Service Quote Request
Please fill out the form below with your detailed inquiry.
-
PHONE: 1-866-918-6812
Viral Vector Technologies for Gene Expression
Viral vectors are engineered viruses used to deliver genetic material into target cells. They are essential tools in gene therapy, disease modeling, and target validation, enabling efficient and reliable manipulation of gene expression in vitro and in vivo
Key advantages include:
- High gene delivery efficiency compared with non-viral methods
- Precise targeting of specific cell types or tissues
- Stable, long-term gene expression, supporting sustained biological or therapeutic effects
An Introduction to AAV
What is AAV
Adeno-Associated Virus (AAV) is a small, non-enveloped virus belonging to the Dependoparvovirus genus. In biotechnology, it is the premier vector for gene therapy due to its ability to infect both dividing and non-dividing cells without integrating significantly into the host genome.
Key Biological Features
- Genomic Structure: Contains a ~4.7kb single-stranded DNA (ssDNA) genome flanked by Inverted Terminal Repeats (ITRs).
- Non-Pathogenic: AAV is not known to cause human disease and requires a helper virus (like Adenovirus) for replication.
- Serotype Diversity: Various capsids (e.g., AAV9, AAV2) exhibit specific tissue tropism, allowing targeted delivery to the heart, liver, or CNS.
By replacing viral genes (rep and cap) with a therapeutic expression cassette, scientists leverage AAV to provide long-term gene expression and treat genetic disorders at their molecular source.
AAV Genome
The AAV2 genome includes genes required for genome replication (Rep78/68), genome packaging (Rep52/40), and capsid proteins (VP 1/2/3).
To understand how scientists use AAVs in the lab, we first need to understand the genome and life cycle.
The AAV genome is viral genome is single-stranded DNA, ~ 4.7 kb long, and consists of three main parts:
- Inverted terminal repeat (ITRs): Located at both ends of the DNA strand, the ITR sequences flank the viral genome.
- Rep: The Rep (replication) genes encode 4 non-structural proteins involved in genome replication and packaging. However, because the AAV genome doesn’t encode a polymerase, AAVs still require the host cell’s polymerase to replicate its genome.
- Cap: The Cap (capsid) genes encode 3 viral capsid proteins that form the outer “shell” of the virus, encapsulating the AAV genome and determining tissue tropism by allowing the virus to bind to specific cell receptors
AAV Lifecycle
AAVs depend on their viral capsid to enter host cells. The capsid interacts with receptors on the host cell’s surface before it can be internalized into the cell. The exact receptors required depend on the AAV’s serotype; AAV2 requires heparan sulfate proteoglycan. Once internalized, the virus enters the nucleus of the cell.
There, AAV can follow one of two distinct and interchangeable pathways of its life cycle: the lytic cycle and the lysogenic cycle.
If the AAV is infected along with a helper virus, such as adenovirus or herpes simplex virus, the AAV enters the lytic cycle. During this process, the AAV gene expression program is activated, and new AAV particles are produced within the cell. These cells burst and release newly formed viral particles into the environment, where they can infect other cells to continue the lytic cycle.
The lysogenic life cycle occurs when the AAV enters the host cell without co-infection of a helper virus. Its gene expression program is auto-repressed and the AAV genome integrates into a specific region on the long arm of human chromosome 19. This integration involves the AAV ITRs and Rep proteins (Rep78, Rep68, Rep52, and Rep40). The AAV can also enter the lytic cycle after being integrated into the host chromosome when a helper virus is introduced. Rep proteins can excise the AAV genome from the host chromosome, where it can be replicated and packaged in the lytic cycle.
How to Make a Recombinant AAV
There are many characteristics that make AAVs a great tool for gene editing.: This includes:
- Low pathogenicity: Wild-type AAVs are non-pathogenic. For example, 80-90% of adults are seropositive with AAV2, but infection hasn’t been associated with any symptoms or disease
- Low immunogenicity: AAV produces the lowest immune response compared to other viral vectors.
- Broad tropism: AAV can infect and transduce many tissues including liver, muscle, heart, kidney, retina, lungs and brain a wide range of tissues.
- Multiple serotypes: The AAV has more than 20 different serotypes that allow for tissue- and wide-spread specificity increases specificity to certain cell types or can be more broad. AAVs can also infect both dividing and quiescent cells.
