Analysis of distinct variants of immunoglobulin G binding protein EAG on humoral immunity and bacterial clearance of Streptococcus equi subspecies equi.

Overview

• This study investigates how different genetic variants of the immunoglobulin G binding protein EAG from Streptococcus equi subspecies equi affect immune responses and bacterial clearance in the context of strangles, a contagious horse disease.
• The research focuses on the impact of specific mutations in EAG on its ability to stimulate antibody production and promote bacterial clearance through opsonophagocytosis.

Background

• Streptococcus equi subspecies equi (S. equi) is the bacterial pathogen responsible for strangles, a common and highly contagious disease affecting horses with major health and economic consequences.
• The bacterium uses immune evasion strategies to persist and cause infection, one of which involves a protein called EAG that binds immunoglobulin G (IgG) and alpha 2-macroglobulin.
• EAG is a critical antigen in the host-pathogen interaction and thus a target for vaccine development.

Research Aims

• To analyze the genetic variation (polymorphisms) within the EAG protein in different bacterial strains.
• To assess how these variations affect the protein’s function in immune evasion and its immunogenicity.
• To evaluate the protection offered by these variants in animal models, including mice and horses.

Methods

• Phylogenetic analysis was conducted to classify different EAG variants, which grouped into two distinct clades.
• Three specific recombinant EAG mutants were produced and purified for testing:
  • EAG5012 (with mutations K123E and changes at amino acids 155-169)
  • EAG823 (mutation A73V)
  • EAGHT1112 (mutation T196I)
• Mouse and equine models were immunized with these mutants to measure:
  • Antibody (IgG, IgG1, and IgG2a) production
  • Phagocytosis inhibition
  • Opsonophagocytosis (a process where antibodies promote the uptake and destruction of bacteria by immune cells)
  • Protection efficacy after challenge with S. equi strains

Key Findings

• EAG5012 mutant:
  • Induced the highest levels of specific IgG, IgG1, and IgG2a antibodies.
  • Conferred protection rates between 80% and 86.7% against infection by multiple S. equi strains in mice.
  • Showed significantly enhanced inhibition of phagocytosis (i.e., better immune evasion blockade).
  • Antisera from this mutant had increased opsonophagocytic activity, helping immune cells clear bacteria more effectively.
• EAG823 mutant:
  • Produced the lowest antibody responses and opsonophagocytic activity.
  • Offered lower protection rates (66.7% to 73%) in mice.
• Data suggest that specific amino acid changes at positions K123E and in the 155-169 region are critical in modifying the structural epitope of EAG, which affects immune recognition.

Conclusions and Implications

• The study highlights that certain EAG variants differ substantially in their ability to stimulate protective immune responses.
• Alterations in EAG’s conformational epitopes induced by mutations can dramatically influence IgG antibody responses and the antibody’s ability to mediate opsonophagocytosis.
• These findings provide valuable insights for vaccine design, suggesting that including variants like EAG5012 could improve multivalent subunit vaccines targeting strangles by enhancing antibody-mediated bacterial clearance.
• Overall, understanding the impact of genetic diversity in bacterial antigens can aid the development of more effective vaccines and disease control strategies in horses.