A Poly-Lysine-Based RBD Mucosal Vaccine Induces Potent Antibody Responses in Mice

(1) Background: The COVID-19 pandemic highlights the critical necessity for the development of mucosal vaccines. (2) Objective: In this study, we aimed to develop mucosal vaccines based on the receptor-binding domain (RBD) of the SARS-CoV-2 Spike protein. (3) Methods: We engineered the RBD of the Spike protein by incorporating ten lysine residues (K10), thereby enhancing its positive charge under physiological conditions. (4) Results: Although this modification did not directly augment the immunogenicity of the antigen, its combination with the mucosal adjuvant cholera toxin B subunit (CTB) and administration via the pulmonary route in BALB/c mice resulted in the induction of robust neutralizing antibody titers. Antigen-specific antibody responses were observed in both serum and bronchoalveolar lavage fluid. Importantly, serum IgG antibody titers remained above 10⁴ six months following third immunization, suggesting the establishment of sustained long-term immunity. Additionally, the incorporation of five lysine residues (K5) into the RBD, in conjunction with CTB, significantly increased serum IgG and IgA antibody titers. (5) Conclusions: Adding poly-lysine to RBD and combining it with CTB can stimulate robust mucosal and humoral immune responses in mice. These findings offer valuable insights for the design of subunit mucosal vaccines.