Synthesis and secretion of bacterial antigens by attenuated Salmonella via the Escherichia coli hemolysin secretion system.

We describe a plasmid system which allows the secretion of foreign antigens in attenuated Salmonella aroA strains by the secretion apparatus of E. coli hemolysin. The gene (or gene fragment) encoding the antigen is inserted in frame into a residual position of the hlyA gene, encoding the HlyA secretion signal (HlyAs). Generally, the fused gene is efficiently expressed and the synthesized antigen is in part secreted into the culture supernatant and in part exposed on the surface of the producing Salmonella strain. The successful use of this approach is demonstrated with two antigens of Salmonella typhimurium, PagC and SlyA, both of which are potent virulence factors but produced only in small amounts under in vitro culture conditions and two virulence proteins of Listeria monocytogenes, p60 and listeriolysin. Interestingly the listeriolysin fusion protein proved to be cytolytically active and allowed, when expressed in Salmonella, the escape of these bacteria into the cytoplasm of infected macrophages.