Escherichia coli enterotoxin B subunit triggers apoptosis of CD8(+) T cells by activating transcription factor c-myc

Different types of cell death induced by enterotoxins

The infection of bacterial organisms generally causes cell death to facilitate microbial invasion and immune escape, both of which are involved in the pathogenesis of infectious diseases. In addition to the intercellular infectious processes, pathogen-produced/secreted enterotoxins (mostly exotoxins) are the major weapons that kill host cells and cause diseases by inducing different types of cell death, particularly apoptosis and necrosis. Blocking these enterotoxins with synthetic drugs and vaccines is important for treating patients with infectious diseases. Studies of enterotoxin-induced apoptotic and necrotic mechanisms have helped us to create efficient strategies to use against these well-characterized cytopathic toxins. In this article, we review the induction of the different types of cell death from various bacterial enterotoxins, such as staphylococcal enterotoxin B, staphylococcal alpha-toxin, Panton-Valentine leukocidin, alpha-hemolysin of Escherichia coli, Shiga toxins, cytotoxic necrotizing factor 1, heat-labile enterotoxins, and the cholera toxin, Vibrio cholerae. In addition, necrosis caused by pore-forming toxins, apoptotic signaling through cross-talk pathways involving mitochondrial damage, endoplasmic reticulum stress, and lysosomal injury is discussed.

Cholera toxin: A paradigm for multi-functional engagement of cellular mechanisms (Review)

Cholera toxin (Ctx) from Vibrio cholerae and its closely related homologue, heat-labile enterotoxin (Etx) from Escherichia coli have become superb tools for illuminating pathways of cellular trafficking and immune cell function. These bacterial protein toxins should be viewed as conglomerates of highly evolved, multi-functional elements equipped to engage the trafficking and signalling machineries of cells. Ctx and Etx are members of a larger family of A-B toxins of bacterial (and plant) origin that are comprised of structurally and functionally distinct enzymatically active A and receptor-binding B sub-units or domains. Intoxication of mammalian cells by Ctx and Etx involves B pentamer-mediated receptor binding and entry into a vesicular pathway, followed by translocation of the enzymatic A1 domain of the A sub-unit into the target cell cytosol, where covalent modification of intracellular targets leads to activation of adenylate cyclase and a sequence of events culminating in life-threatening diarrhoeal disease. Importantly, Ctx and Etx also have the capacity to induce a wide spectrum of remarkable immunological processes. With respect to the latter, it has been found that these toxins activate signalling pathways that modulate the immune system. This review explores the complexities of the cellular interactions that are engaged by these bacterial protein toxins, and highlights some of the new insights to have recently emerged.

The Escherichia coli heat-labile enterotoxin induces apoptosis of immature lymphocytesin vivo via a glucocorticoid-dependent pathway

Escherichia coli heat-labile enterotoxin (LT) exhibits a broad range of immunomodulatory activities, including the induction of lymphocyte-programmed cell death. However, the nature of the lymphoid populations sensitive to LT-induced apoptosis and the mechanisms used by this toxin to promote such activity are still unclear. In this study, we demonstrate that LT induces in mice a rapid increase in the levels of circulating corticosterone, resulting in a dramatic induction of cell death of immature CD4+CD8+, B220+IgM- and IgM+IgD- T and B cell progenitors, respectively. Apoptosis of these cell populations is similar to that reported after experimental treatment with corticosteroids, it is inhibited by mifepristone, a glucocorticoid receptor antagonist, and does not occur in adrenalectomized animals. These results clearly indicate that endogenous glucocorticoids are the mediators of the LT-induced cell death, which involves Bcl-2-dependent apoptotic pathways. The LT-mediated programmed cell death requires systemic exposure and the enzymatic activity of LT, since a mutant devoid of any enzymatic activity have no pro-apoptotic effect at any dose tested.

