The lack of responsiveness of fibroblasts to TRAIL might be due to the lack of TRAIL ligand (as a consequence of decreased expression and/or myeloid cell recruitment into the lungs, as revealed by decreased CD33 cells), internalization of the TRAIL receptors, and increased presence of IL-13 or other intrinsic differences between IPF and normal lung fibroblasts. This evidence concerns the gene CD33 and idiopathic pulmonary fibrosis.