Lung pathology Figure 1 Lung pathology. H&E stained lung tissue, shown at ×20 and ×100 magnifications. Untreated control rats show normal lung histology; SO rats given intratracheal SV/luc also show normal histological appearance; 2CLP rats given intratracheal PBS or SV/ luc show distinct ARDS pathology: alveoli filled with proteinaceous fluid, septal thickening, interstitial lymphocytic and neutrophilic infiltration. compare with Figure 3 ). This suggests that vector penetration into lung epithelial cells is efficient, but transgene expression in many of the cells is below detection level. It is possible that the SV40 promoter used in the present vector is weak and therefore gene expression is lower than gene delivery. A stronger, lung specific promoter may significantly improve expression of a therapeutic gene in alveolar cells.