A Murine Genital-Challenge Model Is a Sensitive Measure of Protective Antibodies against Human Papillomavirus Infection

Abstract

The available virus-like particle (VLP)-based prophylactic vaccines against specific human papillomavirus (HPV) types afford close to 100% protection against the type-associated lesions and disease. Based on papillomavirus animal models, it is likely that protection against genital lesions in humans is mediated by HPV type-restricted neutralizing antibodies that transudate or exudate at the sites of genital infection. However, a correlate of protection was not established in the clinical trials because few disease cases occurred, and true incident infection could not be reliably distinguished from the emergence or reactivation of prevalent infection. In addition, the current assays for measuring vaccine-induced antibodies, even the gold standard HPV pseudovirion (PsV) in vitro neutralization assay, may not be sensitive enough to measure the minimum level of antibodies needed for protection. Here, we characterize the recently developed model of genital challenge with HPV PsV and determine the minimal amounts of VLP-induced neutralizing antibodies that can afford protection from genital infection in vivo after transfer into recipient mice. Our data show that serum antibody levels >100-fold lower than those detectable by in vitro PsV neutralization assays are sufficient to confer protection against an HPV PsV genital infection in this model. The results clearly demonstrate that, remarkably, the in vivo assay is substantially more sensitive than in vitro PsV neutralization and thus may be better suited for studies to establish correlates of protection.Cervical cancer, the second most common cause of cancer death in women worldwide, is associated with high-risk types of human papillomavirus (HPV) infections (27). HPV vaccines based on L1 virus-like particles (VLPs) have been shown to be safe and efficient at preventing infections and precancerous lesions caused by HPV vaccine-related types (26, 33) and now have been commercialized, specifically the HPV6/11/16/18 VLP Gardasil and the HPV16/18 VLP Cervarix vaccines. Neutralizing antibodies (Ab) are thought to be the primary immune mechanism of protection by HPV vaccination, primarily based on preclinical papillomavirus (PV) animal models showing that the passive transfer of immunized sera is protective in naïve rabbits and dogs against skin and oral mucosal challenge, respectively (3, 31). In addition, clinical trials showed that vaccinated individuals developed robust anti-VLP antibody titers in serum (15, 32) and in cervicovaginal secretions (21, 23), and that antibody-mediated cross-type neutralization in in vitro assays largely parallels the cross-type protection in the trials. However, these trials did not allow the establishment of antibody concentrations or thresholds that could be correlated to protection, mainly because too few disease cases occurred (26, 33) and because breakthrough infections could not be unambiguously distinguished from the emergence or reactivation of prevalent infection. In addition, the serological assays that were used in the trials (pr...

A Human Papillomavirus (HPV)In VitroNeutralization Assay That Recapitulates theIn VitroProcess of Infection Provides a Sensitive Measure of HPV L2 Infection-Inhibiting Antibodies

“…Transfer of 2 l of serum resulted in intermediate in vivo protection (35% to 89%) and in vitro titers from 154 to 228. In vitro titers were below the limit of detection for 4/5 animals at the 0.2-l dose, whereas 3 animals were partially protected in vivo (16,23, and 70% protection). Therefore, the in vivo assay may still be slightly more sensitive when considering serum concentrations that offer a lower level of protection.…”

“…A recent mouse study identified substantial differences in the relative sensitivities of the in vivo genital HPV challenge assay and the standard L1-based in vitro assay as measures of protective L1 antibodies (23). These authors demonstrated that the in vivo detection of protection was slightly more sensitive (30-fold) than the in vitro assay for a murine L1-specific monoclonal antibody.…”

Comparison of HPV prevalence between HPV-vaccinated and non-vaccinated young adult women (20–26 years)

“…This may be because protection of the vaccine is provided through the production of serum neutralizing anti-HPV IgG antibodies binding to viral particles, [17][18][19] which only requires small amounts of antibody to be present. 20,21 One interesting finding was that vaccinated young adult women had a higher prevalence than unvaccinated women of www.tandfonline.comhigh-risk types other than HPV 16 and 18, and thus are still at risk of cervical cancer and other HPV-related cancers. This is consistent with clinical trials on the quadrivalent vaccine which showed it provided some protection against HPV 31 and 59, but not other types.…”

“…Details about NHANES can be found elsewhere. 27 Our study included young adult women (20)(21)(22)(23)(24)(25)(26) was first approved for use in 2006, all women in this age range would have received the vaccine after 12 years of age. This age group was selected because few data are available on those vaccinated after the recommended age of 11-12 years.…”

Current Perspectives on HPV Vaccination: A Focus on Targeting the L2 Protein

“…While a passive transfer of immune serum clearly demonstrates the presence of neutralizing antibodies, a challenge of an immunized animal is less clear as immune mechanisms other than antibodies may be mediating protection. Of note, Longet and colleagues already demonstrated that passive transfer of mouse L1 VLP (Gardasil) immune sera >100-fold lower (in titer) than those detectable by standard in vitro neutralization assays are sufficient to confer protection against HPV pseudovirion genital infection in recipient mice [143]. While the effect may not be as pronounced for most L2 immune sera, the study provides further conviction that the standard in vitro neutralization assay actually de-emphasizes protective antibody titers.…”