Fig. 5. Double labeling for T1 (brown) and lacZ expression (blue) in donor (A-C) and recipient (D,E) lungs. A-C show donor lungs from lacZ-positive transgenic mice that constitutively express lacZ in all cells. These panels demonstrate that: (1) in the lacZ mouse X-gal histochemistry stains all lacZ-expressing lung cells blue; (2) immunohistochemical brown staining of a type I pneumocyte membrane protein produces a characteristic linear staining pattern; (3) X-gal staining does not alter the specificity of the T1 antibody. Consistent with the specificity of this type I cell marker, type II cells (A, arrow), endothelium (B, arrow), and macrophages (C, arrow) are T1 negative. D and E show sections from an experimental wild-type recipient mouse that was injected with donor lacZ-positive marrow-derived cells. A representative alveolus contains blue lacZ-positive flattened engrafted cells (D). This section was then re-stained with the T1 monoclonal antibody (E; brown staining). All donor-derived blue cells are also positive for T1 protein staining (arrows) identifying them as type I pneumocytes. An endogenous type I pneumocyte (lacZ negative, T1 positive) is also indicated (arrowheads). Bars, 20 µm (A-C); 9 µm (D,E).