Polymeric display of immunogenic epitopes from herpes simplex virus and transmissible gastroenteritis virus surface proteins on an enteroadherent fimbria.

The strong immunogenicity of bacterial fimbriae results from their polymeric and proteinaceous nature, and the protective role of these immunogens in experimental or commercial vaccines is associated with their capacity to induce antiadhesive antibodies. Fimbria-mediated intestinal colonization by enteropathogens typically leads to similar antibody responses. The possibility of taking advantage of these properties was investigated by determining whether enteroadhesive fimbriae, like the 987P fimbriae of enterotoxigenic Escherichia coli, can serve as carriers for foreign antigens without losing their adhesive characteristics. Random linker insertion mutagenesis of the fasA gene encoding the major 987P subunit identified five different mutants expressing wild-type levels of fimbriation. The linker insertion sites of these mutants were used to introduce three continuous segments of viral surface glycoproteins known to be accessible to antibodies. These segments encode residues 11 to 19 or 272 to 279 of herpes simplex virus type 1 (HSV-1) glycoprotein D [gD(11-19) and gD(272-279), respectively] or residues 379 to 388 of the transmissible gastroenteritis virus (TGEV) spike protein [S(379-388)]. Studies of bacteria expressing fimbriae incorporating mutated FasA subunits alone or together with wild-type FasA subunits (hybrid fimbriae) indicated that foreign epitopes were best exported and displayed on assembled fimbriae when they were inserted near the amino terminus of FasA. Fimbriated bacteria expressing FasA subunits carrying the HSV gD(11-19) or the TGEV S(379-388) epitope inserted between the second and third residues of mature FasA elicited high levels of foreign epitope antibodies in all rabbits immunized parenterally. Antibodies against the HSV epitope were also shown to recognize the epitope in the context of the whole gD protein. Because the 987P adhesive subunit FasG was shown to be present on mutated fimbriae and to mediate bacterial attachment to porcine intestinal receptors, polymeric display of foreign epitopes on 987P offers new opportunities to test the potential beneficial effect of enteroadhesion for mucosal immunization and protection against various enteric pathogens.