Live, attenuated Salmonella enterica serovar Typhi strain Ty21a, a licensed oral typhoid fever vaccine,
has also been employed for use as a vector to deliver protective antigens of Shigella and other pathogens.
Importantly, lipopolysaccharide (LPS) alone has been shown to be a potent antigen for specific protection
against shigellosis. We reported previously the plasmid cloning of heterologous LPS biosynthetic genes
and the expression in Ty21a of either S. sonnei or of S. dysenteriae 1 LPS’s. The resulting plasmids encoding
Shigella LPS’s were reasonably stable for >50 generations of growth in nonselective media, but still contained
an antibiotic resistance marker that is objectionable to vaccine regulatory authorities. Deletion of
this antibiotic-resistance marker inexplicably resulted in significant plasmid instability. Thus, we sought
a method to insert the large
∼12 kb S. sonnei LPS gene region into the chromosome, that would allow
for subsequent removal of a selectable marker and would result in 100% genetic stability. Toward this
objective, we optimized an existing recombination method to mediate the insertion of a
∼12 kb region
encoding the S. sonnei LPS genes into the Ty21a genome in a region that is nonfunctional due to mutation.
The resulting strain Ty21a-Ss simultaneously expresses both homologous Ty21a and heterologous
S. sonnei O-antigens. This chromosomal insert was shown to be 100% genetically stable in vitro and in
vivo. Moreover, Ty21a-Ss elicited strong dual anti-LPS serum immune responses and 100% protection
in mice against a virulent S. sonnei challenge. This new vaccine candidate, absolutely stable for vaccine
manufacture, should provide combined protection against enteric fevers due to Salmonella serovar Typhi
as shown previously (and some Paratyphi infections) and against shigellosis due to S. sonnei.