Photo courtesy of UCSF
Researchers have reported progress in developing an “on/off switch” to temper the over-active immune response and severe toxicities that can result from chimeric antigen receptor (CAR) T-cell therapy.
The team created CAR T cells that are “off” by default, homing to CD19-expressing cancer cells but remaining inactive until a small molecule is administered.
This system effectively targeted leukemia and lymphoma cells in preclinical experiments.
But the researchers said it’s not ready for clinical testing, as the small-molecule “trigger” is expensive and lasts only 4 hours.
Still, the team believes this type of CAR T-cell therapy could eventually help doctors gradually increase the immune response to treatment and therefore avoid toxicities such as cytokine release syndrome and tumor lysis syndrome.
Wendell Lim, PhD, of University of California, San Francisco, and his colleagues described this work in Science.
“T cells are really powerful beasts, and they can be lethal when they’re activated,” Dr Lim said. “We’ve needed a remote control system that retains the power of these engineered T cells but allows us to communicate specifically with them and manage them while they’re in the body.”
To that end, he and his colleagues created a CAR that requires both an antigen and a small molecule for activation. They dubbed it the “ON-switch CAR.”
ON-switch CAR
The researchers explained that the ON-switch CAR consists of 2 parts that assemble in a small molecule-dependent manner.
Part 1 consists of a CD8α signal sequence, Myc epitope, anti-CD19 single-chain variable fragment, CD8α hinge and transmembrane domain, 4-1BB costimulatory motif, and FK506 Binding Protein (FKBP) domain for heterodimerization.
Part 2 consists of the ectodomain of DNAX-activating protein 10 (DAP10) for homodimerization, CD8α transmembrane domain for membrane anchoring, 4-1BB costimulatory motif, T2089L mutant of FKBP-rapamycin binding (FRB*) domain, T-cell receptor CD3ζ signaling chain, and mCherry tag.
The FKBP and FRB* domains heterodimerize in the presence of the rapamycin analog AP21967, referred to as the “rapalog.”
The researchers conducted in vitro experiments with this ON-switch CAR in cells expressing CD19 (K562, Raji, and Daudi).
The ON-switch CAR T cells homed to CD19-expressing cells but did nothing else until the rapalog was added. Once the rapalog was added, CD19-expressing cells were killed off in a dose-dependent manner.
The team observed similar results in mice with leukemia. Leukemia cells (K562) were selectively eliminated by the ON-switch CARs only after the rapalog had been administered.
Dr Lim stressed that this work should be considered a proof of principle, as the rapalog has too short a half-life to be clinically useful. Nevertheless, he believes the research provides the foundation for practical remote control of CAR T cells.
Members of his lab are exploring other techniques to accomplish this goal, such as controlling CAR T-cell activation with light.
The team is also working to reduce side effects of CAR T-cell therapy by introducing multiple CARs into T cells so the cells will respond to multiple characteristics that are distinctive to an individual patient’s tumor, rather than to a single protein that may also be found on normal cells.
“That we can engineer CAR T cells to have slightly different, quite powerful effects—even if for a subset of patients or for certain types of cancer—is really remarkable,” Dr Lim said. “And this is just the tip of the iceberg.”
