Rapid generation of an anthrax immunotherapeutic from goats using a novel non-toxic muramyl dipeptide adjuvant
© Kelly et al; licensee BioMed Central Ltd. 2007
Received: 24 July 2007
Accepted: 22 October 2007
Published: 22 October 2007
There is a clear need for vaccines and therapeutics for potential biological weapons of mass destruction and emerging diseases. Anthrax, caused by the bacterium Bacillus anthracis, has been used as both a biological warfare agent and bioterrorist weapon previously. Although antibiotic therapy is effective in the early stages of anthrax infection, it does not have any effect once exposed individuals become symptomatic due to B. anthracis exotoxin accumulation. The bipartite exotoxins are the major contributing factors to the morbidity and mortality observed in acute anthrax infections.
Using recombinant B. anthracis protective antigen (PA83), covalently coupled to a novel non-toxic muramyl dipeptide (NT-MDP) derivative we hyper-immunized goats three times over the course of 14 weeks. Goats were plasmapheresed and the IgG fraction (not affinity purified) and F(ab')2 derivatives were characterized in vitro and in vivo for protection against lethal toxin mediated intoxication.
Anti-PA83 IgG conferred 100% protection at 7.5 μg in a cell toxin neutralization assay. Mice exposed to 5 LD50 of Bacillus anthracis Ames spores by intranares inoculation demonstrated 60% survival 14 d post-infection when administered a single bolus dose (32 mg/kg body weight) of anti-PA83 IgG at 24 h post spore challenge. Anti-PA83 F(ab')2 fragments retained similar neutralization and protection levels both in vitro and in vivo.
The protection afforded by these GMP-grade caprine immunotherapeutics post-exposure in the pilot murine model suggests they could be used effectively to treat post-exposure, symptomatic human anthrax patients following a bioterrorism event. These results also indicate that recombinant PA83 coupled to NT-MDP is a potent inducer of neutralizing antibodies and suggest it would be a promising vaccine candidate for anthrax. The ease of production, ease of covalent attachment, and immunostimulatory activity of the NT-MDP indicate it would be a superior adjuvant to alum or other traditional adjuvants in vaccine formulations.
Bacillus anthracis, the causative agent of anthrax, has been the focus of much research and attention following the release of spores through the US mail system in 2001. 22 cases of infection resulted in 5 deaths, causing much concern regarding treatment, therapeutics and vaccine efficacy. Recently, the CDC discontinued the administration of the current anthrax vaccine (Anthrax Vaccine Adsorbed -AVA) due to adverse side effects observed in a large percentage of volunteers. This revocation of available vaccine has left healthcare workers, laboratory personnel and first responders with only limited means of protection following potential exposures to anthrax spores.
In humans, the anthracis bacilli can cause three types of infections: cutaneous via abrasions in the skin, gastrointestinal through ingestion of spores in contaminated meat and inhalation when spores less than 5 uM um are deposited into the lungs . The mortality rates vary between each form of the disease with cutaneous anthrax presenting as a self-limiting and treatable infection with only a 20% case fatality rate. When left untreated gastrointestinal infections can progress rapidly and have over 80% case fatality rates. Inhalation anthrax infections are rare but have a high case fatality rate (over 75%) even with antibiotic treatment.
Treatment options for patients presenting with symptoms of inhalational anthrax infections are limited and are generally ineffective at reducing mortality. Although antibiotic therapy is effective in the early stages of infection, it does not have any effect on the bipartite exotoxins, which are the major contributing factors to the mortality observed in acute anthrax infections . The current lack of an approved, available vaccine puts laboratory workers, military personnel and first responders at an increased risk of inhalational anthrax should another terrorist event, similar to the anthrax mailings in 2001, occur. Clearly there is a need for an effective vaccine as well as a well-tolerated, economical, post-exposure therapeutic for the treatment of human anthrax infections.
