Functional characterization of protease-treated Bacillus anthracis protective antigen.

Characterization of the functional domains of Bacillus anthracis protective antigen (PA, 83-kDa), the common cellular binding molecule for both anthrax edema toxin and anthrax lethal toxin, is important for understanding the mechanism of entry and action of the anthrax toxins. In this study, we generated both biologically active (facilitates killing of J774A.1 cells in combination with lethal factor, LF) and inactive preparations of PA by protease treatment. Limited proteolytic digestion of PA in vitro with trypsin generated a 20-kDa fragment and a biologically active 63-kDa fragment. In contrast, limited digestion of PA with chymotrypsin yielded a preparation containing 37- and 47-kDa fragments defective for biological activity. Treatment with both chymotrypsin and trypsin generated three major fragments, 20, "17," and 47 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This PA preparation was also biologically inactive. To investigate the nature of the defect resulting from chymotrypsin treatment, we assayed PA preparations for the ability to bind to the cellular receptor and to bind and internalize 125I-LF. All radiolabeled PA preparations bound with specificity to J774A.1 cells and exhibited affinities similar to native 83-kDa PA. Once bound to the cell surface receptor, both trypsin-treated PA and chymotrypsin/trypsin-treated PA specifically bound 125I-LF with high affinity. Finally, these PA preparations delivered 125I-LF to a Pronase-resistant cellular compartment in a time- and temperature-dependent fashion. Thus, the biological defect exhibited by chymotrypsin-treated PA is not at the level of cell binding or internalization but at a step later, such as toxin routing or processing by J774A.1 cells. These protease-treated preparations of PA should prove useful in both elucidating the intracellular processing of anthrax lethal toxin and determining the structure-function relationship of PA and LF.     

Proteome analysis of mouse macrophages treated with anthrax lethal toxin

Anthrax toxin produced by Bacillus anthracis is a tripartite toxin comprising of protective antigen (PA), lethal factor (LF) and edema factor (EF). PA is the receptor-binding component, which facilitates the entry of LF or EF into the cytosol. EF is a calmodulin-dependent adenylate cyclase that causes edema whereas LF is a zinc metalloprotease and leads to necrosis of macrophages. It is also important to note that the exact mechanism of LF action is still unclear. With this view in mind, in the present study, we investigated a proteome wide effect of anthrax lethal toxin (LT) on mouse macrophage cells (J774A.1). Proteome analysis of LT-treated and control macrophages revealed 41 differentially expressed protein spots, among which phosphoglycerate kinase I, enolase I, ATP synthase (beta subunit), tubulin beta2, gamma-actin, Hsp70, 14-3-3 zeta protein and tyrosine/tryptophan-3-monooxygenase were found to be down-regulated, while T-complex protein-1, vimentin, ERp29 and GRP78 were found to be up-regulated in the LT-treated macrophages. Analysis of up- and down-regulated proteins revealed that primarily the stress response and energy generation proteins play an important role in the LT-mediated macrophage cell death.     

Monoclonal antibodies directed against protective antigen of Bacillus anthracis enhance lethal toxin activity in vivo

Protective antigen (PA) from Bacillus anthracis binds to cellular receptors, combines with lethal factor (LF) forming lethal toxin (LeTx), and facilitates the translocation of LF into the cytosol. LeTx is cytotoxic for J774A.1 cells, a murine macrophage cell line, and causes death of Fisher 344 rats when injected intravenously. PA is also the major protective component in anthrax vaccines. Antibody-dependent enhancement has been reported for several viral diseases, a bacterial infection, and for B. anthracis LeTx in vitro cytotoxicity. Further screening of our 73 PA monoclonal antibodies (mAbs) identified a total of 17 PA mAbs that enhanced in vitro cytotoxicity at suboptimal concentrations of LeTx. A competitive binding enzyme-linked immunosorbent assay showed that these 17 PA mAbs identified eight different antigenic regions on PA. Eight of the 17 PA mAbs that enhanced LeTx in vitro cytoxicity were examined for their activity in vivo. Of the eight mAbs that were injected intravenously with a sublethal concentration of LeTx into male Fisher 344 rats, four mAbs enhanced the lethality of LeTx and resulted in the death of animals, whereas control animals did not succumb to intoxication. This is the first demonstration that PA mAbs can enhance LeTx intoxication in vivo.     

