Protective antigen and extractable antigen 1 based chimeric protein confers protection against Bacillus anthracis in mouse model

Design and biosynthesis of functional protein nanostructures

Proteins are one of the major classes of biomolecules that execute biological functions for maintenance of life. Various kinds of nanostructures self-assembled from proteins have been created in nature over millions of years of evolution, including protein nanowires, layers and nanocages. These protein nanostructures can be reconstructed and equipped with desired new functions. Learning from and manipulating the self-assembly of protein nanostructures not only help to deepen our understanding of the nature of life but also offer new routes to fabricate novel nanomaterials for diverse applications. This review summarizes the recent research progress in this field, focusing on the characteristics, functionalization strategies, and applications of protein nanostructures.

Functional characterization and evaluation of protective efficacy of EA752-862 monoclonal antibody against B. anthracis vegetative cell and spores

The most promising means of controlling anthrax, a lethal zoonotic disease during the early infection stages, entail restricting the resilient infectious form, i.e., the spores from proliferating to replicating bacilli in the host. The extractible antigen (EA1), a major S-layer protein present on the vegetative cells and spores of Bacillus anthracis, is highly immunogenic and protects mice against lethal challenge upon immunization. In the present study, mice were immunized with r-EA1C, the C terminal crystallization domain of EA1, to generate a neutralizing monoclonal antibody EA752-862, that was evaluated for its anti-spore and anti-bacterial properties. The monoclonal antibody EA752-862 had a minimum inhibitory concentration of 0.08 mg/ml, was bactericidal at a concentration of 0.1 mg/ml and resulted in 100% survival of mice against challenge with B. anthracis vegetative cells. Bacterial cell lysis as observed by scanning electron microscopy and nucleic acid leakage assay could be attributed as a possible mechanism for the bactericidal property. The association of mAb EA752-862 with spores inhibits their subsequent germination to vegetative cells in vitro, enhances phagocytosis of the spores and killing of the vegetative cells within the macrophage, and subsequently resulted in 90% survival of mice upon B. anthracis Ames spore challenge. Therefore, owing to its anti-spore and bactericidal properties, the present study demonstrates mAb EA752-862 as an efficient neutralizing antibody that hinders the establishment of early infection before massive multiplication and toxin release takes place.

A Bivalent Protein r-PAbxpB Comprising PA Domain IV and Exosporium Protein BxpB Confers Protection Against B. anthracis Spores and Toxin

Anthrax vaccines primarily relying only on protective antigen (PA), the cell binding component in anthrax toxins provide incomplete protection when challenged with spores of virulent encapsulated Bacillus anthracis strains. Alternatively, formaldehyde inactivated spores (FIS) or recombinant spore components generate anti-spore immune responses that inhibit the early stages of infection and augment the PA protective efficacy. In the present study domain IV of PA was spliced with exosporium antigen BxpB via a flexible G4S linker to generate a single functional antigen r-PAbxpB that was further assessed for its protective efficacy against anthrax toxins and spore infection. Immunization of mice with r-PAbxpB elicited significantly high titer antibodies comprising IgG1:IgG2a isotypes in 1:1 ratio, balanced up-regulation of both Th1 (IL2, IL12, IFN-γ) and Th2 (IL4, IL5, IL10) cytokines and high frequencies of CD4+ and CD8+ T cell subsets. The anti-r-PAbxpB antibodies significantly enhanced spore phagocytosis, and killing within macrophages; inhibited their germination to vegetative cells and completely neutralized the anthrax toxins as evidenced by the 100% protection in passive transfer studies. Active immunization with r-PAbxpB provided 100 and 83.3% protection in mice I.P. challenged with 5 × LD₁₀₀ LD of toxins and 5 × 10⁴ cfu/ml Ames spores, respectively while the sham immunized group succumbed to infection in 48 h. Therefore, the ability of r-PAbxpB to generate protective immune responses against both spores and toxin and provide significant protection suggests it as an efficient vaccine candidate against B. anthracis infection.

A bivalent protein r-PB, comprising PA and BclA immunodominant regions for comprehensive protection against Bacillus anthracis

Anthrax infection is primarily initiated by B. anthracis endospores that on entry into the host germinate to vegetative cells and cause severe bacteremia and toxaemia employing an array of host colonisation factors and the lethal tripartite toxin. The protective efficacy of conventional protective antigen (PA) based anthrax vaccines is improved by co-administration with inactivated spores or its components. In the present study, using structural vaccinology rationale we synthesized a bivalent protein r-PB encompassing toxin (PAIV) and spore components (BclACTD) and characterized its protective efficacy against B. anthracis infection. Active immunization of mice with r-PB generated high titer circulating antibodies which facilitated the phagocytic uptake of spores, inhibited their germination to vegetative cells and completely neutralized anthrax toxins in vivo resulting in 100 % survival against anthrax toxin challenge. Proliferation of CD4+ T cell subsets with up-regulation of Th1 (IFN-γ, IL-2, and IL-12), Th2 (IL-5, IL-10) cytokines and balanced expression of IgG1:IgG2a antibody isotypes indicated the stimulation of both Th1 and Th2 subsets. The immunized mice exhibited 100 % survival upon challenge with B. anthracis spores or toxin indicating the ability of r-PB to provide comprehensive protection against anthrax. Our results thus demonstrate r-PB an efficient vaccine candidate against anthrax infection.

