BACKGROUND: The M2 ectodomain (M2e) of influenza A virus (IAV) strains that have circulated in humans during the past 90 years shows remarkably little structural diversity. Since M2e-specific antibodies (Abs) are capable of restricting IAV replication in vivo but are present only at minimal concentration in human sera, efforts are being made to develop a M2e-specific vaccine. We are exploring a synthetic multiple antigenic peptide (MAP) vaccine and here report on the role of adjuvants (cholera toxin and immunostimulatory oligodeoxynucleotide) and route of immunization on Ab response and strength of protection. RESULTS: Independent of adjuvants and immunization route, on average 87% of the M2e-MAP-induced Abs were specific for M2e peptide and a variable fraction of these M2e(pep)-specific Abs (average 15%) cross-reacted with presumably native M2e expressed by M2-transfected cells. The titer of these cross-reactive M2e(pep-nat)-specific Abs in sera of parenterally immunized mice displayed a sigmoidal relation to level of protection, with EC50 of approximately 20 microg Ab/ml serum, though experiments with passive M2e(pep-nat) Abs indicated that serum Abs did not fully account for protection in parenterally vaccinated mice, particularly in upper airways. Intranasal vaccination engendered stronger protection and a higher proportion of G2a Abs than parenteral vaccination, and the strength of protection failed to correlate with M2e(pep-nat)-specific serum Ab titers, suggesting a role of airway-associated immunity in protection of intranasally vaccinated mice. Intranasal administration of M2e-MAP without adjuvant engendered no response but coadministration with infectious IAV slightly enhanced the M2e(pep-nat) Ab response and protection compared to vaccination with IAV or adjuvanted M2e-MAP alone. CONCLUSION: M2e-MAP is an effective immunogen as approximately 15% of the total M2e-MAP-induced Ab response is of desired specificity. While M2e(pep-nat)-specific serum Abs have an important role in restricting virus replication in trachea and lung, M2e-specific T cells and/or locally produced Abs contribute to protection in upper airways. Intranasal vaccination is preferable to parenteral vaccination, presumably because of induction of local protective immunity by the former route. Intranasal coadministration of M2e-MAP with infectious IAV merits further investigation in view of its potential applicability to human vaccination with live attenuated IAV.