Expression and immunogenicity of a recombinant diphtheria toxin fragment A in Streptococcus gordonii

A nontoxic mutant diphtheria toxin fragment A (DTA) was genetically fused in single, double, or triple copy to the major surface protein antigen P1 (SpaP) and surface expressed in Streptococcus gordonii DL-1. The expression was verified by Western immunoblotting. Mouse antisera raised against the recombinant S. gordonii recognized the native diphtheria toxinm suggesting the recombinant DTA was immunogenic. When given intranasally to mice with cholera toxin subunit B as the adjuvant, the recombinant S. gordonii expressing double copies of DTA (SpaP-DTA₂) induced a mucosal immunoglobulin A response and a weak systemic immunoglobulin G response. S. gordonii SpaP-DTA₂ was able to orally colonize BALB/c mice for a 15-week period and elicited a mucosal response, but a serum immunoglobulin G response was not apparent. The antisera failed to neutralize diphtheria toxin cytotoxicity in a Vero cell assay.