Rapamycin-activated cytokine receptor (RACR)

The Rapamycin-activated cytokine receptor (RACR) is a synthetic signaling system engineered to provide external, pharmacological control over cellular activities, particularly within adoptive immunotherapy. This receptor architecture consists of two separate polypeptide chains: one containing the FK506-binding protein (FKBP12) and the other containing the FKBP-rapamycin binding (FRB) domain of mTOR, both localized to the extracellular space (Stavrou et al., 2018). The system functions through a mechanism known as chemical induction of dimerization (CID), where the small molecule rapamycin (or its analogs) acts as a molecular bridge, binding both FKBP12 and FRB simultaneously to bring the receptor subunits into close proximity. This dimerization mimics the natural activation of cytokine receptors, such as the IL-2 or IL-7 receptors, thereby initiating intracellular signaling through the JAK/STAT pathway to promote cell survival and proliferation (Pellegrini et al., 2018). In clinical settings, RACRs allow for the "remote control" of engineered T-cells, enabling clinicians to trigger cell expansion or activity only when the drug is administered, which can help mitigate toxicities like cytokine release syndrome (Wu et al., 2015). While highly effective for controlling cell therapy, the use of rapamycin as an inducer carries the inherent challenge of its systemic immunosuppressive effects, which must be balanced against the desired activation of the engineered cells.

Other names

Synthetic rapamycin-activated cytokine receptor composed of extracellular FRB and FKBP12 domainsSynthetic rapamycin-induced receptorFKBP-FRB dimerizable cytokine receptorSyCyRRapamycin-induced dimerization receptor

Chemical induction of dimerization (CID) where rapamycin acts as a molecular bridge between extracellular FKBP12 and FRB domains, triggering intracellular signaling.