Enhanced papillomavirus-like particle production in insect cells.

Human papillomavirus (HPV) L1 self-assembles into virus-like particles (VLPs), which are the basis for the two commercially available prophylactic vaccines. For one of them (Cervarix) HPV 16 and 18 VLPs are being produced in insect cells using the baculovirus expression system. However, due to low yield, production of VLPs remains challenging for certain other PV types. Here we report that employment of a modified baculovirus-based (MultiBac) expression system (Berger, I., Fitzgerald, D. J., and Richmond, T. J. (2004). Baculovirus expression system for heterologous multiprotein complexes. Nat. Biotechnol. 22(12), 1583-7) permits substantially improved VLP production of several PV types up to 40-fold. Highest VLP yields were achieved when two copies of the L1 gene were expressed from independently controlled cassettes. We have evaluated the production of HPV 57 L1 VLPs by the MultiBac system in more detail. Whereas the level of the HPV 57 L1 protein was only slightly increased in comparison to the standard protocol we monitored a strongly enhanced yield of HPV 57 VLPs. Our results imply that a critical concentration of L1 within the producer cell is required for efficient VLPs assembly. We show evidence that in addition a dominant negative factor in conventionally produced recombinant baculoviruses contributes to differences in VLP yield. This phenomenon might be attributable to the absence of the viral cysteine protease V-CATH in the modified baculovirus system. We anticipate that use of the MultiBac expression system will facilitate capsid production for papillomaviruses and thereby enable the generation of vaccines against infections by many of the as yet untargeted HPV types.