Is germinal center selection required for influenza vaccination?

Murine Cross-Reactive Nonneutralizing Polyclonal IgG1 Antibodies Induced by Influenza Vaccine Inhibit the Cross-Protective Effect of IgG2 against Heterologous Virus in Mice

Annual vaccination against influenza viruses is the most reliable and efficient way to prevent and control annual epidemics and protect from severe influenza disease. However, current split influenza vaccines are generally not effective against antigenically mismatched (heterologous) strains. To broaden the protective spectrum of influenza vaccines, adjuvants that can induce cross-reactive antibodies with cross-protection via Fc-mediated effector functions are urgently sought. Although IgG2 antibodies are generally more efficient than IgG1 antibodies in Fc-mediated effector functions, it is not yet clear which IgG isotypes show superior cross-protection against heterologous strains. It also remains unclear whether these IgG isotypes interfere with each other's protective effects. Here, we found that influenza split vaccine adjuvanted with aluminum salts, which predominantly induce cross-reactive IgG1, did not confer cross-protection against heterologous virus challenge in mice. In contrast, split vaccine adjuvanted with CpG oligodeoxynucleotides, which predominantly induce cross-reactive IgG2, showed cross-protection through the interaction of cross-reactive nonneutralizing IgG2 and alveolar macrophages, indicating the importance of cross-reactive nonneutralizing IgG2 for cross-protection. Furthermore, by using serum samples from immunized mice and isolated polyclonal antibodies, we show that vaccine-induced cross-reactive nonneutralizing IgG1 suppress the cross-protective effects of IgG2 by competitively inhibiting the binding of IgG2 to virus. Thus, we demonstrate the new concept that cross-reactive IgG1 may interfere with the potential for cross-protection of influenza vaccine. We propose that adjuvants that selectively induce virus-specific IgG2 in mice, such as CpG oligodeoxynucleotides, are optimal for heterologous protection.IMPORTANCE Current influenza vaccines are generally effective against highly similar virus strains by inducing neutralizing antibodies. However, these antibodies fail to neutralize antigenically mismatched (heterologous) strains and therefore provide limited protection against them. Efforts are being made to develop vaccines with cross-protective ability that would protect broadly against heterologous strains, because the mismatch between predicted and epidemic strains cannot always be avoided, resulting in low vaccine efficacy. Here, we show that nonneutralizing IgG2 antibodies induced by an optimal adjuvant play a crucial role in cross-protection against heterologous virus challenge in mice. Furthermore, nonneutralizing polyclonal IgG1 suppressed the cross-protective effects of nonneutralizing polyclonal IgG2 by competitively blocking the binding of IgG2 to its antigen. These data shed new light on the importance of IgG isotypes and the selection of appropriate adjuvants for the development of universal influenza vaccines. Furthermore, our findings are applicable to the rational design of vaccines against other pathogens.

Breadth of Antibody Responses during Influenza Virus Infection and Vaccination

Influenza viruses are a major public health problem, causing severe respiratory diseases. Vaccines offer the effective protective strategy against influenza virus infection. However, the systemic and adaptive immune responses to infection and vaccination are quite different. Inactivated vaccines are the best available countermeasure to induce effective antibodies against the emerged virus, but the response is narrow compared with potential breadth of virus infection. There is solid evidence to indicate that antibody responses to natural infection are relatively broad and exhibit quite different immunodominance patterns. Furthermore, T follicular helper cells (T_FH) and germinal center (GC) responses play a central role in generating broad protective antibodies. In this review, we discuss recent advances on the contribution of T_FH and GC responses to the breadth of antibody responses.