Macrophage apoptosis by anthrax lethal factor through p38 MAP kinase inhibition

Inhibition of MAPK and NF-kappa B pathways is necessary for rapid apoptosis in macrophages infected with Yersinia

Macrophages respond to infection with pathogenic Yersinia species by activating MAPK- and NF-kappaB-signaling pathways. To counteract this response, Yersiniae secrete a protease (Yersinia outer protein J (YopJ)) that is delivered into macrophages, deactivates MAPK- and NF-kappaB-signaling pathways, and induces apoptosis. NF-kappaB promotes cell survival by up-regulating expression of several apoptosis inhibitor genes. Previous studies show that deactivation of the NF-kappaB pathway by YopJ is important for Yersinia-induced apoptosis. To determine whether deactivation of the NF-kappaB pathway is sufficient for Yersinia-induced apoptosis, two inhibitors of the NF-kappaB pathway, IkappaBalpha superrepressor or A20, were expressed in macrophages. Macrophages expressing these proteins were infected with Yersinia pseudotuberculosis strains that secrete functionally active or inactive forms of YopJ. Apoptosis levels were substantially higher (5- to 10-fold) when active YopJ was delivered into macrophages expressing IkappaBalpha superrepressor or A20, suggesting that deactivation of the NF-kappaB pathway is not sufficient for rapid Yersinia-induced apoptosis. When macrophages expressing A20 were treated with specific inhibitors of MAPKs, similar levels of apoptosis (within approximately 2-fold) were observed when active or inactive YopJ were delivered during infection. These results suggest that MAPK and NF-kappaB pathways function together to up-regulate apoptosis inhibitor gene expression in macrophages in response to Yersinia infection and that YopJ deactivates both pathways to promote rapid apoptosis. In addition, treating macrophages with a proteasome inhibitor results in higher levels of infection-induced apoptosis than can be achieved by blocking NF-kappaB function alone, suggesting that proapoptotic proteins are stabilized when proteasome function is blocked in macrophages.

The role of host cell death in Salmonella infections

Salmonella enterica is an important enteric pathogen of humans and a variety of domestic and wild animals. Infection is initiated in the intestinal tract, and severe disease produces widespread destruction of the intestinal mucosa. Salmonella strains can also disseminate from the intestine and produce serious, sometimes fatal infections with considerable cytopathology in a number of systemic organs. A combination of bacterial genetic and cell biology studies have shown that Salmonella uses specific virulence mechanisms to induce host cell death during infection. Salmonella produces one set of virulence proteins to promote invasion of the intestine and a different set to mediate systemic disease. Significantly, each set of virulence factors mediates a distinct mechanism of host cell death. The Salmonella pathogenicity island-1 (SPI-1) locus encodes a type III protein secretion system (TTSS) that delivers effector proteins required for intestinal invasion and the production of enteritis. The SPI-1 effector SipB activates caspase-1 in macrophages, releasing IL-1beta and IL-18 and inducing rapid cell death by a mechanism that has features of both apoptosis and necrosis. Caspase-1 is required for Salmonella to infect Peyer's patches and disseminate to systemic tissues in mice. Progressive Salmonella infection in mice requires the SPI-2 TTSS and associated effector proteins as well as the SpvB cytotoxin. Apoptosis of macrophages in the liver is found during systemic infection. In cell culture, Salmonella strains induce delayed apoptosis dependent on SPI-2 function in macrophages from a variety of sources. This delayed apoptosis also requires activation of TLR4 on macrophages by the bacterial LPS. Downstream activation of kinase pathways leads to balanced pro- and antiapoptotic regulatory factors in the cell. NF-kappaB and p38 mitogen-activated protein kinase (MAPK) are particularly important for the induction of antiapoptotic factors, whereas the kinase PKR is required for bacterial-induced apoptosis. The Salmonella SPI-2 TTSS is essential for altering the balance in favor of apoptosis during intracellular infection, but the effectors involved remain poorly characterized. The SpvB cytotoxin has been shown to play a role in apoptosis in human macrophages by depolymerizing the actin cytoskeleton. A model for the role of bacteria-induced host cell death in Salmonella pathogenesis is proposed. In the intestine, the Salmonella SPI-1 TTSS and SipB mediate macrophage death by caspase-1 activation, which also releases IL-1beta and IL-18, promoting inflammation and subsequent phagocytosis by incoming macrophages and leading to dissemination to systemic tissues. Intracellular secretion of virulence effector proteins by the SPI-2 TTSS facilitates growth of Salmonella in these macrophages and the delayed onset of apoptosis in extraintestinal tissues. These infected, apoptotic cells are targeted for engulfment by incoming macrophages, thus perpetuating the cycle of cell-to-cell spread that is the hallmark of systemic Salmonella infection.

Chrysin induces G1 phase cell cycle arrest in C6 glioma cells through inducing p21Waf1/Cip1 expression: involvement of p38 mitogen-activated protein kinase

Flavonoids are a broadly distributed class of plant pigments, universally present in plants. They are strong anti-oxidants that can inhibit carcinogenesis in rodents. Chrysin (5,7-dihydroxyflavone) is a natural and biologically active compound extracted from many plants, honey, and propolis. It possesses potent anti-inflammatory, anti-oxidant properties, promotes cell death, and perturbing cell cycle progression. However, the mechanism by which chrysin inhibits cancer cell growth remains poorly understood. Therefore, we developed an interest in the relationship between MAPK signaling pathways and cell growth inhibition after chrysin treatment in rat C6 glioma cells. Cell viability assay and flow cytometric analysis suggested that chrysin exhibited a dose-dependent and time-dependent ability to block rat C6 glioma cell line cell cycle progression at the G1 phase. Western blotting analysis showed that the levels of Rb phosphorylation in C6 glioma cells exposed to 30 microM chrysin for 24h decreased significantly. We demonstrated the expression of cyclin-dependent kinase inhibitor, p21(Waf1/Cip1), to be significantly increased, but the p53 protein level did not change in chrysin-treated cells. Both cyclin-dependent kinase 2 (CDK2) and 4 (CDK4) kinase activities were reduced by chrysin in a dose-dependent manner. Furthermore, chrysin also inhibited proteasome activity. We further showed that chrysin induced p38-MAPK activation, and using a specific p38-MAPK inhibitor, SB203580, attenuated chrysin-induced p21(Waf1/Cip1) expression. These results suggest that chrysin exerts its growth-inhibitory effects either through activating p38-MAPK leading to the accumulation of p21(Waf1/Cip1) protein or mediating the inhibition of proteasome activity.