利用转基因小鼠筛选全人源炭疽致死因子中和抗体

炭疽是由炭疽芽孢杆菌引起的严重威胁人类健康的传染病。炭疽毒素包括3种蛋白质成分:保护性抗原(PA)、致死因子(LF)和水肿因子(EF)。PA与LF形成致死毒素(LT),与EF形成水肿毒素(ET)。由于致死毒素(LT)在感染者损伤及死亡中发挥主要作用,因此在炭疽感染晚期单纯使用抗生素治疗难以发挥疗效,治疗性中和抗体成为目前最有效的炭疽治疗药物。目前国外获得的炭疽毒素抗体多为炭疽PA抗体,美国FDA已批准瑞西巴库(人源PA单抗)用于吸入性炭疽的治疗。一旦炭疽芽孢杆菌被人为改构或PA中和表位发生突变,针对PA单一表位的抗体将可能失效,因此针对LF的抗体将成为炭疽治疗的有效补充。目前国外已有的LF抗体多为鼠源抗体和嵌合抗体,而全人源抗体可以避免鼠源抗体免疫原性高等缺点。本研究首先用LF抗原免疫人抗体转基因小鼠,利用流式细胞仪从小鼠脾淋巴细胞中分选抗原特异的记忆B细胞,通过单细胞PCR方法快速获得两株具有结合活性的抗LF单抗1D7和2B9。瞬时转染Expi 293F细胞制备抗体,通过毒素中和实验(TNA)发现1D7和2B9在细胞模型中均显示较好的中和活性,并且与PA单抗联合使用时,表现出较好的协同作用。总之,本文利用转基因小鼠、流式分选技术和单细胞PCR技术的优势,快速筛选到全人源LF抗体,为快速筛选全人源单克隆抗体开辟了新的思路与方法。     

Effect of pH on stability of anthrax lethal factor: correlation between denaturation and activity

Anthrax is caused by Gram positive bacterium Bacillus anthracis. Pathogenesis is result of production of three protein components, protective antigen (PA), lethal factor (LF), and edema factor (EF). PA in combination with LF (lethal toxin) is lethal to animals, while PA in combination with EF (edema toxin), causes edema. PA, LF, and EF are very thermolabile. Differential scanning calorimetry (DSC) was used to unravel the energetics of LF denaturation as a function of pH ranging from 7.8 to 5.5. Transition temperature (T(m)) of LF was found to be approximately equal to 42 degrees C and onset of denaturation occurs at approximately equal to 30 degrees C. The ratio of calorimetric to van't Hoff's enthalpy was nearly equal to unity at pH 7.0, indicative of presence of single structural domain in LF at pH 7.0, unlike PA which has been structurally observed to consist of 4 domains. It was found by cytotoxicity studies using J774A.1 macrophage like cells that LF was most stable at pH approximately 6.5. This paper reports for the first time the denaturation of LF at different pH values at 37 degrees C and tries to establish a correlation between denaturation and loss of LF activity at different pH values.     

Role of the Protective Antigen Octamer in the Molecular Mechanism of Anthrax Lethal Toxin Stabilization in Plasma

Anthrax is caused by strains of Bacillus anthracis that produce two key virulence factors, anthrax toxin (Atx) and a poly-γ-D-glutamic acid capsule. Atx is comprised of three proteins: protective antigen (PA) and two enzymes, lethal factor (LF) and edema factor (EF). To disrupt cell function, these components must assemble into holotoxin complexes, which contain either a ring-shaped homooctameric or homoheptameric PA oligomer bound to multiple copies of LF and/or EF, producing lethal toxin (LT), edema toxin, or mixtures thereof. Once a host cell endocytoses these complexes, PA converts into a membrane-inserted channel that translocates LF and EF into the cytosol. LT can assemble on host cell surfaces or extracellularly in plasma. We show that, under physiological conditions in bovine plasma, LT complexes containing heptameric PA aggregate and inactivate more readily than LT complexes containing octameric PA. LT complexes containing octameric PA possess enhanced stability, channel-forming activity, and macrophage cytotoxicity relative to those containing heptameric PA. Under physiological conditions, multiple biophysical probes reveal that heptameric PA can prematurely adopt the channel conformation, but octameric PA complexes remain in their soluble prechannel configuration, which allows them to resist aggregation and inactivation. We conclude that PA may form an octameric oligomeric state as a means to produce a more stable and active LT complex that could circulate freely in the blood.     

