Single domain antibody–alkaline phosphatase fusion proteins for antigen detection — Analysis of affinity and thermal stability of single domain antibody

Docking guided phase display to develop fusion protein with novel scFv and alkaline phosphatase for one-step ELISA salbutamol detection

Introduction

Salbutamol (SAL) is a β2 adrenergic receptor agonist which has potential hazardous effects for human health. It is very important to establish a sensitive and convenient method to monitor SAL.

Methods

Here we introduce a method to combine the information from docking and site specific phage display, with the aim to obtain scFv with high affinity to SAL. First, single chain variable fragment (scFv) antibodies against SAL were generated through phage display. By using molecular docking approach, the complex structure of SAL with antibody was predicted and indicated that H3 and L3 contribute to the binding. Then new libraries were created by randomization specific residues located on H3 and L3 according to the docking results.

Results and discussion

Anti-SAL scFv antibodies with high efficiency were finally identified. In addition, the selected scFv was fused with alkaline phosphatase and expressed in E coli to develop a rapid and low-cost one step ELISA to detect SAL.

Recombinant antibodies and their use for food immunoanalysis

Antibodies are widely employed as biorecognition elements for the detection of a plethora of compounds including food and environmental contaminants, biomarkers, or illicit drugs. They are also applied in therapeutics for the treatment of several disorders. Recent recommendations from the EU on animal protection and the replacement of animal-derived antibodies by non-animal-derived ones have raised a great controversy in the scientific community. The application of recombinant antibodies is expected to achieve a high growth rate in the years to come thanks to their versatility and beneficial characteristics in comparison to monoclonal and polyclonal antibodies, such as stability in harsh conditions, small size, relatively low production costs, and batch-to-batch reproducibility. This review describes the characteristics, advantages, and disadvantages of recombinant antibodies including antigen-binding fragments (Fab), single-chain fragment variable (scFv), and single-domain antibodies (V_HH) and their application in food analysis with especial emphasis on the analysis of biotoxins, antibiotics, pesticides, and foodborne pathogens. Although the wide application of recombinant antibodies has been hampered by a number of challenges, this review demonstrates their potential for the sensitive, selective, and rapid detection of food contaminants.

NanoLuc luciferase as a suitable fusion partner of recombinant antibody fragments for developing sensitive luminescent immunoassays

Enzyme-linked immunosorbent assays (ELISAs) are essential for monitoring various biomarkers. Competitive and noncompetitive (sandwich) assay formats are used to determine hapten and macromolecule levels, respectively. Both formats require more sensitive detection of reporter enzymes for greater assay sensitivities. We previously reported the utility of wild-type Gaussia luciferase (wtGLuc) as a fusion partner with antibody single-chain Fv fragments (scFvs) for developing sensitive luminescent ELISAs. Here, we evaluated utility of NanoLuc luciferase (NLuc), a recently developed luciferase, as fusion partner with scFvs from the view of comparison with wtGLuc and a mutant of alkaline phosphatase (ALP'). Thyroxine (T₄) and T₄-labeled albumin were chosen as model haptenic and macromolecular antigens, respectively. An in-house-prepared anti-T₄ scFv was fused with NLuc, wtGLuc, or ALP'. The scFv-NLuc fusion protein showed 47-fold and 29-fold lower limit of detection [LOD; 59 zmol (per assay)] than the wtGLuc- and ALP'-fusions, respectively. In a competitive T₄ ELISA, the NLuc-fusion showed 9.3- and 6.3-fold lower LOD, (0.67 pg) than the wtGLuc- and ALP'-fusions, respectively, with a higher specificity in clinical applications. A typical colorimetric ELISA using a peroxidase-labeled second antibody showed 70-fold higher LOD than NLuc-based ELISA. Another advantage of the NLuc-fusion was shown in the sandwich assays; the LOD of T₄-labeled albumin (5.0 fmol) was >6-fold lower than that of the other luminescent ELISAs. In an additional sandwich assay developed to count bacteriophage particles, NLuc enabled more sensitive determination than wtGLuc, whereas ALP' showed nearly equivalent performance. Its slowest alteration rate for light intensity after starting the enzyme reaction should enable robust batch-by-batch assay operations. Thus, we concluded that scFv-NLuc fusions serve as suitable probes in various types of immunoassays and may facilitate higher sensitivities with practical specificities.

