Essential sequence of influenza B virus hemagglutinin DNA to provide protection against lethal homologous viral infection.

Hemagglutinin (HA) is the main surface glycoprotein of influenza B virus. The B/Ibaraki/2/85 virus HA gene is 1758 bp in length, including signal peptide sequence, HA1 sequence, and HA2 sequence. We previously proved that B/Ibaraki/2/85 HA DNA induced immune response and provided effective protection in mice against challenge with homologous virus. In this study, a series of recombinant plasmids encoding truncated HA gene were constructed by PCR. BALB/c mice were immunized with the plasmids and challenged with a lethal dose of homologous virus. The essential sequence of HA DNA against influenza virus was explored by evaluation of survival rate, lung virus titer, bodyweight change, and serum anti-HA antibody titer of mice. The result showed that serial deletion did not deprive HA DNA of its protective ability until 885 nucleotides (295 amino acids) at 3'-terminal or 9 nucleotides of the signal peptide sequence at 5'-terminal were deleted. When the signal peptide sequence was kept intact and the 5'-terminal deletion started at the beginning of the HA1 sequence, deletion of 51 nucleotides (17 amino acids) made HA DNA lose its protective ability. This suggests that the sequence nt94-876 of B/Ibaraki/2/85 virus HA DNA played an important role in protection against infection.