Protective and vaccine dose-sparing efficacy of Poly I:C-functionalized calcium phosphate nanoparticle adjuvants in inactivated influenza vaccination.

Adjuvants are required to increase the immunogenicity and efficacy of vaccination and enable vaccine dose sparing. Polyinosinic-polycytidylic acid (Poly I:C), a toll-like receptor 3 agonist, is a promising adjuvant candidate that can induce cell-mediated immune responses; however, it remains unlicensed owing to its low stability and toxicity. Calcium phosphate (CaP), a biocompatible and biodegradable nanoparticle, is widely used in biomedicine for stable and targeted drug delivery. In this study, we developed Poly I:C-functionalized CaP (Poly-CaP) and evaluated its vaccine adjuvant efficacy in vitro and in vivo. A half dose of Poly-CaP nanoparticles showed similar efficacy to a full dose of soluble Poly I:C in stimulating bone marrow-derived dendritic cells and macrophages to secrete proinflammatory cytokines and express their activation markers. Immunization with a half dose of inactivated influenza vaccine in the presence of Poly I:C or Poly-CaP adjuvants induced sufficient antigen-specific humoral responses after boost immunization. Immunization with Poly I:C, CaP, or Poly-CaP-adjuvanted with a half dose of influenza vaccine showed comparable protective efficacy against lethal virus infection, with lower weight loss and virus titer than a full dose of influenza vaccine. The Poly-CaP adjuvant was effective in stimulating antigen-specific CD4+ T cell proliferation in the lungs. Collectively, our results showed that the Poly-CaP adjuvant enhanced antigen-specific cell-mediated immunity and humoral immune responses with vaccine dose-sparing effects, suggesting its potential as a novel vaccine adjuvant candidate.