Poster Presentation Lancefield International Symposium for Streptococci and Streptococcal Diseases 2025

Functional immunoassay performance across the life course in the Gambia: association with binding IgG titre to vaccine antigens (#270)

Alexander J Keeley 1 2 3 , Fatoumata Camara 3 , Edwin Armitage 1 3 , Gabrielle de Crombrugghe 3 4 , Jainaba Sillah 3 , Elina Senghore 3 , Musukoi Jammeh 3 , Alana L Whitcombe 5 , Amat Bittaye 3 , Haddy Ceesay 3 , Isatou Ceesay 3 , Bunja Samateh 3 , Muhammed Manneh 3 , Martina Carducci 6 , Luca Rovetini 6 , Elena Boero 6 , Luisa Massai 6 , Miren Iturriza 6 , Danilo Moriel Gomes 6 , Nicole J Moreland 5 , Ed Clarke 3 , Beate Kampmann 3 7 , Michael Marks 1 , Omar Rossi 6 , Henrick Salje 8 , Claire E Turner 9 , Thushan I de Silva 2 3
  1. London School of Hygiene & Tropical Medicine, London, LONDON, United Kingdom
  2. Division of Clinical Medicine, University of Sheffield, Sheffield, UK
  3. Vaccines and Immunity, MRC Unit The Gambia at LSHTM, Fajara, The Gambia
  4. Molecular Bacteriology Laboratory, Université Libre de Bruxelles, Brussels, Belgium
  5. Faculty of Medical and Health Sciences, , The University of Auckland, Auckland, New Zealand
  6. GSK Vaccines Institute for Global Health (GVGH), Siena, Italy
  7. Charité Centre for Global Health, Berlin, Germany
  8. Department of Genetics, University of Cambridge, Cambridge, UK.
  9. School of Biosciences, University of Sheffield, Sheffield, United Kingdom

Background:

Understanding the functional capacity of antibodies to conserved Streptococcus pyogenes vaccine antigens is critical for vaccine development. We studied performance of functional immunoassays in a setting with high disease burden.

Methods:

Sera (n=114) from two cross sectional cohorts in The Gambia spanning infancy to adulthood were randomly selected from strata of binding IgG titres to SLO, SpyAD and SpyCEP. Functional immunoassays measured inhibition of SLO-mediated hemolysis, SpyCEP-mediated IL-8 cleavage, and opsonophagocytosis of SpyAD- and GAC-coated beads as well as FITC-labelled whole M1 bacteria into THP-1 cells1,2,3.

Results:

Stronger correlations were observed between IgG binding levels to SLO and inhibition of haemolysis (Spearman’s coefficient=0.78) than between IgG to SpyCEP and inhibition of IL-8 cleavage (coefficient=0.59). Opsonophagocytosis of antigen-coated beads correlated strongly with binding IgG for GAC (coefficient=0.73) and SpyAD (coefficient=0.81). A higher proportion of samples with IgG levels above previously identified 50% protective thresholds demonstrated opsonophagocytosis of M1 bacteria compared to those below the threshold for SLO (74% vs 34%, p=0.00078), SpyAD (93% vs 35%, p<0.0001), and SpyCEP (70% vs 36%, p=0.011).

Discussion:

We observed substantial heterogeneity, with broad correlation between binding IgG and functionality. SpyCEP-mediated Il-8 cleavage inhibition demonstrated a less clear correlation with anti-SpyCEP IgG, suggesting binding of antibodies to non-functional regions of SpyCEP. Given IgG to M1 was low, observed opsonophagocytosis is assumed to have occurred mediated by antibodies directed at targets other than the M1 hypervariable region. Further assessment in clinical vaccine trials and human challenge models will further clarify the utility of these assays.

  1. Carducci, M. et al. Development and characterization of a hemolysis inhibition assay to determine functionality of anti-Streptolysin O antibodies in human sera. J. Immunol. Methods 526, 113618 (2024).
  2. Massai, L. et al. Characterization of an IL-8 cleavage inhibition assay to determine the functionality of anti-SpyCEP antibodies in human sera. J. Immunol. Methods 536, 113786 (2025).
  3. Boero, E. et al. A flow cytometry-based assay to determine the ability of anti-Streptococcus pyogenes antibodies to mediate monocytic phagocytosis in human sera. J. Immunol. Methods 528, 113652 (2024).