Endocrine perturbation increases susceptibility of mice to anthrax lethal toxin

Bacillus anthracis lethal toxin (LT) causes vascular collapse and high lethality in BALB/cJ mice, intermediate lethality in C57BL/6J mice, and no lethality in DBA/2J mice. We found that adrenalectomized (ADX) mice of all three strains had increased susceptibility to LT. The increased susceptibility of ADX-DBA/2J mice was not accompanied by changes in their macrophage sensitivity or cytokine response to LT. DBA/2J mice showed no change in serum corticosteroid levels in response to LT injection, while BALB/cJ mice showed a fivefold increase in serum corticosterone. However, LT inhibited dexamethasone (DEX)-induced glucocorticoid receptor gene activation to similar extents in all three strains. DEX treatment did not rescue ADX mice from LT-mediated mortality. Surprisingly, oral DEX treatment also sensitized adrenally intact DBA/2J mice to LT lethality at all doses tested and also exacerbated LT-mediated pathogenesis and mortality in BALB/cJ mice. Aldosterone did not protect ADX mice from toxin challenge. These results indicate that susceptibility to anthrax LT in mice depends on a fine but easily perturbed balance of endocrine functions. Thus, the potentially detrimental consequences of steroid therapy for anthrax must be considered in treatment protocols for this disease.

I{kappa}B kinase (IKK){beta}, but not IKK{alpha}, is a critical mediator of osteoclast survival and is required for inflammation-induced bone loss

Transcription factor, nuclear factor kappaB (NF-kappaB), is required for osteoclast formation in vivo and mice lacking both of the NF-kappaB p50 and p52 proteins are osteopetrotic. Here we address the relative roles of the two catalytic subunits of the IkappaB kinase (IKK) complex that mediate NF-kappaB activation, IKKalpha and IKKbeta, in osteoclast formation and inflammation-induced bone loss. Our findings point out the importance of the IKKbeta subunit as a transducer of signals from receptor activator of NF-kappaB (RANK) to NF-kappaB. Although IKKalpha is required for RANK ligand-induced osteoclast formation in vitro, it is not needed in vivo. However, IKKbeta is required for osteoclastogenesis in vitro and in vivo. IKKbeta also protects osteoclasts and their progenitors from tumor necrosis factor alpha-induced apoptosis, and its loss in hematopoietic cells prevents inflammation-induced bone loss.

Efficient synthetic inhibitors of anthrax lethal factor

Inhalation anthrax is a deadly disease for which there is currently no effective treatment. Bacillus anthracis lethal factor (LF) metalloproteinase is an integral component of the tripartite anthrax lethal toxin that is essential for the onset and progression of anthrax. We report here on a fragment-based approach that allowed us to develop inhibitors of LF. The small-molecule inhibitors we have designed, synthesized, and tested are highly potent and selective against LF in both in vitro tests and cell-based assays. These inhibitors do not affect the prototype human metalloproteinases that are structurally similar to LF. Initial in vivo evaluation of postexposure efficacy of our inhibitors combined with antibiotic ciprofloxacin against B. anthracis resulted in significant protection. Our data strongly indicate that the scaffold of inhibitors we have identified is the foundation for the development of novel, safe, and effective emergency therapy of postexposure inhalation anthrax.

Molecular basis for improved anthrax vaccines

The current vaccine for anthrax has been licensed since 1970 and was developed based on the outcome of human trials conducted in the 1950s. This vaccine, known as anthrax vaccine adsorbed (AVA), consists of a culture filtrate from an attenuated strain of Bacillus anthracis adsorbed to aluminum salts as an adjuvant. This vaccine is considered safe and effective, but is difficult to produce and is associated with complaints about reactogenicity among users of the vaccine. Much of the work in the past decade on generating a second generation vaccine is based on the observation that antibodies to protective antigen (PA) are crucial in the protection against exposure to virulent anthrax spores. Antibodies to PA are thought to prevent binding to its cellular receptor and subsequent binding of lethal factor (LF) and edema factor (EF), which are required events for the action of the two toxins: lethal toxin (LeTx) and edema toxin (EdTx). The bacterial capsule as well as the two toxins are virulence factors of B. anthracis. The levels of antibodies to PA must exceed a certain minimal threshold in order to induce and maintain protective immunity. Immunity can be generated by vaccination with purified PA, as well as spores and DNA plasmids that express PA. Although antibodies to PA address the toxemia component of anthrax disease, antibodies to additional virulence factors, including the capsule or somatic antigens in the spore, may be critical in development of complete, sterilizing immunity to anthrax exposure. The next generation anthrax vaccines will be derived from the thorough understanding of the interaction of virulence factors with human and animal hosts and the role the immune response plays in providing protective immunity.

Phosphatidylinositol-Specific Phospholipase C ofBacillus anthracisDown-Modulates the Immune Response

Phosphatidylinositol-specific phospholipases (PI-PLCs) are virulence factors produced by many pathogenic bacteria, including Bacillus anthracis and Listeria monocytogenes. Bacillus PI-PLC differs from Listeria PI-PLC in that it has strong activity for cleaving GPI-anchored proteins. Treatment of murine DCs with Bacillus, but not Listeria, PI-PLC inhibited dendritic cell (DC) activation by TLR ligands. Infection of mice with Listeria expressing B. anthracis PI-PLC resulted in a reduced Ag-specific CD4 T cell response. These data indicate that B. anthracis PI-PLC down-modulates DC function and T cell responses, possibly by cleaving GPI-anchored proteins important for TLR-mediated DC activation.

Antiapoptotic role of p38 mitogen activated protein kinase in Jurkat T cells and normal human T lymphocytes treated with 8-methoxypsoralen and ultraviolet-A radiation

A combination of 8-methoxypsoralen and ultraviolet-A radiation (320-400 nm) (PUVA) is used for the treatment of T cell-mediated disorders, including chronic graft-versus-host disease, autoimmune disorders, and cutaneous T-cell lymphomas. The mechanisms of action of this therapy, referred to as extracorporeal phototherapy, have not been fully elucidated. PUVA is known to induce apoptosis in T lymphocytes collected by apheresis, however no information is available concerning the underlying signaling pathways which are activated by PUVA. In this study, we found that PUVA treatment of Jurkat cells and human T lymphocytes up-regulates the p38 MAPK pathway but not the p42/44 MAPK or the SAPK/JNK signaling networks. The use of a pharmacological inhibitor selective for the p38 MAPK pathway, SB203580, allowed us to demonstrate that this network exerts an antiapoptotic effect in PUVA-treated Jurkat cells and T lymphocytes from healthy donors. Moreover, the effect of SB203580 was not due to a down-regulation of the Akt survival pathway which was not activated in response to PUVA. These results may suggest that p38 MAPK-dependent signaling is very important for the regulation of survival genes after exposure to PUVA. Since the therapeutic effect of PUVA seems to depend, at least in part, on apoptosis, further studies on the apoptosis signaling networks activated by this treatment might lead to the use of signal transduction modulators in combination with PUVA, to increase the efficacy of this form of therapy.

