Identification of antigens which stimulate T lymphocytes of Salmonella enteritidis 11RX immunized mice

Adaptive Immune Responses during Salmonella Infection

The interaction betweenSalmonella and its host is complex and dynamic: the host mounts an immune defense against the pathogen, which in turn acts to reduce, evade, or exploit these responses to successfully colonize the host. Although the exact mechanisms mediating protective immunity are poorly understood, it is known that T cells are a critical component of immunity to Salmonella infection, and a robust T-cell response is required for both clearance of primary infection and resistance to subsequent challenge. B-cell functions, including but not limited to antibody production, are also required for generation of protective immunity. Additionally, interactions among host cells are essential. For example, antigen-presenting cells (including B cells) express cytokines that participate in CD4+ T cell activation and differentiation. Differentiated CD4+ T cells secrete cytokines that have both autocrine and paracrine functions, including recruitment and activation of phagocytes, and stimulation of B cell isotype class switching and affinity maturation. Multiple bacterium-directed mechanisms, including altered antigen expression and bioavailability and interference with antigen-presenting cell activation and function, combine to modify Salmonella's "pathogenic signature" in order to minimize its susceptibility to host immune surveillance. Therefore, a more complete understanding of adaptive immune responses may provide insights into pathogenic bacterial functions. Continued identification of adaptive immune targets will guide rational vaccine development, provide insights into host functions required to resist Salmonella infection, and correspondingly provide valuable reagents for defining the critical pathogenic capabilities of Salmonella that contribute to their success in causing acute and chronic infections.

Protective immunity of biodegradable nanoparticle-based vaccine against an experimental challenge with Salmonella Enteritidis in mice

Salmonella spp. infections transmitted by contaminated poultry and eggs represent a major global health burden. Salmonella enterica serovar. Enteritidis is the leading cause of human salmonellosis worldwide. The cell surface antigens of Salmonella Enteritidis play an important role in the host-pathogen interactions and as such represent potential candidates for subunit-vaccine development. An immunogenic subcellular extract obtained from whole Salmonella Enteritidis cells (HE) was encapsulated in nanoparticles made with the polymer Gantrez (HE-NP). Proteomics was used to investigate the complex protein nature of the HE extract. Immunogenicity and protection studies against lethal Salmonella Enteritidis challenge were performed in BALB/c mice. Increased survival was observed in vaccinated mice as compared to a control group; 80% of the mice immunized with the HE-NP formulation survived even when administered 49 days before the lethal challenge. The cytokines released from in vitro-stimulated spleens showed a strong gamma interferon response in all immunized groups at day 10 post-immunization. However, the immunity induced by HE-NP at day 49 post-immunization suggests the involvement of a TH2 subclass in the protective effect. The potential for mucosal vaccination suggests that HE-nanoparticles may represent an important alternative to the conventional attenuated vaccines against Salmonella Enteritidis.

Development of acquired immunity to Salmonella

Salmonella enterica serovar Typhi (S. typhi) causes human typhoid fever, a serious and widespread disease in developing countries. Other Salmonella serovars are associated with food-borne infections. The recent emergence of multi-drug-resistant Salmonella strains highlights the need for better preventive measures, including vaccination. The available vaccines against Salmonella infection do not confer optimal protection. The design of new Salmonella vaccines must be based on the identification of suitable virulence genes and on knowledge of the immunological mechanisms of resistance to the disease. Control and clearance of a vaccine strain rely on the phagocyte oxidative burst, reactive nitrogen intermediates, inflammatory cytokines and CD4(+) TCR-alphabeta(+) T cells and are controlled by genes including NRAMP1 and MHC class II. Vaccine-induced resistance to reinfection requires the presence of Th1-type immunological memory and anti-Salmonella antibodies. The interaction between T and B cells is essential for the development of resistance following vaccination. The identification of immunodeficiencies that render individuals more susceptible to salmonellosis must be taken into consideration when designing and testing live attenuated Salmonella vaccines. An ideal live Salmonella vaccine should therefore be safe, regardless of the immunological status of the vaccinee, but still immunogenic.

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.

