Exploiting retrograde transport of Shiga-like toxin 1 for the delivery of exogenous antigens into the MHC class I presentation pathway

Therapeutic Uses of Bacterial Subunit Toxins

The B subunit pentamer verotoxin (VT aka Shiga toxin-Stx) binding to its cellular glycosphingolipid (GSL) receptor, globotriaosyl ceramide (Gb₃) mediates internalization and the subsequent receptor mediated retrograde intracellular traffic of the AB5 subunit holotoxin to the endoplasmic reticulum. Subunit separation and cytosolic A subunit transit via the ER retrotranslocon as a misfolded protein mimic, then inhibits protein synthesis to kill cells, which can cause hemolytic uremic syndrome clinically. This represents one of the most studied systems of prokaryotic hijacking of eukaryotic biology. Similarly, the interaction of cholera AB5 toxin with its GSL receptor, GM1 ganglioside, is the key component of the gastrointestinal pathogenesis of cholera and follows the same retrograde transport pathway for A subunit cytosol access. Although both VT and CT are the cause of major pathology worldwide, the toxin–receptor interaction is itself being manipulated to generate new approaches to control, rather than cause, disease. This arena comprises two areas: anti neoplasia, and protein misfolding diseases. CT/CTB subunit immunomodulatory function and anti-cancer toxin immunoconjugates will not be considered here. In the verotoxin case, it is clear that Gb₃ (and VT targeting) is upregulated in many human cancers and that there is a relationship between GSL expression and cancer drug resistance. While both verotoxin and cholera toxin similarly hijack the intracellular ERAD quality control system of nascent protein folding, the more widespread cell expression of GM1 makes cholera the toxin of choice as the means to more widely utilise ERAD targeting to ameliorate genetic diseases of protein misfolding. Gb₃ is primarily expressed in human renal tissue. Glomerular endothelial cells are the primary VT target but Gb₃ is expressed in other endothelial beds, notably brain endothelial cells which can mediate the encephalopathy primarily associated with VT2-producing E. coli infection. The Gb₃ levels can be regulated by cytokines released during EHEC infection, which complicate pathogenesis. Significantly Gb₃ is upregulated in the neovasculature of many tumours, irrespective of tumour Gb₃ status. Gb₃ is markedly increased in pancreatic, ovarian, breast, testicular, renal, astrocytic, gastric, colorectal, cervical, sarcoma and meningeal cancer relative to the normal tissue. VT has been shown to be effective in mouse xenograft models of renal, astrocytoma, ovarian, colorectal, meningioma, and breast cancer. These studies are herein reviewed. Both CT and VT (and several other bacterial toxins) access the cell cytosol via cell surface ->ER transport. Once in the ER they interface with the protein folding homeostatic quality control pathway of the cell -ERAD, (ER associated degradation), which ensures that only correctly folded nascent proteins are allowed to progress to their cellular destinations. Misfolded proteins are translocated through the ER membrane and degraded by cytosolic proteosome. VT and CT A subunits have a C terminal misfolded protein mimic sequence to hijack this transporter to enter the cytosol. This interface between exogenous toxin and genetically encoded endogenous mutant misfolded proteins, provides a new therapeutic basis for the treatment of such genetic diseases, e.g., Cystic fibrosis, Gaucher disease, Krabbe disease, Fabry disease, Tay-Sachs disease and many more. Studies showing the efficacy of this approach in animal models of such diseases are presented.

Endoplasmic Reticulum-Targeted Subunit Toxins Provide a New Approach to Rescue Misfolded Mutant Proteins and Revert Cell Models of Genetic Diseases

Many germ line diseases stem from a relatively minor disturbance in mutant protein endoplasmic reticulum (ER) 3D assembly. Chaperones are recruited which, on failure to correct folding, sort the mutant for retrotranslocation and cytosolic proteasomal degradation (ER-associated degradation-ERAD), to initiate/exacerbate deficiency-disease symptoms. Several bacterial (and plant) subunit toxins, retrograde transport to the ER after initial cell surface receptor binding/internalization. The A subunit has evolved to mimic a misfolded protein and hijack the ERAD membrane translocon (dislocon), to effect cytosolic access and cytopathology. We show such toxins compete for ERAD to rescue endogenous misfolded proteins. Cholera toxin or verotoxin (Shiga toxin) containing genetically inactivated (± an N-terminal polyleucine tail) A subunit can, within 2–4 hrs, temporarily increase F508delCFTR protein, the major cystic fibrosis (CF) mutant (5-10x), F508delCFTR Golgi maturation (<10x), cell surface expression (20x) and chloride transport (2x) in F508del CFTR transfected cells and patient-derived F508delCFTR bronchiolar epithelia, without apparent cytopathology. These toxoids also increase glucocerobrosidase (GCC) in N370SGCC Gaucher Disease fibroblasts (3x), another ERAD–exacerbated misfiling disease. We identify a new, potentially benign approach to the treatment of certain genetic protein misfolding diseases.

