Oligomerization of anthrax toxin protective antigen and binding of lethal factor during endocytic uptake into mammalian cells

Polyelectrolyte-coated liposomes microfluidically assembled in one-step for enhancing cell endocytosis and in-vivo immune responses

To enhance the cellular uptake of liposomes, we prepared conventional liposomes with targeting molecules and surface-charged liposomes and evaluated their potential as nano-carriers and vaccine adjuvants by comparing their endocytosis efficiencies using immune cells. Surface-charged liposomes were synthesized via a one-step microfluidic method, which provided a novel, simple, fast, and highly reproducible method for preparing liposomes. Flow cytometry revealed that cationic polyelectrolyte-coated liposomes exhibited higher endocytosis efficiencies (of up to a factor of 100) in A774A.1 cells and JAWs II cells compared with uncoated liposomes or those coated with anionic polyelectrolytes. Positively charged liposomes exhibited some cytotoxicity at quaternary-chitosan coating concentrations higher than 6 mg/mL; however, significantly lower cytotoxicities (by a factor of almost ten) were obtained by protein mixing. Furthermore, BALB/c mice vaccinated with a mixture of Anthrax vaccine adsorbed (AVA) and quaternary chitosan-coated liposomes showed faster and stronger anti-PA IgG inductions compared to those vaccinated with AVA alone, with titers positively correlating with the amount of cationic liposome used. This finding clearly reveals that quaternary chitosan-coated liposomes act as both nano-carriers and vaccine adjuvants that significantly enhance in-vivo immune responses to vaccines with low immunogenicities.

A genome-based investigation of the Priestia species isolated from anthrax endemic regions in Kruger National Park

Priestia is a genus that was renamed from the genus Bacillus based on the conserved signature indels (CSIs) in protein sequences that separate Priestia species from Bacillus, with the latter only including species closely related to B. subtilis and B. cereus. Diagnosis of anthrax, a zoonotic disease, is implicated by tripartite anthrax virulence genes (lef, pagA, and cya) and poly-γ-D-glutamic acid capsular genes cap-ABCDE of Bacillus anthracis. Due to the amplification of anthrax virulence genes in Priestia isolates, the search for homologous anthrax virulence genes within the Priestia genomes (n = 9) isolated from animal blood smears was embarked upon through whole genome sequencing. In silico taxonomic identification of the isolates was conducted using genome taxonomy database (GTDB), average nucleotide identity (ANI), and multi-locus sequence typing (MLST), which identified the genomes as P. aryabhattai (n = 5), P. endophytica (n = 2) and P. megaterium (n = 2). A pan-genome analysis was further conducted on the Priestia genomes, including the screening of virulence, antibiotic resistance genes and mobile genetic elements on the sequenced genomes. The oligoribonuclease NrnB protein sequences showed that Priestia spp. possess a unique CSI that is absent in other Bacillus species. Furthermore, the CSI in P. endophytica is unique from other Priestia spp. Pan-genomic analysis indicates that P. endophytica clusters separately from P. aryabhattai and P. megaterium. In silico BLASTn genome analysis using the SYBR primers, Taqman probes and primers that target the chromosomal marker (Ba-1), protective antigen (pagA), and lethal factor (lef) on B. anthracis, showed partial binding to Priestia regions encoding for hypothetical proteins, pyridoxine biosynthesis, hydrolase, and inhibitory proteins. The antibiotic resistance genes (ARG) profile of Priestia spp. showed that the genomes contained no more than two ARGs. This included genes conferring resistance to rifamycin and fosfomycin on P. endophytica, as well as clindamycin on P. aryabhattai and P. megaterium. Priestia genomes lacked B. anthracis plasmids and consisted of plasmid replicon types with unknown functions. Furthermore, the amplification of Priestia strains may result in false positives when qPCR is used to detect the virulence genes of B. anthracis in soil, blood smears, and/or environmental samples.

Structure of the anthrax protective antigen D425A dominant negative mutant reveals a stalled intermediate state of pore maturation

The tripartite protein complex produced by anthrax bacteria (Bacillus anthracis) is a member of the AB family of β-barrel pore-forming toxins. The protective antigen (PA) component forms an oligomeric prepore that assembles on the host cell surface and serves as a scaffold for binding of lethal and edema factors. Following endocytosis, the acidic environment of the late endosome triggers a pH-induced conformational rearrangement to promote maturation of the PA prepore to a functional, membrane spanning pore that facilitates delivery of lethal and edema factors to the cytosol of the infected host. Here, we show that the dominant-negative D425A mutant of PA stalls anthrax pore maturation in an intermediate state at acidic pH. Our 2.7 Å cryo-EM structure of the intermediate state reveals structural rearrangements that involve constriction of the oligomeric pore combined with an intramolecular dissociation of the pore-forming module. In addition to defining the early stages of anthrax pore maturation, the structure identifies asymmetric conformational changes in the oligomeric pore that are influenced by the precise configuration of adjacent protomers.

Imaging of anthrax intoxication in mice reveals shared and individual functions of surface receptors CMG-2 and TEM-8 in cellular toxin entry

Bacillus anthracis lethal toxin and edema toxin are binary toxins that consist of a common cell-binding moiety, protective antigen (PA), and the enzymatic moieties, lethal factor (LF) and edema factor (EF). PA binds to either of two receptors, capillary morphogenesis protein-2 (CMG-2) or tumor endothelial marker-8 (TEM-8), which triggers the binding and cytoplasmic translocation of LF and EF. However, the distribution of functional TEM-8 and CMG-2 receptors during anthrax toxin intoxication in animals has not been fully elucidated. Herein, we describe an assay to image anthrax toxin intoxication in animals, and we use it to visualize TEM-8- and CMG-2-dependent intoxication in mice. Specifically, we generated a chimeric protein consisting of the N-terminal domain of LF fused to a nuclear localization signal-tagged Cre recombinase (LFn-NLS-Cre). When PA and LFn-NLS-Cre were coadministered to transgenic mice expressing a red fluorescent protein in the absence of Cre and a green fluorescent protein in the presence of Cre, intoxication could be visualized at single-cell resolution by confocal microscopy or flow cytometry. Using this assay, we found that: (a) CMG-2 is critical for intoxication in the liver and heart, (b) TEM-8 is required for intoxication in the kidney and spleen, (c) CMG-2 and TEM-8 are redundant for intoxication of some organs, (d) combined loss of CMG-2 and TEM-8 completely abolishes intoxication, and (e) CMG-2 is the dominant receptor on leukocytes. The novel assay will be useful for basic and clinical/translational studies of Bacillus anthracis infection and for clinical development of reengineered toxin variants for cancer treatment.

Assembly and Function of the Anthrax Toxin Protein Translocation Complex

Anthrax toxin is a major virulence factor of Bacillus anthracis, a Gram-positive bacterium which can form highly stable spores that are the causative agents of the disease, anthrax. While chiefly a disease of livestock, spores can be "weaponized" as a bio-terrorist agent, and can be deadly if not recognized and treated early with antibiotics. The intracellular pathways affected by the enzymes are broadly understood and are not discussed here. This chapter focuses on what is known about the assembly of secreted toxins on the host cell surface and how the toxin is delivered into the cytosol. The central component is the "Protective Antigen", which self-oligomerizes and forms complexes with its pay-load, either Lethal Factor or Edema Factor. It binds a host receptor, CMG2, or a close relative, triggering receptor-mediated endocytosis, and forms a remarkably elegant yet powerful machine that delivers toxic enzymes into the cytosol, powered only by the pH gradient across the membrane. We now have atomic structures of most of the starting, intermediate and final assemblies in the infectious process. Together with a major body of biophysical, mutational and biochemical work, these studies reveal a remarkable story of both how toxin assembly is choreographed in time and space.

