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

## TL;DR

This study shows that a modified RBD vaccine with poly-lysine and CTB induces strong and long-lasting antibody responses in mice when administered via the lungs.

## Contribution

A novel RBD vaccine design with poly-lysine and CTB adjuvant that induces potent mucosal and systemic immunity in mice.

## Key findings

- Combining poly-lysine-modified RBD with CTB and pulmonary delivery induced robust neutralizing antibodies in mice.
- Antibody titers remained high for six months after the third immunization, indicating long-term immunity.
- Adding five lysine residues to RBD with CTB significantly increased IgG and IgA antibody levels in serum.

## Abstract

(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.