Anthrax lethal toxin has direct and potent inhibitory effects on B cell proliferation and immunoglobulin production

Protective host immune responses to anthrax infection in humans and animal models are characterized by the development of neutralizing Abs against the receptor-binding anthrax protective Ag (PA), which, together with the lethal factor (LF) protease, composes anthrax lethal toxin (LT). We now report that B cells, in turn, are targets for LT. Anthrax PA directly binds primary B cells, resulting in the LF-dependent cleavage of the MAPK kinases (MAPKKs) and disrupted signaling to downstream MAPK targets. Although not directly lethal to B cells, anthrax LT treatment causes severe B cell dysfunction, greatly reducing proliferative responses to IL-4-, anti-IgM-, and/or anti-CD40 stimulation. Moreover, B cells treated with anthrax LT in vitro or isolated from mice treated with anthrax LT in vivo have a markedly diminished capacity to proliferate and produce IgM in response to TLR-2 and TLR-4 ligands. The suppressive effects of anthrax LT on B cell function occur at picomolar concentrations in vitro and at sublethal doses in vivo. These results indicate that anthrax LT directly inhibits the function of B cells in vitro and in vivo, revealing a potential mechanism through which the pathogen could bypass protective immune responses.     

A dominant negative mutant of Bacillus anthracis protective antigen inhibits anthrax toxin action in vivo

PA63, a proteolytically activated 63-kDa form of anthrax protective antigen (PA), forms heptameric oligomers and has the ability to bind and translocate the catalytic moieties, lethal factor (LF), and edema factor (EF) into the cytosol of mammalian cells. Acidic pH triggers oligomerization and membrane insertion by PA63. A disordered amphipathic loop in domain II of PA (2beta2-2beta3 loop) is involved in membrane insertion by PA63. Because conditions required for membrane insertion coincide with those for oligomerization of PA63 in mammalian cells, residues constituting the 2beta2-2beta3 loop were replaced with the residues of the amphipathic membrane-inserting loop of its homologue iota-b toxin secreted by Clostridium perfringens. It was hypothesized that such a molecule might assemble into hetero-heptameric structures with wild-type PA ultimately leading to the inhibition of cellular intoxication. The mutation blocked the ability of PA to mediate membrane insertion and translocation of LF into the cytosol but had no effect on proteolytic activation, oligomerization, or binding LF. Moreover, an equimolar mixture of purified mutant PA (PA-I) and wild-type PA showed complete inhibition of toxin activity both in vitro on J774A.1 cells and in vivo in Fischer 344 rats thereby exhibiting a dominant negative effect. In addition, PA-I inhibited the channel-forming ability of wild-type PA on the plasma membrane of CHO-K1 cells thereby indicating protein-protein interactions between the two proteins resulting in the formation of mixed oligomers with defective functional activity. Our findings provide a basis for understanding the mechanism of translocation and exploring the possibility of the use of this PA molecule as a therapeutic agent against anthrax toxin action in vivo.     

Cross-reactivity of anthrax and C2 toxin: protective antigen promotes the uptake of botulinum C2I toxin into human endothelial cells

