Immunogenicity and Protective Efficacy of Bacillus anthracis Poly-γ-d-glutamic Acid Capsule Covalently Coupled to a Protein Carrier Using a Novel Triazine-based Conjugation Strategy

Poly-γ-glutamic acid nanoparticles as adjuvant and antigen carrier system for cancer vaccination

Vaccination is an innovative strategy for cancer treatment by leveraging various components of the patients' immunity to boost an anti-tumor immune response. Rationally designed nanoparticles are well suited to maximize cancer vaccination by the inclusion of immune stimulatory adjuvants. Also, nanoparticles might control the pharmacokinetics and destination of the immune potentiating compounds. Poly-γ-glutamic acid (γ-PGA) based nanoparticles (NPs), which have a natural origin, can be easily taken up by dendritic cells (DCs), which leads to the secretion of cytokines which ameliorates the stimulation capacity of T cells. The intrinsic adjuvant properties and antigen carrier properties of γ-PGA NPs have been the focus of recent investigations as they can modulate the tumor microenvironment, can contribute to systemic anti-tumor immunity and subsequently inhibit tumor growth. This review provides a comprehensive overview on the potential of γ-PGA NPs as antigen carriers and/or adjuvants for anti-cancer vaccination.

An Acid-Sensitive Bone Targeting Delivery System Carrying Acacetin Prevents Osteoporosis in Ovariectomized Mice

One effective treatment for postmenopausal osteoporosis is to inhibit osteoclasts and subsequent bone resorption. In our study, we demonstrated that acacetin, a flavone with potential therapeutic effects in infections, cancers, and several metabolic disorders, inhibited osteoclast differentiation and bone resorption in vitro. For improving the efficacy of acacetin in vivo, we developed an acid-sensitive bone-targeting delivery system composed of an acid-sensitive linker (N-ε-maleimidocaproic acid hydrazide, EMCH) for ensuring an effective release of acacetin at the site of action and a hydrophilic aspartic acid hexapeptide ((Asp)6, D6) as the effective bone targeting agent. Our results revealed that Acacetin-EMCH-D6 specifically bound to the bone surface once administrated in vivo, prolonged the retention time in bone and released acacetin at the osteoclastic bone resorption sites where the acidity is higher. We further demonstrated that, in ovariectomy-induced osteoporosis mice, treatment with Acacetin-EMCH-D6 inhibited osteoclast formation and increased trabecular bone mass. On the contrary, neither acacetin nor EMCH-D6 with the same dosage alone showed significant anti-osteoporosis effects in vivo. Mechanistically, targeted delivery of acacetin to the bone resorption sites by Acacetin-EMCH-D6 inhibited autophagy through activating PI3K/AKT/mTOR pathway in osteoclasts, while the activation of autophagy by rapamycin partially reversed the inhibitory effects of acacetin in vitro and in vivo. In summary, our study, for the first time, showed that the acid-sensitive bone-targeting delivery system carrying acacetin was effective for the treatment of postmenopausal osteoporosis. Thus, targeted delivery of acacetin using Acacetin-EMCH-D6 to bone resorption sites is a promising therapy for osteoporosis.

Synthesis and preliminary immunologic properties of di-/trisaccharide-conjugates related to Bacillus anthracis

Herein, the di- and trisaccharide mimics of the hexasaccharide antigen related to Bacillus anthracis were synthesized and covalently coupled with carrier proteins, such as keyhole limpet hemocyanin (KLH) and bovine serum albumin (BSA), to form the corresponding glycoconjugates 1-6. 2,3,4,6-Tetra-O-benzyl thioglycoside and 2-deoxyl-2-phthalylamino-3,4,6-tri-O-benzyl thioglycoside were applied as glycosyl donors to guarantee α or β-configuration of the newly formed glycosidic bonds. Glutaraldehyde was used as a homobifunctional cross-linker for high-efficiency coupling. The synthetic KLH-glycoconjugates 2, 4 and 6 were also used to vaccinate female Balb/c mice and the preliminary results of ELISA uncovered that all three KLH-conjugates could induce immune responses and generate oligosaccharide-specific total IgG antibodies. The trisaccharide 8, the glycosyl part of glycoconjugate 4, is of great immunogenicity.

Staphylococcus aureus Putative Vaccines Based on the Virulence Factors: A Mini-Review

Since the 1960s, the frequency of methicillin-resistant Staphylococcus aureus as a recurrent cause of nosocomial infections has increased. Since multidrug-resistant Staphylococcus has overcome antimicrobial treatment, the development of putative vaccines based on virulence factors could be a great help in controlling the infections caused by bacteria and are actively being pursued in healthcare settings. This mini-review provides an overview of the recent progress in vaccine development, immunogenicity, and therapeutic features of some S. aureus macromolecules as putative vaccine candidates and their implications against human S. aureus-related infections. Based on the reviewed experiments, multivalent vaccines could prevent the promotion of the diseases caused by this bacterium and enhance the prevention chance of S. aureus infections.

Toxin-neutralizing antibodies elicited by naturally acquired cutaneous anthrax are elevated following severe disease and appear to target conformational epitopes

Understanding immune responses to native antigens in response to natural infections can lead to improved approaches to vaccination. This study sought to characterize the humoral immune response to anthrax toxin components, capsule and spore antigens in individuals (n = 46) from the Kayseri and Malatya regions of Turkey who had recovered from mild or severe forms of cutaneous anthrax infection, compared to regional healthy controls (n = 20). IgG antibodies to each toxin component, the poly-γ-D-glutamic acid capsule, the Bacillus collagen-like protein of anthracis (BclA) spore antigen, and the spore carbohydrate anthrose, were detected in the cases, with anthrax toxin neutralization and responses to Protective Antigen (PA) and Lethal Factor (LF) being higher following severe forms of the disease. Significant correlative relationships among responses to PA, LF, Edema Factor (EF) and capsule were observed among the cases. Though some regional control sera exhibited binding to a subset of the tested antigens, these samples did not neutralize anthrax toxins and lacked correlative relationships among antigen binding specificities observed in the cases. Comparison of serum binding to overlapping decapeptides covering the entire length of PA, LF and EF proteins in 26 cases compared to 8 regional controls revealed that anthrax toxin-neutralizing antibody responses elicited following natural cutaneous anthrax infection are directed to conformational epitopes. These studies support the concept of vaccination approaches that preserve conformational epitopes.

