Self-silencing adenovirus enables precise infectious titration of recombinant adeno-associated viral vectors

Robust and accurate quantification of recombinant adeno-associated virus (rAAV) vectors' infectivity is essential for pre-clinical and clinical development of AAV gene therapy programs. The industry standard method for rAAV titration is the 50% tissue culture infectious dose (TCID50) assay using HeLa-based cell lines that stably encode the <i>rep</i> and <i>cap</i> genes from AAV serotype 2. Co-infection with wild-type (WT) adenoviruses provides the helper functions for expression of these genes, and the use of quantitative PCR (qPCR)/droplet digital PCR (ddPCR) serves as the endpoint method for the detection of infectious events. However, TCID50 assays using these HeLa-based <i>rep cap trans</i>-complementing cell lines have traditionally been regarded as challenging due to high variability, stability of the integrated genes, and safety concerns associated with the use of WT helper viruses. Here we developed a novel method for infectious titration of rAAV using our vector "tetracycline-enabled self-silencing adenovirus" (TESSA); we engineered it to deliver and express the AAV2 <i>rep</i> genes and adenoviral helper functions for rAAV genome replication, independent of the cell type. This approach allows the infectious titration of rAAV serotypes in cell lines permissive to adenovirus but without the production of adenoviral particles for improved safety, therefore benefiting GMP analytical requirements for rAAV gene therapies.