- A conformation-specific ON-switch for controlling CAR T cells with an orally available drug (Fig 1)
The lack of reversible control of CAR T cell function is addressed by constructing molecular ON-switches. Molecular ON-switches are modules in which a protein-protein interaction can be triggered with a small molecule. ON-switches based on clinically applicable components would have the potential to be used for functional control of cellular therapeutics such as CAR T cells by administration of orally available drugs. In principle, our molecular ON-switch system is based on the human lipocalin retinol binding protein 4 (hRBP4), which slightly changes its conformation when its hydrophobic pocket is loaded with a selected small molecule drug. Next, binding scaffolds are engineered to specifically recognize hRBP4 only when loaded with the small molecule drug, thereby yielding a drug-induced protein-protein interaction, i.e., a molecular ON-switch. The development of such ON-switches based on clinically applicable components thus has the potential to be used for the functional control of CAR T cells and other cellular therapeutics directly in the patient.
- Avidity-controlled CARs (AvidCARs) for inducible and combinatorial CAR control (Fig. 2)
In addition to molecular ON-switches, we are also exploring novel strategies based on our newly developed avidity-controlled CAR (AvidCAR) platform with inducible and logic control functions. These AvidCARs can be used (i) to control CAR T cell activity in the patient and (ii) to improve tumor specificity of CAR T cells. Here, our approach is based on the combination of (i) an improved CAR design which enables controlled dimerization and (ii) a significant reduction of antigen binding affinities to introduce dependence on bivalent interaction, i.e. the exploitation of the avidity-effect resulting from amplification of individual low affinities. For our concept of AvidCARs we postulated that a bispecific CAR with two binders with sufficiently low affinity for target antigens A and B, respectively, generates a signal only by the combined interaction with antigens A AND B. A crucial feature of the avidity-based AND gate function of AvidCARs is the fact that antigens A and B must be co-expressed on the same cell. By merging our concept of AvidCARs with our ON-switch system we can generate switchable CAR T cells that at same time depend on the presence of two different antigens on a target cell. Thus, we expect that AvidCARs will be a highly valuable platform for the development of controllable CAR therapies with improved tumor specificity.