Characterization of the antiphagocytic activity of equine fibrinogen for Streptococcus equi subsp. equi.

This study investigates the ability of equine fibrinogen, a protein found in horses, to protect the bacteria Streptococcus equi from being destroyed by immune cells. The results suggest that fibrinogen increases the bacteria’s resistance to immune attacks and that this property doesn’t depend on the inhibition of a specific immune response. The study also suggests that a large part of the fibrinogen that adheres to the bacteria binds to a particular bacterial protein.

Research Methodology and Findings

• The research starts by investigating equine fibrinogen’s antiphagocytic property for Streptococcus equi, a bacterium causing diseases in horses. Special focus is on strain CF32, and all the tests are conducted in vitro, meaning they take place outside of a living organism.
• Bactericidal assays show that the presence of fibrinogen enhances the bacteria’s resistance to equine neutrophils, immune cells responsible for consuming and eliminating pathogens. This result depicts that fibrinogen can multiply the bacterium’s resistance by 12 times for overnight cultures and seven times for early log-phase cultures.
• Further investigations find out that fibrinogen-coated bacteria treated with fibrinogen specific F(ab’)2 fragments are more prone to neutrophil attacks after being coated in serum. This implies that particular parts of fibrinogen (epitopes) could be crucial for its antiphagocytic property.
• Prior research indicated that complement deposition, an important part of the immune response, is inhibited in Streptococcus equi. However, this study found no significant difference in the amount of C3 (a key immune protein) deposited on the bacteria’s surface after treatment with serum, serum plus fibrinogen, or plasma, suggesting that fibrinogen’s protection is unrelated to the inhibition of complement deposition.
• Last, the study finds that pretreatment of the bacteria with M protein specific antibody reduces fibrinogen binding by 72%, and a strain of the bacteria expressing low levels of M protein bounds 64% less fibrinogen. This implies that a considerable portion of the fibrinogen attaching to the bacteria binds specifically to the M protein.

Conclusion and Implications

• This research sheds light on the enigmatic interactions between bacteria and the host’s immune response. Fibrinogen emerges as a vital player, enhancing the bacterium’s resistance to immune attacks.
• The antiphagocytic property’s independence from the inhibition of complement response entails the existence of alternate mechanisms through which fibrinogen protects the bacterium, inviting further exploration.
• The significant role of the M protein in fibrinogen binding suggests that it may serve as a promising target for therapeutic interventions to counter bacterial infections.