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.

Dual-Readout Sandwich Immunoassay for Device-Free and Highly Sensitive Anthrax Biomarker Detection

We report a dual-readout, AuNP-based sandwich immunoassay for the device-free colorimetric and sensitive scanometric detection of disease biomarkers. An AuNP-antibody conjugate serves as a signal transduction and amplification agent by promoting the reduction and deposition of either platinum or gold onto its surface, generating corresponding colorimetric or light scattering (scanometric) signals, respectively. We apply the Pt-based colorimetric readout of this assay to the discovery of a novel monoclonal antibody (mAb) sandwich pair for the detection of an anthrax protective antigen (PA83). The identified antibody pair detects PA83 down to 1 nM in phosphate-buffered saline and 5 nM in human serum, which are physiologically relevant concentrations. Reducing gold rather than platinum onto the mAb-AuNP sandwich enables scanometric detection of subpicomolar PA83 concentrations, over 3 orders of magnitude more sensitive than the colorimetric readout.

Potential Therapeutic Targeting of Coronavirus Spike Glycoprotein Priming

Processing of certain viral proteins and bacterial toxins by host serine proteases is a frequent and critical step in virulence. The coronavirus spike glycoprotein contains three (S1, S2, and S2′) cleavage sites that are processed by human host proteases. The exact nature of these cleavage sites, and their respective processing proteases, can determine whether the virus can cross species and the level of pathogenicity. Recent comparisons of the genomes of the highly pathogenic SARS-CoV2 and MERS-CoV, with less pathogenic strains (e.g., Bat-RaTG13, the bat homologue of SARS-CoV2) identified possible mutations in the receptor binding domain and in the S1 and S2′ cleavage sites of their spike glycoprotein. However, there remains some confusion on the relative roles of the possible serine proteases involved for priming. Using anthrax toxin as a model system, we show that in vivo inhibition of priming by pan-active serine protease inhibitors can be effective at suppressing toxicity. Hence, our studies should encourage further efforts in developing either pan-serine protease inhibitors or inhibitor cocktails to target SARS-CoV2 and potentially ward off future pandemics that could develop because of additional mutations in the S-protein priming sequence in coronaviruses.

Nanoparticles assembled from fucoidan and trimethylchitosan as anthrax vaccine adjuvant: In vitro and in vivo efficacy in comparison to CpG

Fucoidan/trimethylchitosan nanoparticles (FUC-TMC-NPs) have the potential to improve the immunostimulating efficiency of anthrax vaccine adsorbed (AVA). FUC-TMC-NPs with positive (+) or negative (-) surface charges were prepared via polyelectrolyte complexation, both charged NP types permitted high viability and presented no cytotoxicity on L929, A549 and JAWS II dendritic cells. Flow cytometry measurements indicated lower (+)-FUC-TMC-NPs internalization levels than (-)-FUC-TMC-NPs, yet produced high levels of pro-inflammatory cytokines IFN-γ, IL12p40, and IL-4. Moreover, fluorescence microscope images proved that both charged NP could deliver drugs into the nucleus. In vivo studies on A/J mice showed that (+)-FUC-TMC-NPs carrying AVA triggered an efficient response with a higher IgG anti-PA antibody titer than AVA with CpG oligodeoxynucleotides, and yielded 100 % protection when challenged with the anthracis spores. Furthermore, PA-specific IgG1 and IgG2a analysis confirmed that (+)-FUC-TMC-NPs strongly stimulated humoral immunity. In conclusion, (+)-FUC-TMC-NP is promising anthrax vaccine adjuvant as an alternative to CpG.

Rapid Discovery and Characterization of Synthetic Neutralizing Antibodies against Anthrax Edema Toxin

Anthrax, a lethal, weaponizable disease caused by Bacillus anthracis, acts through exotoxins that are primary mediators of systemic toxicity and also targets for neutralization by passive immunotherapy. The ease of engineering B. anthracis strains resistant to established therapy and the historic use of the microbe in bioterrorism present a compelling test case for platforms that permit the rapid and modular development of neutralizing agents. In vitro antigen-binding fragment (Fab) selection offers the advantages of speed, sequence level molecular control, and engineering flexibility compared to traditional monoclonal antibody pipelines. By screening an unbiased, chemically synthetic phage Fab library and characterizing hits in cell-based assays, we identified two high-affinity neutralizing Fabs, A4 and B7, against anthrax edema factor (EF), a key mediator of anthrax pathogenesis. Engineered homodimers of these Fabs exhibited potency comparable to that of the best reported neutralizing monoclonal antibody against EF at preventing EF-induced cyclic AMP production. Using internalization assays in COS cells, B7 was found to block steps prior to EF internalization. This work demonstrates the efficacy of synthetic alternatives to traditional antibody therapeutics against anthrax while also demonstrating a broadly generalizable, rapid, and modular screening pipeline for neutralizing antibody generation.

Bismaleimide cross-linked anthrax toxin forms functional octamers with high specificity in tumor targeting

In recent years, anthrax toxin has been reengineered to act as a highly specific antiangiogenic cancer therapeutic, shown to kill tumors in animal models. This has been achieved by modifying protective antigen (PA) so that its activation and toxicity require the presence of two proteases, matrix metalloproteinase (MMP) and urokinase plasminogen activator (uPA), which are upregulated in tumor microenvironments. These therapeutics consist of intercomplementing PA variants, which are individually nontoxic, but form functional toxins upon complementary oligomerization. Here, we have created a dual-protease requiring PA targeting system which utilizes bismaleimide cross-linked PA (CLPA) rather than the intercomplementing PA variants. Three different CLPA agents were tested and, as expected, found to exclusively form octamers. Two of the CLPA agents have in vitro toxicities equal to those of previous intercomplementing agents, while the third CLPA agent had compromised in vitro cleavage and was significantly less cytotoxic. We hypothesize this difference was due to steric hindrance caused by cross-linking two PA monomers in close proximity to the PA cleavage site. Overall, this work advances the development and use of the PA and LF tumor-targeting system as a practical cancer therapeutic, as it provides a way to reduce the drug components of the anthrax toxin drug delivery system from three to two, which may lower the cost and simplify testing in clinical trials. HIGHLIGHT: Previously, anthrax toxin has been reengineered to act as a highly specific antiangiogenic cancer therapeutic. Here, we present a version, which utilizes bismaleimide cross-linked protective antigen (PA) rather than intercomplementing PA variants. This advances the development of anthrax toxin as a practical cancer therapeutic as it reduces the components of the drug delivery system to two, which may lower the cost and simplify testing in clinical trials.

Bacillus anthracis Protective Antigen Shows High Specificity for a UV Induced Mouse Model of Cutaneous Squamous Cell Carcinoma

Squamous cell carcinoma (SCC) accounts for the majority of non-melanoma skin cancer related deaths, particularly in immunosuppressed persons. Identification of biomarkers that could be used to identify or treat SCC would be of significant benefit. The anthrax toxin receptors, Tumor Endothelial Marker 8 (TEM8) and Capillary Morphogenesis Gene 2 (CMG2), are endothelial receptors involved in extracellular matrix homeostasis and angiogenesis that are selectively upregulated on numerous tumors. One method of targeting these receptors is Protective Antigen (PA), a protein produced by B. anthracis that mediates binding and translocation of anthrax toxins into cells. PA targeted toxins have been demonstrated to selectively inhibit tumor growth and angiogenesis, but tumor selectivity of PA is currently unknown. In this work fluorescently labeled PA was shown to maintain receptor dependent binding and internalization in vitro. Utilizing a human papillomavirus transgenic mouse model that develops cutaneous SCC in response to ultraviolet irradiation we identified tumor uptake of PA in vivo. The intravenously administered PA resulted in tumor specific localization, with exclusive tumor detection 24 h post injection. Ex vivo analysis identified significantly higher fluorescence in the tumor compared to adjacent healthy tissue and major clearance organs, demonstrating low non-specific uptake and rapid clearance. While both TEM8 and CMG2 were observed to be overexpressed in SCC tumor sections compared to control skin, the intravenously administered PA was primarily co-localized with TEM8. These results suggest that PA could be systemically administered for rapid identification of cutaneous SCC, with potential for further therapeutic development.