Human Innate Immune Cells Respond Differentially to Poly-γ-Glutamic Acid Polymers from Bacillus anthracis and Nonpathogenic Bacillus Species

The poly-γ-glutamic acid (PGA) capsule produced by Bacillus anthracis is composed entirely of d-isomer glutamic acid, whereas nonpathogenic Bacillus species produce mixed d-, l-isomer PGAs. To determine if B. anthracis PGA confers a pathogenic advantage over other PGAs, we compared the responses of human innate immune cells to B. anthracis PGA and PGAs from nonpathogenic B. subtilis subsp. chungkookjang and B. licheniformis Monocytes and immature dendritic cells (iDCs) responded differentially to the PGAs, with B. anthracis PGA being least stimulatory and B. licheniformis PGA most stimulatory. All three elicited IL-8 and IL-6 from monocytes, but B. subtilis PGA also elicited IL-10 and TNF-α, whereas B. licheniformis PGA elicited all those plus IL-1β. Similarly, all three PGAs elicited IL-8 from iDCs, but B. subtilis PGA also elicited IL-6, and B. licheniformis PGA elicited those plus IL-12p70, IL-10, IL-1β, and TNF-α. Only B. licheniformis PGA induced dendritic cell maturation. TLR assays also yielded differential results. B. subtilis PGA and B. licheniformis PGA both elicited more TLR2 signal than B. anthracis PGA, but only responses to B. subtilis PGA were affected by a TLR6 neutralizing Ab. B. licheniformis PGA elicited more TLR4 signal than B. anthracis PGA, whereas B. subtilis PGA elicited none. B. anthracis PGA persisted longer in high m.w. form in monocyte and iDC cultures than the other PGAs. Reducing the m.w. of B. anthracis PGA reduced monocytes' cytokine responses. We conclude that B. anthracis PGA is recognized less effectively by innate immune cells than PGAs from nonpathogenic Bacillus species, resulting in failure to induce a robust host response, which may contribute to anthrax pathogenesis.

Outer membrane protein complex as a carrier for malaria transmission blocking antigen Pfs230

Malaria transmission blocking vaccines (TBV) target the mosquito stage of parasite development by passive immunization of mosquitoes feeding on a vaccinated human. Through uptake of vaccine-induced antibodies in a blood meal, mosquito infection is halted and hence transmission to another human host is blocked. Pfs230 is a gametocyte and gamete surface antigen currently under clinical evaluation as a TBV candidate. We have previously shown that chemical conjugation of poorly immunogenic TBV antigens to Exoprotein A (EPA) can enhance their immunogenicity. Here, we assessed Outer Membrane Protein Complex (OMPC), a membrane vesicle derived from Neisseria meningitidis, as a carrier for Pfs230. We prepared Pfs230-OMPC conjugates with varying levels of antigen load and examined immunogenicity in mice. Chemical conjugation of Pfs230 to OMPC enhanced immunogenicity and functional activity of the Pfs230 antigen, and OMPC conjugates achieved 2-fold to 20-fold higher antibody titers than Pfs230-EPA/AdjuPhos^® at different doses. OMPC conjugates were highly immunogenic even at low doses, indicating a dose-sparing effect. EPA conjugates induced an IgG subclass profile biased towards a Th2 response, whereas OMPC conjugates induced a strong Th1-biased immune response with high levels of IgG2, which can benefit Pfs230 antibody functional activity, which depends on complement activation. OMPC is a promising carrier for Pfs230 vaccines.

Malaria transmission blocking vaccines (TBV) target Plasmodium stages that transmit between human and mosquitos in order to interrupt the parasite’s life cycle and reduce spread. One TBV antigen currently under clinical development is Pf230, which is expressed on sexual Plasmodium stages. In this study, led by Patrick Duffy from the NIAID, researchers improve immunogenicity of Pf230. They chemically conjugate a part of Pf230 to membrane vesicles derived from bacteria, so-called outer membrane protein complexes (OMPC). Immunization of mice with Pf230-OMPC elicits a higher antibody response and a more balanced IgG subclass profile than control immunizations. Serum from Pf230-OMPC-vaccinated mice efficiently blocks infection of mosquitoes. These results with mice encourage further pre-clinical and clinical characterization of OMPC as a carrier for TBV antigens.

Cyclopeptide scaffolds in carbohydrate-based synthetic vaccines

Cyclopeptides have been recently used successfully as carriers for the multivalent presentation of carbohydrate and peptide antigens in immunotherapy. Beside their synthetic versatility, these scaffolds are indeed interesting due to their stability against enzyme degradation and low immunogenicity. This mini-review highlights the recent advances in the utilization of cyclopeptides to prepare fully synthetic vaccines prototypes against cancers and pathogens.

