Remote control of therapeutic T cells through a small molecule–gated chimeric receptor

Keeping a leash on cancer-killing cells Redirecting the immune system to attack tumor cells is proving to be an effective therapy against cancer. However, when patients are exposed to T cells engineered to recognize and attack cancer cells, there is a risk of runaway or excessive activity or of off-target effects, both of which can themselves be deadly. Wu et al. designed T cells expressing chimeric antigen receptors that recognize and attack cancer cells with an additional control system. This mechanism would allow a doctor administering the therapy to turn the engineered T cell “on” or “off” by administering a small molecule that is required along with cancer cell antigen to stimulate the T cells and activate their tumor cell–killing properties. Science, this issue p. 10.1126/science.aab4077 Engineering a fail-safe control mechanism in cancer-targeted T cells. INTRODUCTION Cell-based therapies have emerged as a promising treatment modality for diseases such as cancer and autoimmunity. T cells engineered with synthetic receptors known as chimeric antigen receptors (CARs) have proven effective in eliminating chemotherapy-resistant forms of B cell cancers. Such CAR T cells recognize antigens on the surface of tumor cells and eliminate them. However, CAR T cells also have adverse effects, including life-threatening inflammatory side effects associated with their potent immune activity. Risks for severe toxicity present a key challenge to the effective administration of such cell-based therapies on a routine basis. RATIONALE Concerns about the potential for severe toxicity of cellular therapeutics primarily stem from a lack of precise control over the activity of the therapeutic cells once they are infused into patients. Exogenously imposed specific regulation over the location, duration, and intensity of the therapeutic activities of engineered cells would therefore be desirable. One way to achieve the intended control is to use small molecules to gate cellular functions. Small molecules with desired pharmacologic properties could be systemically or locally administered at varying dosages to achieve refined temporal and spatial control over engineered therapeutic cells. RESULTS We developed an ON-switch CAR that enables small molecule–dependent, titratable, and reversible control over CAR T cell activity. ON-switch CAR T cells required not only a cognate antigen but also a priming small molecule to activate their therapeutic functions. Depending on the amount of small molecule present, ON-switch CAR T cells exhibited titratable therapeutic activity, from undetectable to as strong as that of conventional CAR T cells. The ON-switch CAR was constructed by splitting key signaling and recognition modules into distinct polypeptides appended to small molecule–dependent heterodimerizing domains. The ON-switch CAR design is modular; different antigen recognition domains and small-molecule dimerizing modules can be swapped in. CONCLUSION The ON-switch CAR exemplifies a simple and effective strategy to integrate cell-autonomous decision-making (e.g., detection of disease signals) with exogenous, reversible user control. The rearrangement and splitting of key modular components provides a simple strategy for achieving integrated multi-input regulation. This work also highlights the importance of developing optimized bio-inert, orthogonal control agents such as small molecules and light, together with their cellular cognate response components, in order to advance precision-controlled cellular therapeutics. Titratable control of engineered therapeutic T cells through an ON-switch chimeric antigen receptor. A conventional CAR design activates T cells upon target cell engagement but can yield severe toxicity due to excessive immune response. The ON-switch CAR design, which has a split architecture, requires a priming small molecule, in addition to the cognate antigen, to trigger therapeutic functions. The magnitude of responses such as target cell killing can be titrated by varying the dosage of small molecule to mitigate toxicity. scFv, single-chain variable fragment; ITAM, immunoreceptor tyrosine-based activation motif. There is growing interest in using engineered cells as therapeutic agents. For example, synthetic chimeric antigen receptors (CARs) can redirect T cells to recognize and eliminate tumor cells expressing specific antigens. Despite promising clinical results, these engineered T cells can exhibit excessive activity that is difficult to control and can cause severe toxicity. We designed “ON-switch” CARs that enable small-molecule control over T cell therapeutic functions while still retaining antigen specificity. In these split receptors, antigen-binding and intracellular signaling components assemble only in the presence of a heterodimerizing small molecule. This titratable pharmacologic regulation could allow physicians to precisely control the timing, location, and dosage of T cell activity, thereby mitigating toxicity. This work illustrates the potential of combining cellular engineering with orthogonal chemical tools to yield safer therapeutic cells that tightly integrate cell-autonomous recognition and user control.