The L1 Major Capsid Protein of Human Papillomavirus Type 16 Variants Affects Yield of Virus-Like Particles Produced in an Insect Cell Expression System

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Abstract
The L1 major capsid proteins of six human papillomavirus type 16 (HPV-16) strains were expressed in insect cells by using recombinant baculoviruses. Virus-like particles (VLPs) which appeared similar to empty virions were identified by electron microscopy for all HPV strains investigated. However, the yield of VLPs produced varied in a range from 1 to 79 depending on the HPV-16 strain. The L1 proteins of these strains differed by up to 15 amino acids from the L1 protein of the prototype HPV-16 strain. Mutations in the amino acid region from residues 83 to 97 seemed to affect the level of expression of the L1 protein. These results are important when considering the development of HPV vaccines and serological tests. They indicate that strains inducing high levels of VLP production must be selected for the development of vaccines. Moreover, the L1 proteins of all strains investigated were able to bind with DNA. We also investigated the seroreactivities of VLPs derived from three different HPV-16 strains from Algeria, Senegal, and the Philippines by testing sera from women from 11 countries in immunoglobulin G-specific enzyme-linked immunosorbent assays. We observed a strong correlation between the reactivities of the three different VLP variants, independent of the geographical origin of the sera investigated. These results indicate that the three strains investigated are serologically cross-reactive despite the fact that their L1 proteins differ in 14 amino acids and suggest that VLPs derived from only one HPV-16 strain could be sufficient for the development of an HPV-16 vaccine and anti-HPV-16 tests.