Quantitative measurement of anthrax toxin receptor messenger RNA in primary mononuclear phagocytes

Anthrax Edema and Lethal Toxins Differentially Target Human Lung and Blood Phagocytes

Bacillus anthracis, the causative agent of inhalation anthrax, is a serious concern as a bioterrorism weapon. The vegetative form produces two exotoxins: Lethal toxin (LT) and edema toxin (ET). We recently characterized and compared six human airway and alveolar-resident phagocyte (AARP) subsets at the transcriptional and functional levels. In this study, we examined the effects of LT and ET on these subsets and human leukocytes. AARPs and leukocytes do not express high levels of the toxin receptors, tumor endothelium marker-8 (TEM8) and capillary morphogenesis protein-2 (CMG2). Less than 20% expressed surface TEM8, while less than 15% expressed CMG2. All cell types bound or internalized protective antigen, the common component of the two toxins, in a dose-dependent manner. Most protective antigen was likely internalized via macropinocytosis. Cells were not sensitive to LT-induced apoptosis or necrosis at concentrations up to 1000 ng/mL. However, toxin exposure inhibited B. anthracis spore internalization. This inhibition was driven primarily by ET in AARPs and LT in leukocytes. These results support a model of inhalation anthrax in which spores germinate and produce toxins. ET inhibits pathogen phagocytosis by AARPs, allowing alveolar escape. In late-stage disease, LT inhibits phagocytosis by leukocytes, allowing bacterial replication in the bloodstream.

Large-scale investigation of Leishmania interaction networks with host extracellular matrix by surface plasmon resonance imaging

We have set up an assay to study the interactions of live pathogens with their hosts by using protein and glycosaminoglycan arrays probed by surface plasmon resonance imaging. We have used this assay to characterize the interactions of Leishmania promastigotes with ~70 mammalian host biomolecules (extracellular proteins, glycosaminoglycans, growth factors, cell surface receptors). We have identified, in total, 27 new partners (23 proteins, 4 glycosaminoglycans) of procyclic promastigotes of six Leishmania species and 18 partners (15 proteins, 3 glycosaminoglycans) of three species of stationary-phase promastigotes for all the strains tested. The diversity of the interaction repertoires of Leishmania parasites reflects their dynamic and complex interplay with their mammalian hosts, which depends mostly on the species and strains of Leishmania. Stationary-phase Leishmania parasites target extracellular matrix proteins and glycosaminoglycans, which are highly connected in the extracellular interaction network. Heparin and heparan sulfate bind to most Leishmania strains tested, and 6-O-sulfate groups play a crucial role in these interactions. Numerous Leishmania strains bind to tropoelastin, and some strains are even able to degrade it. Several strains interact with collagen VI, which is expressed by macrophages. Most Leishmania promastigotes interact with several regulators of angiogenesis, including antiangiogenic factors (endostatin, anastellin) and proangiogenic factors (ECM-1, VEGF, and TEM8 [also known as anthrax toxin receptor 1]), which are regulated by hypoxia. Since hypoxia modulates the infection of macrophages by the parasites, these interactions might influence the infection of host cells by Leishmania.

Transcriptional response of bovine monocyte-derived macrophages after the infection with different Argentinean Mycobacterium bovis isolates

Infection of bovines with Mycobacterium bovis causes important financial hardship in many countries presenting also a risk for humans. M. bovis is known to be adapted to survive and thrive within the intramacrophage environment. In spite of its relevance, at present the information about macrophage expression patterns is scarce, particularly regarding the bovine host. In this study, transcriptomic analysis was used to detect genes differentially expressed in macrophages derived from peripheral blood mononuclear cells at early stages of infection with two Argentinean strains of M. bovis, a virulent and an attenuated strains. The results showed that the number of differentially expressed genes in the cells infected with the virulent strain (5) was significantly lower than those in the cells infected with the attenuated strain (172). Several genes were more strongly expressed in infected macrophages. Among them, we detected encoding transcription factors, anthrax toxin receptor, cell division and apoptosis regulator, ankyrin proteins, cytoskeleton proteins, protein of cell differentiation, and regulators of endocytic traffic of membrane. Quantitative real-time PCR of a selected group of differentially expressed genes confirmed the microarrays results. Altogether, the present results contribute to understanding the mechanisms involved in the early interaction of M. bovis with the bovine macrophage.

