Supernatants of HeLa-prME or its parent HeLa cells were harvested and cleared by centrifugation at 3,000 rpm for 15 minutes and 10,000 rpm for 30 minutes. Clarified supernatants were then concentrated by ultracentrifugation at 28,000 rpm for 2.5 hours. Pellets were then resuspended in 100 ml of Phosphate buffered saline (PBS). For the production of immature RSPs, the HeLa-prME cells were cultured in medium containing 20 mM of ammonium chloride (NH 4 Cl).