Selection and Characterization of Single-Domain Antibodies for Detection of Lassa Nucleoprotein

Lassa virus is the etiologic agent of Lassa fever, an acute and often fatal illness endemic to West Africa. It is important to develop new reagents applicable either for the specific diagnosis or as improved therapeutics for the treatment of Lassa fever. Here, we describe the development and initial testing of llama-derived single-domain antibodies that are specific for the Lassa virus nucleoprotein. Four sequence families based on complementarity-determining region (CDR) homology were identified by phage-based enzyme-linked immunosorbent assays, however, the highest affinity clones all belonged to the same sequence family which possess a second disulfide bond between Framework 2 and CDR3. The affinity and thermal stability were evaluated for each clone. A MagPlex-based homogeneous sandwich immunoassay for Lassa virus-like particles was also demonstrated to show their potential for further development as diagnostic reagents.

Development of a highly thermostable immunoassay based on a nanobody-alkaline phosphatase fusion protein for carcinoembryonic antigen detection

Carcinoembryonic antigen-related cell adhesion molecule 5 (CEACAM-5) assays are employed in routine clinical settings to diagnose tumor. We selected two nanobodies with high-affinity to CEACAM-5, termed Nb11C12 and Nb2D5, using phage-display technology. The Nb2D5 fused with calf intestinal alkaline phosphatase (CAP), human placental alkaline phosphatase (HAP), or Pyrococcus abyssi alkaline phosphatase (PAP) were expressed in human embryonic kidney (HEK293) cells. The enzymatic activity of Nb2D5-HAP fusion protein was the best and remained stable at 60 °C for 7 days. The affinity of Nb2D5-HAP fusion protein to CEACAM-5 reached 42 pM. A chemiluminescent enzyme immunoassay (CLEIA) based on Nb2D5-HAP fusion protein was established for quantitative CEACAM-5 assay in clinical settings. The CLEIA exhibited a wide linear range of 0.31-640 ng/mL toward CEACAM-5, with a limit of detection (LOD) of 0.85 ng/mL. No cross-reactivity occurred with CEACAM-1, CEACAM-3, CEACAM-6, or CEACAM-8, and no interference was observed with rheumatoid factors. The CLEIA based on Nb2D5-HAP fusion protein was stable for 8 weeks at 37 °C and 50% relative humidity. The CLEIA developed from Nb2D5-HAP fusion protein had much better stability and linearity with similar reproducibility compared with the enzyme-linked immunosorbent assay developed from conventional monoclonal antibodies, which have been widely used in clinics over the past several decades. Graphical abstract.

A rapid and simple fluorescence enzyme-linked immunosorbent assay for tetrabromobisphenol A in soil samples based on a bifunctional fusion protein

Tetrabromobisphenol A (TBBPA) is the largest brominated flame retardant which can be released to environment and cause long-term hazard. In this work, we developed a rapid and highly sensitive fluorescence enzyme-linked immunosorbent assay (FELISA) for monitoring of TBBPA in soil samples. TBBPA specific nanobody derived from camelid was fused with alkaline phosphatase to obtain the bi-functional fusion protein, which enable the specific binding of TBBPA and the generation of detection signal simultaneously. The assay showed an IC₅₀ of 0.23 ng g^-1, limit detection of 0.05 ng g^-1 and linear range from 0.1 to 0.55 ng g^-1 for TBBPA in soil samples. Due to the high resistance to organic solvents of the fusion protein, a simple pre-treatment by using 40% dimethyl sulfoxide (DMSO) as extract solvent can eliminate matrix effect and obtain good recoveries (ranging from 93.4% to 112.4%) for spiked soil samples. Good relationship between the results of the proposed FELISA and that of liquid chromatography tandem mass spectrometry (LC-MS/MS) was obtained, which indicated it could be a powerful analytical tool for determination of TBBPA to monitor human and environmental exposure.

