Structural relatedness of enteric bacterial porins assessed with monoclonal antibodies to Salmonella typhimurium OmpD and OmpC

Design and in silico analysis of mRNA vaccine construct against Salmonella

Salmonella infections are continuously growing. Causative serovars have gained enhanced drug resistance and virulence. Current vaccines have fallen short of providing sufficient protection. mRNA vaccines have come up with huge success against SARS-CoV-2; Pfizer-BioNTech and Moderna vaccines have resulted in >90% efficacy with efficient translocation, expression, and presentation of antigen to the host immune system. Herein, based on the same approach a mRNA vaccine construct has been designed and analyzed against Salmonella by joining regions of genes of outer membrane proteins C and F of S. Typhi through a flexible linker. Construct was flanked by regulatory regions that have previously shown better expression and translocation of encoded protein. GC content of the construct was improved to attain structural and thermodynamic stability and smooth translation. Sites of strong binding miRNAs were removed through codon optimization. Protein encoded by this construct is structurally plausible, highly antigenic, non-allergen to humans, and does not cross-react to the human proteome. It is enriched in potent, highly antigenic, and conserved linear and conformational epitopes. Most conserved conformational epitopes of core protein lie on extended beta hairpins exposed to the cellular exterior. Stability and thermodynamic attributes of the final construct were found highly comparable to the Pfizer-BioNTech vaccine construct. Both contain a stable stem-loop structure downstream of the start codon and do not offer destabilizing secondary structures upstream of the start codon. Given structural and thermodynamic stability, effective immune response, and epitope composition the construct is expected to provide broad-spectrum protection against clinically important Salmonella serovars.Communicated by Ramaswamy H. Sarma.

Immunosuppressive Mechanisms in Brucellosis in Light of Chronic Bacterial Diseases

Brucellosis is considered one of the major zoonoses worldwide, constituting a critical livestock and human health concern with a huge socio-economic burden. Brucella genus, its etiologic agent, is composed of intracellular bacteria that have evolved a prodigious ability to elude and shape host immunity to establish chronic infection. Brucella’s intracellular lifestyle and pathogen-associated molecular patterns, such as its specific lipopolysaccharide (LPS), are key factors for hiding and hampering recognition by the immune system. Here, we will review the current knowledge of evading and immunosuppressive mechanisms elicited by Brucella species to persist stealthily in their hosts, such as those triggered by their LPS and cyclic β-1,2-d-glucan or involved in neutrophil and monocyte avoidance, antigen presentation impairment, the modulation of T cell responses and immunometabolism. Attractive strategies exploited by other successful chronic pathogenic bacteria, including Mycobacteria, Salmonella, and Chlamydia, will be also discussed, with a special emphasis on the mechanisms operating in brucellosis, such as granuloma formation, pyroptosis, and manipulation of type I and III IFNs, B cells, innate lymphoid cells, and host lipids. A better understanding of these stratagems is essential to fighting bacterial chronic infections and designing innovative treatments and vaccines.

Transient plant production of Salmonella Typhimurium diagnostic antibodies

Salmonella Typhimurium is one of the most important zoonotic pathogens worldwide and a major cause of economic losses in the pig production chain. The emergence of multi-drug resistant strains over the past years has led to considerations about an enhanced surveillance of bacterial food contamination. Currently, ELISA is the method of choice for high throughput identification of S. Typhimurium. The sensitivity and specificity of this assay might be improved by application of new diagnostic antibodies. We focused on plant-based expression of candidate diagnostic TM43-E10 antibodies discovered using as antigen the S. Typhimurium OmpD protein. The scFv-TM43-E10 and scFv-Fc-TM43-E10 antibody derivatives have been successfully produced in N. benthamiana using a deconstructed movement-deficient PVX vector supplemented with the γb silencing suppressor from Poa semilatent virus. The plant-made antibodies showed the same antigen-binding specificity as that of the microbial/mammalian cell-produced counterparts and could recognize the OmpD antigen in S. Typhimurium infected plant samples.

QCM-based immunosensor for rapid detection of Salmonella Typhimurium in food

Salmonella Typhimurium is one of the main causes of outbreaks and sporadic cases of human gastroenteritis. At present, the rapid detection of this pathogen is a major goal of biosensing technology applied to food safety. In fact, ISO standardized culture method takes up to ten days to provide a reliable response. In this paper, we describe a relatively simple protocol for detecting Salmonella Typhimurium in chicken meat based on a Quartz-Crystal Microbalance (QCM), which leads to a limit of detection (LOD) less than of 10° CFU/mL and requires a pre-enrichment step lasting only 2 h at 37 °C. The reliability of the proposed immunosensor has been demonstrated through the validation of the experimental results with ISO standardized culture method. The cost-effectiveness of the procedure and the rapidity of the QCM-based biosensor in providing the qualitative response make the analytical method described here suitable for applications in food inspection laboratory and throughout the chain production of food industry.

