Evaluation of Salmonella porins as a broad spectrum vaccine candidate

Modulation of host cellular responses by gram-negative bacterial porins

The outer membrane of a gram-negative bacteria encapsulates the plasma membrane thereby protecting it from the harsh external environment. This membrane acts as a sieving barrier due to the presence of special membrane-spanning proteins called "porins." These porins are β-barrel channel proteins that allow the passive transport of hydrophilic molecules and are impermeable to large and charged molecules. Many porins form trimers in the outer membrane. They are abundantly present on the bacterial surface and therefore play various significant roles in the host-bacteria interactions. These include the roles of porins in the adhesion and virulence mechanisms necessary for the pathogenesis, along with providing resistance to the bacteria against the antimicrobial substances. They also act as the receptors for phage and complement proteins and are involved in modulating the host cellular responses. In addition, the potential use of porins as adjuvants, vaccine candidates, therapeutic targets, and biomarkers is now being exploited. In this review, we focus briefly on the structure of the porins along with their important functions and roles in the host-bacteria interactions.

Fetal Macrophages Exposed to Salmonella Antigens Elicit Protective Immunity Against Overwhelming Salmonella Challenge in A Murine Model

Despite the evidence for fetal immunization following maternal infection, it remained a mystery how the fetal immune system was primed by vertically-transmitted pathogens or microbial antigens, especially before its full maturation. We previously demonstrated the capacity of fetal macrophages for endocytosing oncoprotein and allergens to bridge towards adaptive immunity in postnatal life. To investigate the immunological consequences of fetal contact with microbial antigens and the role of fetal macrophages in the defense against infection before T-cell development, we exposed gestational day 14 murine fetuses and their macrophages to flagellin and heat-killed Salmonella Typhimurium. Recipients with in utero exposure to Salmonella antigens or adoptive transfer of microbial antigen-loaded fetal macrophages were examined for immune responses to Salmonella antigens and resistance to virulent Salmonella challenge. Fetal exposure to microbial antigens or adoptive transfer of microbial antigen-loaded fetal macrophages could confer antigen-specific adaptive immunity. However, protective immunity against lethal Salmonella challenge was only granted to those receiving heat-killed Salmonella antigens, presenting as heightened recall responses of serum anti-lipopolysaccharide immunoglobulins and interferon-gamma. In immunized recipients surviving Salmonella challenge, their serum transfer to succeeding recipients provided immediate protection from lethal Salmonella challenge in preference to lymphocyte transfer, indicating a more active role of humoral immunity in the prevention of Salmonella invasiveness. Our study sheds insight on the role of fetal macrophages in immunogenicity to transplacental pathogens regardless of fetal lymphocyte maturity, paving the way for fetal macrophage therapies to enhance vaccine responsiveness or increase resistance to pathogenic microorganisms in perinatal life.

Surface display of OmpC of Salmonella serovar Pullorum on Bacillus subtilis spores

Salmonellosis is a major public health problem throughout the world. Thus, there is a huge need for diversified control strategies for Salmonella infections. In this work, we have assessed the potential use of Bacillus subtilis (B. subtilis) spores for the expression of a major protective antigen of Salmonella serovar Pullorum, OmpC. The expression of OmpC on the surface of spores was determined by immunofluorescence microscopy. Mice immunized with recombinant spores expressing the OmpC antigen presented significant levels of OmpC-specific serum IgG and mucosal SIgA antibodies than in mice immunized with non-recombinant spores (p<0.01). In addition, oral immunization with recombinant spores was able to induce a significant level of protection in mice against lethal challenge with Salmonella serovar Typhimurium. These results suggest that B. subtilis spores have promising potential in the development of mucosal vaccines against Salmonella infections.

Immunogenicity and Cross-Protective Efficacy Induced by Outer Membrane Proteins from Salmonella Typhimurium Mutants with Truncated LPS in Mice

Lipopolysaccharide (LPS) is a major virulence factor present in the outer membrane of Salmonella enterica serovar Typhimurium (S. Typhimurium). Outer membrane proteins (OMPs) from Salmonella show high immunogenicity and provide protection against Salmonella infection, and truncated LPS alters the outer membrane composition of the cell wall. In our previous study, we demonstrated that Salmonella mutants carrying truncated LPS failed to induce strong immune responses and cross-reaction to other enteric bacteria, due to their high attenuation and low colonization in the host. Therefore, we plan to investigate whether outer membrane proteins from Salmonella mutants with truncated LPS resulting from a series of nonpolar mutations, including ∆waaC12, ∆waaF15, ∆waaG42, ∆rfaH49, ∆waaI43, ∆waaJ44, ∆waaL46, ∆wbaP45 and ∆wzy-48, affect immunogenicity and provide protection against diverse Salmonella challenge. In this study, the immunogenicity and cross-protection efficiency of purified OMPs from all mutants were investigated to explore a potential OMP vaccine to protect against homologous or heterologous serotype Salmonella challenge. The results demonstrated that OMPs from three Salmonella mutants (∆waaC12, ∆waaJ44 and ∆waaL46) induced higher immune responses and provided good protection against homologous S. Typhimurium. The OMPs from these three mutants were also selected to determine the cross-protective efficacy against homologous and heterologous serotype Salmonella. Our results indicated that the mutant ∆waaC12 can elicit higher cross-reactivity and can provide good protection against S. Choleraesuis and S. Enteritidis infection and that the cross-reactivity may be ascribed to an antigen of approximately 18.4–30 kDa.

