Oral Presentation Lancefield International Symposium for Streptococci and Streptococcal Diseases 2025

A novel approach to assess antigen specific Fc-mediated antibody functions induced by Streptococcus pyogenes pharyngitis (118113)

Kaneka Chheng 1 2 , Natalie Caltabiano 1 , Kristy Azzopardi 1 , Josh Osowicki 1 3 4 , Michael Morici 2 5 , Jill Gilmour 2 6 , Reuben McGregor 7 , Nikki Moreland 7 , Georgia Tomaras 8 , Amy Chung 9 , Alma Fulurija 2 5 , Jonathan Carapetis 2 5 , Andrew Steer 1 2 3 4 , Hannah Frost 1 2
  1. Tropical Diseases Research Group, Murdoch Children's Research Institute, Parkville, VIC, Australia
  2. The Australian Strep A Vaccine Initiative, Australia
  3. Department of Paediatrics, University of Melbourne, Melbourne, Australia
  4. Infectious Diseases Unit, Department of General Medicine, Royal Children's Hospital, Melbourne, Australia
  5. The Kids Research Institute, University of Western Australia, Perth, Australia
  6. JWG Global Lab Solutions, Dumfries, Scotland, UK
  7. Maurice Wilkins Centre for Biodiscovery and School of Medical Sciences, Faculty of Medical and Health Sciences, The University of Auckland, Auckland, New Zealand
  8. Duke Center for Humans Systems Immunology, Durham, North Carolina, USA
  9. Department of Microbiology and Immunology , Peter Doherty Institute for Infection and Immunity, University of Melbourne, Melbourne, Australia

Background: There is a clear and pressing need for a vaccine against Streptococcus pyogenes. Induction of functional antibodies is essential for immune protection and functional immunoassays are central to vaccine development. Functional immunoassays for S. pyogenes are labour-intensive, low throughput and difficult to scale.

Methods: We have developed scalable antibody dependent cellular phagocytosis (ADCP) and neutrophil phagocytosis (ADNP) assays for S. pyogenes candidate vaccine antigens: GAC, SLO, SpyAD, and SpyCEP. Monocytes (THP-1) were used in ADCP assays and neutrophils (DMF-differentiated HL-60 cells) in ADNP assays. Plasma samples from healthy adults pre- and 1-month post-challenge, in the CHIVAS-M75 pharyngitis study, were incubated with fluorescent microspheres conjugated to antigens, and phagocytosis was measured by flow cytometry and confirmed by microscopy.

Results: ADCP and ADNP responses for all antigens were highly variable before and after challenge. Participants who did not develop pharyngitis had higher baseline antibody activity compared to those who did, especially for the SLO ADCP assay (p = 0.05). Increased functional antibodies were observed in participants with pharyngitis, most significantly in the SpyCEP ADCP (p < 0.0001). Functional antibodies in ADCP and ADNP were highly correlated with SpyAD IgG titres (r > 0.7) and moderately for SLO (r > 0.3) and SpyCEP (r > 0.5). IgG titres and GAC antibody-mediated phagocytosis correlated poorly (r < 0.2).

Conclusion: Adults have a diverse repertoire of functional antibodies against S. pyogenes, influenced by infection. This can be measured in these reproducible, sample sparing, high-throughput, antigen-specific assays.