Residues on Adeno-associated Virus Capsid Lumen Dictate Interactions and Compatibility with the Assembly-Activating Protein

Implications of AAV affinity column reuse and vector stability on product quality attributes

In this work, the implications of AAV9 capsid design and column reuse on AAV9 vector product quality were assessed with POROS CaptureSelect (PCS) AAVX and AAV9 resins using sf9 insect cell-derived model AAV9 vectors with varying viral protein (VP) ratios. Chromatographic experiments with purified drug substance AAV9 model feeds indicated consistent vector elution profiles, independent of adeno-associated virus (AAV) VP ratio, or cycle number. In contrast, the presence of process impurities in the clarified lysate feeds resulted in clear changes in the elution patterns. This included increased aggregate content in the vector eluates over multiple cycles as well as clear differences in the performance of these affinity resin systems. The AAV9-serotype specific PCS AAV9 column, with lower vector elution pH, resulted in higher aggregate content over multiple cycles as compared to the serotype-independent PCS AAVX column. Further, the results with vectors of varying VP ratio indicated that while one vector type eluate displayed higher aggregation in both affinity columns over column reuse, the eluate with the other vector type did not exhibit changes in the aggregation profile. Interestingly, vector aggregates in the affinity eluates also contained double-stranded DNA impurities and histone proteins, with similar trends to the aggregate levels. This behavior upon column reuse indicates that these host cell impurities are likely carried over to subsequent runs due to incomplete clean-in-place (CIP). These results indicate that feed impurities, affinity resin characteristics, elution pH, column CIP, and vector stability can impact the reusability of AAV affinity columns and product quality.

Unravelling the essential elements for recombinant adeno-associated virus (rAAV) production in animal cell-based platforms

Recombinant adeno-associated viruses (rAAVs) stand at the forefront of gene therapy applications, holding immense significance for their safe and efficient gene delivery capabilities. The constantly increasing and unmet demand for rAAVs underscores the need for a more comprehensive understanding of AAV biology and its impact on rAAV production. In this literature review, we delved into AAV biology and rAAV manufacturing bioprocesses, unravelling the functions and essentiality of proteins involved in rAAV production. We discuss the interconnections between these proteins and how they affect the choice of rAAV production platform. By addressing existing inconsistencies, literature gaps and limitations, this review aims to define a minimal set of genes that are essential for rAAV production, providing the potential to advance rAAV biomanufacturing, with a focus on minimizing the genetic load within rAAV-producing cells.

Development of Stable Packaging and Producer Cell Lines for the Production of AAV Vectors

Today, recombinant adeno-associated virus (rAAV) vectors represent the vector systems which are mostly used for in vivo gene therapy for the treatment of rare and less-rare diseases. Although most of the past developments have been performed by using a transfection-based method and more than half of the authorized rAAV-based treatments are based on transfection process, the tendency is towards the use of stable inducible packaging and producer cell lines because their use is much more straightforward and leads in parallel to reduction in the overall manufacturing costs. This article presents the development of HeLa cell-based packaging/producer cell lines up to their use for large-scale rAAV vector production, the more recent development of HEK293-based packaging and producer cell lines, as well as of packaging cell lines based on the use of Sf9 cells. The production features are presented in brief (where available), including vector titer, specific productivity, and full-to-empty particle ratio.

Adeno-associated virus receptor complexes and implications for adeno-associated virus immune neutralization

Adeno-associated viruses (AAV) are among the foremost vectors for in vivo gene therapy. A number of monoclonal antibodies against several serotypes of AAV have previously been prepared. Many are neutralizing, and the predominant mechanisms have been reported as the inhibition of binding to extracellular glycan receptors or interference with some post-entry step. The identification of a protein receptor and recent structural characterization of its interactions with AAV compel reconsideration of this tenet. AAVs can be divided into two families based on which domain of the receptor is strongly bound. Neighboring domains, unseen in the high-resolution electron microscopy structures have now been located by electron tomography, pointing away from the virus. The epitopes of neutralizing antibodies, previously characterized, are now compared to the distinct protein receptor footprints of the two families of AAV. Comparative structural analysis suggests that antibody interference with protein receptor binding might be the more prevalent mechanism than interference with glycan attachment. Limited competitive binding assays give some support to the hypothesis that inhibition of binding to the protein receptor has been an overlooked mechanism of neutralization. More extensive testing is warranted.