Salmonella Choleraesuis outer membrane vesicles: Proteomics and immunogenicity

Salmonella enterica serotype Choleraesuis (S. Choleraesuis), Gram-negative facultative intracellular pathogen is capable of inducing the cholera in pigs whose symptoms manifest as fever, depression, septicemia, arthritis, and diarrhea. Infections with S. Choleraesuis has resulted in great economic loss for the swine breeding operations. Bacterial outer membrane vesicles (OMVs) play an important role in pathogenicity and host-pathogen interaction. In this study, we purified OMVs released by S. Choleraesuis strain χ3545 and characterized their lipopolysaccharide (LPS) profile. The OMVs contained intact LPS molecules. By using LC-MS/MS, we identified 192 proteins in the OMVs. In addition, the subcellular location and biological functions of the vesicles was predicted. The proteins were mainly derived from outer membranes and cytoplasm. Several proteins were immunoreactive and associated with the secretion pathway. Some putative multi-drug resistance-associated proteins were also identified. Furthermore, immunization experiment via intranasal or intraperitoneal route in mice demonstrated that S. Choleraesuis OMVs could elicit strong humoral and mucosal immune responses. Although OMVs as vaccine did not provide strong protection against clinical strain of wild-type S. Choleraesuis, immunization of OMVs still prolonged the survival time of vaccinated mice after high dose of S. Choleraesuis infection. Overall, this study provides valuable fundamental information toward elucidating the pathogenicity and functions of OMVs secreted from S. Choleraesuis.

Salmonella Infection Drives Promiscuous B Cell Activation Followed by Extrafollicular Affinity Maturation

The B cell response to Salmonella typhimurium (STm) occurs massively at extrafollicular sites, without notable germinal centers (GCs). Little is known in terms of its specificity. To expand the knowledge of antigen targets, we screened plasmablast (PB)-derived monoclonal antibodies (mAbs) for Salmonella specificity, using ELISA, flow cytometry, and antigen microarray. Only a small fraction (0.5%-2%) of the response appeared to be Salmonella-specific. Yet, infection of mice with limited B cell receptor (BCR) repertoires impaired the response, suggesting that BCR specificity was important. We showed, using laser microdissection, that somatic hypermutation (SHM) occurred efficiently at extrafollicular sites leading to affinity maturation that in turn led to detectable STm Ag-binding. These results suggest a revised vision of how clonal selection and affinity maturation operate in response to Salmonella. Clonal selection initially is promiscuous, activating cells with virtually undetectable affinity, yet SHM and selection occur during the extrafollicular response yielding higher affinity, detectable antibodies.

Characterization of surface proteins of Cronobacter muytjensii using monoclonal antibodies and MALDI-TOF Mass spectrometry

BACKGROUND

Cronobacter spp. is a newly emerging pathogen that causes meningitis in infants and other diseases in elderly and immunocompromised individuals. This study was undertaken to investigate surface antigenic determinants in Cronobacter spp. using monoclonal antibodies (MAbs) and MALDI-TOF Mass spectrometry.

RESULTS

Spleenocytes from mice that were immunized with heat-killed (20 min, 80°C) Cronobacter cells were fused with SP2 myeloma cells. Five desirable MAbs (A1, B5, 2C2, C5 and A4) were selected. MAbs A1, B5, 2C2 and C5 were of IgG2a isotype while A4 was an IgM. Specificity of the MAbs was determined by using immunoblotting with outer membrane protein preparations (OMPs) extracted from 12 Cronobacter and 6 non-Cronobacter bacteria. All MAbs recognized proteins with molecular weight ranging between 36 and 49 kDa except for one isolate (44) in which no OMPs were detected. In addition, MAbs recognized two bands (38-41 kDa) in four of the non-Cronobacter bacteria. Most of the proteins recognized by the MAbs were identified by MALDI-TOF peptide sequencing and appeared to be heterogeneous with the identities of some of them are still unknown. All MAbs recognized the same epitope as determined by an additive Index ELISA with their epitopes appeared to be conformational rather than sequential. Further, none of the MAbs recognized purified LPS from Cronobacter spp. Specificity of the MAbs toward OMPs was further confirmed by transmission electron microscopy.

CONCLUSIONS

Results obtained in this study highlight the immunological cross-reactivity among Cronobacter OMPs and their Enterobacteriaceae counterparts. Nevertheless, the identity of the identified proteins appeared to be different as inferred from the MALDI-TOF sequencing and identification.

