Self-Assembling Sulfated Lactobacillus Exopolysaccharide Nanoparticles as Adjuvants for SARS-CoV-2 Subunit Vaccine Elicit Potent Humoral and Cellular Immune Responses

Coronavirus disease 2019 (COVID-19) has caused a global pandemic since its onset in 2019, and the development of effective vaccines for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) to induce potent and long-lasting immunity remains a priority. Herein, we prepared two Lactobacillus exopolysaccharide (EPS) nanoparticle adjuvants (NPs 7-4 and NPs 8-2) that were constructed by using sulfation-modified EPS and quaternization-modified chitosan. These two NPs displayed a spherical morphology with sizes of 39 and 47 nm. Furthermore, the zeta potentials of NPs 7-4 and NPs 8-2 were 50.40 and 44.40 mV, respectively. In vitro assays demonstrated that NPs could effectively adsorb antigenic proteins and exhibited a sustained release effect. Mouse immunization tests showed that the NPs induced the expression of cytokines and chemokines at the injection site and promoted the uptake of antigenic proteins by macrophages. Mechanically, the NPs upregulated the expression of pattern recognition receptors (toll-like receptors and nod-like receptors) and activated the immune response of T cells and the production of neutralizing antibodies. In addition, the NP adjuvants had favorable immune-enhancing effects in cats, which are of great significance for controlling the trans-host transmission and re-endemicity of SARS-CoV-2. Overall, we demonstrated that NP-adjuvanted SARS-CoV-2 receptor binding domain proteins could induce robust specific humoral and cellular immunity.