A deleted variant of Bacillus anthracis protective antigen is non-toxic and blocks anthrax toxin action in vivo

Anthrax toxin is the only protein secreted by Bacillus anthracis that contributes to the virulence of this bacterium. An obligatory step in the action of anthrax toxin on eukaryotic cells is cleavage of the receptor-bound protective antigen (PA) protein (83 kilodaltons) to produce a 63-kilodalton, receptor-bound COOH-terminal fragment. A similar fragment can be obtained by limited treatment with trypsin. This proteolytic processing event exposes a site with high affinity for the other two anthrax toxin proteins, lethal factor and edema factor. Terminal sequencing of the purified fragment showed that the activating cleavage occurred in the sequence Arg164-Lys165-Lys166-Arg167. The gene encoding PA was mutagenized to delete residues 163-168, and the deleted PA was purified from a Bacillus subtilis host. The deleted PA was not cleaved by either trypsin or the cell-surface protease, and was non-toxic when administered with lethal factor. Purified, deleted PA protected rats when administered 90 min before injection of 20 minimum lethal doses of toxin. This mutant PA may be useful as a replacement for the PA that is the major active ingredient in the current human anthrax vaccine, because deleted PA is expected to have normal immunogenicity, but would not combine with trace amounts of LF and EF to cause toxicity.

Adenylate cyclase toxins from Bacillus anthracis and Bordetella pertussis. Different processes for interaction with and entry into target cells

Adenylate cyclase (AC) toxins produced by Bacillus anthracis and Bordetella pertussis were compared for their ability to interact with and intoxicate Chinese hamster ovary cells. At 30 degrees C, anthrax AC toxin exhibited a lag of 10 min for measurable cAMP accumulation that was not seen with pertussis AC toxin. This finding is consistent with previous data showing inhibition of anthrax AC toxin but not pertussis AC toxin entry by inhibitors of receptor-mediated endocytosis (Gordon, V. M., Leppla, S. H., and Hewlett, E. L. (1988) Infect. Immun. 56, 1066-1069). Treatment of target Chinese hamster ovary cells with trypsin or cycloheximide reduced anthrax AC toxin-induced cAMP accumulation by greater than 90%, but was without effect on pertussis AC toxin. In contrast, incubation of the AC toxins with gangliosides prior to addition to target cells inhibited cAMP accumulation by pertussis AC toxin, but not anthrax AC toxin. To evaluate the role of lipids in the interaction of pertussis AC toxin with membranes, multicompartmental liposomes were loaded with a fluorescent marker and exposed to toxin. Pertussis AC toxin elicited marker release in a time- and concentration-dependent manner and required a minimal calcium concentration of 0.2 mM. These data demonstrate that the requirements for intoxication by the AC toxins from B. anthracis and B. pertussis are fundamentally different and provide a perspective for new approaches to study the entry processes.

Transcriptional regulation of the protective antigen gene of Bacillus anthracis

Bicarbonate is required for production of the major virulence factors, the toxins and capsule, of Bacillus anthracis. In this study we examined the basis for stimulation of production of protective antigen (PA), a central component of the two anthrax toxins encoded by plasmid pXO1. RNA prepared from B. anthracis grown in media with and without added bicarbonate was probed for PA mRNA. Data showed that bicarbonate was required for increased transcription of the PA gene (pag) in minimal medium. Transcription of pag was low in rich medium and could not be stimulated by the addition of bicarbonate. To characterize further the factors required for transcriptional regulation of pag, the promoter region of pag was fused to the chloramphenicol acetyltransferase gene (cat-86) of vector pPL703 and transformed by electroporation into pXO1+ (Tox+) and pXO1- (Tox-) strains of B. anthracis. Analysis of chloramphenicol acetyltransferase produced by the pag-cat-86 fusion in each of these backgrounds confirmed the results obtained by hybridization. Data obtained with this fusion also revealed that the large toxin plasmid, pXO1, found in virulent strains of B. anthracis, was required for stimulation of transcription of pag by bicarbonate. This result suggests the existence of a trans-acting factor that is involved in the activation of pag transcription.

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.

