IdeE reduces the bactericidal activity of equine neutrophils for Streptococcus equi.

This article investigates the role of the protein IdeE in the infection process of Streptococcus equi (S. equi), a bacteria that causes the condition equine strangles in horses. The research finds that IdeE reduces the ability of equine neutrophils (white blood cells) to kill S. equi, thus facilitating the spread of the disease.

Identity and Role of IdeE

• The protein IdeE is identified as an homologue of the leucocyte receptor Mac-1, also known as CD11b. It functions as an IgG-endopeptidase, which can cleave proteins into smaller peptides.
• IdeE is expressed in significant levels during S. equi infection, prompting both serum and mucosal antibody responses in horses. These responses retain their significance for over 200 days.
• The protein is released from S. equi into the culture medium during the exponential phase of bacterial growth, suggesting that its production is linked to rapid bacterial replication.

IdeE in Different Bacterial Species

• Investigation of closely related bacteria, Streptococcus zooepidemicus, shows that while this species stores IdeE, it does not actively release the protein. This difference may shed light on diversity in virulence strategies among closely related bacterial species.

Affect on Equine Neutrophils

• The study reveals that IdeE hinders the bactericidal (bacteria-killing) activity of equine neutrophils. This anti-phagocytic (preventing the ingestion of bacteria by immune cells) action is found to be dose-dependent, meaning it increases with the amount of IdeE present.
• The presence of IdeE-specific antiserum neutralizes this anti-bactericidal activity, demonstrating a possible means of countering the protein’s effect on immune response.
• IdeE is observed to bind, with differing strengths, to about 77% of purified equine neutrophils, suggesting a direct interaction with the immune cells and potential mechanism for its suppression of immune function.