Production and characterization of monoclonal antibodies to staphylococcal enterotoxins: use in immunodetection and immunopurification

In vitro selection of DNA aptamers against staphylococcal enterotoxin A

Staphylococcal enterotoxin A (SEA) is the most frequently reported in staphylococcal food poisoning (SFP) outbreaks. Aptamers are single-stranded nucleic acids that are seen as promising alternatives to antibodies in several areas, including diagnostics. In this work, systematic evolution of ligands by exponential enrichment (SELEX) was used to select DNA aptamers against SEA. The SELEX protocol employed magnetic beads as an immobilization matrix for the target molecule and real-time quantitative PCR (qPCR) for monitoring and optimizing sequence enrichment. After 10 selection cycles, the ssDNA pool with the highest affinity was sequenced by next generation sequencing (NGS). Approximately 3 million aptamer candidates were identified, and the most representative cluster sequences were selected for further characterization. The aptamer with the highest affinity showed an experimental dissociation constant (KD) of 13.36 ± 18.62 nM. Increased temperature negatively affected the affinity of the aptamer for the target. Application of the selected aptamers in a lateral flow assay demonstrated their functionality in detecting samples containing 100 ng SEA, the minimum amount capable of causing food poisoning. Overall, the applicability of DNA aptamers in SEA recognition was demonstrated and characterized under different conditions, paving the way for the development of diagnostic tools.

A novel tandem repeat cloning technique for creation of multiple short peptide repeats to differentiate closely related antigens

Antibody cross-reactivity is a problem often associated with closely related antigens. This study was aimed to develop a method enabling differentiation of closely related toxins based on antigen designing strategy. The method involves identification of disparate amino acids (AA) confined to target antigen in comparison with two or more closely related antigens, their assembly into a DNA oligomer and further cloning as six tandem repeats (TR) using restriction and ligation strategy into a desired vector and finally generation of antigen specific antibodies. The practical utility of this method was demonstrated by generating and testing the specificity of polyclonal antibodies against staphylococcal enterotoxin C (SEC). Cross-reactivity is a problem often associated with SEC in immunoassays due to its amino acid sequence identity with staphylococcal enterotoxin B (SEB) (40-60%). To circumvent the same, the above-mentioned strategy was applied. Unique AA of SEC (36 AA) in comparison to SEB were selected, reassembled and with deduced corresponding nucleotides, an oligomer of 117 bases was designed. Using primers with restriction overhangs, three constructs were created each with two repeats using a common restriction site. The resulting three constructs were sequentially cloned into alternating restriction sites of pRSET A vector in directional orientation, expressed in E. coli for rTR/SEC protein which was used to generate specific polyclonal antibodies against SEC. Specificity was compared with antibody raised against whole SEC recombinant protein using Western blot and dot blot assays. High specificity was achieved through the developed strategy signifying its possible application to address cross-reactivity problem associated with closely related antigens.

Intralaboratory validation according to the EN ISO 16 140 Standard of the Vidas SET2 detection kit for use in official controls of staphylococcal enterotoxins in milk products

AIM

Immunological tools used to detect staphylococcal enterotoxins (SEs) in foods are numerous. The aim of this study was to evaluate, on naturally contaminated milk product samples, the performance of the Vidas SET2, in comparison to the Transia plate SET.

METHODS AND RESULTS

The Vidas SET2 was compared with the Transia plate SET on supernatants of Staphylococcus aureus isolates and on naturally contaminated milk products. It is noteworthy that when using IgG rabbit treatment, both kits can be considered as equivalent to detect enterotoxins in naturally contaminated milk products.

CONCLUSIONS

This study demonstrated that the Vidas SET2 performance is similar to that of Transia plate SET kit, when a rabbit IgG treatment step is used before detection step. This additional treatment significantly decreased, from 42% to 8%, the rate of positive deviations observed using the Transia plate SET detection kit.

SIGNIFICANCE AND IMPACT OF THE STUDY

The Vidas SET2 was clearly found as more specific, when no preliminary rabbit IgG treatment was used, and which results in a better workflow when a large number of samples have to be analysed within a few days. Considering the results obtained, the Vidas SET2 detection kit can be used to assess the safety of milk products for SEs.

