Measurement of HPV antibodies in unvaccinated individuals has been used as measure of lifetime exposure to HPV. In the absence of commercial assays and immune correlates of protection, laboratories have developed a variety of assays, most commonly based on ELISAs with conformationally intact HPV L1 viral-like particles (VLPs) as antigen. With regulatory approval and implementation of HPV vaccines, serology assays, i.e. immunoequivalency in terms of seroconversion and titer, are now being relied on as endpoints in clinical trials of reduced and altered dosing schedules, as well as of new vaccine formulations and biosimilar vaccines. Thus, there is a need to standardize serology assays to improve comparability of results between assay formats and platforms. The WHO HPV Labnet (2006-2011) had taken the first steps to harmonize HPV serology assays through inter-laboratory comparisons and development and validation of International Standards (IS) for HPV 16 and 18. IS allow titers to be expressed in International Units (IU), greatly facilitating comparison of results. Additional needs for standardization include IS for the additional 7 HPV types targeted by current HPV vaccines, reagents and protocols to validate type-specific VLPs, serum panels for setting negative cut-off values and serum panels for ongoing inter-laboratory comparisons. NCI is sponsoring an international meeting on HPV Serology Standardization in March 2016 to assist in assay harmonization, and their recommendations and action plan will be discussed.
To address the public health needs for HPV serology, CDC searched for a high-throughput multiplex assay platform for reproducible type-specific detection. We used the Meso Scale Discovery (MSD) electrochemiluminescent detection platform to develop a 9-plex direct VLP IgG ELISA. Samples and standard are tested at three point titrations. Antibody titers are calculated in IU or Arbitrary Units/ml in reference to the standard, using the parallel line method. The assay range of detection is ~1000 fold and precision is ≤25%. Performance was evaluated by comparison with neutralization and competitive Luminex results. Advantages include reduced capture antigen and sample, low backgrounds, fast read times and a large dynamic range. However, multi-spot printing must be performed by the vendor and requires evaluation of spotting quality. The assay has been used to estimate population and cohort based seroprevalence as well as vaccine response in special populations.
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