Antibody Fab display system that can perform open-sandwich ELISA

Detection of calcitonin gene-related peptide based on increased antigen-driven interaction with antibody variable regions

Introduction: Calcitonin gene-related peptide (CGRP) is involved in trigeminal neuralgia and migraine, and measuring the CGRP concentration in the serum is crucial for the early prediction of these conditions. Current methods for CGRP detection are primarily radioimmunoassay, which needs radioactive substances and enzyme-linked immunosorbent assays (ELISAs) which need long detection time and some have a narrow detection range. Methods: The genes of anti-CGRP antibody variable regions were cloned into pDong1 vector to obtain pDong1/Fab-CGRP, with which phage-Fab was prepared, and the concentration of CGRP was detected by competitive ELISA. The pDong1/Fab-CGRP was modified to obtain pDong1/OS-CGRP, with which the co-expression solution containing phage-displayed heavy chain variable fragments (phage-VH) and light chain was obtained. CGRP was detected by OS-ELISA based on phage-VH, antibody light chain, and anti-light chain antibody. The VL gene was cloned into the pMAL vector to obtain pMAL-VL (CGRP), with which maltose binding protein fused with VL (MBP-VL) was prepared. CGRP was detected by OS-ELISA employing MBP-VL and phage-VH. Results: OS-ELISAs that measure the CGRP concentration by quantifying the interaction between variable regions were investigated. OS-ELISA using phage-VH and secreted light chains in the same culture system exhibited a limit of detection (LOD) of 0.05 nM, offering higher sensitivity than competitive assay with an LOD of 0.75 nM, whereas using phage-VH and separately prepared MBP-VL exhibited an LOD of 0.15 nM and a broader detection range of 0.15-500 nM than competitive ELISA, whose detection range was 0.75-10 nM. Discussion: The combination of the two OS assays achieved high sensitivity and a broad detection range for CGRP, which may have significance in clinical applications.

A breakthrough of immunoassay format for hapten: recent insights into noncompetitive immunoassays to detect small molecules

Immunoassay based on the antibodies specific for targets has advantages of high sensitivity, simplicity and low cost, therefore it has received more attention in recent years, especially for the rapid detection of small molecule chemicals present in foods, diagnostics and environments. However, limited by low molecular weight and only one antigenic determinant existed, immunoassays for these small molecule chemicals, namely hapten substances, were commonly performed in a competitive immunoassay format, whose sensitivities were obviously lower than the sandwich enzyme-linked immunosorbent assay generally adaptable for the protein targets. In order to break through the bottleneck of detection format, researchers have designed and established several novel noncompetitive immunoassays for the haptens in the past few years. In this review, we focused on the four representative types of noncompetitive immunoassay formats and described their characteristics and applications in rapid detection of small molecules. Meanwhile, a systematic discussion on the current technologies challenges and the possible solutions were also summarized. This review aims to provide an updated overview of the current state-of-the-art in noncompetitive immunoassay for small molecules, and inspire the development of novel designs for small molecule detection.

Recent progress in homogeneous immunosensors based on fluorescence or bioluminescence using antibody engineering

Homogeneous immunosensors integrate the advantages of both biosensors and immunoassays; they include speed, high sensitivity, and accuracy. They have been developed rapidly in the past few years and offer a cost-effective alternative technology with rapidity, sensitivity, and user-friendliness, which has been applied in a wide variety of applications. This review introduces the current directions of immunosensor development, focusing on fluorescent and bioluminescent immunosensors and highlighting the advantages, improvements, and key approaches to overcome the limitations of each.

