Amyloid-beta (A beta) has been identified as a key component in Alzheimer's disease (AD). Significant in vitro and human pathological data suggest that intraneuronal accumulation of A beta peptides plays an early role in the neurodegenerative cascade. We hypothesized that targeting an antibody-based therapeutic to specifically abrogate intracellular A beta accumulation could prevent or slow disease onset. A beta 42-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 A beta 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 A beta. Chronic expression of the ER-targeted intrabody (IB) led to partial clearance of A beta 42 deposits and interestingly, in reduced staining for a pathologic phospho-Tau epitope (Thr231). This approach may provide insights into the functional relevance of intraneuronal A beta accumulation in early AD and potentially lead to the development of new therapeutics.