An ELISA using a recombinant chimera of protective antigen and lethal factor for serodiagnosis of cutaneous anthrax in India

In this study, an ELISA was developed for simultaneous detection of antibodies against both the important toxins of B. anthracis i.e. protective antigen (PA) and lethal factor (LF). A chimera of PA and LF was made by fusion and cloned and expressed in E. coli. The purified recombinant protein was used in plate ELISA for serodiagnosis of anthrax. The chimera could detect antibodies against both the toxins of Bacillus anthracis. The human serum samples (n = 98) collected from anthrax endemic and non-endemic areas were tested employing ELISA. The ELISA gave sensitivity of 100% (95% Confidence Interval [CI], 92.13 to 100) and specificity of 97.78% (95% Confidence Interval [CI], 88.23 to 99.94) with a J index of 0.97. The efficiency of ELISA was found to be 98.9% with the positive predictive value (PPV) and negative predictive value (NPV) of 97.8% and 100%, respectively. The chimera of PA and LF could be a better diagnostic antigen for serodiagnosis as the assay detects antibodies against both the toxins in early as well delayed infection cases of anthrax. Therefore, it can be a very useful tool for the surveillance as well as for confirmation of cutaneous anthrax cases.

Design and use of a novel substrate for simple, rapid, and specific early detection of anthrax infection

Bacillus anthracis is a major biological warfare threat. The inhalation form of infection can kill quickly. While antibiotic treatment is effective, if diagnosis is delayed, the rapidly produced toxin may already be present in lethal amounts. This report describes a fast, sensitive, specific and accurate method for detection of active infection by Bacillus anthracis in plasma. One of the virulence factors, anthrax lethal factor, is an endopeptidase present in blood early in the infection. However, the use of peptidic substrates to detect endopetidases is problematic in plasma due to the presence of other proteases and the likelihood of nonspecific cleavage of the substrate. The fluorescently labeled peptide substrate MAPKKide Plus designed in this study is not cleaved by plasma proteases and thus is specific for lethal factor. Three detection strategies are described. Two include enrichment by capture from plasma using lethal factor antibody-coated microtiter plates or similarly coated immuno-tubes. The captured lethal factor is exposed to the MAPKKide Plus, and the amount of cleavage is determined either by HPLC or microplate reader. Concentration of lethal factor using the antibody-coated plates aplnd HPLC allows for detection of less than 5 pg lethal factor/ml of neat plasma after 2 hours of incubation. Using antibody-coated immuno-tubes, 20 pg lethal factor/ml plasma can be detected in 5 hours by a simple end point read of fluorescence in a microplate reader. For a third strategy, the substrate is added directly to diluted plasma, and cleavage is monitored by the increase in fluorescence as a function of time. The limit of detection by this simple method is 25 ng lethal factor/ml of plasma in 15 minutes, 5 ng/ml after 45 minutes, and <1 ng lethal factor/ml of plasma after 5 hours.

Anti-proliferative role of recombinant lethal toxin of Bacillus anthracis on primary mammary ductal carcinoma cells revealing its therapeutic potential

Bacillus anthracis secretes three secretary proteins; lethal factor (LF), protective antigen (PA) and edema factor (EF). The LF has ability to check proliferation of mammary tumors, chiefly depending on mitogen activated protein kinase (MAPK) signaling pathway. Evaluation of therapeutic potential of recombinant LF (rLF), recombinant PA (rPA) and lethal toxin (rLF + rPA = LeTx) on the primary mammary ductal carcinoma cells revealed significant (p < 0.01) reduction in proliferation of tumor cells with mean inhibition indices of 28.0 ± 1.37% and 19.6 ± 1.47% respectively. However, treatment with rPA alone had no significant anti-proliferative effect as evident by low mean inhibition index of 3.4 ± 3.87%. The higher inhibition index observed for rLF alone as compared to LeTx is contrary to the existing knowledge on LF, which explains the requirement of PA dependent endocytosis for its enzymatic activity. Therefore, the plausible existence of PA independent mode of action of LF including direct receptor mediated endocytosis or modulation of signal transduction cascade via unknown means is hypothesized. In silico protein docking analysis of other cellular receptors for any plausibility to play the role of receptor for LF revealed c-Met receptor showing strongest affinity for LF (H bond = 19; Free energy = −773.96), followed by nerve growth factor receptor (NGFR) and human epidermal growth factor receptor (HER)-1. The study summarizes the use of rLF or LeTx as therapeutic molecule against primary mammary ductal carcinoma cells and also the c-Met as potential alternative receptor for LF to mediate and modulate PA independent signal transduction.

The pag Gene of pXO1 Is Involved in Capsule Biosynthesis of Bacillus anthracis Pasteur II Strain

The poly-γ-D-glutamic acid capsule and anthrax toxins are major virulence factors of Bacillus anthracis. Genes responsible for capsule biosynthesis are located on pXO2, whereas genes encoding the toxins, which are composed of edema factors, lethal factors, and protective antigens (PA), are located on pXO1. In this study, we found that the pag null mutation not only eliminated the production of the protective antigen, it also eliminated the ability of the B. anthracis Pasteur II strain to form capsules. qPCR analysis revealed that the deletion of pag decreased the transcription levels of the capABCD operon and its regulatory genes acpA and acpB. The introduction of the acpA or acpB plasmid complemented the effect of the pag null mutation on capsule formation. Taken together, the above results suggest that PA probably affects capsule biosynthesis by altering the expression of acpA and acpB. In addition, we found that the deletion mutation of pag remarkably attenuated bacterial pathogenicity in a mouse model of infection. Our results indicate that besides encoding the protective antigen, the pag gene of pXO1 is also involved in the modulation of capsule biosynthesis. Our findings provide new insight into the regulation mechanisms of capsule formation in B. anthracis Pasteur II strain.

Pharmacokinetics and Tolerability of Obiltoxaximab: A Report of 5 Healthy Volunteer Studies

PURPOSE

This report describes the safety, immunogenicity, and pharmacokinetic results of obiltoxaximab treatment in healthy subjects from 5 clinical trials.

METHODS

Healthy men and women were enrolled in randomized, double-blind studies of obiltoxaximab versus placebo (studies 1-3), an open-label, parallel-group study of obiltoxaximab alone versus obiltoxaximab and ciprofloxacin (study 4), or a randomized, double-blind, placebo-controlled study involving administration of a second dose of obiltoxaximab 13 or 119 days after an initial dose (study 5). Obiltoxaximab was administered intravenously in all studies. The safety profile was characterized by physical examinations, including focused examinations of the skin and infusion sites; study drug infusion discontinuations; and assessment of adverse events, vital signs, electrocardiographic findings, laboratory parameters, and immunogenicity. Studies 3 to 5 were the primary safety profile studies. Pharmacokinetic parameters were calculated using noncompartmental methods.

