Control by H-2 genes of the Th1 response induced against a foreign antigen expressed by attenuated Salmonella typhimurium

Animal models paving the way for clinical trials of attenuated Salmonella enterica serovar Typhi live oral vaccines and live vectors

Attenuated Salmonella enterica serovar Typhi (S. Typhi) strains can serve as safe and effective oral vaccines to prevent typhoid fever and as live vectors to deliver foreign antigens to the immune system, either by the bacteria expressing antigens through prokaryotic expression plasmids or by delivering foreign genes carried on eukaryotic expression systems (DNA vaccines). The practical utility of such live vector vaccines relies on achieving a proper balance between minimizing the vaccine's reactogenicity and maximizing its immunogenicity. To advance to clinical trials, vaccine candidates need to be pre-clinically evaluated in relevant animal models that attempt to predict what their safety and immunogenicity profile will be when administered to humans. Since S. Typhi is a human-restricted pathogen, a major obstacle that has impeded the progress of vaccine development has been the shortcomings of the animal models available to assess vaccine candidates. In this review, we summarize the usefulness of animal models in the assessment of the degree of attenuation and immunogenicity of novel attenuated S. Typhi strains as vaccine candidates for the prevention of typhoid fever and as live vectors in humans.

MHC heterozygosity confers a selective advantage against multiple-strain infections

Genetic heterozygosity is thought to enhance resistance of hosts to infectious diseases, but few tests of this idea exist. In particular, heterozygosity at the MHC, the highly polymorphic loci that control immunological recognition of pathogens, is suspected to confer a selective advantage by enhancing resistance to infectious diseases (the "heterozygote advantage" hypothesis). To test this hypothesis, we released mice into large population enclosures and challenged them with multiple strains of Salmonella and one of Listeria. We found that during Salmonella infections with three avirulent strains, MHC heterozygotes had greater survival and weight than homozygotes (unlike sham controls), and they were more likely to clear chronic Salmonella infection than homozygotes. In laboratory experiments, we found that MHC heterozygosity enhanced the clearance of multiple-strain Salmonella infections. Yet, contrary to what is widely assumed, the benefits of heterozygosity were due to resistance being dominant rather than overdominant, i.e., heterozygotes were more resistant than the average of parental homozygotes, but they were not more resistant than both. The fact that MHC heterozygotes were more resistant to infection and had higher fitness than homozygotes provides a functional explanation for MHC-disassortative mating preferences.

Overproduction of monokines by leukocytes after stimulation with lipopolysaccharide in the elderly

The elderly suffer from an impaired immune function being obvious in a higher susceptibility to infections. Especially the rate of complications after infection with Salmonella, normally confined to the gastrointestinal tract, is raised. We compared in a whole blood assay and in vitro stimulation of peripheral blood mononuclear cell (PBMC) the secretion of interleukin (IL)-1-beta, IL-6, IL-8 and TNF-alpha, after stimulation with lipopolysaccharid (LPS) from Salmonella abortus equi and Escherichia coli, phytohaemagglutinin (PHA) and toxic shock syndrome toxin-1 (TSST-1) by leukocytes of healthy young donors, healthy elderly and healthy elderly fulfilling the SENIEUR protcol. Significantly higher secretion of IL-1-beta, IL-6 and IL-8 after stimulation with LPS were found in the SENIEUR elderly compared to young donors. IL-1-beta, IL-6 and IL-8 were elevated in the whole blood samples of the healthy elderly controls as well. After stimulation of whole blood samples from these healthy elderly with LPS, IL-1 and IL-6 secretion was significantly elevated, but stimulation of their PBMCs showed lower amounts of produced cytokines compared to the PBMCs of healthy young donors. The results suggest that the elvated cytokine releases are caused by an interaction of LPS with a serum factor in the blood of the elderly. Such an overproduction of these inflammatory cytokines by moncytes and neutrophils may be in part responsible for many symptoms elderly people suffer from during an infection with Salmonella.

