Textile foreign body in a Green Iguana (Iguana iguana): Diagnostic imaging for localisation.

INTRODUCTION: This case report includes different diagnostic imaging methods for localization of textile foreign bodies in reptiles and shows the limitations and advantages of these methods. A six-year-old, male, green iguana was presented to our clinic after ingesting a sock 5 days earlier. Ultrasound, contrast x-ray, computed tomography and endoscopy were used to locate the foreign body before surgery. Attempts to remove the sock endoscopically failed. The sock was surgically removed via celiotomy and enterotomy.

Characterization of an internal permissive site in the cholera toxin B-subunit and insertion of epitopes from human immunodeficiency virus-1, hepatitis B virus and enterotoxigenic Escherichia coli.

We previously described the construction of novel hybrid proteins based on the B-subunit of cholera toxin (CTB) [Bäckström et al., Gene 149 (1994) 211-217], in which a neutralizing B-cell epitope from the third variable (V3) loop in the envelope glycoprotein gp120 from human immunodeficiency virus type 1 (HIV-1) was inserted within a surface-exposed region between amino acids (aa) 55 and 64. The resulting protein retained properties of native CTB and could induce strong anti-CTB antibody (Ab) responses, but the inserted gp120 epitope was only modestly immunogenic. In this study, the potential use of this internal permissive site in CTB for the insertion of heterologous epitopes has been further investigated. Six additional plasmids were constructed encoding HIV::CTB hybrid proteins with ten to fourteen aa from the V3 loop of gp120 genetically inserted at different positions between aa 52 and 65, with deletions of different CTB aa. Plasmids encoding proteins with peptides inserted between aa 53 and 64 in CTB gave rise to stable proteins which reacted with CTB-specific monoclonal antibodies (mAb) and bound to GM1 gangliosides (GM1), indicating that insertions between these positions do not drastically alter the conformation or the receptor-binding properties of native CTB. Plasmids were also constructed encoding CTB hybrid proteins which had either an 11-aa peptide from hepatitis B virus (HBV) pre-S(2) or one of two peptides related to the heat-stable toxin (STa) of enterotoxigenic Escherichia coli inserted between aa 55 and 64 of CTB. This resulted in the production of HBV::CTB or ST::CTB hybrid proteins and illustrated that the internal permissive site can be used for insertion of peptides of varying aa composition. The reactivity of the inserted epitopes with epitope-specific mAb in GM1-ELISA and immunoblots varied greatly between hybrid proteins and depended on the position in CTB and the aa composition of the inserted peptides. Despite these differences, all the HIV::CTB, ST::CTB and HBV::CTB hybrid proteins could induce low, but significant, levels of serum Ab in mice against gp120, STa or pre-S(2), in addition to strong serum Ab responses against CTB. The Ab response against the internally inserted gp120 peptide was similar to that against the same peptide fused to the N-terminus of CTB, indicating that internally placed peptides had similar immunogenicity to the same peptides added terminally.

Insertion of a HIV-1-neutralizing epitope in a surface-exposed internal region of the cholera toxin B-subunit.

The non-toxic B-subunit of cholera toxin (CTB) is a powerful immunogen and has been investigated as a carrier for foreign peptide epitopes, with peptides genetically fused to either the N- or C terminus of CTB. In the present study, we have constructed a plasmid encoding a novel intrachain CTB fusion protein with a peptide epitope inserted into an internal region of CTB: eight amino acids (aa) in CTB (56-63) were substituted with a 10-aa peptide from the third variable (V3) loop of the HIV-1 envelope protein gp120. The resulting chimeric protein retained important functional characteristics of the native CTB including pentamerization and GM1 ganglioside receptor binding. The internal hybrid protein was also shown to be resistant to proteolytic degradation during production in Vibrio cholerae, whereas a terminal hybrid protein, where the same gp120-epitope was fused to the N terminus of CTB, was rapidly cleaved during culture. The inserted epitope, which is known to give rise to HIV-1 neutralizing Ab, could be detected with a V3 loop-specific monoclonal Ab when the chimeric protein was analyzed in ELISA and immunoblot, indicating that the epitope inserted at this site is presented on the surface of the protein. Consistent with these observations, immunization of mice with the CTB::HIV hybrid protein elicited a high titered serum Ab response to the CTB moiety and also, in some but not all animals, a detectable response to the inserted gp120 epitope.

Structure and arrangement of the cholera toxin genes in Vibrio cholerae O139.