Comparison between ultrafiltration and trichloroacetic acid precipitation method for concentration of Staphylococcus aureus enterotoxin in dairy samples

Staphylococcus aureus enterotoxin that may be contained at low concentrations in milk and dairy products can cause food poisoning. To detect this enterotoxin at low concentrations, samples should be concentrated. We evaluated the performance of centrifugal ultrafiltration method (UF) in comparison with trichloroacetic acid precipitation method (TCA) for the concentration of S. aureus enterotoxin in milk and dairy products. S. aureus enterotoxin A (SEA) were added at various concentrations to ultra high-temperature heating process (UHT) milk, UHT concentrated skim milk, UHT skim milk powder, low heat-treated (LH) skim milk powder, and raw milk. SEA was concentrated by TCA and UF once a day on a total of 3 days by different researchers to prepare test solutions. The fluorescence value (TV) of test solutions was determined using an immunofluorescence autoanalyzer (miniVIDAS), and the linearity and slope of the regression line, relative standard deviation (RSD(RW)) at each added concentration, detection limit (DL), quantification limit (QL), and the recovery rate by each concentration method were obtained according to the guidelines of the International Conference on Harmonization (ICH). The slope of the regression line obtained by UF was steeper than that by TCA for all dairy samples excluding LH (74 degrees C, 20 s) skim milk powder. RSDRW, DL, and QL obtained by UF were comparable to or more excellent than those obtained by TCA. The procedure of UF was simpler than that of TCA. The recovery rate and rapidity were similar between the two methods. The DL and QL of enterotoxins other than SEA in dairy products by UF or TCA were estimated based on the DL and QL of SEA. In this estimation, consideration was given to reactions between each enterotoxin and its antibody, and also to the immunoactivity maintenance rate of each enterotoxin after addition of trichloroacetic acid in TCA. The estimated values were similar to those obtained by experiments using enterotoxin C1 (SEC1). UF using a centrifugal ultrafiltration membrane can be more readily performed and similar to or more reliable than TCA. UF combined with a miniVIDAS can be used for quantitative routine analysis.

A method for the generation of combinatorial antibody libraries using pIX phage display

For more than a decade, phage displayed combinatorial antibody libraries have been used to generate and select a wide variety of antibodies. We previously reported that the phage coat proteins pVII and pIX could be used to display the heterodimeric structure of the antibody Fv region. Herein, aspects of this technology were invoked and extended to construct a large, human single-chain Fv (scFv) library of 4.5 x 10(9) members displayed on pIX of filamentous bacteriophage. Furthermore, the diversity, quality, and utility of the library were demonstrated by the selection of scFv clones against six different protein antigens. Notably, more than 90% of the selected clones showed positive binding for their respective antigens after as few as three rounds of panning. Analyzed scFvs were also found to be of high affinity. For example, kinetic analysis (BIAcore) revealed that scFvs against staphylococcal enterotoxin B and cholera toxin B subunit had a nanomolar and subnanomolar dissociation constant, respectively, affording affinities comparable to, or exceeding that, of mAbs obtained from immunization. High specificity was also attained, not only between very distinct proteins, but also in the case of the Ricinus communis ("ricin") agglutinins (RCA(60) and RCA(120)), despite >80% sequence homology between the two. The results suggested that the performance of pIX-display libraries can potentially exceed that of the pIII-display format and make it ideally suited for panning a wide variety of target antigens.

Evaluation of an alternative extraction procedure for enterotoxin determination in dairy products

A concentration protocol based on trichloroacetic acid precipitation was evaluated and compared with the reference method using dialysis concentration. Different quantities of purified staphylococcal enterotoxins were added to pasteurized Camembert-type cheeses. Detection of enterotoxins in these cheeses was performed using an automated detection system. Raw goat milk Camembert-type cheeses involved in a staphylococcal food poisoning were also tested. Both enterotoxin extraction methods allowed detection of the lowest enterotoxin concentration level used in this study (0.5 ng g-1). Compared with the dialysis concentration method, TCA precipitation of staphylococcal enterotoxins was 'user-friendly' and less time-consuming. These results suggest that TCA precipitation is a rapid (1 h), simple and reliable method of extracting enterotoxin from food which gives excellent recovery from dairy products.

