Antigenic Drift of H7N9 Viral Hemagglutinin.

Background: Since its emergence in 2013, there have been five H7N9 influenza epidemic waves in China. However, evolution of the hemagglutinin (HA) antigenicity has not been systematically investigated.

Methods: To better understand how antigenic drift in HA proteins of H7N9 occurs, 902 H7N9 HA sequences from public database were retrieved and analyzed. 53 mutants with single amino acid substitutions in HA were introduced into pseudoviruses and their antigenic characteristics were analyzed using pseudovirus-based assays.

Results: Nine mutations incrementally increased over the past 5 years, with mutations identified at multiple sites. While mean neutralization titers of most variants remain unchanged, three mutations, A143V, A143T and R148K displayed a median 4-fold lower susceptibility to neutralization by antisera against A/Anhui/1/2013(H7N9) (A/Anhui). Notably, A143V and A143T were located outside the previously reported antigenic sites. The most dominant variant (A143V/R148K) in the most recent season constitutes 74.11% of all mutations and demonstrated a 10-fold reduction in its reactivity to A/Anhui antisera. Importantly, compared to DNA construct without the corresponding HA mutation, DNA vaccine encoding A143V/R148K mutant induced a 5-fold increase in the neutralizing activity against this circulating virus.

Conclusions: Appropriate vaccine strain should be considered in response to increasing antigenic drift in H7N9 HA.