Calcium is required for the expression of anthrax lethal toxin activity in the macrophagelike cell line J774A.1

Anthrax lethal toxin, which consists of two separate proteins, protective antigen (Mr, 82,700) and lethal factor (Mr, approximately 83,000), is cytotoxic to the macrophagelike cell line J774A.1. Removal of calcium from the culture medium protected cells against the action of lethal toxin. Calcium depletion during the binding phase of intoxication afforded only partial protection. Further analysis showed that calcium removal caused some inhibition of protective antigen binding but that it had minimal effect on proteolytic conversion of protective antigen to the active 63-kilodalton fragment and that it had no effect on lethal factor binding. Cells to which lethal toxin had bound in the presence of calcium were protected when transferred to calcium-depleted culture medium, indicating a role for calcium at a postbinding stage. When ammonium chloride is present with lethal toxin, toxin accumulates in intracellular vesicles. Calcium-free medium protected these cells upon removal of the amine block, suggesting that calcium is also required at a step after internalization of lethal toxin. Calcium channel blockers inhibited 45Ca2+ uptake and protected cells against cytotoxicity. Calmodulin inhibitors also protected against the action of lethal toxin, suggesting involvement of calmodulin at a step during intoxication. We conclude that calcium is required at several steps in the intoxication of cells by anthrax lethal toxin.

Nucleotide sequence of the Bacillus anthracis edema factor gene (cya): a calmodulin-dependent adenylate cyclase

The nucleotide sequence of the Bacillus anthracis edema factor (EF) gene (cya), which encodes a calmodulin-dependent adenylate cyclase, has been determined. EF is part of the tripartite protein exotoxin of B. anthracis. An ATG start codon, immediately upstream from codons which specify the first 15 amino acids (aa) of EF, was preceded by an AAAGGAGGT sequence which is its probable ribosome-binding site. Starting at this ATG codon, there was a continuous 2400-bp open reading frame which encodes the 800-aa EF-precursor protein with a Mr of 92,464. The mature, secreted protein (767 aa; Mr 88,808) was preceded by a 33-aa signal peptide which has characteristics in common with leader peptides for other secreted proteins of the Bacillus species. A consensus amino acid sequence (Gly-X-X-X-X-Gly-Lys-Ser,X = any aa), which was part of the presumed ATP binding site for EF, was also present. The codon usage of the EF gene reflected the high A + T (71%) base composition for its DNA. B. anthracis EF was not related to the Escherichia coli or yeast adenylate cyclases, but was related to the Bordetella pertussis calmodulin-dependent adenylate cyclase.

Sequence and analysis of the DNA encoding protective antigen of Bacillus anthracis

The nucleotide sequence of the protective antigen (PA) gene from Bacillus anthracis and the 5' and 3' flanking sequences were determined. PA is one of three proteins comprising anthrax toxin; and its nucleotide sequence is the first to be reported from B. anthracis. The open reading frame (ORF) is 2319 bp long, of which 2205 bp encode the 735 amino acids of the secreted protein. This region is preceded by 29 codons, which appear to encode a signal peptide having characteristics in common with those of other secreted proteins. A consensus TATAAT sequence was located at the putative -10 promoter site. A Shine-Dalgarno site similar to that found in genes of other Bacillus sp. was located 7 bp upstream from the ATG start codon. The codon usage for the PA gene reflected its high A + T (69%) base composition and differed from those of genes for bacterial proteins from most other sequences examined. The TAA translation stop codon was followed by an inverted repeat forming a potential termination signal. In addition, a 192-codon ORF of unknown significance, theoretically encoding a 21.6-kDa protein, preceded the 5' end of the PA gene.

Antibodies to anthrax toxin in humans and guinea pigs and their relevance to protective immunity

A forerunning study on the relationship between antibodies to the protective antigen (PA) and lethal factor (LF) components of anthrax toxin and protective immunity has been expanded and extended to include the third toxin component, the edema factor (EF). It was found that protection against the "vaccine resistant" Ames strain was possible in the absence of detectable anti-LF and anti-EF antibodies. Evidence is given that PA may be the essential anthrax-derived antigen for protection, but that equally essential is that it be presented to the host's immune system in such a manner as to provide stimulation of more than just production of antibody to PA. Titers to the three components in sera of individuals with histories of clinically diagnosed anthrax as well as from human vaccinees are included in the report.

