Mucosal and systemic immune responses to chimeric fimbriae expressed by Salmonella enterica serovar Typhimurium vaccine strains

Recombinant live oral vaccines expressing pathogen-derived antigens offer a unique set of attractive properties, Among these are the simplicity of adm inistration, the capacity to induce mucosal and systemic immunity, and the advantage of permitting genetic manipulation for optimal antigen presentati on, In this study, the benefit of having a heterologous antigen expressed o n the surface of a live vector rather than intracellularly was evaluated. A ccordingly, the immune response of mice immunized with a Salmonella enteric a serovar Typhimurium vaccine strain expressing the Escherichia coli 987P f imbrial antigen on its surface (Fas(+)) was compared with the expression in the periplasmic compartment (Fas(-)). Orally immunized BALB/c mice showed that 987P fimbriated Salmonella serovar Typhimurium CS3263 (aroA asd) with pCS151 (fas(+) asd(+)) elicited a significantly higher level of 987P-specif ic systemic immunoglobulin G (IgG) and mucosal IgA than serovar Typhimurium CS3263 with pCS152 (fasD mutant, asd(+)) expressing 987P periplasmic antig en, Further studies were aimed at determining whether the 987P fimbriae exp ressed by serovar Typhimurium chi 4550 (cya crp asd) could be used as carri ers of foreign epitopes. For this, the vaccine strain was genetically engin eered to express chimeric fimbriae carrying the transmissible gastroenterit is virus (TGEV) C (379-388) and A (521-531) epitopes of the spike protein i nserted into the 987P major fimbrial subunit FasA, BALB/c mice administered orally serovar Typhimurium chi 4550 expressing the chimeric fimbriae from the tet promoter in pCS154 (fas(+) asd(+)) produced systemic antibodies aga inst both fimbria and the TGEV C epitope but not against the TGEV A epitope , To improve the immunogenicity of the chimeric fimbriae, the in vivo induc ible nirB promoter was inserted into pCS154, upstream of the fas genes, to create pCS155. In comparison with the previously used vaccine, BALB/c mice immunized orally with serovar Typhimurium chi 4550/pCS155 demonstrated sign ificantly higher levels of serum IgG and mucosal IgA against 987P fimbria, Moreover, mucosal IgA against the TGEV C epitope was only detected with ser ovar Typhimurium chi 4550/pCS155. The induced antibodies also recognized th e epitopes in the context of the full-length TGEV spike protein. Hence, imm une responses to heterologous chimeric fimbriae on Salmonella vaccine vecto rs can be optimized by using promoters known to be activated in vivo.