Passive immunotherapy is a non-chemical therapeutic providing immediate immunity to infectious agents and toxins. This treatment option has been shown to be effective against many diseases including anthrax [2–6] and other biothreat agents [7, 8]. Several approaches have been used previously for the production of immunotherapeutics specific for B. anthracis although they all have significant drawbacks. The pooling of immune serum from previously vaccinated volunteers yields highly protective anti-sera in very small quantities, limiting its use as a source of therapeutics for the Strategic National Stockpile or as a commercially available product. Monoclonal antibodies are highly specific, limiting their application to a single antigenic target and have a high cost associated with their development further limiting their feasibility for mass production and stockpiling. In the past animal vaccination has successfully been used to generate immunotherapeutic antiserum specific for infectious and toxic agents including snake venom, botulism toxin and Ebola virus [9–12] but limitations in quantity and safety have prevented their widespread use in the development of human therapeutics. Horses can provide large amounts of antiserum but are costly to maintain. Mice, rabbits and guinea pigs are inexpensive to maintain but yield limited volumes of anti-sera. Goats provide a renewable source of plasma and serum; however they have not been traditionally used in the generation of passive immunotherapeutics. We have plasmapheresed hyper immunized goats to successfully produce liters of GMP-grade antisera following a short immunization schedule (3 immunizations over 14 weeks), with minimal cost.
Bacillus anthracis produces two separate exotoxins, edema toxin (EdTx) and lethal toxin (LeTx). The two exotoxins utilize a common cell binding component termed protective antigen (PA83, 83 kDa) which binds to the ubiquitous anthrax toxin receptor (ATR) found on most cell surfaces. Once PA83 is bound to the host cell surface, a furin-like protease cleaves the full-length, inactive protein into the active form, PA63 (63 kDa), thereby exposing the binding sites for the catalytic components of the exotoxins (edema factor, EF or lethal factor, LF). A heptamer composed of PA63 + three LF/EF moieties [13, 14] forms on the cell surface and is internalized via receptor mediated endocytosis. The subsequent decrease in pH within the endosome causes conformational changes in PA63, so that it inserts into the endosomal membrane, forming a protease-stable pore; formation of this pore allows EF and LF to enter the cell and exert their toxic effects . LeTx is formed when PA63 is combined with LF, and is responsible for the most severe intoxicative effects of anthrax infection. EF is an adenylate cyclase capable of causing severe disregulation of cellular cAMP levels . LF has been shown to be a zinc-dependant metalloprotease with specificity for mitogen-activated protein kinase kinases (MAPKKs) capable of disrupting several cell signaling cascades; however, its specific mode of action is still unclear [17, 18]. Disruption of the binding of PA to ATR or LF would disrupt internalization of functional LeTx and would thereby prevent toxin-mediated death of the host following rapid multiplication of the bacilli.
Here we immunized goats with recombinant PA83, coupled to a novel non-toxic muramyl dipeptide derivative (NT-MDP) capable of inducing both innate and humoral immunity and does not induce clotting even when administered at high concentrations. The resulting polyclonal anti-sera conferred protection against in vitro and in vivo intoxication with the anthrax lethal toxin (LeTx) and in vivo intranasal challenge with virulent B. anthracis spores. Recently, we have shown that the passive transfer of goat-derived anti-HIV antibodies to failing therapy AIDS patients has been well tolerate, safe and effective [19–21].
In order to circumvent any hypersensitivity reactions associated with goat IgG, we have explored the use of F(ab')2 antibodies lacking the Fc region of the IgG molecule. The Fc region of the IgG is involved in the activation of complement, and patients with a pre-developed sensitivity to goat proteins may be at a higher risk of developing fatal allergic reactions following the administration of a goat-based antibody therapy. Removal of the Fc region allows for the retention of the dimeric antigen binding sites while increasing the safety of the immunotherapeutic without a significant loss in neutralizing capabilities.
Our data suggests that the administration of anti-PA83 goat IgG or F(ab')2 would provide an efficacious and well-tolerated passive immunotherapy for post-exposure treatment of acute human anthrax infections. Most notable is the rapidity with which the anti-sera were produced in goats and the volume of anti-sera generated from a single plasmapheresis. In addition, this data serves a proof of concept that a rapid, inexpensive, GMP-grade immunotherapeutic can be produced in a short enough timeframe for an emerging disease event like SARS-CoV.
