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The protective M proteins of the equine group C streptococci
1Gluck Equine Research Center, University of Kentucky, Lexington.
Abstract:
The group C streptococci are the most commonly isolated bacteria from disease states in the horse. Important virulence factors of S. equi and S. zooepidemicus are the hyaluronic acid capsule and the antiphagocytic fibrillar M protein located on the surface of the cell wall and extending into and through the capsule. The hyaluronic acid capsule is non-antigenic and so is not involved in protective immunity. The M protein, a superantigen, elicits very strong B and T cell responses that may result in protective immunity mediated by opsonic antibodies in plasma and by locally synthesized IgG and IgA on the pharyngeal mucosa. However, vaccines based on acid or mutanolysin extracted M protein do not confer a high level of protection against field exposure. Protective antibodies to S. equi or S. zooepidemicus can in part be assayed by the bactericidal test that measures opsonization for equine neutrophils. A mouse-challenge model has also been used to test immunizing potency of streptococcal extracts and in a passive protection test for protective antibody. There is as yet no means of distinguishing protective opsonic or mucosal antibodies from other antibodies produced against the many epitopes on the M molecule.
Insights
Group C streptococci cause horse diseases, with M protein being a key virulence factor. Current vaccines offer limited protection, highlighting the need for better understanding of protective antibodies against Streptococcus equi and Streptococcus zooepidemicus.
Area of Science:
- Veterinary Microbiology
- Equine Infectious Diseases
- Bacterial Pathogenesis
Background:
- Group C streptococci, specifically Streptococcus equi and Streptococcus zooepidemicus, are primary causes of disease in horses.
- Key virulence factors include the hyaluronic acid capsule and the antiphagocytic M protein, crucial for bacterial survival and host interaction.
Purpose of the Study:
- To investigate the role of M protein in Streptococcus equi and Streptococcus zooepidemicus infections in horses.
- To evaluate the efficacy of current vaccines and identify challenges in achieving protective immunity against these pathogens.
Main Methods:
- Analysis of bacterial virulence factors, including hyaluronic acid capsule and M protein.
- Assessment of immune responses, including B and T cell activation and antibody production (IgG, IgA).
- Evaluation of protective immunity using bactericidal tests with equine neutrophils and a mouse-challenge model.
Main Results:
- Hyaluronic acid capsule is non-antigenic and not involved in protective immunity.
- M protein elicits strong immune responses, potentially leading to protective immunity via opsonic antibodies.
- Vaccines based on extracted M protein provide insufficient protection against field exposure.
Conclusions:
- M protein is a critical target for protective immunity against group C streptococci in horses.
- Current vaccine strategies are limited in conferring adequate protection.
- Further research is needed to differentiate protective antibodies from non-protective ones against M protein epitopes.