Immunization with live aroA recombinant Salmonella typhimurium producing invasin inhibits intestinal translocation of Yersinia pseudotuberculosis

Salmonella: immune responses and vaccines

Salmonella infections are a serious medical and veterinary problem world-wide and cause concern in the food industry. Vaccination is an effective tool for the prevention of Salmonella infections. Host resistance to Salmonella relies initially on the production of inflammatory cytokines leading to the infiltration of activated inflammatory cells in the tissues. Thereafter T- and B-cell dependent specific immunity develops allowing the clearance of Salmonella microorganisms from the tissues and the establishment of long-lasting acquired immunity to re-infection. The increased resistance that develops after primary infection/ vaccination requires T-cells cytokines such as IFNgamma TNFalpha and IL12 in addition to opsonising antibody. However for reasons that are not fully understood seroconversion and/or the presence of detectable T-cell memory do not always correlate with the development of acquired resistance to infection.Whole-cell killed vaccines and subunit vaccines are used in the prevention of Salmonella infection in animals and in humans with variable results. A number of early live Salmonella vaccines derived empirically by chemical or u.v. mutagenesis proved to be immunogenic and protective and are still in use despite the need for repeated parenteral administration. Recent progress in the knowledge of the genetics of Salmonella virulence and modern recombinant DNA technology offers the possibility to introduce multiple defined attenuating and irreversible mutations into the bacterial genome. This has recently allowed the development of Salmonella strains devoid of significant side effects but still capable of inducing solid immunity after single oral administration. Live attenuated Salmonella vaccines have been used for the expression of heterologous antigens/proteins that can be successfully delivered to the immune system. Furthermore Salmonella can transfer plasmids encoding foreign antigens under the control of eukaryotic promoters (DNA vaccines) to antigen-presenting cells resulting in targeted delivery of DNA vaccines to these cells. Despite the great recent advances in the development of Salmonella vaccines a large proportion of the work has been conducted in laboratory rodents and more research in other animal species is required.

Superantigen YPMa exacerbates the virulence of Yersinia pseudotuberculosis in mice

Yersinia pseudotuberculosis, a gram-negative bacterium responsible for enteric and systemic infection in humans, produces a superantigenic toxin designated YPMa (Y. pseudotuberculosis-derived mitogen). To assess the role of YPMa in the pathogenesis of Y. pseudotuberculosis, we constructed a superantigen-deficient mutant and compared its virulence in a mouse model of infection to the virulence of the wild-type strain. Determination of the survival rate after intravenous (i.v.) bacterial inoculation of OF1 mice clearly showed that inactivation of ypmA, encoding YPMa, reduced the virulence of Y. pseudotuberculosis. Mice infected i.v. with 10(4) and 10(5) wild-type bacteria died within 9 days, whereas mice infected with the ypmA mutant survived 12 and 3 days longer, respectively. This decreased virulence of the ypmA mutant strain was not due to an impaired colonization of the spleen, liver, or lungs. In contrast to i.v. challenge, bacterial inoculation by the intragastric (i.g.) route did not reveal any difference in virulence between wild-type Y. pseudotuberculosis and the ypmA mutant since the 50% lethal doses were identical for both strains. Moreover, inactivation of ypmA gene did not affect the bacterial growth of Y. pseudotuberculosis in Peyer's patches, mesenteric lymph nodes (MLNs), and spleen after oral infection. Histological studies of spleen, liver, lungs, heart, Peyer's patches, and MLNs after i.v. or i.g. challenge with the wild type or the ypmA mutant did not reveal any feature that can be specifically related to YPMa. Our data show that the superantigenic toxin YPMa contributes to the virulence of Y. pseudotuberculosis in systemic infection in mice.