Oral absorption of peptides and nanoparticles across the human intestine: Opportunities, limitations and studies in human tissues

In this contribution, we review the molecular and physiological barriers to oral delivery of peptides and nanoparticles. We discuss the opportunities and predictivity of various in vitro systems with special emphasis on human intestine in Ussing chambers. First, the molecular constraints to peptide absorption are discussed. Then the physiological barriers to peptide delivery are examined. These include the gastric and intestinal environment, the mucus barrier, tight junctions between epithelial cells, the enterocytes of the intestinal epithelium, and the subepithelial tissue. Recent data from human proteome studies are used to provide information about the protein expression profiles of the different physiological barriers to peptide and nanoparticle absorption. Strategies that have been employed to increase peptide absorption across each of the barriers are discussed. Special consideration is given to attempts at utilizing endogenous transcytotic pathways. To reliably translate in vitro data on peptide or nanoparticle permeability to the in vivo situation in a human subject, the in vitro experimental system needs to realistically capture the central aspects of the mentioned barriers. Therefore, characteristics of common in vitro cell culture systems are discussed and compared to those of human intestinal tissues. Attempts to use the cell and tissue models for in vitro-in vivo extrapolation are reviewed.

Modular Three-component Delivery System Facilitates HLA Class I Antigen Presentation and CD8(+) T-cell Activation Against Tumors

Cell-mediated immunotherapies have potential as stand-alone and adjuvant therapies for cancer. However, most current protocols suffer from one or more of three major issues: cost, safety, or efficacy. Here we present a nanoparticle delivery system that facilitates presentation of an immunogenic measles antigen specifically in cancer cells. The delivery system does not contain viral particles, toxins, or biologically derived material. Treatment with this system facilitates activation of a secondary immune response against cancer cells, bypassing the need to identify tumor-associated antigens or educate the immune system through a primary immune response. The delivery system consists of a stealth liposome displaying a cancer-specific targeting peptide, named H1299.3, on its exterior surface and encapsulating H250, an immunogenic human leukocyte antigen class 1 restricted peptide. This targeted-nanoparticle facilitates presentation of the H250 peptide in major histocompatibility complex class I molecules. Activation is dependent on the targeting peptide, previous antigen exposure, and utilizes a novel autophagy-mediated mechanism to facilitate presentation. Treatment with this liposome results in a significant reduction of tumor growth using an aggressive LLC1 model in vaccinated C57BL/6 mice. These data provide proof-of-principle for a novel cell-mediated immunotherapy that is scalable, contains no biologically derived material, and is an efficacious cancer therapy.

Ribosome-inactivating proteins with an emphasis on bacterial RIPs and their potential medical applications

Ribosome-inactivating proteins (RIPs) are toxic due to their N-glycosidase activity catalyzing depurination at the universally conserved α-sarcin loop of the 60S ribosomal subunit. In addition, RIPs have been shown to also have other enzymatic activities, including polynucleotide:adenosine glycosidase activity. RIPs are mainly produced by different plant species, but are additionally found in a number of bacteria, fungi, algae and some mammalian tissues. This review describes the occurrence of RIPs, with special emphasis on bacterial RIPs, including the Shiga toxin and RIP in Streptomyces coelicolor recently identified in S. coelicolor. The properties of RIPs, such as enzymatic activity and targeting specificity, and how their unique biological activity could be potentially turned into medical or agricultural tools to combat tumors, viruses and fungi, are highlighted.

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.

Tracing the retrograde route in protein trafficking

Retrograde transport, in which proteins and lipids are shuttled between endosomes and biosynthetic/secretory compartments such as the Golgi apparatus, is crucial for a diverse range of cellular functions. Mechanistic studies that explore the molecular machinery involved in this retrograde trafficking route are shedding light on the functions of transport proteins and are providing fresh insights into possible new therapeutic directions.

