Structure-activity relationship of lipid, cyclic peptide and antigen rearrangement of physically mixed vaccines

Currently there is no approved vaccine to prevent and/or treat group A Streptococcus (GAS) infection. With increasing reports of GAS antibiotic resistance, vaccine adjuvants and targeted delivery systems which induce a strong immune response are a widely acknowledged unmet need. Through extensive structure-activity studies, we investigated a cyclic decapeptide physically mixed with a GAS B cell peptide epitope (J8), a universal T helper epitope (PADRE), and different synthetic lipidic moieties as a conceivable self-adjuvanting GAS vaccine. We explored the structure (orientation)-relationship of the chemically-conjugated B cell epitope and T helper epitope peptide as part of this physically-mixed vaccine. Following in vivo assessment in mice, these cyclic lipopeptide vaccines showed successful induction of J8-specific systemic IgG antibodies when administered subcutaneously without additional adjuvant. Interestingly, an exposed C-terminus of the GAS B cell epitope and a 16-carbon alpha-amino fatty acid lipid was required for strong immunoreactivity, capable of effectively opsonising multiple strains of clinically-isolated GAS bacteria. Physicochemical assessment proved the alpha helix structure of the GAS B cell epitope was retained, impacting particle self-assembly and vaccine immunoreactivity. This study showed the capability for a self-adjuvanting cyclic delivery system to act as a vehicle for the delivery of GAS peptide antigens to treat GAS infection.