Interactions between anthrax toxin receptors and protective antigen

Defensive strategies of Bacillus anthracis that promote a fatal disease

Bacillus anthracis is a Gram-positive bacterium that causes anthrax. Bacterial spores that enter the host germinate into metabolically active bacilli that disseminate throughout the body and replicate to high numbers. Two virulence factors are essential for this unrestrained growth. The first is a weakly immunogenic poly gamma-D-glutamic acid capsule that surrounds the bacilli and confers resistance to phagocytosis. The second virulence factor, anthrax toxin, disrupts multiple host functions to diminish the immune response.

EMILINs interact with anthrax protective antigen and inhibit toxin action in vitro

The informational spectrum method (ISM) is a virtual spectroscopy method for the fast analysis of potential protein-protein relationships. By applying the ISM approach to the GeneBank protein database the vascular proteins EMILIN1 (Elastin Microfibril Interface Located ProteIN), EMILIN2, MMN1, and MMN2 were identified as additional anthrax PA antigen interacting molecules. This virtual molecular interaction was formally proven by solid phase assays using recombinant proteins. The interaction is independent of the presence of divalent cations and does not involve PA aspartic residue at 683, a critical residue in receptor binding. In fact, the D683A point mutation fully prevented the cell intoxication ability of PA in the presence of Lethal Factor, but it was fully ineffective on the binding of mutated PA to EMILIN1 and EMILIN2. The ISM approach also led to the identification of the potential interaction sites between PA and EMILINs. A PA mutant with a deletion at residue D425 and solid phase protein-protein interaction studies as well as deletion mutant of EMILIN2 confirmed the hypothesized interaction site. Our findings imply that the PA-cell surface receptor interaction is not likely to provide the full explanation for the vascular lesions and prominent hemorrhages that follow Bacillus anthracis infection and spreading and call into play vascular associated proteins such as EMILINs as potential inhibitory proteins.

Anthrax toxin: receptor binding, internalization, pore formation, and translocation

Anthrax toxin consists of three nontoxic proteins that self-assemble at the surface of receptor-bearing mammalian cells or in solution, yielding a series of toxic complexes. Two of the proteins, called Lethal Factor (LF) and Edema Factor (EF), are enzymes that act on cytosolic substrates. The third, termed Protective Antigen (PA), is a multifunctional protein that binds to receptors, orchestrates the assembly and internalization of the complexes, and delivers them to the endosome. There, the PA moiety forms a pore in the endosomal membrane and promotes translocation of LF and EF to the cytosol. Recent advances in understanding the entry process include insights into how PA recognizes its two known receptors and its ligands, LF and EF; how the PA:receptor interaction influences the pH-dependence of pore formation; and how the pore functions in promoting translocation of LF and EF across the endosomal membrane.

The role of the inflammasome in cellular responses to toxins and bacterial effectors

Invading pathogens are recognized by mammalian cells through dedicated receptors found either at the cell surface or in the cytoplasm. These receptors, like the trans-membrane Toll-like Receptors (TLR) or the cytosolic Nod-like Receptors (NLR), initiate innate immunity after recognition of molecular patterns found in bacteria or viruses, such as LPS, flagellin, or double-stranded RNA. Recognition of molecules produced only by a specific pathogen, such as a viral envelop protein or a bacterial adhesin does not appear to occur. Bacterial protein toxins, however, might compose an intermediate class. Considering the diversity of toxins in terms of structure, it is unlikely that cells respond to them via specific molecular recognition. It rather appears that different classes of toxins trigger cellular changes that are sensed by the cells as danger signals, such as changes in cellular ion composition after membrane perforation by pore-forming toxins or type III secretion systems. The signaling pathways triggered through toxin-induced cell alterations will likely play a role in modulating host responses to virulent bacteria. We will here describe the few studied cases in which detection of the toxin by the host cell was addressed. The review will include not only toxins but also bacteria effectors secreted by the bacterium in to the host cell cytoplasm.

