VLPs released in the transfected-cell SUP were harvested and clarified by centrifugation at 3,500 rpm for 30 minutes at 4uC and concentrated by sucrose density gradient centrifugation based on previous descriptions [44, 45, 58] . Briefly, the clarified SUPs were concentrated by ultracentrifugation through 20% sucrose cushion in TN buffer (0.1 M NaCl; 0.05 M Tris-HCL, pH 7.4) at 200,0006 g for 8 hours at 4uC. The resulting VLP pellet in ,0.5 ml volume was purified on a discontinuous sucrose gradient formed by layering 80%, 65%, 50% and 10% sucrose in TN buffer. After centrifugation at 186,0006 g for 8 hours, the top ,1.5 ml of the gradient (which included the VLP-containing band at the interface between the 10% and 50% sucrose layers) was resuspended in 20% sucrose buffer and centrifuged once more at 160,0006 g for one hour. The resulting pellet was resuspended in 20% sucrose solution in endotoxin-free TN bufffer and stored at 4uC for subsequent analysis. Supernatant of 293T cells transfected with empty pCAGGS plasmid and processed similarly (referred to as ''mock'' particles) served as negative control when needed.