Development of high-throughput assay of lethal factor using native substrate

The design of inhibitors for anthrax lethal factor (LF) is currently of interest as an approach for the treatment of anthrax because LF plays a major role in the cytotoxicity of target cells. LF is a zinc-dependent metalloprotease that specifically cleaves the mitogen-activated protein kinase kinase (MKK) family. Current assay systems for the screening of LF inhibitor use the optimized synthetic peptide coupled with various kinds of fluorophores, enabling fast, sensitive, and robust assays suited to high-throughput screening. However, evidence suggests that the regions beside the cleavage site are also involved in specificity and proteolytic activity of LF. In the current study, we tried to develop a high-throughput assay for LF activity based on native substrate, mitogen-activated ERK kinase 1 (MEK1). The assay system relies on the enhanced chemiluminescence signal resulting from a specific antibody against the C-terminal region of native substrate. A glutathione-coated multiwell plate was used as a solid support to immobilize the native substrate by its N-terminal glutathione-S-transferase moiety. Immobilized substrate increases the specificity and sensitivity of LF-catalyzed substrate hydrolysis compared with the solution phase assay. This assay system might be used to discover a wide spectrum of anthrax inhibitors.

Anthrax lethal toxin blocks MAPK kinase-dependent IL-2 production in CD4+ T cells

Anthrax lethal toxin (LT) is a critical virulence factor that cleaves and inactivates MAPK kinases (MAPKKs) in host cells and has been proposed as a therapeutic target in the treatment of human anthrax infections. Despite the potential use of anti-toxin agents in humans, the standard activity assays for anthrax LT are currently based on cytotoxic actions of anthrax LT that are cell-, strain-, and species-specific, which have not been demonstrated to occur in human cells. We now report that T cell proliferation and IL-2 production inversely correlate with anthrax LT levels in human cell assays. The model CD4+ T cell tumor line, Jurkat, is a susceptible target for the specific protease action of anthrax LT. Anthrax LT cleaves and inactivates MAPKKs in Jurkat cells, whereas not affecting proximal or parallel TCR signal transduction pathways. Moreover, anthrax LT specifically inhibits PMA/ionomycin- and anti-CD3-induced IL-2 production in Jurkat cells. An inhibitor of the protease activity of anthrax LT completely restores IL-2 production by anthrax LT-treated Jurkat cells. Anthrax LT acts on primary CD4+ T cells as well, cleaving MAPKKs and leading to a 95% reduction in anti-CD3-induced proliferation and IL-2 production. These findings not only will be useful in the development of new human cell-based bioassays for the activity of anthrax LT, but they also suggest new mechanisms that facilitate immune evasion by Bacillus anthracis. Specifically, anthrax LT inhibits IL-2 production and proliferative responses in CD4+ T cells, thereby blocking functions that are pivotal in the regulation of immune responses.

Bacterial toxins and the immune system: show me the in vivo targets

Microorganisms that cause persistent infection often exhibit specific adaptations that allow them to avoid the adaptive immune response. Recently, several bacterial toxins have been shown in vitro to disrupt immune cell functions. However, it remains to be established whether these activities are relevant during infection and whether these toxins have specifically evolved to disrupt the adaptive immune system.

Anthrax toxins suppress T lymphocyte activation by disrupting antigen receptor signaling

Anthrax is an infection caused by pathogenic strains of Bacillus anthracis, which secretes a three-component toxic complex consisting of protective antigen (PA), edema factor (EF), and lethal factor (LF). PA forms binary complexes with either LF or EF and mediates their entry into host cells. Although the initial phases of bacterial growth occur in the lymph node, the host fails to mount an effective immune response. Here, we show that LT and ET are potent suppressors of human T cell activation and proliferation triggered through the antigen receptor. Both LT and ET inhibit the mitogen-activated protein and stress kinase pathways, and both toxins inhibit activation of NFAT and AP-1, two transcription factors essential for cytokine gene expression. These data identify a novel strategy of immune evasion by B. anthracis, based on both effector subunits of the toxic complex, and targeted to a key cellular component of adaptive immunity.

IKKalpha limits macrophage NF-kappaB activation and contributes to the resolution of inflammation

Inflammation and innate immunity involve signalling pathways leading to the production of inflammatory mediators. Usually such responses are self-limiting, but aberrant resolution of inflammation results in chronic diseases. Much attention has focused on pro-inflammatory signalling but little is known about the mechanisms that resolve inflammation. The IkappaB kinase (IKK) complex contains two catalytic subunits, IKKalpha and IKKbeta, and controls the activation of NF-kappaB transcription factors, which play a pivotal role in inflammation. Ample evidence indicates that IKKbeta mediates NF-kappaB activation in response to pro-inflammatory cytokines and microbial products. IKKalpha regulates an alternative pathway important for lymphoid organogenesis, but the role of IKKalpha in inflammation is unknown. Here we describe a new role for IKKalpha in the negative regulation of macrophage activation and inflammation. IKKalpha contributes to suppression of NF-kappaB activity by accelerating both the turnover of the NF-kappaB subunits RelA and c-Rel, and their removal from pro-inflammatory gene promoters. Inactivation of IKKalpha in mice enhances inflammation and bacterial clearance. Hence, the two IKK catalytic subunits have evolved opposing but complimentary roles needed for the intricate control of inflammation and innate immunity.

Francisella tularensis LVS initially activates but subsequently down-regulates intracellular signaling and cytokine secretion in mouse monocytic and human peripheral blood mononuclear cells

Monocytic cells constitute an important defense mechanism against invading pathogens by recognizing conserved pathogens components. The recognition leads to activation of intracellular pathways involving nuclear factor kappa B (NF-kappaB) and mitogen-activated protein kinases (MAPK), such as the c-Jun NH2-terminal kinase (JNK), and p38. We show that in vitro infection with Francisella tularensis results in activation of NF-kappaB, phosphorylation of p38 and c-Jun, and secretion of TNF-alpha in adherent mouse peritoneal cells, in the mouse macrophage-like cell line J774A.1, in the human macrophage cell line THP-1, and in human peripheral blood monocytic cells. This occurred after infection with the human live vaccine strain, F. tularensis LVS or a mutant strain denoted deltaiglC, which lacks expression of a 23-kDa protein, or after addition of killed F. tularensis LVS. Addition of purified F. tularensis LPS resulted in no discernible effects on the cells. When the effects were followed up to 5 h, activation persisted in cultures with killed bacteria or infected with the deltaiglC strain. In contrast, the signal transduction activation and secretion of TNF-alpha were down-regulated within the 5h period in mouse peritoneal cells, J774 cells or human peripheral blood mononuclear cells infected with F. tularensis LVS. Together, the results suggest that infection with live F. tularensis LVS bacteria leads to a rapid induction of a proinflammatory response in mouse and human cells but after internalization of bacteria, this response is completely or partly down-regulated in most cell types. This down-regulation does not occur when cells are infected with the mutant deltaiglC.