Conformations and molecular interactions of poly-γ-glutamic acid as a soluble microbial product in aqueous solutions

Soluble microbial products (SMPs) are of significant concern in the natural environment and in engineered systems. In this work, poly-γ-glutamic acid (γ-PGA), which is predominantly produced by Bacillus sp., was investigated in terms of pH-induced conformational changes and molecular interactions in aqueous solutions; accordingly, its sedimentation coefficient distribution and viscosity were also elucidated. Experimental results indicate that pH has a significant impact on the structure and molecular interactions of γ-PGA. The conformation of the γ-PGA acid form (γ-PGA-H) is rod-like while that of the γ-PGA sodium form (γ-PGA-Na) is sphere-like. The transformation from α-helix to random coil in the γ-PGA secondary structure is primarily responsible for this shape variation. The intramolecular hydrogen bonds in the γ-PGA-H structure decrease and intramolecular electrostatic repulsion increases as pH increases; however, the sedimentation coefficient distributions of γ-PGA are dependent on intermolecular interactions rather than intramolecular interactions. Concentration has a more substantial effect on intermolecular electrostatic repulsion and chain entanglement at higher pH values. Consequently, the sedimentation coefficient distributions of γ-PGA shift significantly at pH 8.9 from 0.1 to 1.0 g/L, and the viscosity of γ-PGA (5% w/v) significantly increases as pH increases from 2.3 to 6.0.

Protection of rhesus macaques against inhalational anthrax with a Bacillus anthracis capsule conjugate vaccine

The efficacy of currently licensed anthrax vaccines is largely attributable to a single Bacillus anthracis immunogen, protective antigen. To broaden protection against possible strains resistant to protective antigen-based vaccines, we previously developed a vaccine in which the anthrax polyglutamic acid capsule was covalently conjugated to the outer membrane protein complex of Neisseria meningitidis serotype B and demonstrated that two doses of 2.5μg of this vaccine conferred partial protection of rhesus macaques against inhalational anthrax . Here, we demonstrate complete protection of rhesus macaques against inhalational anthrax with a higher 50μg dose of the same capsule conjugate vaccine. These results indicate that B. anthracis capsule is a highly effective vaccine component that should be considered for incorporation in future generation anthrax vaccines.

Anthrax Pathogenesis

Anthrax is caused by the spore-forming, gram-positive bacterium Bacillus anthracis. The bacterium's major virulence factors are (a) the anthrax toxins and (b) an antiphagocytic polyglutamic capsule. These are encoded by two large plasmids, the former by pXO1 and the latter by pXO2. The expression of both is controlled by the bicarbonate-responsive transcriptional regulator, AtxA. The anthrax toxins are three polypeptides-protective antigen (PA), lethal factor (LF), and edema factor (EF)-that come together in binary combinations to form lethal toxin and edema toxin. PA binds to cellular receptors to translocate LF (a protease) and EF (an adenylate cyclase) into cells. The toxins alter cell signaling pathways in the host to interfere with innate immune responses in early stages of infection and to induce vascular collapse at late stages. This review focuses on the role of anthrax toxins in pathogenesis. Other virulence determinants, as well as vaccines and therapeutics, are briefly discussed.

Bacillus anthracis Capsular Conjugates Elicit Chimpanzee Polyclonal Antibodies That Protect Mice from Pulmonary Anthrax

The immunogenicity of Bacillus anthracis capsule (poly-γ-D-glutamic acid [PGA]) conjugated to recombinant B. anthracis protective antigen (rPA) or to tetanus toxoid (TT) was evaluated in two anthrax-naive juvenile chimpanzees. In a previous study of these conjugates, highly protective monoclonal antibodies (MAbs) against PGA were generated. This study examines the polyclonal antibody response of the same animals. Preimmune antibodies to PGA with titers of >10(3) were detected in the chimpanzees. The maximal titer of anti-PGA was induced within 1 to 2 weeks following the 1st immunization, with no booster effects following the 2nd and 3rd immunizations. Thus, the anti-PGA response in the chimpanzees resembled a secondary immune response. Screening of sera from nine unimmunized chimpanzees and six humans revealed antibodies to PGA in all samples, with an average titer of 10(3). An anti-PA response was also observed following immunization with PGA-rPA conjugate, similar to that seen following immunization with rPA alone. However, in contrast to anti-PGA, preimmune anti-PA antibody titers and those following the 1st immunization were ≤300, with the antibodies peaking above 10(4) following the 2nd immunization. The polyclonal anti-PGA shared the MAb 11D epitope and, similar to the MAbs, exerted opsonophagocytic killing of B. anthracis. Most important, the PGA-TT-induced antibodies protected mice from a lethal challenge with virulent B. anthracis spores. Our data support the use of PGA conjugates, especially PGA-rPA targeting both toxin and capsule, as expanded-spectrum anthrax vaccines.

Anthrax vaccines: present status and future prospects

The management of anthrax remains a top priority among the biowarfare/bioterror agents. It was the Bacillus anthracis spore attack through the US mail system after the September 11, 2001, terrorist attacks in the USA that highlighted the potential of B. anthracis as a bioterrorism agent and the threat posed by its deliberate dissemination. These attacks invigorated the efforts toward understanding the anthrax pathogenesis and development of more comprehensive medical intervention strategies for its containment in case of both natural disease and manmade, accidental or deliberate infection of a non-suspecting population. Currently, efforts are directed toward the development of safe and efficacious vaccines as well as intervention tools for controlling the disease in the advanced fulminant stage when toxemia has already developed. This work presents an overview of the current understanding of anthrax pathogenesis and recent advances made, particularly after 2001, for the successful management of anthrax and outlines future perspectives.

Potential biological targets ofBacillus anthracisin anti-infective approaches against the threat of bioterrorism

The terrorist attacks of 2001 involving anthrax underscore the imperative that safe and effective medical countermeasures should be readily available. Vaccination appears to be the most effective form of mass protection against a biological attack, but the current vaccines have drawbacks that justify the enormous amount of effort currently being put into developing more effective vaccines and other treatment modalities. After providing a comprehensive overview of the organism Bacillus anthracis as a biological weapon and its pathogenicity, this review briefly summarizes the current knowledge vital to the management of anthrax disease. This knowledge has been acquired since 2001 as a result of the progress on anthrax research and focuses on the possible development of improved human anti-infective strategies targeting B. anthracis spore components, as well as strategies based on host-pathogen interactions.