FINDINGS

Results of 2 multiple dose studies (studies 1 and 2) revealed that obiltoxaximab exposure increased proportionally. Pharmacokinetic results were consistent across studies. After administration of 16 mg/kg of obiltoxaximab, serum concentrations decreased in a biexponential or multiexponential fashion with a terminal half-life of 17 to 23 days. Mean volume of distribution was approximately 6.3 to 7.5 L, suggesting obiltoxaximab distribution outside the vascular compartment and potentially into tissues. Mean systemic clearance was approximately 0.27 L/d, suggesting that hepatic metabolism and/or renal excretion are not critical to obiltoxaximab elimination. Obiltoxaximab was generally well tolerated. Hypersensitivity reactions were the most common adverse reactions in the safety profile clinical trials, occurring in 34 of 320 subjects (10.6%) receiving obiltoxaximab and 4 of 70 subjects (5.7%) receiving placebo. The most common adverse events were headache, pruritus, upper respiratory tract infection, cough, infusion site swelling, bruising and/or pain, nasal congestion, urticaria, and extremity pain. Of the 320 subjects in the primary safety profile studies who received ≥1 dose of 16 mg/kg of obiltoxaximab, 8 (2.5%) tested positive for a exposure-emergent antiobiltoxaximab response; however, quantitative titers were low (1:20-1:320).

IMPLICATIONS

On the basis of consistent results of 5 clinical trials in healthy volunteers, the pharmacokinetic properties of obiltoxaximab after a 16-mg/kg IV infusion can be considered adequately characterized, a criteria of the Animal Rule. Obiltoxaximab appears to be generally well tolerated. ClinicalTrials.gov identifiers: NCT00829582, NCT01453907, NCT01929226, NCT01952444, NCT01932242.

Tumor Targeting and Drug Delivery by Anthrax Toxin

Anthrax toxin is a potent tripartite protein toxin from Bacillus anthracis. It is one of the two virulence factors and causes the disease anthrax. The receptor-binding component of the toxin, protective antigen, needs to be cleaved by furin-like proteases to be activated and to deliver the enzymatic moieties lethal factor and edema factor to the cytosol of cells. Alteration of the protease cleavage site allows the activation of the toxin selectively in response to the presence of tumor-associated proteases. This initial idea of re-targeting anthrax toxin to tumor cells was further elaborated in recent years and resulted in the design of many modifications of anthrax toxin, which resulted in successful tumor therapy in animal models. These modifications include the combination of different toxin variants that require activation by two different tumor-associated proteases for increased specificity of toxin activation. The anthrax toxin system has proved to be a versatile system for drug delivery of several enzymatic moieties into cells. This highly efficient delivery system has recently been further modified by introducing ubiquitin as a cytosolic cleavage site into lethal factor fusion proteins. This review article describes the latest developments in this field of tumor targeting and drug delivery.

Identification of the pXO1 plasmid in attenuated Bacillus anthracis vaccine strains

Anthrax toxins and capsule are the major virulence factors of Bacillus anthracis. They are encoded by genes located on the plasmids pXO1 and pXO2, respectively. The vaccine strain Pasteur II was produced from high temperature subcultures of B. anthracis, which resulted in virulence attenuation through the loss of the plasmid pXO1. However, it is unclear whether the high temperature culture completely abolishes the plasmid DNA or affects the replication of the plasmid pXO1. In this study, we tested 3 B. anthracis vaccine strains, including Pasteur II from France, Qiankefusiji II from Russia, and Rentian II from Japan, which were all generated from subcultures at high temperatures. Surprisingly, we detected the presence of pXO1 plasmid DNA using overlap PCR in all these vaccine strains. DNA sequencing analysis of overlap PCR products further confirmed the presence of pXO1. Moreover, the expression of the protective antigen (PA) encoded on pXO1 was determined by using SDS-PAGE and western blotting. In addition, we mimicked Pasteur's method and exposed the A16R vaccine strain, which lacks the pXO2 plasmid, to high temperature, and identified the pXO1 plasmid in the subcultures at high temperatures. This indicated that the high temperature treatment at 42.5°C was unable to eliminate pXO1 plasmid DNA from B. anthracis. Our results suggest that the attenuation of the Pasteur II vaccine strain is likely due to the impact of high temperature stress on plasmid replication, which in turn limits the copy number of pXO1. Our data provide new insights into the mechanisms of the remaining immunogenicity and toxicity of the vaccine strains.

[Molecular model of anthrax toxin translocation into target-cells]

Anthrax toxin is formed from three components: protective antigen (PA), lethal (LF) and edema (EF) factors. PA83 is cleaved by cell surface protease furin to produce a 63-kDa fragment (PA63). PA63 and LF/EF molecules are assembled to anthrax toxin complexes: oligomer PA63 x 7 + LF/EF x 3. Assembly is occurred during of binding with cellular receptor or near surface of target-cell. This toxin complex forms pore and induces receptor-mediated endocytosis. Formed endosome consists extracellular liquid with LF/EF and membrane-associated ferments (H+ and K+/Na+-ATPases) and proteins (receptors and others). H+ concentration is increased into endosome as result of K/Na-ATPase-dependent- activity of H+-ATPase. Difference of potentials (between endosome and intracellular liquid) is increased and LF/EF molecules are moved to pore and bound with PA63-oligomer to PA63 x 7 + LF/EF x 7 and full block pore (ion-selective channel). Endosome is increased in volume and induces increasing of PA63-oligomer pore to.size of effector complex: LF/EF x 7 + PAl7 x 7 = 750 kDa. Effector complex is translocated from endosome to cytosol by means high difference of potentials (H+) and dissociates from PA47 x 7 complex after cleavage of FFD315-sait by intracellular chymotrypsin-like proteases in all 7 molecules PA63. PA47 x 7 complex (strongly fixed in membrane with debris of hydrophobic loops) return into endosome and pore is destroyed. Endosome pH is decreased rapidly and PA47 x 7 complex is destroyed by endosomal/lysosomal proteases. Receptor-mediated endocytosis is ended by endosome recycling in cell-membrane.

A field usable qualitative anti-protective antigen enzyme-linked immunosorbent assay for serodiagnosis of human anthrax

Although all mammals, including humans, are vulnerable when they come into direct contact with infected animals, anthrax is primarily a disease of herbivorous animals. In countries like India, cutaneous anthrax is a public health problem in several regions. Hence, a simple and efficacious serodiagnostic assay for large scale surveillance of endemic populations is required. In the present study, a field-usable, qualitative ELISA was developed for serodiagnosis of human anthrax. Results are assessed on a visual basis and no sophisticated instruments are required. Anti-protective antigen (PA) IgG was determined by visual examination of ELISA results of 225 human serum samples (160 from healthy humans, 5 from PA vaccinated individuals and 60 from confirmed anthrax cases). Comparison of the ELISA results with the results obtained from optical density values showed compatible sensitivity and specificity. Assay sensitivity, specificity, and positive and negative predictive values were found to be 100%. The developed assay could be a very useful tool for serological diagnosis of anthrax infection in humans.

Zinc regulates the activity of kinase-phosphatase pair (BasPrkC/BasPrpC) in Bacillus anthracis

Bacillus anthracis Ser/Thr protein kinase PrkC (BasPrkC) is important for virulence of the bacterium within the host. Homologs of PrkC and its cognate phosphatase PrpC (BasPrpC) are the most conserved mediators of signaling events in diverse bacteria. BasPrkC homolog in Bacillus subtilis regulates critical processes like spore germination and BasPrpC modulates the activity of BasPrkC by dephosphorylation. So far, biochemical and genetic studies have provided important insights into the roles of BasPrkC and BasPrpC; however, regulation of their activities is not known. We studied the regulation of BasPrkC/BasPrpC pair and observed that Zn(2+) metal ions can alter their activities. Zn(2+) promotes BasPrkC kinase activity while inhibits the BasPrpC phosphatase activity. Concentration of Zn(2+) in growing B. anthracis cells was found to vary with growth phase. Zn(2+) was found to be lowest in log phase cells while it was highest in spores. This variation in Zn(2+) concentration is significant for understanding the antagonistic activities of BasPrkC/BasPrpC pair. Our results also show that BasPrkC activity is modulated by temperature changes and kinase inhibitors. Additionally, we identified Elongation Factor Tu (BasEf-Tu) as a substrate of BasPrkC/BasPrpC pair and assessed the impact of their regulation on BasEf-Tu phosphorylation. Based on these results, we propose Zn(2+) as an important regulator of BasPrkC/BasPrpC mediated phosphorylation cascades. Thus, this study reveals additional means by which BasPrkC can be activated leading to autophosphorylation and substrate phosphorylation.

