Papillomaviruses (PVs) infect the epithelia of animals and man, where they generally induce benign proliferations at the sites of infection. In addition, persistent infection by some human papillomaviruses (HPV), especially HPV16, can induce malignant progression of human genital lesions and certain human papillomavirus (HPV) types. Our research is concerned with the biology of papillomavirus infection, elucidation of the PV life cycle, and development of vaccines and other strategies to inhibit HPV infection. We previously developed a simple and efficient strategy for generating high titers of infectious papillomavirus particles that transduce encapsidated marker plasmids, i.e. pseudovirions. We have exploited this technology in our basic virologic and translational research efforts, leading to development of the first cervicovaginal challenge model for HPVs. Using the mouse model, we determined that transient disruption of epithelial integrity is required for HPV infection, that the first step in the virus life cycle involves binding of the virus to the basement membrane, and that the virus transfers to target epithelial cells only after the L2 protein in the capsid has undergone an enzymatic cleavage while on the basement membrane. The cleavage of L2 is associated with a conformational change in the capsid, leading to exposure of highly conserved L2 cross-neutralization epitopes that lie C-terminal to the cleavage site. In a process that takes several hours, the virions transfer from the basement membrane to the surface of keratinocytes invading the site of trauma, and the virions are then internalized. The mouse model also enabled us to determine that antibodies induced by L1 VLP vaccines and L2 vaccines block in vivo infection by distinct mechanisms. At high concentrations, L1 VLP-induced antibodies block virion binding to the basement membrane, while at low concentrations, they permit basement membrane binding but prevent binding to target keratinocytes. Since the L2 cross-neutralization epitopes are cryptic, and are not exposed until after the virions have been bound to the basement membrane, L2 antibodies that broadly cross-neutralize HPV types do not interfere with initial basement membrane binding. Once the L2 epitopes are exposed, while the virions remain on the basement membrane, the L2 antibodies can bind to the virions, and abrogate the infectious process by preventing transfer of the virions to target keratinocytes. Both in the mouse model and in an analogous model developed for the rhesus macaque, infection is prevented by local application of carrageenan, which is an algal polysaccharide widely used in processed food and cosmetics and is the main gelling agent in some over-the-counter lubricants. Based on our data, clinical trials of carrageenan as a microbicide to prevent genital HPV infection in young women are underway. Using HPV pseudoviruses, we previously developed a high throughput in vitro HPV neutralization assay that has become the gold standard for the field. Based on the improved understanding of the in vivo infectious process and the mechanism by which L2 vaccines conferred protection, we developed an alternative in vitro neutralization assay that is approximately three orders of magnitude more sensitive than the standard assay in detecting L2 neutralizing antibodies. The alternative assay has considerable potential for determining the immunogenicity of candidate L2 vaccines and for clinical trials of L2 vaccines. Our development of a method to induce efficient HPV pseudovirus infection of the female genital tract after transient epithelial disruption with nonoxynol-9 (N-9) proved to be the key to our development of an effective intravaginal vaccination strategy. In patent pending studies, we found that intravaginal pseudovirus vaccination of N-9-treated mice induces strong systemic and mucosal T and B cell responses to target antigens transduced by the pseudovirions. Systemic responsesrival those induced by previously optimized Ad5 vectors. Intravaginal responses are remarkably strong, with up to 80% of intravaginal CD8 T cells staining tetramer positive for the targeted antigen. Most of the induced T cells appear to be intraepithelial and are maintained in the vaginal tract at least 100 days after vaccination. Intravaginal pseudovirus vaccination is a promising approach for focusing immune responses to the female genital tract that may increase the effectiveness of therapeutic vaccines directed at herpes simplex virus infection, HIV infection, and HPV infection. HIV-positive women and men have a higher risk of HPV-induced cancers. In NCI human clinical trials of the bivalent HPV vaccine (manufactured by GlaxoSmithKline) conducted in Costa Rica, post-hoc analyses of women who received fewer than the standard 3 dose regimen indicated they were as protected, during 7 years of study, against infection by the HPV types targeted by the vaccine (HPV16/18) as the women who received all 3 doses. In addition, the women who received only one dose had stable serum HPV16/18 antibody levels between years 1 and 7, suggesting that the strong protection may be durable. This surprising result raised the possibility that a single vaccine dose might confer long-term protection. A rigorously controlled clinical trial that tests the efficacy of one vaccine dose vs. two doses has now started in Costa Rica.
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