Isolation of scFv fragments specific to OmpD of Salmonella Typhimurium

Pork meat is one of the major sources for human infections with Salmonella enterica subspecies enterica serovars. Further, zoonoses caused by S. enterica subspecies enterica serovars are responsible for substantial economical losses in industrial countries. Quick and reliable detection of this infection is urgently needed to improve consumer security. Due to its capability to identify infections independent of the species, a competitive ELISA is the preferable method for the detection of anti-Salmonella antibodies in serum. Recombinant antibody fragments (scFvs) were isolated from the naive human antibody gene library HAL7 by phage display. Recombinant produced outer membrane protein D (OmpD) of Salmonella Typhimurium was used as antigen. The characterization of the isolated single chain Fv (scFv) antibodies was done by enzyme-linked immunosorbent assay (ELISA), immunoblot, sequencing, epitope mapping and size exclusion chromatography (SEC). The detection of anti-OmpD IgGs in swine sera by competitive ELISA was shown in a proof of principle concept. Furthermore, the developed competitive ELISA would be compatible to a recently published DIVA vaccine, allow to distinguish between infected and vaccinated pigs.

The porin OmpD from nontyphoidal Salmonella is a key target for a protective B1b cell antibody response

Invasive nontyphoidal Salmonella (NTS), including Salmonella typhimurium (STm), are major yet poorly-recognized killers of infants in sub-Saharan Africa. Death in these children is usually associated with bacteremia, commonly in the absence of gastrointestinal symptoms. Evidence from humans and animal studies suggest that severe infection and bacteremia occur when specific Ab is lacking. Understanding how Ab responses to Salmonella are regulated will help develop vaccines against these devastating infections. STm induces atypical Ab responses characterized by prominent, accelerated, extrafollicular T-independent (TI) Ab against a range of surface antigens. These responses develop without concomitant germinal centers, which only appear as infection resolves. Here, we show STm rapidly induces a population of TI B220(+)CD5(-) B1b cells during infection and TI Ab from B1b cells targets the outer membrane protein (Omp) porins OmpC, OmpD and OmpF but not flagellin. When porins are used as immunogens they can ablate bacteremia and provide equivalent protection against STm as killed bacterial vaccine and this is wholly B cell-dependent. Furthermore Ab from porin-immunized chimeras, that have B1b cells, is sufficient to impair infection. Infecting with porin-deficient bacteria identifies OmpD, a protein absent from Salmonella Typhi, as a key target of Ab in these infections. This work broadens the recognized repertoire of TI protein antigens and highlights the importance of Ab from different B cell subsets in controlling STm infection. OmpD is a strong candidate vaccine target and may, in part, explain the lack of cross-protection between Salmonella Typhi and STm infections.

Low molecular weight proteins of outer membrane of Salmonella typhimurium are immunogenic in Salmonella induced reactive arthritis revealed by proteomics

In patients with reactive arthritis (ReA)/undifferentiated spondyloarthropathy (uSpA), synovial fluid mononuclear cells (SFMC) show proliferation to bacterial antigens that trigger ReA, i.e. Chlamydia, Yersinia, Campylobactor, Shigella and Salmonella species. We have shown previously that SFMC proliferate significantly to outer membrane proteins of S typhimurium in Salmonella induced ReA. In the present study we characterized the immunoreactive fractions of outer membrane protein (Omp) of S typhimurium in Salmonella induced ReA. Omp of Salmonella was isolated and fractionated by continuous elution sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) using Prep-Cell into eight Omp fractions based on molecular weight. Twenty-three patients with ReA were screened for the bacterial trigger using the SFMC proliferative response to crude lysates of Y enterocolitica, S flexneri, C jejuni and S typhimurium using thymidine uptake assay. SFMC from patients with salmonella induced ReA were tested against eight fractions. Seven of 23 patients with ReA had S typhimurium-induced ReA. Of these seven patients, five patients SFMC had a significant stimulation index (SI) against < 22, 22-26, 25-35 and 28-40 kDa fractions of Omp. These fractions were analysed by SDS-PAGE and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, which revealed 10 proteins. These proteins were 37 kDa OmpA, 33 kDa TsX, 28 kDa putative Omp, 28 kDa Vac J, 39 kDa OmpD, 18 kDa OmpX, 23 kDa OmpW, 43 kDa OmpS1 and 19 kDa peptidoglycan-associated lipoprotein. In conclusion, for the first time we have identified some low molecular weight proteins in the Omps of Salmonella which are T cells immunoreactive in patients with salmonella induced ReA/uSpA.

