Extracellular vesicles improve gene transfer with adeno-associated virus vectors

Gene therapy, the ability to treat a disease at the level of nucleic acid, has journeyed from science fiction, to hard lessons learned from early clinical trials, to improved technologies with efficacy in patients for several diseases. Adeno-associated virus (AAV) vectors are currently a leader for direct in vivo gene therapy. To date, AAV is safe in patients, with clinical benefit in trials to treat blindness, hemophilia, and a lipid disorder, with many more trials underway. Despite this remarkable progress, barriers exist for AAV vectors to be effective gene transfer vehicles in all organ/cell targets, as well as patient subpopulations. Extracellular vesicles (EVs, e.g., exosomes, microvesicles) are natural lipid particles released by many cell types. They have been reported to mediate cell to cell communication via transferred contents including proteins, nucleic acids, and metabolites. These properties of EV attracted the attention of Harvard Medical School researchers to help solve certain gene transfer issues encountered by AAV vectors. The researchers made the initial discovery that a subpopulation of AAV vectors isolated from media directly interacted with EVs [referred to as exosome-associated AAV (exo-AAV)]. In following reports, they have demonstrated that exo-AAV has advantages over the conventional AAV vector in areas such as anti-AAV antibody evasion and transduction of cells of the eye and cochlea in preclinical models. The work of others using EVs as therapeutics as well as their continued development of the exo-AAV platform may advance the field towards useful clinical applications.

Exo-AAV2 outperforms conventional AAV2 transduction of retina
following intravitreal (IVT) injection (2 × 10⁹ g.c./eye).

(a) Fundus images at 4 weeks post-injection. The inset in one of the eyes injected with exo-AAV2 shows the same fundus image with lower gain. (b) Left panel: total GFP intensity on the fundus images, **, P < 0.01, Mann–Whitney U-test, numbers in bars represent number of analyzed samples. Right panel: quantitative real-time PCR (qrt-PCR) for GFP mRNA at 4 weeks post-injection. Expression was normalized to GAPDH expression level. *, P < 0.05, Mann–Whitney U-test, numbers in bars represent number of analyzed samples. (c) Two representative sections of retinas from eyes injected with AAV2 (left) or exo-AAV2 (right), scale bar represents 100 µm. (d) Bipolar cell targeting of exo-AAV2 and conventional AAV2. Bipolar cells were stained with antibodies against PKCα (rod bipolar cells, left) or CaBP5 (all bipolar cells, right). Arrows show GFP-positive cells, scale bar represents 20 µm. Higher amounts of bipolar cells were transduced by exo-AAV2 compared to AAV2.