Construction, genotypic and phenotypic characterization, and immunogenicity of attenuated Delta guaBA Salmonella enterica serovar Typhi strain CVD 915

A promising live attenuated typhoid vaccine candidate strain for mucosal im munization was developed by introducing a deletion in the guaBA locus of pa thogenic Salmonella enterica serovar Typhi strain Ty2. The resultant Delta guaBA mutant, serovar Typhi CVD 915, has a gene encoding resistance to arse nite replacing the deleted sequence within guaBA, thereby providing a marke r to readily identify the vaccine strain. CVD 915 was compared in in vitro and in vivo assays with wild-type strain Ty2, licensed live oral typhoid va ccine strain Ty21a, or attenuated serovar Typhi vaccine strain CVD 908-htrA (harboring mutations in aroC, aroD, and htrA). CVD 915 was less invasive t han CVD 908-htrA in tissue culture and was more crippled in its ability to proliferate after invasion. In mice inoculated intraperitoneally with serov ar Typhi and hog gastric mucin (to estimate the relative degree of attenuat ion), the 50% lethal dose of CVD 915 (7.7 x 10(7) CFU) was significantly hi gher than that of wild-type Ty2 (1.4 x 10(2) CFU) and was only slightly low er than that of Ty21a (1.9 x 10(8) CFU). Strong serum O and H antibody resp onses were recorded in mice inoculated intranasally with CVD 915, which wer e higher than those elicited by Ty21a and similar to those stimulated by CV D 908-htrA. CVD 915 also elicited potent proliferative responses in splenoc ytes from immunized mice stimulated with serovar Typhi antigens. Used as a live vector, CVD 915(pTETlpp) elicited high titers of serum immunoglobulin G anti-fragment C. These encouraging preclinical data pave the way for phas e 1 clinical trials with CVD 915.