The secreted fructose 1,6-bisphosphate aldolase as a broad spectrum vaccine candidate against pathogenic bacteria in aquaculture

The development of aquaculture has been hampered by different aquatic pathogens that can cause edwardsiellosis, vibriosis, or other diseases. Therefore, developing a broad spectrum vaccine against different fish diseases is necessary. In this study, fructose 1,6-bisphosphate aldolase (FBA), a conserved enzyme in the glycolytic pathway, was demonstrated to be located in the non-cytoplasmic components of five aquatic pathogenic bacteria and exhibited remarkable protection and cross-protection against these pathogens in turbot and zebrafish. Further analysis revealed that sera sampled from vaccinated turbot had a high level of specific antibody and bactericidal activity against these pathogens. Meanwhile, the increased expressions of immune response-related genes associated with antigen recognition and presentation indicated that the adaptive immune response was effectively aroused. Taken together, our results suggest that FBA can be utilized as a broad-spectrum vaccine against various pathogenic bacteria of aquaculture in the future.

Vibrio cholerae porin OmpU mediates M1-polarization of macrophages/monocytes via TLR1/TLR2 activation

Polarization of the monocytes and macrophages toward the M1 and M2 states is important for hosts' defense against the pathogens. Moreover, it plays a crucial role to resolve the overwhelming inflammatory responses that can be harmful to the host. Polarization of macrophages/monocytes can be induced by pathogen-associated molecular patterns (PAMPs). PAMP-mediated monocyte/macrophage polarization is important during the infection, as pathogen can suppress host immune system by altering the polarization status of the macrophages/monocytes. OmpU, an outer membrane porin protein of Vibrio cholerae, possesses the ability to induce pro-inflammatory responses in monocytes/macrophages. It is also able to down-regulate the LPS-mediated activation of the monocytes/macrophages. Such observation leads us to believe that OmpU may induce a state that can be called as M1/M2-intermediate state. In the present study, we evaluated a set of M1 and M2 markers in RAW 264.7 murine macrophage cell line, and THP-1 human monocytic cell line, in response to the purified OmpU protein. We observed that OmpU, as a PAMP, induced M1-polarization by activating the Toll-like receptor (TLR) signaling pathway. OmpU induced formation of TLR1/TLR2-heterodimers. OmpU-mediated TLR-activation led to the MyD88 recruitment to the TLR1/TLR2 complex. MyD88, in turn, recruited IRAK1. Ultimately, OmpU-mediated signaling led to the activation and subsequent nuclear translocation of the NFκB p65 subunit. We also observed that blocking of the TLR1, TLR2, IRAK1, and NFκB affected OmpU-mediated production of M1-associated pro-inflammatory cytokines such as TNFα and IL-6.

The Mycobacterium tuberculosis outer membrane channel protein CpnT confers susceptibility to toxic molecules

Mycobacterium tuberculosis, the causative agent of tuberculosis, is protected from toxic solutes by an effective outer membrane permeability barrier. Recently, we showed that the outer membrane channel protein CpnT is required for efficient nutrient uptake by M. tuberculosis and Mycobacterium bovis BCG. In this study, we found that the cpnT mutant of M. bovis BCG is more resistant than the wild type to a large number of drugs and antibiotics, including rifampin, ethambutol, clarithromycin, tetracycline, and ampicillin, by 8- to 32-fold. Furthermore, the cpnT mutant of M. bovis BCG was 100-fold more resistant to nitric oxide, a major bactericidal agent required to control M. tuberculosis infections in mice. Thus, CpnT constitutes the first outer membrane susceptibility factor in slow-growing mycobacteria. The dual functions of CpnT in uptake of nutrients and mediating susceptibility to toxic molecules are reflected in macrophage infection experiments: while loss of CpnT was detrimental for M. bovis BCG in macrophages that enable bacterial replication, presumably due to inadequate nutrient uptake, it conferred a survival advantage in macrophages that mount a strong bactericidal response. Importantly, the cpnT gene showed a significantly higher density of nonsynonymous mutations in drug-resistant clinical M. tuberculosis strains, indicating that CpnT is under selective pressure in human tuberculosis and/or during chemotherapy. Our results indicate that the CpnT channel constitutes an outer membrane gateway controlling the influx of nutrients and toxic molecules into slow-growing mycobacteria. This study revealed that reducing protein-mediated outer membrane permeability might constitute a new drug resistance mechanism in slow-growing mycobacteria.

