Overview

A viral vector is a genetically modified virus designed to deliver a therapeutic gene into cells without causing disease. In gene therapy, viral vectors serve as delivery vehicles for a transgene cassette, which usually includes a promoter (controls gene expression), a coding sequence (gene of interest) and regulatory elements (enhancers, polyA signal).

Why use viral vectors for gene therapy?

Viruses have evolved mechanisms to transport their genome into the host organism they infect. Once they enter the target cell, they hijack its biological machinery to express their genes and multiply. Scientists have used this mechanism to develop gene therapies and produce biologics. The process of delivering a gene using a viral vector is called transduction. Through transduction, the newly delivered genes can elicit the expression of therapeutic proteins essential for treating a patient's condition or correcting genetic anomalies to cure the underlying disease.

Viral vectors are preferred over non-viral vectors when efficient delivery and sustained expression are required for gene therapy.

How do viral vectors work?

• Binding – The viral vector attaches to receptors on the target cell
• Entry – The vector gains entry into the cell through endocytosis or membrane fusion
• Uncoating – The viral capsid releases its genetic material inside the cell
• Intracellular trafficking – The DNA is transported towards the nucleus
• Nuclear entry – The genetic material enters the nucleus
• Expression – The transgene is expressed, leading to the production of the therapeutic protein

Common Viral Vectors Used in Gene Therapy

Currently, many viruses are being studied for their potential use as viral vectors. These vectors can deliver genes of interest to targeted cells for permanent or temporary transgene expression. Some commonly studied viral vectors include retroviruses such as γ-retroviruses and lentiviruses, herpes simplex viruses, adenoviruses and adeno-associated viruses (AAVs).

Each of these viral vectors has unique advantages and disadvantages.

• Retroviral Vectors: These vectors integrate into the host genome through reverse transcriptase, widely used to deliver therapeutic genes. They stabilize the gene in target cells but face challenges like small insert sizes and immunogenicity.
• Lentiviral Vectors (LVVs): LVVs effectively deliver genes to non-dividing and hard-to-transduce cells, facilitating stable gene expression and being used in CAR T therapies. However, they pose a risk of off-target effects due to random genomic integration.
• Herpes Simplex Virus Vectors: These vectors can deliver significant DNA amounts to various cells, including non-dividing cells and neurons. They offer high transgene dosage per cell and can carry large inserts. However, challenges include cytotoxicity and the need for helper viruses.
• Adenovirus Vectors: Adenovirus vectors are versatile for delivering genes to various cell types, useful for in vitro and in vivo research. They are easy to construct and handle large cargo, but are not suitable for stable gene expression.
• Adeno-Associated Virus (AAV): The vector, once a contaminant in adenovirus cultures, is now vital for gene editing, silencing and replacement due to its high transduction efficiency, making it ideal for gene therapy. However, factors such as tropism, limited cargo capacity, immunogenicity and manufacturing must be considered.

Integrating vs non-integrating vectors

Integrating and non-integrating viral vectors differ in how they interact with the host genome and the durability of expression they provide. Integrating vectors like lentivirus and retrovirus insert DNA into the host genome, enabling long-term expression but risking insertional mutagenesis. Non-integrating vectors, such as AAV, adenovirus and HSV, deliver episomal DNA, reducing genomic risk but often resulting in transient expression. Integrating vectors are preferred for durable therapies (e.g., ex vivo), while non-integrating vectors prioritize safety.

AAV serotypes and tropism: what to consider

AAV serotypes are capsid variants that define which cells a vector can enter, while tropism is their natural preference for tissues such as the liver, CNS or muscle. Choosing the right serotype depends on target tissue specificity, pre-existing antibodies and the therapeutic dose. Thus, serotype selection is crucial for optimizing efficacy, safety and dosing.

Considerations in Viral Vector Production

The development process for viral vectors involves several key considerations independent of viral species. Designing a suitable viral vector determines its efficacy and function in gene therapy applications. Here are some considerations that should be accounted for during viral vector production:

• Viral vector selection: This requires in-depth comprehension of the in vivo delivery mechanism and cell type specificity. Each vector has its advantages and limitations.
• pDNA/plasmid creation: Plasmids are crucial components of viral vector production. This requires them to be free of impurities and to maintain consistent batch-to-batch quality. Plasmid design and development impact downstream vector production and transgene expression.
• Choice of cell line: Cell lines can impact the overall timeline, cost and efficiency of viral vector production. Commonly used cell lines include HEK293, Sf9, BHK21 and MDCK, with different cell lines demonstrating various efficiencies in producing specific vectors.
• Cell expansion adherent vs. suspension: The decision to use adherent or suspension cell culture depends on the required amount of viral production. Adherent cells are less amenable to scaling up, while suspension cells may require more rounds of transfection optimization.
• Capsid biology: Characterizing and engineering capsids can lower immune response risks and enhance tissue targeting. However, new capsid biology may complicate purification and concentration, requiring separation of full capsids from empty or partial capsids, which are impurities lacking genomic material. Sometimes, existing technologies might not suffice for certain serotypes or novel capsid biology.

Viral vector manufacturing workflow

Quality control (QC) and release testing for viral vectors

Consistent manufacturing is supported by rigorous quality control (QC), which is crucial for reducing batch variability, ensuring reproducibility and meeting regulatory standards. These processes help maintain product consistency and support the safe and effective use of viral vectors in clinical applications.

QC testing is typically organized into key categories:

• Identity - Confirms vector genome sequence

• Purity - Host cell DNA/protein impurities

• Safety - Sterility, endotoxin testing

• Potency - Functional transduction efficiency

• Titer measurements:

  • Genome titer (vg/mL)
  • Infectious titer

• AAV-specific:

  • Empty vs full capsid ratio

• Safety testing:

  • Replication-competent virus (RCL/RCA)

Gene Therapy Current Challenges

Viral vectors have tremendous potential for gene therapy, yet the research gains don’t always translate into in vivo efficacy.

Here are a few challenges surrounding gene therapy:

• Gene expression levels may not be sufficient for symptom treatment or disease cure due to delivery inefficiencies, incorrect gene copies or cell type limitations
• Drug developers still struggle to eliminate off-target effects despite extensive risk research
• Gene therapy may not be feasible for all diseases due to access limitations to disease sites
• Treatment costs can be a significant obstacle for low-income patients, as they often remain prohibitively high

Safety risks and mitigation (high-level)

Viral vectors pose several safety risks, including immune responses that may lead to neutralization or toxicity, insertional mutagenesis from integrating vectors, dose-dependent toxicity at high exposure levels and manufacturing variability that can impact product quality. To address these risks, consistent manufacturing processes and strict quality control (QC) are crucial, helping to reduce batch variability, ensure reproducibility and facilitate regulatory approval.