Bacterial toxins as immunomodulators

Bacterial toxins are the causative agent at pathology in a variety of diseases. Although not always the primary target of these toxins, many have been shown to have potent immunomodulatory effects, for example, inducing immune responses to co-administered antigens and suppressing activation of immune cells. These abilities of bacterial toxins can be harnessed and used in a therapeutic manner, such as in vaccination or the treatment of autoimmune diseases. Furthermore, the ability of toxins to gain entry to cells can be used in novel bacterial toxin based immuno-therapies in order to deliver antigens into MHC Class I processing pathways. Whether the immunomodulatory properties of these toxins arose in order to enhance bacterial survival within hosts, to aid spread within the population or is pure serendipity, it is interesting to think that these same toxins potentially hold the key to preventing or treating human disease.

Human heat shock protein 70 enhances tumor antigen presentation through complex formation and intracellular antigen delivery without innate immune signaling

Heat shock proteins (HSPs) have shown promise for the optimization of protein-based vaccines because they can transfer exogenous antigens to dendritic cells and at the same time induce their maturation. Great care must be exercised in interpretating HSP-driven studies, as by-products linked to the recombinant generation of these proteins have been shown to mediate immunological effects. We generated highly purified human recombinant Hsp70 and demonstrated that it strongly enhances the cross-presentation of exogenous antigens resulting in better antigen-specific T cell stimulation. Augmentation of T cell stimulation was a direct function of the degree of complex formation between Hsp70 and peptides and correlated with improved antigen delivery to endosomal compartments. The Hsp70 activity was independent of TAP proteins and was not inhibited by exotoxin A or endosomal acidification. Consequently, Hsp70 enhanced cross-presentation of various antigenic sequences, even when they required different post-uptake processing and trafficking, as exemplified by the tumor antigens tyrosinase and Melan-A/MART-1. Furthermore, Hsp70 enhanced cross-presentation by different antigen-presenting cells (APCs), including dendritic cells and B cells. Importantly, enhanced cross-presentation and antigen-specific T cell activation were observed in the absence of innate signals transmitted by Hsp70. As Hsp70 supports the cross-presentation of different antigens and APCs and is inert to APC function, it may show efficacy in various settings of immune modulation, including induction of antigen-specific immunity or tolerance.

Noncovalent Shiga-like Toxin Assemblies: Characterization by Means of Mass Spectrometry and Tandem Mass Spectrometry

Shiga-like toxin 1 (SLTx), produced by enterohemorrhagic strains of Escherichia coli (EHEC), belongs to a family of structurally and functionally related AB(5) protein toxins that are associated with human disease. EHEC infection often gives rise to hemolytic colitis, while toxin-induced kidney damage is one of the major causes of hemolytic uremic syndrome (HUS) and acute renal failure in children. As such, an understanding and analysis of the noncovalent interactions that maintain the quaternary structure of this toxin are fundamentally important since such interactions have significant biochemical and medical implications. This paper reports on the analysis of the noncovalent homopentameric complex of Shiga-like toxin B chain (SLTx-B(5)) using electrospray ionization (ESI) triple-quadrupole (QqQ) mass spectrometry (MS) and tandem mass spectrometry (MS/MS) and the analysis of the noncovalent hexameric holotoxin (SLTx-AB(5)) using ESI time-of-flight (TOF) MS. The triple-quadrupole analysis revealed highly charged monomer ions dissociate from the multiprotein complex to form dimer, trimer, and tetramer product ions, which were also seen to further dissociate. The ESI-TOFMS analysis of SLTx-AB(5) revealed the complex remained intact and was observed in the gas phase over a range of pHs. Theses findings demonstrate that the gas-phase structure observed for both the holotoxin and the isoloated B chains correlates well with the structures reported to exist in the solution phase for these proteins. Such analysis provides a rapid screening technique for assessing the noncovalent structure of this family of proteins and other structurally related toxins.

