The novel H7N9 avian influenza A virus has caused human infections in China since 2013; some isolates from the fifth wave of infections have emerged as highly pathogenic avian influenza viruses. Recombinant hemagglutinin proteins of H7N9 viruses can be rapidly and efficiently produced with low-level biocontainment facilities. In this study, recombinant H7 antigen was obtained from engineered stable clones of Chinese Hamster Ovary (CHO) cells for subsequent large-scale production. The stable CHO cell clones were also adapted to grow in serum-free suspension cultures. To improve the immunogenicity of the recombinant H7 antigens, we evaluated the use of a novel combination adjuvant of PELC and CpG (PELC/CpG) to augment the anti-H7N9 immune responses in mice. We compared the effects with other adjuvants such as alum, AddaVax (MF59-like), and several Toll-like receptor ligands such as R848, CpG, and poly (I:C). With the PELC/CpG combination adjuvant, CHO cell-expressed rH7 antigens containing terminally sialylated complex type N-glycans were able to induce high titers of neutralizing antibodies in sera and conferred protection following live virus challenges. These data indicate that the CHO cell-expressed recombinant H7 antigens and a PELC/CpG combination adjuvant can be used for H7N9 subunit vaccine development.