Macrophage Cell Death Due toSalmonella entericaSerovar Typhi and Its Acid Stress Protein Has Features of Apoptosis

Salmonella spp. have been shown to cause apoptosis of various host cell types as a part of their infection process. However, the induction of apoptosis remains to be looked into under the different host environments including acidic stress experienced by the pathogen. In order to simulate the in vivo acidic conditions, we studied the potential of S. typhi and its protein expressed under in vitro acidic conditions to induce apoptosis in macrophages. Murine macrophages were isolated and interacted with serovar Typhi and its acid stress protein for different time periods. The assessment of nucleosomal DNA, and nuclear staining with H-33342 dye and flow cytometry indicated the occurrence of characteristic features of apoptosis. Analysis of data revealed that S. typhi caused apoptotic cell death in 61% of macrophages whereas stress-induced protein alone accounted for apoptotic cell death in 45% of macrophages. The present study, for the first time demonstrates the potential of stress-induced outermembrane component of S. typhi to induce apoptosis. Identification of such factors may offer new insights for understanding the pathophysiology of the disease during the host-pathogen interactions.

Vibrio cholerae porin OmpU induces pro-inflammatory responses, but down-regulates LPS-mediated effects in RAW 264.7, THP-1 and human PBMCs

Vibrio cholerae porin OmpU plays a crucial role in the survival of the organism in the human gut. Various observations suggest critical involvement of OmpU in V. cholerae pathogenesis. However, OmpU is poorly characterized in terms of its ability to evoke cellular responses, particularly in the context of host immune system. Therefore, towards characterizing V. cholerae OmpU for its host immunomodulatory functions, we have studied the ability of OmpU to elicit pro-inflammatory responses in a range of immune cells which include, mouse RAW 264.7 macrophages, human THP-1 monocytes and human PBMCs. We have observed that purified OmpU induces pro-inflammatory responses in terms of production of NO, TNFα and IL-6. Interestingly, pre-treatment of the cells with OmpU suppresses the production of NO, TNFα, IL-6 as well as IL-12 upon subsequent activation with LPS. Our results therefore suggest that V. cholerae OmpU may have a differential regulatory role in terms of host immunomodulatory function: it can induce pro-inflammatory responses in target host immune cells, whereas it can also exert suppressive effect on LPS-induced pro-inflammatory responses. In addition, our study indicates that purified OmpU may have the ability to skew the Th1 response towards the Th2 response, presumably via suppression of IL-12 production.

Mouse models to assess the efficacy of non-typhoidal Salmonella vaccines: revisiting the role of host innate susceptibility and routes of challenge

Non-typhoidal Salmonella enterica (NTS) serovars Typhimurium and Enteritidis are important causes of bacterial gastroenteritis in the USA and worldwide. In sub-Saharan Africa these two serovars are emerging as agents associated with lethal invasive disease (e.g., bacteremia, meningitis). The development of NTS vaccines, based on mucosally administered live attenuated strains and parenteral non-living antigens, could diminish the NTS disease burden globally. Mouse models of S. Typhimurium and S. Enteritidis invasive disease can accelerate the development of NTS vaccines. Live attenuated NTS vaccines elicit both cellular and humoral immunity in mice and their efficacy is well established. In contrast, non-living vaccines that primarily elicit humoral immunity have demonstrated variable efficacy. An analysis of the reported studies with non-living vaccines against S. Typhimurium and S. Enteritidis reveals that efficacy is influenced by two important independent variables: (1) the innate susceptibility to NTS infection that differs dramatically between commonly used mouse strains and (2) the virulence of the NTS strain used for challenge. Protection by non-living vaccines has generally been seen only in host-pathogen interactions where a sub-lethal infection results, such as challenging resistant mice with either highly virulent or weakly virulent strains or susceptible mice with weakly virulent strains. The immunologic basis of this discrepancy and the implications for human NTS vaccine development are reviewed herein.