The sequence of the ctxB gene encoding the B subunit of cholera toxin has been determined for a strain of Vibrio cholerae of the novel O139 serotype associated with recent outbreaks of severe cholera throughout South-East Asia and found to be identical to the ctxB gene in V. cholerae O1 of the El Tor biotype. Analyses by Southern hybridization and PCR showed that all strains of the O139 serotype V. cholerae tested carried cholera toxin genes and other genes associated with a virulence cassette DNA region at two loci identical or homologous to those identified in the Classical rather than the El Tor biotype of V. cholerae serotype O1 although these loci in O139 could reside on restriction fragments of variable size.

Large-scale production of Vibrio cholerae toxin B subunit for use in oral vaccines.

By systematically manipulating promoter and ribosome binding structures, plasmid copy number and the structure of the cholera toxin B (CTB) subunit gene, we were able to develop a plasmid expression system that, when used in conjunction with an optimized growth medium, provided yields of CTB approaching one gram per liter. The CTB protein which was secreted to > 95%, could readily be purified from the growth medium of a V. cholerae production strain and was shown to be immunologically indistinguishable from previously used vaccine preparations of native or recombinant CTB.

Mucosal vaccines based on the use of cholera toxin B subunit as immunogen and antigen carrier.

Stimulation of strong mucosal IgA immune responses as a basis for vaccine-induced protection against various pathogens has proved difficult. Most soluble protein antigens administered either parenterally or oral-mucosally have given disappointing results. A notable exception in this regard are cholera toxin (CT) and, particularly in humans, its non-toxic B subunit pentamer moiety (CTB) both of which stimulate a strong intestinal IgA antibody response and long-lasting immunological memory. Based on this, CTB has become an important component in recently developed oral vaccines against cholera and diarrhea caused by enterotoxigenic E. coli. The strong immunogenicity of CT and CTB can to a large extent be explained by their ability to bind to receptors on the intestinal mucosal surface. This has promoted much recent interest in the use of CTB as an oral delivery carrier for other vaccine-relevant antigens. Oral administration of antigens coupled to CTB either chemically or genetically has in several systems been found to markedly potentiate both intestinal and extra-intestinal IgA immune responses against the CTB-coupled antigens and to elicit substantial circulating antibody responses. In contrast to CTB, CT also has strong adjuvant properties for stimulating mucosal IgA immune responses to unrelated, non-coupled antigens after oral co-immunization. This adjuvant activity appears to be closely linked to the A subunit-catalyzed ADP-ribosylating action of CT leading to enhanced cyclic AMP formation in the affected cells.

Cloning and sequencing of Vibrio cholerae mannose-sensitive haemagglutinin pilin gene: localization of mshA within a cluster of type 4 pilin genes.

The mannose-sensitive haemagglutinin (MSHA) pilus that is associated with Vibrio cholerae strains of El Tor biotype has been shown to be a potential colonization factor and protective antigen. The gene encoding the structural subunit of MSHA pili was cloned from size-fractionated SacI-cleaved chromosomal DNA in the expression phage vector lambda ZAPII. Positive clones carried a c. 5.3 kb SacI fragment and were identified on the basis of MSHA expression and hybridization with a synthetic oligonucleotide probe based upon the N-terminus of MshA, the structural subunit of MSHA. The mshA gene was localized to a 2.6 kb SalI-EcoRI fragment, which was subcloned and shown to express MshA from its own promoter in Escherichia coli. Nucleotide sequencing of the entire fragment revealed six open reading frames (ORFs) of which four were complete. The mshA gene encodes an 18,094 Da prepilin protein, which in its mature form has a size of 17,436 Da. MshA is a type 4 (N-MePhe) pilin protein that is more homologous to pilins produced by Pseudomonas aeruginosa and Neisseria gonorrhoeae than to TcpA, the structural subunit of the toxin-coregulated pilus of V. cholerae. The protein seems to be directly involved in receptor binding, as an in-frame mutation in the mshA gene was found to abolish both D-mannose-dependent haemagglutination and binding of V. cholerae bacteria to D-mannose-containing agarose beads. Three additional ORFs, all in the same transcriptional orientation as mshA, were found to encode type 4 pilin-like proteins. A potential promoter with a sequence homologous to that of cAMP-CRP-activated promoters in E. coli was identified upstream of ORF3, the gene preceding mshA.

TOL plasmid pWW15 contains two nonhomologous, independently regulated catechol 2,3-oxygenase genes.

