Production and purification of a recombinant Staphylococcal enterotoxin B vaccine candidate expressed in Escherichia coli

A Sensitive Immunodetection Assay Using Antibodies Specific to Staphylococcal Enterotoxin B Produced by Baculovirus Expression

Staphylococcal enterotoxin B (SEB) is a potent bacterial toxin that causes inflammatory stimulation and toxic shock, thus it is necessary to detect SEB in food and environmental samples. Here, we developed a sensitive immunodetection system using monoclonal antibodies (mAbs). Our study is the first to employ a baculovirus expression vector system (BEVS) to produce recombinant wild-type SEB. BEVS facilitated high-quantity and pure SEB production from suspension-cultured insect cells, and the SEB produced was characterized by mass spectrometry analysis. The SEB was stable at 4 °C for at least 2 years, maintaining its purity, and was further utilized for mouse immunization to generate mAbs. An optimal pair of mAbs non-competitive to SEB was selected for sandwich enzyme-linked immunosorbent assay-based immunodetection. The limit of detection of the immunodetection method was 0.38 ng/mL. Moreover, it displayed higher sensitivity in detecting SEB than commercially available immunodetection kits and retained detectability in various matrices and S. aureus culture supernatants. Thus, the results indicate that BEVS is useful for producing pure recombinant SEB with its natural immunogenic property in high yield, and that the developed immunodetection assay is reliable and sensitive for routine identification of SEB in various samples, including foods.

Effective Treatment of Staphylococcal Enterotoxin B Aerosol Intoxication in Rhesus Macaques by Using Two Parenterally Administered High-Affinity Monoclonal Antibodies

Staphylococcal enterotoxin B (SEB) is a protein exotoxin found on the cell surface of Staphylococcus aureus that is the source for multiple pathologies in humans. When purified and concentrated in aerosol form, SEB can cause an acute and often fatal intoxication and thus is considered a biological threat agent.

Staphylococcal enterotoxin B (SEB) is a protein exotoxin found on the cell surface of Staphylococcus aureus that is the source for multiple pathologies in humans. When purified and concentrated in aerosol form, SEB can cause an acute and often fatal intoxication and thus is considered a biological threat agent. There are currently no vaccines or treatments approved for human use. Studies with rodent models of SEB intoxication show that antibody therapy may be a promising treatment strategy; however, many have used antibodies only prophylactically or well before any clinical signs of intoxication are apparent. We assessed and compared the protective efficacies of two monoclonal antibodies, Ig121 and c19F1, when administered after aerosol exposure in a uniformly lethal nonhuman primate model of SEB intoxication. Rhesus macaques were challenged using small-particle aerosols of SEB and then were infused intravenously with a single dose of either Ig121 or c19F1 (10 mg/kg of body weight) at either 0.5, 2, or 4 h postexposure. Onset of clinical signs and hematological and cytokine response in untreated controls confirmed the acute onset and potency of the toxin used in the challenge. All animals administered either Ig121 or c19F1 survived SEB challenge, whereas the untreated controls succumbed to SEB intoxication 30 to 48 h postexposure. These results represent the successful therapeutic in vivo protection by two investigational drugs against SEB in a severe nonhuman primate disease model and punctuate the therapeutic value of monoclonal antibodies when faced with treatment options for SEB-induced toxicity in a postexposure setting.

Leaf-Encapsulated Vaccines: Agroinfiltration and Transient Expression of the Antigen Staphylococcal Endotoxin B in Radish Leaves

Transgene introgression is a major concern associated with transgenic plant-based vaccines. Agroinfiltration can be used to selectively transform nonreproductive organs and avoid introgression. Here, we introduce a new vaccine modality in which Staphylococcal enterotoxin B (SEB) genes are agroinfiltrated into radishes (Raphanw sativus L.), resulting in transient expression and accumulation of SEB in planta. This approach can simultaneously express multiple antigens in a single leaf. Furthermore, the potential of high-throughput vaccine production was demonstrated by simultaneously agroinfiltrating multiple radish leaves using a multichannel pipette. The expression of SEB was detectable in two leaf cell types (epidermal and guard cells) in agroinfiltrated leaves. ICR mice intranasally immunized with homogenized leaves agroinfiltrated with SEB elicited detectable antibody to SEB and displayed protection against SEB-induced interferon-gamma (IFN-γ) production. The concept of encapsulating antigens in leaves rather than purifying them for immunization may facilitate rapid vaccine production during an epidemic disease.