Anthrax Edema Toxin Cooperates with Lethal Toxin to Impair Cytokine Secretion during Infection of Dendritic Cells

Bacillus anthracis secretes two critical virulence factors, lethal toxin (LT) and edema toxin (ET). In this study, we show that murine bone marrow-derived dendritic cells (DC) infected with B. anthracis strains secreting ET exhibit a very different cytokine secretion pattern than DC infected with B. anthracis strains secreting LT, both toxins, or a nontoxinogenic strain. ET produced during infection selectively inhibits the production of IL-12p70 and TNF-alpha, whereas LT targets IL-10 and TNF-alpha production. To confirm the direct role of the toxins, we show that purified ET and LT similarly disrupt cytokine secretion by DC infected with a nontoxinogenic strain. These effects can be reversed by specific inhibitors of each toxin. Furthermore, ET inhibits in vivo IL-12p70 and IFN-gamma secretion induced by LPS. These results suggest that ET produced during infection impairs DC functions and cooperates with LT to suppress the innate immune response. This may represent a new strategy developed by B. anthracis to escape the host immune response.

Anthrax toxin: the long and winding road that leads to the kill

The past five years have led to a tremendous increase in our molecular understanding of the mode of action of the anthrax toxin, one of the two main virulence factors produced by Bacillus anthracis. The structures of each of the three components of the toxin--lethal factor (LF), edema factor (EF) and protective antigen (PA)--have been solved not only in their monomeric forms but, depending on the subunit, in a heptameric form, bound to their substrate, co-factor or receptor. The endocytic route followed by the toxin has also been unraveled and the enzymatic mechanisms of EF and LF elucidated.

Anthrolysin O and other gram-positive cytolysins are toll-like receptor 4 agonists

Exposure of bone marrow–derived macrophages (BMDMs) to low concentrations of Bacillus anthracis lethal toxin (LT), whose catalytic subunit is lethal factor (LF), results in induction of a robust apoptotic response dependent on activation of Toll-like receptor (TLR)4. A similar TLR4-dependent apoptotic response is observed when BMDMs are infected with live B. anthracis (Sterne strain). However, TLR4 is considered to be a specific signaling receptor for lipopolysaccharide (LPS), a typical product of gram-negative bacteria, whereas B. anthracis is gram-positive. To understand how B. anthracis can activate TLR4, we analyzed its culture supernatants and found them to contain a potent TLR4-stimulating activity that can also induce apoptosis in macrophages in which the antiapoptotic p38 MAP kinase (whose activation is prevented by LF) was inhibited. Purification of this activity suggested it consists of anthrolysin O (ALO), a member of the cholesterol-dependent cytolysin (CDC) family. We show that recombinant ALO can activate TLR4 in a manner independent of LPS contamination and, together with LT, can induce macrophage apoptosis. We also provide genetic evidence that ALO is required for induction of macrophage apoptosis in response to infection with live B. anthracis and that other CDC family members share the ability to activate TLR4.

Human alpha-defensins neutralize anthrax lethal toxin and protect against its fatal consequences

Anthrax caused by Bacillus anthracis represents a major bioterroristic threat. B. anthracis produces lethal toxin (LeTx), a combination of lethal factor (LF) and protective antigen that plays a major role in anthrax pathogenesis. We demonstrate that human neutrophil alpha-defensins are potent inhibitors of LF. The inhibition of LF by human neutrophil protein (HNP-1) was noncompetitive. HNP-1 inhibited cleavage of a mitogen-activated protein kinase kinase and restored impaired mitogen-activated protein kinase signaling in LeTx-treated macrophages. HNP-1 rescued murine macrophages from B. anthracis-induced cytotoxicity, and in vivo treatment with HNP-1-3 protected mice against the fatal consequences of LeTx.

Differential susceptibility of macrophage cell lines to Bacillus anthracis–Vollum 1B

Bacillus anthracis (BA) is a spore forming bacterium and the causative agent of anthrax disease. Macrophages (Mphis) play a central role in anthrax disease. An important step in disease progression is the ability of BA to secrete lethal toxin (LeTx) that kills Mphis. LeTx is a heterodimer composed of protective antigen (PA) and lethal factor (LF). Researchers have shown that Mphi cell lines demonstrate differential susceptibility to purified LeTx; for example RAW264.7 and J774A.1 Mphis are sensitive to LeTx whereas IC-21 Mphis are resistant. Research has also suggested that exogenous factors, including other BA proteins, can influence the activity of LeTx. For this reason, the objective of the current work was to examine if RAW264.7, J774A.1, and IC-21 Mphis demonstrated differential susceptibility when cultured with a LeTx-producing strain of BA. Here, we co-cultured Mphis with LeTx+ Vollum 1B (V1B) spores for >15 h and assayed for Mphi cell death by morphology, trypan blue (TB) staining, neutral red (NR) activity, and lactate dehydrogenase (LDH) activity in the culture media. Following the addition of V1B spores, necrosis (approximately 50% mortality) was observed in RAW264.7 and J774A.1 Mphis at 7.5 and 10 h, respectively. By 15 h, both RAW264.7 and J774A.1 Mphis demonstrated 100% mortality. In contrast, IC-21 Mphis, under identical culture conditions, remained viable (98%) and activated throughout the course of the experiment (>24 h). The mechanism of RAW264.7 cell death appeared to involve LeTx because the V1B-induced cytotoxicity was dose-dependently reversed by the addition of anti-PA antibody to the culture media. These observations suggest there is differential susceptibility of Mphi cell lines to the LeTx+ V1B strain of BA. Further development of this in vitro model may be useful to further characterize the interactions between Mphis and BA spores.

Manipulation of mitogen-activated protein kinase/nuclear factor-kappaB-signaling cascades during intracellular Toxoplasma gondii infection

The intracellular protozoan Toxoplasma gondii exerts profound effects on nuclear factor-kappaB (NF-kappaB)- and mitogen-activated protein kinase (MAPK)-signaling cascades in macrophages. During early infection, nuclear translocation of NF-kappaB is blocked, and later, the cells display defects in lipopolysaccharide (LPS)-induced MAPK phosphorylation after undergoing initial activation in response to Toxoplasma itself. Infected macrophages that are subjected to triggering through Toll-like receptor 4 (TLR4) with LPS display defective production of tumor necrosis factor-alpha and IL-12 (IL-12) that likely reflects interference with NF-kappaB- and MAPK-signaling cascades. Nevertheless, T. gondii possesses molecules that themselves induce eventual proinflammatory cytokine synthesis. For interleukin-12, this occurs through both myeloid differentiation factor 88-dependent and chemokine receptor CCR5-dependent pathways. The balance between activation and interference with proinflammatory signaling is likely to reflect the need to achieve an appropriate level of immunity that allows the host and parasite to maintain a stable interaction.

Interactions between anthrax toxin receptors and protective antigen

Since the anthrax mail attacks of 2001, much has been learned about the interactions between anthrax toxin and its receptors. Two distinct cellular receptors for anthrax toxin have been identified and are designated capillary morphogenesis protein 2 (CMG2) and anthrax toxin receptor/tumor endothelial marker 8 (ATR/TEM8). The molecular details of the toxin-receptor interactions have been revealed through crystallographic, biochemical and genetic studies. In addition, a novel pathway by which anthrax toxin enters cells is starting to be uncovered.

