SARS-CoV-2 RBD Conjugated to Polyglucin, Spermidine, and dsRNA Elicits a Strong Immune Response in Mice

Abstract

Despite the rapid development and approval of several COVID vaccines based on the full-length spike protein, there is a need for safe, potent, and high-volume vaccines. Considering the predominance of the production of neutralizing antibodies targeting the receptor-binding domain (RBD) of S-protein after natural infection or vaccination, it makes sense to choose RBD as a vaccine immunogen. However, due to its small size, RBD exhibits relatively poor immunogenicity. Searching for novel adjuvants for RBD-based vaccine formulations is considered a good strategy for enhancing its immunogenicity. Herein, we assess the immunogenicity of severe acute respiratory syndrome coronavirus 2 RBD conjugated to a polyglucin:spermidine complex (PGS) and dsRNA (RBD-PGS + dsRNA) in a mouse model. BALB/c mice were immunized intramuscularly twice, with a 2-week interval, with 50 µg of RBD, RBD with Al(OH)3, or conjugated RBD. A comparative analysis of serum RBD-specific IgG and neutralizing antibody titers showed that PGS, PGS + dsRNA, and Al(OH)3 enhanced the specific humoral response in animals. There was no significant difference between the groups immunized with RBD-PGS + dsRNA and RBD with Al(OH)3. Additionally, the study of the T-cell response in animals showed that, unlike adjuvants, the RBD-PGS + dsRNA conjugate stimulates the production of specific CD4+ and CD8+ T cells in animals.

Approaches to Improve the Immunogenicity of Plasmid DNA-Based Vaccines against COVID-19

“…The polyglucine-spermidine conjugate was used previously to enhance the immunogenicity of the RBD protein [54], and here, we demonstrate the efficacy of using a PGS conjugate to deliver the pVAX-rbd DNA vaccine.…”

“…As a result, complexes presenting RBD on the surface and dsRNA inside were obtained due to self-assembly of conjugate and RNA. Such complexes induced virus neutralizing antibody level against SARS-COV-2 much more effectively than RBD with AL(OH)3 or RBD without adjuvant [54].…”

“…The polyglucin-spermidine conjugate can be used both for the delivery of proteins, including the recombinant RBD protein [54] and nucleic acids. Previously, polyglucin-spermidine has proven to be an effective delivery vehicle for experimental DNA vaccines against Ebola and HIV-1 [35,36,55].…”

“…For this purpose, we decided to use polyglucin-spermidine-protein RBD conjugate (PGS-RBD) for DNA vaccine packaging. The synthesis technique was proposed by Lebedev et al [54,55,58,59]. We previously used this approach to increase the immunogenicity of an experimental vaccine based only on the recombinant RBD protein [54].…”

“…The synthesis technique was proposed by Lebedev et al [54,55,58,59]. We previously used this approach to increase the immunogenicity of an experimental vaccine based only on the recombinant RBD protein [54]. We attached the protein to a polyglucine-spermidine conjugate and mixed it with dsRNA (as adjuvant).…”