To generate recombinant AAV (rAAV), the following are required:
- Cis plasmid: The transfercis plasmid contains the two ITRs from the AAV genome, with the Rep and Cap genes replaced with the desired “payload.” This payload is the gene that you want to introduce into your cells (along with a selective or ubiquitous promoter upstream of the gene for expression) and must be less than 4.5 kb in length so that it can fit inside the capsid.
- Transfer plasmid: The transfer plasmid provides the Rep and Cap genes that direct the AAV serotype.were originally on the AAV. This plasmid does not include the e ITRs. aren’t included in this plasmid, so these genes won’t be packaged into viral particles.
- Helper virus genes: There are five helper viral genes needed to create rAAVs. Three of these genes are typically provided on a helper plasmid (E4orf6, E2a, and VA RNA). Two other helper genes (E1a and E1b55k) are typically expressed by the HEK293 cell line commonly used to produce rAAV. It’s possible to use one plasmid that provides both the adenoviral helper genes as well as the Rep and Cap genes.
AAV Production Protocol
The first step is to subclone the gene of interest (GOI), shRNA, or gRNA in the final pAAV cis-plasmid, which is then amplified in preparation for viral production. The common approach to produce rAAV is by a triple transfection (cis-, trans-, and helper- plasmids) in a producer cell line. Vector Biolabs currently uses HEK293 cells. The HEK293 cells are harvested ~3 days post-transfection, followed by purification by CsCl combined with ultracentrifugation. Once the rAAV is purified, Vector BioLabs performs a series of bioanalyses, for example: (1) viral purity, (2) determination of physical titer, (3) aggregation by DLS and SLS, (4) empty/full capsid ratio by AUC or mass photometer, (5) endotoxin testing by LAL, and more.
AAV Serotypes & Tropism
AAV serotypes are determined by the composition of the AAV capsid used to enter host cells. Since different cell types can have different receptors or other surface components. Although AAV2 has been the most extensively studied serotype, it still has low transduction efficiency in some cell types. In attempts to overcome this, scientists have developed hybrid rAAVs were developed that combine a capsid from oneother serotype with a Rep gene & genomeITRs from another serotypeAAV2. E.g., AAV2/5 uses the Rep gene from AAV2 and the Cap gene from AAV5. This approach makes it possible to package the same construct into multiple AAV capsids without additional cloning work for each capsid packaged.
There are also engineered serotypes that have been designed to infect new tissues or evade the immune system. Many of these capsids are engineered using directed capsid evolution, capsid DNA shuffling, or peptide library insertion technologies. , E,g, AAV-DJ is a hybrid capsid created through DNA shuffling that combines eight different AAV capsids AAV-DJ combines capsid elements from eight different serotypes and has higher transduction efficiency in vitro compared to any naturally of its parental occurring serotypecapsids.
AAV can be tailored by over 20 different serotypes (naturally occurring and engineered) that allow for (1) tissue-specific and widespread targeting, and (2) systemic injections that can penetrate the blood-brain barrier.
The question is: among these serotypes of AAV, which serotype(s) is appropriate or best for a particular study model?
To begin, consider the cell type you are targeting. Our Vector Selection Guide can help you choose serotypes depending on your cell type and species. Once you have selected your potential serotypes, you can carry out experiments using a reporter gene such as GFP or LacZ to verify transduction efficiency in your exact study.
When to Use AAVs
As mentioned above, AAVs have low immunogenicity and don’t have any diseases associated with humans. These vectors can give prolonged and stable expression in many animal models without notable toxicity. Therefore, AAV is mainly used for in vivo studies. If you need a virus for most in vitro studies, use adenovirus or lentivirus.
However, one major drawback to AAVs is that they can only hold a transgene DNA payload of less than 4.5 kb in length. Larger transgenes can be spliced into multiple fragments for each fragment to be carried by its own AAV virus, and then combined with a second and even third AAV virus carrying all fragments of the gene for injection. Beyond that lengthAlternatively, you may need to consider another viral vector, such as lentivirus or adenovirus, to carry a larger gene.
An Introduction to Lentivirus
What is Lentivirus
A human HIV-1-based lentivirus is a recombinant Retroviridae vector derived from the Human Immunodeficiency Virus type 1, engineered to function as a sophisticated gene delivery vehicle. Scientifically, it is a replication-incompetent system that utilizes the virus’s natural machinery to achieve stable transduction of mammalian cells.