One-step immunoassay for the insecticide carbaryl using a chicken single-chain variable fragment (scFv) fused to alkaline phosphatase

Immunoassays provide a high-throughput method for monitoring pesticides in foods and the environment. Due to easy generation and capable of being manipulated, chicken single-chain variable fragment (scFv) is attractive in the development of immunoassays for pesticides. Two scFvs (X1 and X2) against the insecticide carbaryl were generated from a chicken immunized with hapten C1 conjugated to keyhole limpet hemocyanin and fused with alkaline phosphatase (AP) to develop a rapid one-step enzyme-linked immunosorbent assay for this pesticide. X2-AP showed higher binding affinity to carbaryl than X1-AP. The X2-AP-based ELISA had a half-maximum signal inhibition concentration of 15 ng mL^-1 and a limit of detection of 1.6 ng mL^-1. This assay showed negligible cross-reactivity with other carbamate pesticides (<0.1%) and low cross-reactivity with 1-naphthol (5%). The average recoveries of carbaryl spiked in soil, apple and pear samples by the one-step assay ranged from 90% to 114% and agreed well with those of high-performance liquid chromatography. The chicken scFv-based assay showed promise as a high-throughput screening tool for carbaryl in environmental and food matrices.

Development of a one-step immunoassay for triazophos using camel single-domain antibody-alkaline phosphatase fusion protein

Triazophos is mainly used in Asian and African countries for the control of insects in agricultural production. Camelid variable domains of heavy-chain antibodies (VHHs) show great promise in monitoring environmental chemicals such as pesticides. To improve the rate of success in the generation of VHHs against triazophos, genes specifically encoding VHH fragments from the unique allotype IgG3a of an immunized Camelus bactrianus were amplified by using a pair of novel primers and introduced to construct a diverse VHH library. Five out of seven isolated positive clones, including the VHH T1 with the highest affinity to triazophos, were derived from the allotype IgG3a. A one-step enzyme-linked immunosorbent assay (ELISA) using VHH T1 genetically fused with alkaline phosphatase (AP) had a half-maximum inhibition concentration of 6.6 ng/mL for triazophos. This assay showed negligible cross-reactivity with a list of important organophosphate pesticides (< 0.1%). The average recoveries of triazophos from water, soil, and apple samples determined by the one-step ELISA ranged from 83 to 108%, having a good correlation with those by a gas chromatography mass spectrometry (R² = 0.99). The VHH-AP fusion protein shows potential for the analysis of triazophos in various matrices.

Selection and Characterization of Anti-Dengue NS1 Single Domain Antibodies

Reliable detection and diagnosis of dengue virus (DENV) is important for both patient care and epidemiological control. Starting with a llama immunized with a mixture of recombinant nonstructural protein 1 (NS1) antigen from the four DENV serotypes, a phage display immune library of single domain antibodies was constructed and binders selected which exhibited specificity and affinity for DENV NS1. Each of these single domain antibodies was evaluated for its binding affinity to NS1 from the four serotypes, and incorporated into a sandwich format for NS1 detection. An optimal pair was chosen that provided the best combination of sensitivity for all four DENV NS1 antigens spiked into 50% human serum while showing no cross reactivity to NS1 from Zika virus, yellow fever virus, tick-borne encephalitis virus, and minimal binding to NS1 from Japanese encephalitis virus and West Nile virus. These rugged and robust recombinant binding molecules offer attractive alternatives to conventional antibodies for implementation into immunoassays destined for resource limited locals.

Selection of Single-Domain Antibodies towards Western Equine Encephalitis Virus

In this work, we describe the selection and characterization of single-domain antibodies (sdAb) towards the E2/E3E2 envelope protein of the Western equine encephalitis virus (WEEV). Our purpose was to identify novel recognition elements which could be used for the detection, diagnosis, and perhaps treatment of western equine encephalitis (WEE). To achieve this goal, we prepared an immune phage display library derived from the peripheral blood lymphocytes of a llama that had been immunized with an equine vaccine that includes killed WEEV (West Nile Innovator + VEWT). This library was panned against recombinant envelope (E2/E3E2) protein from WEEV, and seven representative sdAb from the five identified sequence families were characterized. The specificity, affinity, and melting point of each sdAb was determined, and their ability to detect the recombinant protein in a MagPlex sandwich immunoassay was confirmed. Thus, these new binders represent novel recognition elements for the E2/E3E2 proteins of WEEV that are available to the research community for further investigation into their applicability for use in the diagnosis or treatment of WEE.

