Understanding influenza B humoral immunity to improve vaccine design

Abstract

Annual influenza epidemics cause significant morbidity and mortality globally. Although influenza B virus (IBV) is responsible for approximately 25% of the global influenza burden, it remains understudied compared to influenza A virus (IAV). Current influenza vaccines elicit mostly strain- specific antibody responses, so vaccine efficacy is dramatically reduced when mutated variants dominate in circulation. Improved IBV vaccines require a better understanding of humoral immunity against IBV in order to inform improved vaccine design. In this thesis, we examined the human IBV-specific humoral response following influenza vaccination and investigated the potential utility of ferritin nanoparticles and IBV HA stem antigens to induce broader protective immune responses.

IBV-specific antibody responses have been described in previous studies, but the knowledge we have regarding the specificities, protection and epitopes of cross-reactive antibodies remains limited. Using a flow cytometry-based approach, we delineated different B cell populations with either single-lineage or cross-lineage specificities from samples collected following seasonal influenza immunization clinical trials. Both neutralizing and non-neutralizing antibodies protected mice from lethal challenges with IBV, but protection by non-neutralizing antibodies was non-sterile and dependent on Fc-effector functions. We also localized neutralizing epitopes of both lineage-specific and cross-lineage antibodies on IBV HA by sequencing viral escape mutants. The comprehensive information we gathered from this study may guide future efforts to design broadly protective IBV vaccines.

Nanoparticles as a novel vaccine carrier system has drawn increasing attention over the past decade. The self-assembling ferritin nanoparticles loaded with IAV HA have been proven to induce robust and broad humoral response in mice and ferrets. We investigated the feasibility and protective potential of displaying IBV HA on ferritin nanoparticles. The results demonstrated that ferritin nanoparticles significantly boosted the immunogenicity of IBV HA. Furthermore, we found that co-loading HAs of both IBV lineages led to antibody responses against both lineages and conferred similar level of protection in comparison of mixing two monovalent nanoparticles. Overall, ferritin nanoparticles presenting IBV HA, either monovalent or multivalent, may provide some immunological advantages over the conventional influenza vaccines, hence are promising foundations to build improved IBV vaccines upon.

The highly conserved stem domain of HA has long been considered as a major target for generating cross-reactive antibody response. Due partially to the difficulty in preparing stable IBV stem proteins, the protective potential of the IBV stem is poorly understood. Despite problematic expression and/or misfolding, IBV stem proteins we generated were immunogenic in mice and elicited robust cross-lineage antibody responses. Mice vaccinated with IBV stem proteins were partially protected from simultaneous challenge with IBV of both antigenic lineages. The broad protection of IBV stem demonstrated by these experiments suggests that it is a promising candidate for universal IBV vaccines.

In summary, this thesis improves the understanding of humoral immunity against IBV both by elucidating important aspects of HA-specific antibodies and exploring different approaches to improve current influenza vaccines. Our findings add to the field of IBV research and inform development of better IBV vaccines.