Unlike standard retroviruses, HIV-1 vectors possess a nuclear localization signal (NLS) on their pre-integration complex, allowing them to bypass the nuclear envelope and infect both dividing and non-dividing cells (e.g., neurons or stem cells). The molecular architecture is optimized for safety: pathogenicity is eliminated by deleting accessory genes (vif, vpr, vpu, nef) and the tat transactivator. Through reverse transcription, the ssRNA payload is converted to dsDNA, which the enzyme integrase then covalently integrates into the host’s chromatin. This ensures permanent, heritable expression of the therapeutic transgene across all future cell generations.
Lentivirus Genome
A recombinant lentiviral vector is a highly modified system derived from HIV-1, designed to safely deliver genetic material into host cells. To transform a pathogen into a tool, scientists split the genome into separate components to prevent the production of new infectious particles.
The Multi-Plasmid System
In a standard third-generation system, the viral genome is divided across four plasmids:
- Transfer Plasmid: Contains the gene of interest (GOI) flanked by Long Terminal Repeats (LTRs) and the psi (psi) packaging signal. This is the only part that integrates into the target cell.
- Packaging Plasmids: Two separate plasmids encoding the Gag-Pol (structural/enzymatic) and Rev (regulatory) proteins.
- Envelope Plasmid: Usually expresses VSV-G to allow the virus to infect a wide range of cell types.
Safety Features
Modern recombinant genomes include Self-Inactivating (SIN) LTRs. By deleting a portion of the 3′ UTR, the virus loses its ability to replicate after the first round of infection. This ensures the modified genetic material stays put without creating more virus, making it a gold standard for stable, long-term gene therapy.
Lentivirus Lifecycle
The lentivirus lifecycle is a strategic process of genetic hijacking, distinguished by its ability to infect non-dividing cells.
- Entry: The virus binds to specific surface receptors, triggering membrane fusion and releasing the viral core into the cytoplasm.
- Reverse Transcription: Inside the cell, the enzyme Reverse Transcriptase converts the viral RNA into double-stranded DNA.
- Integration: The DNA enters the nucleus—even through an intact nuclear envelope—where Integrase inserts it into the host’s genome. The virus is now a permanent “provirus.”
- Replication: The host cell’s machinery is forced to transcribe viral RNA and translate it into long polyproteins.
- Assembly & Budding: New viral components gather at the cell membrane and push outward, wrapping themselves in the host’s lipid bilayer.
- Maturation: After budding, Protease clips the polyproteins into functional units, transforming the particle into a mature, infectious virus.
This efficient integration makes lentiviruses premier tools for long-term gene therapy.
How to Make a Recombinant Lentivirus
Developing a recombinant lentivirus is a precise process of “packaging” a gene of interest into a viral shell using a multi-plasmid system. This ensures the resulting virus can deliver genetic cargo but cannot replicate.
The Development Process
- Plasmid Preparation: In a 3rd-generation system, you prepare four plasmids: the Transfer plasmid (carrying your gene), two Packaging plasmids (Gag-Pol and Rev), and an Envelope plasmid (usually VSV-G).
- Transfection: These plasmids are mixed with a transfection reagent and added to HEK293T producer cells.
- Viral Assembly: For 48–72 hours, the cells act as factories, transcribing the transfer RNA and translating viral proteins that self-assemble into particles.
- Harvesting: The virus-laden supernatant is collected and filtered (0.45 µm) to remove cellular debris.
- Concentration & Titering: The virus is often concentrated via ultracentrifugation. Finally, the “titer” (concentration) is measured to ensure effective gene delivery in future experiments.
When to Use Lentivirus
Lentiviral vectors are the “workhorses” of modern molecular biology. You should choose them when your experiment or therapy requires permanent genetic changes or the ability to infect stubborn, non-dividing cells.
Here is a breakdown of when to reach for Lentivirus over other options like AAV or Adenovirus:
1. When You Need Stable, Long-Term Expression
Because lentiviruses integrate their DNA directly into the host genome, the gene is passed down to all future generations of that cell.
- Best for: Creating stable cell lines or treating diseases where the cell divides frequently (like blood or skin).
- Contrast: AAV and Adenovirus remain “episomal” (floating outside the DNA), meaning their effect is diluted and eventually lost as cells divide.
2. When Infecting Non-Dividing Cells
Lentiviruses are famous for their ability to slip through the nuclear pores of cells that aren’t dividing.
- Best for: Neurons, muscle cells, and hematopoietic stem cells (HSCs).
