Novel chimeric antigen receptor‐expressing T cells targeting the malignant mesothelioma‐specific antigen sialylated HEG1

Abstract The selection of highly specific target antigens is critical for the development of clinically efficient and safe chimeric antigen receptors (CARs). In search of diagnostic marker for malignant mesothelioma (MM), we have established SKM9‐2 monoclonal antibody (mAb) which recognizes a MM‐specific molecule, sialylated Protein HEG homolog 1 (HEG1), with high specificity and sensitivity. In this study, to develop a novel therapeutic approach against MM, we generated SKM9‐2 mAb‐derived CARs that included the CD28 (SKM‐28z) or 4‐1BB (SKM‐BBz) costimulatory domain. SKM‐28z CAR‐T cells showed continuous growth and enhanced Tim‐3, LAG‐3, and PD‐1 expression in vitro, which might be induced by tonic signaling caused by self‐activation; however, these phenotypes were not observed in SKM‐BBz CAR‐T cells. In addition, SKM‐BBz CAR‐T cells exhibited slightly stronger in vitro killing activity against MM cell lines than SKM‐28z CAR‐T cells. More importantly, only SKM‐BBz CAR‐T cells, but not SKM‐28z CAR‐T cells, significantly inhibited tumor growth in vivo in a MM cell line xenograft mouse model. Gene expression profiling and reporter assays revealed differential signaling pathway activation; in particular, SKM‐BBz CAR‐T cells exhibited enhanced NF‐kB signaling and reduced NFAT activation. In addition, SKM‐BBz CAR‐T cells showed upregulation of early memory markers, such as TCF7 and CCR7, as well as downregulation of pro‐apoptotic proteins, such as BAK1 and BID, which may be associated with phenotypical and functional differences between SKM‐BBz and SKM‐28z CAR‐T cells. In conclusion, we developed novel SKM9‐2‐derived CAR‐T cells with the 4‐1BB costimulatory domain, which could provide a promising therapeutic approach against refractory MM.