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Background Adoptive cellular immunotherapeutics, including CAR T cells and T cell engager bispecifics, have had limited success against solid tumors. Hypotheses proposed to explain such limited success include antigen heterogeneity, tumor-mediated immunosuppression, and lack of support for T cell survival, proliferation and differentiation. We introduce novel and highly effective biologics that target multiple tumor antigens and provide natural costimulatory signals to T cells.Methods We developed solid tumor targeting CAR-T Engager proteins (CAR-TE) that act by binding tumor cells while also binding to and activating CD19-CAR T cells. A CAR-TE was created using the CD19 extracellular domain, llama VHH that bind to IL13Rα2 and to B7-H3 and CD58 binding domains. To further armor this CAR-TE a monomeric TGFβR2 protein was added. To extend the technology, a T cell engager version of the protein was engineered by substituting an anti-CD3 antibody sequence for CD19. Antigen-positive and cell lines were used to evaluate potency and T cell phenotypes in serial cytotoxicity assays; PDX models were used for translational validation.Results CAR19 T cells have best-in-class expansion and persistence characteristics. To leverage these exemplary properties we have created CAR19-T Cell Engager proteins (CAR19-TE) that contain the CD19 protein targeted by CAR19 T cells. When linked to VHH that recognize the IL13Rα2 and B7-H3 tumor antigens the CAR19-TE enables tumor cell killing by CAR19 T cells. Tumor cell killing by CAR19 T cells is amplified by the presence of CD58 binding domains in the CAR19-TE protein. IL13Ra2-positive/B7-H3-positive cell lines and PDX tumor samples were eliminated by the multi-antigen targeting CAR19-TE added with CAR19 T cells. A TCE version of the costimulatory engager proteins enabled PBMC-mediated killing of the same targeted tumor cells. Potencies at low and sub-pM concentrations were measured. Increased T cell mediated anti-tumor activity was associated with secretion of key cytokines and T cell differentiation. An overview of clinical development and supporting collaborations will be presented.Conclusions Costimulatory CAR19-TE and costimulatory TCE proteins have been characterized and found to have remarkable potency and to improve T cell survival, proliferation and differentiation. Models representing IL13Ra2/B7-H3-positive cancers including melanoma, glioma and GBM produced robust translational results. The lead CAR19-TE and TCE programs are poised for clinical development.