- Contrast: Simple retroviruses (like MMLV) can only infect cells that are actively dividing.
3. When Your Gene is “Large”
Lentiviruses have a relatively generous “trunk” capacity for genetic cargo.
- Capacity: Roughly 8–10 kb.
- Contrast: AAV is limited to about 4.7 kb, which is too small for many human genes or complex CRISPR systems.
| Applications | Primary Use Case | Target Cells | Real-World Examples / Products |
| Cancer Therapy | CAR-T Cell Therapy | T-Cells | Kymriah (for Leukemia); Yescarta (for Lymphoma). |
| Genetic Disorders | Ex Vivo Gene Addition | Hematopoietic Stem Cells (HSCs) | Zynteglo (?-thalassemia); Skysona (Adrenoleukodystrophy). |
| Basic Research | Stable Cell Line Generation | HEK293T, HeLa, CHO, etc. | Creating “immortalized” cell lines for long-term protein production. |
| Gene Silencing | shRNA Delivery | Various (Dividing & Non-dividing) | Targeted knockdown of specific genes to study their function. |
| Functional Genomics | CRISPR Screen Libraries | Pooled populations | Genome-wide “knockout” screens to find drug resistance genes. |
| Neuroscience | In Vivo Gene Delivery | Neurons / Brain Tissue | Delivery of fluorescent markers (GFP) or optogenetic tools to the CNS. |
| Vaccine Dev. | Antigen Presentation | Dendritic Cells | Experimental vaccines for HIV, Malaria, and Zika. |
An Introduction to Adenovirus
What is Adenovirus
A recombinant Adenovirus (rAd) is a wild-type adenovirus that has been genetically engineered to serve as a delivery vehicle for foreign genetic material. A human serotype 5 based recombinant adenovirus (Ad5) is the most widely used viral vector in research and medicine. It is engineered from the wild-type Ad5 virus by removing key replication genes (usually E1 and E3), making it safe for use in humans.
- Structure: It retains the iconic icosahedral shell composed of hexon and penton proteins, with characteristic long fibers protruding from each vertex.
- Mechanism: These fibers bind to the Coxsackievirus and Adenovirus Receptor (CAR) on human cells, allowing the virus to inject its DNA cargo into the nucleus.
- Capacity: It can carry up to 8 kb of foreign DNA, such as a gene for a therapeutic protein or a vaccine antigen.
- High Efficiency: They are exceptionally good at “infecting” a wide range of cell types, including both dividing and non-dividing cells.
- Episomal Expression: The delivered DNA stays in the nucleus as an independent “episome” and does not integrate into the host’s genome, reducing the risk of unintended mutations.
- High Payload: They can carry relatively large segments of foreign DNA compared to other viral vectors like AAV.
- Gene Therapy: Used to deliver functional genes to treat genetic disorders or cancer.
Adenovirus Genome
The human adenovirus serotype 5 (Ad5) genome is a linear, double-stranded DNA molecule approximately 36 kb in length. In its recombinant form, it is engineered as a replication-deficient vector, primarily through the deletion of the E1 (early gene 1) region, which is essential for viral transcription and replication. This deletion not only renders the virus safe for clinical use but also creates space for the insertion of a transgene expression cassette.
Key structural components include:
- Inverted Terminal Repeats (ITRs): Located at both ends, these serve as origins of replication.
- Packaging Signal (?): A cis-acting sequence required for encapsidation.
- E3 Deletion: Often removed to increase the cloning capacity (up to ~8 kb) and reduce the host immune response.
- Late Genes (L1–L5): These remain largely intact to encode structural proteins like the hexon, penton base, and fiber, ensuring successful virion assembly in complementing producer cell lines.
Adenovirus Lifecycle
The adenovirus lifecycle is a highly choreographed sequence of events typically divided into Early and Late phases, demarcated by the onset of viral DNA replication.
- Attachment and Entry: The cycle begins with the high-affinity binding of the viral fiber knob to the Coxsackievirus and Adenovirus Receptor (CAR). This is followed by a secondary interaction between the penton base and cellular αv integrins (like αvβ3, αvβ5), triggering clathrin-mediated endocytosis.
- Uncoating and Nuclear Import: As the endosome acidifies, the capsid partially disassembles, releasing the virion into the cytosol. It hijacks microtubules to reach the nuclear pore complex, where the linear dsDNA genome is injected into the nucleus.