A novel staphylococcal enterotoxin B subunit vaccine candidate elicits protective immune response in a mouse model

Staphylococcal enterotoxin B (SEB), produced by the gram-positive bacterium Staphylococcus aureus, is responsible for food poisoning and toxic shock syndrome, and is considered a potential bioterrorism agent. Unfortunately, still now no approved vaccines are available against SEB. In this study, we constructed a series of nontoxic SEB mutants (mSEBs) and examined whether these mSEBs provide protective immunity against SEB challenge. These mSEB vaccine candidates did not demonstrate superantigen activity in mouse splenocyte cultures. Immunization with the vaccine candidates triggered the production of IgG-antibodies with neutralizing activity. In addition, increased production of IgG1 and IgG3 was observed after immunization, which signifies both Th1- and Th2-induced immune responses. Among the vaccine candidates tested, S9, a double mutant (N23A and Y90A) and S19, a quadruple mutant (N23A, Y90A, R110A, and F177A), demonstrated complete protection against a lethal SEB challenge. Altogether, our results strongly suggest that these mSEBs could be an effective recombinant SEB vaccine candidates for further/future preclinical and clinical studies.

Safety and Immunogenicity of a Parenterally Administered, Structure-Based Rationally Modified Recombinant Staphylococcal Enterotoxin B Protein Vaccine, STEBVax

Staphylococcus aureus produces several enterotoxins and superantigens, exposure to which can elicit profound toxic shock. A recombinant staphylococcal enterotoxin B (rSEB) containing 3 distinct mutations in the major histocompatibility complex class II binding site was combined with an alum adjuvant (Alhydrogel) and used as a potential parenteral vaccine named STEBVax. Consenting healthy adult volunteers (age range, 23 to 38 years) participated in a first-in-human open-label dose escalation study of parenteral doses of STEBVax ranging from 0.01 μg up to 20 μg. Safety was assessed by determination of the frequency of adverse events and reactogenicity. Immune responses to the vaccination were determined by measurement of anti-staphylococcal enterotoxin B (anti-SEB) IgG by enzyme-linked immunosorbent assay and a toxin neutralization assay (TNA). Twenty-eight participants were enrolled in 7 dosing cohorts. All doses were well tolerated. The participants exhibited heterogeneous baseline antibody titers. More seroconversions and a faster onset of serum anti-SEB IgG toxin-neutralizing antibodies were observed by TNA with increasing doses of STEBVax. There was a trend for a plateau in antibody responses with doses of STEBVax of between 2.5 and 20 μg. Among the participants vaccinated with 2.5 μg to 20 μg of STEBVax, ∼93% seroconverted for SEB toxin-neutralizing antibody. A strong correlation between individual SEB-specific serum IgG antibody titers and the neutralization of gamma interferon production was found in vitro STEBvax appeared to be safe and immunogenic, inducing functional toxin-neutralizing antibodies. These data support its continued clinical development. (This study has been registered at ClinicalTrials.gov under registration no. NCT00974935.).

Sublethal staphylococcal enterotoxin B challenge model in pigs to evaluate protection following immunization with a soybean-derived vaccine