Murine macrophage transcriptional responses to Bacillus anthracis infection and intoxication

Interactions between Bacillus anthracis and host macrophages represent critical early events in anthrax pathogenesis, but their details are not clearly understood. Here we report the first genomewide characterization of the transcriptional changes within macrophages infected with B. anthracis and the identification of several hundred host genes that were differentially expressed during this intracellular stage of infection. These loci included both genes that are known to be regulated differentially in response to many other bacterial pathogens and those that appear to be differentially regulated in response to B. anthracis but not other bacterial species that have been tested. These data provide a transcriptional basis for a variety of physiological changes observed during infection, including the induction of apoptosis caused by the infecting bacteria. The expression patterns underlying B. anthracis-induced apoptosis led us to test further the importance of one very highly induced macrophage gene, that for ornithine decarboxylase. Our data show that this enzyme plays an important and previously unrecognized role in suppressing apoptosis in B. anthracis-infected cells. We have also characterized the transcriptional response to anthrax lethal toxin in activated macrophages and found that, following toxin treatment, many of the host inflammatory response pathways are dampened. These data provide insights into B. anthracis pathogenesis as well as potential leads for the development of new diagnostic and therapeutic options.

Purified Bacillus anthracis lethal toxin complex formed in vitro and during infection exhibits functional and biological activity

Anthrax protective antigen (PA, 83 kDa), a pore-forming protein, upon protease activation to 63 kDa (PA(63)), translocates lethal factor (LF) and edema factor (EF) from endosomes into the cytosol of the cell. The relatively small size of the heptameric PA(63) pore (approximately 12 angstroms) raises questions as to how large molecules such as LF and EF can move through the pore. In addition, the reported high binding affinity between PA and EF/LF suggests that EF/LF may not dissociate but remain complexed with activated PA(63). In this study, we found that purified (PA(63))(7)-LF complex exhibited biological and functional activities similar to the free LF. Purified LF complexed with PA(63) heptamer was able to cleave both a synthetic peptide substrate and endogenous mitogen-activated protein kinase kinase substrates and kill susceptible macrophage cells. Electrophysiological studies of the complex showed strong rectification of the ionic current at positive voltages, an effect similar to that observed if LF is added to the channels formed by heptameric PA(63) pore. Complexes of (PA(63))(7)-LF found in the plasma of infected animals showed functional activity. Identifying active complex in the blood of infected animals has important implications for therapeutic design, especially those directed against PA and LF. Our studies suggest that the individual toxin components and the complex must be considered as critical targets for anthrax therapeutics.

Activation of liver X receptors and retinoid X receptors prevents bacterial-induced macrophage apoptosis

Microbe-macrophage interactions play a central role in the pathogenesis of many infections. The ability of some bacterial pathogens to induce macrophage apoptosis has been suggested to contribute to their ability to elude innate immune responses and successfully colonize the host. Here, we provide evidence that activation of liver X receptors (LXRs) and retinoid X receptors (RXRs) inhibits apoptotic responses of macrophages to macrophage colony-stimulating factor (M-CSF) withdrawal and several inducers of apoptosis. In addition, combined activation of LXR and RXR protected macrophages from apoptosis caused by infection with Bacillus anthracis, Escherichia coli, and Salmonella typhimurium. Expression-profiling studies demonstrated that LXR and RXR agonists induced the expression of antiapoptotic regulators, including AIM/CT2, Bcl-X(L), and Birc1a. Conversely, LXR and RXR agonists inhibited expression of proapoptotic regulators and effectors, including caspases 1, 4/11, 7, and 12; Fas ligand; and Dnase1l3. The combination of LXR and RXR agonists was more effective than either agonist alone at inhibiting apoptosis in response to various inducers of apoptosis, and it acted synergistically to induce expression of AIM/CT2. Inhibition of AIM/CT2 expression in response to LXR/RXR agonists partially reversed their antiapoptotic effects. These findings reveal unexpected roles of LXRs and RXRs in the control of macrophage survival and raise the possibility that LXR/RXR agonists may be exploited to enhance innate immunity to bacterial pathogens that induce apoptotic programs as a strategy for evading host responses.

Activation of liver X receptors and retinoid X receptors prevents bacterial-induced macrophage apoptosis

Microbe-macrophage interactions play a central role in the pathogenesis of many infections. The ability of some bacterial pathogens to induce macrophage apoptosis has been suggested to contribute to their ability to elude innate immune responses and successfully colonize the host. Here, we provide evidence that activation of liver X receptors (LXRs) and retinoid X receptors (RXRs) inhibits apoptotic responses of macrophages to macrophage colony-stimulating factor (M-CSF) withdrawal and several inducers of apoptosis. In addition, combined activation of LXR and RXR protected macrophages from apoptosis caused by infection with Bacillus anthracis, Escherichia coli, and Salmonella typhimurium. Expression-profiling studies demonstrated that LXR and RXR agonists induced the expression of antiapoptotic regulators, including AIM/CT2, Bcl-X(L), and Birc1a. Conversely, LXR and RXR agonists inhibited expression of proapoptotic regulators and effectors, including caspases 1, 4/11, 7, and 12; Fas ligand; and Dnase1l3. The combination of LXR and RXR agonists was more effective than either agonist alone at inhibiting apoptosis in response to various inducers of apoptosis, and it acted synergistically to induce expression of AIM/CT2. Inhibition of AIM/CT2 expression in response to LXR/RXR agonists partially reversed their antiapoptotic effects. These findings reveal unexpected roles of LXRs and RXRs in the control of macrophage survival and raise the possibility that LXR/RXR agonists may be exploited to enhance innate immunity to bacterial pathogens that induce apoptotic programs as a strategy for evading host responses.

Differentiation of human monocytic cell lines confers susceptibility to Bacillus anthracis lethal toxin

Anthrax lethal toxin (LT) is comprised of protective antigen and lethal factor. Lethal factor enters mammalian cells in a protective antigen-dependent process and cleaves mitogen-activated protein kinase kinases. Although LT has no observable effect on many cell types, it causes necrosis in macrophages derived from certain mouse strains and apoptosis in activated mouse macrophages. In this study, we observed that LT treatment of three different human monocytic cell lines U-937, HL-60 and THP-1 did not induce cell death. Cells did become susceptible to the toxin, however, after differentiation into a macrophage-like state. Treatment with LT resulted in decreased phosphorylation of p38, ERK1/2 and JNK in both undifferentiated and differentiated HL-60 cells, suggesting that the change in susceptibility does not result from differences in toxin delivery or substrate cleavage. Death of differentiated HL-60 cells was accompanied by chromosome condensation and DNA fragmentation, but was not inhibited by the pan-caspase inhibitor Z-VAD-FMK. In addition, we observed that the macrophage differentiation process could be inhibited by LT. Our results indicate that LT-mediated death of mouse and human macrophages may occur through distinct processes and that the differentiation state of human cells can determine susceptibility or resistance to LT.

