Production of Neuraminidase Virus Like Particles by Stably Transformed Insect Cells: A Simple Process for NA-Based Influenza Vaccine Development

Neuraminidase (NA) is a second major surface protein of the influenza virus and has recently been suggested as a supplemental antigen to the major immunodominant hemagglutinin (HA) antigen in the influenza vaccine. NA is less affected by antigenic drift compared to the HA, induces strong anti-neuraminidase immune responses, and provides broader protection against many influenza strains. However, the NA amount in currently licensed influenza virus vaccines is much lower than that of HA, and not standardized. A platform to produce NA antigen, in the form of virus-like particles (VLPs), was thus developed, to facilitate supplementation of NA antigen in the influenza vaccine formula. Stably transformed Sf9 insect cells had been engineered to express the influenza A virus (H5N1) NA gene under a baculovirus OpMNPV IE2 promoter. Recombinant NA protein was synthesized and assembled into VLPs, in the intact cellular environment provided by insect cells. Approximately 150 µg/ml of NA-VLPs was obtained in the culture medium. Purification of the NA-VLPs was achieved by a sucrose density gradient ultracentrifugation. The purified NA-VLPs effectively induced anti-NA antibodies with neuraminidase inhibition activities in mice. This work demonstrates a simple process to produce an immunocompetent NA-VLPs antigen, exclusively made of only neuraminidase, by insect cells.