The antibody responses to swine influenza virus (SIV) recombinant matrix 1 (rM1), matrix 2 (M2), and hemagglutinin (HA) proteins in pigs with different SIV exposure.

The influenza invariant matrix 2 (M2) protein is a potential subunit vaccine candidate to induce protective immunity against broader strains of influenza A viruses (IAV). Antibodies to M2 protein have not been well characterized in IAV natural hosts. To characterize M2-specific antibodies in pigs, an ELISA to the extracellular region of the M2 (M2e) protein was developed. Sera from pigs experimentally infected with three different swine influenza virus (SIV) subtypes, immunized with an SIV inactivated vaccine, or positive for SIV maternally derived antibodies (MDA) in the absence of SIV infection were tested in assay. Confirmation of antibody titer status of pigs, was determined using a hemagglutination-inhibition (HI) test and the presence of antibodies to matrix 1 (M1) protein was measured by a recombinant M1 (rM1)-based ELISA. The antibody titers to the HA and M2e proteins but not to the rM1 were directly correlated to the dose of virus used to infect the pigs and the level of antibodies detected by the HI assay varied according to SIV subtype. Pigs experimentally infected with SIV produced low levels of M2e antibodies compared to antibodies detected by the HI and rM1 assays. Vaccination alone followed by infection did not increase the levels of M2e antibodies in contrast to HA and rM1 antibodies. Pigs with MDA had different levels of HA antibodies and were positive to M2e antibodies, but results were not correlated to HA antibodies levels and inconsistently present.