Murine antibody responses to the verotoxin 1 B subunit: demonstration of major histocompatibility complex dependence and an immunodominant epitope involving phenylalanine 30

Shiga Toxins as Antitumor Tools

Shiga toxins (Stxs), also known as Shiga-like toxins (SLT) or verotoxins (VT), constitute a family of structurally and functionally related cytotoxic proteins produced by the enteric pathogens Shigella dysenteriae type 1 and Stx-producing Escherichia coli (STEC). Infection with these bacteria causes bloody diarrhea and other pathological manifestations that can lead to HUS (hemolytic and uremic syndrome). At the cellular level, Stxs bind to the cellular receptor Gb3 and inhibit protein synthesis by removing an adenine from the 28S rRNA. This triggers multiple cellular signaling pathways, including the ribotoxic stress response (RSR), unfolded protein response (UPR), autophagy and apoptosis. Stxs cause several pathologies of major public health concern, but their specific targeting of host cells and efficient delivery to the cytosol could potentially be exploited for biomedical purposes. Moreover, high levels of expression have been reported for the Stxs receptor, Gb3/CD77, in Burkitt's lymphoma (BL) cells and on various types of solid tumors. These properties have led to many attempts to develop Stxs as tools for biomedical applications, such as cancer treatment or imaging, and several engineered Stxs are currently being tested. We provide here an overview of these studies.

Mucosal Immune Responses to Escherichia coli and Salmonella Infections

The best-characterized mucosa-associated lymphoid tissue (MALT), and also the most relevant for this review, is the gastrointestinal-associated lymphoid tissue (GALT). The review reviews our understanding of the importance of mucosal immune responses in resisting infections caused by E. coli and Salmonella spp. It focuses on the major human E. coli infections and discusses whether antigen-specific mucosal immune responses are important for resistance against primary infection or reinfection by pathogenic E. coli. It analyzes human data on mucosal immunity against E. coli, a growing body of data of mucosal responses in food production animals and other natural hosts of E. coli, and more recent experimental studies in mice carrying defined deletions in genes encoding specific immunological effectors, to show that there may be considerable conservation of the effective host mucosal immune response against this pathogen. The species Salmonella enterica contains a number of serovars that include pathogens of both humans and animals; these bacteria are frequently host specific and may cause different diseases in different hosts. Ingestion of various Salmonella serovars, such as Typhimurium, results in localized infections of the small intestine leading to gastroenteritis in humans, whereas ingestion of serovar Typhi results in systemic infection and enteric fever. Serovar Typhi infects only humans, and the review discusses the mucosal immune responses against serovar Typhi, focusing on the responses in humans and in the mouse typhoid fever model.

Shiga toxins

Shiga toxins are virulence factors produced by the bacteria Shigella dysenteriae and certain strains of Escherichia coli. There is currently no available treatment for disease caused by these toxin-producing bacteria, and understanding the biology of the Shiga toxins might be instrumental in addressing this issue. In target cells, the toxins efficiently inhibit protein synthesis by inactivating ribosomes, and they may induce signaling leading to apoptosis. To reach their cytoplasmic target, Shiga toxins are endocytosed and transported by a retrograde pathway to the endoplasmic reticulum, before the enzymatically active moiety is translocated to the cytosol. The toxins thereby serve as powerful tools to investigate mechanisms of intracellular transport. Although Shiga toxins are a serious threat to human health, the toxins may be exploited for medical purposes such as cancer therapy or imaging.