Expression of furin-linked Fab fragments against anthrax toxin in a single mammalian expression vector

Human anti-recombinant protective antigen (rPA) Fab genes were previously cloned from single B cells of a donor immunized with anthrax vaccine using fluorescence activated cell sorting with fluorescein labeled rPA and single-cell PCR. The light and heavy chains were sub-cloned individually into mammalian expression vectors pSecTag2B or pEXPR44, respectively, and expressed in the same CHOK1 cells. Alternatively, the same heavy and light chains were linked together, using PCR, with an in-frame sequence coding for a furin cleavage site. This construct was cloned into pSecTag2B and expressed in CHOK1 cells. Once expressed, the individual chains combined in vivo to form a Fab fragment which was purified as a single protein when either method was utilized. The human Fab antibodies produced by this technique were functional when tested in Western blots using the recombinant PA antigen as the target.

Manipulation of host signalling pathways by anthrax toxins

Infectious microbes face an unwelcoming environment in their mammalian hosts, which have evolved elaborate multicelluar systems for recognition and elimination of invading pathogens. A common strategy used by pathogenic bacteria to establish infection is to secrete protein factors that block intracellular signalling pathways essential for host defence. Some of these proteins also act as toxins, directly causing pathology associated with disease. Bacillus anthracis, the bacterium that causes anthrax, secretes two plasmid-encoded enzymes, LF (lethal factor) and EF (oedema factor), that are delivered into host cells by a third bacterial protein, PA (protective antigen). The two toxins act on a variety of cell types, disabling the immune system and inevitably killing the host. LF is an extraordinarily selective metalloproteinase that site-specifically cleaves MKKs (mitogen-activated protein kinase kinases). Cleavage of MKKs by LF prevents them from activating their downstream MAPK (mitogen-activated protein kinase) substrates by disrupting a critical docking interaction. Blockade of MAPK signalling functionally impairs cells of both the innate and adaptive immune systems and induces cell death in macrophages. EF is an adenylate cyclase that is activated by calmodulin through a non-canonical mechanism. EF causes sustained and potent activation of host cAMP-dependent signalling pathways, which disables phagocytes. Here I review recent progress in elucidating the mechanisms by which LF and EF influence host signalling and thereby contribute to disease.

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.

Palmitoylation: policing protein stability and traffic

Palmitate modifies both peripheral and integral membrane proteins and its addition can be permanent or transient, which makes it unique among the lipid modifications of proteins. The presence of palmitate on a protein affects how the protein interacts with lipids and proteins in a membrane compartment, and the reversibility of palmitoylation allows different modes of trafficking between membrane compartments. Here, we review recent studies that have provided insights into the mechanisms that mediate the functional consequences of this versatile modification.

Contribution of toxins to the pathogenesis of inhalational anthrax

Inhalational anthrax is a life-threatening infectious disease of considerable concern, especially as a potential bioterrorism agent. Progress is gradually being made towards understanding the mechanisms used by Bacillus anthracis to escape the immune system and to induce severe septicaemia associated with toxaemia and leading to death. Recent advances in fundamental research have revealed previously unsuspected roles for toxins in various cell types. We summarize here pathological data for animal models and macroscopic histological examination data from recent clinical records, which we link to the effects of toxins. We describe three major steps in infection: (i) an invasion phase in the lung, during which toxins have short-distance effects on lung phagocytes; (ii) a phase of bacillus proliferation in the mediastinal lymph nodes, with local effects of toxins; and (iii) a terminal, diffusion phase, characterized by a high blood bacterial load and by long-distance effects of toxins, leading to host death. The pathophysiology of inhalational anthrax thus involves interactions between toxins and various cell partners, throughout the course of infection.