In an effort to develop a sustainable platform for manufacturing protein-based vaccine candidates, we expressed a triple mutant of staphylococcal enterotoxin B carrying the L45R, Y89A, and Y94A modifications in transgenic soybean seeds (soy-mSEB). Soy-mSEB possessed no detectable superantigen activity in vitro. We found that this soybean-derived, nontoxic mutant of SEB could be stably expressed, stored in seeds for extended periods at room temperature without degradation, and easily purified from contaminating soy proteins. Vaccination of pigs with purified soy-mSEB, or the identical triple mutant expressed in Escherichia coli (E. coli-mSEB), resulted in high antibody titers against the native toxin in immunized animals. In fact, titers were indistinguishable regardless of the immunogen used, demonstrating the equivalence of soy-mSEB and E. coli-mSEB vaccinations. Antisera from either immunized group were able to block native SEB superantigen activity in an in vitro neutralization assay. Similar results were obtained when immunized animals were challenged with a sublethal dose of native toxin. Significant reductions in toxin-induced serum cytokine levels were observed in soy-mSEB- and E. coli-mSEB-immunized pigs compared to control animals. The reductions in SEB-induced cytokine responses were similar regardless of the immunogen used for vaccination. Surprisingly, however, some clinical symptoms, such as prostration, lethargy, emesis, and/or diarrhea, were still observed in all immunized animals. These studies demonstrate the potential for soybean-derived proteins as a platform technology for sustainable vaccine manufacturing and the usefulness of a sublethal challenge model in pigs for evaluating the efficacy of potential SEB vaccine candidates.

Synthetic human monoclonal antibodies toward staphylococcal enterotoxin B (SEB) protective against toxic shock syndrome

Staphylococcal enterotoxin B (SEB) is a potent toxin that can cause toxic shock syndrome and act as a lethal and incapacitating agent when used as a bioweapon. There are currently no vaccines or immunotherapeutics available against this toxin. Using phage display technology, human antigen-binding fragments (Fabs) were selected against SEB, and proteins were produced in Escherichia coli cells and characterized for their binding affinity and their toxin neutralizing activity in vitro and in vivo. Highly protective Fabs were converted into full-length IgGs and produced in mammalian cells. Additionally, the production of anti-SEB antibodies was explored in the Nicotiana benthamiana plant expression system. Affinity maturation was performed to produce optimized lead anti-SEB antibody candidates with subnanomolar affinities. IgGs produced in N. benthamiana showed characteristics comparable with those of counterparts produced in mammalian cells. IgGs were tested for their therapeutic efficacy in the mouse toxic shock model using different challenge doses of SEB and a treatment with 200 μg of IgGs 1 h after SEB challenge. The lead candidates displayed full protection from lethal challenge over a wide range of SEB challenge doses. Furthermore, mice that were treated with anti-SEB IgG had significantly lower IFNγ and IL-2 levels in serum compared with mock-treated mice. In summary, these anti-SEB monoclonal antibodies represent excellent therapeutic candidates for further preclinical and clinical development.

Novel structurally designed vaccine for S. aureus α-hemolysin: protection against bacteremia and pneumonia

Staphylococcus aureus (S. aureus) is a human pathogen associated with skin and soft tissue infections (SSTI) and life threatening sepsis and pneumonia. Efforts to develop effective vaccines against S. aureus have been largely unsuccessful, in part due to the variety of virulence factors produced by this organism. S. aureus alpha-hemolysin (Hla) is a pore-forming toxin expressed by most S. aureus strains and reported to play a key role in the pathogenesis of SSTI and pneumonia. Here we report a novel recombinant subunit vaccine candidate for Hla, rationally designed based on the heptameric crystal structure. This vaccine candidate, denoted AT-62aa, was tested in pneumonia and bacteremia infection models using S. aureus strain Newman and the pandemic strain USA300 (LAC). Significant protection from lethal bacteremia/sepsis and pneumonia was observed upon vaccination with AT-62aa along with a Glucopyranosyl Lipid Adjuvant-Stable Emulsion (GLA-SE) that is currently in clinical trials. Passive transfer of rabbit immunoglobulin against AT-62aa (AT62-IgG) protected mice against intraperitoneal and intranasal challenge with USA300 and produced significant reduction in bacterial burden in blood, spleen, kidney, and lungs. Our Hla-based vaccine is the first to be reported to reduce bacterial dissemination and to provide protection in a sepsis model of S. aureus infection. AT62-IgG and sera from vaccinated mice effectively neutralized the toxin in vitro and AT62-IgG inhibited the formation of Hla heptamers, suggesting antibody-mediated neutralization as the primary mechanism of action. This remarkable efficacy makes this Hla-based vaccine a prime candidate for inclusion in future multivalent S. aureus vaccine. Furthermore, identification of protective epitopes within AT-62aa could lead to novel immunotherapy for S. aureus infection.