Intraperitoneal administration of a novel chimeric immunogen (rOP) elicits IFN-γ and IL-12p70 protective immune response in BALB/c mice against virulent Brucella

Recombinant engineering of immunologically active chimeric protein consisting of Omp19 and P39 domains of B. abortus (rOP), was purified under denaturing conditions upon expression in E. coli BL21 (DE3) and refolded to dynamic form. The immuno-protective efficacy of rOP was evaluated by challenging the BALB/c mice intraperitoneally (I.P) with the infective species of Brucella in the absence or presence of adjuvants, such as Aluminum hydroxide gel (Al), or Freund's Complete Adjuvant (FCA)/Incomplete Freund's Adjuvant (IFA). Surprisingly, after second boosting, mice received rOP per se were found to be immunogenic in terms of IgG response with the dominant expression of IgG2a and significant IFN-γ by splenic T cells, suggesting that rOP is a strong inducer of anti-Brucella immunity. The resulted anti-rOP antibodies recognized native Omp19 and P39 among species of Brucella with distinct double bands and single band against chimera in immunoblotting. An enhanced and comparable antibody response with varied IgG isotype combinations were noticed in the mice primed and boosted with rOP in adjuvants. However, rOP+FCA/IFA formulation was found to be the most effective in lymphocyte recall assays at inducing significant (P<0.001) proliferation index (P.I.) as well as increased Th1-coupled cytokines (IFN-γ, IL-2 and IL-12p70) than rOP+Al in response to rOP re-stimulation. Furthermore, in vitro defensive assay revealed that compared to anti-rOP antisera, the polyclonal anti-sera from rOP+adjuvants exhibited enhanced protection of RAW264.7 cells against virulent challenge by B. melitensis 16M and B. abortus 544. In addition, compared to sham group, enumeration of Brucella CFU after challenge with the above species showed a significant (P<0.01) reduction of bacteria (log CFU) in the macrophage cell lines and organs of vaccinated mice. On the whole, a relatively higher and faster reduction was noticed in the mice vaccinated with similar amount of purified antigen in Freund's adjuvant. Ability of inducing Th1 directed immune protection in the absence of adjuvant support, postulated rOP as a plausible entrant for developing a chimeric based subunit vaccine against Brucella.

Use of human immunoglobulins as an anti-infective treatment: the experience so far and their possible re-emerging role

INTRODUCTION

Pooled human immunoglobulins (IGs) are prepared from plasma obtained from healthy donors as a concentrated antibody-containing solution. In addition, high-titer IGs (hyperimmune) against a specific pathogen can be obtained from vaccinated or convalescing donors. Currently, IGs can be used for the treatment of a variety of infections for which no specific therapy exists or that remain difficult to treat. Moreover, the recent pathogen outbreaks for which there is no approved treatment have renewed attention to the role of convalescent plasma and IGs. Areas covered: In this review, a historical perspective of the use of sera and IGs in humans as anti-infective agents (any viral, bacterial, parasitic infection), excluding immunodeficient patients, is presented from early development to the latest clinical studies. A Medline search was conducted to examine the peer-reviewed literature, with no date limits. Expert commentary: Human pooled plasma-derived IG products benefit from the polyclonal response of every individual donor and from the interindividual variability in such response. The trend to increased availability of vaccines for infectious diseases also opens new potential applications of hyperimmune IGs for emerging or re-emerging infectious diseases (e.g.: Ebola, Zika, Dengue), for the prevention and treatment in the general population, healthcare personnel and caregivers.

A S-Layer Protein of Bacillus anthracis as a Building Block for Functional Protein Arrays by In Vitro Self-Assembly

S-layer proteins create a cell-surface layer architecture in both bacteria and archaea. Because S-layer proteins self-assemble into a native-like S-layer crystalline structure in vitro, they are attractive building blocks in nanotechnology. Here, the potential use of the S-layer protein EA1 from Bacillus anthracis in constructing a functional nanostructure is investigated, and apply this nanostructure in a proof-of-principle study for serological diagnosis of anthrax. EA1 is genetically fused with methyl parathion hydrolase (MPH), to degrade methyl parathion and provide a label for signal amplification. EA1 not only serves as a nanocarrier, but also as a specific antigen to capture anthrax-specific antibodies. As results, purified EA1-MPH forms a single layer of crystalline nanostructure through self-assembly. Our chimeric nanocatalyst greatly improves enzymatic stability of MPH. When applied to the detection of anthrax-specific antibodies in serum samples, the detection of our EA1-MPH nanostructure is nearly 300 times more sensitive than that of the unassembled complex. Together, it is shown that it is possible to build a functional and highly sensitive nanosensor based on S-layer protein. In conclusion, our present study should serve as a model for the development of other multifunctional nanomaterials using S-layer proteins.