Selection pressure on the hemagglutinin gene of influenza A (H1N1) virus: adaptation to human and swine hosts in Asia.

UNLABELLED: Influenza A (H1N1) virus is an important pathogen that can be transmitted in the enzootic cycle involved in influenza epidemics. In this study, we analyzed natural selection on the 260 sequences of subtype 1 hemagglutinin (HA1) gene of H1N1 viruses isolated from humans and swine in Asia. Nonsynonymous and synonymous substitution rates (dN, dS) were determined for each amino acid site in the HA1 coding region. Four and two positively selected sites were identified in human and swine viruses, respectively. These sites might be involved in the change of antigenic epitopes to evade the host immune system. The results indicate that more diversifying and less purifying selection controls HA1 of human viruses compared with the swine viruses. Furthermore, thirteen sites evolving differently in human and swine viruses were identified in the HA1. These sites may play a role in the cross-species transmission. The analysis of adaptive evolution of the HA1 can provide valuable information for accelerating vaccine and drug development for prevention and treatment of influenza A (H1N1) in Asia. KEYWORDS: Influenza A virus; H1N1; hemagglutinin; positive selection; antigenic epitope.