Salmonella-macrophage interactions upon manganese supplementation

Various studies indicate the role of manganese (Mn) in the virulence of pathogens. Salmonella is an intracellular pathogen which is able to multiply within the macrophages. The present study was therefore, designed to assess the effect of Mn supplementation on Salmonella-macrophage interactions particularly in reference to Salmonella virulence and macrophage functions. A 50-fold decrease in the lethal dose 50 (LD(50)) of Salmonella typhimurium was observed when mice were infected with Salmonella grown in the presence of Mn as compared to the LD(50) in the absence of Mn indicating an increase in the virulence of the organism. A significant increase was observed in the levels of superoxide dismutase (SOD) of S. typhimurium grown in presence of manganese. Upon Mn supplementation, macrophage functions were also found to be altered. Decreased phagocytic activity of macrophages interacted with Salmonella was observed in presence of Mn as compared to the activity in the absence of Mn. A significant increase was observed in the extent of lipid peroxidation along with significant decreases in the activities of SOD and catalase as well as nitrite levels of macrophages interacted with S. typhimurium upon supplementation with Mn. These observations indicate that Mn supplementation might have increased the expression of Mn transporters in Salmonella resulting in increased levels of its superoxide dismutase. The altered Salmonella function in turn might have been responsible for inhibiting phagocytosis and impairing the balance between the oxidant and antioxidant profile of macrophages, thus protecting itself by exhibiting exalted virulence.

Recombinant SpaO and H1a as immunogens for protection of mice from lethal infection with Salmonella paratyphi A: implications for rational design of typhoid fever vaccines

The Vi capsular polysaccharide vaccine is one of two vaccines against typhoid recommended worldwide and is the vaccine generally used in China. However, in recent years a Salmonella paratyphi A strain that is naturally devoid of capsule has caused frequent outbreaks of typhoid fever in Southern China, leading to the need for identification of additional antigens that could be incorporated into new vaccines. SpaO acts as a major invasion factor of Salmonella enterica spp. and H1a is the unique flagellin subunit ofS. paratyphi A. In this study, the two prokaryotic recombinant antigens, rSpaO and rH1a, were expressed and their immunogenicity was demonstrated by the slide agglutination test and Western blot assays. Using PCR and sequencing analysis as well as ELISA, we find that the spaO and h1a genes are widely distributed in 196 S. paratyphi A isolates (97.5 and 100%, respectively), with high expression frequencies for the SpaO (98.0%) and H1a (100%) antigens. The two genes also show high sequence conservation (similarities from 99.31 to 99.88% for both genes). In sera from 172 paratyphoid A patients, anti-SpaO and anti-H1a IgGs were detectable by ELISA, in 94.8 and 98.8% of patients, respectively. Furthermore, 41.7-66.7% of mice immunized with rSpaO or rH1a alone were protected against subsequent infection, and the protection rate rose to 75.0-91.7% in mice co-immunized with the two antigens. As the spaO and h1a genes of S. paratyphi A are sequence conserved, extensively distributed and highly expressed, the rSpaO and rH1a immunogens should be considered in the development of novel vaccines to prevent S. paratyphi A-caused typhoid fever.

Tracking the dynamics of salmonella specific T cell responses

Over the last decade, significant advances have been made in the methodology for studying immune responses in vivo. It is now possible to follow almost every aspect of pathogen-specific immunity using in vivo models that incorporate physiological infectious doses and natural routes of infection. This new ability to study immunity in a relevant physiological context will greatly expand our understanding of the dynamic interplay between host and pathogen. Visualizing the resolution of primary infection and the development of long-term immunological memory should also aid the development of new vaccines and therapeutics for infectious diseases. In this review, we will describe the application of in vivo visualization technology to Salmonella infection, describe our current understanding of Salmonella-specific immunity, and discuss some unanswered questions that remain in this model.

"Gently rough": the vaccine potential of a Salmonella enterica regulatory lipopolysaccharide mutant

BACKGROUND

An alternative to multivalent vaccines could be to construct strains capable of conferring broad protection through shared antigens. Down-regulation of immunodominant major antigens has been proposed to enhance the immunogenicity of conserved antigens.

METHODS

The protection provided by an aroA as well as structural and regulatory lipopolysaccharide (LPS) mutants of Salmonella enterica serovar Typhimurium against homologous and heterologous challenges was assessed in the murine model of typhoid. The reactivity and cross-reactivity of the immune sera raised was tested by enzyme-linked immunospot assay and immunoblots. Conserved outer membrane proteins were identified by mass spectrometry.

RESULTS

Unlike any structural LPS mutants, the regulatory mutant lacking RfaH was finely balanced between safety and immunogenicity, and its vaccine potential was comparable to that of the well-characterized DeltaaroA mutant. Loss of the transcriptional antiterminator RfaH resulted in a heterogeneous length of LPS chains, designated here as the "gently rough" phenotype. Our study also provides evidence that the rough phenotype enhances the immunogenicity of minor antigens, which may improve cross-protection against heterologous bacteria. A panel of conserved antigens shared by members of the Enterobacteriaceae family was identified as abundant porins and lipoprotein antigens.

CONCLUSIONS

Fine-tuned down-regulation of immunodominant epitopes can create live vaccine strains that are not only desirably attenuated but that also exhibit an improved cross-protective potential.