Selection and characterization of protective anti-chikungunya virus single domain antibodies

Chikungunya virus (CHIKV) is a mosquito-borne alphavirus that causes an arthralgia febrile illness that has affected millions of people on three continents. Previously, neutralizing monoclonal antibodies that have prophylactic and therapeutic activity were found to remove virus in joint tissues, thereby reducing the severity of symptoms in mice and non-human primates. In this study, we sought to develop thermostable small recombinant antibodies against CHIKV for future diagnostic, prophylactic and therapeutic applications. To develop these single domain antibodies (sdAb) a CHIKV immune library was constructed by displaying the consortium of variable heavy domains (VHH) amplified from peripheral white blood cells isolated from llamas immunized with CHIKV virus-like particles (VLPs). Five anti-CHIKV sdAb isolated using bio-panning were evaluated for their affinity and thermal stability. Their ability to detect CHIKV VLPs was demonstrated in both MagPlex- and ELISA- based assays. Finally, the ability of two sdAb, CC3 and CA6, to inhibit CHIKV infection were tested using a plaque reduction and neutralization test (PRNT), yielding PRNT₅₀ values of 0.6 and 45.6 nM, respectively.

Development of a Highly Sensitive Direct Competitive Fluorescence Enzyme Immunoassay Based on a Nanobody-Alkaline Phosphatase Fusion Protein for Detection of 3-Phenoxybenzoic Acid in Urine

3-Phenoxybenzoic acid (3-PBA) is a human urinary metabolite of many pyrethroid insecticides and can be used as a biomarker to monitor human exposure to these pesticides. A rapid and sensitive direct competitive fluorescence enzyme immunoassay (dc-FEIA) for detecting 3-PBA on the basis of a nanobody (Nb)-alkaline phosphatase (AP) fusion protein was developed. The anti-3-PBA Nb-AP fusion protein was expressed and purified. The 50% inhibitory concentration (IC₅₀) and linear range of dc-FEIA were 0.082 and 0.015-0.447 ng/mL, respectively, with a detection limit of 0.011 ng/mL. The IC₅₀ of dc-FEIA was improved by nearly ten times compared with those of one-step and three-step direct competitive enzyme-linked immunosorbent assay (dc-ELISA). Spiked urine samples were detected by both dc-FEIA and liquid chromatography-mass spectrometry (LC-MS), and the results showed good consistency between the two analysis methods, indicating the reliability of dc-FEIA based on the Nb-AP fusion protein for detecting 3-PBA in urine.

Genetic Fusion of an Anti-BclA Single-Domain Antibody with Beta Galactosidase

The Bacillus collagen-like protein of anthracis (BclA), found in Bacillus anthracis spores, is an attractive target for immunoassays. Previously, using phage display we had selected llama-derived single-domain antibodies that bound to B. anthracis spore proteins including BclA. Single-domain antibodies (sdAbs), the recombinantly expressed heavy domains from the unique heavy-chain-only antibodies found in camelids, provide stable and well-expressed binding elements with excellent affinity. In addition, sdAbs offer the important advantage that they can be tailored for specific applications through protein engineering. A fusion of a BclA targeting sdAb with the enzyme Beta galactosidase (β-gal) would enable highly sensitive immunoassays with no need for a secondary reagent. First, we evaluated five anti-BclA sdAbs, including four that had been previously identified but not characterized. Each was tested to determine its binding affinity, melting temperature, producibility, and ability to function as both capture and reporter in sandwich assays for BclA. The sdAb with the best combination of properties was constructed as a fusion with β-gal and shown to enable sensitive detection. This fusion has the potential to be incorporated into highly sensitive assays for the detection of anthrax spores.

In silico-designed mutations increase variable new-antigen receptor single-domain antibodies for VEGF165 neutralization

The stability, binding, and tissue penetration of variable new-antigen receptor (VNAR) single-domain antibodies have been tested as part of an investigation into their ability to serve as novel therapeutics. V13 is a VNAR that recognizes vascular endothelial growth factor 165 (VEGF₁₆₅). In the present study V13 was used as a parental molecule into which we introduced mutations designed in silico. Two of the designed VNAR mutants were expressed, and their ability to recognize VEGF₁₆₅ was assessed in vitro and in vivo. One mutation (Pro98Tyr) was designed to increase VEGF₁₆₅ recognition, while the other (Arg97Ala) was designed to inhibit VEGF₁₆₅ binding. Compared to parental V13, the Pro98Tyr mutant showed enhanced VEGF₁₆₅ recognition and neutralization, as indicated by inhibition of angiogenesis and tumor growth. This molecule thus appears to have therapeutic potential for neutralizing VEGF₁₆₅ in cancer treatment.

