Survey of Human Antibody Responses to Influenza Virus Matrix Protein 2 by Use of a Sensitive Flow Cytometric Method

Conventional influenza vaccines are designed to elicit antibodies to strainspecific antigens, leaving a public health gap when novel viruses break out unexpectedly. As examples, strain-matched vaccine became available too late in the pandemic of 2009 to protect against the fall wave, and drift viruses are sometimes divergent enough to cause vaccine failure (eg, A/Sydney in 1997). Available vaccines are also inadequate to protect against various zoonotic strains, including avian influenza A(H5N1), influenza A(H7N7), and most recently influenza A(H7N9). For this reason, there is much interest in developing vaccines based on conserved influenza virus features that can provide protection regardless of strain. These are usually designed for influenza A virus, but such vaccines can also be made for influenza B virus. Universal influenza vaccines can induce immune protection dependent upon antibody or T-cell responses or both, and the target antigens explored have included nucleoprotein, matrix proteins (M1 and M2), the hemagglutinin (HA) stem, polymerase PB1, and other antigens, as reviewed previously [1]. M2, the focus of Zhong et al in the current issue of the Journal, has long been known as a target of antibodies that reduce viral replication and spread [2, 3]. Vaccines based on many forms of M2 (fusion proteins, M2 multiple antigenic peptides, peptide conjugates, and M2 expressed from viral vectors) provide protective immunity in animals [4–10], and some have been tested in humans. A clinical trial of recombinant M2 shows that this antigen is immunogenic when administered with adjuvant [11]. Human anti-M2 antibodies induced by natural infection have been reported, but data are scanty. Black et al reported that 6 of 17 pairs of acute-phase and convalescent-phase serum specimens showed increased anti-M2 activity by enzyme-linked immunosorbent assay (ELISA), while 12 of 17 convalescent-phase serum specimens demonstrated some signal by Western blot [12]. In a study by Feng et al, an increase in anti-M2 activity was found for 11 of 24 such serum pairs [13]. In that study, of the antibodies detectable by assay on cell surface tetrameric M2, only a minority also recognized M2e peptide. Thus, the majority of antibodies appeared to be conformational [13]. These antibodies may be biologically very important, so assays on native M2 are needed. However, measuring antibodies to native M2 has presented technical problems, with high background encountered in cell surface ELISA. To overcome these problems, Zhong et al developed a flow cytometric assay (M2-FCA) using a panel of 293FT transfected cell lines (M2-293FT) stably expressing full-length tetrameric forms of M2 from various viral strains [14]. With this sensitive assay, some mouse antibodies recognize strain-specific epitopes and some see epitopes cross-reactive among viral strains [14]. The present article uses the assay to analyze collections of human sera. Transfection efficiency and expression levels are normalized through the use of a positive human serum pool as a control. Unlike most serological assays in which a titer reflects the dilution at which a defined end point is reached, the unit of M2 antibody is defined for a single (1:40) dilution of serum. This allows large numbers of sera to be analyzed within a single run, but does not measure titers or other antibody properties revealed by dilution series. Results for healthy donors of different ages showed that antibodies to M2 were found in a higher percentage of, and at higher levels, in adults aged ≥40 years, compared with younger donors. If anti-M2 antibodies were present, they usually recognized both seasonal and swine-origin M2. Received 5 December 2013; accepted 5 December 2013. Correspondence: Suzanne Epstein, PhD, OCTGT, CBER, FDA, 1401 Rockville Pk, HFM-730, Rockville, MD 20852-1448 (suzanne.epstein@fda.hhs.gov). The Journal of Infectious Diseases Published by Oxford University Press on behalf of the Infectious Diseases Society of America 2013. This work is written by (a) US Government employee(s) and is in the public domain in the US. DOI: 10.1093/infdis/jit813