Salmonella porins induce a sustained, lifelong specific bactericidal antibody memory response

We examined the ability of porins from Salmonella enterica serovar typhi to induce a long-term antibody response in BALB/c mice. These porins triggered a strong lifelong production of immunoglobulin G (IgG) antibody in the absence of exogenous adjuvant. Analysis of the IgG subclasses produced during this antibody response revealed the presence of the subclasses IgG2b, IgG1, IgG2a and weak IgG3. Despite the high homology of porins, the long-lasting anti-S. typhi porin sera did not cross-react with S. typhimurium. Notably, the antiporin sera showed a sustained lifelong bactericidal-binding activity to the wild-type S. typhi strain, whereas porin-specific antibody titres measured by enzyme-linked immunosorbent assay (ELISA) decreased with time. Because our porin preparations contained the outer membrane proteins C and F (OmpC and OmpF), we evaluated the individual contribution of each porin to the long-lasting antibody response. OmpC and OmpF induced long-lasting antibody titres, measured by ELISA, which were sustained for 300 days. In contrast, although OmpC induced sustained high bactericidal antibody titres for 300 days, postimmunization, the bactericidal antibody titre induced by OmpF was not detected at day 180. These results indicate that OmpC is the main protein responsible for the antibody-mediated memory bactericidal response induced by porins. Taken together, our results show that porins are strong immunogens that confer lifelong specific bactericidal antibody responses in the absence of added adjuvant.

OmpD but not OmpC is involved in adherence ofSalmonella entericaserovar Typhimurium to human cells

Conflicting reports exist regarding the role of porins OmpC and OmpD in infections due to Salmonella enterica serovar Typhimurium. This study investigated the role of these porins in bacterial adherence to human macrophages and intestinal epithelial cells. ompC and ompD mutant strains were created by transposon mutagenesis using P22-mediated transduction of Tn10 and Tn5 insertions, respectively, into wild-type strain 14028. Fluorescein-labeled wild-type and mutant bacteria were incubated with host cells at various bacteria to cell ratios for 1 h at 37 °C and analyzed by flow cytometry. The mean fluorescence intensity of cells with associated wild-type and mutant bacteria was used to estimate the number of bacteria bound per host cell. Adherence was also measured by fluorescence microscopy. Neither assay showed a significant difference in binding of the ompC mutant and wild-type strains to the human cells. In contrast, the ompD mutant exhibited lowered binding to both cell types. Our findings suggest that OmpD but not OmpC is involved in the recognition of Salmonella serovar Typhimurium by human macrophages and intestinal epithelial cells.Key words: Salmonella, adherence, porins, intestinal epithelial cells, macrophage.

Identification of an I-Ed-restricted T-cell epitope of Escherichia coli outer membrane protein F

A predominant T-cell epitope of Escherichia coli outer membrane protein F (OmpF) that encompasses amino acids 295 to 314 was identified in H-2(d) mice. BALB/c-derived T-cell hybridomas generated against this region were CD3(+), CD4(+), CD8(-), and T-cell receptor alphabeta(+) and secreted TH-1-associated cytokines (interleukin-2 [IL-2] and gamma interferon), but not a TH-2-associated cytokine (IL-4), when restimulated with peptide 295-314. Class II(+) mouse lymphoma (A20) cells, but not class II(-) mouse mastocytoma (P815) cells, supported IL-2 secretion of hybridomas when substituted for syngeneic splenocytes as antigen-presenting cells (APCs). Antibodies specific for I-E(d) blocked IL-2 secretion by hybridomas, but I-A(d)-specific antiserum did not. When transfected L cells expressing I-A(d) (AalphaAbeta(d)), I-E(d) (EalphaEbeta(d)), or the hybrid molecule I-EalphaAbeta(d) were used as APCs, hybridomas recognized peptide only when presented by the I-E(d)-transfected cells. When peptide 295-314 truncated at either the C or the N terminus of the sequence was used, the minimal epitope was determined. Critical residues were determined by using alanine-substituted peptide analogues. T-cell hybridomas were only stimulated by peptides that encompassed amino acids 295 to 303 (9-mer), and the core sequence required a minimum of three additional amino acids at either the amino or the carboxy terminus to induce IL-2 secretion. Critical residues were determined to be phenylalanine at position 295, threonine at position 300, and tyrosines at positions 301 and 302. This study is the first to identify a minimal T-cell epitope and major histocompatibility complex restriction element of the OmpF protein and confirms previous observations that there is considerable degeneracy in the length of peptides that can bind I-E(d) and variability in the amino acid composition of the C and N termini of these peptides.

Recent advances in the diagnosis and classification of inflammatory bowel disease

The availability of an increasing number of inflammatory bowel disease (IBD)-specific serologic antibodies, the discovery of disease-susceptibility genes, the introduction of pharmacogenetic markers, and the recent application of wireless capsule enteroscopy to the evaluation of patients with IBD are providing new types of information that must be integrated with more traditional IBD paradigms. The challenge facing researchers and clinicians is to determine how to incorporate these potentially clinically relevant insights into our understanding of disease pathogenesis and to define the spectrum of potential applications to the management of patients with IBD. The ultimate diagnostic and predictive value of these tests will likely be optimized when they are applied in combination, rather than individually. As the clinical relevance of this expanding diagnostic armamentarium is defined, it is hoped that these tests will enable clinicians not only to diagnose IBD accurately but also to determine disease patterns prospectively, suggest prognoses, and allow for individualization of therapeutic regimens.

