Influenza (flu) is a highly infectious acute respiratory disease that poses a huge economic burden to all societies across the world and is responsible for hundreds of thousand cases of severe illness and deaths every year. Current seasonal vaccines comprise of trivalent or quadrivalent formulations that are based on prediction of circulating influenza A and B strains in the Northern and Southern hemispheres. This vaccination strategy has shown limited effectiveness especially when there is a mismatch between predicted and circulating strains. There is hence an urgent need to develop a universal flu vaccine that could provide basal protection to supplement seasonal flu annual vaccination, and in outbreak/pandemic situations caused by zoonotic influenza viruses.
The 23-amino acid ectodomain of the M2 viral protein (M2e) has been extensively studied as a promising “universal” flu antigen vaccine candidate owing to its highly conserved amino acid sequence across all the IAV subtypes. However, its inherent low immunogenicity has represented a major bottleneck in the development of M2e-based vaccines, with disappointing results in clinical trials. To overcome M2e’s weak immunogenicity, we have employed a powerful dendritic cell (DC) -targeting strategy to deliver M2e to a specific DC subset termed conventional Dendritic Cells 1 (cDC1), which excels in antigen uptake, processing, and cross-presentation. The vaccine construct consists of an anti-Clec9A monoclonal antibody where three copies of M2e have been genetically fused in tandem at the C-terminus of each heavy chain. Clec9A is a C-type lectin receptor that is exclusively expressed on cDC1. Here, we have explored the pulmonary route of immunization with the rationale that inducing a strong immunity at the primary site of infection (respiratory tract) is expected to confer strong protection and prevent both infection and transmission of IAV.
We show that a prime-boost regimen with 2µg only of the Clec9A-M2e construct (adjuvanted with poly I:C) induced very high systemic M2e-specific IgG titres that afforded full protection against lethal H1N1/PR8 challenge in mice, with minimal body weight loss. We also employed an antibody-dependent cell cytotoxicity (ADCC) reporter assay to demonstrate the functionality of immune sera collected up to 2 months post-booster shot. The Clec9A-M2e prime-boost immunization also generated significant M2e-specific T cell responses both systemically (spleen) and locally (lungs). Taken together, these results support that the Clec9A-targeting strategy represents a promising vaccine delivery platform able to overcome the weak immunogenicity of M2e and induce strong immune responses upon respiratory immunization.