Creation of a quick and sensitive fluorescent immunosensor for detecting the mineralocorticoid steroid hormone aldosterone

Aldosterone (ALD) is a steroid hormone secreted by the zona glomerulosa of the adrenal cortex that mainly acts on the kidney to regulate sodium ion and water reabsorption. Detection of ALD plays an important role in the diagnosis of primary aldosteronism in patients with hypertension. For the first time, the gene encoding the anti-ALD antibody, A2E11, was successfully cloned and analyzed using phage display technology. The antibody had an affinity of 2.5 nM against ALD, and after binding to ALD, it reached saturation within 5 s. Using this antibody, a Quenchbody (Q-body) was constructed by labeling the N-termini of heavy and light chains of the antigen-binding fragment of A2E11 with the fluorescent dye ATTO520 to detect ALD based on the principle of photoinduced electron transfer. The sensor detected ALD in 2 min, and the limit of detection was 24.1 pg/mL with a wide detection range from 24.1 pg/mL to 10 µg/mL and a half-maximal effective concentration of 42.3 ng/mL. At the highest concentration of ALD in the assay, the fluorescence intensity increased by 5.0-fold compared to the original fluorescence intensity of the Q-body solution. The Q-body could be applied to analyze 50% of human serum without a significant influence of the matrix. The recoveries of ALD in spiked serum samples with the Q-body assay were confirmed to range from 90.3% to 98.2%, suggesting their potential applications in the diagnosis of diseases, such as essential hypertension.

Isolation of a Monoclonal Antibody and its Derived Immunosensor for Rapid and Sensitive Detection of 17β-Estradiol

Estrogens are effective for stimulating several functions in living organisms and for regulating cancer development by promoting cell proliferation. Estradiol can disrupt the reproductive and endocrine systems, leading to the development of various diseases. In this study, the monoclonal antibody ESC9 was developed by immunizing mice with a 17β-estradiol (E2) conjugate, preparing an antibody phage display library, and screening monoclonal antibodies from the prepared library. An antibody with the same sequence as that of ESC9 has not been reported previously. The equilibrium dissociation constant between ESC9 and E2 was found to be 43.3 nM. Additionally, we generated an ESC9-derived immunosensor named as the ESC9 Quenchbody (Q-body), which can rapidly and sensitively detect E2. The assay can be completed within 2 min with a limit of detection of 3.9 pg/ml and half-maximal effective concentration of 154.0 ng/ml. Serum E2 levels were measured using the ESC9 Q-body without pretreatment with serum and with a high recovery rate of 83.3–126.7%. The Q-body immunosensor shows potential for clinical applications based on its excellent detection speed and sensitivity.

Immunoassay technology: Research progress in microcystin-LR detection in water samples

Increasing global warming and eutrophication have led to frequent outbreaks of cyanobacteria blooms in freshwater. Cyanobacteria blooms cause the death of aquatic and terrestrial organisms and have attracted considerable attention since the 19th century. Microcystin-LR (MC-LR) is one of the most typical cyanobacterial toxins. Therefore, the fast, sensitive, and accurate determination of MC-LR plays an important role in the health of humans and animals. Immunoassay refers to a method that uses the principle of immunology to determine the content of the tested substance in a sample using the tested substance as an antigen or antibody. In analytical applications, the immunoassay technology could use the specific recognition of antibodies for MC-LR detection. In this review, we firstly highlight the immunoassay detection of MC-LR over the past two decades, including classical enzyme-link immunosorbent assay (ELISA), modern immunoassay with optical signal, and modern immunoassay with electrical signal. Among these detection methods, the water environment was used as the main detection system. The advantages and disadvantages of the different detection methods were compared and analyzed, and the principles and applications of immunoassays in water samples were elaborated. Furthermore, the current challenges and developmental trends in immunoassay were systematically introduced to enhance MC-LR detection performance, and some critical points were given to deal with current challenges. This review provides novel insight into MC-LR detection based on immunoassay method.

An enhanced open sandwich immunoassay by molecular evolution for noncompetitive detection of Alternaria mycotoxin tenuazonic acid

Open sandwich enzyme-linked immunosorbent assay (OS-ELISA), a novel noncompetitive immunoassay format, has shown great potential in rapid detection for small molecules compared with traditional competitive format. Here, an enhanced OS-ELISA towards the mycotoxin tenuazonic acid (TeA) was developed for the first time based on heavy chain variable region (V_H) and light chain variable region (V_L) from the hybridoma cells (3F10) producing anti-TeA monoclonal antibody (mAb). The established OS-ELISA exhibited a limit of detection of 0.08 ng/mL, and was 13 times more sensitive than mAb-based indirect competitive ELISA (ic-ELISA). The proposed assay was also applied to detect TeA contents in juice, flour and tomato ketchup samples with satisfactory recoveries of 87.6%-111.3%. Finally, the great accuracy of the established OS-ELISA method was validated by the standard ultra-performance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS).

