Chimeric L1-L2 Virus-Like Particles as Potential Broad-Spectrum Human Papillomavirus Vaccines

The amino (N) terminus of the human papillomavirus (HPV) minor capsid protein L2 can induce low-titer, cross-neutralizing antibodies. The aim of this study was to improve immunogenicity of L2 peptides by surface display on highly ordered, self-assembled virus-like particles (VLP) of major capsid protein L1, and to more completely characterize neutralization epitopes of L2. Overlapping peptides comprising amino acids (aa) 2 to 22 (hereafter, chimera or peptide 2-22), 13 to 107, 18 to 31, 17 to 36, 35 to 75, 75 to 112, 115 to 154, 149 to 175, and 172 to 200 of HPV type 16 (HPV16) L2 were genetically engineered into the DE surface loop of bovine papillomavirus type 1 L1 VLP. Except for chimeras 35-75 and 13-107, recombinant fusion proteins assembled into VLP. Vaccination of rabbits with Freund''s adjuvanted native VLP induced higher L2-specific antibody titers than vaccination with corresponding sodium dodecyl sulfate-denatured proteins. Immune sera to epitopes within residues 13 to 154 neutralized HPV16 in pseudovirion neutralization assays, whereas chimera 17-36 induced additional cross-neutralization to divergent high-risk HPV18, -31, -45, -52, and -58; low-risk HPV11; and beta-type HPV5 (titers of 50 to 10,000). Aluminum hydroxide-monophosphoryl lipid A (Alum-MPL)-adjuvanted VLP induced similar patterns of neutralization in both rabbits and mice, albeit with 100-fold-lower titers than Freund''s adjuvant. Importantly, Alum-MPL-adjuvanted immunization with chimeric HPV16L1-HPV16L2 (peptide 17-36) VLP induced neutralization or cross-neutralization of HPV16, -18, -31, -45, -52, and -58; HPV6 and -11; and HPV5 (titers of 50 to 100,000). Immunization with HPV16 L1-HPV16 L2 (chimera 17-36) VLP in adjuvant applicable for human use induces broad-spectrum neutralizing antibodies against HPV types evolutionarily divergent to HPV16 and thus may protect against infection with mucosal high-risk, low-risk, and beta HPV types and associated disease.The more than 100 types of human papillomaviruses (HPV) identified to date (14) are the etiological agents of skin and mucosal papillomas or warts. Persistent infection with high-risk mucosal types, most often HPV type 16 (HPV16) and HPV18, causes cervical cancer, which constitutes the second leading fatal cancer in women worldwide, causing 274,000 deaths per year. Substantial morbidity results from other noncervical HPV-related conditions, such as anogenital warts or anal cancer (23).The development of current prophylactic papillomavirus vaccines was launched by observations that recombinantly expressed major capsid protein L1 self-assembles into virus-like particles (VLP). These empty viral capsids are composed of 360 L1 molecules and resemble native virions in both structure and immunogenicity, yet are nononcogenic and noninfectious. Moreover, VLP cannot replicate because the cells in which VLP are made contain only L1 and no other papillomavirus genes. Subunit VLP vaccines induce high-titer and type-restricted antibody responses to conformational L1 epitopes (12, 26, 39, 44). When applied to women prior to infection, available vaccines targeting the most prevalent high-risk types, HPV16 and HPV18, have demonstrated up to 100% efficacy against persistent infection and associated disease caused by the included types and thus are potentially able to prevent ∼70% of cervical high-grade dysplasias and probably cancers (22, 46). Therefore, use of currently licensed L1 vaccines necessitates continuation of cytological cervical screening of women. The prevention of 96% of cervical cancer would require immunity to seven high-risk HPV types (HPV16, -18, -31, -33, -45, -52, and -58) (32) and the development of more highly multivalent (and presumably costly) L1 VLP vaccines.The search for alternative broader-spectrum immunogens drew attention to the minor capsid protein L2, which is immunogenically subdominant in the context of coexpressed L1-L2 capsids (38). Immunization of animals with the amino (N)-terminal peptide of L2 demonstrated its ability to elicit low-titer neutralizing antibodies that protect against challenge with cognate papillomavirus types in vivo (16, 19), cross-neutralize heterologous types in vitro (25, 33, 38), and confer cross-protection in vivo (17).This study addresses two major issues that may further the development of L2-based broader-spectrum vaccines. First, the N terminus of L2 is more closely examined for potential neutralization epitopes, by incorporating peptides into papillomavirus VLP as peptide-presenting platforms (7, 21, 42). Moreover, we take advantage of the immunogenic characteristics of virion surfaces, such as the dense repetitive surface array of VLP, to induce strong and enduring immune responses to displayed L2 epitopes.