Engineered Toxin Bodies delivering immunogenic MHC class I peptides to tumor cells summon polyfunctional and relevant CTL responses against cancers

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Abstract Immunotherapeutic approaches such as CAR-T and T cell engaging bispecific antibodies have shown positive clinical outcomes and demonstrate the value of harnessing cytotoxic T lymphocyte (CTL) activity for tumor intervention. Here we describe Engineered Toxin Bodies (ETBs) with Antigen Seeding Technology (AST) as a unique tumor targeted therapeutic approach to recruit endogenous polyfunctional memory CTL responses against cancers. Molecular Templates’ ETB platform combines antibody domains with a proprietary form of the Shiga like Toxin A (SLTA) subunit to specifically target cells and exploit SLTA’s intrinsic ability to self-internalize and destroy ribosomes. With the addition of a foreign class I antigenic peptide fused to an ETB (AST), antigens present in complex with endogenous MHC I to initiate a high avidity T cell response to target cells. Utilizing this approach, we demonstrate ETBs targeted to clinically relevant markers can seed antigenic peptides to hematological tumor, solid tumor, and primary cells. Expanded CTLs from donors with preexisting Human Cytomegalovirus (CMV) reactivity phenotypically resembled memory expansion from natural infection. We next delivered class I CMV peptide to tumor cells by AST with ETBs targeted to PD-L1 and co-cultured tumor cells with expanded CTLs. This led to activation of CMV specific T cells, direct tumor-T cell engagement, and potent cell kill activity above that of parental ETBs. CTL phenotyping and cytokine profiling revealed polyfunctional T cell activation driven by AST enabled ETBs. Thus, ETBs delivering peptide via AST provide additive mechanisms of action: direct cell kill via ribosomal destruction, and antigen specific polyfunctional memory CTL activation.