Recombinant anthrax toxin proteins
High-purity, histidine-tagged rLF and rPA83 were supplied by the Northeast Biodefense Center Protein Expression Core. Functional lethal toxin (LeTx) was formed by the combination of purified rLF and rPA83 at a 1:1 (w/w) ratio diluted in sterile PBS.
Purified rPA83 was supplied to Virionyx Corporation Ltd (Auckland, NZ) for caprine immunizations as follows. A novel muramyl dipeptide adjuvant (NT-MDP) was oxidized with sodium meta periodate (0.5 M) for 1 h and excess sodium meta periodate was removed by centrifugation followed by a water wash. 1 mg of rPA83 in sodium carbonate buffer (0.1 M, pH 9.5) was added to 10 mg of activated NT-MDP and incubated overnight at room temperature. The resulting Schiff's base was reduced by the addition of ascorbic acid to achieve a pH of 7.0. Three goats were immunized with 100 μg rPA83-NT-MDP conjugates emulsified in Freund's complete adjuvant and were subsequently boosted three additional times with immunogen in Freund's incomplete adjuvant over a 13-week period. Hyper-immune plasma was collected from each animal two weeks following the last immunization. Plasma was pooled and IgG was purified using a standard octanoic acid precipitation technique. Purified anti-PA83 IgG was supplied at a concentration of 15 mg/ml.
Generation of F(ab')2antibody fragments
F(ab')2 fragments were generated by pepsin digestion (100 U/mg IgG) at pH 3.5 in 0.1 M glycine buffer for 24 h. Reactivity was demonstrated using an Ouchterlony gel diffusion assay and demonstrated reactivity at 1 mg/ml against rabbit anti-goat IgG (data not shown). Purity and extent of digestion was determined by SDS-PAGE analysis (data not shown).
Anti-sera titer determination
ELISAs were performed in microtiter plates coated with rPA83 (10 nM) in 10 mM carbonate/bicarbonate buffer (pH 8.5) with a final coating volume of 50 μl. Plates were coated for 1 h then washed in water and blocked with 5% non-fat milk powder. Antibody titers were measured by reacting (2 h) serially diluted anti-PA83 IgG with the rPA83-coated microtiter wells. The wells were then washed with water and reacted (2 h) with horseradish peroxidase-labeled rabbit anti-goat IgG. Following one water wash, the wells were reacted (30 min) with the substrate, orthophenylenediamine. The reaction was stopped by the addition of sulfuric acid and absorbance was measured at 492 nm. Anti-PA83 IgG titers were measured and expressed as the reciprocal of the antibody dilution which produced an absorbance value equal to 50% maximum absorbance.
Cell lines and media
Murine macrophage-like cells, J774A.1, were obtained from the American Type Cell Culture Collection (ATCC TIB-67). Cells were cultured in complete medium: Dulbecco's Modified Eagle Medium (DMEM) supplemented with 10% fetal bovine serum, Glutamax, and penicillin/streptomycin at 37°C with 5% CO2.
In vitrocytotoxicity and protection assays
Macrophage-like cells were harvested by gentle scraping (no trypsin) and were seeded in 96-well plates at a density of 6 × 104 cells/well in 100 μl of complete medium. Cells were incubated for 18–24 h or until > 90% confluency had been achieved. Medium was removed, and cells were washed once in sterile PBS before addition of toxin or anti-sera. For toxicity assays, 100 μl of LeTx was added to the cells at final concentrations of 1000 ng, 100 ng, 10 ng and 0.1 ng (data not shown). For protection assays, 50 ng of LeTx (2 TCEC50) was combined with varying dilutions of anti-PA83 IgG or F(ab')2 and incubated at 37°C, while shaking for 1 h prior to the addition of 100 μl per well. Cells with LeTx alone or in combination with anti-sera were incubated at 37°C and 5% CO2 for 4 h. Cell viability was determined using Sigma's Cell Growth Determination Kit, an MTT-based assay. Briefly, 10 μl of MTT dye was added to cells and incubated for 15 h at 37°C and 5% CO2. 100 μl of solubilization solution was added to each well after removal of media, and cell viability was measured at 570 nm. Percent relative cell viability was calculated as the ratio between LeTx-treated cells (LeTx) and untreated control cells (100 μl PBS). Percent protection conferred by caprine anti-PA83 IgG or F(ab')2 was measured as follows:
(1-((PBS - α PA83 IgG)/(PBS - 50 ng LeTx))) × 100.
