Targeting Group A Streptococcus with a Recombinant Chimeric Vaccine: Integrating ScpA and SpeA Fragments

Our objective was to develop a vaccine against group A streptococci ( Streptococcus pyogenes , GAS), the causative agent of a broad spectrum of infections with varying severity. The expression vectors pET27 and pQE30 were used to clone genes encoding recombinant proteins, which were subsequently affinity-purified. Female mice were immunized subcutaneously twice with the purified polypeptides (20 μg/mouse) formulated with Alum adjuvant (2:1) at three-week intervals. Immune sera were analyzed using ELISA to evaluate antigen-specific responses. Three weeks after the final immunization, mice were challenged intraperitoneally with GAS M1 serotype at a dose of 5 × 10 7 CFU/mouse. Vaccination efficacy was determined by comparing bacterial clearance in vaccinated versus control animals, assessed by bacterial loads in the spleen at 3 and 15 hours post-infection. The vaccine candidate is a hybrid recombinant protein comprising fragments from two essential GAS virulence factors: C5a peptidase (ScpA) and SpeA exotoxin. T and B cell epitopes from conserved regions, common across multiple GAS serotypes, were identified and included into the construct using bioinformatics tools. The integration of these epitopes is designed to confer broad-spectrum protection against GAS strains carrying exotoxin, particularly those linked to invasive infections. Immunogenicity studies in mice revealed a robust humoral immune response targeting both components of the hybrid protein. Further evaluation of protective efficacy demonstrated accelerated bacterial clearance in vaccinated animals, with the SpeA fragment playing a significant protective role. These findings emphasize the potential of this recombinant chimeric vaccine as a promising candidate for the prevention of group A streptococcal infections, addressing the critical need for effective prophylactic strategies against GAS-associated diseases.