Tumor-specific targeting of pancreatic cancer with Shiga toxin B-subunit

Pancreatic carcinoma is one of the most aggressive tumor entities, and standard chemotherapy provides only modest benefit. Therefore, specific targeting of pancreatic cancer for early diagnosis and therapeutic intervention is of great interest. We have previously shown that the cellular receptor for Shiga toxin B (STxB), the glycosphingolipid globotriaosylceramide (Gb(3) or CD77) is strongly increased in colorectal adenocarcinoma and their metastases. Here, we report an upregulation of Gb(3) in pancreatic adenocarcinoma (21 of 27 cases) as compared with matched normal tissue (n = 27). The mean expression was highly significantly increased from 30 ± 16 ng Gb(3)/mg tissue in normal pancreas to 61 ± 41 ng Gb(3)/mg tissue (mean ± SD, P = 0.0006), as evidenced by thin layer chromatography. Upregulation of Gb(3) levels did not depend on tumor stage or grading and showed no correlation with clinical outcome. Tumor cells and endothelial cells were identified as the source of increased Gb(3) expression by immunocytochemistry. Pancreatic cancer cell lines showed rapid intracellular uptake of STxB to the Golgi apparatus, following the retrograde pathway. The therapeutic application of STxB was tested by specific delivery of covalently coupled SN38, an active metabolite of the topoisomerase I inhibitor irinotecan. The cytotoxic effect of the STxB-SN38 compound in pancreatic cancer cell lines was increased more than 100-fold compared with irinotecan. Moreover, this effect was effectively blocked by competing incubation with nonlabeled STxB, showing the specificity of the targeting. Thus, STxB constitutes a promising new tool for specific targeting of pancreatic cancer.

Verotoxin-1 treatment or manipulation of its receptor globotriaosylceramide (gb3) for reversal of multidrug resistance to cancer chemotherapy

A major problem with anti-cancer drug treatment is the development of acquired multidrug resistance (MDR) of the tumor cells. Verotoxin-1 (VT-1) exerts its cytotoxicity by targeting the globotriaosylceramide membrane receptor (Gb3), a glycolipid associated with multidrug resistance. Gb3 is overexpressed in many human tumors and tumor cell lines with inherent or acquired MDR. Gb3 is co-expressed and interplays with the membrane efflux transporter P-gp encoded by the MDR1 gene. P-gp could act as a lipid flippase and stimulate Gb3 induction when tumor cells are exposed to cancer chemotherapy. Recent work has shown that apoptosis and inherent or acquired multidrug resistance in Gb3-expressing tumors could be affected by VT-1 holotoxin, a sub-toxic concentration of the holotoxin concomitant with chemotherapy or its Gb3-binding B-subunit coupled to cytotoxic or immunomodulatory drug, as well as chemical manipulation of Gb3 expression. The interplay between Gb3 and P-gp thus gives a possible physiological approach to augment the chemotherapeutic effect in multidrug resistant tumors.

Human colorectal tumors and metastases express Gb3 and can be targeted by an intestinal pathogen-based delivery tool

The targeting of solid tumors requires delivery tools that resist intracellular and extracellular inactivation, and that are taken up specifically by tumor cells. We have shown previously that the recombinant nontoxic B-subunit of Shiga toxin (STxB) can serve as a delivery tool to target digestive tumors in animal models. The aim of this study was to expand these experiments to human colorectal cancer. Tissue samples of normal colon, benign adenomas, colorectal carcinomas, and liver metastases from 111 patients were obtained for the quantification of the expression of the cellular STxB receptor, the glycosphingolipid globotriaosyl ceramide (Gb(3) or CD77). We found that compared with normal tissue, the expression of Gb(3) was strongly increased in colorectal adenocarcinomas and their metastases, but not in benign adenomas. Short-term primary cultures were prepared from samples of 43 patients, and STxB uptake was studied by immunofluorescence microscopy. Of a given tumor sample, on average, 80% of the cells could visibly bind STxB, and upon incubation at 37 degrees C, STxB was transported to the Golgi apparatus, following the retrograde route. This STxB-specific intracellular targeting allows the molecule to avoid recycling and degradation, and STxB could consequently be detected on tumor cells even 5 days after initial uptake. In conclusion, the targeting properties of STxB could be diverted for the delivery of contrast agents to human colorectal tumors and their metastases, whose early detection and specific targeting remains one of the principal challenges in oncology.

