Antiserum against the conserved nine amino acid N-terminal peptide of influenza A virus matrix protein 2 is not immunoprotective

The recent emergence and rapid spread of the pandemic H1N1 swine influenza virus reminded us once again of the need for a universal influenza vaccine that can elicit heterosubtypic protection. Here, we show the superior immunogenicity and immunoprotective capacity of the full-length matrix protein 2 ectodomain (M2e) peptide coupled to keyhole limpet haemocyanin (KLH) compared with the N-terminal 9 aa residues of M2e (SP1). Immunization with M2e KLH protected mice against a lethal challenge with influenza A virus and significantly reduced weight loss and lung virus titres. In addition, passive transfer of serum raised in rabbits against M2e KLH protected mice against a lethal influenza virus challenge, whereas serum from rabbits immunized with SP1 KLH did not. Nevertheless, immunofluorescence staining revealed that rabbit serum raised against SP1 KLH bound specifically to infected Madin Darby canine kidney cells. We conclude that the peptide SP1 contains an immunogenic epitope that is not sufficient for immunoprotection