Binary toxins are among the most potent bacterial protein toxins performing a cooperative mode of translocation and exhibit fatal enzymatic activities in eukaryotic cells. Anthrax and C2 toxin are the most prominent examples for the AB(7/8) type of toxins. The B subunits bind both host cell receptors and the enzymatic A polypeptides to trigger their internalization and translocation into the host cell cytosol. C2 toxin is composed of an actin ADP-ribosyltransferase (C2I) and C2II binding subunits. Anthrax toxin is composed of adenylate cyclase (EF) and MAPKK protease (LF) enzymatic components associated to protective antigen (PA) binding subunit. The binding and translocation components anthrax protective antigen (PA(63)) and C2II of C2 toxin share a sequence homology of about 35%, suggesting that they might substitute for each other. Here we show by conducting in vitro measurements that PA(63) binds C2I and that C2II can bind both EF and LF. Anthrax edema factor (EF) and lethal factor (LF) have higher affinities to bind to channels formed by C2II than C2 toxin's C2I binds to anthrax protective antigen (PA(63)). Furthermore, we could demonstrate that PA in high concentration has the ability to transport the enzymatic moiety C2I into target cells, causing actin modification and cell rounding. In contrast, C2II does not show significant capacity to promote cell intoxication by EF and LF. Together, our data unveiled the remarkable flexibility of PA in promoting C2I heterologous polypeptide translocation into cells.     

Internalization and processing of Bacillus anthracis lethal toxin by toxin-sensitive and -resistant cells

Anthrax lethal toxin consists of two separate proteins, protective antigen and lethal factor (LF). Certain macrophages and a mouse macrophage-like cell line, J774A.1, are lysed by low concentrations of lethal toxin. In contrast, another macrophage cell line, IC-21, and all other cell types tested were resistant to this toxin. To discover the basis for this difference, each step in the intoxication process was examined. No differences between sensitive and resistant cells were found in receptor binding or proteolytic activation of protective antigen, steps that are required prior to LF binding. To determine whether resistance results from a defect in translocation to the cytosol, we introduced LF into J774A.1 and IC-21 cells and a nonmacrophage cell line (L6 myoblast) by osmotic lysis of pinocytic vesicles. Only J774A.1 cells were lysed; no effect was observed in IC-21 and L6 cells. These results suggest that resistant cells either lack the intracellular target of LF or fail to process LF to an active form. The relatively low potency of LF introduced into J774A.1 cells by osmotic lysis suggests that protective antigen may also be required at a stage subsequent to endocytosis.     

Thermostabilization of protective antigen—the binding component of anthrax lethal toxin

Protective antigen (PA) is the binding component of anthrax lethal toxin produced by Bacillus anthracis, and constitutes a major ingredient of the vaccine against anthrax. PA and lethal factor when added together are cytolytic to mouse macrophages and J774G8 macrophage cell line. This in vitro lethal toxicity assay is very useful in understanding the molecular mechanism of action of lethal toxin. Effective utilization of PA is, however, hampered due to its thermolability. On prolonged storage at 37 ° C, PA was found to lose its activity almost completely. The effect of solvent additives like trehalose, sorbitol, xylitol, sodium citrate and magnesium sulphate on the thermal stabilization of PA was examined. The results indicated an increase in the stability of PA when the incubation at 37 ° C was carried out in the presence of solvent additives used in the 1–3 M range. Magnesium sulphate helped retain the activity up to 82.7% against the control in which no additive was used, as judged by cytolytic assay using J774G8 macrophage cell line. Trehalose or sodium citrate also showed an appreciable protection of PA activity, while sorbitol or xylitol were not very effective. Competitive binding assay using radiolabeled PA showed that PA had lost capacity of binding to macrophage cells on prolonged incubation at 37 ° C. Circular dichroism results at 4, 18 and 37 ° C indicated an increase in secondary structure at 37 ° C relative to that at 4 or 18 ° C, supporting the activity data.     

Bacillus anthracis Lethal Toxin Disrupts TCR Signaling in CD1d-Restricted NKT Cells Leading to Functional Anergy

Exogenous CD1d-binding glycolipid (α-Galactosylceramide, α-GC) stimulates TCR signaling and activation of type-1 natural killer–like T (NKT) cells. Activated NKT cells play a central role in the regulation of adaptive and protective immune responses against pathogens and tumors. In the present study, we tested the effect of Bacillus anthracis lethal toxin (LT) on NKT cells both in vivo and in vitro. LT is a binary toxin known to suppress host immune responses during anthrax disease and intoxicates cells by protective antigen (PA)-mediated intracellular delivery of lethal factor (LF), a potent metalloprotease. We observed that NKT cells expressed anthrax toxin receptors (CMG-2 and TEM-8) and bound more PA than other immune cell types. A sub-lethal dose of LT administered in vivo in C57BL/6 mice decreased expression of the activation receptor NKG2D by NKT cells but not by NK cells. The in vivo administration of LT led to decreased TCR-induced cytokine secretion but did not affect TCR expression. Further analysis revealed LT-dependent inhibition of TCR-stimulated MAP kinase signaling in NKT cells attributable to LT cleavage of the MAP kinase kinase MEK-2. We propose that Bacillus anthracis–derived LT causes a novel form of functional anergy in NKT cells and therefore has potential for contributing to immune evasion by the pathogen.     