IgG subclass and heavy chain domains contribute to binding and protection by mAbs to the poly γ-D-glutamic acid capsular antigen of Bacillus anthracis

Bacterial capsules are common targets for antibody-mediated immunity. The capsule of Bacillus anthracis is unusual among capsules because it is composed of a polymer of poly-γ-d-glutamic acid (γdPGA). We previously generated murine IgG3 monoclonal antibodies (mAbs) to γdPGA that were protective in a murine model of pulmonary anthrax. IgG3 antibodies are characteristic of the murine response to polysaccharide antigens. The goal of the present study was to produce subclass switch variants of the γdPGA mAbs (IgG3 → IgG1 → IgG2b → IgG2a) and assess the contribution of subclass to antibody affinity and protection. Subclass switch antibodies had identical variable regions but differed in their heavy chains. The results showed that a switch from the protective IgG3 to IgG1, IgG2b or IgG2a was accompanied by i) a loss of protective activity ii) a change in mAb binding to the capsular matrix, and iii) a loss of affinity. These results identify a role for the heavy chain constant region in mAb binding. Hybrid mAbs were constructed in which the CH1, CH2 or CH3 heavy chain constant domains from a non-protective, low binding IgG2b mAb were swapped into the protective IgG3 mAb. The IgG3 mAb that contained the CH1 domain from IgG2b showed no loss of affinity or protection. In contrast, swapping the CH2 or CH3 domains from IgG2b into IgG3 produced a reduction in affinity and a loss of protection. These studies identify a role for the constant region of IgG heavy chains in affinity and protection against an encapsulated bacterial pathogen.

Panning of a phage display library against a synthetic capsule for peptide ligands that bind to the native capsule of Bacillus anthracis

Bacillus anthracis is the causative agent of anthrax with the ability to not only produce a tripartite toxin, but also an enveloping capsule comprised primarily of γ-D-glutamic acid residues. The purpose of this study was to isolate peptide ligands capable of binding to the native capsule of B. anthracis from a commercial phage display peptide library using a synthetic form of the capsule consisting of 12 γ-D-glutamic acid residues. Following four rounds of selection, 80 clones were selected randomly and analysed by DNA sequencing. Four clones, each containing a unique consensus sequence, were identified by sequence alignment analysis. Phage particles were prepared and their derived 12-mer peptides were also chemically synthesized and conjugated to BSA. Both the phage particles and free peptide-BSA conjugates were evaluated by ELISA for binding to encapsulated cells of B. anthracis as well as a B. anthracis capsule extract. All the phage particles tested except one were able to bind to both the encapsulated cells and the capsule extract. However, the peptide-BSA conjugates could only bind to the encapsulated cells. One of the peptide-BSA conjugates, with the sequence DSSRIPMQWHPQ (termed G1), was fluorescently labelled and its binding to the encapsulated cells was further confirmed by confocal microscopy. The results demonstrated that the synthetic capsule was effective in isolating phage-displayed peptides with binding affinity for the native capsule of B. anthracis.

Expression of either lethal toxin or edema toxin by Bacillus anthracis is sufficient for virulence in a rabbit model of inhalational anthrax

The development of therapeutics against biothreats requires that we understand the pathogenesis of the disease in relevant animal models. The rabbit model of inhalational anthrax is an important tool in the assessment of potential therapeutics against Bacillus anthracis. We investigated the roles of B. anthracis capsule and toxins in the pathogenesis of inhalational anthrax in rabbits by comparing infection with the Ames strain versus isogenic mutants with deletions of the genes for the capsule operon (capBCADE), lethal factor (lef), edema factor (cya), or protective antigen (pagA). The absence of capsule or protective antigen (PA) resulted in complete avirulence, while the presence of either edema toxin or lethal toxin plus capsule resulted in lethality. The absence of toxin did not influence the ability of B. anthracis to traffic to draining lymph nodes, but systemic dissemination required the presence of at least one of the toxins. Histopathology studies demonstrated minimal differences among lethal wild-type and single toxin mutant strains. When rabbits were coinfected with the Ames strain and the PA- mutant strain, the toxin produced by the Ames strain was not able to promote dissemination of the PA- mutant, suggesting that toxigenic action occurs in close proximity to secreting bacteria. Taken together, these findings suggest that a major role for toxins in the pathogenesis of anthrax is to enable the organism to overcome innate host effector mechanisms locally and that much of the damage during the later stages of infection is due to the interactions of the host with the massive bacterial burden.

Sortase-conjugation generates a capsule vaccine that protects guinea pigs against Bacillus anthracis

Capsules protect bacteria against phagocytic clearance. Capsular polysaccharides or polyglutamates have evolved also to resist antigen presentation by immune cells, thereby interfering with the production of opsonophagocytic antibodies. Linking capsular material to a carrier protein stimulates its presentation to the immune system. For many conjugate vaccines this is achieved by a process of random chemical cross-linking. Here we describe a new technology, designated sortase-conjugation, which generates a single amide bond between the C-terminal end of a carrier protein and the capsular material. Sortase-conjugation was used to link the poly-D-γ-glutamic acid (PDGA) capsule of Bacillus anthracis to the receptor binding domain (D4) of protective antigen (PagA). When used as a vaccine, PDGA-D4 conjugate elicited robust antibody responses against both capsule and D4. Immunization with PDGA-D4 afforded guinea pigs complete protection against anthrax challenge with wild-type or pagA mutant B. anthracis Ames.

