Immunological Correlates of Opsonophagocytosis and Killing for Streptococcus pyogenes

Abstract

Background: Streptococcus pyogenes (Strep A) causes a wide spectrum of diseases, ranging from superficial pharyngitis and invasive disease to post-immune sequelae such as acute rheumatic fever. There is currently no licensed vaccine for Strep A, and development has been hindered by a lack of known correlates of protection. Identifying immune features associated with in vitro killing of Strep A may help to define candidate correlates of protection. The overall goal of this project was to establish a correlate of killing for the emm12 Strep A strain, which is frequently associated with serious Strep A infections in Aotearoa New Zealand and globally. Methods: Immune-mediated killing of emm12 was quantified in vitro using an opsonophagocytic killing assay (OPKA) that uses HL-60 cells as a source of neutrophils. This was applied to a cohort of serum samples from children with culture-confirmed emm12 and non-emm12 Strep A pharyngitis (n=61). Sera was also analysed using a multiplex immunoassay of conserved and type-specific antigens to measure antibody responses. This incorporated detectors for all antibody isotypes (IgA, IgM, and IgG), IgG subclasses (IgG1-4), and FcγR receptors. Finally, the OPKA and immunoassay data were combined in a systems serology analysis to identify antibody features associated with killing. Results: 70% of serum samples from children that had emm12 pharyngitis were able to kill Strep A, compared with 33-55% of serum from children that had non-emm12 infections. IgG titres were the highest isotype overall, with type-specific responses to the hypervariable region of M12 (M12 HVR) and T12 the most elevated in sera from children with emm12 pharyngitis compared to healthy children. The IgG1 subclass was the most elevated across the conserved antigens, while IgG1 and IgG3 were elevated for type-specific antigens. Multivariate analysis indicated that T12 and M12 HVR-specific antibodies that bind FcγR receptors on phagocytes were enriched in the killing-positive group. Conclusion: Type-specific antibody engagement with FcγR receptors was associated with emm12 killing and appeared to align more strongly with a type-specific functional response than overall antibody magnitude alone. Future studies should determine whether killing of other Strep A strain types is similarly type-specific. These findings provide a step towards identifying a correlate of protection, which would be valuable for Strep A vaccine development