Multiple mechanisms contribute to resistance to fratricide. (A) Inhibition of cytotoxicity of CD5 CAR T cells against autologous T cells and Jurkat cells by blocking either FasL (brefeldin A + aFasL), perforin (CMA + EGTA), or both pathways. Cell death was measured by Annexin V after 2 hours of coculture. (B) Expression of PI-9 protein in CD5 CAR T cells and malignant T-cell lines was measured by intracellular staining and flow cytometry (left). Bar graphs show MFI of PI-9 (right). (C) Expression of cathepsin B transcript in CD5 CAR T cells and target cell lines was measured by quantitative polymerase chain reaction. (D) Levels of Bcl-2 transcript in CD5 CAR T cells and target cell lines. (E) Protein expression of Bcl-2 was measured by intracellular staining and flow cytometry. (F) Bid expression in CD5 CAR T cells and target cell lines was measured by quantitative polymerase chain reaction. Error bars denote SD for 3 different T cell donors. MFI, mean fluorescence intensity.