Inhibition of anthrax protective antigen outside and inside the cell

In the course of Bacillus anthracis infection, B. anthracis lethal factor (LF) and edema factor bind to a protective antigen (PA) associated with cellular receptors ANTXR1 (TEM8) or ANTXR2 (CMG2), followed by internalization of the complex via receptor-mediated endocytosis. A new group of potential antianthrax drugs, beta-cyclodextrins, has recently been described. A member of this group, per-6-(3-aminopropylthio)-beta-cyclodextrin (AmPrbetaCD), was shown to inhibit the toxicity of LF in vitro and in vivo. In order to determine which steps in lethal factor trafficking are inhibited by AmPrbetaCD, we developed two targeted fluorescent tracers based on LFn, a catalytically inactive fragment of LF: (i) LFn site specifically labeled with the fluorescent dye AlexaFluor-594 (LFn-Al), and (ii) LFn-decorated liposomes loaded with the fluorescent dye 8-hydroxypyrene-1,3,6-trisulfonic acid (LFn-Lip). Both tracers retained high affinity to PA/ANTXR complexes and were readily internalized via receptor-mediated endocytosis. Using fluorescent microscopy, we found that AmPrbetaCD inhibits receptor-mediated cell uptake but not the binding of LFn-Al to PA/ANTXR complexes, suggesting that AmPrbetaCD works outside the cell. Moreover, AmPrbetaCD and LFn-Al synergistically protect RAW 264.7 cells from PA-mediated LF toxicity, confirming that AmPrbetaCD did not affect the binding of LFn-Al to receptor-associated PA. In contrast, AmPrbetaCD did not inhibit PA-mediated internalization of LFn-Lip, suggesting that multiplexing of LFn on the liposomal surface overcomes the inhibiting effects of AmPrbetaCD. Notably, internalized LFn-Al and LFn-Lip protected cells that overexpressed anthrax receptor TEM8 from PA-induced, LF-independent toxicity, suggesting an independent mechanism for PA inhibition inside the cell. These data suggest the potential for the use of beta-cyclodextrins in combination with LFn-Lip loaded with antianthrax drugs against intracellular targets.

Tumour-associated angiogenesis in human colorectal cancer

Tumour angiogenesis is a critical step in the growth, metastatic spread and regrowth of colorectal cancer. Angiogenesis specific to tumour is a complicated process, the mechanisms of which remain unclear. Metastasis of colorectal cancer may result from passive entry into the circulation secondary to the effect of angiogenic factors. The survival and growth of colorectal tumour and thus their metastases are dependent on the balance of endogenous angiogenic and anti-angiogenic factors such that the outcome favours increased angiogenesis. Angiogenesis has become an attractive target for anticancer drug development, based on its important roles in tumour growth, invasion and metastasis. Several growth factors have been identified that regulate angiogenesis in colorectal cancer; the most important of these are vascular endothelial growth factors (VEGF), and of the several angiogenic factors, VEGF expression at the deepest invasive site of tumour is the most statistically significant prognostic indicator in advanced colorectal carcinoma. In this review article, we provide an overview on angiogenic factors and their receptors, and discuss the role of newly identified tumour endothelial markers (TEMs) that are involved in tumour-associated angiogenesis in colorectal cancer.

Innate recognition of intracellular bacteria

The molecular repertoire for innate recognition of bacterial pathogens has expanded rapidly in the past decade. These immunosensors include Toll-like receptors and the more recently defined NOD-like receptors (NLRs): NODs, NALPs, NAIP and IPAF. Toll-like receptors signal from the cell surface or endosome upon ligand binding, whereas NLRs are activated by characteristic bacterially derived molecules, such as peptidoglycan, RNA, toxins and flagellin, in the cytosol. Studies using animal and culture models of bacterial infection indicate a pro-inflammatory role for NLRs, mediated by signaling through nuclear transcription factor kappaB and activation of caspase-1 by the inflammasome. These data also support a synergistic role for extracellular and intracellular bacterial sensing in regulating inflammation. In humans, NLR mutations are often associated with autoinflammatory syndromes, suggesting a complex role for cytosolic surveillance in systemic innate immunity.