Broad expression analysis of human ANTXR1/TEM8 transcripts reveals differential expression and novel splizce variants

Tumor endothelial marker 8 (TEM8; ANTXR1) is one of two anthrax toxin receptors; the other is capillary morphogenesis gene 2 protein (CMG2; ANTXR2). TEM8 shows enhanced expression in certain tumor endothelia, and is thought to be a player in tumor vasculature formation. However, a comprehensive expression profile of individual TEM8 variants in normal or cancerous tissues is lacking. In this work we carried out an extensive analysis of all splice variants of human TEM8 in 12 digestive tissues, and 8 each fetal and adult tissues, 6 of them cognate pairs. Using variant-specific primers, we first ascertained the status of full-length transcripts by nested PCR. We then carried out quantitative analysis of each transcript by real-time PCR. Three splice variants of TEM8 were reported before, two single-pass integral membrane forms (V1 and V2) and one secreted (V3). Our analysis has revealed two new variants, one encoding a membrane-bound form of the receptor and the other secreted, which we have designated V4 and V5, respectively. All tissues had V1, V2, V3, and V4, but only prostate had V5. Real-time PCR revealed that all variants are present at different levels in various tissues. V3 appeared the most abundant of all. To ascertain its functionality for anthrax toxin, we expressed the newly identified form V4 in a receptor-negative host cell, and included V1 and V2 for comparison. Cytotoxicity, toxin binding, and internalization assays showed V4 to be as efficient a receptor as V1 and V2.

Tumor endothelial marker 8 expression levels in dendritic cell-based cancer vaccines are related to clinical outcome

Previous studies have shown that tumor endothelial markers (TEMs 1-9) are up modulated in immunosuppressive, pro-angiogenic dendritic cells (DCs) found in tumor microenvironments. We recently reported that monocyte-derived DCs used for vaccination trials may accumulate high levels of TEM8 gene transcripts. Here, we investigate whether TEM8 expression in DC preparations represents a specific tumor-associated change of potential clinical relevance. TEM8 expression at the mRNA and protein level was evaluated by quantitative real-time RT-PCR and cytofluorimetric analysis in human clinical grade DCs utilized for the therapeutic vaccination of 17 advanced cancer patients (13 melanoma and 4 renal cell carcinoma). The analyses revealed that DCs from patients markedly differ in their ability to up-modulate TEM8. Indeed, mDCs from eight non-progressing patients [median overall survival (OS) = 32 months, all positive to the delayed-type hypersensitivity test (DTH)], had similar TEM8 mRNA expression levels [mDCs vs. immature iDCs; mean fold increase (mfi) = 1.97] to those found in healthy donors (mfi = 2.7). Conversely, mDCs from nine progressing patients (OS < 5 months, all but one with negative DTH) showed an increase in TEM8 mRNA levels (mfi = 12.88, p = 0.0018). The present observations suggest that TEM8 expression levels in DC-based therapeutic vaccines would allow the selection of a subgroup of patients who are most likely to benefit from therapeutic vaccination.

Resistance of human alveolar macrophages to Bacillus anthracis lethal toxin

The etiologic agent of inhalational anthrax, Bacillus anthracis, produces virulence toxins that are important in the disease pathogenesis. Current studies suggest that mouse and human macrophages are susceptible to immunosuppressive effects of one of the virulence toxins, lethal toxin (LT). Thus a paradigm has emerged that holds that the alveolar macrophage (AM) does not play a significant role in the innate immune response to B. anthracis or defend against the pathogen as it is disabled by LT. This is inconsistent with animal models and autopsy studies that show minimal disease at the alveolar surface. We examined whether AM are immunosuppressed by LT. We found that human AM were relatively resistant to LT-mediated innate immune cytokine suppression, MEK cleavage, and induction of apoptosis as compared with mouse RAW 264.7 macrophages. Mouse AM and murine bone marrow-derived macrophages were also relatively resistant to LT-mediated apoptosis despite intermediate sensitivity to MEK cleavage. The binding component of LT, protective Ag, does not attach to human AM, although it did bind to mouse AM, murine bone marrow-derived macrophages, and RAW 264.7 macrophages. Human AM do not produce significant amounts of the protective Ag receptor anthrax toxin receptor 1 (TEM8/ANTXR1) and anthrax toxin receptor 2 (CMG2/ANTXR2). Thus, mature and differentiated AM are relatively resistant to the effects of LT as compared with mouse RAW 264.7 macrophages. AM resistance to LT may enhance clearance of the pathogen from the alveolar surface and explain why this surface is relatively free of B. anthracis in animal models and autopsy studies.