Rabbit and nonhuman primate models of toxin-targeting human anthrax vaccines

The intentional use of Bacillus anthracis, the etiological agent of anthrax, as a bioterrorist weapon in late 2001 made our society acutely aware of the importance of developing, testing, and stockpiling adequate countermeasures against biological attacks. Biodefense vaccines are an important component of our arsenal to be used during a biological attack. However, most of the agents considered significant threats either have been eradicated or rarely infect humans alive today. As such, vaccine efficacy cannot be determined in human clinical trials but must be extrapolated from experimental animal models. This article reviews the efficacy and immunogenicity of human anthrax vaccines in well-defined animal models and the progress toward developing a rugged immunologic correlate of protection. The ongoing evaluation of human anthrax vaccines will be dependent on animal efficacy data in the absence of human efficacy data for licensure by the U.S. Food and Drug Administration.

Binary bacterial toxins: biochemistry, biology, and applications of common Clostridium and Bacillus proteins

Certain pathogenic species of Bacillus and Clostridium have developed unique methods for intoxicating cells that employ the classic enzymatic "A-B" paradigm for protein toxins. The binary toxins produced by B. anthracis, B. cereus, C. botulinum, C. difficile, C. perfringens, and C. spiroforme consist of components not physically associated in solution that are linked to various diseases in humans, animals, or insects. The "B" components are synthesized as precursors that are subsequently activated by serine-type proteases on the targeted cell surface and/or in solution. Following release of a 20-kDa N-terminal peptide, the activated "B" components form homoheptameric rings that subsequently dock with an "A" component(s) on the cell surface. By following an acidified endosomal route and translocation into the cytosol, "A" molecules disable a cell (and host organism) via disruption of the actin cytoskeleton, increasing intracellular levels of cyclic AMP, or inactivation of signaling pathways linked to mitogen-activated protein kinase kinases. Recently, B. anthracis has gleaned much notoriety as a biowarfare/bioterrorism agent, and of primary interest has been the edema and lethal toxins, their role in anthrax, as well as the development of efficacious vaccines and therapeutics targeting these virulence factors and ultimately B. anthracis. This review comprehensively surveys the literature and discusses the similarities, as well as distinct differences, between each Clostridium and Bacillus binary toxin in terms of their biochemistry, biology, genetics, structure, and applications in science and medicine. The information may foster future studies that aid novel vaccine and drug development, as well as a better understanding of a conserved intoxication process utilized by various gram-positive, spore-forming bacteria.

Membrane insertion of anthrax protective antigen and cytoplasmic delivery of lethal factor occur at different stages of the endocytic pathway

The protective antigen (PA) of anthrax toxin binds to a cell surface receptor, undergoes heptamerization, and binds the enzymatic subunits, the lethal factor (LF) and the edema factor (EF). The resulting complex is then endocytosed. Via mechanisms that depend on the vacuolar ATPase and require membrane insertion of PA, LF and EF are ultimately delivered to the cytoplasm where their targets reside. Here, we show that membrane insertion of PA already occurs in early endosomes, possibly only in the multivesicular regions, but that subsequent delivery of LF to the cytoplasm occurs preferentially later in the endocytic pathway and relies on the dynamics of internal vesicles of multivesicular late endosomes.

Mitogen activated protein kinases as targets for development of novel anti-inflammatory drugs

Given the prevalence and debilitating nature of chronic inflammatory diseases there is a never ending quest for identification of novel targets for the rational development of anti-inflammatory drugs. Although the major signalling pathway that controls inflammation associated gene expression is the one leading to activation of transcription factor NF-kappaB, considerable attention has also been given to mitogen activated protein kinases (MAPKs) as likely targets for development of novel anti-inflammatory therapeutics. Indeed, inhibitors targeting these pathways have been developed and preliminary preclinical data suggest that they exhibit anti-inflammatory activity. This report focuses on the possible mechanisms through which such inhibitors may interfere with inflammation and discusses the pros and cons of targeting MAPKs in the treatment of chronic inflammatory disease.

Antitoxins: novel strategies to target agents of bioterrorism

Never before has there been such a strong possibility that biological agents might be used indiscriminately on civilian populations. This review focuses on the use of antitoxins - antibodies, receptor decoys, dominant-negative inhibitors of translocation, small-molecule inhibitors and substrate analogues - to counteract those biological weapons for which toxins are an important mechanism of disease pathogenesis.

IKKbeta links inflammation and tumorigenesis in a mouse model of colitis-associated cancer

A link between inflammation and cancer has long been suspected, but its molecular nature remained ill defined. A key player in inflammation is transcription factor NF-kappaB whose activity is triggered in response to infectious agents and proinflammatory cytokines via the IkappaB kinase (IKK) complex. Using a colitis-associated cancer model, we show that although deletion of IKKbeta in intestinal epithelial cells does not decrease inflammation, it leads to a dramatic decrease in tumor incidence without affecting tumor size. This is linked to increased epithelial apoptosis during tumor promotion. Deleting IKKbeta in myeloid cells, however, results in a significant decrease in tumor size. This deletion diminishes expression of proinflammatory cytokines that may serve as tumor growth factors, without affecting apoptosis. Thus, specific inactivation of the IKK/NF-kappaB pathway in two different cell types can attenuate formation of inflammation-associated tumors. In addition to suppressing apoptosis in advanced tumors, IKKbeta may link inflammation to cancer.

Mouse susceptibility to anthrax lethal toxin is influenced by genetic factors in addition to those controlling macrophage sensitivity

Bacillus anthracis lethal toxin (LT) produces symptoms of anthrax in mice and induces rapid lysis of macrophages (M phi) derived from certain inbred strains. We used nine inbred strains and two inducible nitric oxide synthase (iNOS) knockout C57BL/6J strains polymorphic for the LT M phi sensitivity Kif1C locus to analyze the role of M phi sensitivity (to lysis) in LT-mediated cytokine responses and lethality. LT-mediated induction of cytokines KC, MCP-1/JE, MIP-2, eotaxin, and interleukin-1 beta occurred only in mice having LT-sensitive M phi. However, while iNOS knockout C57BL/6J mice having LT-sensitive M phi were much more susceptible to LT than the knockout mice with LT-resistant M phi, a comparison of susceptibilities to LT in the larger set of inbred mouse strains showed a lack of correlation between M phi sensitivity and animal susceptibility to toxin. For example, C3H/HeJ mice, harboring LT-sensitive M phi and having the associated LT-mediated cytokine response, were more resistant than mice with LT-resistant M phi and no cytokine burst. Toll-like receptor 4 (Tlr4)-deficient, lipopolysaccharide-nonresponsive mice were not more resistant to LT. We also found that CAST/Ei mice are uniquely sensitive to LT and may provide an economical bioassay for toxin-directed therapeutics. The data indicate that while the cytokine response to LT in mice requires M phi lysis and while M phi sensitivity in the C57BL/6J background is sufficient for BALB/cJ-like mortality of that strain, the contribution of M phi sensitivity and cytokine response to animal susceptibility to LT differs among other inbred strains. Thus, LT-mediated lethality in mice is influenced by genetic factors in addition to those controlling M phi lysis and cytokine response and is independent of Tlr4 function.

