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By Dr Julian Seago, The Pirbright Institute

Schematic representation of a FMDV virion (146S) dissociating into its constituent pentameric subunits (12S) and RNA genome. Positions of the three structural proteins (VP1-3) on the external surface of the capsid are shown, the fourth (VP4) is positioned internally.

The following work is part of an ongoing collaboration between the research teams of Jenner Investigators Dr Bryan Charleston at The Pirbright Institute and Professor David Stuart at the Division of Structural Biology (STRUBI), University of Oxford.

The effectiveness of FMD vaccines is influenced by multiple factors such as route of vaccination, dose, similarity to circulating virus and adjuvant. Antigen integrity is also imperative for vaccine efficacy; FMD vaccines that are comprised of dissociated viral particles do not elicit adequate levels of protective neutralising antibodies. This problem is exacerbated by the fact that FMDV capsids (146S particles) readily dissociate at mild acidic conditions and at room temperatures into their constituent subunits (12S particles).

In addition, chemical inactivation further decreases the stability of the FMDV capsid (Doel and Baccarini 1981). A research team headed by Bryan Charleston at The Pirbright Institute has recently focused on (i) the development of alternative FMDV vaccines based on virus-like particles (VLPs), and (ii) the generation and characterization of more stable infectious virions and VLPs. To quantify the antigen content and integrity rent assays have been developed. The first, termed the thermofluor assay, is qPCR-based technique that monitors viral genome release as an indicator of capsid disassembly using a dye sensitive to the presence of nucleic acid during a slow increase in temperature.

The second assay is ELISA-based and uses llama single-domain antibodies (VHHs) that are specific for intact viral capsids. Michiel Harmsen and Aldo Dekker at the Central Veterinary Institute Wageningen UR, The Netherlands (CVI) have developed the selection of such 146s specific VHHs using phage display libraries derived from llamas immunized against FMDV. Both assays are userfriendly and can be quickly performed with minimal training using standard laboratory equipment. In combination, the techniques complement each other and have the potential to be applied for quality control of FMDV vaccines, both during and after the production process, as well as for the characterisation of optimal vaccine storage conditions.

At the 2014 EuFMD meeting in Croatia, the use of both techniques as diagnostic tools for FMDV capsid stability was presented, generating a lot of interest amongst the FMDV community. This has led to a successful application for EuFMDFAR funding. The collaborative project combines the technical expertise of Julian Seago and Eva Perez at The Pirbright Institute in performing the assays with established methodologies at CVI in the production of VHHs.

The project will develop and assess the assays on FMD vaccine strains currently being produced in East Africa by vaccine companies, with the intention of transferring the technologies for their future  application; this will be mediated through Nick Lyons at The Pirbright Institute.