Anthrax biosensor, protective antigen ion channel asymmetric blockade

The significant threat posed by biological agents (e.g. anthrax, tetanus, botulinum, and diphtheria toxins) (Inglesby, T. V., O'Toole, T., Henderson, D. A., Bartlett, J. G., Ascher, M. S., Eitzen, E., Friedlander, A. M., Gerberding, J., Hauer, J., Hughes, J., McDade, J., Osterholm, M. T., Parker, G., Perl, T. M., Russell, P. K., and Tonat, K. (2002) J. Am. Med. Assoc. 287, 2236-2252) requires innovative technologies and approaches to understand the mechanisms of toxin action and to develop better therapies. Anthrax toxins are formed from three proteins secreted by fully virulent Bacillus anthracis, protective antigen (PA, 83 kDa), lethal factor (LF, 90 kDa), and edema factor (EF, 89 kDa). Here we present electrophysiological measurements demonstrating that full-length LF and EF convert the current-voltage relationship of the heptameric PA63 ion channel from slightly nonlinear to highly rectifying and diode-like at pH 6.6. This effect provides a novel method for characterizing functional toxin interactions. The method confirms that a previously well characterized PA63 monoclonal antibody, which neutralizes anthrax lethal toxin in animals in vivo and in vitro, prevents the binding of LF to the PA63 pore. The technique can also detect the presence of anthrax lethal toxin complex from plasma of infected animals. The latter two results suggest the potential application of PA63 nanopore-based biosensors in anthrax therapeutics and diagnostics.     

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.     

Antidotes to anthrax lethal factor intoxication. Part 3: Evaluation of core structures and further modifications to the C2-side chain

Four core structures capable of providing sub-nanomolar inhibitors of anthrax lethal factor (LF) were evaluated by comparing the potential for toxicity, physicochemical properties, in vitro ADME profiles, and relative efficacy in a rat lethal toxin (LT) model of LF intoxication. Poor efficacy in the rat LT model exhibited by the phenoxyacetic acid series (3) correlated with low rat microsome and plasma stability. Specific molecular interactions contributing to the high affinity of inhibitors with a secondary amine in the C2-side chain were revealed by X-ray crystallography.     

Lethal factor of anthrax toxin binds monomeric form of protective antigen

Anthrax toxin consists of three components: the enzymatic moieties edema factor (EF) and the lethal factor (LF) and the receptor-binding moiety protective antigen (PA). These toxin components are released from Bacillus anthracis as unassociated proteins and form complexes on the surface of host cells after proteolytic processing of PA into PA20 and PA63. The sequential order of PA heptamerization and ligand binding, as well as the exact mechanism of anthrax toxin entry into cells, are still unclear. In the present study, we provide direct evidence that PA63 monomers are sufficient for binding to the full length LF or its LF-N domain, though with lower affinity with the latter. Therefore, PA oligomerization is not a necessary condition for LF/PA complex formation. In addition, we demonstrated that the PA20 directly interacts with the LF-N domain. Our data points to an alternative process of self-assembly of anthrax toxin on the surface of host cells.     

