For propidium iodine (PI) viability studies, cells were washed twice in PBS (five minutes, 10006g), resuspended in a 0.5 mg/mL PI solution, and incubated for 15 minutes. Following incubation, cells were centrifuged (five minutes, 10006g) and washed in a solution of PBS, 2% fetal calf serum, and 0.1% NaN 3 (buffer A). Cells were fixed in 1% paraformaldehyde, and analyzed by flow cytometry. For two parameter viability analysis, cells were washed in 1X annexin V binding buffer (BD Bioscience, San Jose, CA), resuspended in 0.5 mg/mL PI and 5 mL annexin V-FITC (BD Bioscience), incubated for 15 minutes, washed in binding buffer, fixed in 1% paraformaldehyde, and analyzed by flow cytometry.