Protein toxins: intracellular trafficking for targeted therapy

The immunotoxin approach is based on the use of tumor-targeting ligands or antibodies that are linked to the catalytic (toxic) moieties of bacterial or plant protein toxins. In this review, we first discuss the current state of clinical development of immunotoxin approaches describing the results obtained with the two toxins most frequently used: diphtheria and Pseudomonas toxin-derived proteins. In the second part of the review, a novel concept will be presented in which the roles are inverted: nontoxic receptor-binding toxin moieties are used for the targeting of therapeutic and diagnostic compounds to cancer or immune cells. The cell biological basis of these novel types of toxin-based therapeutics will be discussed, and we will summarize ongoing preclinical and clinical testing.

Production of IgA monoclonal antibody against Shiga toxin binding subunits employing nasal-associated lymphoid tissue

We established an IgA monoclonal antibody (mAb) against Shiga toxin 1 B subunits (Stx1B) from mouse nasal-associated lymphoid tissues (NALT) of BALB/c mice. We have developed an improved protocol in which cross-linked Stx1B is intranasally administered together with cholera toxin. Surface IgA-positive NALT lymphocytes from mice immunized in this manner were enriched and then fused with mouse myeloma cells to produce hybridoma cells. Hybridoma culture supernatants were examined to see if they contain IgA against Stx1B and if they can inhibit carbohydrate recognition by Stx1B. For the latter purpose, we prepared carbohydrate ligands in which globotriose is present on the poly-lysine backbone. The established IgA mAb exhibited saturable and dose-dependent binding to the immobilized Stx1B. Inversely, the binding of the carbohydrate ligands to the immobilized Stx1B was inhibited by the mAb pretreatment. Immunoblotting and SDS-PAGE analysis revealed dimeric IgA. The IgA mAb inhibited the binding of digoxigenin-conjugated Stx1B to natural ligands displayed on a Burkitt's lymphoma cell line, Ramos. These results suggested that surface IgA-positive B cells in the inductive sites of the mucosal immune system in the upper respiratory tract are a potent source for producing IgA mAb against protein antigens with weak immunogenicity such as Stx1B.

Facilitated production of secretory IgA against Shiga toxin B subunits by intranasal application of antigen-coated polystyrene microspheres

We examined the effects of microspheres as antigen carriers in mucosal immunization. Shiga toxin B subunits (Stx1B) were adsorbed on 6 mum polystyrene microspheres, which were then intranasally administered to mice together with cholera toxin (CT). Stx1B-specific serum IgG production and secretory IgA production at local mucosal sites were enhanced by the use of microspheres. When OVA was used as a model antigen, secretory IgA production but not serum IgG production was enhanced on the use of microspheres. These results indicated that microspheres provide a useful means of potentiating the immune response against Stx1B with weak immunogenicity.

Production of secretory immunoglobulin A against Shiga toxin-binding subunits in mice by mucosal immunization

The toxicity of Shiga toxins (Stx) depends on the binding of their B subunits to carbohydrate ligands on host cells. The production of antibodies against B subunits, especially immunoglobulin A (IgA) secreted on the mucosal surface, should contribute to host defense. One of the major problems in attempts to produce IgA against Stx was the poor immunogenicity of B subunits. We were able to produce serum IgA as well as IgG against Stx1B in mice of the H-2d haplotype by means of intranasal immunization with recombinant B subunits of Stx (Stx1B) together with cholera toxin as a mucosal adjuvant. Secretory IgA (S-IgA) was detected in nasal washes but not in feces. We prepared chemically cross-linked Stx1B for use as an immunogen, and the formation of stable oligomers was revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and mass spectrometry. When the cross-linked Stx1B was used together with cholera toxin for the intranasal immunization of BALB/c mice, strong enhancement of the immune response was observed. The S-IgA titers in nasal washes were 16- to more than 64-fold higher than those in mice immunized with native Stx1B plus cholera toxin. Furthermore, fecal IgA was detectable when the cross-linked Stx1B was used. The use of cholera toxin was necessary for the induction of high titers of S-IgA in the nasal washes. However, the effect of cross-linking was dependent on the major histocompatibility complex haplotype; that is, no enhancement of IgA production was observed in C57BL/6 mice. The present results provide a practical means of producing IgA against Stx1B in BALB/c mice.