Construction of a full-length antibody phage display vector

Antibody phage display technology plays an important role in the development of monoclonal antibodies, humanization, and affinity evolution of antibodies. Thus far, antibody phage display mainly focuses on the display of antibody variable region or antigen-binding fragments. In this study, we constructed a new phage display system that can display full-length IgG antibodies on M13 phage. The phage display vector contains open reading frames (ORFs) encoding full-length the heavy and light chains of the antibody. NcoI/XhoI restriction enzyme sites were used to clone the variable region of the heavy chain into the heavy chain ORF, and SalI/NotI sites were used to clone the light chain variable region. SnaBI and SbfI restriction enzyme sites were designed between the cloning sites of heavy and light chains, respectively, to increase the cloning efficiency. The full-length antibodies of nivolumab against programmed death factor 1, trastuzumab against human epidermal growth factor 2, diL2K against the cluster of differentiation 3 epsilon, and adalimumab against tumor necrosis factor- alpha were displayed on phage with the vector. Phage-displayed antibodies showed their original antigen-binding activity. An amber codon shifted the vector to express IgG in non-suppressed Escherichia coli. The heavy and light chains of the E. coli-expressed antibodies could be detected through western blotting, and the antigen-binding activity was confirmed using an enzyme-linked immunosorbent assay. Biopanning was carried out with a model phage display antibody library, and the results showed that the novel phage system could be used for antibody library construction and highly efficient antibody screening. The reported system is the first full-length antibody phage display system.

A novel murine antibody and an open sandwich immunoassay for the detection of clenbuterol

Clenbuterol (CLEN) is a sympathomimetic amine used as a decongestant and bronchodilator while treating breathing disorders. It is also used in food-producing animals as it improves the rate of red meat production. However, it is prohibited in many countries nowadays due to human health and safety concerns. Unfortunately, the illegal use of CLEN is still rampant. Thus, monitoring it in food and livestock is important. Here, we report a novel murine antibody and an open sandwich enzyme linked immunosorbent assay (OS-ELISA) to detect CLEN based on antigen-antibody reactions. The genes of antibody variable regions in mice immunized with CLEN conjugated with bovine serum albumin were cloned into a phagemid (pDong1/Fab) to construct a phage-display antibody library, from which a novel antibody, A12, was selected. Then, an OS-ELISA was developed to detect CLEN using separated variable regions of the A12 antibody. The limit of detection of the assay was found to be 8 ng/mL, which was useful for monitoring CLEN usage.

An open sandwich immunoassay for detection of 13(R,S)-hydroxy-9(E),11(E)-octadecadienoic acid

Lipid peroxidation is involved in many disorders and diseases such as cardiovascular disease, cancers, neurodegenerative diseases, and even aging.

Optimal fusion of antibody binding domains resulted in higher affinity and wider specificity

Antibody is a very important protein in biotechnological and biomedical fields because of its high affinity and specificity to various antigens. Due to the rise of human antibody therapeutics, its cost-effective purification is an urgent issue for bio-industry. In this study, we made novel fusion proteins PAxPG with a flexible (DDAKK)n linker between the two Ig binding domains derived from Staphylococcus protein A and Streptococcus protein G. The fusion proteins bound human and mouse IgGs and their fragments with up to 58-times higher affinity and wider specificity than the parental binding domains. Interestingly, the optimal linker for human Fab fragment was n = 4, which was close to the modeled distance between the termini of domains bound to heavy chain, implying increased avidity as a possible mechanism. For binding to Fc, the longest n=6 linker gave the highest affinity, implying longer interchain distance between the two binding sites. The novel fusion protein with optimized interdomain linker length will be a useful tool for the purification and detection of various IgGs including mouse IgG1 that binds only weakly to natural protein A.