Multiplicity of the interactions of Wnt proteins and their receptors

Wnts are secreted proteins that are essential for a wide array of developmental and physiological processes. They signal across the plasma membranes by interacting with serpentine receptors of the Frizzled (Fz) family and members of the low-density-lipoprotein receptor-related protein (LRP) family. Recent advances in the Wnt signaling field have revealed that Wnt-unrelated proteins activate or suppress Wnt signaling by binding to Fzs or LRP5/6 and that atypical receptor tyrosine kinases mediate Wnt signaling independently of Fz and/or function as a Fz co-receptor. This review highlights recent progress in our understanding of the multiplicity of Wnts and their receptors. We discuss how the interaction between the ligands and receptors activate distinct intracellular signaling pathways. We also discuss how intracellular trafficking of Wnt signaling components can regulate the sensitivity of cells to Wnts.

The Bacteroides fragilis toxin binds to a specific intestinal epithelial cell receptor

The Bacteroides fragilis toxin (BFT) is the only known virulence factor of enterotoxigenic B. fragilis. BFT has previously been shown to act, at least in part, through cleavage of the intercellular adhesion protein E-cadherin. A specific cellular receptor for BFT has not been identified. The goal of this study was to determine if the initial interaction of BFT with intestinal epithelial cells was consistent with binding to a specific cellular receptor. Purified BFT was labeled with a fluorophore or iodide to assess specific cellular binding and the properties of BFT cellular binding. BFT binds specifically to intestinal epithelial cell lines in vitro in a polarized manner. However, specific binding occurs only at 37 degrees C and requires BFT metalloprotease activity. The BFT receptor is predicted to be a membrane protein other than E-cadherin or a known protease-activated receptor (PAR1 to PAR4). BFT binding is resistant to acid washing, suggesting an irreversible interaction. Sugar or lipid residues do not appear to be involved in the mechanism of BFT cellular binding, but binding is sensitive to membrane cholesterol depletion. We conclude that intestinal epithelial cells in vitro possess a specific membrane BFT receptor that is distinct from E-cadherin. The data favor a model in which the metalloprotease domain of BFT processes its receptor protein, initiating cellular signal transduction that mediates the biological activity of BFT. However, activation of recognized protease-activated receptors does not mimic or block BFT biological activity or binding, suggesting that additional protease-activated receptors on intestinal epithelial cells remain to be identified.

Anthrax Edema Factor, Voltage-dependent Binding to the Protective Antigen Ion Channel and Comparison to LF Binding

Anthrax toxin complex consists of three different molecules, the binding component protective antigen (PA, 83 kDa), and the enzymatic components lethal factor (LF, 90 kDa) and edema factor (EF, 89 kDa). The 63-kDa N-terminal part of PA, PA(63), forms a heptameric channel that inserts at low pH in endosomal membranes and that is necessary to translocate EF and LF in the cytosol of the target cells. EF is an intracellular active enzyme, which is a calmodulin-dependent adenylate cyclase (89 kDa) that causes a dramatic increase of intracellular cAMP level. Here, the binding of full-length EF on heptameric PA(63) channels was studied in experiments with artificial lipid bilayer membranes. Full-length EF blocks the PA(63) channels in a dose, temperature, voltage, and ionic strength-dependent way with half-saturation constants in the nanomolar concentration range. EF only blocked the PA(63) channels when PA(63) and EF were added to the same side of the membrane, the cis side. Decreasing ionic strength and increasing transmembrane voltage at the cis side of the membranes resulted in a strong decrease of the half-saturation constant for EF binding. This result suggests that ion-ion interactions are involved in EF binding to the PA heptamer. Increasing temperature resulted in increasing half-saturation constants for EF binding to the PA(63) channels. The binding characteristics of EF to the PA(63) channels are compared with those of LF binding. The comparison exhibits similarities but also remarkable differences between the bindings of both toxins to the PA(63) channel.

