Serum Immune Responses to Group A Streptococcal Antigens following Pharyngeal Acquisitions among Children in Cape Town, South Africa

There is limited information on the human immune response following infection with group A Streptococcus (Strep A). Animal studies have shown, in addition to the M protein, that shared Strep A antigens elicit protective immunity. This study aimed to investigate the kinetics of antibody responses against a panel of Strep A antigens in a cohort of school-aged children in Cape Town, South Africa. Participants provided serial throat cultures and serum samples at two-monthly follow-up visits. Strep A recovered were emm-typed, and serum samples were analyzed by enzyme-linked immunosorbent assay (ELISA) to assess immune responses to thirty-five Strep A antigens (10-shared and 25-M peptides). Serologic evaluations were performed on serial serum samples from 42 selected participants (from 256 enrolled) based on the number of follow-up visits, the frequency of visits, and throat culture results. Among these, there were 44 Strep A acquisitions, 36 of which were successfully emm-typed. Participants were grouped into three clinical event groups based on culture results and immune responses. A preceding infection was most convincingly represented by a Strep A-positive culture with an immune response to at least one shared antigen and M peptide (11 events) or a Strep A-negative culture with antibody responses to shared antigens and M peptides (9 events). More than a third of participants demonstrated no immune response despite a positive culture. This study provided important information regarding the complexity and variability of human immune responses following pharyngeal acquisition of Strep A, as well as demonstrating the immunogenicity of Strep A antigens currently under consideration as potential vaccine candidates. IMPORTANCE There is currently limited information regarding the human immune response to group A streptococcal throat infection. An understanding of the kinetics and specificity of antibody responses against a panel of Group A Streptococcus (GAS) antigens will serve to refine diagnostic approaches and contribute to vaccine efforts, which together will serve to reduce the burden of rheumatic heart disease, a major source of morbidity and mortality especially in the developing world. This study, utilizing an antibody-specific assay, uncovered three patterns of response profiles following GAS infection, among 256 children presenting with sore throat to local clinics. Overall, the response profiles were complex and variable. Of note, a preceding infection was most convincingly represented by a GAS-positive culture with an immune response to at least one shared antigen and M peptide. Also, more than a third of participants demonstrated no immune response despite a positive culture. All antigens tested were immunogenic, providing guidance for future vaccine development.