Immune responses dependent on antigen location in recombinant attenuated Salmonella typhimurium vaccines following oral immunization

The subcellular location of a recombinant antigen in recombinant attenuated Salmonella vaccines may influence immunogenicity dependent on exposure of the recombinant antigen to cells involved in systemic immune responses. It has been shown that a recombinant attenuated Salmonella vaccine secreting the recombinant Streptococcus pneumoniae PspA (rPspA) antigen specified by pYA3494 induced protective anti-rPspA-specific immune responses (Kang et al. (2002) Infect. Immun. 70, 1739-1749). A recombinant plasmid pYA3496 specifying a His(6)-tagged rPspA (His(6)-rPspA) protein (no apparent signal sequence) caused the rPspA antigen to localize to the cytoplasm of Salmonella. Salmonella vaccines carrying pYA3494 or pYA3496 expressed similar amounts of rPspA. After a single oral immunization in BALB/c mice with 10(9) colony-forming units (CFU) of the recombinant Salmonella vaccines carrying pYA3494 or pYA3496, IgG antibody responses were stimulated to both rPspA and Salmonella lipopolysaccharide (LPS) antigens. The anti-rPspA IgG titer induced by Salmonella carrying pYA3494 (1.9 x 10(7)) was 10(4) times higher than induced by Salmonella carrying pYA3496 (<2.4 x 10(3)).

The C-terminal domain of Salmonella enterica serovar typhimurium OmpA is an immunodominant antigen in mice but appears to be only partially exposed on the bacterial cell surface

We examined the way the major outer membrane protein OmpA of Salmonella enterica serovar Typhimurium is recognized by the mouse immune system, by raising a panel of 12 monoclonal antibodies (MAbs) against this protein. Interaction between OmpA and these MAbs is competitively inhibited with several-hundredfold dilutions of mouse polyclonal sera obtained by immunization with live or heat-killed whole cells, suggesting that OmpA is one of the immunodominant antigens of serovar Typhimurium. All of the MAbs were specific for an identical epitope(s) located on the C-terminal domain of OmpA, as indicated by the use of OmpA fragments generated by protease or cyanogen bromide treatment and by competitive inhibition enzyme-linked immunosorbent assay. This epitope was highly conserved within (but not outside) the family Enterobacteriaceae: The strong immunogenicity of this epitope was surprising because the C-terminal domain of OmpA, usually thought to be located in the periplasm, is not expected to be exposed on the bacterial cell surface. A MAb, however, reacted in a cytofluorometry assay more strongly with outer-membrane-permeabilized cells than with untreated cells, a result supporting the predominantly periplasmic localization of the epitope. Significant, though low-level, reactivity of intact cells nevertheless suggests that in some cells the C-terminal domain of OmpA is exposed on the surface, a result consistent with the proposal that OmpA can fold into one of the two alternate conformations.

The C-terminal domain of Salmonella enterica serovar typhimurium OmpA is an immunodominant antigen in mice but appears to be only partially exposed on the bacterial cell surface

We examined the way the major outer membrane protein OmpA of Salmonella enterica serovar Typhimurium is recognized by the mouse immune system, by raising a panel of 12 monoclonal antibodies (MAbs) against this protein. Interaction between OmpA and these MAbs is competitively inhibited with several-hundredfold dilutions of mouse polyclonal sera obtained by immunization with live or heat-killed whole cells, suggesting that OmpA is one of the immunodominant antigens of serovar Typhimurium. All of the MAbs were specific for an identical epitope(s) located on the C-terminal domain of OmpA, as indicated by the use of OmpA fragments generated by protease or cyanogen bromide treatment and by competitive inhibition enzyme-linked immunosorbent assay. This epitope was highly conserved within (but not outside) the family Enterobacteriaceae: The strong immunogenicity of this epitope was surprising because the C-terminal domain of OmpA, usually thought to be located in the periplasm, is not expected to be exposed on the bacterial cell surface. A MAb, however, reacted in a cytofluorometry assay more strongly with outer-membrane-permeabilized cells than with untreated cells, a result supporting the predominantly periplasmic localization of the epitope. Significant, though low-level, reactivity of intact cells nevertheless suggests that in some cells the C-terminal domain of OmpA is exposed on the surface, a result consistent with the proposal that OmpA can fold into one of the two alternate conformations.

