In vivo characterization of the murine intranasal model for immunologic assessment of Salmonella typhi vaccine and live vector strains and use of this model to assess an S. typhi live vector strain expressing a Plasmodium falciparum antigen

Abstract

Attenuated Salmonella typhi live vector vaccine strains are highly immunogenic in mice following intranasal, but not orogastric inoculation. To elucidate the relationship between organs within which vaccine organisms are found and the induction of specific serum IgG antibodies, we examined the in vivo distribution of S. typhi vaccine strain CVD 908-htrA following intranasal administration. Vaccine organisms were cultured from the nasal-associated lymphoid tissue (NALT), lungs, and Peyer's patches two minutes after intranasal inoculation. Vaccine organisms persisted longer in NALT than in other organs. By decreasing the volume of intranasal inoculum containing 109 CFU (from a single 30 mul or 10 mul dose to four 2.5 mul doses given over the course of one hour), we were able to significantly reduce the number of vaccine organisms isolated from the lungs (p < 0.05), without reducing the number of vaccine organisms in NALT. Reducing the number of vaccine organisms in the lungs resulted in a significant decrease in the serum tetanus antitoxin response elicited by CVD 908-htrA expressing tetanus toxin fragment C under the control of the redox-responsive nir15 promoter. In contrast, a similar construct expressing tetanus toxin fragment C under control of the constitutive lpp promoter stimulated a strong serum IgG tetanus antitoxin response with both inoculation regimens. The data suggest that following intranasal inoculation, the NALT is a sufficient inductive site for elicitation of an immune response against both the live vector and heterologous antigen and, as occurs following oral inoculation of humans, attenuated S. typhi vaccine organisms elicit serum IgG responses. The murine intranasal model was used to assess the immunogenicity of CVD 908-htrA expressing an antigen from Plasmodium falciparum, the etiological agent of human malaria. A truncated version of the circumsporozoite protein (tCSP) was expressed as a translational fusion to tetanus toxin fragment C, under the control of either the nir15 or the ompC promoter. Mice inoculated with both constructs mounted appreciable serum IgG responses against S. typhi LPS; serum IgG responses against the fusion protein were weak or non-existent. However mice inoculated with both constructs exhibited specific lymphoproliferative responses against the live vector, tetanus toxoid and CSP.