1st class ticket to class I: protein toxins as pathfinders for antigen presentation

A number of bacterial toxins have evolved diverse strategies for crossing membrane barriers in order to reach their substrates in the mammalian cytosol. Recent studies show that this property can be exploited for the delivery of fused antigens into the major histocompatibility complex class I-restricted presentation pathway, with the goal of eliciting a specific immune response. Here we discuss the peculiarities of the trafficking pathways of a variety of toxins, and how these may allow the toxins to be used as delivery vehicles for therapeutic and diagnostic purposes.

Mouse toxicity and cytokine release by verotoxin 1 B subunit mutants

The crystal structure of the verotoxin 1 (VT1) B subunit complexed with a globotriaosylceramide (Gb(3)) analogue showed the presence of three receptor binding sites per monomer. We wished to study the effects of altering the three sites, singly or in combination, on animal toxicity and cytokine induction in vitro. We found that while the site 1 and 2 mutants were modestly (two- to sevenfold) reduced in their ability to cause disease in BALB/c mice, the site 3 mutant, W34A, was as toxic as VT1. However, all the double-mutant proteins, irrespective of which two sites were mutated, exhibited approximately a 100-fold reduction in their 50% lethal doses for mice. These results suggest that multivalent receptor binding is important in vivo and that all three binding sites make a similar contribution to the latter process. The triple-mutant holotoxin, F30A G62T W34A, administered intraperitoneally without adjuvant, stimulated a strong antibody response in BALB/c mice, and the immune sera neutralized the activity of VT1 in vitro. Induction of tumor neurosis factor alpha release from differentiated human monocytes (THP-1 cells) was relatively impaired for site 1 and site 2 but not site 3 mutants, suggesting an auxiliary role for the latter site in mediation of cytokine release in vitro. Cytotoxicity assays on undifferentiated THP-1 cells have also demonstrated the importance of sites 1 and 2 and the relatively small role played by site 3 in causing cell death. These data suggest an association between the cytotoxicity of the protein and its ability to induce cytokine release.

Antiviral activity of shiga toxin 1: suppression of bovine leukemia virus-related spontaneous lymphocyte proliferation

Human infections with Shiga toxin (Stx)-producing Escherichia coli (STEC) cause hemorrhagic colitis. The Stxs belong to a large family of ribosome-inactivating proteins (RIPs) that are found in a variety of higher plants and some bacteria. Many RIPs have potent antiviral activity for the plants that synthesize them. STEC strains, both virulent and nonvirulent to humans, are frequently isolated from healthy cattle. Interestingly, despite intensive investigations, it is not known why cattle carry STEC. We tested the hypothesis that Stx has antiviral properties for bovine viruses by assessing the impact of Stx type 1 (Stx1) on bovine peripheral blood mononuclear cells (PBMC) from cows infected with bovine leukemia virus (BLV). PBMC from BLV-positive animals invariably displayed spontaneous lymphocyte proliferation (SLP) in vitro. Stx1 or the toxin A subunit (Stx1A) strongly inhibited SLP. Toxin only weakly reduced the pokeweed mitogen- or interleukin-2-induced proliferation of PBMC from normal (BLV-negative) cows and had no effect on concanavalin A-induced proliferation. The toxin activity in PBMC from BLV-positive cattle was selective for viral SLP and did not abrogate cell response to pokeweed mitogen- or interleukin-2-induced proliferation. Antibody to virus or Stx1A was most effective at inhibiting SLP if administered at the start of cell culture, indicating that both reagents likely interfere with BLV-dependent initiation of SLP. Stx1A inhibited expression of BLV p24 protein by PBMC. A well-defined mutant Stx1A (E167D) that has decreased catalytic activity was not effective at inhibiting SLP, suggesting the inhibition of protein synthesis is likely the mechanism of toxin antiviral activity. Our data suggest that Stx has potent antiviral activity and may serve an important role in BLV-infected cattle by inhibiting BLV replication and thus slowing the progression of infection to its malignant end stage.

