Amyloid-beta (Abeta) has been identified as a key component in Alzheimer's disease (AD). Significant in vitro and human pathological data suggest that intraneuronal accumulation of Abeta peptides plays an early role in the neurodegenerative cascade. We hypothesized that targeting an antibody-based therapeutic to specifically abrogate intracellular Abeta accumulation could prevent or slow disease onset. Abeta42-specific intracellular antibodies (intrabodies) with and without an intracellular trafficking signal were engineered from a previously characterized single-chain variable fragment (scFv) antibody. The intrabodies, one with an endoplasmic reticulum (ER) targeting signal and one devoid of a targeting sequence, were assessed in cells harboring a doxycycline (Dox)-regulated mutant human amyloid precursor protein Swedish mutant (hAPP(swe)) transcription unit for their abilities to prevent Abeta peptide egress. Adeno-associated virus (AAV) vectors expressing the engineered intrabodies were administered to young adult 3xTg-AD mice, a model that develops amyloid and Tau pathologies, prior to the initial appearance of intraneuronal Abeta. Chronic expression of the ER-targeted intrabody (IB) led to partial clearance of Abeta42 deposits and interestingly, in reduced staining for a pathologic phospho-Tau epitope (Thr231). This approach may provide insights into the functional relevance of intraneuronal Abeta accumulation in early AD and potentially lead to the development of new therapeutics.Molecular Therapy (2009); doi:10.1038/mt.2009.174.