In vivoprotection assays
Lethal toxin challenge
Female Balb/c mice (average weight 17.5 g) were injected with 100 μg LeTx in 200 μl saline via intraperitoneal injection (5 per group). Five minutes following toxin injection mice were injected on the opposite side with 8 mg/kg anti-PA83 IgG or F(ab')2 in 200 μl saline. Control mice (3 in group) received LeTx followed by saline injections. Mice were observed for signs of illness and distress for 11 days at which point all surviving mice were sacrificed.
Virulent B. anthracisspore intranasal challenge
Female Swiss Webster mice (average weight 25.2 g) were infected with approximately 5 × 104 B. anthracis Ames spores (5 LD50) by 20 μl installations in each nares. Groups of 10 mice received saline at 1 hour post-infection or anti-PA83 IgG at 24 h post-infection (32 mg/kg) by intraperitoneal injection. Mice were monitored twice daily for 14 d for signs of illness and death. To evaluate synergistic effects of antibiotic treatment post-exposure, low-dose Ciprofloxacin was administered twice daily at 0.9 mg/day via intraperitoneal injection for the first six days post spore challenge.
Statistical Analysis of in vivoresults
Statistical analysis (logrank test) of the in vivo survival data was performed using GraphPad Prism (version 4.03), GraphPad Software, San Diego, CA.
Results and Discussion
Anthrax lethal toxin activity
Purified rLF (90 kDa) and rPA83 (83 kDa) showed high product purity, with no significant breakdown products by SDS PAGE, trypsin digestion and mass spectroscopy (> 95% purity for both, data not shown). In vitro bioactivity of LeTx was confirmed by treating J774A.1 murine macrophage-like cells with varying doses of LeTx (10 – 0.001 ng/μl), and cell viability determined via toxin neutralization assay. Cell viability experiments established a TCEC50 of 25 ng LeTx (equivalent to 2.85 nM, data not shown). This dose of LeTx is within the range of previously reported TCEC50s [22–25]. Based on this data, all subsequent in vitro protection assays were performed at 2× TCEC50 equivalent to a total of 50 ng LeTx per well.
Generation and evaluation of anti-PA83 caprine immunoglobulin
Anti-PA83 IgG and F(ab')2protect cells against LeTx-induced cytotoxicity
The protective efficacy of the anti-PA83 IgG and the F(ab')2 derivative was evaluated in the J774A.1 LeTx in vitro model. Cells were exposed to 0.5 ng/μl of LeTx and dilutions of anti-PA83 IgG or F(ab')2. MTT-based cell viability assays were used to determine percent protection as described in Materials and Methods. Control included untreated cells (i.e., PBS substituted for LeTx), cells treated with IgG alone (7.5 μg α PA83 Ig with no LeTx), or cells treated with 0.5 ng/μl LeTx alone (LeTx). LeTx treated cells demonstrated a statistically significant decrease in cell viability (p < 0.001) as compared to the untreated PBS control cells, while standard concentrations of anti-PA83 IgG (7.5 μg) had no effect on cell viability (data not shown). The use of higher concentrations of anti-PA83 IgG (up to 250 μg) produced no significant differences in cell viability (data not shown). These results confirm that caprine IgG exhibits no inherent cytotoxic effects in vitro and does not interfere with the observed cytotoxicity of the recombinant LeTx.
In vivoprotection of mice following LeTx challenge
Passive protection of mice 24 hours post-infection with Ames spores
Many groups have shown the efficacy of polyclonal, animal-derived sera for use as a passive immunotherapeutic against anthrax infections, however these groups have relied on smaller animal models (e.g., mice, rabbits, guinea pigs) to generate the antisera [3, 4, 26, 27]. Smaller animals are typically terminally bled in order to produce larger volumes of serum. Yields from a terminal bleed typically range from 0.5 ml for mice up to 200 ml for terminally bled rabbits. The large number of animals required to produce the therapeutic quantities needed for a useful medical countermeasure stockpile (e.g., the SNS) makes these animal models prohibitively expensive. Caprine plasmapheresis does not require the animals to be euthanized/terminally bled in order to generate large volumes of antisera. Additionally, the goats can be plasmapheresed up to four times per year for several years making for a nearly endless source of antisera. Plasmapheresis of three goats generated liters of anti-PA83 serum within a very short time frame. Additionally, the goats used to produce this material are part of a certified pathogen-free herd and the antisera produced are of GMP grade. Comparably produced IgG against HIV has been previously approved for clinical trials in humans [19–21].