Pathways followed by protein toxins into cells

A number of protein toxins have an enzymatically active part, which is able to modify a cytosolic target. Some of these toxins, for instance ricin, Shiga toxin and cholera toxin, which we will focus on in this article, exert their effect on cells by first binding to the cell surface, then they are endocytosed, and subsequently they are transported retrogradely all the way to the ER before translocation of the enzymatically active part to the cytosol. Thus, studies of these toxins can provide information about pathways of intracellular transport. Retrograde transport to the Golgi and the ER seems to be dependent not only on different Rab and SNARE proteins, but also on cytosolic calcium, phosphatidylinositol 3-kinase and cholesterol. Comparison of the three toxins reveals differences indicating the presence of more than one pathway between early endosomes and the Golgi apparatus or, alternatively, that transport of different toxin-receptor complexes present in a certain subcompartment is differentially regulated.

New tools for antigen delivery to the MHC class I pathway

At the beginning of this new millennium, pathogens and cancer remain the leading causes of death worldwide. The development of vaccines to prevent diseases for which no vaccine currently exists, such as AIDS or malaria, or to treat chronic infections or cancers, as well as the improvement of efficacy and safety of existing vaccines, remains a high priority. In most cases, the development of such vaccines requires strategies capable of stimulating CD8(+) cytotoxic T lymphocytes (CTLs) and thus, to deliver antigen to MHC class I molecules. There exists several different pathways for loading antigenic peptides onto MHC class I molecules, either based on the endogenous cytosolic MHC I pathway or on cross-presentation. The understanding of the relevance of each of these mechanisms in CTL activation will help vaccine design to progress more rationally.

Membrane traffic exploited by protein toxins

A large number of protein toxins having enzymatically active A- and B-moieties that bind to cell surface receptors must be endocytosed before the A-moiety is translocated into the cytosol where it exerts its cytotoxic action. The accumulated information about the most well-studied toxins has provided a detailed picture of how they exploit the membrane trafficking systems of cells, and studies of toxin trafficking have revealed the existence of new pathways. The complexity of different endocytic mechanisms, as well as the multiple routes between endosomes and the Golgi apparatus and retrogradely to the endoplasmic reticulum (ER), are being unravelled by investigations of how toxins gain access to their targets. With increasing information about the internalization and intracellular trafficking of these opportunistic toxins, new avenues have been opened for their application in areas of medicine such as drug delivery and therapy.

CD8+ T-cell mediated tumor protection by Pseudomonas exotoxin fused to ovalbumin in C57BL/6 mice

BACKGROUND

Pseudomonas exotoxin (PE) is a 66 kDa bacterial toxin that is able to bind to mammalian cells, undergo receptor mediated endocytosis, and translocate its C-terminal catalytic domain into the cytosol. We investigated whether PE could be used in vivo to deliver CD8+ T-cell epitopes to the MHC-class I antigen presentation pathway to trigger a specific cytotoxic T-lymphocyte (CTL) response.

METHODS

Amino acid 553 of PE was deleted to eliminate toxin catalytic activity, and amino acids 204-386 of ovalbumin were fused near the nontoxic PE C-terminus to produce PE(D)-OVA200. Mice were vaccinated with 100 microg of PE(D)-OVA200 3 times at 21 day intervals. Splenocytes were harvested 1 week later, and stimulated in vitro with ovalbumin expressing EG7 murine thymoma cells. In vivo tumor protection experiments were performed by vaccinating groups of mice as before, followed by a lethal dose of ovalbumin expressing tumor cells (MO5) injected subcutaneously.

RESULTS

Splenocytes from PE(D)-OVA200 vaccinated mice lysed (51)Cr labeled EG7 cells but not the untransfected EL4 parent cell line, whereas splenocytes from mice immunized with PBS, PE(D), or ovalbumin were unable to lyse EG7 cells. Cytotoxicity in vitro was mediated by CD8+ T-cells. PE(D)-OVA200 vaccinated mice survived (88%) a lethal subcutaneous challenge of ovalbumin expressing MO5 cells. Depletion of CD8+ cells from PE(D)-OVA200 vaccinated mice abolished this protection, indicating that this cell population is required for tumor rejection in vivo.

CONCLUSIONS

Our results indicate that PE(D) may be used as a vehicle to stimulate a protective CTL response to heterologous antigen in vivo.

