Genetic detoxification of an aroA Salmonella enterica serovar Typhimurium vaccine strain does not compromise protection against virulent Salmonella and enhances the immune responses towards a protective malarial antigen.

Live Salmonella vaccines are limited in use by the inherent toxicity of the lipopolysaccharide. The waaN gene encodes a myristyl transferase required for the secondary acylation of lipid A in lipopolysaccharide. A waaN mutant exhibits reduced induction of the inflammatory cytokines associated with lipopolysaccharide toxicity. Here the characteristics of a Salmonella enterica serovar Typhimurium aroA waaN mutant (SK100) in vitro and in vivo compared with its parent aroA strain (SL3261) were described. Phenotypic analysis of purified lipopolysaccharide obtained from SK100 confirmed that the physical and biological activities of the lipopolysaccharide had been altered. Nevertheless both strains had similar patterns of colonization and persistence in mice and significantly the aroA waaN mutant was equally as effective as the parent at protecting against challenge with wild-type S. Typhimurium. Furthermore, a SK100 strain was constructed expressing both tetanus toxin fragment C and the circumsporozoite protein of a malaria parasite. In marked contrast to its isogenic parent, the new attenuated strain induces significantly enhanced immune responses against the circumsporozoite protein. The waaN mutation enhances the ability of this strain to elicit immune responses towards guest antigens. This study provides important insights into the development of safe and effective multivalent Salmonella vaccines.

Processing of viable Salmonella typhimurium for presentation of a CD4 T cell epitope from the Salmonella invasion protein C (SipC).

We have identified Salmonella invasion protein C (SipC) as a target antigen for CD4 T cell recognition in mice infected with Salmonella typhimurium. SipC is a product of the type III secretion system encoded by S. typhimurium pathogenicity island 1. A SipC-specific T cell response was induced by infection with either the C5 wild type or attenuated SL3261 vaccine strain of S. typhimurium. We localized the response of T cell lines from infected mice to an epitope near the carboxyl terminus of SipC (SipC(381-394)) and studied the way it was processed from viable S. typhimurium. We demonstrated that CD4 T cell recognition of this epitope required actin-dependent uptake of S. typhimurium. Presentation also occurred when transport of newly synthesized MHC class II from the endoplasmic reticulum was disrupted and when the pH of intracellular compartments was raised, suggesting presentation by mature MHC class II recycled from the macrophage surface into neutral intracellular compartments. Salmonellae are known to colonize macrophages by localizing to compartments that do not make contact with the bactericidal environment of late endosomes or lysosomes, and thus might avoid lysosomal antigen processing. However, we demonstrate that a CD4 T cell response to S. typhimurium-secreted proteins may be induced by an alternative pathway capable of antigen presentation in conditions similar to those in the compartments where Salmonella localize.

Attenuated Salmonella typhimurium htrA mutants cause fatal infections in mice deficient in NADPH oxidase and destroy NADPH oxidase-deficient macrophage monolayers.

Salmonella live vaccine strains harbouring mutations in htrA, a stress protein gene, display increased susceptibility to oxidative stress in vitro. This is believed to be connected to their reduced virulence, perhaps due to impaired survival inside phagocytes, although this has never been formally proven. We report that the in vitro phenotype of increased susceptibility to oxidative stress of Salmonella typhimurium htrA mutants newly prepared by transduction is rapidly lost on subculture, with the mutants becoming as resistant as the parent for reasons that remain unclear. However, despite this change, htrA mutants are still attenuated in normal mice. In contrast, they were found to be lethal for gene targeted gp91phox-/- mice deficient in NADPH oxidase, as was a S. typhimurium SPI-2 mutant known to be virulent in gp9lphox-/- mice. Infection with htrA mutants caused little damage to primary bone marrow macrophage cultures from normal mice; conversely, they caused extensive damage to macrophages from gp9lphox-/- mice, with more than 60% reduction in cell numbers 2.5h after being infected. The parental wild type strain similarly caused extensive damage to macrophages from both normal and gp9lphox-/- mice, whereas an aroA live vaccine strain had no effect on either normal or gp9lphox-/- macrophages. Taken collectively, the present results suggest that htrA is somehow involved in resistance to oxidative stress in vivo, with the avirulence of htrA mutants in mice being due to mechanisms which involve NADPH oxidase and suppression of bacterial growth within macrophages.

Expression of disulphide-bridge-dependent conformational epitopes and immunogenicity of the carboxy-terminal 19 kDa domain of Plasmodium yoelii merozoite surface protein-1 in live attenuated Salmonella vaccine strains.

The 19 kDa carboxy-terminal domain of Plasmodium yoelii merozoite surface protein-1 (MSP1(19)) was expressed in Salmonella vaccine strains as a carboxy-terminal fusion to fragment C of tetanus toxin (TetC). This study demonstrates that antibodies that recognize disulphide-dependent conformational epitopes in native MSP1 react with the TetC-MSP1(19) fusion protein expressed in Salmonella. The proper folding of MSP1(19) polypeptide is dependent on both the Salmonella host strain and the protein to which the MSP1(19) polypeptide is fused. Serum from mice immunized with Salmonella typhimurium C5aroD expressing TetC-MSP1(19) recognized native MSP1 as shown by immunofluorescence with P. yoelii-infected erythrocytes. Antibody levels to MSP1(19) were highest in out-bred mice immunized with S. typhimurium C5aroD carrying pTECH2-MSP1(19) and antibody was mostly directed against reduction-sensitive conformational epitopes. However, antibody levels were lower than in BALB/c mice immunized with a glutathione S-transferase (GST)-MSP1(19) fusion protein in Freund's adjuvant, and which were protected against P. yoelii challenge infection. In challenge experiments with P. yoelii the Salmonella-immunized mice were not protected, probably reflecting the magnitude of the antibody response. The results of this study have important implications in the design of live multivalent bacterial vaccines against eukaryotic pathogens.