Progress in the development of immunoanalytical methods incorporating recombinant antibodies to small molecular weight biotoxins

Rapid immunoanalytical screening of food and environmental samples for small molecular weight (hapten) biotoxin contaminations requires the production of antibody reagents that possess the requisite sensitivity and specificity. To date animal-derived polyclonal (pAb) and monoclonal (mAb) antibodies have provided the binding element of the majority of these assays but recombinant antibodies (rAb) isolated from in vitro combinatorial phage display libraries are an exciting alternative due to (1) circumventing the need for experimental animals, (2) speed of production in commonly used in vitro expression systems and (3) subsequent molecular enhancement of binder performance. Short chain variable fragments (scFv) have been the most commonly employed rAb reagents for hapten biotoxin detection over the last two decades but antibody binding fragments (Fab) and single domain antibodies (sdAb) are increasing in popularity due to increased expression efficiency of functional binders and superior resistance to solvents. rAb-based immunochromatographic assays and surface plasmon resonance (SPR) biosensors have been reported to detect sub-regulatory levels of fungal (mycotoxins), marine (phycotoxins) and aquatic biotoxins in a wide range of food and environmental matrices, however this technology has yet to surpass the performances of the equivalent mAb- and pAb-based formats. As such the full potential of rAb technology in hapten biotoxin detection has yet to be achieved, but in time the inherent advantages of engineered rAb are set to provide the next generation of ultra-high performing binder reagents for the rapid and specific detection of hapten biotoxins.

Open-sandwich immunoassay for sensitive and broad-range detection of a shellfish toxin gonyautoxin

At present, the analytical method for paralytic shellfish poisoning (PSP) toxins in shellfish is the mouse bioassay (MBA), which is an official method of the Association of Analytical Communities (AOAC [8]). However, the low sensitivity and concerns over the number of live animals required for testing have been cited as the major reason for seeking its replacement. In this report, we employed an open-sandwich immunoassay (OS-IA) to detect gonyautoxin (GTX2/3), a kind of PSP toxins. OS-IA, which utilizes the antigen-induced enhancement of antibody VH/VL interaction, can measure a small molecule antigen in a noncompetitive format. Hence it has a wider working range and shorter measurement time. We isolated anti-GTX2/3 antibody gene from a hybridoma GT-13A by screening a Fab-displaying phage library. Then the vectors for OS-IA were constructed, and examined for antigen concentration-dependency of the VH/VL interaction by OS-ELISA. As a result, in each case, signal intensity increases notably in a wide concentration range (0.1 to >1000 ng mL(-1)) of free GTX2/3, which was enough to cover its regulation value (80 μg 100 g(-1)) in many countries. So OS-IA will be widely applicable to detect PSP toxins in shellfish meats and in drinking water.

Isolation of recombinant phage antibodies targeting the hemagglutinin cleavage site of highly pathogenic avian influenza virus

Highly pathogenic avian influenza (HPAI) H5N1 viruses, which have emerged in poultry and other wildlife worldwide, contain a characteristic multi-basic cleavage site (CS) in the hemagglutinin protein (HA). Because this arginine-rich CS is unique among influenza virus subtypes, antibodies against this site have the potential to specifically diagnose pathogenic H5N1. By immunizing mice with the CS peptide and screening a phage display library, we isolated four antibody Fab fragment clones that specifically bind the antigen peptide and several HPAI H5N1 HA proteins in different clades. The soluble Fab fragments expressed in Escherichia coli bound the CS peptide and the H5N1 HA protein with nanomolar affinity. In an immunofluorescence assay, these Fab fragments stained cells infected with HPAI H5N1 but not those infected with a less virulent strain. Lastly, all the Fab clones could detect the CS peptide and H5N1 HA protein by open sandwich ELISA. Thus, these recombinant Fab fragments will be useful novel reagents for the rapid and specific detection of HPAI H5N1 virus.

