Development of a tripartite vector system for live oral immunization using a gram-negative probiotic carrier.

The mucosa represents the primary target site and thus the first barrier for most microbial pathogens. Nevertheless, nearly all present-day vaccines are applied by an invasive route, target the systemic immune system, and do not confer efficient mucosal protection. Currently, mucosal immunity can only be achieved by the delivery of antigens via the mucosal route. Therefore, multiple efforts are under way to develop mucosal vaccines and particularly live oral vaccines as these would confer considerable advantages. We have engineered the AIDA autotransporter system for the surface presentation and/or release of heterologous polypeptides. This study is focused on the development and evaluation of a tripartite live bacterial vector system for oral vaccination based on the AIDA autotransporter, heterologous virulence factor-derived (poly-)peptides, and the apathogenic Escherichia coli Nissle 1917 strain as a live carrier. Potentially with this system also attenuated Salmonella or Shigella strains might be employed as carriers. Model antigens included e.g. the p60 antigen of Listeria monocytogenes, the OspA/OspG antigens of B. burgdorferi, the LT-B subunit of E. coli, and Stx-B subunits of enterohemorrhaghic E. coli (EHEC), all representing crucial virulence factors of important bacterial pathogens. Exemplary oral immunization studies were conducted using different regimes in BALB/c mice with candidate vaccines expressing Stx B-subunits and OspA and OspG proteins. To monitor the induction of immune responses, specific antibody titers in serum as well as secreted mucosal antibodies of local and distal mucosal surfaces were determined. Antigen-specific mucosal as well as systemic antibodies could be induced; however, thus far the response turned out to be heterogeneous and appeared not to be sufficient to mediate protection.

Use of attenuated bacteria as delivery vectors for DNA vaccines.

Live, attenuated bacterial vaccines (LBV) are promising candidates for the induction of a broad-based immune response directed at recombinant heterologous antigens and the corresponding pathogen. LBVs allow vaccination through the mucosal surfaces and specific targeting of professional antigen-presenting cells located at the inductive sites of the immune system. A novel approach exploits attenuated intracellular bacteria as delivery vectors for eukaryotic antigen-expression plasmids (so-called DNA vaccines). Candidate carrier bacteria include attenuated strains of Gram-positive and Gram-negative bacteria. These bacteria have been shown to deliver DNA vaccines to human cells in vitro and have also proven their in vivo efficacy in several experimental animal models of infectious diseases and different cancers. The clinical assessment of the safety, immunogenicity and efficacy of these candidate strains will be the next challenging step towards live bacterial DNA vaccines.