Nasal immunity to staphylococcal toxic shock is controlled by the nasopharynx-associated lymphoid tissue

The nasopharynx-associated lymphoid tissue (NALT) of humans and other mammals is associated with immunity against airborne infections, though it is generally considered to be a secondary component of the mucosa-associated lymphoid system. We found that protective immunity to a virulence factor of nasal mucosa-colonizing Staphylococcus aureus, staphylococcal enterotoxin B (SEB), requires a functional NALT. We examined the role of NALT using intranasal (IN) vaccination with a recombinant SEB vaccine (rSEBv) combined with an adjuvant in a mouse model of SEB-induced toxic shock. The rSEBv was rapidly internalized by NALT cells at the mucosal barrier, and transport into NALT was accelerated by inclusion of a Toll-like receptor 4 (TLR4) agonist. Vaccine-induced germinal centers of B cells formed within NALT, accompanied by elevated levels of IgA(+) and IgG(+) cells, and these were further increased by TLR4 activation. The NALT was the site of specific anti-rSEBv IgA and IgG production but was also influenced by intraperitoneal (IP) inoculation and perhaps other isolated lymphoid follicles observed within the nasal cavity. Vaccination by the IN route generated robust levels of anti-rSEBv IgA in saliva, nasal secretions, and blood compared to much lower levels after IP vaccination. IN vaccination also induced secretion of anti-rSEBv IgG in the blood and nasal secretions. Significantly, the efficacy of IN vaccination was dependent on NALT, as surgical removal resulted in greater sensitivity to IN challenge with wild-type SEB. Thus, protective immunity to SEB within the nasal sinuses was elicited by responses originating in NALT.

Oral vaccine formulations stimulate mucosal and systemic antibody responses against staphylococcal enterotoxin B in a piglet model

Despite the potential for its use as an agent of biowarfare or bioterrorism, no approved vaccine against staphylococcal enterotoxin B (SEB) exists. Nontoxic, mutant forms of SEB have been developed; however, it has been difficult to determine the efficacy of such subunit vaccine candidates due to the lack of superantigen activity of native SEB in rodents and due to the limitations of primate models. Since pigs respond to SEB in a manner similar to that of human subjects, we utilized this relevant animal model to investigate the safety and immunogenicity of a triple mutant of SEB carrying the amino acid changes L45R, Y89A, and Y94A. This recombinant mutant SEB (rmSEB) did not possess superantigen activity in pig lymphocyte cultures. Furthermore, rmSEB was unable to compete with native SEB for binding to pig leukocytes. These in vitro studies suggested that rmSEB could be a safe subunit vaccine. To test this possibility, piglets immunized orally with rmSEB formulations experienced no significant decrease in food consumption and no weight loss during the vaccination regimen. Oral vaccination with 1-mg doses of rmSEB on days 0, 7, 14, and 24 resulted in serum IgG and fecal IgA levels by day 36 that cross-reacted with native SEB. Surprisingly, the inclusion of cholera toxin adjuvant in vaccine formulations containing rmSEB did not result in increased antibody responses compared to formulations using the immunogen alone. Taken together, these studies provide additional evidence for the potential use of nontoxic forms of SEB as vaccines.

