AAV Empty Capsids: For Better or for Worse?

Adeno-associated virus (AAV) vectors expressing therapeutic genes continue to demonstrate great promise for treatment of a wide variety of diseases. A peculiar feature of AAV vector generation in cell culture is the formation of an excess of “empty” capsids, which lack the vector genome and are therefore unable to provide a therapeutic benefit. The effect of the empty capsids on clinical outcome is unclear, but the potential for increasing innate or adaptive immune responses to the vector is a major concern.