(A and B) WT mice received a single orotracheal dose of vehicle or 1.25 U/kg pharmacological grade bleomycin and were followed for 14 days. After the mice were euthanized, the lungs underwent immunohistochemical detection of NTN1 (brown stain). Vehicle- (A) and bleomycin-challenged lungs (B) contained multiple populations of NTN1⁺ cells that appeared more numerous in the bleomycin condition. Slides were counterstained with hematoxylin and are shown as ×10 original magnification. (C–F) Flow cytometric detection of NTN1⁺ macrophages (C, identified with the markers F4/80⁺CD11b⁺CD11c⁺), fibroblasts (D, identified with the markers CD45^–ColIα1⁺), epithelial cells (E, identified with the markers CD45^–CD31^–EpCAM⁺) and endothelial cells (F, identified with the markers CD45^–EpCAM^–CD31⁺) in single-cell suspensions prepared from lungs of WT mice challenged with vehicle (round symbol) or bleomycin (triangle). Left axis shows percentages, and right axis shows quantities. (G–J) Relative to mice with “intact” expression of Ntn1 in macrophages (LysM-Cre, Ntn1^+/+, round symbol), mice with macrophage Ntn1 “deleted” (LysM-Cre, Ntn1^fl/fl, triangle) show reduced right lung collagen (G, P = 0.0056, 2-tailed Student’s t test) and, in a separate group of mice, both MAS (H, P = 0.0013 by 2-tailed Student’s t test) and trichrome staining (I, “intact” and J, “deleted”). (K–N) Relative to mice with “intact” expression of Ntn1 in fibroblasts (corn oil–treated ColIα2-CreER, Ntn1^fl/fl animals, round symbol), mice with Ntn1 “deleted” in fibroblasts (tamoxifen-treated ColIα2-CreER, Ntn1^fl/fl mice, triangle) show unchanged right lung collagen (K), and in a separate group of mice, both MAS (L) and trichrome staining (M, “intact” and N, “deleted”). *P < 0.05, **P < 0.01, ***P < 0.001, and ****P < 0.0001. Data are shown as mean ± SEM.