VLP production in Vero and MRC-5 cells. Cells infected with the FPL1 recombinant were fixed 3 days p.i., and VLP formation was examined at ultrastructural level by electron microscopy. Complete VLPs (T7, 72 pentamers) were found in Vero cells mainly enclosed in pseudo-vacuoles (A). Unassembled T1-particles (12 pentamers) were present in the nuclei of MRC-5 cells (B). No VLPs were detected in FPL1-infected CEFs (data not shown). Negative-stained VLPs from the commercial HPV Gardasil® vaccine (inset of panel A) were used as a control. An enlargement of the nuclear T1 particles is shown in the inset of panel B. Bar in Panels A and B: 300 nm. Bar in the insets: 100 nm.