VHH Antibodies: Reagents for Mycotoxin Detection in Food Products

Mycotoxins are the toxic secondary metabolites produced by fungi and they are a worldwide public health concern. A VHH antibody (or nanobody) is the smallest antigen binding entity and is produced by heavy chain only antibodies. Compared with conventional antibodies, VHH antibodies overcome many pitfalls typically encountered in clinical therapeutics and immunodiagnostics. Likewise, VHH antibodies are particularly useful for monitoring mycotoxins in food and feedstuffs, as they are easily genetic engineered and have superior stability. In this review, we summarize the efforts to produce anti-mycotoxins VHH antibodies and associated assays, presenting VHH as a potential tool in mycotoxin analysis.

Bglbrick strategy for the construction of single domain antibody fusions

Single domain antibodies, recombinantly expressed variable domains derived from camelid heavy chain antibodies, are often expressed as multimers for detection and therapeutic applications. Constructs in which several single domain antibodies are genetically fused serially, as well as those in which single domain antibodies are genetically linked with domains that naturally form multimers, yield improvement in apparent binding affinity due to avidity. Here, using a single domain antibody that binds envelope protein from the Dengue virus, we demonstrated the construction of single domain antibody dimers using the Bglbrick cloning strategy. Constructing single domain antibodies and multimerization domains as Bglbrick parts enables the easy mixing and matching of parts. The dimeric constructs provided enhanced fluorescent signal in assays for detection of Dengue virus like particles over the monomeric single domain antibody.

Development of NanoLuc bridging immunoassay for detection of anti-drug antibodies

Anti-drug antibodies (ADAs) are generated in-vivo as an immune response to therapeutic antibody drugs and can significantly affect the efficacy and safety of the drugs. Hence, detection of ADAs is recommended by regulatory agencies during drug development process. A widely accepted method for measuring ADAs is "bridging" immunoassay and is frequently performed using enzyme-linked immunosorbent assay (ELISA) or electrochemiluminescence (ECL) platform developed by Meso Scale Discovery (MSD). ELISA is preferable due to widely available reagents and instruments and broad familiarity with the technology; however, MSD platform has gained wide acceptability due to a simpler workflow, higher sensitivity, and a broad dynamic range but requires proprietary reagents and instruments. We describe the development of a new bridging immunoassay where a small (19kDa) but ultra-bright NanoLuc luciferase enzyme is used as an antibody label and signal is luminescence. The method combines the convenience of ELISA format with assay performance similar to that of the MSD platform. Advantages of the NanoLuc bridging immunoassay are highlighted by using Trastuzumab and Cetuximab as model drugs and developing assays for detection of anti-Trastuzumab antibodies (ATA) and anti-Cetuximab antibodies (ACA). During development of the assay several aspects of the method were optimized including: (a) two different approaches for labeling drugs with NanoLuc; (b) sensitivity and dynamic range; and (c) compatibility with the acid dissociation step for improved drug tolerance. Assays showed high sensitivity of at least 1.0ng/mL, dynamic range of greater than four log orders, and drug tolerance of >500.

Integrating scFv into xMAP Assays for the Detection of Marine Toxins

Marine toxins, such as saxitoxin and domoic acid are associated with algae blooms and can bioaccumulate in shell fish which present both health and economic concerns. The ability to detect the presence of toxin is paramount for the administration of the correct supportive care in case of intoxication; environmental monitoring to detect the presence of toxin is also important for prevention of intoxication. Immunoassays are one tool that has successfully been applied to the detection of marine toxins. Herein, we had the variable regions of two saxitoxin binding monoclonal antibodies sequenced and used the information to produce recombinant constructs that consist of linked heavy and light variable domains that make up the binding domains of the antibodies (scFv). Recombinantly produced binding elements such as scFv provide an alternative to traditional antibodies and serve to "preserve" monoclonal antibodies as they can be easily recreated from their sequence data. In this paper, we combined the anti-saxitoxin scFv developed here with a previously developed anti-domoic acid scFv and demonstrated their utility in a microsphere-based competitive immunoassay format. In addition to detection in buffer, we demonstrated equivalent sensitivity in oyster and scallop matrices. The potential for multiplexed detection using scFvs in this immunoassay format is demonstrated.