Immunization with Outer Membrane Proteins of Pasteurella multocida (6:B) Provides Protection in Mice

The immunoprotective efficacy of Pasteurella multocida (6:B) outer membrane proteins (OMPs) was examined in the mouse model. Bacterial OMPs were extracted using sarkosyl method and analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and immunoblotting. Prototype vaccines were prepared using OMPs with adjuvants including dioleoyl phosphatidyl choline-based liposome and Montanide ISA206 water-in oil-in water emulsion. Antibody response to the vaccine was monitored using indirect enzyme linked immunosorbent assay. The results of the study showed that immunized mice had high titre with both the formulations. The vaccinated mice were able to survive a live virulent bacterial challenge. Based on the findings of the study it can be inferred that OMPs are important determinants of immunoprotection hence can serve as vaccine candidates against haemorrhagic septicaemia.

55 kDa outer-membrane protein from short-chain fatty acids exposed Salmonella enterica serovar Typhi induces apoptosis in macrophages

Various pathogens including Salmonella species are known to induce apoptosis in host cell types during their infection processes. However, the bacterial components capable of inducing apoptosis have not been fully understood. It is now known that in vivo expression of virulence determinants differ from the expression under in vitro conditions. Therefore, in the present study, attempts were made to evaluate the apoptotic potential of outer-membrane protein (OMP) from short-chain fatty acids (SCFA) exposed Salmonella enterica serovar Typhi. Short-chain fatty acids exposure is one of the in vivo stresses encountered by the pathogen in the intestine. Therefore, to simulate the in vivo condition, S. enterica serovar Typhi was grown in the presence of SCFA and its OMP profile was analyzed. The apoptotic potential of 55 kDa protein expressed with enhanced intensity under the SCFA stress was evaluated. Murine peritoneal macrophages interacted with 55 kDa protein showed DNA fragmentation, changes in fluorescence and exposure of phosphatidylserine on their outer leaflets. Levels of nitrite and citrulline were found to be increased in the supernatant of macrophages after interacting them with 55 kDa protein. However, the enzymatic activity of superoxide dismutase was found to be decreased as compared to that of the control (uninteracted) macrophages. These observations indicate that increased levels of nitrite and decreased levels of superoxide dismutase may be one of the mechanisms to induce apoptosis in macrophages by SCFA induced 55 kDa OMP. These findings may help us better understand the pathophysiology of the disease during the host pathogen interactions.

Cellular immune response induced by Salmonella enterica serotype Typhi iron-regulated outer-membrane proteins at peripheral and mucosal levels

The role of purified iron-regulated outer-membrane proteins (IROMPs) from Salmonella enterica serotype Typhi in modulation of specific T-cell responses was studied. The cellular immune response induced by IROMPs was measured by assessing the delayed-type hypersensitivity (DTH) response, lymphocyte proliferation, T-cell phenotyping and cytokine-producing cells using lymphocytes isolated from the spleen and Peyer's patches of IROMPs-immunized, immunized-challenged, infected and control mice. IROMPs immunization resulted in an enhanced DTH response and exhibited a significant increase in the protein-specific proliferative response of lymphocyte from the spleen as well as Peyer's patches. A significant increase was also observed in the ratio of CD4+/CD8+ cells in the immunized mice as compared to the infected mice. Results of the cytokine analysis revealed that during the initial period there was increased production of interleukin (IL)-2- and interferon (IFN)-gamma-producing cells in the spleen and Peyer's patches, indicating a Th1 type response, whereas in the later period of the study, increased production of IL-4-producing cells suggested a Th2 type response. The results of this study suggest a role for S. Typhi IROMPs in modulating the cellular immune response at peripheral and mucosal levels.

Molecular cloning, sequencing and characterization of omp48, the gene encoding for an antigenic outer membrane protein from Aeromonas veronii

AIMS

To clone, sequence and characterize the gene encoding the Omp48, a major outer membrane protein from Aeromonas veronii.

METHODS AND RESULTS

A genomic library of Aer. veronii was constructed and screened to detect omp48 gene sequences, but no positive clones were identified, even under low stringency conditions. The cloned gene probably was toxic to the host Escherichia coli strain, so the cloning of omp48 was achieved by inverse PCR. The nucleotide sequence of omp48 consisted of an open reading frame of 1278 base pairs. The predicted primary protein is composed of 426 amino acids, with a 25-amino-acid signal peptide and common Ala-X-Ala cleavage site. The mature protein is composed of 401 amino acids with a molecular mass of 44,256 Da.

CONCLUSIONS

The omp48 gene from Aer. veronii was cloned, sequenced and characterized in detail. BLAST analysis of Omp48 protein showed sequence similarity (over 50%) to the LamB porin family from other pathogenic Gram-negative bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

Bacterial diseases are a major economic problem for the fish farming industry. Outer membrane proteins are potentially important vaccine components. The characterization of omp48 gene will allow further investigation of the potential of Omp48 as recombinant or DNA vaccine component to prevent Aer. veronii and related species infections in reared fish.