Probing the donor and acceptor substrate specificity of the γ-glutamyl transpeptidase

γ-Glutamyl transpeptidase (GGT) is a two-substrate enzyme that plays a central role in glutathione metabolism and is a potential target for drug design. GGT catalyzes the cleavage of γ-glutamyl donor substrates and the transfer of the γ-glutamyl moiety to an amine of an acceptor substrate or water. Although structures of bacterial GGT have revealed details of the protein-ligand interactions at the donor site, the acceptor substrate site is relatively undefined. The recent identification of a species-specific acceptor site inhibitor, OU749, suggests that these inhibitors may be less toxic than glutamine analogues. Here we investigated the donor and acceptor substrate preferences of Bacillus anthracis GGT (CapD) and applied computational approaches in combination with kinetics to probe the structural basis of the enzyme's substrate and inhibitor binding specificities and compare them with human GGT. Site-directed mutagenesis studies showed that the R432A and R520S variants exhibited 6- and 95-fold decreases in hydrolase activity, respectively, and that their activity was not stimulated by the addition of the l-Cys acceptor substrate, suggesting an additional role in acceptor binding and/or catalysis of transpeptidation. Rat GGT (and presumably HuGGT) has strict stereospecificity for L-amino acid acceptor substrates, while CapD can utilize both L- and D-acceptor substrates comparably. Modeling and kinetic analysis suggest that R520 and R432 allow two alternate acceptor substrate binding modes for L- and D-acceptors. R432 is conserved in Francisella tularensis, Yersinia pestis, Burkholderia mallei, Helicobacter pylori and Escherichia coli, but not in human GGT. Docking and MD simulations point toward key residues that contribute to inhibitor and acceptor substrate binding, providing a guide to designing novel and specific GGT inhibitors.

Efficacy of a capsule conjugate vaccine against inhalational anthrax in rabbits and monkeys

Bacillus anthracis, the causative agent of anthrax, is recognized as one of the most serious bioterrorism threats. The current human vaccines are based on the protective antigen component of the anthrax toxins. Concern about possible vaccine resistant strains and reliance on a single antigen has prompted the search for additional immunogens. Bacterial capsules, as surface-expressed virulence factors, are well-established components of several licensed vaccines. In a previous study we showed that an anthrax vaccine consisting of the B. anthracis poly-γ-D-glutamic acid capsule covalently conjugated to the outer membrane protein complex of Neisseria meningitidis serotype B protected mice against parenteral B. anthracis challenge. Here we tested this vaccine in rabbits and monkeys against an aerosol spore challenge. The vaccine induced anti-capsule antibody responses in both species, measured by ELISA and a macrophage opsono-adherence assay. While rabbits were not protected against a high aerosol challenge dose, significant protection was observed in monkeys receiving the capsule conjugate vaccine. The results confirm that the capsule is a protective immunogen against anthrax, being the first non-toxin antigen shown to be efficacious in monkeys and suggest that addition of capsule may broaden and enhance the protection afforded by protective antigen-based vaccines.

Monoclonal antibody therapies against anthrax

Anthrax is a highly lethal infectious disease caused by the spore-forming bacterium Bacillus anthracis. It not only causes natural infection in humans but also poses a great threat as an emerging bioterror agent. The lethality of anthrax is primarily attributed to the two major virulence factors: toxins and capsule. An extensive effort has been made to generate therapeutically useful monoclonal antibodies to each of the virulence components: protective antigen (PA), lethal factor (LF) and edema factor (EF), and the capsule of B. anthracis. This review summarizes the current status of anti-anthrax mAb development and argues for the potential therapeutic advantage of a cocktail of mAbs that recognize different epitopes or different virulence factors.

Progress and novel strategies in vaccine development and treatment of anthrax

The lethal anthrax disease is caused by spores of the gram-positive Bacillus anthracis, a member of the cereus group of bacilli. Although the disease is very rare in the Western world, development of anthrax countermeasures gains increasing attention due to the potential use of B. anthracis spores as a bio-terror weapon. Protective antigen (PA), the non-toxic subunit of the bacterial secreted exotoxin, fulfills the role of recognizing a specific receptor and mediating the entry of the toxin into the host target cells. PA elicits a protective immune response and represents the basis for all current anthrax vaccines. Anti-PA neutralizing antibodies are useful correlates for protection and for vaccine efficacy evaluation. Post exposure anti-toxemic and anti-bacteremic prophylactic treatment of anthrax requires prolonged antibiotic administration. Shorter efficient postexposure treatments may require active or passive immunization, in addition to antibiotics. Although anthrax is acknowledged as a toxinogenic disease, additional factors, other than the bacterial toxin, may be involved in the virulence of B. anthracis and may be needed for the long-lasting protection conferred by PA immunization. The search for such novel factors is the focus of several high throughput genomic and proteomic studies that are already leading to identification of novel targets for therapeutics, for vaccine candidates, as well as biomarkers for detection and diagnosis.

An in-depth analysis of polymer-analogous conjugation using DMTMM

Combinatorial libraries have become increasingly popular in the field of functional biomaterials. One approach for creating diverse polymer libraries is polymer-analogous conjugation of functional groups to polymer scaffolds. In this study, we show that a water-soluble condensing agent, 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMTMM), can be employed to conjugate two disparate model ligands, d-(+)-galactosamine (Gal) and agmatine (Agm), to the side chains of either poly(methacrylic acid) (pMAA) or poly(acrylic acid) (pAA) at various substitution ratios. The degree of substitution was found to be directly influenced by media pH, polymer concentration, structure of ligands, and polymer precursor. A nearly 2-fold increase in conjugation efficiencies for both ligands to pAA was achieved as compared to pMAA under identical conditions reaching up to 56% and 78% of Gal and Agm of total content, respectively. These two structurally similar polymers showed remarkably different performances, which reveals that the selection of a polymer precursor is crucial for the optimal design of polymeric libraries, particularly when complex structural ligands are involved. The approach employed provides a basis from which larger and more diverse combinatorial libraries of functionalized polymers with multiple moieties can be generated.

