For example, we have observed that slow IPF fibroblasts are less responsive to IL-4/IL-13 compared with rapid IPF fibroblasts14,23 and these findings were confirmed in the present study with the observation that the immunotoxin IL13-PE had no anti-fibrotic effects in the humanized SCID model of IPF induced by the introduction of stable IPF fibroblasts. Here, IL13 is linked to idiopathic pulmonary fibrosis.