A Competitive Serological Assay Shows Naturally Acquired Immunity to Human Papillomavirus Infections in the Guanacaste Natural History Study

Almost all cervical cancers are caused by persistent infections due to carcinogenic human papillomavirus (HPV) types [1]. Cervical cancer develops through characteristic stages, from HPV infection to cervical precancer and cancer [2]. Most HPV infections are transient and resolve after a few months. The host immune system plays an important role in preventing, controlling, and eliminating HPV infection of the cervix [3]. Neutralizing antibodies prevent the initial internalization of the virus in basal cells [4, 5], whereas clearance of a transient infection is thought to be mainly mediated by cellular immune components [6, 7]. Because antibody responses are central to preventing HPV infections, serological assays measuring antibodies directed against HPV may be helpful to identify individuals exposed to HPV and/or protected from subsequent HPV infection [8]. Virus-like particle (VLP) enzyme-linked immunosorbent assays (ELISAs) measure a polyclonal response against HPV VLPs and cannot differentiate between neutralizing and nonneutralizing antibodies. Only approximately one-half of women with results positive for HPV DNA at the cervix experience seroconversion to that HPV type; higher seroconversion rates have been observed with longer duration of infection [9–11]. Most studies have reported similar seroprevalence for HPV16 and HPV18, despite the much higher prevalence of HPV16 infection [10]. Our previous studies in the Guanacaste Natural History Study have not convincingly demonstrated that natural antibody titers against HPV are protective against subsequent infections [9]. A recent VLP ELISA study conducted in the Costa Rica Vaccine Trial has suggested that there is some protection by natural antibody responses, especially at higher antibody titers [12]. HPV vaccination induces high antibody titers against HPV L1 epitopes that correlate with protection from new infections among virtually all women naive to that type of HPV [13]. The vaccine-induced antibodies are polyclonal, and only a subset of the polyclonal response represents neutralizing antibodies [14, 15]. Natural HPV antibody titers are lower than vaccine-induced titers, and it is unclear whether the composition of natural serologic responses is similar to that of vaccine-induced responses. We do not know whether protection observed in vaccinated women is mainly related to high titers or to a higher proportion of neutralizing antibodies. Neutralization assays can measure protective neutralizing antibodies with high specificity but suboptimal sensitivity; they are not applicable in large epidemiologic studies [16]. A new high-throughput competitive Luminex Immunoassay (cLIA) permits epidemiologic studies of HPV serology [17]. The assay measures the competition of antibodies in serum samples with neutralizing antibodies targeting HPV6, HPV11, HPV16, and HPV18 in parallel. We compared results obtained with the cLIA assay in a cohort of women from the Guanacaste Natural History Study with previous results obtained with a VLP ELISA assay.