Alkaline phosphatase-fused repebody as a new format of immuno-reagent for an immunoassay

Enzyme-linked immunoassays based on an antibody-antigen interaction are widely used in biological and medical sciences. However, the conjugation of an enzyme to antibodies needs an additional chemical process, usually resulting in randomly cross-linked molecules and a loss of the binding affinity and enzyme activity. Herein, we present the development of an alkaline phosphatase-fused repebody as a new format of immuno-reagent for immunoassays. A repebody specifically binding to human TNF-α (hTNF-α) was selected through a phage display, and its binding affinity was increased up to 49 nM using a modular engineering approach. A monomeric alkaline phosphatase (mAP), which was previously isolated from a metagenome library, was genetically fused to the repebody as a signal generator, and the resulting repebody-mAP fusion protein was used for direct and sandwich immunoassays of hTNF-α. We demonstrate the utility and potential of the repebody-mAP fusion protein as an immuno-reagent by showing the sensitivity of 216 pg mL^-1 for hTNF-α in a sandwich immunoassay. Furthermore, this repebody-mAP fusion protein enabled the detection of hTNF-α spiked in a serum-supplemented medium with high accuracy and reproducibility. It is thus expected that a mAP-fused repebody can be broadly used as an immuno-reagent in immunoassays.

Single-domain antibody based thermally stable electrochemical immunosensor

Conventional monoclonal and polyclonal antibodies are sensitive to changes in environmental factors such as temperature, pH, humidity, etc. This limits the current cost-effective and portable electrochemical immunosensors in harsh environments. Using Ricin Chain-A, a naturally occurring toxin, as a model analyte we report fabrication of a thermally stable electrochemical immunosensor. Single-domain antibodies (sdAb) or nanobodies have been employed as recognition elements for direct detection of Ricin at temperatures great than 4°C. Immunosensor fabricated using the conventional Ricin monoclonal and polyclonal antibodies have also been demonstrated for comparison. In the case of sdAb immunosensor, Ricin was detected in a linear range of 1log(fg/mL)-1log(μg/mL) with a sensitivity of 0.07μA/log(g/mL)/cm(2) using cyclic voltammetry. The fabricated miniaturized sensors have demonstrated higher shelf life and stability at temperatures up to 40°C. Therefore these electrochemical sensors can be integrated as a part of a portable device for point-of-care immunosensing.

NanoLuc Luciferase - A Multifunctional Tool for High Throughput Antibody Screening

Based on the recent development of NanoLuc luciferase (Nluc), a small (19 kDa), highly stable, ATP independent, bioluminescent protein, an extremely robust and ultra high sensitivity screening system has been developed whereby primary hits of therapeutic antibodies and antibody fragments could be characterized and quantified without purification. This system is very versatile allowing cellular and solid phase ELISA but also homogeneous BRET based screening assays, relative affinity determinations with competition ELISA and direct Western blotting. The new Nluc protein fusion represents a “swiss army knife solution” for today and future high throughput antibody drug screenings.

One-step immunoassay for tetrabromobisphenol a using a camelid single domain antibody-alkaline phosphatase fusion protein

Tetrabromobisphenol A (TBBPA) is a ubiquitous brominated flame retardant, showing widespread environmental and human exposures. A variable domain of the heavy chain antibody (VHH), naturally occurring in camelids, approaches the lower size limit of functional antigen-binding entities. The ease of genetic manipulation makes such VHHs a superior choice to use as an immunoreagent. In this study, a highly selective anti-TBBPA VHH T3-15 fused with alkaline phosphatase (AP) from E. coli was expressed, showing both an integrated TBBPA-binding capacity and enzymatic activity. A one-step immunoassay based on the fusion protein T3-15-AP was developed for TBBPA in 5% dimethyl sulfoxide (DMSO)/phosphate buffered saline (PBS, pH 7.4), with a half-maximum signal inhibition concentration (IC50) of 0.20 ng mL(-1). Compared to the parental VHH T3-15, T3-15-AP was able to bind to a wider variety of coating antigens and the assay sensitivity was slightly improved. Cross-reactivity of T3-15-AP with a set of important brominated analogues was negligible (<0.1%). Although T3-15-AP was susceptible to extreme heat (90 °C), much higher binding stability at ambient temperature was observed in the T3-15-AP-based assay for at least 70 days. A simple pretreatment method of diluting urine samples with DMSO was developed for a one-step assay. The recoveries of TBBPA from urine samples via this one-step assay ranged from 96.7% to 109.9% and correlated well with a high-performance liquid chromatography-tandem mass spectroscopy (HPLC-MS/MS) method. It is expected that the dimerized fusion protein, VHH-AP, will show promising applications in human exposure and environmental monitoring.

