecular mechanism inferred by the authors.

    The pathology-related parameter measured as “amyloid burden,” equated to “amyloid plaques” in the text, was measured by immunohistochemistry (IHC) using the 3D6 antibody. This is actually the same as used for the treatment, which is somewhat remarkable. Biochemical data are provided only for soluble and insoluble Aβ40/42 in the Tg2576-β1 study, which are not affected. Biochemical data on various subtypes of amyloid peptides, from 38 to 42 and/or N-truncated or otherwise derivatized, and of the physical form known as oligomeric, would have been welcome. Their relevance eclipses that of “amyloid burden,” which is more and more depreciated as contributing essentially to the actual pathological process and cognitive demise in AD. 

    The highly scattered, variable levels of amyloid burden observed (Fig. 1a and Suppl. Fig. 2a) demonstrate this point even more since amyloid burden did not correlate with Ca2+ in the same mice (compare Fig 1a and b and Suppl Fig 2a and b). A scatterplot of both sets of data would have been illustrative and much appreciated. 

    Besides the molecular and physical diversity of amyloid peptides, we must consider intact APP as well as its major metabolites resulting from cleavage by BACE and ADAM10-C99 or beta-stubs, and C83 or alpha-stubs, respectively. Intact APP and C99 fragments are recognized by the 3D6 and β1 antibodies, which then would eventually decorate neurons and/or become internalized, contributing to the observed Ca2+ effects. Finally, one must also not forget the most recently reported metabolic cleavage of APP, called η-cleavage, as potentially contributing to the proposed mechanism involving cell-bound APP-metabolites. 

    Secondarily, the common induction of high neuronal activity by both antibodies was not observed in WT mice, nor was it related to inflammation, as measured by levels of pro-inflammatory cytokines in the brains of untreated and treated Tg2576 mice and in acute experiments using LPS and dexamethasone.

    While the contribution of inflammation should not be considered as refuted by the data, the lack of human APP and all its metabolites named above in WT mice would explain the complete absence of any effects of these antibodies and supports the hypothesis that neuroinflammation was not involved.

    Not least, the study offers advice or even warning about clinical applications of passive vaccination in AD patients, and forces us to (re)consider the importance of the selected epitope on the amyloid sequence to avoid or minimalize side effects. In this respect one is reminded that the Crenezumab antibody binds to the middle part of the amyloid peptide, inviting it to be tested in the same context as this study.

    View all comments by Fred Van Leuven
  2. I wonder whether the explanation that these antibodies bind partially with APP and APP CTF, which are heavily overexpressed in these mouse models, does not explain this finding. I would recommend expanding this study to include the knock-in mice from Takaomi Saido's group before making conclusions about what happens in people, in whom APP expression is much lower.

    View all comments by Bart De Strooper

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