Detection of protective antigen, an anthrax specific toxin in human serum by using surface plasmon resonance

In this study, surface plasmon resonance (SPR) technology was used for the sensitive detection of protective antigen (PA), an anthrax specific toxin in spiked human serum samples. A monoclonal antibody raised against Bacillus anthracis PA was immobilized on carboxymethyldextran-modified gold chip, and its interaction with PA was characterized in situ by SPR. By using kinetic evaluation software, KD (equilibrium constant) and Bmax (maximum binding capacity of analyte) were found to be 20 fM and 18.74 m°, respectively. The change in Gibb's free energy (∆G= -78.04 kJ/mol) confirmed the spontaneous interaction between antigen and antibody. The assay could detect 1 pg/mL purified PA. In PA-spiked human serum samples, 10 pg/mL of PA could be detected. Presence of PA in blood samples serves as an important early diagnostic marker for B. anthracis infections. Thus, SPR test can be a sensitive assay for detection of anthrax at early stages of infection.

Engineering anthrax toxin variants that exclusively form octamers and their application to targeting tumors

Anthrax toxin protective antigen (PA) delivers its effector proteins into the host cell cytosol through formation of an oligomeric pore, which can assume heptameric or octameric states. By screening a highly directed library of PA mutants, we identified variants that complement each other to exclusively form octamers. These PA variants were individually nontoxic and demonstrated toxicity only when combined with their complementary partner. We then engineered requirements for activation by matrix metalloproteases and urokinase plasminogen activator into two of these variants. The resulting therapeutic toxin specifically targeted cells expressing both tumor associated proteases and completely stopped tumor growth in mice when used at a dose far below that which caused toxicity. This scheme for obtaining intercomplementing subunits can be employed with other oligomeric proteins and potentially has wide application.

Rapid vascular responses to anthrax lethal toxin in mice containing a segment of chromosome 11 from the CAST/Ei strain on a C57BL/6 genetic background

Host allelic variation controls the response to B. anthracis and the disease course of anthrax. Mouse strains with macrophages that are responsive to anthrax lethal toxin (LT) show resistance to infection while mouse strains with LT non-responsive macrophages succumb more readily. B6.CAST.11M mice have a region of chromosome 11 from the CAST/Ei strain (a LT responsive strain) introgressed onto a LT non-responsive C57BL/6J genetic background. Previously, B6.CAST.11M mice were found to exhibit a rapid inflammatory reaction to LT termed the early response phenotype (ERP), and displayed greater resistance to B. anthracis infection compared to C57BL/6J mice. Several ERP features (e.g., bloat, hypothermia, labored breathing, dilated pinnae vessels) suggested vascular involvement. To test this, Evan’s blue was used to assess vessel leakage and intravital microscopy was used to monitor microvascular blood flow. Increased vascular leakage was observed in lungs of B6.CAST.11M mice compared to C57BL/6J mice 1 hour after systemic administration of LT. Capillary blood flow was reduced in the small intestine mesentery without concomitant leukocyte emigration following systemic or topical application of LT, the latter suggesting a localized tissue mechanism in this response. Since LT activates the Nlrp1b inflammasome in B6.CAST.11M mice, the roles of inflammasome products, IL-1β and IL-18, were examined. Topical application to the mesentery of IL-1β but not IL-18 revealed pronounced slowing of blood flow in B6.CAST.11M mice that was not present in C57BL/6J mice. A neutralizing anti-IL-1β antibody suppressed the slowing of blood flow induced by LT, indicating a role for IL-1β in the response. Besides allelic differences controlling Nlrp1b inflammasome activation by LT observed previously, evidence presented here suggests that an additional genetic determinant(s) could regulate the vascular response to IL-1β. These results demonstrate that vessel leakage and alterations to blood flow are part of the rapid response in mice resistant to B. anthracis infection.

Anti-protective antigen IgG enzyme-linked immunosorbent assay for diagnosis of cutaneous anthrax in India

Anthrax caused by Bacillus anthracis is a public health problem in several developing countries whose main source of income is farming. Anthrax is a disease of herbivorous animals, and humans can be infected by handling infected animals or contaminated animal products. Specific diagnostic tests are unavailable in India for the detection and confirmation of cutaneous anthrax in humans. Here, we describe the development of an enzyme-linked immunosorbent assay (ELISA) for detection of serum antibodies against Bacillus anthracis protective antigen in the Indian population. A total of 405 serum samples collected from different groups were tested by the developed ELISA. The assay provided a specificity of 99.41% (95% confidence interval [CI], 97.89 to 99.93) and a sensitivity of 100% (CI, 94.4 to 100) using a cutoff value of 0.29 ELISA unit (EU). The positive predictive value (PPV) and negative predictive value (NPV) of the assay were 97% and 100%, respectively. The efficiency and J index for the reliability of the assay were 99.5% and 0.994, respectively. The assay can be a very useful tool for surveillance as well as for diagnosis of cutaneous anthrax cases in India.

Small molecule inhibitors of anthrax toxin-induced cytotoxicity targeted against protective antigen

Two molecular scaffolds were designed using the CAVEAT molecular design package to inhibit the oligomerization of protective antigen (PA(63) ), a key protein component of anthrax toxin. The inhibitors were designed to prevent heptamerization of PA(63) by mimicking key residues of PA(63) needed for the intermolecular interactions that stabilize the heptamer. Using the scaffolds identified by CAVEAT, seven candidate inhibitors were synthesized and tested for their ability to inhibit anthrax toxin-induced cytotoxicity, with three of the agents demonstrating modest inhibition in murine J774A.1 macrophage cells.

The cell surface structure of tumor endothelial marker 8 (TEM8) is regulated by the actin cytoskeleton

Tumor endothelial marker 8 (TEM8) is an integrin-like cell surface protein upregulated on tumor blood vessels and a potential vascular target for cancer therapy. Here, we found that the ability of an anti-TEM8 antibody, clone SB5, to recognize the extracellular domain of TEM8 on the cell surface depends on other host-cell factors. By taking advantage of SB5's ability to distinguish different forms of cell surface TEM8, we identified alpha-smooth muscle actin and transgelin, an actin binding protein, as intracellular factors able to alter TEM8 cell surface structure. Overexpression of either of these proteins in cells converted TEM8 from an SB5-exposed to an SB5-masked form and protected cells from SB5-saporin immunotoxins. Because the predominant form of TEM8 on the cell surface is not recognized by SB5, we also developed a new monoclonal antibody, called AF334, which is able to recognize both the SB5-exposed and the SB5-masked forms of TEM8. AF334-saporin selectively killed TEM8-positive cells independent of TEM8 cell surface structure. These studies reveal that TEM8 exists in different forms at the cell surface, a structure dependent on interactions with components of the actin cytoskeleton, and should aid in the rational design of the most effective diagnostic and therapeutic anti-TEM8 monoclonal antibodies.