The chicken, the egg and Salmonella enteritidis

Salmonella enterica serovar Enteritidis is the cause of the food-borne salmonellosis pandemic in humans, in part because it has the unique ability to contaminate eggs without causing discernible illness in the birds infected. The infection route to humans involves colonization, survival and multiplication of the pathogen in the hen house environment, the bird and, finally, the egg. This review highlights the stages of transmission and discusses evidence that altered bacterial growth patterns and specific cell surface characteristics contribute to the adaptation of S. enteritidis to these diverse environments.

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.

Immune recognition of porin and lipopolysaccharide epitopes of Salmonella typhimurium in mice

We investigated the antigenic specificity of the humoral immune response to infection by Salmonella typhimurium, by competitive inhibition enzyme-linked immunosorbent assay and Western immunoblots. A panel of eight murine monoclonal antibodies, raised to OmpC and OmpD porins and lipopolysaccharide (LPS)-O antigens, was used to define the specificity of the polyclonal immune response in mice. The monoclonal antibody panel recognized five distinct epitopes; these were localized to surface-exposed loops of OmpC and OmpD porin, to the "eye-let" forming loop L3 of OmpC/OmpD, and to LPS-O4 and O5 factors. The immune mouse serum raised to infections with S. typhimurium LT-2 strain WB600 (wild-type) competitively inhibited the binding of biotin-labelled monoclonal antibodies to the epitopes that they recognize, indicating that all five epitopes were targets of the host immune response to natural infection. However, only two epitopes, one within a surface-exposed loop of OmpC porin, and the other in the LPS-O4 factor, were immunodominant. Furthermore, the bacterial LPS core and O-antigen structure influenced the immune response to the porins. Surface epitopes of porins were dominant in the rough strain SH5014 (rfa), whereas the immune recognition of LPS epitopes was predominant in mice infected with the smooth, wild-type strain (WB600). Finally, the immune response to LPS epitopes O4 and O5 was more pronounced in mice immunized with heat-killed cells than those infected with live S. typhimurium.

Characterization of CD4+ T cell responses during natural infection with Salmonella typhimurium

CD4+ T cells are important for resistance to infection with Salmonella typhimurium. However, the Ag specificity of this T cell response is unknown. Here, we demonstrate that a significant fraction of Salmonella-specific CD4+ T cells respond to the flagellar filament protein, FliC, and that this Ag has the capacity to protect naive mice from lethal Salmonella infection. To characterize this Ag-specific response further, we generated FliC-specific CD4+ T cell clones from mice that had resolved infection with an attenuated strain of Salmonella. These clones were found to respond to an epitope from a constant region of FliC, enabling them to cross-react with flagellar proteins expressed by a number of distinct Salmonella serovars.

The effect of immunization with porins on gut pathophysiological response in rats infected with Salmonella typhimurium

Attachment of Salmonella typhimurium to epithelial surfaces elicit significant alterations in different cell signalling events which lead to the development of disease. The present investigation was conducted to evaluate the effect of immunization of rats with porins, on gut physiologic markers following challenge with S. typhimurium. Male albino Wistar rats were immunized with purified porins and challenged by intragastric infection with S. typhimurium. Electrolyte transport, levels of different second messengers and inflammatory mediators were studied. A net absorption of transepithelial fluxes of Na+ and Cl- in immunized-challenged group and secretion in infected group was found. Ca2+ and 3-O-methyl-D-glucose fluxes did not show any change. Significant increase in the levels of [Ca2+]i, cAMP, membrane form of protein kinase C, prostaglandins, NADPH oxidase, Glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, total oxygen free radicals, reactive nitrogen intermediates, citrulline and lipid peroxidation was found in the infected group. However, in the immunized-challenged group, the values of all the parameters were found to be almost the same as that of control as well as immunized groups. Na+, K+-ATPase and calmodulin levels were unaltered in all the groups of animals. The results of this study thus suggest that immunization of rats with purified Salmonella porins followed by subsequent challenge with the organism might be helpful for the prevention of multiple physiologic derangements in isolated ileal cells.