Production and characterization of anti-peptide monoclonal antibodies with specificity for staphylococcal enterotoxins A and B

A synthetic peptide containing selected epitopes from staphylococcal enterotoxin A (SEA) and enterotoxin B (SEB) was used to produce monoclonal antibodies (Mabs) to respective enterotoxins in a single fusion procedure. The peptide inhibited the reaction of polyclonal anti-SEA or anti-SEB antisera with their homologous enterotoxin, thus showing that the chosen epitopes are part of the antibody-inducing enterotoxin sequences. Two Mabs, Mab-A and Mab-B, reacted with both the peptide and with either SEA or SEB. Used in a double antibody sandwich ELISA, the Mabs were able to quantitate the native SEA or SEB toxins at nanogram levels.

Selection of phage-displayed superantigen by binding to cell-surface MHC class II

We have expressed the superantigen staphylococcal enterotoxin A (SEA) on the surface of bacteriophage as a fusion with the gene VIII protein (gVIIIp). This phage-displayed superantigen retains the properties inherent in the natural protein. It binds to MHC class II and activates T-cells bearing appropriate V beta regions. A flexible 5-amino acid linker sequence between the SEA molecule and the phage coat protein improved the production of functional phage-displayed SEA. Binding to MHC class II-expressing cells effectively selected SEA-phage from non-SEA-phage background. This indicates that this will be an effective method for selecting new specificities of superantigen from libraries of SEA mutants and for cloning of novel superantigens.

Double-sandwich enzyme-linked immunosorbent assay (ELISA) with murine monoclonal antibodies for detection of staphylococcal enterotoxin B

Three monoclonal antibodies (MAbs) which react specifically with staphylococcal enterotoxin B (SEB) were studied for their suitability for use in ELISA. One of them (MAb B14) worked well as a coating antibody; MAb B12 was shown to be a good probing antibody (conjugated with peroxidase) when ELISA plates were coated with MAb B14. This effective pair of MAbs (B14-B12PO) is able to detect 0.625 ng SEB ml, and to distinguish between SEB and other proteins present in food extracts.

T-cell receptor beta-chain binding to enterotoxin superantigens

The last few years have seen an enormous jump in our knowledge and understanding of T-cell activation by superantigens. Clearly, a great number of infectious and parasitic organisms utilize superantigens as part of a strategy to evade the immune response of their host. The ability to modulate superantigen effects will give us new means to fight infections, and the knowledge of T-cell activation that we have gained from study of superantigens will, in turn, allow us to modulate the immune system in new ways.

Murine monoclonal antibodies against staphylococcal enterotoxin B: production and characterization

A group of 14 monoclonal antibodies (mAbs) to staphylococcal enterotoxin B (SEB) were obtained by fusion of Sp2/O myeloma cells with spleen cells from female BALB/c mice immunized with commercial SEB. The antibodies belonged to IgG1 and IgG2b subclasses. We evaluated the anti-SEB titres, competition assays and sensitivity of detection by indirect ELISA. Reactivity and cross-reactivity were also studied by indirect ELISA and confirmed by immunoblotting. All the mAbs reacted with SEB and with a second band which had a different electrophoretic mobility and probably represents an aggregate of SEB or SEB bound to membranes. Three mAbs reacted only with SEB and the rest showed cross-reactions with SEC1. No reactions were observed against any other serovar (SEA, SED and SEE) or other proteins.

Purification of staphylococcal enterotoxins A and E by immunoaffinity chromatography using a murine monoclonal antibody with dual specificity for both of these toxins

An immunosorbent column was prepared by coupling a murine monoclonal antibody (MAb) with dual specificity for staphylococcal enterotoxins A (SEA) and E (SEE) to Affi-Gel 10. Purification of both SEA and SEE from culture supernatants was carried out with the immunosorbent column using 0.2 M acetic acid containing 0.15 M NaCl as eluant. The yields obtained were approximately 76% for SEA and 70% for SEE. Purified SEA and SEE were found to be immunologically and electrophoretically homogeneous. Immunoaffinity chromatography using a MAb with dual specificity proved to be valuable in the purification of SEA and SEE, not only from the standpoint of percentage recovery, but also because of the degree of purity and the ease of purification.