Development of recombinant B subunit of Shiga-like toxin 1 as a probe to detect carbohydrate ligands in immunochemical and flowcytometric application

Measurement of carbohydrate binding activity of Escherichia coli Shiga-like toxin in a simple and quantitative way is an important step for evaluation of antibodies with therapeutic value and of effectiveness of vaccine treatment. We constructed a plasmid vector (pVT1-B5) to express carbohydrate binding (B) subunit of Shiga-like toxin 1 without expression of toxic (A) subunit, and established a simple method to purify the recombinant B subunit, which was then labeled with digoxigenin. The binding specificity of the digoxigenin-labeled B subunit for globotriaosylceramide was established by thin-layer chromatography immunostaining. We developed an enzyme-linked immunosorbent assay using immobilized glycolipids, demonstrating high sensitivity and clear-cut specificity of the assay. The digoxigenin-labeled B subunit was also readily applicable to the detection of cell surface carbohydrate ligands by flow cytometry.

The identification of three biologically relevant globotriaosyl ceramide receptor binding sites on the Verotoxin 1 B subunit

The Verotoxin 1 (VT1) B subunit binds to the glycosphingolipid receptor globotriaosylceramide (Gb3). Receptor-binding specificity is associated with the terminally linked Galalpha(1-4) Galbeta disaccharide sequence of the receptor. Recently, three globotriose (Galalpha[1-4] Galbeta [1-4] Glcbeta) binding sites per B-subunit monomer were identified by crystallography. Two of these sites (sites I and II) are located adjacent to phenylalanine-30. Site I was originally predicted as a potential Gb3 binding site on the basis of sequence conservation, and site II was additionally predicted based on computer modelling and receptor docking. The third (site III) was also identified by crystallography and is located at the N-terminal end of the alpha-helix. To determine the biological significance of sites II and III, and to support our previous findings of the significance of site I, we examined the binding properties and cytotoxicity of VT1 mutants designed to block Gb3 binding at each site selectively. The Scatchard analysis of saturation-binding data for each mutant revealed that only the amino acid substitutions predicted to affect site I (D-17E) or site II (G-62T) caused reductions in the binding affinity and capacity of VT1 for Gb3. Similarly, those mutations at sites I and II also caused significant reductions in both Vero and MRC-5 cell cytotoxicity (by seven and five logs, respectively, for G-62T and by four and two logs, respectively, for D-17E). In contrast, the substitution of alanine for W-34 at site III did not reduce the high-affinity binding of the B subunit, despite causing a fourfold reduction in the receptor-binding capacity. The corresponding mutant W-34A holotoxin had a two-log reduction in cytotoxicity on Vero cells and no statistically significant reduction on MRC-5 cells. We conclude that the high-affinity receptor binding most relevant for cell cytotoxicity occurs at sites I and II. In contrast, site III appears to mediate the recognition of additional Gb3 receptor epitopes but with lower affinity. Our results support the significance of the indole ring of W-34 for binding at this site.

Bacterial infections of the small intestine and colon

Enterotoxigenic Escherichia coli, Vibrio cholerae O1, Campylobacter jejuni, Salmonella species, and Shigella species are major causes of morbidity and death in diarrheal disease. More recently recognized pathogens are V. cholerae O139 and enterohemorrhagic E. coli. In addition to this, several presumptive virulence factors have been identified in diarrheagenic E. coli and in other species. To confirm these as virulence factors we need good diagnostic tools and good epidemiological studies. These are of vital importance to create vaccines for diarrheal diseases.