Adapter CAR-based immunotheranostics for personalized immunotherapy and diagnosis
After genetic manipulation of immune effector cells (NK and T cells) with artificial 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) derived from antibodies), a transmembrane and intracellular signaling domains. Via CARs immune cells can recognize a certain tumor-associated antigen on tumor cells resulting in T cell activation and tumor cell killing. Due to their impressive clinical success, in 2017 such “living drugs” were approved for the first time by the the US Food and Drug Administration for treatment of patients with certain CD19-positive malignancies.
However, during CAR T cell therapy in some cases even life-threatening side effects can occur due to e.g. massive cytokine release (cytokine storm), elimination of tumor cells (tumor lysis syndrome) or on-target/off-tumor toxicities. In this regard, safety management is challenging since activated CAR-modified T cells expand in an unpredictable manner and cannot be switched off after their adoptive transfer into patients. Another problem is that CARs have a fixed specificity and CAR-armed T cells become ineffective when tumor cells down-regulate the targeted antigen as an escape mechanism. Furthermore the immunosuppressive tumor microenvironment is challenging for CAR T cell therapy against solid tumors.
In order to overcome these limitations, our group has established modular Adapter CAR platforms, termed the UniCAR and RevCAR system. In contrast to conventional CARs, UniCARs are not directed against an antigen on the tumor cell surface. As a consequence, UniCAR-modified T cells are inert after their adoptive transfer into the patient. UniCARs recognize a well characterized, for humans largely non-immunogenic peptide epitope (UniCAR epitope) that is derived from the human nuclear La/SS-B protein. UniCAR T cells can be redirected to tumor cells via target modules (TMs) that are able to mediate the cross-linkage between T cells and tumor cells. Therefore, the bifunctional TM on the one hand has an antigen-binding motif that binds to the tumor cell and on the other hand contains the UniCAR epitope recognized by UniCARs. 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 tumor targeting molecules including scFvs, nanobodies, peptide ligands and small molecules (e.g. well-characterized PET tracers). In order to increase their half-life, TMs can be also constructed based on an IgG-Fc backbone. 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.
Besides the UniCARs, we have developed the RevCAR platform that functions in a similar manner showing high anti-tumor efficacy, safety, controllability, and flexibility. In contrast to conventional CARs and UniCARs, RevCARs lack the extracelluar antigen-binding motif and express only the short RevCAR peptide epitope. RevCAR T cells require a bispecific target module (termed RevTM) to be redirected to tumor cells. Therefore, bispecific RevTMs bind with one arm to the RevCAR peptide epitope and with the other arm to a tumor antigen. So far, we have shown that the RevCAR platform can be used for combinatorial tumor targeting following the OR and AND gate logic of Boolean Algebra in order to pave the way for an improved personalized immunotherapy.
Our Adapter UniCAR/RevCAR platforms are broadly applicable. Besides tumor immunotherapy, the system can be also applied to treat infectious diseases, autoimmune diseases or transplantation reactions (e.g. graft versus host diseases) by e.g. antigen-specific retargeting of regulatory T cells (Tregs) armed with our Adapter UniCARs/RevCARs.