Improved cell surface display of Salmonella enterica serovar Enteritidis antigens in Escherichia coli.

ABSTRACT

BACKGROUND: Salmonella enterica serovar Enteritidis (SE) is one of the most potent pathogenic Salmonella serotypes causing food-borne diseases in humans. We have previously reported the use of the ß-autotransporter AIDA-I to express the Salmonella flagellar protein Hgm and the SE serotype-specific fimbrial protein SefA at the surface of E. coli as live bacterial vaccine vehicles. While SefA was successfully displayed at the cell surface, virtually no full-length Hgm was exposed to the medium due to extensive proteolytic cleavage of the N-terminal region. In the present study, we addressed this issue by expressing a truncated Hgm variant (Hgmd) covering only the serotype-specific central region. This protein was also expressed in fusion to SefA (HgmdSefA) to understand if the excellent translocation properties of SefA could be used to enhance the secretion and immunogenicity. RESULTS: Hgmd and HgmdSefA were both successfully translocated to the E. coli outer membrane as full-length proteins using the AIDA-I system. Whole-cell flow cytometric analysis confirmed that both antigens were displayed and accessible from the extracellular environment. In contrast to Hgm, the Hgmd protein was not only expressed as full-length protein, but it also seemed to promote the display of the protein fusion HgmdSefA. Moreover, the epitopes appeared to be recognized by HT-29 intestinal cells, as measured by induction of the pro-inflammatory interleukin 8. CONCLUSIONS: We believe this study to be an important step towards a live bacterial vaccine against Salmonella due to the central role of the flagellar antigen Hgm and SefA in Salmonella infections and the corresponding immune responses against Salmonella.