Constitutive expression of the vi polysaccharide capsular antigen in attenuated Salmonella enterica serovar typhi oral vaccine strain CVD 909

Live oral Ty21a and parenteral Vi polysaccharide vaccines provide significa nt protection against typhoid fever, albeit by distinct immune mechanisms. Vi stimulates serum immunoglobulin G Wi antibodies, whereas Ty21a, which do es not express Wi, elicits humoral and cell-mediated immune responses other than Wi antibodies. Protection may be enhanced if serum Wi antibody as wel l as cell-mediated and humoral responses can be stimulated. Disappointingly , several new attenuated Salmonella enterica serovar Typhi oral vaccines (e .g., CVD 908-htrA and Ty800) that elicit serum O and H antibody and cell-me diated responses following a single dose do not stimulate serum Wi antibody . Tli expression is regulated in response to environmental signals such as osmolarity by controlling the transcription of tviA in the viaB locus. To i nvestigate if Wi antibodies can be stimulated if Vi expression is rendered constitutive, we replaced P-tviA in serovar Typhi vaccine CVD 908-htrA with the constitutive promoter P-tac, resulting in CVD 909. CVD 909 expresses W i even under high-osmolarity conditions and is less invasive for Henle 407 cells. In mice immunized with a single intranasal dose, CVD 909 was more im munogenic than CVD 908-htrA in eliciting serum Vi antibodies (geometric mea n titer of 160 versus 49, P = 0.0007), whereas O antibody responses were vi rtually identical (geometric mean titer of 87 versus 80). In mice challenge d intraperitoneally with wild-type serovar Typhi 4 weeks after a single int ranasal immunization, the mortality of those immunized with CVD 909 (3 of 8 ) was significantly lower than that of control mice (10 of 10, P = 0.043) o r mice given CVD 908-htrA (9 of 10, P = 0.0065).