Immunotoxins in the treatment of refractory hairy cell leukemia

An increasing number of patients who have hairy cell leukemia (HCL) have persistent disease that requires treatment, despite purine analogs, splenectomy, interferon, and rituximab. Many of these patients have been treated successfully with immunotoxins. An immunotoxin contains a protein toxin connected to a cell-binding ligand, such as an antibody. An immunotoxin recognizes the target cell, internalizes, and the toxin translocates to the cytosol where it inhibits protein synthesis enzymatically. Immunotoxins that show activity in HCL contain truncated Psedomonas exotoxin fused to the Fv fragments of anti-CD25 or anti-CD22 monoclonal antibodies. Both agents, termed LMB-2 and BL22, respectively, have been tested in patients who have HCL after failure of purine analogs and other therapies; major responses have been achieved in most patients.

About lipids and toxins

Many mono or multicellular organisms secrete soluble proteins, referred to as protein toxins, which alter the behavior of foreign, or target cells, possibly leading to their death. These toxins affect either the cell membrane by forming pores or modifying lipids, or some intracellular target. To reach this target, they must cross one of the cellular membranes, generally that of an intracellular organelle. As described in this minireview, lipids play crucial roles in the intoxication process of most if not all toxins, by allowing/promoting binding, endocytosis, trafficking and/or translocation into the cytoplasm.

Quantitative analysis of the effect of cell type and cellular differentiation on protective antigen binding to human target cells

We quantitatively measured protective antigen (PA) binding to human cells targeted by anthrax lethal toxin (LT). Affinities were less than 50 nM for all cells, but differentiated cells (macrophages and neutrophils) had significantly increased PA binding and endothelial cells demonstrated the most binding. Combined with the function of such cells, this suggests that PA receptors interact with the extracellular matrix and that differentiation increases the number of PA-specific receptors, which supports previously observed differentiation-induced LT susceptibility. Our results quantifiably confirm that the generality of PA binding will complicate its use as a tumor targeting agent.

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.

Onset of anthrax toxin pore formation

Protective antigen (PA) is the anthrax toxin protein recognized by capillary morphogenesis gene 2 (CMG2), a transmembrane cellular receptor. Upon activation, seven ligand-receptor units self-assemble into a heptameric ring-like complex that becomes endocytozed by the host cell. A critical step in the subsequent intoxication process is the formation and insertion of a pore into the endosome membrane by PA. The pore conversion requires a change in binding between PA and its receptor in the acidified endosome environment. Molecular dynamics simulations totaling approximately 136 ns on systems of over 92,000 atoms were performed. The simulations revealed how the PA-CMG2 complex, stable at neutral conditions, becomes transformed at low pH upon protonation of His-121 and Glu-122, two conserved amino acids of the receptor. The protonation disrupts a salt bridge important for the binding stability and leads to the detachment of PA domain II, which weakens the stability of the PA-CMG2 complex significantly, and subsequently releases a PA segment needed for pore formation. The simulations also explain the great strength of the PA-CMG2 complex achieves through extraordinary coordination of a divalent cation.

Furin regulates the intracellular activation and the uptake rate of cell surface-associated MT1-MMP

Invasion-promoting membrane type-1 matrix metalloproteinase (MT1-MMP) functions in cancer cells as an oncogene and as a mediator of proteolytic events on the cell surface. To exert its functional activity, MT1-MMP requires proteolytic removal of the prodomain sequence. There are two potential furin cleavage motifs, R(89)-R-P-R-C(93) and R(108)-R-K-R-Y(112), in the prodomain sequence of MT1-MMP. Our data suggest an important role of furin and related proprotein convertases (PCs) in mediating both the activation of MT1-MMP and the levels of functionally active MT1-MMP at the surface of cancer cells. We have determined that the peptide sequence that spans the first cleavage site is susceptible to furin and PC5/6, whereas the second sequence is susceptible to furin and also to PC5/6, PC7 and PACE4. In the structure of the MT1-MMP proenzyme, the R(89)-R-P-R-C(93) site, however, is inaccessible to PCs. Our studies also demonstrated a direct functional link between the activation and the uptake rate of the proenzyme and the enzyme of MT1-MMP. Thus, the uptake rate of the latent MT1-MMP proenzyme noticeably exceeded that of the active enzyme. We conclude that furin and related PCs are the essential components of the specialized cellular machinery that controls the levels of the functionally active, mature, MT1-MMP enzyme on the cell surface to continually support the potency of pericellular proteolysis.