A femtomol range FRET biosensor reports exceedingly low levels of cell surface furin: implications for the processing of anthrax protective antigen

Furin, a specialized endoproteinase, transforms proproteins into biologically active proteins. Furin function is important for normal cells and also in multiple pathologies including malignancy and anthrax. Furin is believed to cycle between the Golgi compartment and the cell surface. Processing of anthrax protective antigen-83 (PA83) by the cells is considered thus far as evidence for the presence of substantial levels of cell-surface furin. To monitor furin, we designed a cleavage-activated FRET biosensor in which the Enhanced Cyan and Yellow Fluorescent Proteins were linked by the peptide sequence SNSRKKR↓STSAGP derived from anthrax PA83. Both because of the sensitivity and selectivity of the anthrax sequence to furin proteolysis and the FRET-based detection, the biosensor recorded the femtomolar levels of furin in the in vitro reactions and cell-based assays. Using the biosensor that was cell-impermeable because of its size and also by other relevant methods, we determined that exceedingly low levels, if any, of cell-surface furin are present in the intact cells and in the cells with the enforced furin overexpression. This observation was in a sharp contrast with the existing concepts about the furin presentation on cell surfaces and anthrax disease mechanism. We next demonstrated using cell-based tests that PA83, in fact, was processed by furin in the extracellular milieu and that only then the resulting PA63 bound the anthrax toxin cell-surface receptors. We also determined that the biosensor, but not the conventional peptide substrates, allowed continuous monitoring of furin activity in cancer cell extracts. Our results suggest that there are no physiologically-relevant levels of cell-surface furin and, accordingly, that the mechanisms of anthrax should be re-investigated. In addition, the availability of the biosensor is a foundation for non-invasive monitoring of furin activity in cancer cells. Conceptually, the biosensor we developed may serve as a prototype for other proteinase-activated biosensors.

Effects of introducing a single charged residue into the phenylalanine clamp of multimeric anthrax protective antigen

Multimeric pores formed in the endosomal membrane by the Protective Antigen moiety of anthrax toxin translocate the enzymatic moieties of the toxin to the cytosolic compartment of mammalian cells. There is evidence that the side chains of the Phe(427) residues come into close proximity with one another in the lumen of the pore and form a structure, termed the Phe clamp, that catalyzes the translocation process. In this report we describe the effects of replacing Phe(427) in a single subunit of the predominantly heptameric pore with a basic or an acidic amino acid. Incorporating any charged residue at this position inhibited cytotoxicity >or=1,000-fold in our standard assay and caused strong inhibition of translocation in a planar phospholipid bilayer system. His and Glu were the most strongly inhibitory residues, ablating both cytotoxicity and translocation. Basic residues at position 427 prevented the Phe clamp from interacting with a translocation substrate to form a seal against the passage of ions and accelerated dissociation of the substrate from the pore. Acidic residues, in contrast, allowed the seal to form and the substrate to remain firmly bound, but blocked its passage, perhaps via electrostatic interactions with the positively charged N-terminal segment. Our findings are discussed in relation to the role of the Phe clamp in a Brownian ratchet model of translocation.

Proteomic studies of Bacillus anthracis

Bacillus anthracis is a Gram-positive, spore-forming bacterium representing the etiological cause of anthrax, a rare lethal disease of animals and humans. Development of anthrax countermeasures has gained increasing attention owing to the potential use of B. anthracis spores as a bioterror weapon. The various forms of infection by B. anthracis are characterized both by toxemia and septicemia, both of which are the result of spore entry into the host followed by their germination into rapidly multiplying, toxin-producing bacilli. Following the publication of the bacterial genome, proteomic studies were carried out to determine the protein composition of the spore and identify exposed vegetative (membrane-located or secreted) proteins. These studies included comparison of strains differing in their virulence, cultured under different conditions and, in some cases, were complemented by serological inspection, which addressed expression during infection of proteomically identified proteins and their immunogenicity. The proteomic approach emerged as a valuable strategy for the generation of a pool of potential B. anthracis protein targets for further evaluation in detection, diagnostics, therapy and prophylaxis, and contributed to the elucidation of some aspects of the pathogenesis of the disease.

Novel and unique diagnostic biomarkers for Bacillus anthracis infection

A search for bacterium-specific biomarkers in peripheral blood following infection with Bacillus anthracis was carried out with rabbits, using a battery of specific antibodies generated by DNA vaccination against 10 preselected highly immunogenic bacterial antigens which were identified previously by a genomic/proteomic/serologic screen of the B. anthracis secretome. Detection of infection biomarkers in the circulation of infected rabbits could be achieved only after removal of highly abundant serum proteins by chromatography using a random-ligand affinity column. Besides the toxin component protective antigen, the following three secreted proteins were detected in the circulation of infected animals: the chaperone and protease HtrA (BA3660), an NlpC/P60 endopeptidase (BA1952), and a protein of unknown function harboring two SH3 (Src homology 3) domains (BA0796). The three proteins could be detected in plasma samples from infected animals exhibiting 10(3) to 10(5) CFU/ml blood and also in standard blood cultures at 3 to 6 h post-bacterial inoculation at a bacteremic level as low as 10(3) CFU/ml. Furthermore, the three biomarkers appear to be present only in the secretome of B. anthracis, not in those of the related pathogens B. thuringiensis and B. cereus. To the best of our knowledge, this is the first report of direct detection of B. anthracis-specific proteins, other than the toxin components, in the circulation of infected animals.

Sequential B-cell epitopes of Bacillus anthracis lethal factor bind lethal toxin-neutralizing antibodies

The bipartite anthrax lethal toxin (LeTx) consisting of protective antigen (PA) and lethal factor (LF) is a major virulence factor contributing to death from systemic Bacillus anthracis infection. The current vaccine elicits antibodies directed primarily to PA; however, in experimental settings serologic responses to LF can neutralize LeTx and contribute to protection against infection. The goals of the present study were to identify sequential B-cell epitopes of LF and to determine the capacity of these determinants to bind neutralizing antibodies. Sera of recombinant LF-immunized A/J mice exhibited high titers of immunoglobulin G anti-LF reactivity that neutralized LeTx in vitro 78 days after the final booster immunization and protected the mice from in vivo challenge with 3 50% lethal doses of LeTx. These sera bound multiple discontinuous epitopes, and there were major clusters of reactivity on native LF. Strikingly, all three neutralizing, LF-specific monoclonal antibodies tested bound specific peptide sequences that coincided with sequential epitopes identified in polyclonal antisera from recombinant LF-immunized mice. This study confirms that LF induces high-titer protective antibodies in vitro and in vivo. Moreover, the binding of short LF peptides by LF-specific neutralizing monoclonal antibodies suggests that generation of protective antibodies by peptide vaccination may be feasible for this antigen. This study paves the way for a more effective anthrax vaccine by identifying discontinuous peptide epitopes of LF.