Anthrax toxins: a weapon to systematically dismantle the host immune defenses

Successful colonization of the host by bacterial pathogens relies on their capacity to evade the complex and powerful defenses opposed by the host immune system, at least in the initial phases of infection. The two toxins of Bacillus anthracis, lethal toxin and edema toxin, appear to have been shaped by evolution to assist the microorganism in this crucial function, in addition to act as general toxins acting on almost all cell types. Edema toxin causes a consistent elevation of cAMP, an important second messenger the production of which is normally strictly controlled in mammalian cells, whereas lethal toxin cleaves most isoforms of mitogen-activated protein kinase kinases. By disrupting or subverting central modules common to all the principal signaling networks which control immune cell activation, effector function and migration, the anthrax toxins effectively and systematically dismantle both the innate and the adaptive immune defenses of the host. Here, we review the specific effects of the lethal and edema toxins of B. anthracis on the activation and function of phagocytes, dendritic cells and lymphocytes. We also discuss some open issues which should be addressed to gain a comprehensive insight into the complex relationship that B. anthracis establishes with the host.

Mutant anthrax toxin B moiety (protective antigen) inhibits angiogenesis and tumor growth

Bacillus anthracis protective antigen (PA), the B subunit of the binary anthrax toxin, binds to the cellular receptors capillary morphogenesis gene 2 protein and tumor endothelial marker 8 with high affinity. Both receptors are expressed on endothelial cells during angiogenesis. We sought to determine whether one could inhibit angiogenesis by interfering with the binding of these receptors to their endogenous ligands. Here, we show that wild-type PA inhibits both vascular endothelial growth factor-induced and basic fibroblast growth factor-induced angiogenesis at moderate but statistically significant levels. Structure-activity studies identified a PA mutant that exhibited markedly enhanced inhibition of angiogenesis and also inhibited tumor growth in vivo. This mutant, PASSSR, is unable to undergo normal cellular processing and, thus, remains bound to the surface receptor. Further mutation of PASSSR so that it does not bind to these cell surface receptors abolished its ability to inhibit angiogenesis. We conclude that high-affinity anthrax toxin receptor (ATR) ligands, such as PA and PASSSR, are angiogenesis inhibitors and that ATRs are useful targets for antiangiogenic therapy. These results also suggest that endothelial cell-binding proteins from additional pathogens may inhibit angiogenesis and raise the question of the role of such inhibition in pathogenesis.

Transcriptional stimulation of anthrax toxin receptors by anthrax edema toxin and Bacillus anthracis Sterne spore

We used quantitative real-time RT-PCR to not only investigate the mRNA levels of anthrax toxin receptor 1 (ANTXR1) and 2 (ANTXR2) in the murine J774A.1 macrophage cells and different tissues of mice, but also evaluate the effect of anthrax edema toxin and Bacillus anthracis Sterne spores on the expression of mRNA of these receptors. The mRNA transcripts of both receptors were detected in J774A.1 cells and mouse tissues such as the lung, heart, kidney, spleen, stomach, jejunum, brain, skeleton muscle, and skin. The ANTXR2 mRNA level was significantly higher than that of ANTXR1 in J774A.1 cells and all tissues examined. The mRNA expression of both receptors in the lung was the highest among the tissues evaluated. Interestingly, the mRNA expression of both receptors in J774A.1 cells was upregulated by edema toxin. In addition, ANTXR mRNA expression in the lung was downregulated after subcutaneous inoculation of B. anthracis Sterne spores as well as after intranasal administration of anthrax toxin-based vaccine BioThrax. These results suggest that anthrax edema toxin and B. anthracis Sterne spore are involved in the ANTXR mRNA regulation in host cells.