Vaccination of mice with live recombinant Salmonella typhimurium aroA against H. pylori: parameters associated with prophylactic and therapeutic vaccine efficacy

Previously we described a recombinant attenuated Salmonella typhimurium aroA strain (SL3261[pYZ97]) with constitutive expression of plasmid encoded Helicobacter pylori urease subunits A and B (UreAB). Single dose oral vaccination effectively induced prophylactic immunity against bacterial challenge in BALB/c mice. Here we successfully extended this approach to several mouse strains with allelic differences in NRAMP-1 and H-2 genes. The respective host determinants are known to influence the immune response against S. typhimurium. A comparative analysis of the vaccine efficacy in C57BL/6 and BALB/c mice showed that the live vaccine confers long lasting immunity in both strains (>18 weeks). In C57BL/6 mice, protection was still observed 54 weeks while not all vaccinated BALB/c were immune when challenged after this time. BALB/c mice also needed higher doses of SL3261[pYZ97] for full protection. We also demonstrate a therapeutic potential of SL3261[pYZ97] in H. pylori infected BALB/c and C57BL/6 mice. Urease- and carrier-specific serum antibody responses as well as the level of colonization by the Salmonella were analyzed in both mouse strains after immunization with low (4 x 10(7)CFU) or high (1 x 10(9)CFU) vaccine doses. The results are discussed in the context of inoculum size and the mode of antigen supply required for effective vaccination with recombinant Salmonella.

Murine metal-induced systemic autoimmunity: baseline and stimulated cytokine mRNA expression in genetically susceptible and resistant strains

Cytokines play an important and complex role in the pathogenesis of systemic autoimmune diseases. In susceptible H-2s mice, inorganic mercury (Hg) induces lymphoproliferation, antinucleolar antibodies against the 34-kDa-protein fibrillarin, and systemic immune-complex (IC) deposits. Here, we report extensive analysis of cytokine mRNA levels in susceptible A.SW (H-2s) and resistant A.TL (H-2tl) mice under unstimulated conditions and during oral treatment with Hg and/or silver nitrate (Ag). Cytokine mRNA expression in lymphoid tissues was assessed using the ribonuclease protection assay and phosphorimaging. Baseline expression of IL-2 and IFN-gamma mRNA was higher in A.SW than in A.TL mice. In A.SW mice, Hg treatment caused early up-regulation of IL-2 and IFN-gamma levels, followed by substantial expression of IL-4 mRNA, which was significant compared to control A.SW and Hg-treated A.TL mice. Hg-exposed A.TL mice exhibited unchanged IFN-gamma, reduced IL-2 and greatly increased IL-10 mRNA expression. Ag-treated A.SW mice, which develop antifibrillarin antibodies (AFA) but exhibit minimal immune activation and no IC deposits, showed an early increase in IL-2 and IFN-gamma mRNA, but only a small and delayed rise in IL-4 mRNA. In conclusion, H-2-linked resistance to Hg-induced AFA is characterized by low constitutive expression of IL-2 and IFN-gamma mRNA, which is not increased by Hg, and a marked increase in IL-10 expression. Conversely, the key features of H-2-linked susceptibility to Hg- and Ag-induced AFA are up-regulation of IL-2, IFN-gamma and IL-4 mRNA expression, and down-regulation of IL-10 expression.

Use of live bacterial vaccine vectors for antigen delivery: potential and limitations

Most infectious agents are restricted to the mucosal membranes or their transit through the mucosa constitutes a critical step in the infection process. Therefore, the elicitation of an efficient immune response, not only at systemic, but also at mucosal level, after vaccination is highly desirable, representing a significant advantage in order to prevent infection. This goal can be only achieved, when the vaccine formulation is administered by the mucosal route. However, soluble antigens given by this route are usually poorly immunogenic. Among the available approaches to stimulate efficient mucosal responses, the use of bacterial carriers to deliver vaccine antigens, probably, constitutes one of the most successful strategies. The potential and limitations of the most extensively studied bacterial carrier systems will be discussed.

Protection conferred by a fully recombinant sub-unit vaccine against Yersinia pestis in male and female mice of four inbred strains

In this paper, we describe for the first time the use of a fully recombinant sub-unit vaccine for plague. We have compared the protection afforded by the recombinant vaccine against Yersinia pestis in male and female mice of four inbred strains. We also determined the in vivo cellular memory and antibody response after one year. The recombinant vaccine was capable of inducing protective immunity, against subcutaneous and aerosol challenge, in mice from all four strains. Although, there was some breakthrough in the CBA males challenged with 10(7) median lethal dose (MLDs) the other mice regardless of sex or strain were well protected even at this extreme challenge dose. In both male and female mice, the specific IgG titres to both antigens peaked at day 28 and 35 and in female mice these titres were maintained for >1 year.