Changing trends in the development of AAV-based gene therapies: a meta-analysis of past and present therapies

Gene therapy has seen a transformation from a proof-of-concept approach to a clinical reality over the past several decades, with adeno-associated virus (AAV)-mediated gene therapy emerging as the leading platform for in vivo gene transfer. A systematic review of AAV-based gene therapies in clinical development was conducted herein to determine why only a handful of AAV-based gene therapy products have achieved market approval. The indication to be treated, route of administration and vector design were investigated as critical factors and assessed for their impact on clinical safety and efficacy. A shift in recent years towards high-dose systemic administration for the treatment of metabolic, neurological and haematological diseases was identified, with intravenous administration demonstrating the highest efficacy and safety risks in clinical trials. Recent years have seen a decline in favour of traditional AAV serotypes and promoters, accompanied by an increase in favour and higher clinical success rate for novel capsids and tissue-specific promoters. Furthermore, a meta-analysis was performed to identify factors that may inhibit the translation of therapeutic efficacy from preclinical large animal studies to first-in-human clinical trials and a detrimental effect on clinical efficacy was associated with alterations to administration routes.

Adeno-Associated Virus (AAV) Gene Delivery: Dissecting Molecular Interactions upon Cell Entry

Human gene therapy has advanced from twentieth-century conception to twenty-first-century reality. The recombinant Adeno-Associated Virus (rAAV) is a major gene therapy vector. Research continues to improve rAAV safety and efficacy using a variety of AAV capsid modification strategies. Significant factors influencing rAAV transduction efficiency include neutralizing antibodies, attachment factor interactions and receptor binding. Advances in understanding the molecular interactions during rAAV cell entry combined with improved capsid modulation strategies will help guide the design and engineering of safer and more efficient rAAV gene therapy vectors.

Cellular pathways of recombinant adeno-associated virus production for gene therapy

Recombinant adeno-associated viruses (rAAVs) are among the most important vectors for in vivo gene therapies. With the rapid development of gene therapy, current rAAV manufacturing capacity faces a challenge to meet the emerging demand for these therapies in the future. To examine the bottlenecks in rAAV production during cell culture, we focus here on an analysis of cellular pathways of rAAV production, based on an overview of assembly mechanisms first in the wild-type (wt) AAV replication and then in the common methods of rAAV production. The differences analyzed between the wild-type and recombinant systems provide insights into the mechanistic differences that may correlate with viral productivity. Based on these analyses, we identify potential barriers to high productivity of rAAV and discuss future directions for improvement to meet the emerging needs set by the growth of rAAV-based therapy and the needs of patients.

Completion of the AAV Structural Atlas: Serotype Capsid Structures Reveals Clade-Specific Features

The capsid structures of most Adeno-associated virus (AAV) serotypes, already assigned to an antigenic clade, have been previously determined. This study reports the remaining capsid structures of AAV7, AAV11, AAV12, and AAV13 determined by cryo-electron microscopy and three-dimensional image reconstruction to 2.96, 2.86, 2.54, and 2.76 Å resolution, respectively. These structures complete the structural atlas of the AAV serotype capsids. AAV7 represents the first clade D capsid structure; AAV11 and AAV12 are of a currently unassigned clade that would include AAV4; and AAV13 represents the first AAV2-AAV3 hybrid clade C capsid structure. These newly determined capsid structures all exhibit the AAV capsid features including 5-fold channels, 3-fold protrusions, 2-fold depressions, and a nucleotide binding pocket with an ordered nucleotide in genome-containing capsids. However, these structures have viral proteins that display clade-specific loop conformations. This structural characterization completes our three-dimensional library of the current AAV serotypes to provide an atlas of surface loop configurations compatible with capsid assembly and amenable for future vector engineering efforts. Derived vectors could improve gene delivery success with respect to specific tissue targeting, transduction efficiency, antigenicity or receptor retargeting.

Establishment of a Recombinant AAV2/HBoV1 Vector Production System in Insect Cells

We have previously developed an rAAV2/HBoV1 vector in which a recombinant adeno-associated virus 2 (rAAV2) genome is pseudopackaged into a human bocavirus 1 (HBoV1) capsid. Recently, the production of rAAV2/HBoV1 in human embryonic kidney (HEK) 293 cells has been greatly improved in the absence of any HBoV1 nonstructural proteins (NS). This NS-free production system yields over 16-fold more vectors than the original production system that necessitates NS expression. The production of rAAV with infection of baculovirus expression vector (BEV) in the suspension culture of Sf9 insect cells is highly efficient and scalable. Since the replication of the rAAV2 genome in the BEV system is well established, we aimed to develop a BEV system to produce the rAAV2/HBoV1 vector in Sf9 cells. We optimized the usage of translation initiation signals of the HBoV1 capsid proteins (Cap), and constructed a BEV Bac-AAV2Rep-HBoV1Cap, which expresses the AAV2 Rep78 and Rep52 as well as the HBoV1 VP1, VP2, and VP3 at the appropriate ratios. We found that it is sufficient as a trans helper to the production of rAAV2/HBoV1 in Sf9 cells that were co-infected with the transfer Bac-AAV2ITR-GFP-luc that carried a 5.4-kb oversized rAAV2 genome with dual reporters. Further study found that incorporation of an HBoV1 small NS, NP1, in the system maximized the viral DNA replication and thus the rAAV2/HBoV1 vector production at a level similar to that of the rAAV2 vector in Sf9 cells. However, the transduction potency of the rAAV2/HBoV1 vector produced from BEV-infected Sf9 cells was 5-7-fold lower in polarized human airway epithelia than that packaged in HEK293 cells. Transmission electron microscopy analysis found that the vector produced in Sf9 cells had a high percentage of empty capsids, suggesting the pseudopackage of the rAAV2 genome in HBoV1 capsid is not as efficient as in the capsids of AAV2. Nevertheless, our study demonstrated that the rAAV2/HBoV1 can be produced in insect cells with BEVs at a comparable yield to rAAV, and that the highly efficient expression of the HBoV1 capsid proteins warrants further optimization.

