Saponin derivative and MPLA-adjuvanted intranasal protein nanoparticle vaccines induce potent mucosal immunity and durable heterologous influenza protection in mice.

Mucosal immunity is essential for defenses against respiratory infections, such as influenza. Identifying adjuvant formulations that enhance mucosal immune responses in the respiratory tract will facilitate the development of effective mucosal vaccines. In this study, we explored the mucosal application of a novel semisynthetic saponin derivative, VSA-2. An intranasal vaccine formulation, PMV, containing influenza double-layered protein nanoparticles (PNPs) with monophosphoryl lipid A (MPLA) and VSA-2 was compared with PNPs alone or PNPs adjuvanted with MPLA or VSA-2 in mice. Two doses of PMV elicited high levels of systemic and mucosal cross-reactive antibody responses and robust cellular immunity, including tissue-resident memory T cells. PMV also promoted early recruitment and activation of innate immune cells in mucosal tissues. Furthermore, PMV-induced immunity was durable, protecting against heterologous viral challenges for at least 6 months. These findings demonstrate that the MPLA+VSA-2 combination significantly enhances the immunogenicity and protective efficacy of intranasal protein nanoparticle vaccines.