Development of immune responses against influenza virus

DNA immunization provides many advantages as an approach to prevent infectious diseases. However, although previous studies using this approach have demonstrated immune responses in serum, they were not successful in inducing significant levels of antibodies in secretions. In this study, plasmid DNAs expressing the influenza virus hemagglutinin glycoprotein have been evaluated for their ability to induce antibody responses in serum and saliva when used alone or along with either liposomes or bioadhesive polymers as mucosal delivery vehicles. Significant levels of virus-specific Ig in serum as well as secretory IgA in saliva were detected in mice following mucosal DNA immunization. These antibodies were found to block the infectivity of the virus using a plaque reduction assay. Our findings thus indicate that mucosal DNA immunization with specific delivery systems can elicit virus-specific antibody responses in serum as well as IgA responses at mucosal surfaces.; We have also investigated the cellular requirements for immunoglobulin class switching in immunodeficient mice. Through cognate interaction between antigen-specific B cell and CD4+ αβ T cells, the CD4+ αβ T cells secrete cytokines that initiate immunoglobulin class switching. CD4 T helper cells were believed to be essential for induction of a high-affinity antibody response and for efficient isotype switching from IgM to IgG and IgA production. However, recent studies have shown that immunoglobulin (Ig) class switching can occur in the absence of αβ + and/or γδ+ T cells when mice were infected with certain live viruses. In this study, we show that formalin-inactivated influenza PR8 virus induces virus-specific IgM and IgG responses in the absence of CD4+ T cells, and that all four subclasses of IgG are produced. The immunized CD4-deficient mice were also found to be completely protected against lethal infection with live, pathogenic influenza virus. The ability of CD4+ T cell deficient mice to generate these IgG responses was not found to be impaired when these mice were depleted of CD8+ T cells by anti-CD8 mAb. In contrast, αβ T cell deficient mice (TCRβ−/−) were not found to produce significant amounts of IgG upon immunization with formalin-inactivated PR8 virus. These results suggest that CD4 and CD8 double negative αβ T cells are playing a role in regulating immunoglobulin class switching in the absence of CD4 + T cells.; To date, not much information is available about the immune responses after mucosal immunization of CD4+ T cell-deficient mice. In this study, we show that intranasal immunization with formalin-inactivated influenza A/PR/8/34 virus induces IgG and IgA responses in serum and IgA responses in mucosal secretions in CD4+ T cell-deficient mice. We found that all four subclasses of IgG were produced, and IgG1/IgG2a ratios were from 1 to 1.75. The sera and mucosal secretions were found to have neutralizing activity against PR8 virus in vitro. The mucosally immunized CD4+ T cell-deficient mice were protected completely from challenge with a lethal dose of live, pathogenic influenza PR8 virus. To our knowledge, this is the first demonstration that mucosal immunization with an inactivated virus induces immune responses in serum and mucosal secretions in CD4+ T cell-deficient mice.