After HPV infection, single-seropositive antibodies were highly type-specific and neutralizing whereas multi-positive sera were less specific and tended to be non-neutralizing. of antibodies induced by HPV illness (genital HPV pseudovirion challenge, that already very low vaccine-derived HPV-specific antibody levels could protect against HPV infections and that these protecting antibody levels are much below detection limits of the pseudovirion-based neutralization assay [33]. We could not find an association between the level and the neutralizing capacity of naturally induced antibodies. This is definitely in line with a study of Lu et al., showing that the level of HPV16 antibodies was not connected with a lower risk of HPV illness [34]. Apparently, the HPV antibody level after illness will provide limited information about a correlate CD350 of safety. Other immune guidelines than antibody levels, that might correlate with safety, have not been defined and data on antibody characteristics such GW-1100 as avidity are scarce [16]. Antibody avidity generally raises over time following encounter with an antigen. Memory reactions are characterized by the production of high-avidity antibodies. Therefore, the level of antibody avidity could be regarded as a surrogate of successful induction of immunological memory space [35]. Vaccine-derived neutralizing antibody levels correlate with antibody avidity 6 months and one year after HPV vaccination [15,16]. We found that antibody avidity after vaccination was 3 times higher than after HPV illness. After HPV illness, we observed a wide range of antibody avidity levels in sera of naturally infected individuals reflecting a great biological diversity in individual reactions to HPV illness. Naturally induced HPV16 antibodies of low avidity have been associated with possible susceptibility to illness GW-1100 with additional HPV types [14]. Higher avidity indices tended to become associated with naturally derived neutralizing HPV-specific antibodies, albeit tested in small sample sizes. This indicates that levels of antibody avidity might be useful to distinguish between protecting and non-protective HPV-specific antibodies. Therefore, antibody avidity might be a potential immune surrogate of HPV safety, but further study is necessary to confirm this. Our findings on IgG subclasses are in line with Harro et al. reporting that IgG1 was the predominant subclass induced GW-1100 after VLP vaccination [36]. In contrast, Matsumoto et al. found the IgG2 subclass to be dominating in HPV16 seropositive ladies and that IgG2 dominance was associated with the regression of CIN, albeit in a small sample size [37]. IgG1 and IgG3 are generally induced in response to protein antigens, whereas IgG2 is definitely associated with the immune response against polysaccharide antigens and IgG4 with allergy [38,39]. In conclusion, we showed that vaccine-derived antibodies were primarily genotype-specific and cross-reacted only for a smaller part with additional HPV types within the varieties. After HPV illness, single-seropositive antibodies were highly type-specific and neutralizing whereas multi-positive sera were less specific and tended to become non-neutralizing. This might mean that naturally induced HPV-specific antibodies in multi-HPV positive individuals could not or only partially protect against subsequent HPV illness and these individuals are still at risk. Although sample sizes were small, neutralizing antibodies in single-positive sera inclined to be associated with higher antibody avidity indices than non-neutralizing antibodies. Further, we found the avidity of vaccine-derived antibodies to be approximately 3 times higher than after HPV illness. These results imply that the avidity of HPV antibodies might be used like a potential surrogate of safety. However, more studies are needed to set up the part of HPV antibody avidity like a potential surrogate of safety and the use of this immunological tool in sero-epidemiological and vaccine monitoring studies. Acknowledgments The Rwandan samples, utilized for the dedication of IgG subclasses, were kindly supplied by the Project Ubuzima group, in?Kigali,?Rwanda. We would like to say thanks to Sylviane Poncelet and Francis Dessy (GlaxoSmithKline vaccines, Belgium) who coordinated the PBNA analysis. Funding Statement The authors have no support or funding to statement..