Expression, Purification and Characterization of Ricin vectors used for exogenous antigen delivery into the MHC Class I presentation pathway

Disarmed versions of the cytotoxin ricin can deliver fused peptides into target cells leading to MHC class I-restricted antigen presentation [Smith et al. J Immunol 2002; 169:99-107]. The ricin delivery vector must contain an attenuated catalytic domain to prevent target cell death, and the fused peptide epitope must remain intact for delivery and functional loading to MHC class I molecules. Expression in E. coli and purification by cation exchange chromatography of the fusion protein is described. Before used for delivery, the activity of the vector must be characterized in vitro, via an N-glycosidase assay, and in vivo, by a cytotoxicity assay. The presence of an intact epitope must be confirmed using mass spectrometry by comparing the actual mass with the predicted mass.

Transport of protein toxins into cells: pathways used by ricin, cholera toxin and Shiga toxin

Ricin, cholera, and Shiga toxin belong to a family of protein toxins that enter the cytosol to exert their action. Since all three toxins are routed from the cell surface through the Golgi apparatus and to the endoplasmic reticulum (ER) before translocation to the cytosol, the toxins are used to study different endocytic pathways as well as the retrograde transport to the Golgi and the ER. The toxins can also be used as vectors to carry other proteins into the cells. Studies with protein toxins reveal that there are more pathways along the plasma membrane to ER route than originally believed.

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.

Two-dimensional structures of the Shiga toxin B-subunit and of a chimera bound to the glycolipid receptor Gb3

The B-subunit of Shiga toxin has been demonstrated as a powerful vector for carrying attached peptides into cells for intracellular transport studies and for medical research. We have investigated the structure of the B-subunit and of a chimera bearing a peptide extension, bound to the membranous lipidic receptor, the globotriaosylceramide (Gb3). Two-dimensional crystals of both B-subunits have been obtained by the lipid layer method and projection maps have been calculated at 8.5A resolution from ice-embedded samples. The B-subunits as the chimera are organized in a pentameric form similar to the X-ray structure of the B-subunit not bound to Gb3. A difference map of both proteins has been calculated in which no density could be attributed to the peptide extension. Cross-correlations with projections of the B-subunit X-ray structure revealed that pentamers in the 2D crystals were oriented with their binding sites pointing to the lipid layer. Thus, it is likely that the peptide extension was disordered and confined to the surface of the pentamer opposite to the Gb3 binding sites. This location confirms the hypothesis that addition of peptide extension to the C-terminus conserves the ability of the modified B-subunit to bind the membranous receptor Gb3.

Exogenous peptides delivered by ricin require processing by signal peptidase for transporter associated with antigen processing-independent MHC class I-restricted presentation

In this study we demonstrate that a disarmed version of the cytotoxin ricin can deliver exogenous CD8(+) T cell epitopes into the MHC class I-restricted pathway by a TAP-independent, signal peptidase-dependent pathway. Defined viral peptide epitopes genetically fused to the N terminus of an attenuated ricin A subunit (RTA) that was reassociated with its partner B subunit were able to reach the early secretory pathway of sensitive cells, including TAP-deficient cells. Successful processing and presentation by MHC class I proteins was not dependent on proteasome activity or on recycling of MHC class I proteins, but rather on a functional secretory pathway. Our results demonstrated a role for signal peptidase in the generation of peptide epitopes associated at the amino terminus of RTA. We showed, first, that potential signal peptide cleavage sites located toward the N terminus of RTA can be posttranslationally cleaved by signal peptidase and, second, that mutation of one of these sites led to a loss of peptide presentation. These results identify a novel MHC class I presentation pathway that exploits the ability of toxins to reach the lumen of the endoplasmic reticulum by retrograde transport, and suggest a role for endoplasmic reticulum signal peptidase in the processing and presentation of MHC class I peptides. Because TAP-negative cells can be sensitized for CTL killing following retrograde transport of toxin-linked peptides, application of these results has direct implications for the development of novel vaccination strategies.

The epithelial cell cytoskeleton and intracellular trafficking. I. Shiga toxin B-subunit system: retrograde transport, intracellular vectorization, and more

Many intracellular transport routes are still little explored. This is particularly true for retrograde transport between the plasma membrane and the endoplasmic reticulum. Shiga toxin B subunit has become a powerful tool to study this pathway, and recent advances on the molecular mechanisms of transport in the retrograde route and on its physiological function(s) are summarized. Furthermore, it is discussed how the study of retrograde transport of Shiga toxin B subunit allows one to design new methods for the intracellular delivery of therapeutic compounds.