Extending the CD4(+) T-cell epitope specificity of the Th1 immune response to an antigen using a Salmonella enterica serovar typhimurium delivery vehicle

We analyzed the CD4 T-cell immunodominance of the response to a model antigen (Ag), MalE, when delivered by an attenuated strain of Salmonella enterica serovar Typhimurium (SL3261*pMalE). Compared to purified MalE Ag administered with adjuvant, the mapping of the peptide-specific proliferative responses showed qualitative differences when we used the Salmonella vehicle. We observed the disappearance of one out of eight MalE peptides' T-cell reactivity upon SL3261*pMalE immunization, but this phenomenon was probably due to a low level of T-cell priming, since it could be overcome by further immunization. The most striking effect of SL3261*pMalE administration was the activation and stimulation of new MalE peptide-specific T-cell responses that were silent after administration of purified Ag with adjuvant. Ag presentation assays performed with MalE-specific T-cell hybridomas showed that infection of Ag-presenting cells by this intracellular attenuated bacterium did not affect the processing and presentation of the different MalE peptides by major histocompatibility complex (MHC) class II molecules and therefore did not account for immunodominance modulation. Thus, immunodominance of the T-cell response to microorganisms is governed not only by the frequency of the available T-cell repertoire or the processing steps in Ag-presenting cells that lead to MHC presentation but also by other parameters probably related to the infectious process and to the bacterial products. Our results indicate that, upon infection by a microorganism, the specificity of the T-cell response induced against its Ags can be much more effective than with purified Ags and that it cannot completely be mimicked by purified Ags administered with adjuvant.

Immune responses induced by recombinant BCG strains according to level of production of a foreign antigen: malE

A variety of viral, bacterial and parasitic antigens have been expressed in BCG and the capacity of these recombinant bacteria to induce immune responses has been well documented. However, little is known about the parameters influencing the induction of immune responses by recombinant BCG (rBCG), such as level of production and localization of the recombinant antigen. In the present study, we have constructed several rBCG strains expressing the malE gene from Escherichia coli which is either secreted or targeted to the cytoplasm or plasma membrane. Expression of malE was quantified by ELISA and localization was analyzed by flow cytometry. Even when using the same promoter, levels of cytoplasmic or membrane MalE production were far less than those from secreting strains using either mycobacterial or E. coli secretion signals. Stronger and more rapid immune responses were induced by rBCG strains with the highest levels of secreted MalE compared to cytoplasmic or membrane constructs, including both good humoral and proliferative responses in BALB/c, C57BL6 and even C3H mice, previously shown to be poor MalE responders. These results suggest that the levels of foreign antigen production play an important role in the induction of immune responses by rBCG strains.

Host Genetic Determinants of Vaccine-Induced Eosinophilia During Respiratory Syncytial Virus Infection

In BALB/c mice, sensitization with the attachment protein (G) of respiratory syncytial virus (RSV) leads to CD4+ T cell-mediated lung eosinophilia during subsequent challenge with RSV. To determine the host genetic influences on this model of lung eosinophilia, we tested 15 different inbred mouse strains. Eosinophilia developed in all H-2d (BALB/c, DBA/2n, and B10.D2), but not in H-2k (CBA/Ca, CBA/J, C3H, BALB.K, or B10.BR) mouse strains. Among H-2b mice, 129 and BALB.B developed eosinophilia, whereas C57BL/6 and C57BL/10 did not. Testing first generation crosses between sensitive and resistant strains showed that eosinophilia developed in all H-2dxk (n = 5), irrespective of background genes, but not in H-2dxb (n = 2) mice. In vivo depletion of CD8+ T cells or IFN-γ rendered C57BL/6, but not BALB.K mice, susceptible to eosinophilia. Analysis of B10 recombinant mice showed that the Dd allele (in B10.A(5R) mice) prevented CD8+ T cell accumulation in the lung, resulting in intense lung eosinophilia. However, the Db allele (in B10.A(2R) and B10.A(4R) mice) supported CD8+ T cell expansion and prevented eosinophilia. Intracellular cytokine staining showed that lung eosinophilia correlated with reduced IFN-γ and increased IL-10 expression in lung T cells. These results are compatible with the unifying model that Th2 cells mediate the disease but can be inhibited by CD8+ T cells secreting IFN-γ. Our findings have important implications for the development of protective, nonpathogenic vaccines for RSV disease.