Immune responses to recombinant pneumococcal PspA antigen delivered by live attenuated Salmonella enterica serovar typhimurium vaccine

Attenuated Salmonella enterica serovar Typhimurium expressing recombinant antigens from other pathogens elicits primarily a Th1-type dominant immune response to both recombinant and Salmonella antigens. The immunogenicity and appropriate subcellular location of the recombinant antigen in the Salmonella vaccine strain may contribute to augmenting immune responses by facilitating adequate exposure of recombinant antigen to antigen-presenting cells for processing. To allow for secretion from gram-negative bacteria and overexpression of antigen, a DNA fragment encoding a highly antigenic alpha-helical region of PspA (pneumococcal surface protein A) was subcloned downstream from the beta-lactamase signal sequence in the multicopy Asd(+) pYA3493 vector to create pYA3494. pYA3493 was derived from a class of Asd(+) vectors with reduced expression of Asd to minimize selective disadvantage and enhance immunization of expressed recombinant antigens. The S. enterica serovar Typhimurium vaccine strain was constructed by the introduction of deletion mutations Delta crp-28 and Delta asdA16. Approximately 50% of the recombinant PspA (rPspA) expressed in a Salmonella strain harboring pYA3494 was detected in the combined supernatant and periplasmic fractions of broth-grown recombinant Salmonella. After a single oral immunization in BALB/c mice with 10(9) CFU of the recombinant Salmonella vaccine strain carrying pYA3494, immunoglobulin G (IgG) antibody responses were stimulated to both the heterologous antigen rPspA and Salmonella lipopolysaccharide (LPS) and outer membrane proteins (OMPs). About half, and even more at later times after immunization, of the antibodies induced to rPspA were IgG1 (indicating a Th2-type response), whereas 60 to 70% of the antibodies to LPS and 80 to 90% of those to OMPs were IgG2a (indicating a Th1-type response). A sublethal infection with Streptococcus pneumoniae WU2 boosted PspA antibody levels and maintained IgG2a/IgG1 ratios similar to those seen before the challenge. Oral immunization with Salmonella-PspA vaccine protected 60% of immunized mice from death after intraperitoneal challenge with 50 times the 50% lethal dose of virulent S. pneumoniae WU2.

Identification of murine B-cell and T-cell epitopes of Escherichia coli outer membrane protein F with synthetic polypeptides

The major pore-forming outer membrane proteins (Omps) of gram-negative bacteria demonstrate numerous immunomodulating properties and are involved in the virulence of pathogenic strains. Because Escherichia coli OmpF is the best-characterized porin in terms of structural and functional characteristics, in vitro B-cell and T-cell responses to this porin in six different strains of mice were analyzed. Mice were immunized with purified OmpF trimers or overlapping synthetic polypeptides (20-mers) spanning the entire 340-amino-acid sequence of the OmpF monomer. T-cell proliferative responses and immunoglobulin G antibody responses to native OmpF and the peptide analogues were determined. For each strain, patterns of T-cell proliferation were similar regardless of whether native OmpF or synthetic peptides were inoculated, although all strains recognized one or more cryptic determinants. Mice exhibited several haplotype-specific responses, but genetically permissive epitopes were also identified. Four peptides (75-94, 265-284, 295-314, and 305-324) elicited strong T-cell proliferative responses from all strains of mice when mice were presensitized with native OmpF or a homologous peptide. In general, 10 or fewer peptides were recognized by sera from mice immunized with native OmpF or synthetic peptides, and most sera from peptide-immunized mice reacted poorly with the native protein. Four peptides spanning amino acids 45 to 64, 95 to 114, 115 to 134, and 275 to 294 were recognized by sera from all strains immunized with native OmpF but not by sera from peptide-immunized mice. Peptides 245-264 and 305-324 were universally recognized by sera from peptide-immunized mice, but these sera reacted weakly or were negative when tested against the native protein. Based on the pattern of cytokine secretion by proliferating T cells, immunization with native OmpF polarizes T helper cells toward development of a TH1 response. T-cell and B-cell responses have been investigated based on the assumption that differences in epitope specificity could influence protective or pathologic host reactions. Because of the high level of structural homology of OmpF to porins isolated from other enteric pathogens, the identification of T- and B-cell-stimulatory determinants of E. coli OmpF may have broader application.

