Delineating Humoral and Cellular Immune Responses Induced by a Peptide-Based Vaccine for Protection Against Streptococcus pyogenes

Despite the substantial burden of Streptococcus pyogenes infections and their associated sequalae, there is currently no Strep A vaccine. To address this global human health issue, we have developed a combination peptide-based vaccine that utilises conserved epitopes—p* 17 from the M protein and K4S2 from SpyCEP, an anti-neutrophil protease—, each conjugated to diphtheria toxoid and adjuvanted with Alum. This vaccine is now in a phase 1 clinical trial. Transgenic mice expressing human HLA-DR3-DQ2 (HLA-II) were intramuscularly immunised at days 0, 21, and 42, leading to the induction of p*17- and K4S2-specific serum IgG antibodies, which are capable of binding multiple clinical isolate strains of StrepA. However, following a 5-month resting period, a decline in antibody levels and S. pyogenes binding capabilities was observed. To investigate the efficacy of the vaccine in vivo, vaccinated HLA-II mice were challenged intranasally with the S. pyogenes M75 strain. Protection was assessed by measuring bacterial burden in the nasal-associated lymphoid tissue and lungs, as well as via throat swabs, at two time points post-final booster—2 weeks and 5 months. Notably, significant protection against S. pyogenes was maintained in all tissues analysed, regardless of the levels of antigen-specific antibody. Immunological analyses revealed increased systemic IL-1β levels in 5-month-rested vaccinated mice, suggesting a role for sustained innate and cytokine-mediated responses in long-term protection against infection. To further validate these findings, vaccination and challenge studies involving IL-1β knockout mice revealed a loss of vaccine efficacy compared to wild-type controls, despite comparable antibody levels, emphasising the role of IL-1β in mediating vaccine-induced immunity.