Pseudomonas putida MT15 contains a 250-kilobase-pair (kbp) TOL plasmid pWW15, encoding toluene and xylene catabolism, which undergoes large spontaneous deletions to give two classes of mutants with altered catabolic phenotypes (H. Keil and P. A. Williams, J. Gen. Microbiol, 131:1023-1033, 1985). Two structural genes for catechol 2,3-oxygenase (C23O) were cloned from pWW15. The gene for C23OI was located on the 2.1-kbp XhoI fragment Xh, whereas that for C23OII was found on the 11.5-kbp BamHI fragment BJ. The two restriction fragments and the subcloned regions of them showed no similarity in the pattern of restriction digestion, nor did they hybridize with each other. The substrate specificities of the two enzymes were also substantially different. The two structural genes were separated on pWW15 by about 100 kbp. In plasmid pWW15-510 of a B5 mutant, the 90-kbp deletion in the plasmid removed most of the intervening DNA, but it also deleted 80% of the gene for C23OI from its 3' end. Thus, only C23OII was expressed in the host MT15-510. Conversely, in RP4::pWW15 cointegrate plasmid pWW15-1003, only the C23OI gene was present. The expression of C23O activity from these two derivative plasmids and from the wild-type pWW15 showed that only C23OI was induced by growth in the presence of m-toluate, whereas both activities were induced in the only C23OI was induced by growth in the presence of m-toluate, whereas both activities were induced in the presence of m-xylene. These findings cast doubt on the earlier hypothesis that the deletions in B3 and B5 mutants remove a regulatory gene by which m-toluate induces the enzymes necessary for its own catabolism.

Cholera toxin B subunit as a carrier molecule promotes antigen presentation and increases CD40 and CD86 expression on antigen-presenting cells.

Cholera toxin B subunit (CTB) is an efficient mucosal carrier molecule for the generation of mucosal antibody responses and/or induction of systemic T-cell tolerance to linked antigens. CTB binds with high affinity to GM1 ganglioside cell surface receptors. In this study, we evaluated how conjugation of a peptide or protein antigen to CTB by chemical coupling or genetic fusion influences the T-cell-activating capacity of different antigen-presenting cell (APC) subsets. Using an in vitro system in which antigen-pulsed APCs were incubated with antigen-specific, T-cell receptor-transgenic T cells, we found that the dose of antigen required for T-cell activation could be decreased >10,000-fold using CTB-conjugated compared to free antigen. In contrast, no beneficial effects were observed when CTB was simply admixed with antigen. CTB conjugation enhanced the antigen-presenting capacity not only of dendritic cells and B cells but also of macrophages, which expressed low levels of cell surface major histocompatibility complex (MHC) class II and were normally poor activators of naive T cells. Enhanced antigen-presenting activity by CTB-linked antigen resulted in both increased T-cell proliferation and increased interleukin-12 and gamma interferon secretion and was associated with up-regulation of CD40 and CD86 on the APC surface. These results imply that conjugation to CTB dramatically lowers the threshold concentration of antigen required for immune cell activation and also permits low-MHC II-expressing APCs to prime for a specific immune response.

The role of different protein components from the Haemophilus ducreyi cytolethal distending toxin in the generation of cell toxicity.

Cytolethal distending toxin of Haemophilus ducreyi (HdCDT) is a multicomponent toxin, encoded by an operon consisting of three genes, cdtABC. To investigate the role of the individual products in generation of toxicity, recombinant plasmids were constructed allowing expression of each of the genes individually or in different combinations in Escherichia coli and Vibrio cholerae. Expression of all three genes (cdtABC) was necessary to generate toxicity on cells, and no activity was obtained using combinations in which only one or two of the genes were expressed. Of the individual gene products, the CdtA was shown to exist in two forms with an MW of 23 and 17 kDa, respectively. The CdtB protein alone resulted in DNase activity. CdtC purified from both toxic and non-toxic extracts (from strains expressing cdtCAB and cdtC, respectively) had a molecular weight of about 20 kDa and reacted with a CdtC-specific monoclonal antibody. However, the protein isoelectric point (pI) of CdtC from toxic preparations was about 1.5 pH units more basic than from non-toxic ones. Both forms were immunogenic giving rise to toxin-neutralizing antibodies. Toxicity was reconstructed by combining non-toxic cell sonicates from E. coli, expressing CdtA, CdtB and CdtC proteins individually. Only combinations including all three products gave toxicity, indicating that all are actively involved in the generation of toxic activity on cells. The reconstruction resulted in a 1.5 pH unit shift in the PI of CdtC, making it identical to that of the protein isolated from bacteria expressing cdtABC. The results showed that the CdtB component produces DNase activity, but cell toxicity depends on the involvement of the other two components of CDT and is associated with absorption of all three proteins by HEp-2 cells.

