Genetic and antigenic characterization of recent human-like H1 (δ-cluster) swine influenza virus isolates

Objective: To assess genetic and antigenic properties of contemporary human-like H1 (δ-cluster) swine influenza virus (SIV) isolates circulating in US swine herds. Materials and methods: The hemagglutinin genes from 37 δ-cluster SIV isolates were fully sequenced. The isolates were antigenically categorized using a high throughput serum neutralization (HTSN) assay incorporating antisera generated against the four currently circulating clusters of H1 SIV and a commercial vaccine containing a δ-cluster SIV isolate. A subset of the isolates were further characterized with the hemagglutination inhibition (HI) assay using H1 SIV antisera. Results: Genetic analysis of the hemagglutinin gene of 2009-2010 δ-cluster isolates identified five distinct subclusters with 97% to 100% sequence similarity within subclusters and 95% to 97% similarity between subclusters. Antisera generated against SIV representing α, β, and γ clusters failed to neutralize any of 37 δ-cluster viruses in an HTSN assay. Only 46% of the δ-cluster isolates were neutralized by at least one δ-cluster antiserum. Hemagglutination inhibition assay results on a subset of thirteen 2010 isolates were in good agreement with the HTSN assay, with 38% of the isolates positive (HI titer > 40) to at least one δ-cluster antiserum. There was no measurable antibody titer to antiserum generated from the commercial vaccine or the 13 isolates. Implications: Significant genetic and antigenic heterogeneity exists among δ-cluster H1 SIV isolates, suggesting that either multiple representatives of this cluster may be required in commercial vaccines or that herd-specific vaccines may be required to protect swine from influenza virus infection.