M2e-immobilized gold nanoparticles as influenza A vaccine: Role of soluble M2e and longevity of protection.

Influenza virus causes seasonal epidemics and also poses a high risk for pandemics. To develop a broadly cross-protective influenza vaccine we have previously shown that a formulation consisting of the extracellular domain of M2 membrane protein (M2e) immobilized on gold nanoparticles (AuNPs) and soluble CpG as an adjuvant can elicit protective immunity against different influenza A subtypes. The vaccine formulation contains M2e that is immobilized on AuNPs, and an excess amount that is freely dissolved in solution, whose role in inducing protective immunity against virus infection is unclear. Using a mouse model, the current study shows that inclusion of excess soluble M2e antigen along with M2e immobilized on AuNPs is vital for inducing high levels of antibody response, and in providing complete protection against lethal influenza virus challenge. We also show that the vaccine induces long-lasting protection against mortality and morbidity upon lethal challenge with influenza A virus.