Contribution of the flexible loop region to the function of staphylococcal enterotoxin B

Staphylococcal enterotoxin B (SEB), a toxin produced by Staphylococcus aureus, causes food poisoning and other fatal diseases by inducing high levels of pro-inflammatory cytokines. These cytokines are released from CD4+ T cells and major histocompatibility complex (MHC) class II antigen-presenting cells, which are activated through binding of wild-type (WT) SEB to both the MHC class II molecule and specific T-cell receptor Vbeta chains. Here, we focused on a trypsin/cathepsin cleavage site of WT SEB, which is known to be cleaved in vivo between Lys97 and Lys98, located within the loop region. To know the function of the cleavage, an SEB mutant, in which both of these Lys residues have been changed to Ser, was examined. This mutant showed prolonged tolerance to protease cleavage at a different site between Thr107 and Asp108, and structural analyses revealed no major conformational differences between WT SEB and the mutant protein. However, differential scanning calorimetric analysis showed an increase in enthalpy upon thermal denaturation of the mutant protein, which correlated with the speed of cleavage between Thr107 and Asp108. The mutant protein also had slightly increased affinity for MHC. In the in vivo experiment, the SEB mutant showed lower proliferative response in peripheral blood mononuclear cells and had lower cytokine-induction activity, compared with WT SEB. These results highlight the importance of the flexible loop region for the functional, physical and chemical properties of WT SEB, thus providing insight into the nature of WT SEB that was unrevealed previously.

Attenuation of massive cytokine response to the staphylococcal enterotoxin B superantigen by the innate immunomodulatory protein lactoferrin

Staphylococcal enterotoxin B (SEB) is a pyrogenic exotoxin and a potent superantigen which causes massive T cell activation and cytokine secretion, leading to profound immunosuppression and morbidity. The inhibition of SEB-induced responses is thus considered a goal in the management of certain types of staphylococcal infections. Lactoferrin (LF) is a multi-functional glycoprotein with both bacteriostatic and bactericidal activities. In addition, LF is known to have potent immunomodulatory properties. Given the anti-microbial and anti-inflammatory properties of this protein, we hypothesized that LF can modulate T cell responses to SEB. Here, we report that bovine LF (bLF) was indeed able to attenuate SEB-induced proliferation, interleukin-2 production and CD25 expression by human leucocyte antigen (HLA)-DR4 transgenic mouse T cells. This inhibition was not due to bLF's iron-binding capacity, and could be mimicked by the bLF-derived peptide lactoferricin. Cytokine secretion by an engineered SEB-responsive human Jurkat T cell line and by peripheral blood mononuclear cells from healthy donors was also inhibited by bLF. These findings reveal a previously unrecognized property of LF in modulation of SEB-triggered immune activation and suggest a therapeutic potential for this naturally occurring protein during toxic shock syndrome.

Vaccine based on a ubiquitous cysteinyl protease and streptococcal pyrogenic exotoxin A protects against Streptococcus pyogenes sepsis and toxic shock

Background

The gram-positive bacterium Streptococcus pyogenes is a common pathogen of humans that causes invasive infections, toxic-shock syndrome, rheumatic fever, necrotizing fasciitis and other diseases. Detection of antibiotic resistance in clinical isolates has renewed interest in development of new vaccine approaches for control S. pyogenes sepsis. In the study presented, a novel protein vaccine was examined. The vaccine was based on a recombinant protein fusion between streptococcal pyrogenic exotoxin B (SpeB), a cysteinyl protease expressed by all clinical isolates, and streptococcal pyrogenic exotoxin A (SpeA), a superantigen produced by a large subset of isolates.

Results

A novel protein was produced by mutating the catalytic site of SpeB and the receptor binding surface of SpeA in a fusion of the two polypeptides. Vaccination of HLA-DQ8 transgenic mice with the SpeA-SpeB fusion protein protected against a challenge with the wild-type SpeA that was lethal to naïve controls, and vaccinated mice were protected from an otherwise lethal S. pyogenes infection.

Conclusion

These results suggest that the genetically attenuated SpeA-SpeB fusion protein may be useful for controlling S. pyogenes infections. Vaccination with the SpeA-SpeB fusion protein described in this study may potentially result in protective immunity against multiple isolates of S. pyogenes due to the extensive antibody cross-reactivity previously observed among all sequence variants of SpeB and the high frequency of SpeA-producing strains.

