CAR T cells made inside the body through precision gene editing

CAR T cells made inside the body have successfully attacked several types of cancer in experiments using mice carrying human immune cells. The approach uses precision genome editing to reprogram T cells without removing them for modification in a laboratory.

Published in Nature, the study goes beyond demonstrating an antitumor response. Researchers inserted a large therapeutic DNA sequence into a predetermined location in the genome of human T cells while those cells were circulating inside a living organism.

That degree of control is what makes the result distinctive.

Why CAR T-cell therapy requires a manufacturing process

CAR T-cell therapy turns immune cells into cancer-targeting weapons by giving them an artificial receptor, known as a chimeric antigen receptor, or CAR.

A CAR designed to recognize CD19, for example, allows engineered T cells to identify and destroy cells involved in certain leukemias and lymphomas.

The established treatment process is complex. T cells are collected from a patient, transported to a specialized manufacturing facility, genetically modified, expanded and eventually returned to the patient.

This can take weeks. Manufacturing is expensive, and the quality of the final product may vary depending on the patient's starting cells.

Generating CAR T cells directly inside the body could eliminate several of these steps.

The concept is not entirely new. Previous approaches have used nanoparticles to deliver messenger RNA, temporarily instructing T cells to produce CARs. Others have relied on viral vectors that insert genetic material into the cellular genome.

Each strategy has limitations.

RNA-based CAR expression fades as the RNA is degraded. Viral integration can produce longer-lasting effects, but the inserted genetic material may land at unpredictable genomic locations. It is also essential to avoid inadvertently engineering the wrong cells.

The new study addresses both the delivery problem and the question of where the therapeutic gene is inserted.

Two delivery vehicles, one genomic address

The researchers built a system that separates the editing machinery from the DNA sequence being inserted.

One delivery vehicle carries the CRISPR-associated protein Cas9 and its guide RNA. Together, they make a targeted cut in the genome.

A second vehicle, based on an adeno-associated virus, delivers the CAR-encoding DNA template required for precise integration.

The selected genomic destination is TRAC, a region associated with the production of the normal T-cell receptor.

This choice has an important consequence. The inserted CAR construct has no independent promoter. Once integrated at TRAC, it comes under the control of the cell's own T-cell-specific regulatory machinery.

Instead of relying on a randomly positioned genetic switch, the engineered receptor is expressed from a defined genomic location.

The team also modified both delivery vehicles to improve their ability to reach human T cells.

The Cas9-containing particles were equipped with a CD3-binding antibody fragment. This helps direct the particles to T cells while also activating those cells, which supports the DNA repair process required for targeted integration.

For DNA delivery, the researchers developed an engineered adeno-associated virus called AAV-hT7. Its capsid was selected for improved entry into human T cells and resistance to pre-existing serum neutralizing factors.

A cell-surface protein called CD7 emerged as an important factor in this improved delivery.

The two components therefore perform different jobs: one brings the editing machinery, while the other supplies the genetic sequence that must be inserted.

Both must reach the same cell for the intended modification to occur.

From genome editing to tumor elimination

The researchers first tested the system in human T cells in culture, then moved to humanized mice carrying transplanted human immune cells.

With the optimized delivery system, up to 19.7% of splenic T cells carried the integrated CAR construct 14 days after administration.

Those engineered cells were functional.

When programmed to recognize CD19, the newly generated CAR T cells produced complete depletion of detectable B cells in the treated animals, demonstrating that the genetic modification had translated into biological activity.

The team then tested the approach against cancer.

In a model of B-cell acute lymphoblastic leukemia, 18 of 20 mice achieved complete responses across experiments using immune cells from four human donors.

The results extended to multiple myeloma. All eight treated mice in that experiment achieved complete responses.

When some of these animals were challenged again with myeloma cells, three of four maintained tumor control, providing evidence that the engineered T cells could retain functional activity.

The researchers also investigated a solid tumor model, using a CAR directed against the surface protein B7-H3.

In mice bearing sarcoma tumors, complete responses occurred in five of six animals using cells from one donor and three of eight animals using cells from another.

This was particularly notable because generating effective CAR T-cell responses against solid tumors remains much more difficult than targeting certain blood cancers.

The experiments demonstrated that the delivery platform could be adapted to different CAR designs and cancer targets.

The researchers also compared their targeted-integration strategy with lentiviral approaches for generating CAR T cells directly inside the body.

In the tested leukemia model, TRAC-targeted CAR T cells expanded more rapidly and showed more uniform receptor expression than cells generated using the lentiviral vectors.

The findings support the hypothesis that the genomic location of the inserted CAR influences cell behavior and therapeutic activity. They do not, however, establish that precise integration alone explains every difference in efficacy.

What remains unresolved

The cancer experiments were conducted in humanized mice, not patients.

These animals were immunodeficient and carried transplanted human immune cells. They provided a useful experimental system, but they did not reproduce the full complexity of a functioning human immune system.

That distinction matters when evaluating safety.

Questions remain about unintended genome alterations, long-term consequences of the integration process, distribution of the delivery particles across tissues and the immune response to the components.

The investigators found no evidence of a systemic inflammatory cytokine response at the measured time points. However, the immunodeficient mouse model limits how much that observation can establish about human safety.

Repeat administration may also prove difficult. Although AAV-hT7 showed resistance to pre-existing neutralizing factors, antibodies generated after exposure could interfere with subsequent doses.

And while depletion of normal B cells confirms the activity of CD19-directed CAR T cells, prolonged B-cell depletion can itself be an important complication of CAR T-cell therapy.

These uncertainties are especially relevant because the approach aims to produce stable genetic changes rather than short-lived receptor expression.

For now, conventional CAR T-cell treatment remains largely an individualized manufacturing procedure: cells leave a patient's body, undergo genetic engineering and are returned as a therapeutic product.

This study demonstrates another possibility.

Human T cells can receive a genetic editing program inside a living organism, acquire a therapeutic receptor through targeted DNA integration and become capable of attacking malignant cells.

The distinguishing feature is not merely where the CAR T cells are made. It is the ability to control where the new genetic instructions become part of their genome.