Evaluation of Epic® label-free technology to quantify functional recombinant hemagglutinin

Positive charge of Arg-201 on hemagglutinin is required for the binding of H6N1 avian influenza virus to its target through a two-step process

In our previous study, we produced a monoclonal antibody EB2 that recognized an epitope in the HA1 domain on the hemagglutinin (HA) of H6N1 influenza virus (A/chicken/Taiwan/2838 V/00). The residue Arg-201 (R201) on this epitope was protected by the glycan at Asn-167 (N167) from tryptic digestion; therefore, the infectivity of the virus was retained. R201 was extremely conserved in various subtypes of the influenza virus. To explore the role of R201 and the protecting glycan, we developed a bi-cistronic baculovirus expression system for the production of H6HA1 and H6HA0 (nearly full-length HA), which were glycosylated in insect cells. The expressed H6HA1 was mostly found in the trimeric form, and the H6HA0 protein was only found in the monomeric form. The trimeric H6HA1 was resistant to tryptic digestion; however, it could not bind to fetuin, a glycoprotein containing sialylated N-linked and O-linked glycans. By contrast, the monomeric H6HA0 could bind to fetuin but was sensitive to tryptic digestion. We found that the positive charge on R201 was critical for binding HA to the negatively charged surface of host cells because the mutant R201A of H6HA0 lost its binding capacity substantially. Moreover, this binding capacity was dependent on the pH value and inhibited by free electrically charged amino acids. We propose a two-step model for binding the influenza virus with a host cell. The first step involved the specific recognition of the receptor binding site on HA to the sialylated glycan on the host cell. After the virus is engulfed by the acidic endosome, R201 could bind to the cell surface with stronger interactions and trigger the fusion process.

A latex agglutination assay to quantify the amount of hemagglutinin protein in adjuvanted low-dose influenza monovalent vaccines

To formulate inactivated influenza vaccines, the concentration of hemagglutinin (HA) must be accurately determined. The standard test currently used to measure HA in influenza vaccines is the Single Radial Immunodiffusion (SRID) assay. We developed a very rapid, simple and sensitive alternative quantitative HA assay, namely the Latex Agglutination Assay (LAA). The LAA uses the Spherotest® technology, which is based on the agglutination of HA-specific immunoglobulin-coated latex beads. The amount of HA in a sample is calculated from the level of bead agglutination by a simple absorbance measurement at 405nm against a standard curve generated using a monovalent vaccine standard. In less than 2hours, tens of samples could be quantified using the LAA as opposed to 2days for the SRID assay. Ten steps are required to complete an SRID assay as compared to 6 steps for the LAA, from sample preparation through spectrophotometric analysis. Furthermore, the limit of detection of the LAA was found to be approximately 15ng HA/mL, similar to an ELISA, with the quantification of less than 1.8μg HA/mL. The quantification limit of the SRID is usually considered to be approximately 5μg HA/mL. The development of the assay and a comparison of the titers obtained by SRID and LAA for several monovalent vaccines corresponding to various strains were performed. For A/H5N1 and A/H1N1 monovalent vaccines, the LAA was found to be linear and accurate as compared to the SRID. The precision of the LAA was close to that of the standard test, and good reproducibility from one laboratory to another was observed. Moreover, the LAA enabled HA quantification in AlOOH-adjuvanted and in emulsion-adjuvanted low-dose vaccines as well as unadjuvanted vaccines. In conclusion, LAA may be useful to rapidly and accurately measure influenza HA protein in monovalent vaccines, especially in those containing less than 5μg/mL of HA in the presence of an adjuvant.