uPARAP is up-regulated in affected regions of human fibrotic lungs and is closely associated with hyperplastic alveolar epithelial cells expressing high levels of SP-D. (A–C) H&E stain (left) and immunostaining for SP-D and uPARAP (middle and right, respectively) in serial sections from surgically resected nonfibrotic alveolar tissue from patients with emphysema (A; nonfibrotic; n = 5), fibrotic tissue from patients with IPF (B; n = 4), and fibrotic tissue from a patient with radiotherapy-induced fibrosis (C). SP-D was detected in single cells with an epithelial appearance along the alveolar walls of nonfibrotic lung tissue (A, middle, arrows) and was extensively expressed in lung tissue from patients with fibrosis (B and C, middle). Here, strong SP-D staining was observed in hyperplastic alveolar epithelial cells (a) in close proximity with active fibrosing subepithelial fibroblast foci (f). These foci included a large number of highly uPARAP-positive fibroblasts (B and C, right). (D) Top: Quantification of the fraction of SP-D–positive epithelial cells in alveolar walls of nonfibrotic tissue and in alveolar compartments entrapped in fibrotic tissue. Bottom: Visiopharm software was used for automated scoring of SP-D–positive and –negative cells in the alveolar wall (see Materials and methods). (E) Immunostaining for SP-D in a bronchiolized area (b) of fibrotic tissue from patient with IPF. Note that cells of the bronchiolized epithelium did not express SP-D. (F) Internalization of radiolabeled SP-D in the human lung fibroblast cell lines MRC-5 and CCL-210 examined in the absence or presence of the uPARAP function-blocking antibody mAb 5f4. A two-tailed Student’s t test was used to test significance (D and F).