Standardized, mathematical model-based and validated in vitro analysis of anthrax lethal toxin neutralization

Quantification of anthrax lethal toxin (LTx) neutralization activity (TNA) is pivotal in assessing protective antibody responses to anthrax vaccines and for evaluation of immunotherapies for anthrax. We have adapted and redesigned the TNA assay to establish a unifying, standardized, quantitative and validated technology platform for LTx neutralization in the J774A.1 murine cell line. Critical design features of this platform are 1) the application of a free-form or constrained 4 parameter logistic (4-PL) function to model neutralization responses within and between boundary limits of 100% cell survival and 95% cell lysis and 2) to exploit innovative assay curve recognition algorithms for interpretive endpoints. The assay was validated using human serum ED50 (dilution of serum effecting 50% neutralization) as the primary reportable value (RV). Intra-operator and intermediate precision, expressed as the coefficient of variation (%CV), were high at 10.5-15.5%CV and 13.5-14.5%CV respectively. TNA assay dilutional linearity was demonstrated for human sera using linear regression analysis of log(10) transformed data with slope=0.99, intercept=-0.03 and r(2)=0.985. Assay accuracy, inferred from the precision and linearity data and using a spike-recovery approach, was high with a percent error (%E) range of only 3.4-20.5%E. The lower limit of detection (LLOD) was ED50=12 and the lower limit of quantification (LLOQ) was ED50=36. The cell-based assay was robust, tolerating incubation temperatures from 35 to 39 degrees C, CO(2) concentrations from 3% to 7% and reporter substrate (MTT) concentrations of 2.5-7.5 mg/ml. Strict assay quality control parameters were met for up to 25 cell culture passages. The long term (50 month) assay stability, determined using human reference standards AVR414 and AVR801, indicated high precision, consistent accuracy and no detectable assay drift. A customized software program provided two additional assay metrics, Quantification Titer (QT) and Threshold Titer (TT), both of which demonstrate acceptable accuracy, precision and dilutional linearity. The TT was also used to establish the assay reactivity threshold (RT). The application of the assay to sera from humans, Rhesus macaques and rabbits was demonstrated separately and by aggregate dilutional linearity analysis of the ED50 (slope=0.98, intercept=0.003, r(2)=0.989). We propose this TNA assay format with a qualified standard reference serum and customized interpretive software as a unifying platform technology for determination of functional serologic responses to anthrax vaccines and for evaluation of anthrax immunotherapeutics.

The seminal literature of anthrax research

A chronically weak area in research papers, reports, and reviews is the complete identification of seminal background documents that formed the building blocks for these papers. A method for systematically determining these seminal references is presented. Citation-Assisted Background (CAB) is based on the assumption that seminal documents tend to be highly cited. Application of CAB to the field of Anthrax research is presented. While CAB is a highly systematic approach for identifying seminal references, it is not a substitute for the judgment of the researchers, and serves as a supplement.

Cryo-electron microscopy study of bacteriophage T4 displaying anthrax toxin proteins

The bacteriophage T4 capsid contains two accessory surface proteins, the small outer capsid protein (Soc, 870 copies) and the highly antigenic outer capsid protein (Hoc, 155 copies). As these are dispensable for capsid formation, they can be used for displaying proteins and macromolecular complexes on the T4 capsid surface. Anthrax toxin components were attached to the T4 capsid as a fusion protein of the N-terminal domain of the anthrax lethal factor (LFn) with Soc. The LFn-Soc fusion protein was complexed in vitro with Hoc(-)Soc(-)T4 phage. Subsequently, cleaved anthrax protective antigen heptamers (PA63)(7) were attached to the exposed LFn domains. A cryo-electron microscopy study of the decorated T4 particles shows the complex of PA63 heptamers with LFn-Soc on the phage surface. Although the cryo-electron microscopy reconstruction is unable to differentiate on its own between different proposed models of the anthrax toxin, the density is consistent with a model that had predicted the orientation and position of three LFn molecules bound to one PA63 heptamer.

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.

Identification of in vivo-expressed immunogenic proteins by serological proteome analysis of the Bacillus anthracis secretome

In a previous comparative proteomic study of Bacillus anthracis examining the influence of the virulence plasmids and of various growth conditions on the composition of the bacterial secretome, we identified 64 abundantly expressed proteins (T. Chitlaru, O. Gat, Y. Gozlan, N. Ariel, and A. Shafferman, J. Bacteriol. 188:3551-3571, 2006). Using a battery of sera from B. anthracis-infected animals, in the present study we demonstrated that 49 of these proteins are immunogenic. Thirty-eight B. anthracis immunogens are documented in this study for the first time. The relative immunogenicities of the 49 secreted proteins appear to span a >10,000-fold range. The proteins eliciting the highest humoral response in the course of infection include, in addition to the well-established immunogens protective antigen (PA), Sap, and EA1, GroEL (BA0267), AhpC (BA0345), MntA (BA3189), HtrA (BA3660), 2,3-cyclic nucleotide diesterase (BA4346), collagen adhesin (BAS5205), an alanine amidase (BA0898), and an endopeptidase (BA1952), as well as three proteins having unknown functions (BA0796, BA0799, and BA0307). Of these 14 highly potent secreted immunogens, 11 are known to be associated with virulence and pathogenicity in B. anthracis or in other bacterial pathogens. Combining the results reported here with the results of a similar study of the membranal proteome of B. anthracis (T. Chitlaru, N. Ariel, A. Zvi, M. Lion, B. Velan, A. Shafferman, and E. Elhanany, Proteomics 4:677-691, 2004) and the results obtained in a functional genomic search for immunogens (O. Gat, H. Grosfeld, N. Ariel, I. Inbar, G. Zaide, Y. Broder, A. Zvi, T. Chitlaru, Z. Altboum, D. Stein, S. Cohen, and A. Shafferman, Infect. Immun. 74:3987-4001, 2006), we generated a list of 84 in vivo-expressed immunogens for future evaluation for vaccine development, diagnostics, and/or therapeutic intervention. In a preliminary study, the efficacies of eight immunogens following DNA immunization of guinea pigs were compared to the efficacy of a PA DNA vaccine. All eight immunogens induced specific high antibody titers comparable to the titers elicited by PA; however, unlike PA, none of them provided protection against a lethal challenge (50 50% lethal doses) of virulent B. anthracis strain Vollum spores.

Assembly of the small outer capsid protein, Soc, on bacteriophage T4: a novel system for high density display of multiple large anthrax toxins and foreign proteins on phage capsid

Bacteriophage T4 capsid is a prolate icosahedron composed of the major capsid protein gp23*, the vertex protein gp24*, and the portal protein gp20. Assembled on its surface are 810 molecules of the non-essential small outer capsid protein, Soc (10 kDa), and 155 molecules of the highly antigenic outer capsid protein, Hoc (39 kDa). In this study Soc, a "triplex" protein that stabilizes T4 capsid, is targeted for molecular engineering of T4 particle surface. Using a defined in vitro assembly system, anthrax toxins, protective antigen, lethal factor and their domains, fused to Soc were efficiently displayed on the capsid. Both the N and C termini of the 80 amino acid Soc polypeptide can be simultaneously used to display antigens. Proteins as large as 93 kDa can be stably anchored on the capsid through Soc-capsid interactions. Using both Soc and Hoc, up to 1662 anthrax toxin molecules are assembled on the phage T4 capsid under controlled conditions. We infer from the binding data that a relatively high affinity capsid binding site is located in the middle of the rod-shaped Soc, with the N and C termini facing the 2- and 3-fold symmetry axes of the capsid, respectively. Soc subunits interact at these interfaces, gluing the adjacent capsid protein hexamers and generating a cage-like outer scaffold. Antigen fusion does interfere with the inter-subunit interactions, but these interactions are not essential for capsid binding and antigen display. These features make the T4-Soc platform the most robust phage display system reported to date. The study offers insights into the architectural design of bacteriophage T4 virion, one of the most stable viruses known, and how its capsid surface can be engineered for novel applications in basic molecular biology and biotechnology.