Interleukin-15 increases vaccine efficacy through a mechanism linked to dendritic cell maturation and enhanced antibody titers

Interleukin-15 (IL-15) is generally considered to sustain T-cell memory and to be a growth factor for natural killer cells. Previous data from our laboratory demonstrated that IL-15 is also an important factor for developing human dendritic cells. For this study, we investigated the effects of IL-15 on antibody responses in mice to a recombinant staphylococcal enterotoxin B (SEB) vaccine (STEBVax) in a preclinical model of toxic shock syndrome induced by SEB. We observed that mouse spleen cells treated with IL-15 in ex vivo culture gained a dendritic cell-like phenotype. Administration of IL-15 to mice also resulted in an increased number of mature CD11c+ dendritic cells in mouse spleens. A significant, IL-15 dose-dependent increase in antigen-specific antibody was observed after coadministration with the vaccine and an aluminum-based adjuvant (alhydrogel). Furthermore, the coadministration of IL-15 with STEBVax and alhydrogel also protected mice from lethal toxic shock above the levels that obtained without IL-15. Thus, the vaccine response enhanced by IL-15 appears to be mediated by mature dendritic cells and results in prevalent seroconversion to Th2-dependent antibodies. This suggests a potential use of IL-15 as an adjuvant for antibody-dependent responses to vaccines.

Persistence of zinc-binding bacterial superantigens at the surface of antigen-presenting cells contributes to the extreme potency of these superantigens as T-cell activators

Bacterial superantigen intoxication causes massive overactivation of T cells, which can result in potentially lethal toxic shock. Superantigens fall into two groups: superantigens such as staphylococcal enterotoxin B (SEB) that contain a single generic binding site for major histocompatibility complex class II (MHC-II) and more potent superantigens such as SEA with a second, zinc-dependent MHC-II binding site that enables them to cross-link adjacent MHC-II molecules. We found that although all superantigens bound rapidly to the surface of human B cells, zinc-binding superantigens largely remained at the cell surface for at least 40 h. In contrast, single-binding-site superantigens were greatly depleted from the surface by 4 h. Subcellular fractionation and confocal microscopy revealed that some SEB entered lysosomal compartments, but SEA remained almost undetectable inside cells at 20 h. SEA and SEB mutants that do not bind MHC-II were trafficked rapidly to lysosomal compartments. Our findings suggest that the persistence of SEA and other zinc-dependent, cross-linking superantigens on the surface of antigen-presenting cells contributes to their potency as T-cell activators.

Improved methods for prepurification and detection of staphylococcal enterotoxin B from cell-free culture filtrate

An improved ELISA method for the detection of Staphylococcal Enterotoxin B (SEB) in protein A preparations is presented. Fab fragments were obtained by digestion with papain of anti-SEB IgG bound to SEB immobilized on Sepharose 4B. Anti-SEB and peroxidase-labeled Fab fragments from secondary antibodies were successfully used in a modified ELISA of SEB in protein A preparations. SEB-Sepharose was used repeatedly for the production of anti-SEB Fab fragments by papain digestion without loss of affinity. In addition, for the purification of SEB from crude culture filtrates, an initial step utilizing a combined heat and pH treatment for the removal of significant amounts of contaminating proteins without losses of toxin activity is presented. This pretreatment step yielded positive effects in further downstream processing considering both shortened time and an increase in total recovery.

Noncovalently bilayer-coated capillaries for efficient and reproducible analysis of proteins by capillary electrophoresis