Production and characterization of monoclonal antibodies to the protective antigen component of Bacillus anthracis toxin

Thirty-six monoclonal antibodies to the protective antigen protein of Bacillus anthracis exotoxin have been characterized for affinity, antibody subtype, competitive binding to antigenic regions, and ability to neutralize lethal and edema toxin activities. At least 23 antigenic regions were detected on protective antigen by a blocking, enzyme-linked immunosorbent assay. Two clones, 3B6 and 14B7, competed for a single antigenic region and neutralized the activity of both the lethal toxin in vivo (Fisher 344 rat) and the edema toxin in vitro (CHO cells). These two antibodies blocked the binding of 125I-labeled protective antigen to FRL-103 cells. Our results support the proposal that binding of protective antigen to cell receptors is required for expression of toxicity.

Inhibitors of receptor-mediated endocytosis block the entry of Bacillus anthracis adenylate cyclase toxin but not that of Bordetella pertussis adenylate cyclase toxin

Bordetella pertussis and Bacillus anthracis produce extracytoplasmic adenylate cyclase toxins (AC toxins) with shared features including activation by calmodulin and the ability to enter target cells and catalyze intracellular cyclic AMP (cAMP) production from host ATP. The two AC toxins were evaluated for sensitivities to a series of inhibitors of known uptake mechanisms. Cytochalasin D, an inhibitor of microfilament function, abrogated the cAMP response to B. anthracis AC toxin (93%) but not the cAMP response elicited by B. pertussis AC toxin. B. anthracis-mediated intoxication of CHO cells was completely inhibited by ammonium chloride (30 mM) and chloroquine (0.1 mM), whereas the cAMP accumulation produced by B. pertussis AC toxin remained unchanged. The block of target cell intoxication by cytochalasin D could be bypassed when cells were first treated with anthrax AC toxin and then exposed to an acidic medium. These data indicate that despite enzymatic similarities, these two AC toxins intoxicate target cells by different mechanisms, with anthrax AC toxin entering by means of receptor-mediated endocytosis into acidic compartments and B. pertussis AC toxin using a separate, and as yet undefined, mechanism.

Separation of three exotoxic factors of Bacillus anthracis by sequential immunosorbent chromatography

Protective antigen and lethal factor components were isolated directly from crude culture supernatant of Bacillus anthracis by sequential immunosorbent chromatography using immobilized monoclonal antibodies (MAB) against the respective toxins. The immunological activity of protective antigen, lethal factor and edema factor were purified by 1.2-, 6.3- and 2.3-fold, respectively, with recoveries of 63, 70 and 46%, respectively. All three components retained biological activity when combined to form lethal toxin or edema toxin, PA + LF and PA + EF, respectively, after the purification process, and were not contaminated with any of the other components. The order of immunosorbent columns during the purification process was found to be important. The best results were obtained when the protective antigen was removed initially from the crude culture supernatant.

Immunization studies with attenuated strains of Bacillus anthracis

Live, attenuated strains of Bacillus anthracis lacking either the capsule plasmid pXO2, the toxin plasmid pXO1, or both were tested for their efficacy as vaccines against intravenous challenge with anthrax toxin in Fischer 344 rats and against aerosol or intramuscular challenge with virulent anthrax spores in Hartley guinea pigs. Animals immunized with toxigenic, nonencapsulated (pXO1+, pXO2-) strains survived toxin and spore challenge and demonstrated postimmunization antibody titers to the three components of anthrax toxin (protective antigen, lethal factor, and edema factor). Immunization with two nontoxigenic, encapsulated (pXO1-, pXO2+), Pasteur vaccine strains neither provided protection nor elicited titers to any of the toxin components. Therefore, to immunize successfully against anthrax toxin or spore challenge, attenuated, live strains of B. anthracis must produce the toxin components specified by the pXO1 plasmid.