Bacterial toxins as tools for mucosal vaccination

Several studies have demonstrated that the biological properties of secreted bacterial toxins could be harnessed for the induction of mucosal and systemic immunity following application at epithelial surfaces. Although the properties and potential application of several of these toxins will be discussed in this review, special focus will be placed on Pseudomonas aeruginosa exotoxin A (PE). A non-toxic form of PE (ntPE) into which antigenic epitopes can be integrated appears to be a particularly promising vaccination tool, which is able to cross the polarized epithelia of the gastrointestinal, respiratory and reproductive tracts and selectively target macrophages and dendritic cells.

Shiga toxins

Shiga toxin and Shiga-like toxins belong to the group of protein toxins which have a moiety that binds to the cell surface and another enzymatically active moiety that after entry into the cytosol inhibits protein synthesis enzymatically. The toxins can also cause apoptosis by mechanisms that may be different from the effect on the protein synthesis machinery. Shigella dysenteriae, some strains of Escherichia coli as well as other bacteria can secrete such toxins which cause serious complications during infections. An increasing knowledge about the toxins and their interactions with cells is important both for treatment of disease, and for elucidation of pathways of intracellular transport.

Factors affecting the efficiency of CD8+ T cell cross-priming with exogenous antigens

Processing of exogenous protein Ags by APC leads predominantly to presentation of peptides on class II MHC and, thus, stimulation of CD4+ T cell responses. However, "cross-priming" can also occur, whereby peptides derived from exogenous Ags become displayed on class I MHC molecules and stimulate CD8+ T cell responses. We compared the efficiency of cross-priming with exogenous proteins to use of peptide Ags in human whole blood using a flow cytometry assay to detect T cell intracellular cytokine production. CD8+ T cell responses to whole CMV proteins were poorly detected (compared with peptide responses) in most CMV-seropositive donors. Such responses could be increased by using higher doses of Ag than were required to achieve maximal CD4+ T cell responses. A minority of donors displayed significantly more efficient CD8+ T cell responses to whole protein, even at low Ag doses. These responses were MHC class I-restricted and dependent upon proteosomal processing, indicating that they were indeed due to cross-priming. The ability to efficiently cross-prime was not a function of the number of dendritic cells in the donor's blood. Neither supplementation of freshly isolated dendritic cells nor use of cultured, Ag-pulsed dendritic cells could significantly boost CD8 responses to whole-protein Ags in poorly cross-priming donors. Interestingly, freshly isolated monocytes performed almost as well as dendritic cells in inducing CD8 responses via cross-priming. In conclusion, the efficiency of cross-priming appears to be poor in most donors and is dependent upon properties of the individual's APC and/or T cell repertoire. It remains unknown whether cross-priming ability translates into any clinical advantage in ability to induce CD8+ T cell responses to foreign Ags.

Use of bacteria as anticancer agents

Historically, bacteria were used as oncolytic agents for malignant brain tumours. Advances in bacteriology and molecular biology have widened the scope of bacterial approaches to cancer therapy and various possibilities include the use of bacteria as sensitising agents for chemotherapy, as delivery agents for anticancer drugs, and as vectors for gene therapy. Bacterial toxins can be used for tumour destruction and cancer vaccines can be based on immunotoxins of bacterial origin. The most promising approaches are the use of genetically modified bacteria for selective destruction of tumours, and bacterial gene-directed enzyme prodrug therapy. Knowledge gained from study of bacterial genomes forms an important basis of use of bacteria as anticancer agents. TAPET (Tumour Amplified Protein Expression Therapy) uses a genetically altered strain of Salmonella as a bacterial vector, or vehicle, for preferentially delivering anticancer drugs to solid tumours. Verotoxin 1 (VT1) of Escherichia coli has been used for ex vivo purging of human bone marrow of cancer cells before autologous bone marrow transplant. E. coli genes and enzymes have become part of well-known prodrug approaches to cancer in which inert prodrugs can be converted in vivo to highly active species. IL-4 fused with Pseudomonas exotoxin has been administered directly into malignant brain tumours and binds with high affinity to IL-4 receptors, which do not exist on normal brain cells, thus destroying a major part of the tumour without harming the normal brain tissue. It is in Phase I/II clinical trials in patients with glioblastoma. No ideal anticancer agent of bacterial origin that is applicable to all types of cancers has been discovered yet. The most promising approach to malignant brain tumours appears to be the use of genetically engineered bacteria that destroy the tumour selectively while sparing the normal brain tissue.

