Mucosal correlates of cross-protection for live-attenuated influenza virus vaccines in pigs

Controlling swine influenza A virus (IAV) has become increasingly difficult with the emergence of novel reassorted strains and introduction of human seasonal IAV into pigs. In North American swine there are 6 antigenically distinct H1 subtypes currently circulating. Live-attenuated influenza vaccines (LAIV) provide broader cross-protection than whole-inactivated virus (WIV) vaccine making LAIV a candidate for next-generation swine IAV vaccines. However, there is no standardized assay to predict cross-protection following LAIV vaccination. Hemagglutination-inhibiting (HI) immunoglobulin (Ig) in serum has long been the gold standard correlate of protection following WIV vaccination; however, LAIV does not elicit a robust serum HI Ig titer. Oral fluids (OF) have become a rapidly developing diagnostic specimen for a variety of animal pathogens. We evaluated nasal wash (NW) and OF samples for measuring cross-reactive Ig responses to a panel of H1 viruses following vaccination. Both NW and OF had detectable levels of IAV-specific Ig when measured by ELISA. NW from LAIV vaccinated pigs had significant cross-reactive IgA titers, while NW from WIV vaccinated pigs did not have detectable IgA. Samples of OF from either LAIV or WIV vaccinated pigs had high titers of cross-reactive IAV-specific IgG and measurable IgA. OF samples also displayed IAV specific neutralization. These data suggest OF can serve as a sample for evaluating LAIV immunogenicity and predicting cross-protection.