Protective antigen-binding domain of anthrax lethal factor mediates translocation of a heterologous protein fused to its amino- or carboxy-terminus

The edema factor (EF) and lethal factor (LF) components of anthrax toxin require anthrax protective antigen (PA) for binding and entry into mammalian cells. After internalization by receptor-mediated endocytosis, PA facilitates the translocation of EF and LF across the membrane of an acidic intracellular compartment. To characterize the translocation process, we generated chimeric proteins composed of the PA recognition domain of LF (LF_N; residues 1–255) fused to either the amino-terminus or the carboxy-terminus of the catalytic chain of diphtheria toxin (DTA). The purified fusion proteins retained ADP-ribosyltransferase activity and reacted with anti-sera against LF and diphtheria toxin. Both fusion proteins strongly inhibited protein synthesis in CHO-K1 cells in the presence of PA, but not in its absence, and they showed similar levels of activity. This activity could be inhibited by adding LF or the LF_N fragment (which blocked the interaction of the fusion proteins with PA), by adding inhibitors of endo-some acidification known to block entry of EF and LF into cells, or by introducing mutations that attenuated the ADP-ribosylation activity of the DTA moiety. The results demonstrate that LF_N fused to either the amino-terminus or the carboxy-terminus of a heterologous protein retains its ability to complement PA in mediating translocation of the protein to the cytoplasm. Besides its importance in understanding translocation, this finding provides the basis for constructing a translocation vector that mediates entry of a variety of heterologous proteins, which may require a free amino- or carboxy-terminus for biological activity, into the cytoplasm of mammalian cells.     

Participation of residue F552 in domain III of the protective antigen in the biological activity of anthrax lethal toxin

The protective antigen (PA) component of anthrax toxin translocates the catalytic moieties lethal factor (LF) and edema factor (EF) into the cytosol. The proteolytically activated 63 kDa form of PA (PA63) has the ability to oligomerize and bind LF/EF. PA has four distinct domains performing specialized functions; whereas the function of domains I, II and IV has been well characterized, domain III has no known role in the biological activity of PA. Here we report the role of amino acid residues lining an exposed hydrophobic patch of domain III in the biological activity of PA. The residues Phe552, Phe554, lIe562, Leu566 and lle574 were individually substituted with alanine and the effect was studied. All mutant PA proteins except Phe552Ala were equally active as wild-type PA in exhibiting a toxic phenotype to J774A.1 cells in the presence of LF. Substitution of Ala for Phe552 reduced the ability of PA to intoxicate cells by more than 250-fold. However, Phe552Ala was equally active in receptor binding and susceptibility to trypsin and chymotrypsin as wild-type PA, the activities that have been shown to be essential for the biological activity of PA. This mutated PA protein had a decreased ability to bind LF, oligomerize on cells and to induce release of 86Rb+ from Chinese hamster ovary cells. These results suggest that the residue Phe552 in PA plays an important role in LF binding and oligomerization. Our study provides a basis for further exploration of the biological significance of domain III of PA.     

Membrane type-1 matrix metalloproteinase (MT1-MMP) protects malignant cells from tumoricidal activity of re-engineered anthrax lethal toxin

Protective antigen (PA) and lethal factor (LF) are the two components of anthrax lethal toxin. PA is responsible for interacting with cell receptors and for the subsequent translocation of LF inside the cell compartment. A re-engineered toxin comprised of PA and a fusion chimera LF/Pseudomonas exotoxin (FP59) is a promising choice for tumor cell surface targeting. We demonstrated, however, that in vitro in cell-free system and in cultured human colon carcinoma LoVo, fibrosarcoma HT1080 and glioma U251 cells membrane type-1 matrix metalloproteinase (MT1-MMP) cleaves both the PA83 precursor and the PA63 mature protein. Exhaustive MT1-MMP cleavage of PA83 in vitro generates several major degradation fragments with an N-terminus at Glu40, Leu48, and Gln512. In cultured cells, MT1-MMP-dependent cleavage releases the cell-bound PA83 and PA63 species from the cell surface. As a result, MT1-MMP expressing cells have less PA63 to internalize. In agreement, our observations demonstrate that MT1-MMP proteolysis of PA makes the MT1-MMP-expressing aggressive invasive cells resistant to the cytotoxic effect of a bipartite PA/FP59 toxin. We infer from our studies that synthetic inhibitors of MMPs are likely to increase the therapeutic anti-cancer effect of anthrax toxin. In addition, our study supports a unique role of furin in the activation of PA, thereby suggesting that furin inhibitors are the likely specific drugs for short-term therapy of anthrax infection.