Reduced germinal center formation is not sufficient to promote cross-reactive protection against influenza.

Abstract

Abstract Influenza infection continues to be a major health threat with over 9 million cases annually in the United States alone. Vaccination is the best method of preventing infection; however, current vaccines are only 10–60% effective due to a high mutation rate of the virus and difficulty predicting which strains will circulate. Furthermore, current vaccines will have little to no impact on a novel pandemic virus. A universal influenza vaccine would generate an immune response to conserved portions of the virus, allowing for protection against multiple influenza subtypes. We showed that a low dose of rapamycin, given during influenza vaccination, altered the antibody response such that mice were protected against secondary infection with multiple strains of influenza. Rapamycin reduced formation of germinal centers (GC) resulting in a decrease in antibodies specific for the variable region of the vaccine HA. Thus, we hypothesized that rapamycin enhances heterosubtypic protection by reducing generation of high affinity antibodies specific for the variable region of HA, which allows antibodies specific for the conserved regions to be more prevalent. To test this, we investigated whether the reduction in GCs was sufficient to induce cross-reactive antibodies. We depleted GCs during vaccination by removing follicular helper T cells or by transiently blocking CD40L. The reduction of GC reduced the amount of class-switched, influenza-specific antibody, similar to rapamycin-treated mice. However, this was not sufficient to enhance protection following challenge with a virus of a different subtype. These data suggest that while rapamycin reduces germinal center formation, this alone is not sufficient to promote cross-reactive antibodies.