The porin OmpC ofSalmonella typhimuriummediates adherence to macrophages

Macrophages recognize, adhere to, and phagocytose Salmonella typhimurium. The major outer membrane protein OmpC is a candidate ligand for macrophage recognition. To confirm this we used transposon mutagenesis to develop an ompC-deficient mutant in a known virulent strain of S. typhimurium; mutant and wild type were compared in macrophage adherence and association assays. Radiolabeled wild type S. typhimurium bound to macrophages at five-fold higher levels than did the ompC mutant. In association assays, macrophages in monolayers bound and internalized three-fold more wild type than mutant, while macrophages in suspension bound and internalized 40-fold more wild type than mutant. The ompC gene of our test strain of S. typhimurium contains several discrete differences compared with the ompC genes of Salmonella typhi and Escherichia coli. The deduced OmpC amino acid sequence of S. typhimurium shares 77 and 98% identity with OmpC amino acid sequences of E. coli and S. typhi, respectively. Evidence from this study supports a role for the OmpC protein in initial recognition by macrophages and distinguishes regions of this protein that potentially participate in host-cell recognition of bacteria by phagocytic cells.Key words: Salmonella, porin, macrophage, outer membrane protein, DNA sequencing.

Influence of immunopotentiators on the antiporin immunoglobulin G subclass: distribution and protective immunity against murine salmonellosis

To improve the immune potential of porin (a pore-forming protein of Salmonella sp.), different immunopotentiators such as Freund's complete adjuvant (FCA), lipopolysaccharide (LPS) and polyoxydonium (PO) were evaluated by studying the nature of the protective immune response induced against murine Salmonellosis. The nontoxic, synthetic heteropolymer polyoxydonium was as good as LPS at inducing antiporin immunoglobulin G (IgG) antibodies and protective immunity. Analysis of the antiporin IgG subclass pattern revealed a preferential increase in a particular subclass based on the immunopotentiator used. Porin, alone or emulsified in FCA, elicited predominantly antiporin IgG1 antibodies, whereas LPS preferentially evoked antiporin IgG2a, IgG2b and IgG3 antibodies. Polyoxydonium induced a clear shift towards antiporin IgG2b antibodies. The significance of these antiporin IgG subclass antibodies in protection against murine Salmonellosis was studied by passive immunization and by analysing the infected mouse sera.

Involvement of signal transduction pathways in Salmonella typhimurium porin activated gut macrophages

Many membrane proteins are implicated in the regulation of cell functions by triggering specific signaling pathways. Porins are known potential modulators of cell proliferation and differentiation. We explored the possible involvement of this protein in signal transduction pathways in mouse gut macrophages. In the present work we have shown that porins can trigger signal transduction in mouse macrophages infected with S. typhimurium. Activation of macrophages by porins results in an increase in inositol trisphosphate and intracellular Ca2+ mobilization. There is a translocation of protein kinase C to the membrane which is accompanied by nitric oxide release within the macrophages. This effect is the outcome of the expression of nitric oxide synthase, which is dependent on Protein kinase C. Further, we observed that there is an increased binding of the porins on macrophages infected with S. typhimurium which results in activation of macrophages and triggering of specific signaling pathways. These results indicate that porins induce the production of nitric oxide via a protein kinase C dependent pathway. Nitric oxide plays a fundamental role in macrophage effector function where it has both communication and defensive function.

Sequence analysis of Pasteurella multocida major outer membrane protein (OmpH) and application of synthetic peptides in vaccination of chickens against homologous strain challenge

Pasteurella multocida major outer membrane protein (OmpH) has been previously characterized as a porin. The native OmpH from strain X-73 (serotype 1) but not recombinant protein from Escherichia coli induced homologous protection in chickens. In this study OmpH sequences from 15 P. multocida serotypes as well as the CU vaccine strain were compared by sequence alignment and revealed high homology, with major variations confined to two discrete regions which were correspondingly predicted as two largest external loops. Secondary structures of OmpHs were predicted by sequence alignment of OmpHs with well defined porins and analyses of amphiphilicity, hydrophobic moment and antigenic index plots. Several synthetic peptides derived from predicted loop 2 and loop 5 of X-73 OmpH were synthesized as vaccine candidates. Vaccination studies in chickens showed that the cyclic synthetic peptide (Cyclic-L2) mimicking the predicted loop 2 induced 70% protection in chickens against strain X-73 challenge. This is the first report that a synthetic peptide mimicking the conformational epitopes of a native protein provide practical protection in target animal against bacterial infection.

Priming of T-cell responses in mice by porins of Salmonella typhimurium

An imbalance in signals delivered to T cells via T-cell receptor and accessory molecules can lead to anergy, apoptosis, or both. In the present study we have demonstrated that Salmonella typhimurium infection in mice leads to a progressive loss of CD4+ T helper (Th) cell population, abnormal T-cell death by apoptosis and loss of accessory molecules (B7 and intracellular adhesion molecule-1) on macrophages. Quantification of interleukin-2 (IL-2), interleukin-4 (IL-4) and interferon-gamma (IFN-gamma) secretion revealed a Th2-type of response in lymphocytes isolated from spleen. However, preimmunization of mice with porins resulted in an increased CD4+ Th cell population and accessory molecules on the surface of macrophages. Quantification of cytokines revealed a Th1-type of response. We conclude that preimmunization of mice with porins provides a microenvironment in which a well-balanced accessory molecule and cytokine network is established, which results in the prevention of cell death by apoptosis.

