Immune Cross-Opsonization Within emm Clusters Following Group A Streptococcus Skin Infection: Broadening the Scope of Type-Specific Immunity

Background. Group A Streptococcus (GAS) skin infections are particularly prevalent in developing nations. The GAS M protein, by which strains are differentiated into >220 different emm types, is immunogenic and elicits protective antibodies. A major obstacle for vaccine development has been the traditional understanding that immunity following infection is restricted to a single emm type. However, recent evidence has led to the hypothesis of immune cross-reactivity between emm types. Methods. We investigated the human serological response to GAS impetigo in Fijian schoolchildren, focusing on 3 major emm clusters (E4, E6, and D4). Pre-and postinfection sera were assayed by enzyme-linked immunosorbent assay with N-terminal M peptides and bactericidal assays using the infecting-type strain, emm cluster-related strains, and nonrelated strains. Results. Twenty of the 53 paired sera demonstrated a >= 4-fold increase in antibody titer against the infecting type. When tested against all cluster-related M peptides, we found that 9 of 17 (53%) paired sera had a >= 4-fold increase in antibody titer to cluster-related strains as well. When grouped by cluster, the mean change to cluster-related emm types in E4 and E6 was >= 4-fold (5.9-fold and 19.5-fold, respectively) but for D4 was 3.8-fold. The 17 paired sera were tested in bactericidal assays against selected cluster-related and nonrelated strains. While the responses were highly variable, numerous instances of cross-reactive killing were observed. Conclusions. These data demonstrate that M type-specific and cross-reactive immune responses occur following skin infection. The cross-reactive immune responses frequently align with emm clusters, raising new opportunities to design multivalent vaccines with broad coverage.