Salmonella enteritidis temperature-sensitive mutants protect mice against challenge with virulent Salmonella strains of different serotypes

The protection conferred by temperature-sensitive mutants of Salmonella enteritidis against different wild-type Salmonella serotypes was investigated. Oral immunization with the single temperature-sensitive mutant E/1/3 or with a temperature-sensitive thymine-requiring double mutant (E/1/3T) conferred: (i) significant protection against the homologous wild-type Salmonella strains; (ii) significant cross-protection toward high challenge doses of S. typhimurium. Significant antibody levels against homologous lipopolysaccharide and against homologous and heterologous protein antigens were detected in sera from immunized mice. Moreover, a wide range of protein antigens from different Salmonella O serotypes were recognized by sera from immunized animals. Besides, primed lymphocytes from E/1/3 immunized mice recognized Salmonella antigens from different serotypes. Taken together, these results indicate that temperature-sensitive mutants of S. enteritidis are good candidates for the construction of live vaccines against Salmonella.

Role of SefA subunit protein of SEF14 fimbriae in the pathogenesis of Salmonella enterica serovar Enteritidis

In this study, the role of the SefA subunit protein of SEF14 fimbriae in the pathogenesis of Salmonella enterica serovar Enteritidis was investigated. This was accomplished by mutating the sefA gene in the chromosome of two strains of S. enterica serovar Enteritidis by allelic exchange with a copy that has been inactivated by interruption with a nonpolar kanamycin resistance (aphA-3) cassette. The effect of this mutation on the ability of the S. enterica serovar Enteritidis strains to colonize the intestinal epithelium and to invade other tissues was assessed in BALB/c mice and in vitro by adherence and invasion of HeLa cells. Our results show that an avirulent S. enterica serovar Enteritidis vaccine strain, 11RX (no somatic antigen; flagellum antigen phase 1, g,m; flagellum antigen phase 2, -), colonized better and persisted longer in the Peyer's patches of these mice than did its SefA-deficient counterpart. However, no such difference was observed between a highly virulent S. enterica serovar Enteritidis strain, 7314 (somatic antigen, O1, O9, O12; flagellum antigen phase 1, g,m; flagellum antigen phase 2 [1,7]), and its SefA-deficient isogenic mutant. These findings were correlated with in vitro adherence and invasion of HeLa cells. Furthermore, we could not demonstrate a role for SefA in the virulence of S. enterica serovar Enteritidis as assessed by 50% lethal dose determinations. The implications of these findings are discussed.

Human cell-mediated immune responses to antigenic fractions of Salmonella typhi

The proliferative responses of peripheral blood mononuclear cells of 10 subjects that had typhoid fever, and healthy volunteers without history of typhoid fever or immunization against disease, were analysed with antigen fractions from two protein extracts of Salmonella typhi. Fractions from each extract were separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) electrophoresis, transferred to nitrocellulose filters by electroblotting and processed to obtain antigen-bearing nitrocellulose particles for use in lymphocyte cultures. Although the individual proliferative responses were heterogeneous we identified two main immunogenic regions of 29-32 10(3) MW and 45-56 x 10(3) MW for both extracts. Even though there was no one particular antigenic fraction capable of stimulating lymphocytes from all individuals with a previous history of typhoid fever, the combination of three fractions 29-32, 41-45, 63-71 x 10(3) MW could be stimulatory for cells of 90% of these individuals. Also, four subjects that did not respond to unfractionated antigens gave proliferative responses to several fractions of the same extract. We have identified the main immunogenic fractions of S. typhi that might play a role during typhoid infection and postinfection immunity, and merit further purification and characterization.

Protection against oral challenge three months after i.v. immunization of BALB/c mice with live Aro Salmonella typhimurium and Salmonella enteritidis vaccines is serotype (species)-dependent and only partially determined by the main LPS O antigen

The role of the main LPS O antigen in the specificity of protection as mediated by systemic mechanisms following immunization with live attenuated Aro Salmonella vaccines was studied in mice. Innately Salmonella-susceptible (Itys) BALB/c mice were immunized intravenously with a single dose of either Salmonella typhimurium SL3261 aroA (LPS O4,5,12) or Salmonella enteritidis Se795aroA (LPS O1,9,12), and challenged orally 2-3 months later with either S. typhimurium C5 or S. enteritidis Thirsk. Nearly isogenic transductants of the two challenge strains expressing either their own LPS or that of the other serotype (S. typhimurium C5 O4 or O9, and S. enteritidis Thirsk O9 or O4) were also used. Both vaccines conferred similar high protection against the virulent strain of the homologous serotype expressing its own LPS. There was no protection against the heterologous serotype expressing its own LPS. However, when vaccinated mice were challenged with either the same serotype as the vaccine but expressing the heterologous LPS, or with the heterologous serotype expressing the LPS of the vaccine, protection was always lower than protection against the fully homologous serotype. Anti-smooth LPS antibodies showed higher titres against the homologous LPS, but with significant crossreactivity with the heterologous LPS. Antibodies to O-rough S. typhimurium and S. enteritidis LPS were present following immunization with either of the two vaccine strains. The LPS alone cannot fully account for the specificity of protection in this model; other (protein) antigens may be responsible. It remains to be seen whether there is a T-cell mediated component to the specificity of protection conferred by live Salmonella vaccines.

