SDC4 and idiopathic pulmonary fibrosis: Moreover, the ligand-receptor pairs MDK-NCL and MDK-SDC4 were involved in signaling from ciliated cells to ciliated cells or club cells in the control group, whereas in the IPF group, MDK-SDC4 and MDK-NCL exhibited higher expression in basal cells, club cells, and goblet cells compared to ciliated cells.