- Early Phase (E1–E4): Early genes (E1A, E1B, E2, E4) are transcribed to modulate the host cell cycle, inhibit apoptosis, and provide the machinery for DNA polymerase-mediated replication.
- Late Phase and Lysis (L1–L5): Following genome replication, the Major Late Promoter (MLP) drives the expression of structural proteins (L1–L5). New virions assemble in the nucleus via phase separation before being released through virally induced host cell lysis.
Adenovirus Serotype & Tropism
Classification and Tropism by Species
The tropism of an adenovirus is largely determined by its capsid proteins (specifically the fiber and penton base) and the cellular receptors they bind to.
Receptor Usage: The Key to Tropism
The “entry ticket” for the virus into the cell is the binding of the fiber knob to a specific receptor.
- CAR (Coxsackievirus and Adenovirus Receptor): Used by species A, C, D, E, and F. CAR is widely expressed on epithelial cells, which explains the broad range of respiratory and GI infections.
- CD46: Used primarily by species B (and some D types). CD46 is found on nearly all nucleated cells, including hematopoietic cells, giving species B a unique advantage in infecting the blood and immune system.
- Desmoglein 2 (DSG2): A high-affinity receptor for species B serotypes like Ad3, Ad7, and Ad14. Binding to DSG2 can open tight junctions between cells, allowing the virus to spread deeper into tissues.
- Sialic Acid: Some species D types (like Ad37) use sialic acid-containing glycans on the eye surface, leading to severe conjunctivitis.
Clinical Significance
Understanding serotypes is critical for both medicine and biotechnology:
- Epidemics: Serotype Ad14 (the “killer cold”) and Ad7 are known for causing severe, sometimes fatal outbreaks in healthy young adults.
- Vaccines: The U.S. Military uses a live oral vaccine specifically against Ad4 and Ad7 to prevent ARD in barracks.
- Gene Therapy: Most viral vectors are based on Ad5 (Species C) because it has a very high affinity for the liver and lungs, though researchers are switching to “rare” serotypes like Ad26 or Ad35 to avoid pre-existing immunity in humans.
| Species | Primary Serotypes | Main Tissue Tropism | Associated Diseases |
| A | 12, 18, 31 | Gastrointestinal | Cryptic GI infections; linked to obesity (Ad31) |
| B1 | 3, 7, 16, 21, 50 | Respiratory | Pharyngitis, pneumonia, ARD (military recruits) |
| B2 | 11, 14, 34, 35 | Renal / Urinary | Hemorrhagic cystitis, tubulointerstitial nephritis |
| C | 1, 2, 5, 6 | Respiratory / Lymphoid | Common cold, tonsillitis; persists in adenoids |
| D | 8, 19, 37, 53, 54 | Ocular | Epidemic keratoconjunctivitis (EKC), “shipyard eye” |
| E | 4 | Respiratory | Acute respiratory disease (ARD) |
| F | 40, 41 | Gastrointestinal | Infantile gastroenteritis (diarrhea) |
| G | 52 | Gastrointestinal | Gastroenteritis |
When to use Adenovirus
Recombinant adenoviruses are widely used in biomedical research, clinical development, and biotechnology due to their high transduction efficiency and robust transgene expression.
Basic Research:
Recombinant adenoviruses are commonly used to overexpress or knock down genes in mammalian cells, including both dividing and non-dividing cells. They are valuable tools for studying gene function, signaling pathways, and protein–protein interactions in vitro and in vivo.
Gene Therapy:
In gene therapy, recombinant adenoviruses deliver therapeutic genes to treat diseases such as cancer, cardiovascular disorders, and genetic deficiencies. Their large payload capacity enables delivery of complex or multiple genes, although expression is typically transient.
Vaccines:
Adenoviral vectors are widely used in vaccine development, particularly for infectious diseases and cancer vaccines. They efficiently induce strong cellular and humoral immune responses, making them effective vaccine platforms.
Cancer Research and Oncolytic Therapy:
Modified adenoviruses are used to selectively replicate in and kill tumor cells (oncolytic adenoviruses) or to deliver immune-modulating genes to enhance antitumor responses.
Cell and Tissue Engineering:
They are also applied in stem cell research and tissue engineering to transiently express differentiation or reprogramming factors.
Advance Biological Discovery — Today
Contact us for further information or technical questions related to viral vector technologies
Important updates waiting for you!
Enter your email address below and subscribe to our newsletter