Colonic bacteria express an ulcerative colitis pANCA-related protein epitope

Bacteria are a suspected pathogenic factor in inflammatory bowel disease, but the identity of the relevant microbial species remains unresolved. The pANCA autoantibody is associated with most cases of ulcerative colitis (UC) and hence reflects an immune response associated with the disease process. This study addresses the hypothesis that pANCA identifies an antigen(s) expressed by bacteria resident in the human colonic mucosa. Libraries of colonic bacteria were generated using aerobic and anaerobic microbiologic culture conditions, and bacterial pools and clonal isolates were evaluated for cross-reactive antigens by immunoblot analysis using the pANCA monoclonal antibody Fab 5-3. Two major species of proteins immunoreactive to pANCA monoclonal antibodies were detected in bacteria from the anaerobic libraries. Colony isolates of the expressing bacteria were identified as Bacteroides caccae and Escherichia coli. Isolation and partial sequencing of the B. caccae antigen identified a 100-kDa protein without database homologous sequences. The E. coli protein was biochemically and genetically identified as the outer membrane porin OmpC. Enzyme-linked immunosorbent assay with human sera demonstrated elevated immunoglobulin G anti-OmpC in UC patients compared to healthy controls. These findings demonstrate that a pANCA monoclonal antibody detects a recurrent protein epitope expressed by colonic bacteria and implicates colonic bacterial proteins as a target of the disease-associated immune response.

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.

Recognition specificity of monoclonal antibodies which protect mice against Salmonella typhimurium infection

We used enzyme-linked immunosorbent assay (ELISA), competitive inhibition ELISA, flow cytometry and western immunoblots to study the antigenic specificity of two monoclonal antibodies (mAbs) raised against the cell surface antigens of Salmonella typhimurium. These mAbs (SH6.11 and WB60.4) protect CAF1 (Ity(r)) mice against endotoxemia and mouse typhoid. We found that SH6.11 and WB60.4 recognize Salmonella serogroup B-specific lipopolysaccharide O4 and O5 factors, respectively. These mAbs did not bind to Salmonella serotypes that belong to serogroup A, D1, E4, G2, or R and did not cross-react with other enteric and nonenteric bacterial species.

Biochemistry and regulation of a novel Escherichia coli K-12 porin protein, OmpG, which produces unusually large channels

A novel porin, OmpG, is produced in response to a chromosomal mutation termed cog-192. Molecular characterization of cog-192 revealed that it is a large chromosomal deletion extending from the 3' end of pspA through to the 5' end of an open reading frame located immediately upstream of ompG. As a result of this 13.1-kb deletion, the expression of ompG was placed under the control of the pspA promoter. Characterization of OmpG revealed that it is quite different from other porins. Proteoliposome swelling assays showed that OmpG channels were much larger than those of the OmpF and OmpC porins, with an estimated limited diameter of about 2 nm. The channel lacked any obvious solute specificity. The folding model of OmpG suggests that it is the first 16-stranded beta-barrel porin that lacks the large external loop, L3, which constricts the channels of other nonspecific and specific porins. Consistent with the folding model, circular dichroism showed that OmpG contains largely a beta-sheet structure. In contrast to other Escherichia coli porins, there is no evidence that OmpG exists as stable oligomers. Although ompG DNA was present in all E. coli strains examined so far, its expression under laboratory conditions was seen only due to rare chromosomal mutations. Curiously, OmpG was constitutively expressed, albeit at low levels, in Salmonella, Shigella, and Pseudomonas species.

Virulence of a Salmonella typhimurium OmpD mutant

An ompD mutation caused by a Tn10 insertion was transduced into Salmonella typhimurium SL1344 and UK-1. The adherence and invasion capabilities of the resultant ompD mutants were examined by tissue culture analysis. The virulence of the S. typhimurium ompD mutants was ascertained by a 50% lethal dose (LD50) study and by determining colonization ability with BALB/c mice. We found no statistically significant difference in adherence and invasion capacities between the S. typhimurium wild type strains and their corresponding ompD mutants. Furthermore, the LD50 and colonization studies revealed that there is no statistically significant difference in virulence between the S. typhimurium wild type strains and their corresponding ompD mutants. These results differ from those reported previously (C. J. Dorman, S. Chatfield, C. F. Higgins, C. Hayward, and G. Dougan, Infect. Immun. 57:2136-2140, 1989).

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.

An oligomer of the major outer membrane protein of Chlamydia psittaci is recognized by monoclonal antibodies which protect mice from abortion

Monoclonal antibodies (MAbs) were generated against an ovine abortive strain of Chlamydia psittaci. A plaque reduction assay was used to select 19 neutralizing antibodies which appeared to be heterogeneous in isotype, specificity, and recognized proteins. Different neutralizing MAbs were tested for their protective abilities against abortion in a pregnant-mouse model. All of the protective MAbs selected had the same isotype, were serotype 1 specific, and recognized a protein of about 110 kDa by immunoblotting. The recognized epitopes were resistant to sodium dodecyl sulfate and reducing agents, but all of them were heat sensitive. The protein was able to form disulfide-linked polymers. Immunological cross-reaction studies with rabbit sera showed a link between the 110-kDa protein and the major outer membrane protein (MOMP). The 110-kDa protein was purified by immunoaffinity and shown to be dissociated after heating into MOMP by silver staining and immunoblotting. These results show homogeneity among protective MAbs directed to heat-sensitive epitopes located on an oligomer of the MOMP of C. psittaci.