Molecular cloning and expression in Escherichia coli of the lethal factor gene of Bacillus anthracis

We have cloned and expressed in Escherichia coli the lethal factor (LF) gene of Bacillus anthracis. At least two of the six LF recombinant plasmids produce full-length LF protein. Transcription of the LF gene in E. coli appears to be under the control of its own B. anthracis promoter. Recombinant LF protein produced in E. coli remains intracellular and is not secreted. However, this LF protein is biochemically active and displays the same lethal effects as LF secreted by B. anthracis in the mouse macrophage assay. The LF gene, like that of the protective antigen gene, is present on the large B. anthracis toxin plasmid pXO1.

Anthrax toxin components stimulate chemotaxis of human polymorphonuclear neutrophils

Effects of the three-component toxin of Bacillus anthracis on chemotaxis of human polymorphonuclear leukocytes (PMN) were investigated in an effort to determine the basis of the reported antiphagocytic effect of the toxin. The three toxin components, edema factor (EF), protective antigen (PA), and lethal factor (LF), were tested alone and in various combinations for their effect on PMN chemotaxis under agarose to formyl peptides and zymosan-activated serum. No component was active alone; combinations of EF + PA, LF + PA, and EF + LF + PA markedly stimulated chemotaxis (directed migration), but had little or no effect on unstimulated random migration. The toxin components were not themselves chemoattractants. EF in combination with PA had previously been identified as an adenylate cyclase in Chinese hamster ovary (CHO) cells. We found that EF + PA produced detectable cyclic adenosine 3'-5'monophosphate (cAMP) in PMN, but the level of cAMP was less than 1% of that produced in CHO cells by EF + PA, and in PMN by other bacterial adenylate cyclases. LF + PA (which stimulated chemotaxis to an equivalent extent) had no effect on cAMP levels. Thus, the enhancement of chemotaxis by anthrax toxin (at least by LF + PA) does not seem to be related to adenylate cyclase activity.

Crystallization of the protective antigen protein of Bacillus anthracis

The protective antigen protein, one of the three separate proteins constituting the exotoxin system of Bacillus anthracis, has been crystallized in a form suitable for structural studies. The crystal form which is most amenable to x-ray analysis is orthorhombic, space group P2(1)2(1)2(1), a = 101.1 A, b = 95.4 A, c = 87.3 A, with one protective antigen monomer/asymmetric unit. The crystals diffract to approximately 3.0-A resolution.

Immunoelectrophoretic analysis, toxicity, and kinetics of in vitro production of the protective antigen and lethal factor components of Bacillus anthracis toxin

The kinetics of Bacillus anthracis toxin production in culture and its lethal activity in rats, mice, and guinea pigs were investigated. Lethal toxin activity was produced in vitro throughout exponential growth at essentially identical rates in both encapsulated virulent and nonencapsulated avirulent strains. The two toxin proteins which produce lethality when in combination, lethal factor (LF) and protective antigen (PA), could be quantitated directly from culture fluids by rocket immunoelectrophoresis. Using purified preparations of these proteins, we determined that a combination of 8 micrograms of LF and 40 micrograms of PA was required for a maximal rate of killing (39 to 40 min) in Fischer 344 rats (250 to 300 g). Conversely, a minimum of 0.6 microgram of LF and 3 micrograms of PA was required for lethality. The 50% lethal dose for Hartley guinea pigs was 50 micrograms of LF and 250 micrograms of PA, and for Swiss mice it was 2.5 micrograms of LF and 12.5 micrograms of PA. Analyses classically reserved for enzyme kinetic studies were used to study the kinetics of lethal activity in the rat model after intravenous injection of LF-PA mixtures. The amounts of LF and PA which were required to give half the rate of killing (i.e., double the minimum time to death) were 1.2 and 5.8 micrograms, respectively. A theoretical minimum time to death was determined to be 38 min. A third anthrax toxin component, edema factor, was shown to inhibit lethal toxin activity. Edema factor could not be quantitated by rocket immunoelectrophoresis because the protein did not form distinct precipitin bands with available antisera.

