Detailed Protocol for the Novel and Scalable Viral Vector Upstream Process for AAV Gene Therapy Manufacturing

Recombinant adeno-associated viral (rAAV) vector-based gene therapy has been adapted for use in more than 100 clinical trials. This is mainly because of its excellent safety profile, ability to target a wide range of tissues, stable transgene expression, and significant clinical benefit. However, the major challenge is to produce a high-titer, high-potency vector to achieve a better therapeutic effect. Even though the three plasmid-based transient transfection method is currently being used for AAV production in many clinical trials, there are complications associated with scalability and it is not cost-effective. Other methods require either large-scale production of two herpes simplex viruses, rHSV-RepCap and rHSV-GOI (gene of interest), with high titers, or a stable cell line with high titer wild-type adenovirus infection. Both of these options make the process even more complex. To address this issue, we have developed a stable cell line-based production with the use of only one rHSV-RepCap virus. Using this new methodology in small-scale production, we achieved ∼1-6 E + 04 vg/cell of AAV9 in the top producer clones. Large-scale production in 10-CS (10-Cell Stack) of one of the top producing clones resulted in ∼1-2 E + 13 vg/10-CS with 50% of full capsid ratio after purification. This method could potentially be adapted to suspension cells. The major advantage of this novel methodology is that by using the rHSV-RepCap virus, high titer AAV can be produced with any GOI containing a stable adherent or suspension producer cell line. The use of this AAV production platform could be beneficial for the treatment of many diseases.

Rapid Lentiviral Vector Producer Cell Line Generation Using a Single DNA Construct

Stable suspension producer cell lines for the production of vesicular stomatitis virus envelope glycoprotein (VSVg)-pseudotyped lentiviral vectors represent an attractive alternative to current widely used production methods based on transient transfection of adherent 293T cells with multiple plasmids. We report here a method to rapidly generate such producer cell lines from 293T cells by stable transfection of a single DNA construct encoding all lentiviral vector components. The resulting suspension cell lines yield titers as high as can be achieved with transient transfection, can be readily scaled up in single-use stirred-tank bioreactors, and are genetically and functionally stable in extended cell culture. By removing the requirement for efficient transient transfection during upstream processing of lentiviral vectors and switching to an inherently scalable suspension cell culture format, we believe that this approach will result in significantly higher batch yields than are possible with current manufacturing processes and enable better patient access to medicines based on lentiviral vectors.

DROSHA Knockout Leads to Enhancement of Viral Titers for Vectors Encoding miRNA-Adapted shRNAs

RNAi-based gene therapy using miRNA-adapted short hairpin RNAs (shRNA^miR) is a powerful approach to modulate gene expression. However, we have observed low viral titers with shRNA^miR-containing recombinant vectors and hypothesized that this could be due to cleavage of viral genomic RNA by the endogenous microprocessor complex during virus assembly. To test this hypothesis, we targeted DROSHA, the core component of the microprocessor complex, and successfully generated monoallelic and biallelic DROSHA knockout (KO) HEK293T cells for vector production. DROSHA KO was verified by polymerase chain reaction (PCR) and western blot analysis. We produced lentiviral vectors containing Venus with or without shRNA hairpins and generated virus supernatants using DROSHA KO packaging cells. We observed an increase in the fluorescence intensity of hairpin-containing Venus transcripts in DROSHA KO producer cells consistent with reduced microprocessor cleavage of encoded mRNA transcripts, and recovery in the viral titer of hairpin-containing vectors compared with non-hairpin-containing constructs. We confirmed the absence of significant shRNA^miR processing by northern blot analysis and showed that this correlated with an increase in the amount of full-length vector genomic RNA. These findings may have important implications in future production of viral shRNA^miR-containing vectors for RNAi-based therapy.

Characteristics of Minimally Oversized Adeno-Associated Virus Vectors Encoding Human Factor VIII Generated Using Producer Cell Lines and Triple Transfection

Several ongoing clinical studies are evaluating recombinant adeno-associated virus (rAAV) vectors as gene delivery vehicles for a variety of diseases. However, the production of vectors with genomes >4.7 kb is challenging, with vector preparations frequently containing truncated genomes. To determine whether the generation of oversized rAAVs can be improved using a producer cell-line (PCL) process, HeLaS3-cell lines harboring either a 5.1 or 5.4 kb rAAV vector genome encoding codon-optimized cDNA for human B-domain deleted Factor VIII (FVIII) were isolated. High-producing "masterwells" (MWs), defined as producing >50,000 vg/cell, were identified for each oversized vector. These MWs provided stable vector production for >20 passages. The quality and potency of the AAVrh8R/FVIII-5.1 and AAVrh8R/FVIII-5.4 vectors generated by the PCL method were then compared to those prepared via transient transfection (TXN). Southern and dot blot analyses demonstrated that both production methods resulted in packaging of heterogeneously sized genomes. However, the PCL-derived rAAV vector preparations contained some genomes >4.7 kb, whereas the majority of genomes generated by the TXN method were ≤4.7 kb. The PCL process reduced packaging of non-vector DNA for both the AAVrh8R/FVIII-5.1 and the AAVrh8R/FVIII-5.4 kb vector preparations. Furthermore, more DNA-containing viral particles were obtained for the AAVrh8R/FVIII-5.1 vector. In a mouse model of hemophilia A, animals administered a PCL-derived rAAV vector exhibited twofold higher plasma FVIII activity and increased levels of vector genomes in the liver than mice treated with vector produced via TXN did. Hence, the quality of oversized vectors prepared using the PCL method is greater than that of vectors generated using the TXN process, and importantly this improvement translates to enhanced performance in vivo.

Recent advances in recombinant protein production: BAC-based expression vectors, the bigger the better

Designing appropriate expression vectors is one of the critical steps in the generation of stable cell lines for recombinant protein production. Conventional expression vectors are severely affected by the chromatin environment surrounding their integration site into the host genome, resulting in low expression levels and transgene silencing. In the past, a new generation of expression vectors and different strategies was developed to overcome the chromatin effects. Bacterial artificial chromosomes (BACs) are cloning vectors capable of accommodating up to 350 Kb. Thus, BACs can carry a whole eukaryotic locus with all the elements controlling the expression of a gene; therefore, BACs harbor their own chromatin environment. Expression vectors based on BACs containing open/permissive chromatin loci are not affected by the chromatin surrounding their integration site in the host cell genome. Consequently, BAC-based expression vectors containing the appropriate loci confer predictable and high levels of expression over time. These properties make BAC-based expression vectors a very attractive tool applied to the recombinant protein production field.