LRP6 Holds the Key to the Entry of Anthrax Toxin

In this issue of Cell, it is demonstrated that the low-density lipoprotein receptor-related protein 6 (LRP6) promotes endocytosis of the anthrax toxin into cells. LRP6 acts as a coreceptor with either TEM8 or CMG2, the two previously identified receptors for anthrax toxin.

The 2β2–2β3 loop of anthrax protective antigen contains a dominant neutralizing epitope

Anthrax toxin consists of three proteins, protective antigen (PA), lethal factor, and edema factor. PA is the major component in the current anthrax vaccine, but the antigenic epitopes on it are not well-defined. We generated a pool of toxin-neutralizing anti-PA monoclonal antibodies (MAbs) to analyze the neutralizing epitopes of PA. Nine toxin-neutralizing MAbs obtained were found bound to three different domains of PA respectively, among which three MAbs with the strongest toxin-neutralizing activity recognized the same epitope within domain 2. This epitope was fine mapped to the chymotrypsin-sensitive site, (312)SFFD(315), in the 2beta(2)-2beta(3) loop of PA, using phage-displayed random peptide libraries and mutation analysis. The result demonstrated for the first time that the 2beta(2)-2beta(3) loop, which is involved in the transition of PA oligomers from prepore to pore, contains a dominant neutralizing epitope. This work contributes to the immunological and functional analysis of PA and offers perspective for the development of a new epitope vaccine against anthrax.

Both PA63 and PA83 are endocytosed within an anthrax protective antigen mixed heptamer: A putative mechanism to overcome a furin deficiency

Anthrax toxin consists of protective antigen (PA), and lethal (LF) and edema (EF) factors. A 83 kDa PA monomer (PA83) precursor binds to the cell receptor. Furin-like proprotein convertases (PCs) cleave PA83 to generate cell-bound 63 kDa protein (PA63). PA63 oligomerizes to form a ring-shaped heptamer that binds LF-EF and facilitates their entry into the cells. Several additional PCs, as opposed to furin alone, are capable of processing PA83. Following the incomplete processing of the available pool of PA83, the functional heptamer includes both PA83 and PA63. The available structures of the receptor-PA complex imply that the presence of either one or two molecules of PA83 will not impose structural limitations on the formation of the heptamer and the association of either the (PA83)(1)(PA63)(6) or (PA83)(2)(PA63)(5) heteroheptamer with LF-EF. Our data point to the intriguing mechanism of anthrax that appears to facilitate entry of the toxin into the cells which express limiting amounts of PCs and an incompletely processed PA83 pool.

Human monoclonal anti-protective antigen antibody completely protects rabbits and is synergistic with ciprofloxacin in protecting mice and guinea pigs against inhalation anthrax