Cloning of the protective antigen gene of Bacillus anthracis

The tripartite protein toxin of Bacillus anthracis consists of protective antigen (PA), edema factor (EF), and lethal factor (LF). As a first step in developing a more efficacious anthrax vaccine, recombinant plasmids containing the PA gene have been isolated. A library was constructed in the E. coli vector pBR322 from Bam HI-generated fragments of the anthrax plasmid, pBA1. Two clones producing PA were identified by screening lysates with ELISA (enzyme-linked immunosorbent assay). Western blots revealed a full-size PA protein in the recombinant E. coli, and a cell elongation assay demonstrated biological activity. Both positive clones had a 6 kb insert of DNA, which mapped in the Bam HI site of the vector. The two inserts are the same except that they lie in opposite orientations with respect to the vector. Thus PA is encoded by the plasmid pBA1.

Anthrax toxin edema factor: a bacterial adenylate cyclase that increases cyclic AMP concentrations of eukaryotic cells

Anthrax toxin is composed of three proteins: protective antigen (PA), lethal factor (LF), and edema factor (EF). These proteins individually cause no known physiological effects in animals but in pairs produce two toxic actions. Injection of PA with LF causes death of rats in 60 min, whereas PA with EF causes edema in the skin of rabbits and guinea pigs. The mechanisms of action of these proteins have not been determined. It is shown here that EF is an adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] produced by Bacillus anthracis in an inactive form. Activation occurs upon contact with a heat-stable eukaryotic cell material. The specific activity of the resulting adenylate cyclase nearly equals that of the most active known cyclase. In Chinese hamster ovary cells exposed to PA and EF, cAMP concentrations increase without a lag to values about 200-fold above normal, remain high in the continued presence of toxin, and decrease rapidly after its removal. The increase in cAMP is completely blocked by excess LF. It is suggested that PA interacts with cells to form a receptor system by which EF and perhaps LF gain access to the cytoplasm.

Protection against experimental Pseudomonas aeruginosa infection in mice by active immunization with exotoxin A toxoids

The immunoprophylactic effect of chemically inactivated Pseudomonas aeruginosa exotoxin A in experimental pseudomonas infections was studied. Exotoxin A toxoids were prepared by Formalin (f-TXD) or glutaraldehyde (g-TXD) treatment. Immunization of mice with three or four doses (10 micrograms each) of f-TXD and the synthetic adjuvant N-acetylmuramyl-L-alanyl-D-isoglutamine (50 micrograms) induced high levels of antiexotoxin A antibodies as measured by passive hemagglutination assay and enzyme-linked immunosorbent assay. Immunization with toxoid alone did not elicit antitoxin. A significant increase in survival time and survival rate (P less than 0.01) was seen in immunized (f-TXD) and in burned and infected mice (50 to 85%) as compared with control mice immunized with formalinized bovine serum albumin (6 to 20%). Virtually 100% survival was obtained when preinfection immunization weas combined with single-dose gentamicin treatment within 24 h of infection. Immunization with g-TXD increased survival time (P less than 0.01) but did not consistently increase survival rate, and the results were not as satisfactory as those with formalinized exotoxin. The data presented indicate that active immunization with formalinized exotoxin A toxoid and adjuvant induced protective immunity to various degrees against infections in mice and could be potentially useful in prophylaxis of P. aeruginosa infections.

Receptor-mediated internalization and degradation of diphtheria toxin by monkey kidney cells

The receptor-mediated internalization and degradation of radiolabeled diphtheria toxin by cultured monkey kidney cells was studied. The ability of a number of enzymes and chemicals to remove cell surface-bound toxin was tested; the combination of pronase and inositol hexaphosphate (PIHP) proved most effective. Using PIHP, the kinetics of toxin-cell association at 37 degrees C was resolved into two compounds: surface binding and internalization. The PIHP assay also allowed estimation of the half-time of toxin internalization (about 25 min). An assay involving precipitation of culture supernatants with trichloroacetic acid was developed and used to measure the rate of degradation and excretion of cell-associated toxin. Agents which markedly inhibited toxin internalization similarly prevented degradation, implying an intracellular location for the degradative process. The primary radioactive product excreted by Vero cells was monoiodotyrosine. The extent and rate of toxin degradation indicated lysosomal involvement. Finally, agents which blocked internalization or degradation, or both, (e.g. antibody and concanavalin A), protected cells from the cytotoxin action of diphtheria toxin, suggesting that these processes are necessary for expression of biological effect.