Rapid and sensitive sandwich enzyme-linked immunosorbent assay for detection of staphylococcal enterotoxin B in cheese

A rapid and sensitive screening sandwich enzyme-linked immunosorbent assay (ELISA) was developed for the detection of staphylococcal enterotoxin B (SEB) in cheese by using a highly avid anti-SEB antibody (Ab) as the capture Ab (CAb) and as the biotinylated Ab conjugate. The glutaraldehyde fixation method for the immobilization of CAb on polystyrene dipsticks was superior to the adsorption fixation and the adsorption-glutaraldehyde fixation methods. The glutaraldehyde fixation method resulted in a higher surface-saturating CAb concentration as evaluated by the peroxidase saturation technique and by the ability of the CAb-coated dipstick to discriminate between positive and negative controls (index of discrimination). Of nine blocking agents used alone or in pairs, lysine-human serum albumin, bovine serum albumin, human serum albumin, and gelatin effectively saturated available sites on the CAb-coated dipsticks without causing interference with the antigen-Ab reactions. The addition of 1% polyethylene glycol to the diluent of the biotinylated anti-SEB Ab conjugate improved the detection of SEB. A concentration of 4% polyethylene glycol allowed a 5-min reaction time for the streptavidin-biotin-horseradish peroxidase conjugate. Cheddar cheese homogenate reduced the sensitivity of the SEB assay; however, the sensitivity was restored when 1.6% (wt/vol) of either a nonionic detergent (Mega-9) or two zwitterionic detergents (Zwittergent 3-10 and 3-12 detergent) was added to the diluent. By using the rapid sandwich ELISA, a minimum of 0.5 to 1.0 ng of SEB per ml was detected within 45 min. The whole procedure for the analysis of the cheddar cheese samples was completed within 1 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct binding of secreted T-cell receptor beta chain to superantigen associated with class II major histocompatibility complex protein

The interaction of the T-cell receptor (TCR) with peptide antigen plus major histocompatibility complex (MHC) protein requires both alpha and beta chains of the TCR. The "superantigens" are a group of molecules that are recognized in association with MHC class II but that do not appear to conform to this pattern. Superantigens are defined as such because they cause the activation or thymic deletion of many or all T cells bearing specific TCR beta-chain variable region (V beta) elements. The strong association of particular V beta S with T-cell responses to superantigens suggests that their interaction with the TCR is fundamentally different from that of most antigens. We have directly investigated the involvement of the beta chain in recognition of a superantigen by using a secreted, truncated TCR beta chain and the bacterial superantigen staphylococcal enterotoxin A complexed to cell-surface MHC class II. We demonstrate that this interaction is specific for the enterotoxin and is dependent on MHC class II expression by the cell. The reaction can be inhibited by antibodies against the three components of the reaction: V beta, enterotoxin, and class II. This shows that the TCR beta chain is sufficient to mediate the interaction with a superantigen-class II complex. The TCR alpha chain and co-receptors such as CD4 are not required.

A heterogeneous immunoassay performed on a rotating carbon disk electrode with electrocatalytic detection. Mass transfer control of the capture of an enterotoxin

An ELISA procedure for the determination of enterotoxin A from Staphylococcus aureus conducted on the surface of a glassy carbon electrode is described. The electrocatalytic detection of the immobilized labelled second antibody is based upon the electrochemical reaction and the enzymatic catalysis occurring on the same surface. The indirect quantification of the bound antigen is, therefore, very sensitive (10(-15) mol cm-2). This heterogeneous technique was used to study the kinetics of antigen binding to the immunological solid phase, the mass transfer of the antigen being controlled under well-defined hydrodynamic conditions. The experiments were performed with a rotating solid phase disk in such a way that thickness of the diffusion layer was known. We found that the capture of the antigen by the immobilized monoclonal antibody was solely limited by diffusion. A simple theoretical model permitted the amount of bound antigen and the sensitivity of the method to be predicted as a function of the incubation time, the rotational speed of the solid phase and the volume of the sample. Both the theory and the experimental results indicate that the assay may be performed with the sample volume undefined.

Monoclonal antibodies against bacteria

Highlights are presented of most recent work in which monoclonal antibodies have been instrumental in the study of bacteria and their products. Topics summarized pertain to human and veterinary medicines, dentistry, phytopathology, ichthyology, and bacterial ecophysiology, differentiation, evolution and methanogenic biotechnology.