Development of two murine antibodies against Neospora caninum using phage display technology and application on the detection of N. caninum

Neosporosis, caused by an intracellular parasite, Neospora caninum, is an infectious disease primarily of cattle and dogs. It occurs worldwide and causes huge damages to dairy farms. In this study, we immunized mice with recombinant surface-associated protein 1 of N. caninum (rNcSAG1) and developed two novel monoclonal antibodies, A10 and H3, against NcSAG1 using phage-display technology. Both clones bound to purified rNcSAG1 and the half maximal inhibitory concentrations of A10 and H3 are 50 and 72 nM of rNcSAG1, respectively. In immunofluorescence assays, both A10 and H3 Fabs bound to N. caninum parasites. Direct detection of N. caninum parasites was developed firstly using an enzyme-linked immunosorbent assay (ELISA) with A10 and H3. Binding of A10 and H3 antibodies to rNcSAG1 was also inhibited by some certain anti-N. caninum antibodies in the neosporosis-positive cattle sera, suggesting they might bind to the same epitopes of NcSAG1 with those anti-N. caninum antibodies of bovine. These antibodies were demonstrated to have a potential for monitoring the N. caninum parasites in a dairy farm, which may lead to protect livestock from parasite-infection.

Detection of small molecule diagnostic markers with phage-based open-sandwich immuno-PCR

Open sandwich immunoassay (OS-IA) utilizes antigen-dependent stabilization of antibody variable region to quantify various antigens, enabling noncompetitive detection of small molecules with a broad working range. To further improve its detection sensitivity, here we employed phage-based immuno-PCR approach, wherein OS-IA and quantitative PCR methodologies were combined with the use of immobilized V(L) fusion protein and filamentous phages displaying V(H) fragment, whose DNA was extracted for PCR amplification. This approach significantly enhanced the assay sensitivity for small molecule antigens osteocalcin (BGP) peptide and 17beta-estradiol.

Micro open-sandwich ELISA to rapidly evaluate thyroid hormone concentration from serum samples

Background: Thyroxine (T4) is the most commonly measured thyroid hormone for the diagnosis of thyroid function. To elucidate a rapid and sensitive assay for T4, we made a microfluidics-based noncompetitive immunodetection chip system using anti-T4 antibody fragments obtained from a phage display library. Results: Based on the open-sandwich ELISA principle that detects antigen-dependency of the interaction between the two antibody variable regions VH and VL, we could detect less than 1 ng/ml of T4. The assay was also successfully applied to evaluate total T4 concentration in the serum of healthy individuals. Conclusion: This would be the first micro open-sandwich ELISA constructed with antibody fragments directly selected from immunized mice. The system will be applied to the sensitive detection of many diagnostic markers.

Open-sandwich enzyme immunoassay for one-step noncompetitive detection of corticosteroid 11-deoxycortisol

A noncompetitive immunoassay has the potential for improved sensitivity and working range compared with corresponding competitive assays. However, monovalent antigens with less than 1000 in molecular weight are not susceptible to sandwich assays due to their small size. As a noncompetitive immunoassay that can be performed with a clone of an antibody, an open-sandwich immunoassay (OS-IA) based on the antigen-dependent stabilization of the antibody variable region (V(H) + V(L)) was applied to the quantification of 11-deoxycortisol (11-DC; M(r) 346.5), a corticosteroid serving as a diagnostic index for pituitary-adrenal function, as a model target hapten. By one step OS-IA detection of enzyme-labeled V(H) fragment bound to immobilized V(L) in the presence of sample in microplate wells, 11-DC was measured with a femtomolar detection limit and the working range was wider than that with corresponding competitive assay. In addition, the selectivity against analogues was found almost identical to that of conventional assays. The effect of the mutagenesis of a V(H) residue at the V(H)/V(L) interface to reduce background signal was also shown, implying the wider application of OS-IA in small molecule analyses.

Construction of bifunctional phage display for biological analysis and immunoassay

A phage display-based bifunctional display system was developed for simple and sensitive immunoassay. The resulting bifunctional phage could simultaneously display a few single-chain variable fragment (ScFv) and many copies of the gold-binding peptide on its surface, thereby mediating antigen recognition and signal amplification. As a demonstration study, it was possible for bifunctional phage-based immunoassay to identify Bacillus anthracis spores from other Bacillus strains with detection sensitivity 10-fold higher than that of conventional phage enzyme-linked immunosorbent assay (ELISA). This protocol may be applied to build other bifunctional phage clones for broad applications (e.g., immunoassay kits, affinity biosensors, biorecognition assays).