Titration of AAV-2 particles via a novel capsid ELISA: packaging of genomes can limit production of recombinant AAV-2

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Abstract
We demonstrate the rapid and reliable quantification of physical AAV-2 (adeno-associated virus type 2) particles via a novel ELISA based on a monoclonal antibody which selectively recognizes assembled AAV-2 capsids. Titration of a variety of recombinant AAV-2 (rAAV) preparations revealed that at least 80% of all particles were empty, compared with a maximum of 50% in wild-type AAV-2 stocks, indicating that the recombinant genomes were less efficiently encapsidated. This finding was confirmed upon titration of CsCl gradient fractions from recombinant and wild-type AAV-2 stocks. ELISA-based measurement of capsid numbers revealed a large number of physical particles with low densities corresponding to empty capsids in the recombinant, but not in the wild-type AAV-2 preparations. Moreover, additional expression of VP proteins during rAAV production was found to result in an excessive capsid formation, whilst yielding only minor increases in DNA-containing or transducing rAAV particles. We conclude that encapsidation of viral genomes rather than capsid assembly can be limiting for rAAV production, provided that a critical level of VP expression is maintained. The feasibility of quantifying AAV-2 capsid numbers via the ELISA allows determination of physical to DNA-containing or infectious particle ratios. These are important parameters which should help to optimize and standardize the production and application of recombinant AAV-2.