After genetic modification of immune effector cells (T cells, NK cells) with artificial immune receptors (e.g. chimeric antigen receptors, CARs) these cells can specifically recognize and kill tumor cells. CARs are composed of an extracellular antigen binding domain (most commonly a single-chain fragment variable (scFv) of antibodies), a transmembrane domain and intracellular signaling domains derived from activating immune receptors. In light of clinical application, immune cells isolated from the tumor patient have to be firstly manipulated ex vivo to express the CARs and will thereafter be reinfused into the patient (adoptive transfer). Genetically modified T cells bind via the extracellular antibody domain of the CAR to a tumor specific antigen on cancer cells. This leads to a specific activation of CAR-armed T cells and finally results in efficient tumor cell eradication. Due to their impressive clinical success, the US Food and Drug Administration recently approved for the first time two of such “living drugs” for treatment of patients with certain leukemia (CD19-positive leukemia).
However, during tumor therapy with CAR-modified T cells massive, in some cases even life-threatening side effects can occur due to e.g. massive cytokine release (cytokine storm) or elimination of tumor cells (tumor lysis syndrome). Another problem is that CARs are specific against the target antigen not only on malignant cells but also on normal healthy tissues, which even becomes clear on the example of the clinically translated CD19 specific CARs. The herein chosen target antigen CD19 is not only expressed on tumor cells but also on healthy B cells. Therefore, after successful tumor therapy patients will suffer lifelong from the lack of B cells i.e. patients are not able to produce antibodies anymore. As antibodies can be easily substituted, a CD19-targeted therapy is a feasible CAR-based strategy. However, this holds not true for most of the other potential tumor targets. Therefore, other CAR-based tumor therapies will be associated with life-threatening side effects, because CAR-modified T cells cannot be switched off after their adoptive transfer into patients. Although by now approaches were established that lead to deletion of CAR-engrafted T cells from patients, it would be safer if CAR T cell activity can be regulated after adoptive transfer in patients.
In order to achieve this possibility, we developed a novel modular CAR platform termed UniCAR system. In contrast to conventional CARs, UniCARs are not directed against a target structure on the cell surface, but bind to a well characterized, for humans largely non-immunogenic peptide epitope (UniCAR epitope). As a consequence UniCAR-modified T cells are inert after their adoptive transfer into the patient. They can be specifically activated via a bifunctional molecule (termed target module (TM)) that is able to mediate the cross-linkage between T cells and tumor cells. Therefore, the TM on the one hand has to bind to the tumor cell and on the other hand has to contain the UniCAR epitope. If TMs with short eliminations rates are designed, UniCAR T cell activity can be reversibly turned on and off. Meanwhile, we showed that TMs could be successfully constructed by fusion of the UniCAR epitope with different targeting molecules including scFvs, nanobodies and small molecules (e.g. well-characterized PET tracers). Another advantage of the UniCAR systems is its high flexibility because TMs can be easily replaced. In case tumor escape variants occur, it would be possible to restart therapy against an alternative tumor antigen by using an appropriate TM. Moreover, UniCAR-based therapy can be directed simultaneously or successively against multiple targets. This can be achieved either by using a combination of different TMs or appropriate bi- and multispecific TMs including combinational (“gated”) CAR strategies.References:
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- Cartellieri et al., Blood Cancer J. 6(8), 2016
- Koristka et al., Blood 124, 2014
- Kloss et al., Nature Biotechnology, 2013