The previously approved AVA anthrax vaccine required a series of six immunizations followed by annual boosts. The use of a novel non-toxic MDP adjuvant enabled the generation of extremely high-titer antiserum following only two immunizations although for the current study, IgG was isolated from goats immunized four times. With further optimization of the immunization regiment, we may be able to generate an efficacious immunotherapeutic with fewer immunizations, thus shortening the production time and cost. It should also be emphasized that the data presented here used non-affinity-purified IgG or F(ab')2. Studies are underway to evaluate the efficacy of the affinity purified materials, which may significantly reduce the amount of material required to offer significant protection in both animals and humans.
F(ab')2 antibodies have been used for the treatment of rattlesnake bites [28, 29], bee stings  and evaluated for their potential to treat several infectious diseases including respiratory syncitial virus (RSV) . Many monoclonal antibodies (MAbs) have been generated that are specific for the anthrax protective antigen. The majority of these MAbs do not demonstrate significant protection post-exposure and appear to require a blend of several MAbs in order to reduce the mortality associated with anthrax infections [32, 33]. A recent study using a monoclonal antibody against the anthrax protective antigen demonstrated a requirement for the Fc portion of the antibody in order to retain neutralizing capabilities . Our polyclonal immunotherapeutic retained similar neutralizing levels both in vitro and in vivo after removal of the Fc region by pepsin digestion. These findings are consistent with data from other polyclonal antiserum, which indicate most F(ab')2 retain comparable neutralizing and protective abilities to full length IgG [26, 29, 30, 34]. The utility of F(ab')2 antisera derived from goats will reduce the potential for side-effects associated with patients who have a pre-existing sensitivity to goat proteins. In addition, patients requiring multiple treatments with an animal derived therapeutic may also be at increased risk of developing allergic hypersensitivity, so the use of F(ab')2 antibody fragments will decrease this risk and increase the overall safety of this immunotherapeutic for multiple uses within a large population.
This work has shown that pharmaceutical-grade goat polyclonal immunotherapeutics specific for the anthrax protective antigen can be rapidly produced in large quantities. Three goats immunized four times over a 56 day period produced liters of GMP grade, high titer antisera that was capable of neutralizing anthrax lethal toxin both in vitro and in vivo. More importantly the passive transfer of the goat-derived antibodies 24 h post-exposure to virulent anthrax spores provided mice with a substantial survival advantage over untreated mice. A synergistic effect was seen with concomitant antibiotic treatment although levels of protection returned to the levels observed with IgG treatment alone once antibiotic therapy was discontinued. This indicates that a combined treatment approach for patients presenting with clinical signs of anthrax infection could overall increase in survival rates associated with symptomatic disease. Additionally, this immunotherapeutic can be easily produced in quantities large enough to fulfill the requirements for a national medical countermeasures stockpile. The non-toxic MDP adjuvant developed is easily produced; amenable to covalent attachment of antigens, and importantly, renders toxins and pathogens inactive once coupled to the molecule. The use of this novel adjuvant should improve vaccine development and quality control in addition to eliciting significantly higher immune responses than standard adjuvants.
Funding for the intranasal mouse study was provided by the National Institutes of Allergy and Infectious Diseases contract with the University of Texas Medical Branch, Contract # N01-AI-30065. CDK received support from the SUNY Albany Foundation through a Ford Foundation IFW Women in Science Fellowship. Thanks to the Northeast Biodefense Center Protein Core Laboratory for the production and purification of recombinant proteins. We are grateful to Jim Hengst and Michelle Ferreri-Jacobia for their technical assistance.
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