The B subunit of Escherichia coli heat-labile enterotoxin induces both caspase-dependent and -independent cell death pathways in CD8+ T cells

The nontoxic B subunit of Escherichia coli heat-labile enterotoxin (EtxB) is a potent immunomodulatory molecule that acts both as an adjuvant and to stimulate immune deviation processes, resulting in the suppression of Th1-associated inflammatory responses. The ability of EtxB to alter immune reactivity is dependent on its ability to modulate immune cell function through binding to cell surface molecules, the principal receptor of which is the ubiquitous GM1-ganglioside. EtxB activates B cells and antigen-presenting cells and induces the selective apoptosis of murine CD8+ T cells. We postulated that these effects are mediated by the induction of intracellular signaling pathways following EtxB-receptor interaction. We have previously shown that CD8+ T-cell apoptosis induced by EtxB results from the activation of the transcription factor NF-kappaB and caspases. Here we report that while caspase activity is required for apoptosis, additional features of cell death are caspase independent. EtxB induces a rapid loss of mitochondrial membrane potential and cell viability that are unaffected by caspase inhibitors. In addition, our data suggest that these processes are independent of the activity of Bax and Bcl-2 but are mediated by nitric oxide synthase.

Selective induction of CD8+CD4- thymocyte apoptosis mediated by the B-subunit of Escherichia coli heat-labile enterotoxin

Receptor-binding by the B-subunit of Escherichia coli heat-labile enterotoxin (EtxB) induces apoptosis of peripheral CD8(+), but not CD4(+) T-cells. Given that peripheral CD8(+) and CD4(+) T cells arise from a common developmental pathway in the thymus, we investigated the effects of EtxB on different thymocyte populations. We show that the acquisition of sensitivity to EtxB-mediated cell death arises following transition of CD4(+)CD8(+) double positive cells into the CD4(-)CD8(+) pathway. Maturation of T cells into CD4(-)CD8(+) single positive cells is associated with upregulated expression of receptors for EtxB.

The B subunit of Escherichia coli heat-labile enterotoxin enhances CD8+ cytotoxic-T-lymphocyte killing of Epstein-Barr virus-infected cell lines

Epstein-Barr virus (EBV) is associated with a number of important human cancers, including nasopharyngeal carcinoma, gastric carcinoma, and Hodgkin's lymphoma. These tumors express a viral nuclear antigen, EBV nuclear antigen 1 (EBNA1), which cannot be presented to T cells in a major histocompatibility complex class I context, and the viral latent membrane proteins (LMPs). Although the LMPs are expressed in these tumors, no effective immune response is made. We report here that exposure to the cholera-like enterotoxin B subunit (EtxB) in EBV-infected lymphoblastoid cell lines (LCLs) enhances their susceptibility to killing by LMP-specific CD8(+) cytotoxic T lymphocytes (CTLs) in a HLA class I-restricted manner. CTL killing of LCLs is dramatically increased through both transporter-associated protein-dependent and -independent epitopes after EtxB treatment. The use of mutant B subunits revealed that the enhanced susceptibility of LCLs to CTL killing is dependent on the B subunit's interaction with GM(1) but not its signaling properties. These important findings could underpin the development of novel approaches to treating EBV-associated malignancies and may offer a general approach to increasing the presentation of other tumor and viral antigens.

Mutant Escherichia coli heat-labile toxin B subunit that separates toxoid-mediated signaling and immunomodulatory action from trafficking and delivery functions

The homopentameric B-subunit components of Escherichia coli heat-labile enterotoxin (EtxB) and cholera toxin (CtxB) possess the capacity to enter mammalian cells and to activate cell-signaling events in leukocytes that modulate immune cell function. Both properties have been attributed to the ability of the B subunits to bind to GM1-ganglioside receptors, a ubiquitous glycosphingolipid found in the plasma membrane. Here we describe the properties of EtxB(H57S), a mutant B subunit with a His-->Ser substitution at position 57. The mutant was found to be severely defective in inducing leukocyte signaling, as shown by failure to (i) trigger caspase 3-mediated CD8(+)-T-cell apoptosis, (ii) activate nuclear translocation of NF-kappaB in Jurkat T cells, (iii) induce a potent anti-B-subunit response in mice, or (iv) serve as a mucosal adjuvant. However, its GM1 binding, cellular uptake, and delivery functions remained intact. This was further validated by the finding that EtxB(H57S) was as effective as EtxB in delivering a conjugated model class I epitope into the major histocompatibility complex class I pathway of a dendritic cell line. These observations imply that GM1 binding alone is not sufficient to trigger the signaling events responsible for the potent immunomodulatory properties of EtxB. Moreover, they demonstrate that its signaling properties play no role in EtxB uptake and trafficking. Thus, EtxB(H57S) represents a novel tool for evaluating the complex cellular interactions and signaling events occurring after receptor interaction, as well as offering an alternative means of delivering attached peptides in the absence of the potent immunomodulatory signals induced by wild-type B subunits.

