An IgG1 Single-Dilution Avidity ELISA Predicts Cross-Protection Against Heterologous Foot-and-Mouth Disease Virus Challenge after Vaccination : [Preprint]

Serology can be an alternative to challenge animal trials to predict protection against foot-and-mouth disease virus (FMDV) after vaccination, provided they match in-vivo results. Here we analyzed if indirect ELISAs that use purified 146S particles as capture antigen and measure the quality of the antibodies can improve the agreement with challenge. Day-of-challenge sera from animals that had been vaccinated 21 days before with monovalent formulations containing A Iran 96 or A Iran 99 virus strains were used.  Challenge [1, three PD50 trials] and serology were performed with the A22 Iraq strain. Serology included IgG avidity, IgG1 and IgG2 titers and a virus neutralization test (VNT). Additionally, a single dilution IgG1-specific avidity ELISA (IgG1-AE) was developed. VNT titers against A22 Iraq were below the detection limit (VNT< 0.9, a 1:8 dilution of the sample) in half of the samples. Mean values of samples from non-protected (VNT titer=0.91) and protected animals (VNT titer=1.19) were close to the detection limit. IgG2 ELISA titers were similar between animals with different challenge outcomes (p>0.05). IgG1 titers and total-IgG avidity indexes of total IgG were significantly higher in protected animals (p<0.01). Conditional testing combining results from IgG1 titration and avidity ELISAs was applied, improving the statistical agreement with challenge (Kappa value, K=0.7). IgG1-AE results showed that all protected animals had IgG1 OD values higher than 0.5, and 96% of the samples from protected animals had an anti-A22 Iraq IgG1 avidity index greater than 25%. Using these two parameters informed by a single ELISA, we estimated protection with an accuracy of 94% and the agreement with challenge results was strong (K=0.85). These results highlight the role of high-affinity specific IgG1 in protection against FMDV and support IgG1-AE as a candidate to replace challenge trials to predict cross-protection in FMDV-vaccinated cattle.