Immunogenicity and protective efficacy of a recombinant HtsA-SLOdm fusion protein against Group A Streptococcus infection

Group A Streptococcus (GAS) is a major human pathogen responsible for a broad spectrum of diseases, yet no licensed vaccine is currently available. Heme-transfer lipoprotein A (HtsA) and streptolysin O (SLO) are key virulence-associated factors involved in iron acquisition and host cell damage, respectively, making them attractive vaccine targets. Here, we designed and evaluated HtsA-SLOdm, a recombinant fusion protein comprising HtsA and a detoxified mutant of SLO, to assess its immunogenicity and protective efficacy against GAS infection. The fusion protein retained the antigenicity of both components and exhibited negligible hemolytic activity without detectable histopathological toxicity in immunized mice. Immunization with alum-adjuvanted HtsA-SLOdm elicited robust antigen-specific antibody responses, with endpoint IgG titers ranging from 1:312,500 to 1:1,562,500. In a murine skin infection model, HtsA-SLOdm vaccination improved survival, reduced lesion size and bacterial load, and alleviated tissue damage, accompanied by enhanced collagen deposition and angiogenesis. Antisera from immunized mice also mediated opsonophagocytic killing of GAS. Mechanistically, HtsA-SLOdm promoted dendritic cell maturation, B-cell activation, and Th1, Th2, and Th17-associated cytokine responses. Collectively, these findings demonstrate that HtsA-SLOdm induces robust humoral and cellular immunity and confers significant protection against GAS infection, highlighting its potential as a promising candidate for GAS vaccine development.