Synthesis of hybrid molecules between heat-labile enterotoxin and cholera toxin B subunits: potential for use in a broad-spectrum vaccine.

Three variants of the cholera toxin B subunit (CTB) were generated by site-specific mutagenesis in which regions of the mature protein were altered to the composition found at the corresponding positions of the closely related B subunit of the heat-labile enterotoxin of enterotoxigenic Escherichia coli (LTB). The mutant proteins were expressed in Vibrio cholerae and purified from the growth medium. In the first of the mutant proteins, the first 25 amino acids corresponded to the sequence found in LTB, and in the second, changes were made at positions 94 and 95 of the mature protein. The third mutant protein combined the changes made in the first two. Analysis of the immunological properties of these novel proteins by using monoclonal antibodies and absorbed polyclonal antiserum demonstrated that they had acquired LTB-specific epitopes. Immunizations with the mutant proteins resulted in antisera containing LTB-specific as well as CTB-specific and cross-reactive antibodies. The sera were also found to be more strongly cross-reactive in the in vitro neutralization of both cholera toxin and heat-labile enterotoxin than were antisera raised against either CTB or LTB. The results suggest that such hybrid CTB-LTB proteins may be useful in a broad-spectrum vaccine against enterotoxin-induced diarrhea.

Novel carbohydrate binding site recognizing blood group A and B determinants in a hybrid of cholera toxin and Escherichia coli heat-labile enterotoxin B-subunits.

The B-subunits of cholera toxin (CTB) and Escherichia coli heat-labile enterotoxin (LTB) are structurally and functionally related. However, the carbohydrate binding specificities of the two proteins differ. While both CTB and LTB bind to the GM1 ganglioside, LTB also binds to N-acetyllactosamine-terminated glycoconjugates. The structural basis of the differences in carbohydrate recognition has been investigated by a systematic exchange of amino acids between LTB and CTB. Thereby, a CTB/LTB hybrid with a gain-of-function mutation resulting in recognition of blood group A and B determinants was obtained. Glycosphingolipid binding assays showed a specific binding of this hybrid B-subunit, but not CTB or LTB, to slowly migrating non-acid glycosphingolipids of human and animal small intestinal epithelium. A binding-active glycosphingolipid isolated from cat intestinal epithelium was characterized by mass spectrometry and proton NMR as GalNAcalpha3(Fucalpha2)Galbeta4(Fucalpha3)Glc NAcbeta3Galbeta4Glc NAcbeta3Galbeta4Glcbeta1Cer. Comparison with reference glycosphingolipids showed that the minimum binding epitope recognized by the CTB/LTB hybrid was Galalpha3(Fucalpha2)Galbeta4(Fucalpha3)GlcNAc beta. The blood group A and B determinants bind to a novel carbohydrate binding site located at the top of the B-subunit interfaces, distinct from the GM1 binding site, as found by docking and molecular dynamics simulations.

Structural basis for differential receptor binding of cholera and Escherichia coli heat-labile toxins: influence of heterologous amino acid substitutions in the cholera B-subunit.

The closely related B-subunits of cholera toxin (CTB) and Escherichia coli heat-labile enterotoxin (LTB) both bind strongly to GM1 ganglioside receptors but LTB can also bind to additional glycolipids and glycoproteins. A number of mutant CT B-subunits were generated by substituting CTB amino acids with those at the corresponding positions in LTB. These were used to investigate the influence of specific residues on receptor-binding specificity. A mutated CTB protein containing the first 25 residues of LTB in combination with LTB residues at positions 94 and 95, bound to the same extent as native LTB to both delipidized rabbit intestinal cell membranes, complex glycosphingolipids (polyglycosylceramides) and neolactotetraosylceramide, but not to non-GM1 intestinal glycosphingolipids. In contrast, when LTB amino acid substitutions in the 1-25 region were combined with those in the 75-83 region, a binding as strong as that of LTB to intestinal glycosphingolipids was observed. In addition, a mutant LTB with a single Gly-33-->Asp substitution that completely lacked affinity for both GM1 and non-GM1 glycosphingolipids could still bind to receptors in the intestinal cell membranes and to polyglycosylceramides. We conclude that the extra, non-GM1 receptors for LTB consist of both sialylated and non-sialylated glycoconjugates, and that the binding to either class of receptors is influenced by different amino acid residues within the protein.