The suitability of noncovalently bilayer-coated capillaries for the analysis of proteins by capillary electrophoresis (CE) at medium pH was investigated. Fused-silica capillaries were coated simply by successively flushing with a polybrene (PB) and a poly(vinyl sulfonate) (PVS) solution. A protein test mixture was used to evaluate the performance of the coated capillaries. Comparisons with bare fused-silica capillaries were made. Several background electrolytes (BGEs) were tested in combination with the PB-PVS coating, showing that optimum performance was obtained for the proteins using high BGE concentrations. With a 300 mM Tris phosphate buffer (pH 7.0), good plate numbers (150,000-300,000), symmetrical peaks, and favorable migration-time repeatabilities (RSDs below 0.8%) were obtained for the proteins. Using bare fused-silica capillaries, the protein peaks were significantly broadened and the migration-time RSDs often exceeded 5%. It is concluded that the PB-PVS coating effectively minimizes adverse protein adsorption and provides a very stable electroosmotic flow (EOF). We also investigated the potential of a commercially available bilayer coating (CEofix) for protein analysis. It is demonstrated that with this coating, good plate numbers and peak symmetries for proteins can be achieved when the CEofix BGE ("accelerator") is replaced by a common BGE such as sodium or Tris phosphate. Apparently, the negatively charged polymer present in the "accelerator" interacts with the proteins causing band broadening. The utility of the bilayer coatings is further illustrated by the separation of proteins such as interferon-alpha 2b, myoglobin and carbonic anhydrase, by the analysis of a degraded insulin sample in time, and by the profiling of the glycoprotein ovalbumin. In addition, it is demonstrated that even in the presence of concentrations of human serum albumin in the sample of up to 60 mg/mL, the PB-PVS coating still provides reproducible protein separations of good performance.

Vaccines against the category B toxins: Staphylococcal enterotoxin B, epsilon toxin and ricin

The threat of bioterrorism worldwide has accelerated the demand for the development of therapies and vaccines against the Category B toxins: staphylococcal enterotoxin B (SEB), epsilon toxin (ETX) produced by Clostridium perfringens types B and D, and ricin, a natural product of the castor bean. The diverse and unique nature of these toxins poses a challenge to vaccinologists. While formalin-inactivated toxins can successfully induce antibody-mediated protection in animals, their usefulness in humans is limited because of potential safety concerns. For this reason, research is now aimed at developing recombinant, attenuated vaccines based on a detailed understanding of the molecular mechanisms by which these toxins function. Vaccine development is further complicated by the fact that as bioterrorism agents, SEB, ETX and ricin would most likely be disseminated as aerosols or in food/water supplies. Our understanding of the mechanisms by which these toxins cross mucosal surfaces, and importance of mucosal immunity in preventing toxin uptake is only rudimentary.

Simultaneous measurement of specific serum IgG responses to five select agents

Select Agents are defined by CDC and the USDA Animal and Plant Health Inspection Service (APHIS) as biological agents or toxins deemed a threat to public, animal, or plant health, or to animal or plant products. They are classified on the basis of their ease of dissemination, mortality/morbidity rate, and potential for social disruption. A subset of these agents includes Bacillus anthracis, Yersinia pestis, Francisella tularensis, ricin toxin (RT), and staphylococcal enterotoxin B (SEB). Infection or intoxication with these agents has been shown to elicit an antigen-specific serum IgG response. We describe a fluorescent covalent microsphere immunoassay (FCMIA) for measurement of specific IgG antibodies to seven different antigens from five different select agents; B. anthracis [protective antigen (PA) and lethal factor (LF)], Y. pestis (F1 and V antigens), F. tularensis, RT and SEB simultaneously in human B. anthracis vaccinee sera (containing anti-PA and anti-LF IgG) which had been spiked with animal specific IgG antibodies to the other select agents. Inter-assay and intra-assay coefficients of variation were 6.5 and 13.4%, respectively (N = 4). There were no significant differences (P > 0.70) between assay responses when the assays were performed individually or multiplexed. When the observed versus expected interpolated concentrations were compared, highly linear relationships were observed (r2 values from 0.981 to 0.999, P < 0.001). Minimum detectable concentrations (MDC) ranged from 0.3 ng mL(-1) (Y. pestis F1) to 300 ng mL(-1) (RT). Finally, the curves showed responses were linear for most analytes from their MDC to 125 (SEB) to 1,300 (Y. pestis F1) x their MDC. These data indicate that multiplexed FCMIA is a sensitive and accurate method for simultaneous measurement of specific IgG in serum to CDC select agents and may be of value in screening either decontamination workers or the general population for exposure to/infection with these agents.

