Biogenic silver nanoparticle as an adjuvant in an S1 subunit recombinant vaccine

Adjuvants are crucial for maintaining specific, protective, and long-lasting immunity. Here, we aimed to evaluate the antigenic and immunogenic activity of a recombinant form of the S1 domain of the Spike protein, associated with biogenic silver nanoparticles (bio-AgNP) and Alhydrogel^® as an alternative and conventional adjuvant, respectively, for a SARS-CoV-2 subunit vaccine. We produced and evaluated the antigenicity of the recombinant S1 (rS1) protein by testing its recognition by antibodies present in SARS-CoV-2 positive human serum. The immunogenicity of the rS1 protein was assessed in vivo by its ability to induce antibody production in mice. Furthermore, we sought to establish the types of humoral and cellular immune responses generated through intramuscular immunization in BALB/c mice with the rS1 + bio-AgNP vaccine. The recombinant S1 (rS1) protein was successfully cloned, expressed in Escherichia coli, and confirmed to have strong antigenic potential, being recognized by human antibodies up to a 1:51,200 serum dilution. In vivo assays showed that vaccines using rS1 with either bio-AgNP or Alhydrogel^® as adjuvants induced high and consistent antibody titers (1:51,200) and a range of antibody isotypes in mice. Cellular immune response analysis revealed significant IL-10 expression with rS1 + bio-AgNP and both IL-10 and TNFα expression with rS1 + Alhydrogel^®. These results support rS1 + bio-AgNP as a promising vaccine candidate for future development, highlighting its potential not only as a viable alternative to traditional adjuvants but also as an innovative contribution to advancing more effective and accessible immunological strategies.