Dose-dependent effect of IL13-PE on cellular proliferation in tissue cultures of primary normal, Th1-type, and Th2-type fibroblast lines. Fibroblasts were incubated for 24 hours with DMEM-15 alone (control), DMEM-15 plus 200 ng/ml of IL13-PE, or DMEM-15 plus 1000 ng/ml of IL13-PE. Four hours before the conclusion of the experiment, 10 μCi of [3H] thymidine was added to each well. IL13-PE treatment significantly inhibited, in a dose-dependent manner, the proliferation of Th1- and Th2-type but not normal fibroblasts. ‡‡‡, P ≤ 0.001 compared with untreated (media alone) normal fibroblasts. *, P ≤ 0.05; ***, P ≤ 0.001 compared with untreated Th1- or Th2-type fibroblasts.