Se18.9, an anti-phagocytic factor H binding protein of Streptococcus equi.

This research article examines Se18.9, an anti-phagocytic protein secreted by Streptococcus equi, a bacteria causing equine strangles. It highlights how this protein supports the bacteria in escaping phagocytosis and its potential significance in diagnosing the disease and studying the immune response.

Streptococcus equi and Se18.9

• The study is centred around Streptococcus equi, a bacterium that causes equine strangles, a serious infection in horses. One of its subspecies, S. equi subsp. equi is more virulent compared to the closely related S. equi subsp. zooepidemicus.
• Central to this bacteria’s virulence is its capacity to evade phagocytosis, a process where components of the immune system engulf and destroy harmful organisms or particles. This is largely due to Se18.9.
• Se18.9 is a novel H (Complement factor H) factor binding protein secreted by the S. equi bacteria. It reduces the deposition of C3, a protein component of the immune system, on the bacterial surface, thus impeding the bacteria’s detection and destruction by the immune system.

The Role of Se18.9 and its Distinguishing Aspects

• Se18.9 significantly reduces the bactericidal (bacteria-killing) activity of equine neutrophils, a type of white blood cell, in the serum of both S. equi subspecies.
• The protein is produced in large quantities by actively dividing bacterial cells and is also found on the bacterial surface.
• Strong serum and mucosal antibody responses are elicited by S. equi-infected horses, indicating the presence of this protein. This response might provide insight into the body’s immune reaction to the bacteria.

Potential of Se18.9 for Immunodiagnosis and Further Study

• An identical gene to se18.9 was not found in S. zooepidemicus. However, sequences encoding proteins of similar size with similar signal peptide sequences were found in a few selected strains, suggesting a potential avenue of further research and genetic comparison.
• Se18.9 is unique to S. equi, making it a potential target for developing immunodiagnostic tests for S. equi infections.
• Its reactivity with convalescent sera and mucosal IgA (an antibody class commonly found in mucosal areas) makes Se18.9 a useful candidate for studying the mucosal antibody response to S. equi, shedding light on the immunity processes in S. equi infections in horses.