M-cell surface beta1 integrin expression and invasin-mediated targeting of Yersinia pseudotuberculosis to mouse Peyer's patch M cells

Quantitative analysis of Yersinia pseudotuberculosis infection of murine gut loops revealed that significantly more wild-type bacteria associated with Peyer's patch M cells than with dome enterocytes or goblet cells. An invasin-deficient mutant was significantly attenuated for M-cell invasion, while beta1 integrin expression was demonstrated in the apical membranes of M cells but not enterocytes. M-cell targeting by Yersinia pseudotuberculosis in vivo may, therefore, be mediated primarily by the interaction of invasin with cell surface beta1 integrins.

Expression and immunogenicity of an Echinococcus granulosus fatty acid-binding protein in live attenuated Salmonella vaccine strains

Fatty acid-binding proteins (FABPs) are candidate molecules for vaccines against several parasitic platyhelminths. A FABP from the cestode Echinococcus granulosus (EgDf1) was expressed in Salmonella vaccine strains as a C-terminal fusion to fragment C of tetanus toxin (TetC) by using expression vector pTECH. The fusion protein was equally expressed in several attenuated vaccine strains derived from bacteria with different genetic backgrounds and different attenuating mutations. Single-dose immunization experiments with the aroA Salmonella typhimurium strain SL3261 carrying the pTECH-EgDf1 construct were conducted with mice, using both the intravenous and the oral routes. Surprisingly, the antibody response to EgDf1 and the antigen-specific cytokine production in spleen cells were stronger in mice immunized orally. Furthermore, immune mouse sera strongly reacted with fixed sections of the worm's larval stage. Analysis of the isotype distribution of the specific anti-EgDf1 antibodies showed similar production of immunoglobulin G1 (IgG1) and IgG2a together with specific IgA antibodies. In addition, stimulation of spleen cells from mice immunized with the different constructs with either Salmonella lysate, TetC, or EgDf1 showed that, together with Th1-related cytokines (gamma interferon and interleukin 2 [IL-2]), significant levels of a Th2 cytokine (IL-5) were produced specifically, indicating a Th2 component to the response to the Salmonella carrier and to the recombinant antigens. Salmonellae expressing the TetC-rEgDfl fusion are currently under evaluation as potential vaccines against E. granulosus.

Protein p60 participates in intestinal host invasion by Listeria monocytogenes

The role of p60 in intestinal invasion by Listeria monocytogenes was assessed after oral infection of mice with the p60 low-expressing mutant RIII, or with anti-p60 antibody coated wild-type EGD. Invasion by L. monocytogenes RIII bacteria has been unimpaired suggesting that a low density of p60 suffices for entry. Up to 24 h post infection (p.i.), intestinal penetration by L. monocytogenes EGD bacteria was markedly reduced by coating with anti-p60 antibodies. In histological sections, anti-p60 antibody-treated L. monocytogenes EGD, but not uncoated listeriae were still detectable 24 h p.i. at the apical surface of enterocytes in the intestine. We conclude that p60 contributes to host invasion through the natural port of listerial entry, the intestinal epithelium.

Inhibition of the Fc receptor-mediated oxidative burst in macrophages by the Yersinia pseudotuberculosis tyrosine phosphatase

Suppression of host-cell-mediated immunity is a hallmark feature of Yersinia pseudotuberculosis infection. To better understand this process, the interaction of Y. pseudotuberculosis with macrophages and the effect of the virulence plasmid-encoded Yersinia tyrosine phosphatase (YopH) on the oxidative burst was analyzed in a chemiluminescence assay. An oxidative burst was generated upon infection of macrophages with a plasmid-cured strain of Y. pseudotuberculosis opsonized with immunoglobulin G antibody. Infection with plasmid-containing Y. pseudotuberculosis inhibited the oxidative burst triggered by secondary infection with opsonized bacteria. The tyrosine phosphatase activity of YopH was necessary for this inhibition. These results indicate that YopH inhibits Fc receptor-mediated signal transduction in macrophages in a global fashion. In addition, bacterial protein synthesis was not required for macrophage inhibition, suggesting that YopH export and translocation are controlled at the posttranslational level.