IL13-PE cytotoxicity toward an IL-13Rα2-expressing tumor cell line in the presence of mouse sera from various groups of treated mice^a.

aMice were systemically sensitized to IL13-PE as described in the Materials and Methods section. Prior to bleomycin challenge, blood samples were removed from the control (saline) and IL13-PE-treated groups (ie. the pre-bleomyin sample). Mice received bleomycin and 21 days later they were randomized to treatment groups, which received one of saline alone, 200 ng/dose of IL13-PE, 500 ng/dose of IL13-PE, or 1000 ng/dose of IL13-PE. Sera was removed from each group at day 28 after 4 treatments of saline or IL13-PE. For this cytotoxicity assay, 1×104 tumor cells were cultured with IL13-PE and serum sample for 20 hr at 37°C, pulsed with 1 µCi of [3H]-leucine and further incubated for 4 hr. Cells were harvested and counted as described in the Materials and Methods section. bIC50 is the concentration of IL13-PE at which a 50% inhibition of protein synthesis occurs in IL13-PE treated tumor cells compared with untreated tumor cells.