Mn2+-loaded carboxymethyl chitosan nanoparticle as a multifunctional adjuvant to efficiently enhance the performance of inactivated pseudorabies virus vaccines.

Conventional adjuvants (such as Alhydrogel and oil-in-water emulsion) formulated vaccines normally induce an inadequate immune response and poor cellular immunity. In this work, a Mn2+-loaded carboxymethyl chitosan nanoparticle dispersed with sodium alginate (CMS adjuvant) has been designed and used as a novel nanoadjuvant in inactivated pseudorabies virus (PRV) vaccine to overcome above challenges. Comprehensive biocompatibility assessments confirmed the CMS adjuvant's favorable safety profile in both in vitro and in vivo models. CMS-adjuvated PRV vaccine successfully induced higher innate and adaptive immune response as compared with control adjuvants, including higher specific IgG antibody titres and IgG2a/IgG1 ratio, elevated Th1-/Th2-type cytokine secretion, and increased T cell activation and differentiation. The enhanced performance was attributed to three primary factors: (1) sustained antigen release at the injection site, (2) CMS-assisted highly efficient antigen uptake and presentation by dendric cells (DCs), and (3) Mn2+-induced activation of cyclic GMP-AMP synthase-stimulator of interferon gene (cGAS-STING) pathway, leading to increased type I interferons secretion and DCs maturation. In a mouse challenge experiment with a wild-type PRV, the CMS-adjuvanted vaccine demonstrated superior protective efficacy (90 % survival rate) to commercial Alum (40 %) and biphasic 201 (80 %) adjuvants. Thus, the CMS nanoparticles have a great potential to be a promising nanoadjuvant in various vaccines.