Pre- and postexposure protection against virulent anthrax infection in mice by humanized monoclonal antibodies to Bacillus anthracis capsule

One of the two essential virulence factors of Bacillus anthracis is the poly-γ-D-glutamic acid (γDPGA) capsule. Five γDPGA-specific antibody antigen-binding fragments (Fabs) were generated from immunized chimpanzees. The two selected for further study, Fabs 11D and 4C, were both converted into full-length IgG1 and IgG3 mAbs having human IgG1 or IgG3 constant regions. These two mAbs had similar binding affinities, in vitro opsonophagocytic activities, and in vivo efficacies, with the IgG1 and IgG3 subclasses reacting similarly. The mAbs bound to γDPGA specifically with estimated binding affinities (K(d)) of 35-70 nM and effective affinities (effective K(d)) of 0.1-0.3 nM. The LD(50) in an opsonophagocytic bactericidal assay was ≈10 ng/mL of 11D or 4C. A single 30-μg dose of either mAb given to BALB/c mice 18 h before challenge conferred about 50% protection against a lethal intratracheal spore challenge by the virulent B. anthracis Ames strain. More importantly, either mAb given 8 h or 20 h after challenge provided significant protection against lethal infection. Thus, these anti-γDPGA mAbs should be useful, alone or in combination with antitoxin mAbs, for achieving a safe and efficacious postexposure therapy for anthrax.

Medical countermeasures to protect humans from anthrax bioterrorism

The deliberate dissemination of Bacillus anthracis spores via the US mail system in 2001 confirmed their potential use as a biological weapon for mass human casualties. This dramatically highlighted the need for specific medical countermeasures to enable the authorities to protect individuals from a future bioterrorism attack. Although vaccination appears to be the most effective and economical form of mass protection, current vaccines have significant drawbacks that justify the immense research effort to develop improved treatment modalities. After eight years and an expenditure of more than $50 billion, only marginal progress has been made in developing effective therapeutics. This article summarizes the most important medical countermeasures that have mostly been developed since the 2001 events, and highlights current problems and possible avenues for future research.

Anthrax vaccination strategies

The biological attack conducted through the US postal system in 2001 broadened the threat posed by anthrax from one pertinent mainly to soldiers on the battlefield to one understood to exist throughout our society. The expansion of the threatened population placed greater emphasis on the reexamination of how we vaccinate against Bacillus anthracis. The currently-licensed Anthrax Vaccine, Adsorbed (AVA) and Anthrax Vaccine, Precipitated (AVP) are capable of generating a protective immune response but are hampered by shortcomings that make their widespread use undesirable or infeasible. Efforts to gain US Food and Drug Administration (FDA) approval for licensure of a second generation recombinant protective antigen (rPA)-based anthrax vaccine are ongoing. However, this vaccine's reliance on the generation of a humoral immune response against a single virulence factor has led a number of scientists to conclude that the vaccine is likely not the final solution to optimal anthrax vaccine design. Other vaccine approaches, which seek a more comprehensive immune response targeted at multiple components of the B. anthracis organism, are under active investigation. This review seeks to summarize work that has been done to build on the current PA-based vaccine methodology and to evaluate the search for future anthrax prophylaxis strategies.

Association of Bacillus anthracis capsule with lethal toxin during experimental infection

Bacillus anthracis lethal toxin (LT) was characterized in plasma from infected African Green monkeys, rabbits, and guinea pigs. In all cases, during the terminal phase of infection only the protease-activated 63-kDa form of protective antigen (PA(63)) and the residual 20-kDa fragment (PA(20)) were detected in the plasma. No uncut PA with a molecular mass of 83 kDa was detected in plasma from toxemic animals during the terminal stage of infection. PA(63) was largely associated with lethal factor (LF), forming LT. Characterization of LT by Western blotting, capture enzyme-linked immunosorbent assay, and size exclusion chromatography revealed that the antiphagocytic poly-gamma-d-glutamic acid (gamma-DPGA) capsule released from B. anthracis bacilli was associated with LT in animal blood in variable amounts. While the nature of this in vivo association is not understood, we were able to determine that a portion of these LT/gamma-DPGA complexes retained LF protease activity. Our findings suggest that the in vivo LT complexes differ from in vitro-produced LT and that including gamma-DPGA when examining the effects of LT on specific immune cells in vitro may reveal novel and important roles for gamma-DPGA in anthrax pathogenesis.

Capsule anchoring in Bacillus anthracis occurs by a transpeptidation reaction that is inhibited by capsidin

Bacillus anthracis, the causative agent of anthrax, is a dangerous biological weapon, as spores derived from drug-resistant strains cause infections for which antibiotic therapy is no longer effective. We sought to develop an anti-infective therapy for anthrax and targeted CapD, an enzyme that cleaves poly-gamma-D-glutamate capsule and generates amide bonds with peptidoglycan cross-bridges to deposit capsular material into the envelope of B. anthracis. In agreement with the model that capsule confers protection from phagocytic clearance, B. anthracis capD variants failed to deposit capsule into the envelope and displayed defects in anthrax pathogenesis. By screening chemical libraries, we identified the CapD inhibitor capsidin, 4-[(4-bromophenyl)thio]-3-(diacetylamino)benzoic acid), which covalently modifies the active-site threonine of the transpeptidase. Capsidin treatment blocked capsular assembly by B. anthracis and enabled phagocytic killing of non-encapsulated vegetative forms.

