Live Attenuated Shigella as a Diarrheal Vaccine And/or Mucosal Delivery Vector for Dna Vaccines

Several live attenuated Shigella vaccines of different serotypes have been shown to be safe, immunogenic, and in one case effective against challenge with virulent strains. The ability to invade epithelial cells remains critical for the success of these vaccine candidates. Live, orally administered Shigella vaccine derivatives are also being evaluated as multivalent mucosal vaccines able to deliver both bacterial antigens and eukaryotic genes to the gut associated lymphoid tissues of the common mucosal immune system. Fimbrial and enterotoxin antigen genes from enterotoxigenic E. coli (ETEC) were cloned into a plasmid encoding the gene for aspartate semialdehyde dehydrogenase (asd) and expressed within SC608, an asd mutant of Shigella flexneri 2a vaccine strain SC602. Guinea pigs immunized intranasally with SC608 expressing the ETEC antigens demonstrated serum and mucosal immune responses to antigens from both diarrheal pathogens. SC608, in the absence of an asd-based plasmid, lyses within epithelial cells after invasion. This phenotype has also been used to deliver plasmid DNA vaccines containing eukaryotic genes into the host cell cytoplasm for expression. Splenocytes from mice immunized intranasally with asd mutants of Shigella containing plasmid HIV DNA vaccines demonstrated HIV antigen-specific IFN-γ ELISPOTS after in vitro expansion. These preliminary results are encouraging and suggest that live, attenuated Shigella vaccines can be engineered to deliver both prokaryotic and eukaryotic antigens to the mucosal system.