Intranasal immunization with recombinant group A streptococcal M protein fragment fused to the B subunit of Escherichia coli labile toxin protects mice against systemic challenge infections.

A fusion gene named LT-B-M5 was constructed encoding the entire B subunit of Escherichia coli labile toxin (LT-B), a 7 amino acid proline-rich linker, and 15 amino-terminal amino acids of type 5 streptococcal M protein. The purified LT-B-M5 was immunogenic in rabbits and evoked antibodies against a synthetic peptide copy of the amino-terminus of M5 (SM5[1-15]) and the native M5 protein and opsonic antibodies against type 5 streptococci. The hybrid protein retained the ganglioside-binding activity of LT-B and was tested in mice for its immunogenicity after local administration. Mice that were immunized intranasally with LT-B-M5 developed serum antibodies against SM5(1-15) and were significantly protected from death after intraperitoneal challenge infections with type 5 streptococci. The data show that protective systemic immune responses may be evoked after intranasal immunization with a fragment of M protein fused to LT-B.