Microbial Biosynthesis of Polyglutamic Acid Biopolymer and Applications in the Biopharmaceutical, Biomedical and Food Industries

This review article provides an updated critical literature review on the production and applications of Polyglutamic Acid (PGA). alpha-PGA is synthesized chemically, whereas gamma-PGA can be produced by a number of microbial species, most prominently various Bacilli. Great insight into the microbial formation of gamma-PGA has been gained thanks to the development of molecular biological techniques. Moreover, there is a great variety of applications for both isoforms of PGA, many of which have not been discovered until recently. These applications include: wastewater treatment, food products, drug delivery, medical adhesives, vaccines, PGA nanoparticles for on-site drug release in cancer chemotherapy, and tissue engineering.

An oligosaccharide-based HIV-1 2G12 mimotope vaccine induces carbohydrate-specific antibodies that fail to neutralize HIV-1 virions

The conserved oligomannose epitope, Man(9)GlcNAc(2), recognized by the broadly neutralizing human mAb 2G12 is an attractive prophylactic vaccine candidate for the prevention of HIV-1 infection. We recently reported total chemical synthesis of a series of glycopeptides incorporating one to three copies of Man(9)GlcNAc(2) coupled to a cyclic peptide scaffold. Surface plasmon resonance studies showed that divalent and trivalent, but not monovalent, compounds were capable of binding 2G12. To test the efficacy of the divalent glycopeptide as an immunogen capable of inducing a 2G12-like neutralizing antibody response, we covalently coupled the molecule to a powerful immune-stimulating protein carrier and evaluated immunogenicity of the conjugate in two animal species. We used a differential immunoassay to demonstrate induction of high levels of carbohydrate-specific antibodies; however, these antibodies showed poor recognition of recombinant gp160 and failed to neutralize a panel of viral isolates in entry-based neutralization assays. To ascertain whether antibodies produced during natural infection could recognize the mimetics, we screened a panel of HIV-1-positive and -negative sera for binding to gp120 and the synthetic antigens. We present evidence from both direct and competitive binding assays that no significant recognition of the glycopeptides was observed, although certain sera did contain antibodies that could compete with 2G12 for binding to recombinant gp120.

In vivo fate and distribution of poly-gamma-D-glutamic acid, the capsular antigen from Bacillus anthracis

Bacillus anthracis is surrounded by an antiphagocytic capsule composed of poly-gamma-d-glutamic acid (gammaDPGA). Bacterial and fungal capsular polysaccharides are shed into body fluids in large amounts during infection. The goal of our study was to examine the in vivo fate and distribution of the gammaDPGA capsular polypeptide. Mice were injected via the intravenous route with various amounts of purified gammaDPGA. Blood, urine, and various organs were harvested at different times after treatment. Sites of gammaDPGA accumulation were determined by immunoassay using monoclonal antibodies specific for gammaDPGA. The results showed that the liver and spleen were the primary sites for the accumulation of gammaDPGA. As found in previous studies of capsular polysaccharides, the Kupffer cells of the liver and splenic macrophages were sites for the cellular accumulation of gammaDPGA. Unlike capsular polysaccharides, the hepatic sinusoidal endothelial cells were also sites for gammaDPGA accumulation. gammaDPGA was rapidly cleared from serum and was excreted into the urine. gammaDPGA in the urine showed a reduced molecular size relative to native gammaDPGA. The results indicate that in vivo clearance of the polypeptide capsular antigen of B. anthracis shares several features with the clearance of capsular polysaccharides. Key differences between the in vivo behaviors of gammaDPGA and capsular polysaccharides include the accumulation of gammaDPGA in hepatic sinusoidal endothelial cells and a gammaDPGA clearance rate that was more rapid than the clearance reported for capsular polysaccharides.

Nasal immunization with the mixture of PA63, LF, and a PGA conjugate induced strong antibody responses against all three antigens

A new generation anthrax vaccine is expected to target not only the anthrax protective antigen (PA) protein, but also other virulent factors of Bacillus anthracis. It is also expected to be amenable for rapid mass immunization of a large number of people. This study aimed to address these needs by designing a prototypic triantigen nasal anthrax vaccine candidate that contained a truncated PA (rPA63), the anthrax lethal factor (LF), and the capsular poly-gamma-D-glutamic acid (gammaDPGA) as the antigens and a synthetic double-stranded RNA (dsRNA), polyriboinosinic-polyribocytodylic acid (poly(I:C)) as the adjuvant. This study identified the optimal dose of nasal poly(I:C) in mice, demonstrated that nasal immunization of mice with the LF was capable of inducing functional anti-LF antibodies (Abs), and showed that nasal immunization of mice with the prototypic triantigen vaccine candidate induced strong immune responses against all three antigens. The immune responses protected macrophages against an anthrax lethal toxin challenge in vitro and enabled the immunized mice to survive a lethal dose of anthrax lethal toxin challenge in vivo. The anti-PGA Abs were shown to have complement-mediated bacteriolytic activity. After further optimization, this triantigen nasal vaccine candidate is expected to become one of the newer generation anthrax vaccines.

Treatment of experimental anthrax with recombinant capsule depolymerase

Bacillus anthracis produces an antiphagocytic gamma-linked poly-D-glutamic acid capsule that is required for virulence. Capsule depolymerase (CapD) is a membrane-associated poly-gamma-glutamate-specific depolymerase encoded on the B. anthracis capsule plasmid, pX02, that is reported to contribute to virulence by anchoring the capsule to the peptidoglycan and partially degrading high-molecular-weight capsule from the bacterial surface. We previously demonstrated that treatment with CapD effectively removes the capsule from anthrax bacilli, rendering them susceptible to phagocytic killing in vitro. Here we report that CapD promoted in vivo phagocytic killing of B. anthracis bacilli by mouse peritoneal neutrophils and that parenteral administration of CapD protected mice in two models of anthrax infection. CapD conferred significant protection compared with controls when coinjected with encapsulated bacilli from fully virulent B. anthracis Ames or the nontoxigenic encapsulated strain Delta Ames and when injected 10 min after infection with encapsulated bacilli from B. anthracis Ames. Protection was also observed when CapD was administered 30 h after infection with B. anthracis Delta Ames spores, while significant protection could not be demonstrated following challenge with B. anthracis Ames spores. These data support the proposed role of capsule in B. anthracis virulence and suggest that strategies to target anthrax bacilli for neutrophil killing may lead to novel postexposure therapies.

