Antigenic characterization of H5N1 highly pathogenic avian influenza viruses isolated from poultry in India, 2006-2015
Highly pathogenic avian influenza (HPAI) is a major health concern worldwide. In this study, we focused on antigenic analysis of HPAI H5N1 viruses isolated from poultry in India between 2006 and 2015 comprising 25 isolates from four phylogenetic clades 2.2 (1 isolate), 184.108.40.206 (1 isolate), 220.127.116.11a (17 isolates) and 18.104.22.168c (6 isolates). Seven H5N1 isolates from all four clades were selected for production of chicken antiserum, and antigenic analysis was carried out by hemagglutination inhibition (HI) assay. HI data indicated antigenic divergence (6-21 fold reduction in cross-reactivity) between the two recently emerged clades 22.214.171.124a and 126.96.36.199c. These two clades are highly divergent (21-128 fold reduction in HI titre) from the earlier clades 2.2 /188.8.131.52 isolated in India. However, a maximum of 2-fold and 4-fold reduction in cross-reactivity was observed within the isolates of homologous clades 184.108.40.206c and 220.127.116.11a, respectively. The molecular basis of inter-clade antigenic divergence was examined in the haemagglutinin (HA) antigenic sites of the H5N1 virus. Amino acid changes at 8 HA antigenic sites were observed between clades 18.104.22.168a and 22.214.171.124c, whereas 20-23 substitutions were observed between clades 126.96.36.199a/188.8.131.52c and 2.2/184.108.40.206. Therefore, a systematic analysis of antigenic drift of the contemporary field isolates is a pre-requisite for determining the suitable strain(s) for vaccine candidature.