Poster Presentation Lancefield International Symposium for Streptococci and Streptococcal Diseases 2025

CHIVAS-M1: No strain, no gain. Choosing an M1UK human challenge strain. (#283)

Kristy I Azzopardi 1 , Kimberly Davis 1 2 , Natalie Caltabiano 1 , Andrew Hayes 3 , Mark Davies 3 , Hannah Frost 1 , Andrew Steer 1 4 5 , Joshua Osowicki 1 4 5
  1. Tropical Diseases Research Group, The Murdoch Children's Research Institute, Melbourne, Victoria, Australia
  2. Infection and Immunity, Monash Children's Hospital, Clayton, Victoria, Australia
  3. Microbiology and Immunology, The University of Melbourne, Melbourne, Victoria, Australia
  4. Department of Paediatrics, The Univeristy of Melbourne, Melbourne, Victoria, Australia
  5. Department of Infectious Diseases, The Royal Children's Hospital, Melbourne, Victoria, Australia

Background: The CHIVAS-M75 study established the world’s only modern Streptococcus pyogenes pharyngitis human challenge model as a platform for vaccine research and development. Recent surges of invasive S. pyogenes disease have highlighted the emergence of a hypervirulent ‘M1UK’ variant. Here we aimed to characterise an M1UK strain for use in a CHIVAS-M1 trial, expanding the S. pyogenes human challenge portfolio.

Methods: Applying cautious criteria, we considered a shortlist of M1UK strains isolated from children in a pharyngitis surveillance study in Melbourne, and characterised them using a variety of techniques: whole genome sequencing, SpeA superantigen expression by quantitative reverse transcriptase real-time PCR and attachment to Detroit-562 pharyngeal cells. Genome stability was assessed by comparing the parent strain genome with clones produced by serial passage.   

Results: Whole genome sequencing of each M1UK strain confirmed the 27 chromosomal single nucleotide polymorphisms differentiating M1UK from M1global. Sublineages with additional virulence determinants (e.g. scarlet fever prophage) were excluded from further consideration. Strain MCRI0102 displayed prototypic M1UK characteristics, including SpeA expression 9-fold greater than for M1global, and 65% of bacteria adhered to pharyngeal cells after inoculation, with 2% cellular invasion. Mutations of concern were not observed after two passages, suggesting genome stability during the planned challenge strain manufacture.

Conclusion: We have identified and characterised a M1UK suitable for use in a human challenge trial. Following principles of Good Manufacturing Practice, we will now proceed to produce a bank of single-dose vials of MCRI0102 for deployment in the CHIVAS-M1 human challenge trial.