Retrotransposons write CAR genes into T cells inside living mice

Overview

Updated 1 hour ago

CAR-T therapy starts with a patient's T cells being removed, engineered in a lab and reinfused. A paper in Cell reports skipping the removal: engineered retrotransposons, delivered as RNA in lipid nanoparticles, inserted a receptor gene into T cells inside living mice.

The team screened 406 retrotransposon candidates and engineered one, Vingi-1_Acar, to write RNA templates around 2,200 nucleotides long into DNA. In humanized mice given a single 1 mg/kg dose, CD19 CAR-T cells peaked near 0.5% of T cells while CD20 CAR-T cells reached about 15%, and B cell counts stayed low through the 38-day CD20 study. The model uses mice carrying human blood cells, not people.

Why it matters

If it holds in people, CAR-T could be made by infusing RNA instead of extracting, engineering and reinfusing a patient's cells.

Questions about this story

Free account needed to ask — your question is kept and asked for you right after sign-up. Answers are public.

No questions yet — be the first to ask.

Key Indicators

~28%

CD19 CAR expression after lipid nanoparticle delivery in vitro

Share of T cells expressing the receptor when all-RNA components were delivered by lipid nanoparticle, versus about 16% by nucleofection.

~0.5%

Peak CD19 CAR-T cells in humanized mice

Measured on day 11 after a single 1 mg/kg dose; template-only control mice showed none.

~15%

CD20 CAR-T cells at day 11 in mice

Higher than the CD19 result, from a template of similar length.

406

Retrotransposon candidates screened

Drawn from an initial set of roughly 9,000 retroelements identified computationally.

38 days

Length of the CD20 in vivo study

B cell depletion held to the end, with levels near undetectable in blood and spleen.

Voices

Curated perspectives — historical figures and your fellow readers.

People Involved

Donghui Li

First author on the Cell paper

Researcher with the Gene Writers team

Christopher Chase Bolt

Senior author listed last on the Cell paper

Leads the Gene Writers research effort

Andrea Calabria

Co-author on integration-site analysis

Researcher studying where inserted DNA lands in the genome

Organizations Involved

Gene Writers team

Biotechnology research group

Developer of the Vingi-1_Acar element described in the Cell paper

The research group behind the engineered retrotransposons, working with academic collaborators on integration-site analysis.

Cell

Scientific journal

Published the study online on 21 October 2026

Peer-reviewed biology journal that published the paper and its supplementary data.

Timeline

August 2025 October 2026

3 events Latest: October 21st, 2026 · 1 week ago

October 2026
Gene Writers build CAR-T cells inside humanized mice
Latest Research

Cell published a study showing engineered non-LTR retrotransposons inserting CD19 and CD20 CAR templates in vivo via T-cell-targeted lipid nanoparticles, with sustained B cell depletion.
October 21st, 2026
2026
Avian R2 elements catalogued for targeted integration
Research

A Nature Biotechnology paper searched 1,139 avian genomes, identified 159 R2 retrotransposons and engineered them for all-RNA-mediated targeted DNA integration in human cells.
2026
August 2025
Avian R2 retrotransposon engineered to insert DNA in human cells
Research

Researchers at the Chinese Academy of Sciences reported an R2 retrotransposon system, R2Tg, active in human cells at low efficiency before optimization.
August 7th, 2025

Scenarios

First human trial of retrotransposon-written CAR-T opens

Unlikely Resolves by Q2 2029

Discussed by: The paper's authors, who note that retargeting the elements would require extensive additional engineering before therapeutic use

The mouse results would need replication in larger animals and a manufacturing process for clinical-grade RNA. A Phase 1 registration would mark the point where in vivo gene writing moves from a mouse model into patients.

Independent lab reproduces in vivo CAR-T writing in a primate

Possible Resolves by End of 2028

Discussed by: Gene delivery researchers who have followed lipid nanoparticle targeting of T cells

The humanized mouse model limits how far the results generalize. A published primate study from a separate group, showing CAR expression and B cell depletion after RNA delivery, would show the approach travels beyond one team's hands.

Truncated insertions force a template redesign before human testing

Possible Resolves by Q2 2028

Discussed by: The authors, who flag truncated integrants as a safety concern unique to the platform

The paper reports that shorter-than-expected insertions accumulate toward one end of the template, and that these fragments could carry promoter sequences that alter nearby gene expression. A redesign, such as switching cargo orientation, would show up as a changed template in later publications.

Historical Context

3 moments from history that rhyme with this story — and how they unfolded.

1997

Sleeping Beauty transposon reconstructed (1997)

Zoltán Ivics and Zsuzsanna Izsvák rebuilt a working transposase from defective fish DNA sequences, producing Sleeping Beauty, and showed it could insert DNA in mammalian cells.

Then

The element became a tool for insertional mutagenesis screens and, later, for engineering CAR-T cells without viruses.

Now

It established that an ancient, reconstructed mobile element could serve as a gene delivery vehicle in human cells.

Why this matters now

Gene Writers follows the same logic with a different class of element, using an RNA intermediate to carry much larger cargo.

2021

First in vivo CRISPR editing in patients (2021)

Intellia reported interim results from a trial infusing lipid nanoparticles carrying CRISPR components into patients with transthyretin amyloidosis, editing cells inside the body rather than in a lab dish.

Then

The results established lipid nanoparticle RNA delivery as a route to editing inside patients, initially targeting the liver.

Now

In vivo delivery became the field's main alternative to extracting and editing cells, but most programs still aimed at disabling genes rather than adding them.

Why this matters now

Gene Writers uses the same delivery idea to insert a gene-length sequence and to reach T cells rather than the liver.

2017

First CAR-T approval (2017)

The Food and Drug Administration approved tisagenlecleucel for pediatric acute lymphoblastic leukemia, the first gene therapy cleared in the United States.

Then

Viral engineering of a patient's own T cells became a standard manufacturing route for CAR-T therapy.

Now

The approach stayed bespoke: cells are collected, engineered and reinfused, with weeks of processing per patient.

Why this matters now

The step the Gene Writers paper targets is exactly that extraction and lab engineering.