Novel repression of the glucocorticoid receptor by anthrax lethal toxin

Death from anthrax has been reported to occur from systemic shock. The lethal toxin (LeTx) is the major effector of anthrax mortality. Although the mechanism of entry of this toxin into cells is well understood, its actions once inside the cell are not as well understood. LeTx is known to cleave and inactivate MAPKKs. We have recently shown that LeTx represses the glucocorticoid receptor (GR) both in vitro and in vivo. This repression is partial and specific, repressing the glucocorticoid, progesterone, and estrogen receptor alpha, but not the mineralocorticoid or estrogen receptor beta. This toxin does not affect GR ligand or DNA binding, and we have suggested that it may function by removing/inactivating one or more of the many cofactors involved in nuclear hormone receptor signaling. Although the precise involvement of this nuclear hormone receptor repression in LeTx toxicity is unknown, examples of blunted HPA axis and glucocorticoid signaling in numerous autoimmune/inflammatory diseases suggest that such repression of critically important receptors could have deleterious effects on health.

Anthrax toxins

Bacillus anthracis, the etiological agent of anthrax, secretes three polypeptides that assemble into toxic complexes on the cell surfaces of the host it infects. One of these polypeptides, protective antigen (PA), binds to the integrin-like domains of ubiquitously expressed membrane proteins of mammalian cells. PA is then cleaved by membrane endoproteases of the furin family. Cleaved PA molecules assemble into heptamers, which can then associate with the two other secreted polypeptides: edema factor (EF) and/or lethal factor (LF). The heptamers of PA are relocalized to lipid rafts where they are quickly endocytosed and routed to an acidic compartment. The low pH triggers a conformational change in the heptamers, resulting in the formation of cation-specific channels and the translocation of EF/LF. EF is a calcium- and calmodulin-dependent adenylate cyclase that dramatically raises the intracellular concentration of cyclic adenosine monophosphate (cAMP). LF is a zinc-dependent endoprotease that cleaves the amino terminus of mitogen-activated protein kinase kinases (Meks). Cleaved Meks cannot bind to their substrates and have reduced kinase activity, resulting in alterations of the signaling pathways they govern. The structures of PA, PA heptamer, EF, and LF have been solved and much is now known about the molecular details of the intoxication mechanism. The in vivo action of the toxins, on the other hand, is still poorly understood and hotly debated. A better understanding of the toxins will help in the design of much-needed anti-toxin drugs and the development of new toxin-based medical applications.

Enhancement of anthrax lethal toxin cytotoxicity: a subset of monoclonal antibodies against protective antigen increases lethal toxin-mediated killing of murine macrophages

We investigated the ability of using monoclonal antibodies (MAbs) against anthrax protective antigen (PA), an anthrax exotoxin component, to modulate exotoxin cytotoxic activity on target macrophage cell lines. Anthrax PA plays a critical role in the pathogenesis of Bacillus anthracis infection. PA is the cell-binding component of the two anthrax exotoxins: lethal toxin (LeTx) and edema toxin. Several MAbs that bind the PA component of LeTx are known to neutralize LeTx-mediated killing of target macrophages. Here we describe for the first time an overlooked population of anti-PA MAbs that, in contrast, function to increase the potency of LeTx against murine macrophage cell lines. The results support a possible mechanism of enhancement: binding of MAb to PA on the macrophage cell surface stabilizes the PA by interaction of MAb with macrophage Fcgamma receptors. This results in an increase in the amount of PA bound to the cell surface, which in turn leads to an enhancement in cell killing, most likely due to increased internalization of LF. Blocking of PA-receptor binding eliminates enhancement by MAb, demonstrating the importance of this step for the observed enhancement. The additional significance of these results is that, at least in mice, immunization with PA appears to elicit a poly-clonal response that has a significant prevalence of MAbs that enhance LeTx-mediated killing in macrophages.

Systemic cytokine response in murine anthrax

Systemic pro-inflammatory cytokine release has been previously implicated as a major death-causing factor in anthrax, however, direct data have been absent. We determined the levels of IL-1 beta, IL-6 and TNF-alpha in serum of mice challenged with virulent (Ames) or attenuated (Sterne) strains of Bacillus anthracis. More than 10-fold increase in the IL-1beta levels was detected in Ames-challenged Balb/c mice, in contrast to more susceptible C57BL/6 mice, which showed no IL-1beta response. Balb/c mice have also responded with higher levels of IL-6. The A/J mice demonstrated IL-1beta and IL-6 systemic response to either Ames or Sterne strain of B. anthracis, whereas no increase in TNF-alpha was detected in any murine strain. We used RT-PCR for gene expression analyses in the liver which often is a major source of cytokines and one of the main targets in infectious diseases. A/J mice challenged with B. anthracis (Sterne) showed increased gene expression for Fas, FasL, Bax, IL-1 beta, TNF-alpha, TGF-beta, MIP-1alpha, KC and RANTES. These data favour the hypothesis that apoptotic cell death during anthrax infection causes chemokine-induced transmigration of inflammatory cells to vitally important organs such as liver. Administration of caspase inhibitors z-VAD-fmk and ac-YVAD-cmk improved survival in Sterne-challenged mice indicating a pathogenic role of apoptosis in anthrax.

Anthrax lethal factor-cleavage products of MAPK (mitogen-activated protein kinase) kinases exhibit reduced binding to their cognate MAPKs

Anthrax lethal toxin is the major cause of death in systemic anthrax. Lethal toxin consists of two proteins: protective antigen and LF (lethal factor). Protective antigen binds to a cell-surface receptor and transports LF into the cytosol. LF is a metalloprotease that targets MKKs [MAPK (mitogen-activated protein kinase) kinases]/MEKs [MAPK/ERK (extracellular-signal-regulated kinase) kinases], cleaving them to remove a small N-terminal stretch but leaving the bulk of the protein, including the protein kinase domain, intact. LF-mediated cleavage of MEK1 and MKK6 has been shown to inhibit signalling through their cognate MAPK pathways. However, the precise mechanism by which this proteolytic cleavage inhibits signal transmission has been unclear. Here we show that the C-terminal LF-cleavage products of MEK1, MEK2, MKK3, MKK4, MKK6 and MKK7 are impaired in their ability to bind to their MAPK substrates, suggesting a common mechanism for the LF-induced inhibition of signalling.