Antibodies to Escherichia coli 06 porins cross-react with urinary pathogens

Antibodies to partially purified E. coli 06 35-40 KDa porin trimers recognized the reactive epitopes in the intact porin surface molecule present in various wild-type, heterologous, urinary pathogens. The presence of lipopolysaccharide in the membrane did not shield the antibody binding sites. The reactivity was shown to be specific for porins since LPS-absorbed porin antisera reacted with porins on immunoblots and showed no reactivity with LPS. Additionally, the cross-reactions were abolished by absorption of the porin antisera with E. coli 06 containing porin trimers. These data strengthen the rationale for exploring the enhancement of immunoprotection by monoclonal antibodies to specific immunoreactive antigens in the porin molecule.

Purification, pore-forming ability, and antigenic relatedness of the major outer membrane protein of Shigella dysenteriae type 1

The major outer membrane protein (MOMP), the most abundant outer membrane protein, was purified to homogeneity from Shigella dysenteriae type 1. The purification method involved selective extraction of MOMP with sodium dodecyl sulfate in the presence of 0.4 M sodium chloride followed by size exclusion chromatography with Sephacryl S-200 HR. MOMP was found to form hydrophilic diffusion pores by incorporation into artificial liposome vesicles composed of egg yolk phosphatidylcholine and dicetylphosphate, indicating that MOMP of S. dysenteriae type 1 exhibited significant porin activity. However, the liposomes containing heat-denatured MOMP were barely active. The molecular weight of MOMP found by size exclusion chromatography was 130,000, and in sodium dodecyl sulfate-10% polyacrylamide gel it moved as an oligomer of 78,000 molecular weight. Upon boiling, fully dissociated monomers of 38,000 molecular weight were seen for S. dysenteriae type 1. However, among the four Shigella spp., the monomeric MOMP generated upon boiling ranged from 38,000 to 35,000 in molecular weight. Antibody raised in BALB/c mice immunized with MOMP of S. dysenteriae type 1 reacted strongly with purified MOMP of S. dysenteriae type 1 in an enzyme-linked immunosorbent assay (ELISA). The antibody reacted with whole-cell preparations of S. dysenteriae type 1 in an ELISA, suggesting that MOMP possessed surface components. Moreover, MOMP could be visualized on the bacterial surface by immunoelectron microscopy with anti-MOMP antibody. S. dysenteriae type 1 MOMP-specific immunoglobulin eluted from MOMP bound to a nitrocellulose membrane was found to cross-react with MOMP preparations of S. flexneri, S. boydii, and S. sonnei, indicating that MOMPs were antigenically related among Shigella species. The strong immunogenicity, surface exposure, and antigenic relatedness make MOMP of Shigella species an immunologically significant macromolecule for study.

Salmonella-specific monoclonal antibodies against recombinant Salmonella typhi 36-kilodalton porin

Mouse monoclonal antibodies were raised against recombinant Salmonella typhi 36-kDa porin monomer. Specificities of 16 monoclonal antibodies were analyzed as reactivity patterns in dot immunobinding and Western blot (immunoblot) assays using isolated outer membrane proteins of gram-negative bacteria and cloned purified S. typhi porin monomers and trimers. Four monoclonal antibodies were specific for Salmonella spp.

Characterization of the Salmonella typhimurium phoE gene and development of Salmonella-specific DNA probes

In Escherichia coli K-12, the phoE gene, encoding a phosphate-limitation-inducible outer membrane pore protein (PhoE), is closely linked to the genes proA and proB. When the corresponding fragment of the Salmonella typhimurium chromosome was transferred to E. coli K-12 using an RP4::miniMu plasmid, pULB113, no production of S. typhimurium PhoE could be detected. Nevertheless, DNA hybridization studies revealed that the corresponding plasmid did contain S. typhimurium phoE. Production of S. typhimurium PhoE in E. coli was detected only after subcloning the gene in a multicopy vector. Nucleotide (nt) sequence analysis showed extensive homology of S. typhimurium phoE to the E. coli gene and suggested possible explanations for the low expression of S. typhimurium phoE in E. coli. In addition, the sequence information was used to develop Salmonella-specific DNA probes. Two oligodeoxyribonucleotides were synthesized based on nt sequences encoding the fifth and eighth cell-surface-exposed regions of PhoE. When used in polymerase chain reactions, these probes turned out to be specific, i.e., no crossreactions occurred with the non-Salmonella strains, whereas 132 out of 133 tested Salmonella strains were recognized.