Cloning and characterization of the major outer membrane protein gene (ompH) of Pasteurella multocida X-73

The major outer membrane protein (OmpH) of Pasteurella multocida X-73 was purified by selective extraction with detergents, followed by size exclusion chromatography. The planar lipid bilayer assay showed that OmpH has pore-forming function. The average single channel conductance in 1.0 M KCl was 0.62 nS. The gene (ompH) encoding OmpH has been isolated and sequenced by construction of a genomic library and PCR techniques. The coding region of this gene is 1,059 bp long. The predicted primary protein is composed of 353 amino acids, with a 20-amino-acid signal peptide. The mature protein is composed of 333 amino acids with a molecular mass of 36.665 kDa. The ompH gene encoding mature protein has been expressed in Escherichia coli by using a regulatable expression system. The ompH gene was distributed among 15 P. multocida serotypes and strain CU. Protection studies showed that OmpH was able to induce homologous protection in chickens. These findings demonstrate that OmpH is a protective outer membrane porin of strain X-73 and is conserved among P. multocida somatic serotypes.

Macrophage-T cell interaction in murine salmonellosis: selective down-regulation of ICAM-1 and B7 molecules in infected macrophages and its probable role in cell-mediated immunity

Vaccine development and understanding of cellular immune modulatory mechanisms in salmonella infections have been impeded due to the paucity of data on antigens capable of eliciting effective immune responses. The present study was done to evaluate the efficacy of five major purified salmonella antigens (porins, pili, flagella, outer membrane proteins and heat shock proteins) in modulating T cell-macrophage interactions which play a central role in resistance to and recovery from infection with several intracellular pathogens, including salmonella. The results showed that the T cells recovered 10 days post-immunization (D10 T cells) from mice immunized with porins and outer membrane proteins showed maximum proliferation in the presence of macrophages incubated with dead bacteria; however, this response was decreased when T cells were co-cultured with live Salmonella typhimurium-infected macrophages. Delayed-type hypersensitivity responses, as measured by increased footpad thickness at 24 h, though induced effectively by porins, pili and flagella, were completely abrogated when D10 T cells were pre-incubated with macrophages infected with live bacteria. The phagocytic and bactericidal ability of normal macrophages, when grown in presence of T cell supernatants, was not influenced by the immunizing agents, but T cell supernatants obtained from mice immunized with porins and heat-shock protein triggered increased bactericidal activity. Further, the expression of the co-stimulatory molecules ICAM-1 and B7 increased with increasing bacteria (dead):macrophage ratio, but this expression was down-regulated upon incubation with live bacteria.

Antigenic determinants of the OmpC porin from Salmonella typhimurium

The antigenic determinants of Salmonella typhimurium OmpC were investigated by the analysis of cyanogen bromide (CNBr)-generated porin peptides with antiporin monoclonal antibodies (MAbs). We identified six bands (f1 to f6) with estimated molecular masses of 35.5, 31.0, 25.0, 22.5, 13.8, and 10.0 kDa, respectively. In addition, two small fragments (f7 and f8; 3.0 to 6.0 kDa) were detected only infrequently. The OmpC monomer or its CNBr-generated peptides were electrophoretically transferred to a polyvinylidene difluoride membrane and then subjected to amino acid composition analysis and N-terminal sequencing. A comparison of the amino acid composition data with known compositions of Escherichia coli and Salmonella typhi OmpC showed some differences; however, the amino acid sequences of 71 residues identified in S. typhimurium showed 88 and 98% identity with OmpC from E. coli and S. typhi, respectively. The screening of CNBr peptides with the 12 anti-(S. typhimurium) OmpC MAbs by Western blot (immunoblot), in conjunction with the prediction of the OmpC folding pattern based on the known three-dimensional structure of E. coli OmpF, showed that four MAbs reacted with surface-exposed epitopes on loops L2, L8, and L4 to L7, four MAbs reacted with a region in the eyelet structure on loop L3, and four MAbs reacted with the buried epitopes on transmembrane beta strands. The MAbs reacting with surface-exposed loops showed no cross-reaction with E. coli OmpC, whose sequence has diverged extensively from that of S. typhi and (probably) S. typhimurium OmpC only in regions of the externally exposed loops. In contrast, MAbs reacting with transmembrane beta strands, whose sequence is strongly conserved, showed strong cross-reaction with E. coli OmpC. These results show that comparison with the E. coli OmpF structure predicts the folding pattern of S. typhimurium OmpC rather accurately and that evolutionary divergence in sequences is confined to the external loops. The possible roles of these surface-exposed and buried epitopes as potentially useful antigenic regions for diagnostic assays and vaccine development are discussed.