Mitogen-activated protein kinases in apoptosis regulation

Cells are continuously exposed to a variety of environmental stresses and have to decide 'to be or not to be' depending on the types and strength of stress. Among the many signaling pathways that respond to stress, mitogen-activated protein kinase (MAPK) family members are crucial for the maintenance of cells. Three subfamilies of MAPKs have been identified: extracellular signal-regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs), and p38-MAPKs. It has been originally shown that ERKs are important for cell survival, whereas JNKs and p38-MAPKs were deemed stress responsive and thus involved in apoptosis. However, the regulation of apoptosis by MAPKs is more complex than initially thought and often controversial. In this review, we discuss MAPKs in apoptosis regulation with attention to mouse genetic models and critically point out the multiple roles of MAPKs.

Protective role of Bcl-2 on β-amyloid-induced cell death of differentiated PC12 cells: reduction of NF-κB and p38 MAP kinase activation

Activation of the apoptosis program by an increased production of beta-amyloid peptides (Abeta) has been implicated in the neuronal cell death of Alzheimer's disease (AD). Bcl-2 is a well-demonstrated anti-apoptotic protein, however, the mechanisms of anti-apoptotic action of Bcl-2 in Abeta-induced neuronal cell death are not fully understood. In the present study, we therefore have investigated the possibility that overexpression of Bcl-2 may prevent Abeta-induced cell death through inhibition of pro-apoptotic activation of p38 MAP kinase and the transcription factor NF-kappaB in nerve growth factor (NGF)-induced differentiated PC12 cells. Treatment of Abeta into differentiated PC12 cells transfected with plasmid alone resulted in increase of cell death determined by measurement of cytotoxicity and apoptosis in a dose dependent manner. Consistent with the increase of cell death, treatment of Abeta resulted in increase of p38 MAP kinase and NF-kappaB activation. However, overexpression of Bcl-2 reduced Abeta-induced apoptosis, and suppressed the activation of p38 MAP kinase and NF-kappaB. In addition, a p38 MAP kinase specific inhibitor SB 203580 attenuated Abeta-induced apoptosis. This inhibitory effect was correlated well with the inhibition of p38 MAP kniase and NF-kappaB activation. Moreover, inhibition of NF-kappaB activation by sodium salicylates reduced Abeta-induced apoptosis and activation of p38 MAP kinase, and up regulated Bcl-2 expression. These results suggest that Bcl-2 overexpression protects against Abeta-induced cell death of differentiated PC12, and its protective effect may be related to the reduction of Abeta-induced activation of p38 MAP kinase and NF-kappaB.

Crystal structure of the von Willebrand factor A domain of human capillary morphogenesis protein 2: an anthrax toxin receptor

Anthrax toxin is released from Bacillus anthracis as three monomeric proteins, which assemble into toxic complexes at the surface of receptor-bearing host cells. One of the proteins, protective antigen (PA), binds to receptors and orchestrates the delivery of the other two (the lethal and edema factors) into the cytosol. PA has been shown to bind to two cellular receptors: anthrax toxin receptor/tumor endothelial marker 8 and capillary morphogenesis protein 2 (CMG2). Both are type 1 membrane proteins that include an approximately 200-aa extracellular von Willebrand factor A (VWA) domain with a metal ion-dependent adhesion site (MIDAS) motif. The anthrax toxin receptor/tumor endothelial marker 8 and CMG2 VWA domains share approximately 60% amino acid identity and bind PA directly in a metal-dependent manner. Here, we report the crystal structure of the CMG2 VWA domain, with and without its intramolecular disulfide bond, to 1.5 and 1.8 A, respectively. Both structures contain a carboxylate ligand-mimetic bound at the MIDAS and appear as open conformations when compared with the VWA domains from alpha-integrins. The CMG2 structures provide a template to begin probing the high-affinity CMG2-PA interaction (200 pM) and may facilitate understanding of toxin assembly/internalization and the development of new anthrax treatments. The structural data also allow molecular interpretation of known CMG2 VWA domain mutations linked to the genetic disorders, juvenile hyaline fibromatosis, and infantile systemic hyalinosis.

Immune system paralysis by anthrax lethal toxin: the roles of innate and adaptive immunity

Since the deliberate use of anthrax as a bioweapon in the USA in 2001, an enormous amount of attention has been focused on the biology of Bacillus anthracis, the causative bacterium of anthrax. Fatal systemic anthrax involves massive bacteraemia and toxaemia with non-descript early symptoms until the onset of shock and sudden death. The outbreak of fatal symptoms after the incubation period of B anthracis suggests an impairment of the host immune system against this pathogen. Thus, it is likely that B anthracis will possess certain strategies to escape from the host immune system. However, the mechanisms of such immune-evasion strategies are not fully characterised yet. Given the critical role of B anthracis toxins in anthrax pathogenesis, much effort has been made to understand the pathological nature of the toxins. Recent studies have shown the pleiotropic actions of anthrax lethal toxin on host innate immune cells, and that several effects of anthrax lethal toxin may directly account for the mechanism of immune intervention by B anthracis.

Potent inhibitors of anthrax lethal factor from green tea

The anthrax lethal factor (LF) has a major role in the development of anthrax. LF is delivered by the protective antigen (PA) inside the cell, where it exerts its metalloprotease activity on the N-terminus of MAPK-kinases. PA+LF are cytotoxic to macrophages in culture and kill the Fischer 344 rat when injected intravenously. We describe here the properties of some polyphenols contained in green tea as powerful inhibitors of LF metalloproteolytic activity, and how the main catechin of green tea, (-)epigallocatechin-3-gallate, prevents the LF-induced death of macrophages and Fischer 344 rats.

Anthrax lethal toxin rapidly activates caspase-1/ICE and induces extracellular release of interleukin (IL)-1beta and IL-18

Anthrax lethal toxin (LT), a critical virulence factor for Bacillus anthracis, has been demonstrated to cleave and to inactivate mitogen-activated protein kinase kinases (MAPKKs) that propagate prosurvival signals in macrophages (1-5). Whether this action of anthrax LT leads to the production of proinflammatory cytokines by macrophages has been more controversial (6, 7). We now report that anthrax LT treatment leads to the specific extracellular release of interleukin (IL)-1beta and IL-18 by the murine macrophage cell lines, RAW264.7 and J774A.1. Studies of the processing of IL-1beta reveal that the levels of activated/cleaved IL-1beta in RAW264.7 and J774.A1 cells are increased following treatment with anthrax LT. Enhanced processing of IL-1beta directly correlates with increased levels in the activation of its upstream regulator, IL-1beta-converting enzyme/Caspase-1 (ICE). The extracellular release of IL-1beta and IL-18 in response to anthrax LT is ICE-dependent, as an ICE-specific inhibitor blocks this process. These data indicate that ICE, IL-1beta, and IL-18 are downstream effectors of anthrax LT in macrophages, providing the basis for new bioassays for anthrax LT activity and representing potential therapeutic targets.