Vectors in gene therapy are delivery vehicles engineered to carry therapeutic genetic material into your cells. Because naked DNA or RNA can’t easily enter cells on its own and would be quickly destroyed by the body, vectors serve as protective carriers that transport the genetic cargo to the right destination. Most vectors are modified viruses stripped of their ability to cause disease but retaining their natural talent for getting inside cells. Non-viral options like lipid nanoparticles are also increasingly used.
How Vectors Deliver Genes to Cells
Viruses evolved over millions of years to be extremely efficient at entering human cells and depositing genetic material inside them. Gene therapy takes advantage of this by gutting a virus of its harmful components and replacing them with a therapeutic gene or gene-editing tool. The modified virus, now a vector, still “infects” the cell but delivers a treatment instead of causing illness.
Once a vector reaches a cell, it releases its genetic payload into the nucleus. What happens next depends on the type of vector. Some, like those derived from retroviruses, stitch the new gene directly into the cell’s own chromosomes. This means the gene becomes a permanent part of the cell’s DNA and gets copied every time the cell divides. Others, like adenoviral vectors, deposit their DNA in the nucleus without integrating it into the chromosomes, providing gene expression that is typically temporary.
If everything works as intended, the delivered gene instructs the cell to produce a functional protein that was previously missing or defective. In gene-editing approaches, the vector delivers molecular tools that correct the error directly in the cell’s existing DNA.
Two Ways Vectors Reach Your Body
There are two broad strategies for getting vectors into patients. In the first, called in vivo delivery, the vector is injected directly into the body, either into the bloodstream through an IV or into a specific tissue like the eye or brain. Individual cells take up the vector on their own.
In the second approach, called ex vivo delivery, doctors remove a sample of the patient’s cells, expose them to the vector in a laboratory, and then return the modified cells to the patient. This method gives researchers more control over which cells receive the gene and is the basis for treatments like CAR-T cell therapy for certain cancers.
Adeno-Associated Virus (AAV): The Most Common Vector
AAV has become the go-to vector for many gene therapies because of its favorable safety profile. It’s a tiny virus, only about 25 nanometers in diameter, that carries a small single-stranded DNA genome. It causes no known disease in humans, triggers relatively mild immune reactions compared to other viruses, and can produce long-lasting gene expression in cells that don’t divide frequently, like neurons or muscle cells.
The main limitation of AAV is its small cargo capacity. It can only package DNA up to about 4,700 base pairs, which means larger genes simply won’t fit. This rules it out for some genetic diseases caused by mutations in very large genes.
AAV comes in many natural variants called serotypes, each with a preference for different tissues. AAV2, for example, tends to target blood vessel cells in the brain, while AAV9 can cross the blood-brain barrier and reach broad regions of the brain and spinal cord. Researchers choose the serotype based on which tissue they need to reach. Because AAV generally does not integrate its DNA into the host chromosome, the therapeutic gene sits separately in the nucleus and gradually dilutes in tissues where cells are actively dividing.
Lentiviral Vectors: Permanent Integration
Lentiviral vectors are derived from a family of viruses that includes HIV (heavily modified and stripped of disease-causing components). Their key advantage is the ability to integrate their genetic cargo directly into the host cell’s chromosomes, creating a permanent change that persists through cell division. This makes them especially useful for ex vivo therapies where cells like blood stem cells or immune cells need to carry the new gene for the life of the patient.
Unlike older retroviral vectors, lentiviral vectors can enter cells that aren’t actively dividing, which broadens the types of cells they can modify. The tradeoff is that inserting DNA into chromosomes carries a small risk of disrupting an important gene at the insertion site, a phenomenon called insertional mutagenesis. Lentiviral vectors cause this at lower rates than the older generation of retroviral vectors, partly because they tend to integrate in different chromosomal locations, but the risk isn’t zero.
Adenoviral Vectors: Large Capacity, Short Duration
Adenoviruses are double-stranded DNA viruses with a genome of about 36,000 base pairs, leaving considerable room for therapeutic DNA. This large cargo capacity is a major advantage when the gene being delivered is too big for AAV. Adenoviral vectors do not integrate into the host chromosome, so the gene expression they provide is generally temporary. This can be a drawback for chronic genetic diseases but is useful for applications like cancer immunotherapy or vaccine delivery, where short-term protein production is the goal.
The main downside of adenoviral vectors is that they tend to provoke stronger immune responses than AAV. Most people have encountered natural adenoviruses during their lives, so the immune system may already be primed to attack them, reducing the vector’s effectiveness.
Non-Viral Vectors: Lipid Nanoparticles
Not all gene therapy vectors come from viruses. Lipid nanoparticles (LNPs) are tiny fat-based spheres that can wrap around genetic material and carry it into cells. They became widely known through the mRNA COVID-19 vaccines, which used LNPs to deliver messenger RNA. In gene therapy, LNPs can carry both nucleic acids and proteins.
LNPs need to be smaller than about 100 nanometers to pass through the tiny pores in liver blood vessels, which is one reason most current LNP-based therapies naturally end up targeting the liver. They avoid some of the immune complications associated with viral vectors, since the body doesn’t have pre-existing immunity to a fat particle the way it might to a common virus. However, the RNA they carry is fragile and prone to degradation, so chemical modifications to the RNA (such as replacing certain building blocks and capping the ends of the molecule) are essential to keep the payload stable long enough to work.
Immune Reactions and Safety Risks
The biggest challenge with viral vectors is the immune system. Even AAV, considered one of the safer options, can trigger both arms of the immune response. Immune cells called T cells may recognize and destroy cells that have been transduced by the vector, essentially rejecting the therapy. B cells can produce antibodies that neutralize the vector, which not only blocks retreatment with the same vector type but can, in some cases, interfere with conventional protein replacement therapies for the same disease.
At high doses, AAV vectors have caused serious complications including liver toxicity and damage to nerve clusters called dorsal root ganglia, which can produce neurological symptoms. Some of this nerve damage appears to stem from the therapeutic protein itself being overproduced in neurons rather than from an immune attack. In clinical trials, researchers have tried using immunosuppressive drugs to blunt these reactions, with mixed success, particularly at very high vector doses.
Complement activation, part of the body’s innate immune defense, has also emerged as a concern. Early assumptions that AAV triggered only mild innate immune responses have been revised after clinical trials showed that complement activation can cause serious adverse events. These risks are dose-dependent: the more vector particles administered, the higher the likelihood of a severe reaction.
Why Gene Therapies Are Expensive to Produce
Manufacturing viral vectors at scale remains one of the field’s biggest bottlenecks. Most production relies on growing mammalian cells in laboratory systems and then using those cells to generate large quantities of the vector. The industry estimates it needs a 10- to 100-fold increase in manufacturing capacity to meet commercial demand for the diseases currently in clinical development.
Several steps in the process are difficult to standardize. Traditional production uses cells that grow on surfaces (adherent cells), requiring enormous numbers of flasks and roller bottles that must be handled manually in sterile conditions. This is labor-intensive, hard to scale, and introduces variability. A common method for triggering vector production involves transfecting cells with DNA, a step that is sensitive to technique and difficult to perform consistently at large volumes.
Switching to suspension cell cultures (cells floating freely in liquid) would simplify scaling, but not all cell lines maintain high vector output after making that transition. Harvesting certain vectors adds another layer of difficulty: AAV, for instance, often accumulates inside the cells rather than being released into the surrounding liquid, making collection more complex. These manufacturing challenges are a significant reason why approved gene therapies carry price tags that can reach into the millions of dollars per patient.