Development of a nanobody-alkaline phosphatase fusion protein and its application in a highly sensitive direct competitive fluorescence enzyme immunoassay for detection of ochratoxin A in cereal

A rapid and sensitive direct competitive fluorescence enzyme immunoassay (dc-FEIA) for ochratoxin A (OTA) based on a nanobody (Nb)-alkaline phosphatase (AP) fusion protein was developed. The VHH (variable domain of heavy chain antibody) gene of Nb28 was subcloned into the expression vector pecan45 containing the AP double-mutant gene. The Nb28-AP construct was transformed into Escherichia coli BL21(DE3)plysS, and soluble expression in bacteria was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot. Both the Nb properties and AP enzymatic activity were validated by colorimetric and fluorometric analysis. The 50% inhibitory concentration and the detection limit of the dc-FEIA were 0.13 and 0.04 ng/mL, respectively, with a linear range of 0.06-0.43 ng/mL. This assay was compared with LC-MS/MS, and the results indicated the reliability of Nb-AP fusion protein-based dc-FEIA for monitoring OTA contamination in cereal.

Development and evaluation of single domain antibodies for vaccinia and the L1 antigen

There is ongoing interest to develop high affinity, thermal stable recognition elements to replace conventional antibodies in biothreat detection assays. As part of this effort, single domain antibodies that target vaccinia virus were developed. Two llamas were immunized with killed viral particles followed by boosts with the recombinant membrane protein, L1, to stimulate the immune response for envelope and membrane proteins of the virus. The variable domains of the induced heavy chain antibodies were selected from M13 phage display libraries developed from isolated RNA. Selection via biopanning on the L1 antigen produced single domain antibodies that were specific and had affinities ranging from 4×10⁻⁹ M to 7.0×10⁻¹⁰ M, as determined by surface plasmon resonance. Several showed good ability to refold after heat denaturation. These L1-binding single domain antibodies, however, failed to recognize the killed vaccinia antigen. Useful vaccinia binding single domain antibodies were isolated by a second selection using the killed virus as the target. The virus binding single domain antibodies were incorporated in sandwich assays as both capture and tracer using the MAGPIX system yielding limits of detection down to 4×10⁵ pfu/ml, a four-fold improvement over the limit obtained using conventional antibodies. This work demonstrates the development of anti-vaccinia single domain antibodies and their incorporation into sandwich assays for viral detection. It also highlights the properties of high affinity and thermal stability that are hallmarks of single domain antibodies.

Chicken single-chain antibody fused to alkaline phosphatase detects Aspergillus pathogens and their presence in natural samples by direct sandwich enzyme-linked immunosorbent assay

A sensitive and specific analytical method to detect ubiquitous aflatoxigenic Aspergillus pathogens is essential for monitoring and controlling aflatoxins. Four highly reactive chicken single-chain variable fragments (scFvs) against soluble cell wall proteins (SCWPs) from Aspergillus flavus were isolated by phage display. The scFv antibody AfSA4 displayed the highest activity toward both A. flavus and A. parasiticus and specifically recognized a surface target of their cell walls as revealed by immunofluorescence localization. Molecular modeling revealed a unique compact motif on the antibody surface mainly involving L-CDR2 and H-CDR3. As measured by surface plasmon resonance, AfSA4 fused to alkaline phosphatase had a higher binding capability and 6-fold higher affinity compared with AfSA4 alone. Immunoblot analyses showed that the fusion had good binding capacity to SCWP components from the two fungal species. Direct sandwich enzyme-linked immunosorbent assays with mouse antiaspergillus monoclonal antibody mAb2A8 generated in parallel as a capture antibody revealed that the detection limit of the two fungi was as low as 10(-3) μg/mL, 1000-fold more sensitive than that reported previously (1 μg/mL). The fusion protein was able to detect fungal concentrations below 1 μg/g of maize and peanut grains in both artificially and naturally contaminated samples, with at least 10-fold more sensitivity than that reported (10 μg/g) thus far. Thus, the fusion can be applied in rapid, simple, and specific diagnosis of Aspergillus contamination in field and stored food/feed commodities.