A Salmonella enteritidis 11RX pilin induces strong T-lymphocyte responses

Our previous work, using proteins fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to define antigens of Salmonella enteritidis 11RX able to stimulate T cells from S. enteritidis 11RX-primed (BALB/c x C57BL/6)F1 mice, had indicated the presence of a major antigenic determinant of 14 to 18 kDa (H.-M. Vordermeier and I. Kotlarski, Immunol. Cell. Biol. 68:299-305, 1990). The 14-kDa size is similar to that of the monomeric units of one of the fimbrial structures, SEF14, produced by a human enteropathogen, S. enteritidis 27655 (J. Feutrier, W. W. Kay, and T. J. Trust, J. Bacteriol. 168:221-227, 1986). Here we present data which indicate that S. enteritidis 11RX also produces this protein and that it is able to elicit delayed-type hypersensitivity reactions in S. enteritidis 11RX-primed animals and to stimulate in vitro proliferation of, and cytokine release from, T cells obtained from these animals, implying that this fimbrial protein is likely to be an important immunogen of S. enteritidis. The protein was purified to homogeneity and is free from contamination with lipopolysaccharide. Standard immunoblot analysis with unabsorbed S. enteritidis 11RX antiserum and antiserum absorbed with Salmonella typhimurium C5 and various strains of Escherichia coli, as well as a panel of anti-14-kDa-protein monoclonal antibodies, suggests that this fimbrial protein is not the common antigen expressed by a number of organisms belonging to the family Enterobacteriaceae. Immunogold electron microscopy with one of these monoclonal antibodies confirms that the 14-kDa protein and SEF14 are identical.

Proliferative and T-cell specific interleukin (IL-2/IL-4) production responses in spleen cells from mice vaccinated with aroA live attenuated Salmonella vaccines

T-cell responses were studied in mice immunized with the Salmonella typhimurium aroA SL3261 live attenuated vaccine strain. T-cell responses in the spleen, both in whole cell populations and in nylon wool non-adherent (T-cell enriched) cells, were studied in vitro as proliferation by incorporation of tritiated thymidine and production of T-cell specific cytokines [IL-2 (interleukin-2)/IL-4]. Stimulating antigens included whole Salmonella lysates and purified lipopolysaccharide (LPS), both untreated and after alkaline hydrolysis to prevent the non-specific mitogenic effect of LPS. Strong proliferative responses were obtained with untreated whole cell extract and LPS, which were decreased by polymyxin B (PB). Alkaline detoxification of the antigens decreased the proliferative response of nylon-wool non-adherent populations to LPS, but greatly increased their response to the Salmonella extract. Surprisingly, PB also reduced proliferation to detoxified LPS. Little or no IL-2/IL-4 production was seen in response to LPS or purified polysaccharide antigens, while there was a strong IL-2/IL-4 response to whole cell lysate, again markedly increasing after alkaline treatment. The results suggest that the T-cell response elicited by immunization with live Salmonella aroA vaccines in mice recognizes antigens other than LPS determinants, and that estimation of T-cell responses to Salmonella antigens by proliferation alone may yield misleading results.

Partial purification and characterization of low molecular weight antigens of Salmonella enteritidis 11RX

Sephadex G100 chromatography and preparative sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) using 16% polyacrylamide gels were used for the partial purification of 16-18 kDa proteins able to stimulate Salmonella enteritidis 11RX-primed T cells of (BALB/c x C57BL/6J)F1 mice. A soluble antigen (Ag) extract of S. enteritidis 11RX (s11RX) was used as the starting material for purification for two reasons. First, s11RX had been previously shown to induce in vitro proliferation of Salmonella-primed T cells; second, initial analysis of SDS-PAGE fractionated s11RX Ag using the 'T cell western blot' technique indicated that T cell stimulatory activity was located only in the 16-18 kDa region. The partially purified antigens were able to elicit delayed-type hypersensitivity reactions in vivo, and stimulated in vitro proliferation and interleukin-2 release from 11RX-primed T cells and T cell lines and clones derived from these cells, indicating that they are major antigenic determinants of S. enteritidis 11RX. Testing of 16-18 kDa proteins of several other bacteria indicated that these antigens may be 'common' and expressed by a number of organisms belonging to the Enterobacteriaceae.