HPV vaccines for the prevention of cervical cancer Free

LE11-01

The knowledge that genital HPV infection is the central cause of cervical cancer provides an exceptional opportunity for cancer prevention via prevention of infection by these oncogenic viruses. Early development of prophylactic vaccines was limited by the inability to efficiently propagate papillomaviruses in culture. Initial attempts to develop subunit prophylactic vaccines were unsuccessful because short peptides or malfolded versions of the major capsid protein, L1, were employed as immunogens and virion neutralizing antibodies predominantly recognize complex conformation-dependent epitopes. This limitation was overcome when we demonstrated that L1 could, when expressed via recombinant baculovirus-infected insect cells, self-assemble in virus-like particles (VLPs) in the absence of all other viral proteins. These VLPs closely resembled authentic virions morphologically, and, most importantly, mimicked authentic virions in the ability to induce high titers of virion neutralizing antibodies after parenteral injection. This principle was first established in a bovine papillomavirus model, because there were no quantitative infectivity assays for HPV at the time. We and several other labs subsequently developed in vitro neutralization assays for HPVs and established that HPV L1 VLPs also induce high titers of type-restricted antibodies. Initial studies with HPV16, which is detected in a majority of cervical cancers, were problematic, in that its L1 assembled very poorly into VLPs and the aggregates that were produced did not induce neutralizing antibodies. However, we discovered that the widely used prototype clone for this virus, which was isolated from a cancer, contained a mutant L1 gene. Other HPV16 L1 genes, derived from productive lesions, produced abundant VLPs, and these wild type genes were used in subsequent vaccine development. In a series of animal studies that followed, it was established that papillomavirus VLPs could protect against high dose challenge with the homologous, but not heterologous, animal virus, and protection could be passively transferred via immune sera. However, therapeutic vaccination did not induce regression of established lesions.

Based on these promising preclinical results, clinical trials were initiated by three groups, the NCI, Merck, and Medimmune. The NCI and Medimune produced clinical grade VLPs in insect cells while Merck chose to produce their VLPs in yeast. In the first phase 1 dose-escalation trial, we found that intramuscular HPV16 L1 VLP vaccination was safe and highly immunogenic, even in the absence of adjuvant, although adjuvant did boost responses to low dose VLPs. The Merck and Medimune vaccines, which contained conventional aluminum adjuvants, were also shown to be safe and highly immunogenic in early phase trials. These results led to three ongoing efficacy trials sponsored by Merck, GlaxoSmithKline (GSK), which assumed the Medimune project, and NCI, which is testing the GSK vaccine. Results to date indicate that three intramuscular injections of VLPs induce nearly 100% protection against incident persistent infection and development of cervical, vulvar and vaginal displasias that are caused by HPV16 and HPV18, the oncogenic types targeted by the vaccines. However, protection against other types appears limited, and there is no suggestion that the vaccines induce the regression of established infection. The Merck vaccine, which also contains HPV6 and HPV11 VLPs, protects almost completely against genital warts caused by these types. Continued strong protection four years after vaccination, at which time VLP antibody titers have reached a plateau, suggests that protection may be long term. The Merck vaccine received regulatory approval for distribution to females 9-26 in the U.S. and elsewhere in 2006. Approval of the GSK vaccine is expected within a year.
 >We recently developed a technology to efficiently produce high titer papillomavirus pseudovirions, which transduce marker gene plasmids. We used these pseudoviruses to develop high throughput in vitro HPV neutralization assays to facilitate immune monitoring of vaccine trials. Using these assays we were able to discover that vaccines based on L2, the minor capsid protein, could, unlike L1 VLPs, induce broadly type cross-neutralizing antibodies. The pseudoviruses were also used to screen for HPV infection inhibitors that might serve as topical microbicides. Carrageenan, a widely used food and cosmetic additive, was found to be a potent inhibitor of in vitro infection for a wide spectrum of genital HPV types. Remarkably, certain commercial vaginal lubricants based on carrageenan gels were effective inhibitors even when diluted one million-fold. Based on the pseudovirus technology, we recently developed the first in vivo cervicovaginal challenge model for HPVs, and demonstrated that in vivo infection is also potently inhibited by carrageenan, raising the possibility that if shown to be effective in clinical trials, carrageenan might serve as an adjunct to the current HPV vaccine. Interestingly, nonoxynol-9 (N-9) and N-9-containing over-the-counter spermicides greatly potentiate pseudovirus infection, apparently by compromising the integrity of the genital tract epithelium. However, this potentiating effect could be overcome by formulating the nonoxynol-9 in a carrageenan-based gel.