Defensive strategies of Bacillus anthracis that promote a fatal disease

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

Transcriptional profiling of murine organ genes in response to infection with Bacillus anthracis Ames spores

Bacillus anthracis is the Gram-positive, spore-forming etiological agent of anthrax, an affliction studied because of its importance as a potential bioweapon. Although in vitro transcriptional responses of macrophages to either spore or anthrax toxins have been previously reported, little is known regarding the impact of infection on gene expression in host tissues. We infected Swiss-Webster mice intranasally with 5 LD(50) of B. anthracis-virulent Ames spores and observed the global transcriptional profiles of various tissues over a 48 h time period. RNA was extracted from spleen, lung, and heart tissues of infected and control mice and examined by Affymetrix GeneChip analysis. Approximately 580 host genes were significantly over or under expressed among the lung, spleen, and heart tissues at 8 and 48 h time points. Expression of genes encoding for surfactant and major histocompatibility complex (MHC) presentation was diminished during the early phase of infection in lungs. By 48 h, a significant number of genes were modulated in the heart, including up-regulation of calcium-binding-related gene expression, and down-regulation of multiple genes related to cell adhesion, formation of the extracellular matrix, and the cell cytoskeleton. Interestingly, the spleen 8h post-infection showed striking increases in the expression of genes that encode hydrolytic enzymes, and these levels remained elevated throughout infection. Further, genes involving antigen presentation and interferon responses were down-regulated in the spleen at 8 h. In late stages of infection, splenic genes related to the inflammatory response were up-regulated. This study is the first to describe the in vivo global transcriptional response of multiple organs during inhalational anthrax. Although numerous genes related to the host immunological response and certain protection mechanisms were up-regulated in these organs, a vast list of genes important for fully developing and maintaining this response were decreased. Additionally, the lung, spleen, and heart showed differential responses to the infection, further validating the demand for a better understanding of anthrax pathogenesis in order to design therapies against novel targets.

Conjugating recombinant proteins to Pseudomonas aeruginosa ExoProtein A: a strategy for enhancing immunogenicity of malaria vaccine candidates

Conjugation of polysaccharides to carrier proteins has been a successful approach for producing safe and effective vaccines. In an attempt to increase the immunogenicity of two malarial vaccine candidate proteins of Plasmodium falciparum, apical membrane antigen 1 (AMA1) to a blood stage vaccine candidate and surface protein 25 (Pfs25) a mosquito stage vaccine candidate, were each independently chemically conjugated to the mutant, nontoxic Pseudomonas aeruginosa ExoProtein A (rEPA). AMA1 is a large (66kD) relatively good immunogen in mice; Pfs25 is a poorly immunogenic protein when presented on alum to mice. Mice were immunized on days 0 and 28 with AMA1- or Pfs25-rEPA conjugates or unconjugated AMA1 or Pfs25, all formulated on Alhydrogel. Remarkably, sera from mice 14 days after the second immunization with Pfs25-rEPA conjugates displayed over a 1000-fold higher antibody titers as compared to unconjugated Pfs25. In contrast, AMA1 conjugated under the same conditions induced only a three-fold increase in antibody titers. When tested for functional activity, antibodies elicited by the AMA1-rEPA inhibited invasion of erythrocytes by blood-stage parasites and antibodies elicited by the Pfs25-rEPA conjugates blocked the development of the sexual stage parasites in the mosquito midgut. These results demonstrate that conjugation to rEPA induces a marked improvement in the antibody titer in mice for the poor immunogen (Pfs25) and for the larger protein (AMA1). These conjugates now need to be tested in humans to determine if mice are predictive of the response in humans.

Poly-gamma-glutamate in bacteria

Poly-gamma-glutamate (PGA), a natural polymer, is synthesized by several bacteria (all Gram-positive), one archaea and one eukaryote. PGA has diverse biochemical properties, enabling it to play different roles, depending on the organism and its environment. Indeed, PGA allows bacteria to survive at high salt concentrations and may also be involved in virulence. The minimal gene sets required for PGA synthesis were recently defined. There are currently two nomenclatures depending on the PGA final status: cap, for 'capsule', when PGA is surface associated or pgs, for 'polyglutamate synthase', when PGA is released. The minimal gene sets contain four genes termed cap or pgs B, C, A and E. The PGA synthesis complex is membrane-anchored and uses glutamate and ATP as substrates. Schematically, the reaction may be divided into two steps, PGA synthesis and PGA transport through the membrane. PGA synthesis depends primarily on CapB-CapC (or PgsB-PgsC), whereas PGA transport requires the presence, or the addition, of CapA-CapE (or PgsAA-PgsE). The synthesis complex is probably responsible for the stereochemical specificity of PGA composition. Finally, PGA may be anchored to the bacterial surface or released. An additional enzyme is involved in this reaction: either CapD, a gamma-glutamyl-transpeptidase that catalyses anchorage of the PGA, or PgsS, a hydrolase that facilitates release. The anchoring of PGA to the bacterial surface is important for virulence. All cap genes are therefore potential targets for inhibitors specifically blocking PGA synthesis or anchorage.