Figure 3.

(A) Cytotoxicity of MR1 immunotoxin to NR6M cells. Cells were exposed for 24 h to immunotoxin, and incorporation of [³H]leucine was measured. (B) Stability of MR1 immunotoxin. Immunotoxin was incubated at 20 μg/ml in human serum for 0 h (○), 4 h (•), 8 h (□), or 24 h (▪) at 37°C. Cytotoxic activity was then measured as in A. (C) Inhibition by peptide. Cytotoxicity assays were performed with 10 ng/ml MR1 immunotoxin in the presence or absence of 5 μM EGFRvIII peptide. (D) Cytotoxicity of MR1 immunotoxin to human glioblastoma cells transfected with EGFRvIII. ○, U87MG.ΔEGFR transfectant; •, U87MG untransfected parent cell line. Each data point shown is the mean of triplicate determinations; error bars show SEM.