Mechanisms of broad cross-protection provided by influenza virus infection and their application to vaccines.

Mice recovered from influenza A virus infection have been shown to be cross-protected against challenge infection with either drift viruses within a subtype (subtype-specific immunity) or different subtype viruses (heterosubtypic immunity). The mechanisms of broad-spectrum of cross-protection could be explained as follows. (i) Pre-existing S-IgA and IgG antibodies (Abs) induced by infection are involved in the elimination of challenge viruses by forming virus-Ig complexes shortly after re-infection. Due to their polymeric nature, the S-IgA Abs, existing more abundant on the mucosa than are IgG Abs, are strongly cross-reactive with challenge viruses, whereas the IgG Abs are weakly cross-reactive with challenge viruses, due to their monomeric nature. The specificity of Abs is directed mainly at hemagglutinin and neuraminidase. (ii) CD8+ memory T cells induced by infection are involved in the elimination of challenge viruses by the accelerated killing of host cells infected with different subtype viruses from day 3 onwards after re-infection. The specificity of memory T cells is directed against viral internal proteins. (iii) The accelerated IgA and IgG Ab responses, produced by B memory cells after a challenge, are also involved in cross-protection from day 4 onwards after re-infection. (iv) In the epithelial cells of infected mice, dimeric IgA that is trafficked through the epithelial cells can contribute to the prevention of viral assembly by binding to newly synthesized viral proteins. Natural infection is well known to be superior to parenteral inactivated vaccines in inducing the broad-spectrum cross-protection. To improve the efficacy of current inactivated vaccines, many trials have been conducted to mimic natural infection, including intranasal or epidermal administration of inactivated vaccine with or without an adjuvant; such studies are still ongoing. In the near future, some of these trials may provide new, safer and more effective broad-spectrum vaccines than those currently available.