Cyclic adenosine 5'-monophosphate and calcium induce CD152 (CTLA-4) up-regulation in resting CD4+ T lymphocytes

The CTLA-4 (CD152) molecule is up-regulated upon T cell activation and proliferation, and plays a critical role in the inhibition of immune responses. We show in this study that cAMP induces up-regulation of CD152 in human CD4(+) T lymphocytes. This effect occurs in the absence of the up-regulation of CD69 and CD25 activation markers and T cell proliferation. In addition, we found that the Ca(2+) ionophore ionomycin also up-regulates CD152, and that the combination of a cAMP analog or cAMP inducers with ionomycin further enhances the expression of CD152 in resting CD4(+) T lymphocytes. However, cyclosporin A, which inhibits Ca(2+)/calcineurin signaling pathway, fully prevented the ionomycin- but not the cAMP-induced up-regulation of CD152. The effects of cAMP and ionomycin involve increase of both CD152 mRNA transcripts, coding for the membrane and the soluble forms of CD152. Furthermore, we show that CD152 molecules are translocated to the membrane and are functional, as their engagement by specific mAbs prevented NF-kappaB activation by anti-CD3/CD28 stimulation. These findings demonstrate that at least two novel signal pathways regulate CTLA-4 gene expression and CD152 molecule up-regulation in human CD4(+) T lymphocytes, in the absence of full T cell activation.

Contribution of the ADP-ribosylating and receptor-binding properties of cholera-like enterotoxins in modulating cytokine secretion by human intestinal epithelial cells

When epithelial cells first encounter cholera toxin (Ctx) produced by Vibrio cholerae they secrete not only chloride ions responsible for causing diarrhoea, but also a number of cytokines that may contribute to the toxin's potent immunomodulatory properties. Much less is known about the ability of the heat-labile enterotoxin of Escherichia coli (Etx), a close homologue of Ctx, to elicit cytokine secretion by epithelial cells. This study shows that treatment of human intestinal epithelial T84 cells with Etx induces expression of IL-6, IL-10, IL-1R antagonist, as well as IL-1alpha and IL-1beta and low levels of IL-8. Such induction was totally dependent on the intrinsic ADP-ribosylating activity of the toxin A-subunit, and could be mimicked by cAMP-elevating agents, such as forskolin and dibutyryl cAMP. By comparison, neither an enzymically inactive mutant of Etx nor EtxB was able to induce cytokine secretion. The behaviour of Ctx and CtxB was very similar to that of Etx and EtxB, respectively. The spectrum of cytokines released by Etx and Ctx indicates that the toxins may create a local microenvironment that strongly biases the immune response towards an anti-inflammatory and a polarized Th2 response.

New insights into the structure-function relationships and therapeutic applications of cholera-like enterotoxins

Cholera toxin and E. coli heat-labile enterotoxin are structurally homologous proteins comprised of an enzymatically active A-subunit and five B-subunits that bind with high affinity to GM1-ganglioside receptors found on the surface of mammalian cells. The B-subunits have long been thought of simply as trafficking vehicles that trigger entry and subsequent delivery of the 'toxic' A-subunit into cells. Indeed, such is the capacity of the B-subunits to enter cells, that they have been developed as generic carriers for attachment and delivery of a variety of peptides into mammalian cells. However, the B-subunits also appear to possess discrete 'signalling functions', that induce both transcription factor and cell activation. These are thought to be directly responsible for the potent immunomodulatory properties of the B-subunits, and have resulted in their use as adjuvants and as agents to suppress inflammatory immune disorders. The relationship between the signalling properties of the B-subunits and their capacity to act as trafficking vehicles has remained unclear. In an effort to understand the structural requirements for these two functions, a set of mutant B-subunits, with amino acid substitutions at position His-57, have been generated and studied. Importantly, such mutant B-subunits retain an ability to bind with high affinity to GM1 and to traffic into cells, but have entirely lost their capacity to activate immune cell populations. Thus, while binding via GM1 appears to be sufficient to trigger cellular uptake it is not sufficient to activate signal transduction. The His-57 region is therefore speculated to be actively engaged in triggering signalling events, possibly via cognate interaction with other cell surface molecules.