Entry of ricin and Shiga toxin into cells: molecular mechanisms and medical perspectives

A large number of plant and bacterial toxins with enzymatic activity on intracellular targets are now known. These toxins enter cells by first binding to cell surface receptors, then they are endocytosed and finally they become translocated into the cytosol from an intracellular compartment. In the case of the plant toxin ricin and the bacterial toxin Shiga toxin, this happens after retrograde transport through the Golgi apparatus and to the endoplasmic reticulum. The toxins are powerful tools to reveal new pathways in intracellular transport. Furthermore, knowledge about their action on cells can be used to combat infectious diseases where such toxins are involved, and a whole new field of research takes advantage of their ability to enter the cytosol for therapeutic purposes in connection with a variety of diseases. This review deals with the mechanisms of entry of ricin and Shiga toxin, and the attempts to use such toxins in medicine are discussed.

The B subunit of Shiga toxin fused to a tumor antigen elicits CTL and targets dendritic cells to allow MHC class I-restricted presentation of peptides derived from exogenous antigens

Immunization with peptide or recombinant proteins generally fails to elicit CTL, which are thought to play a key role in the control of virus-infected cells and tumor growth. In this study we show that the nontoxic B subunit of Shiga toxin fused to a tumor peptide derived from the mouse mastocytoma P815 can induce specific CTL in mice without the use of adjuvant. The Shiga B subunit acts as a vector rather than as an adjuvant, because coinjection of the tumor peptide and the B subunit as separate entities does not lead to CTL induction. We also demonstrated that in vitro the B subunit mediates the delivery of various exogenous CD8 T cell epitopes into the conventional MHC class I-restricted pathway, as this process is inhibited by brefeldin A and lactacystin and requires a functional TAP system. In contrast to other nonviral methods for transport of exogenous Ags into the endogenous MHC class I pathway that involve macropinocytosis or phagocytosis, the Shiga B subunit targets this pathway in a receptor-dependent manner, namely via binding to the glycolipid Gb3. Because this receptor is highly expressed on various dendritic cells, it should allow preferential targeting of the Shiga B subunit to these professional APCs. Therefore, the Shiga B subunit appears to represent an attractive vector for vaccine development due to its ability to target dendritic cells and to induce specific CTL without the need for adjuvant.

Contrasting effects of HSP72 expression on apoptosis in human umbilical vein endothelial cells and an angiogenic cell line, ECV304

The effect of overexpression of heat shock protein (HSP)72 on apoptosis induced by different stimuli in human umbilical vein endothelial cells (HUVECs) and the angiogenic cell line, ECV304, was studied. Transient overexpression of HSP72 was achieved using an adenoviral vector (Advhsp72) and apoptosis was induced by heat shock, tumour necrosis factor (TNF)-alpha with cycloheximide (CHX), lipopolysaccharide (LPS) with TNF-alpha and verocytotoxin (VT). Apoptosis induced by heat shock was reduced by HSP72 expression. However, HSP72 expression in HUVECs increased apoptosis induced by TNF-alpha/CHX, LPS and VT measured by flow cytometric analysis of propidium iodide (PI)-stained permeabilized cells. In contrast, apoptosis in ECV304 induced by the same stimuli was reduced by HSP72 expression. No difference was seen in cells transduced with a control adenoviral vector expressing beta-galactosidase. These data imply that induction of HSP72 in cells modulates responses to apoptotic stimuli, but that the nature of the response varies with the cell type. However, it is clear that in situations where apoptosis may be part of a pathological process, HSP72 induction, for example by reperfusion injury, may exacerbate the process.

Facing inward from compartment shores: how many pathways were we looking for?

Protein toxins of the Shiga family have become potent tools in studying a number of intracellular transport events such as endocytosis, the communication between endosomes and the biosynthetic/secretory pathway, and retrograde transport from the Golgi apparatus to the endoplasmic reticulum. It seems clear today that most of these transport events can be explained from the toxins' interactions with cellular factors. This review will primarily focus on the discussion of recent data obtained on Shiga toxin and related toxins. We will point out to what extent the study of these proteins has opened new avenues for the development of intracellular targeting tools.