PKA antagonist reverses MSC-mediated cell death protection in BCP-ALL. iMSC#3 cells were grown to confluency before growth medium was changed from MEMα to RPMI with or without Rp-8-Br-cAMPS as indicated. After 24 h, ALL5 cells were cocultured with the iMSC#3 monolayer for 2 h before removal for irradiation with 2 Gy. BCP-ALL blasts were reintroduced to the iMSC#3 monolayer immediately after irradiation and cocultured for 20 h prior to cell death analysis of the CD19+ cell fraction by CD19-FITC/PI co-staining as described in Materials and Methods. The histogram shows mean values of seven independent experiments with error bars indicating SEM values. **P < 0.01, *P < 0.05 by paired samples t test.