Bacteriophage T4 capsid: a unique platform for efficient surface assembly of macromolecular complexes

We report the first description of a macromolecular complex display system using bacteriophage T4. Decorated with two dispensable outer capsid proteins, Hoc (155 copies) and Soc (810 copies), the 120 nm x 86 nm T4 capsid particle offers a unique binding site-rich platform for surface assembly of hetero-oligomeric complexes. To display the 710 kDa anthrax toxin complex, two bipartite functional fusion proteins, LF-Hoc and LFn-Soc, were constructed. Using a defined in vitro binding system, sequential assembly was performed by first attaching LF-Hoc and/or LFn-Soc to hoc-soc- phage, saturating the Hoc and Soc binding sites. Trypsin-nicked PA63 was then assembled into heptamers through specific interaction with the capsid-exposed LFn domain. EF was then attached to the unoccupied sites of PA63 heptamers, completing the assembly of the tripartite anthrax toxin. Negative electron microscopy showed decoration of each capsid with a layer of heptameric PA63 rings. Up to 229 anthrax toxin complexes, equivalent to a total of 2400 protein molecules and a mass of about 133 MDa (2.7 times the mass of capsid shell), were anchored on a single particle, making it the highest density display reported on any virus. The phage T4 capsid lattice provides a stable biological platform allowing maximum display of large hetero-oligomeric complexes in vitro and offers insights for developing novel vaccines, analysis of protein-protein interactions, and structure determination of complexes.

Detection of anthrax toxin in the serum of animals infected with Bacillus anthracis by using engineered immunoassays

Several strategies that target anthrax toxin are being developed as therapies for infection by Bacillus anthracis. Although the action of the tripartite anthrax toxin has been extensively studied in vitro, relatively little is known about the presence of toxins during an infection in vivo. We developed a series of sensitive sandwich enzyme-linked immunosorbent assays (ELISAs) for detection of both the protective antigen (PA) and lethal factor (LF) components of the anthrax exotoxin in serum. The assays utilize as capture agents an engineered high-affinity antibody to PA, a soluble form of the extracellular domain of the anthrax toxin receptor (ANTXR2/CMG2), or PA itself. Sandwich immunoassays were used to detect and quantify PA and LF in animals infected with the Ames or Vollum strains of anthrax spores. PA and LF were detected before and after signs of toxemia were observed, with increasing levels reported in the late stages of the infection. These results represent the detection of free PA and LF by ELISA in the systemic circulation of two animal models exposed to either of the two fully virulent strains of anthrax. Simple anthrax toxin detection ELISAs could prove useful in the evaluation of potential therapies and possibly as a clinical diagnostic to complement other strategies for the rapid identification of B. anthracis infection.

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.

Antitumor efficacy of a urokinase activation–dependent anthrax toxin

Previously, we have generated a potent prodrug consisting of modified anthrax toxins that is activated by urokinase plasminogen activator (uPA). The cytotoxicity of the drug, PrAg-U2 + FP59, is dependent on the presence of receptor-associated uPA activity. Local intradermal administration of PrAg-U2 + FP59 adjacent to the tumor nodules in mice with transplanted solid tumors had a potent antitumor effect. In succession of these experiments, we have now investigated the systemic antitumor efficacy of PrAg-U2 + FP59. C57Bl/6J mice bearing syngenic tumors derived from B16 melanoma, T241 fibrosarcoma, or Lewis lung carcinoma cells were treated with different mass ratios and doses of PrAg-U2 + FP59. Tumor volumes were recorded daily by caliper measurements. In some experiments, dexamethasone was coadministered. Our data show a significant antitumor effect of systemic administration of PrAg-U2 + FP59 in three syngenic tumor models. Optimal antitumor effect and low toxicity was obtained with a 25:1 mass ratio between the two components (PrAg-U2 and FP59). The experiments show that PrAg-U2 + FP59 displays a clear dose-response relationship with regard to both antitumor efficacy and systemic toxicity. Dose-limiting toxicity seemed to be due to activation of the prodrug by uPA and its receptor in the intestinal mucosa. Concurrent treatment with dexamethasone was found to prevent dose-limiting toxicity. Taken together, these data indicate that uPA-activated toxins may be promising candidates for targeted therapy of human cancers that overexpress uPA and its receptor.

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.

In vitro binding of anthrax protective antigen on bacteriophage T4 capsid surface through Hoc-capsid interactions: a strategy for efficient display of large full-length proteins

An in vitro binding system is described to display large full-length proteins on bacteriophage T4 capsid surface at high density. The phage T4 icosahedral capsid features 155 copies of a nonessential highly antigenic outer capsid protein, Hoc, at the center of each major capsid protein hexon. Gene fusions were engineered to express the 83-kDa protective antigen (PA) from Bacillus anthracis fused to the N-terminus of Hoc and the 130-kDa PA-Hoc protein was expressed in Escherichia coli and purified. The purified PA-Hoc was assembled in vitro on hoc(-) phage particles. Binding was specific, stable, and of high affinity. This defined in vitro system allowed manipulation of the copy number of displayed PA and imposed no significant limitation on the size of the displayed antigen. In contrast to in vivo display systems, the in vitro approach allows all the capsid binding sites to be occupied by the 130-kDa PA-Hoc fusion protein. The PA-T4 particles were immunogenic in mice in the absence of an adjuvant, eliciting strong PA-specific antibodies and anthrax lethal toxin neutralizing antibodies. The in vitro display on phage T4 offers a novel platform for potential construction of customized vaccines against anthrax and other infectious diseases.

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.

N-fragment of edema factor as a candidate antigen for immunization against anthrax

The nontoxic N-terminal fragment of Bacillus anthracis edema factor (EF) was evaluated as a candidate antigen in an anthrax vaccine using a replication-incompetent adenoviral vector. An E1/E3 deleted adenovirus (Ad/EFn) encoding the N-terminal region 1-254 amino acids of the edema factor (EFn) was constructed using the native DNA sequence of EFn. Intramuscular immunization three times with 10(8) plaque forming units (pfu)/dose of Ad/EFn in A/J mice resulted in 37% and 57% protection against a subcutaneous challenge with B. anthracis Sterne strain spores at a dosage of 200 x LD50 and 100 x LD50, respectively. EF-specific serum IgG responses (including total IgG, IgG1, and IgG2a isotype titers) were robust in the Ad/EFn immunized animals. Interestingly, anti-EF antibodies cross-reacted with anthrax lethal factor (LF), and had a neutralizing capability against both anthrax lethal toxin (Letx) and edema toxin (Edtx), as demonstrated by in vitro toxin neutralization assays using J774A.1 mouse macrophage and Chinese hamster ovary cell (CHO), respectively. Our data suggest that EF plays a role in eliciting protective immunity against anthrax, and that it should be included in a new generation multi-component subunit vaccine.

Molecular basis for improved anthrax vaccines

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

Pore formation, polymerization, hemolytic and leukotoxic effects of a new Enterobacter cloacae toxin neutralized by antiserum

A new toxin of Enterobacter cloacae was purified and studied by SDS-PAGE electrophoresis with the purpose of investigating its ability to generate polymers and their molecular mass. Monomer of 13.3 kDa and structures of multimeric mass were detected. The toxin of 66 kDa was the most abundant form of toxin. This polymer and the monomer were selected to examine blood cells damage. Membrane pores caused by both toxin forms seemed to be of similar dimension (estimated in 3.6 nm by experiments with osmotic protectors) and were able to lyse erythrocytes and leukocytes. The results obtained indicate that polymerization and pore formation are involved in the molecular events that participate in the cytotoxic effects of E. cloacae toxin. Immunization of rabbits with 13.3kDa toxin generated antibody response capable of inhibiting oxidative stress as well as hemolytic and leukotoxic effects. Immunoblotting indicated that monomer and polymer reacted with antihemolysin serum. The importance of E. cloacae toxin "in vivo" was studied in animals by means of assays performed in peritoneum of rats, inoculated with the hemolytic strain (C1) and a non-hemolytic variant (C4). Both strains stimulated infiltration of leukocytes in peritoneum, but C1 caused cell death and lysis wheras assays with C4 maintained the viability of leukocytes even within 5 h after extraction of samples.