A non-conjugated peptide-adjuvant system of synthetic peptide-vaccine elicited potent antibody response and opsonic protection against clinical GAS strains

Group A Streptococcus (GAS) is a causative agent of life-threatening infections, including pharyngitis, rheumatic fever, and rheumatic heart disease. Currently, no commercial vaccines are licenced to prevent GAS infections. Synthetic peptide-based subunit vaccines composed of minimal antigenic fragments derived from the pathogenic M-protein, represent an attractive approach for mitigating GAS infection transmission. However, such small antigens are poorly immunogenic, necessitating their co-administration with strong immunostimulatory agents (adjuvants). Herein, we proposed the use of self-assembled peptides as the adjuvant by developing non-conjugated peptide-adjuvant system of GAS synthetic peptide-based vaccine in which a synthetic GAS antigen PADRE-J8 (composed of B cell epitope J8 and the universal T-helper epitope PADRE) was physically mixed with a lipopeptide TLR2 agonist (KKSS-C16-C16) and self-assembled peptides. In mice, all vaccine candidates induced antigen-specific systemic immunoglobulin G (IgG) antibodies after subcutaneous immunisation. Among them, a physical mixture of a self-assembled peptide Nap-GFFY, lipopeptide KKSS-C16-C16, and PADRE-J8 antigen (SPV-5) induced the highest antibodies production and produced antibodies were opsonic against GAS clinical isolates. The vaccine formulation lacking the lipopeptide KKSS-C16-C16 (SPV-5(-)) generated J8-specific antibodies; however, their opsonization capacity was poor, indicating that both the self-assembled peptide and the lipopeptide were essential for eliciting strong opsonic responses against bacteria.