Nasal mucosal immunogenicity for the horse of a SeM peptide of Streptococcus equi genetically coupled to cholera toxin.

The research investigates the ability of a combined vaccine (cholera toxin genetically linked to a Streptococcus equi protein segment) administered through the nose to initiate an immune response in horses. Although the vaccine provoked some immune responses, the horses were still susceptible to a common streptococcal infection, suggesting the solution may lack essential protective elements or the produced antibodies were insufficient.

Overview of research methods

• The study was conducted on five young adult Welsh ponies.
• The ponies were immunized intranasally with a solution of cholera toxin (CT) genetically attached to the protein sequence of the Streptococcus equi bacteria (referred as SeMF3 in the study).
• The ponies underwent three rounds of immunization: the first round, the second one on day 14, and the third on day 42.
• Blood (serum) samples and nasal washes were collected periodically to analyze the immune responses, specifically checking for the presence of antibodies.

Main findings

• All ponies rapidly formed serum antibody responses for CTB and SeMF3 after the first immunization.
• Specific nasal IgA responses were found in two ponies 14 days after the first vaccination, in two more ponies 14 days after the second vaccination, and in all ponies 28 days after the third vaccination.
• The SeMF3-specific antibody responses in the serum and nasal washes were maintained for at least 140 days.
• Interestingly, the immune responses were reminiscent of those observed during recovery from equine strangles, a disease caused by the Streptococcus equi bacteria.

Limited protection provided by the vaccine

• Notably, secondary and subsequent immunizations didn’t enhance antibodies in serum.
• In vitro tests, where S. equi bacteria were analyzed in the presence of SeMF3-specific antibodies from the sera, showed initial reduction in bacterial survival. However, survival increased with sera collected after subsequent immunizations.
• Despite these induced immune responses, all immunized ponies were still susceptible to Streptococcus equi when exposed, suggesting that SeMF3 may lack protective epitopes (the part of an antigen that is recognized by the immune system) or the antibodies induced by the chimera (combined vaccine) were not at effective levels.