Antigenic evolution of H3N2 influenza A viruses in swine in the United States from 2012 to 2016.

Background: Six amino acid positions (145, 155, 156, 158, 159, and 189, referred to as the antigenic motif; H3 numbering) in the globular head region of hemagglutinin (HA1 domain) play an important role in defining the antigenic phenotype of swine Clade IV (C-IV) H3N2 IAV, containing an H3 from a late 1990s human-to-swine introduction. We hypothesized that antigenicity of a swine C-IV H3 virus could be inferred based upon the antigenic motif if it matched a previously characterized antigen with the same motif. An increasing number of C-IV H3 genes encoding antigenic motifs that had not been previously characterized were observed in the U.S. pig population between 2012 and 2016.

Objectives: A broad panel of contemporary H3 viruses with uncharacterized antigenic motifs was selected across multiple clades within C-IV to assess the impact of HA1 genetic diversity on the antigenic phenotype.

Methods: Hemagglutination inhibition (HI) assays were performed with isolates selected based on antigenic motif, tested against a panel of swine antisera, and visualized by antigenic cartography.

Results: A previously uncharacterized motif with low but sustained circulation in the swine population demonstrated a distinct phenotype from those previously characterized. Antigenic variation increased for viruses with similar antigenic motifs, likely due to amino acid substitutions outside the motif.

Conclusions: Although antigenic motifs were largely associated with antigenic distances, substantial diversity among co-circulating viruses poses a significant challenge for effective vaccine development. Continued surveillance and antigenic characterization of circulating strains is critical for improving vaccine efforts to control C-IV H3 IAV in U.S. swine.