Prevention of inhalation anthrax requires early and extended antibiotic therapy, and therefore, alternative treatment strategies are needed. We investigated whether a human monoclonal antibody (AVP-21D9) to protective antigen (PA) would protect mice, guinea pigs, and rabbits against anthrax. Control animals challenged with Bacillus anthracis Ames spores by the intranasal route died within 3 to 7 days. AVP-21D9 alone provided minimal protection against anthrax in the murine model, but its efficacy was notably better in guinea pigs. When Swiss-Webster mice, challenged with five 50% lethal doses (LD50s) of anthrax spores, were given a single 16.7-mg/kg of body weight AVP-21D9 antibody dose combined with ciprofloxacin (30 mg/kg/day for 6 days) 24 h after challenge, 100% of the mice were protected for more than 30 days, while ciprofloxacin or AVP-21D9 alone showed minimal protection. Similarly, when AVP-21D9 antibody (10 to 50 mg/kg) was combined with a low, nonprotective dose of ciprofloxacin (3.7 mg/kg/day) and administered to guinea pigs for 6 days, synergistic protection against anthrax was observed. In contrast, a single dose of AVP-21D9 antibody (1, 5, 10, or 20 mg/kg) but not 0.2 mg/kg alone completely protected rabbits against challenge with 100 LD50s of B. anthracis Ames spores, and 100% of the rabbits survived rechallenge. Further, administration of AVP-21D9 (10 mg/kg) to rabbits at 0, 6, and 12 h after challenge with anthrax spores resulted in 100% survival; however, delay of antibody treatment by 24 and 48 h reduced survival to 80% and 60%, respectively. Serological analysis of sera from various surviving animals 30 days postprimary infection showed development of a species-specific PA enzyme-linked immunosorbent assay antibody titer that correlated with protection against reinfection. Taken together, the effectiveness of human anti-PA antibody alone or in combination with low ciprofloxacin levels may provide the basis for an improved strategy for prophylaxis or treatment following inhalation anthrax infection.

Receptor palmitoylation and ubiquitination regulate anthrax toxin endocytosis

The anthrax toxin is composed of three independent polypeptide chains. Successful intoxication only occurs when heptamerization of the receptor-binding polypeptide, the protective antigen (PA), allows binding of the two enzymatic subunits before endocytosis. We show that this tailored behavior is caused by two counteracting posttranslational modifications in the cytoplasmic tail of PA receptors. The receptor is palmitoylated, and this unexpectedly prevents its association with lipid rafts and, thus, its premature ubiquitination. This second modification, which is mediated by the E3 ubiquitin ligase Cbl, only occurs in rafts and is required for rapid endocytosis of the receptor. As a consequence, cells expressing palmitoylation-defective mutant receptors are less sensitive to anthrax toxin because of a lower number of surface receptors as well as premature internalization of PA without a requirement for heptamerization.

Effects of Spontaneous Deamidation on the Cytotoxic Activity of the Bacillus anthracis Protective Antigen

Protective antigen (PA) is a central virulence factor of Bacillus anthracis and a key component in anthrax vaccines. PA binds to target cell receptors, is cleaved by the furin protease, self-aggregates to heptamers, and finally internalizes as a complex with either lethal or edema factors. Under mild room temperature storage conditions, PA cytotoxicity decreased (t(1/2) approximately 7 days) concomitant with the generation of new acidic isoforms, probably through deamidation of Asn residues. Ranking all 68 Asn residues in PA based on their predicted deamidation rates revealed five residues with half-lives of <60 days, and these residues were further analyzed: Asn10 in the 20-kDa region, Asn162 at P6 vicinal to the furin cleavage site, Asn306 in the pro-pore translocation loop, and both Asn713 and Asn719 in the receptor-binding domain. We found that PA underwent spontaneous deamidation at Asn162 upon storage concomitant with decreased susceptibility to furin. A panel of model synthetic furin substrates was used to demonstrate that Asn162 deamidation led to a 20-fold decrease in the bimolecular rate constant (k(cat)/Km) of proteolysis due to the new negatively charged residue at P6 in the furin recognition sequence. Furthermore, reduced PA cytotoxicity correlated with a decrease in PA cell binding and also with deamidation of Asn713 and Asn719. On the other hand, neither deamidation of Asn10 or Asn306 nor impairment of heptamerization could be observed upon prolonged PA storage. We suggest that PA inactivation during storage is associated with susceptible deamidation sites, which are intimately involved in both mechanisms of PA cleavage by furin and PA-receptor binding.