Development of a vaccine against strangles

Abstract: Strangles is an acute upper respiratory tract disease in the horse, which is characterized by purulent pharyngitis and lymphadenitis. It is caused by Streptococcus equi subsp. equi. The severe suffering and economical losses due to Streptococcus equi infections makes it one of the most important bacterial diseases in horses. The aim of this project was to develop a vaccine against strangles. Horses that have undergone strangles do not acquire strangles soon afterwards, suggesting that strangles trigger an immune response resulting in protection. We focused on six different surface located and secreted proteins from S. equi and these were tested for immunogenicity and protection against strangles. The proteins included one surface located fibronectin-binding protein (FNZ) from S. equi subsp. zooepidemicus, and five from subsp. equi including a secreted fibronectin-binding protein (SFS), a surface located á2-macroglobulin, serum albumin and IgG binding protein (EAG), a surface located collagen-binding protein (CNE), a collagen-like protein (SclC) and another fibronectin-binding protein FNEB. We found that horses that had undergone infection caused by S. equi subsp. equi had significantly increased serum antibody titers against all six proteins demonstrating that they are expressed during infection. The proteins were tested in a mouse model of strangles. At first, mice were immunized either subcutaneously or intranasally with recombinant FNZ, SFS and EAG. Nasal colonisation and weight loss due to infection were significantly reduced in the vaccinated groups. This protection was more pronounced after intranasal immunization than after subcutaneous. The same recombinant antigens were used to immunize horses. FNZ and EAG gave a significant immune response but SFS did not. We then tested recombinant CNE, SclC and FNEB using the same mouse model. Immunization with CNE and SclC gave rise to protective antibodies in the mouse model of strangles, whereas no protection following FNEB was found despite a good immune response. Interestingly, a synergistic effect was seen when CNE and EAG were combined. It was clear that CNE could act as an adjuvant for EAG. Taken together these studies identified EAG, CNE and SclC as good candidates for future vaccine development.