Immunogenicity of the histidine-to-tyrosine staphylococcal enterotoxin B mutant protein in C3H/HeJ mice

Staphylococcal enterotoxin B (SEB) is a common cause of food poisoning and toxic shock. A safe and effective vaccine is needed to protect against the superantigenic effects of this toxin. We previously constructed and produced an apparently nontoxic SEB mutant having four histidine-to-tyrosine substitutions in positions 12, 32, 105, and 121. In the present study, we found that this H1.2.3.4 SEB mutant had low toxicity, was able to induce high levels of specific IgG antibodies, and protected mice in both the actinomycin D-primed and intranasal SEB intoxication model systems, despite the absence of detectable specific IgM and IgA antibodies. We propose further development of the H1.2.3.4 recombinant protein as a potential anti-SEB vaccine candidate.

Generation of protective immunity by inactivated recombinant staphylococcal enterotoxin B vaccine in nonhuman primates and identification of correlates of immunity

At this time there are no vaccines or therapeutics to protect against staphylococcal enterotoxin B (SEB) exposure. Here, we report vaccine efficacy of an attenuated SEB in a nonhuman primate model following lethal aerosol challenge and identify several biomarkers of protective immunity. Initial in vitro results indicated that the mutation of key amino acid residues in the major histocompatibility complex (MHC) class II binding sites of SEB produced a nontoxic form of SEB, which had little to no detectable binding to MHC class II molecules, and lacked T-cell stimulatory activities. When examined in a mouse model, we found that the attenuated SEB retained antigenic structures and elicited protective immune responses against wild-type SEB challenge. Subsequently, a vaccine regimen against SEB in a nonhuman primate model was partially optimized, and investigations of immune biomarkers as indicators of protection were performed. SEB-naïve rhesus monkeys were vaccinated two or three times with 5 or 20 microg of the attenuated SEB and challenged by aerosol with wild-type SEB toxin. Unlike exposure to the native toxin, the vaccine did not trigger the release of inflammatory cytokines (TNF alpha, IL6, or IFN gamma). All rhesus monkeys that developed anti-SEB serum titers > or = 10(4) and elicited high levels of neutralizing antibody survived the aerosol challenge. These findings suggest that the attenuated SEB is fully protective against aerosolized toxin when administered to unprimed subjects. Moreover, experiments presented in this study identified various biomarkers that showed substantial promise as correlates of immunity and surrogate endpoints for assessing in vivo biological responses in primates, and possibly in humans, to vaccines against SEs.

Correlation of body temperature with protection against staphylococcal enterotoxin B exposure and use in determining vaccine dose-schedule

The immunoprotective potential of a recombinant vaccine against the incapacitating effect of aerosolized staphylococcal enterotoxin B (SEB) in nonhuman primates is reported. SEB belongs to a family of structurally related superantigens responsible for serious, life threatening pathologies. Injecting the recombinant SEB vaccine did not induce temperature elevation in rhesus monkeys, a classical symptom of toxic-shock syndrome. No temperature elevation was noted following injection with control tetanus toxoid. In addition to 100% survival, we observed a clear correlation between vaccine dose and mitigation of temperature elevation after a lethal SEB aerosol challenge. We conclude that the recombinant SEB vaccine is non-pyrogenic and that monitoring changes in body temperature is an important biomarker of toxic shock in a primate animal model.

Toxicity of the staphylococcal enterotoxin B mutants with histidine-to-tyrosine substitutions

In this study we made a series of site-directed mutants of staphylococcal enterotoxin B (SEB), in which histidine residues in the molecule were replaced by tyrosine. The mutant genes were cloned and expressed, and the corresponding proteins were purified. These mutant proteins were tested for binding to human HLA-DR4 and for mitogenetic activity in mouse splenocyte culture. Toxicity of the proteins in vivo was evaluated in the actinomycin D-primed C3H/HeJ mouse model. We found that SEB mutant proteins with fewer than four histidine-to-tyrosine (his-to-tyr) substitutions retained toxic properties similar to wild-type SEB. However, studies showed that his-to-tyr substitution of four consecutive histidine residues eliminated SEB toxicity. Our results clearly show that this genetically modified SEB protein is non-toxic and justifies its further development as a component of a new, safer vaccine to prevent SEB intoxication.