Therapeutic targeting and diagnostic imaging of Fibroblast Activation Protein expressing cancers using the UniCAR system

Engineered T cells that express chimeric antigen receptors (CARs) show a big potential in cancer immunological research. They are synthetic receptors that can target antigens independently from the MHC complex. However, most tumor associated antigens are not only expressed on the tumor site but also on healthy tissues. To switch off CAR T cells in case of on-target/off-tumor effects occur, an adaptor CAR platform, called the universal CAR (UniCAR), was developed. In contrast to conventional CAR systems this platform consists of the engineered T cells and a linker molecule, called target module (TM), that is both binding to the target antigen and recognized by the UniCAR T cell. With this approach there is the possibility to switch the system on and off when needed.
Fibroblast activation protein (FAP) is a protein of high interest in cancer research as it is upregulated in most of epithelial cancers but shows a comparably low expression in healthy tissues. It is shown to be expressed on cancer associated fibroblasts and therefore plays an important role in the tumor microenvironment where it has pro-tumorigenic activity both in the FAP positive and surrounding cells. Given this, the goal of this work was to develop new TMs against FAP.
To achieve such aim, we cloned two TM formats, either based on a single-chain variable fragment (scFv) or an IgG4 based backbone. The difference in the size of the backbones translates into different half-lifes and kinetics. These different TM formats can be used to customize UniCAR therapy according to the given circumstances. We were able to purify and perform various experiments with the mentioned TMs. We could show binding to the target cells using flow cytometry. We could also show that the TMs could be used to induce specific lysis in vitro on monolayer cells and spheroids. We were also able to show UniCAR T cell infiltration into the spheroids and their consequent activation. Moreover, in mouse models the TMs could be used for PET imaging, showing specific accumulation at FAP-expressing tumor cells, as well as for killing of such tumors using redirected UniCAR T cells.
By that we could show that the anti-FAP scFv and IgG4 based TMs have high theragnostic potential as they can be used for both immunotherapy and diagnostic imaging.