The Carboxyl Terminus of the Porcine Circovirus Type 2 Capsid Protein Is Critical to Virus-Like Particle Assembly, Cell Entry, and Propagation

The capsid protein (Cap) is the sole structural protein and the main antigen of porcine circovirus type 2 (PCV2). Structural loops of the Cap play crucial roles in viral genome packaging, capsid assembly, and virus-host interactions. Although the molecular mechanisms are yet unknown, the carboxyl terminus (CT) of the PCV2 Cap is known to play critical roles in the evolution, pathogenesis, and proliferation of this virus. In this study, we investigated functions of CT. Removal of this loop leads to abrogation of the in vitro Cap self-assembly into virus-like particles (VLPs). Likewise, the mutated virus resists rescue from PK15 cell culture. A conserved PXXP motif in the CT is dispensable for VLP assembly and subsequent cell entry. However, its removal leads to the subsequent failure of virus rescued from PK15 cells. Furthermore, substituting either the PCV1 counterpart or an AXXA for the PXXP motif still supports virus rescue from cell culture but results in a dramatic decrease in viral titers compared with wild type. In particular, a strictly conserved residue (²²⁷K) in the CT is essential for VLP entry into PK15 cells, and its mutation to alanine greatly attenuates cell entry of the VLPs, supporting a mechanism for the failure to rescue a mutated PCV2 infectious DNA clone (K227A) from PK15 cell culture. These results suggest the CT of the PCV2 Cap plays critical roles in virus assembly, viral-host cell interaction(s), and virus propagation in vitro IMPORTANCE The carboxyl terminus (CT) of porcine circovirus type 2 (PCV2) capsid protein (Cap) was previously reported to be associated with immunorecognition, alterations of viral titer in swine sera, and pathogenicity. However, the molecular mechanisms underlying these effects remain unknown. In this study, roles of the critical residues and motifs of the CT are investigated with respect to virus-like particle (VLP) assembly, cell entry, and viral proliferation. The results revealed that the positively charged ²²⁷K of the CT is essential for both cell entry of PCV2 VLPs and virus proliferation. Our findings, therefore, suggest that the CT should be considered one of the key epitopes, recognized by neutralizing antibodies, for vaccine design and a target for drug development to prevent PCV2-associated diseases (PCVADs). Furthermore, it is important to respect the function of ²²⁷K for its role in cell entry if using either PCV2 VLPs for nanoscale DNA/drug cell delivery or using PCV2 VLPs to display a variety of foreign epitopes for immunization.

Adeno-associated virus capsid protein expression in Escherichia coli and chemically defined capsid assembly

Research and clinical applications of recombinant adeno-associated virus (rAAV) significantly increased in recent years alongside regulatory approvals of rAAV gene therapy products. To date, all rAAV vectors as well as AAV empty capsids are produced in eukaryotic cells. We explored a new route to generate AAV capsids with the aim to analyze capsid assembly in a chemically defined setting and pave the way for new production methods and applications based on AAV virus-like particles (VLPs). We generated these empty capsids by bacterial expression and subsequent concomitant protein refolding and VLP formation. AAV serotype 2 structural protein VP3 was expressed in Escherichia coli. VLPs formed as demonstrated by dynamic light scattering, atomic force microscopy, and ELISA. Furthermore, VLPs internalized into human HeLa cells. To extend the application range of the VLPs, we tested peptide insertions, at the genetic level, in a surface loop (amino acid position 587) or at the C-terminus of VP3 and these variants also formed VLPs. VLPs developed without assembly-activating protein (AAP), but adding purified recombinant AAP to the refolding process increased capsid yield. Our findings offer a new route to understand AAV assembly biology and open a toolbox for AAV production strategies that might enable capsid display for vaccination and matching of capsids with cargoes at large scale and low cost.