B7-1 expression in podocytes. (A) Differential display PCR of WT (+/+) and α3_/_ (_/_) podocytes. The arrow indicates the position of B7-1. (B) Induction of B7-1 mRNA expression in α3_/_ podocytes. RT-PCR from three independent samples (a_c) on serial dilution (1:5 to 1:25) of cDNA standardized for β-actin. Con, H2O control. (C) Rescue (Res) of α3_/_ podocytes by stable transfection with human α3 integrin restored B7-1 expression to WT levels. RT-PCR for GAPDH shows equal RNA concentrations. (D) Dose-dependent increase of B7-1 mRNA in WT podocytes in response to puromycin (0.1, 1, 5, 10, and 50 ∝g/ml). (E) Time course of B7-1 and α3 integrin mRNA expression in puromycin aminonucleoside_treated (PAN-treated) WT podocytes. As early as 6 and 24 hours, B7-1 is induced by PAN, whereas α3 integrin levels remain unchanged. (F) B7-1 is not expressed in E18 WT kidneys (left panel) but is expressed in podocytes of α3_/_ kidneys as shown by double labeling with synaptopodin (right panels).