Recombinant antibody-alkaline phosphatase conjugates for diagnosis of human IgGs: application to anti-HBsAg detection

Fabrication of Fragment Antibody–Enzyme Complex as a Sensing Element for Immunosensing

Antibody–enzyme complexes (AECs) are ideal molecular recognition elements for immunosensing applications. One molecule possesses both a binding ability to specific targets and catalytic activity to gain signals, particularly oxidoreductases, which can be integrated into rapid and sensitive electrochemical measurements. The development of AECs using fragment antibodies rather than intact antibodies, such as immunoglobulin G (IgG), has attracted attention for overcoming the ethical and cost issues associated with the production of intact antibodies. Conventionally, chemical conjugation has been used to fabricate AECs; however, controlling stoichiometric conjugation using this method is difficult. To prepare homogeneous AECs, methods based on direct fusion and enzymatic conjugation have been developed, and more convenient methods using Catcher/Tag systems as coupling modules have been reported. In this review, we summarize the methods for fabricating AECs using fragment antibodies developed for sensing applications and discuss the advantages and disadvantages of each method.

Screening Antibody Libraries with Colony Assay Using scFv-Alkaline Phosphatase Fusion Proteins

Screening antibody libraries is an important step in establishing recombinant monoclonal antibodies. The colony assay can identify positive clones without almost any false-positives; however, its antibody library is smaller than those used in other recombinant screening methods such as phage display. Thus, to improve the efficiency of colony assays, it is necessary to increase library size per screening. Here, we report developing a colony assay with single-chain variable fragment (scFv) fused to the N-terminus of bacterial alkaline phosphatase (scFv-PhoA). The scFv-PhoA library was constructed in an expression vector specifically designed for this study. Use of this library allowed the successful and direct detection of positive clones exhibiting PhoA activity, without the need for a secondary antibody. Colony assay screening with scFv-PhoA is simple, rapid, offers a higher success rate than previous methods based on scFv libraries, and—most importantly—it enables high-throughput procedures.

Zipbodyzyme: Development of new antibody-enzyme fusion proteins

A new antibody-enzyme fusion protein, named Zipbodyzyme, composed of a Fab antibody (i.e., an antigen-binding fragment of an antibody) and an enzyme, has been successfully produced in the cytoplasm of Escherichia coli. Zipbodyzymes have a leucine zipper (LZ) pair at the C-termini of the heavy chain (Hc) and the light chain (Lc) of Fab, to promote the association of the Hc and the Lc in E. coli cytoplasm, adjoining a fused enzyme. A Zipbodyzyme containing mouse-derived anti-E. coli O157 Fab and a luciferase or a green fluorescent protein retained both the antigen-binding and an enzymatic activity/fluorescence. The bifunctional proteins were applicable in direct enzyme-linked immunosorbent assay (ELISA) without the need for a secondary antibody, indicating that the utility of the antibody enzyme bifunctional fusion protein will be expanded.

Development of an Immunoassay for Chloramphenicol Based on the Preparation of a Specific Single-Chain Variable Fragment Antibody

Specific antibodies are essential for the immune detection of small molecule contaminants. In the present study, the heavy and light variable regions (V(H )and V(L)) of the immunoglobulin genes from a hybridoma secreting a chloramphenicol (CAP)-specific monoclonal antibody (mAb) were cloned and sequenced. In addition, the light and heavy chains obtained from the monoclonal antibody were separated using SDS-PAGE and analyzed using Orbitrap mass spectrometry. The results of DNA sequencing and mass spectrometry analysis were compared, and the V(H) and V(L) chains specific for CAP were determined and used to construct a single-chain variable fragment (scFv). This fragment was recombinantly expressed as a soluble scFv-alkaline phosphatase fusion protein and used to develop a direct competitive ELISA. Compared with the parent mAb, scFv exhibits lower sensitivity but better food matrix resistance. This work highlights the application of engineered antibodies for CAP detection.

Mutants with higher stability and specific activity from a single thermosensitive variant of T7 RNA polymerase

A single strategy to select RNA polymerase from bacteriophage T7 (T7 RNAP) mutants in Escherichia coli with enhanced thermostability or enzymatic activity is described. T7 RNAP has the ability to specifically transcribe genes under control of T7 phage promoter. By using random mutagenesis of the T7 RNAP gene in combination with an appropriate screening at 25 and 42°C, we have generated and selected E.coli clones with temperature-sensitive phenotype in the presence of chloramphenicol. The resistance to chloramphenicol used to select these clones results from expression control of the chloramphenicol acetyl transferase gene by the T7 promoter. In a second phase, and using the thermosensitive T7 RNAP variants as template, a new round of random mutagenesis was performed. Combined to an appropriate screening strategy, 11 mutations (second-site T7 RNAP revertants) that restore the initial resistance to chloramphenicol at 42°C were identified. Nine of these mutations increase the thermal resistance of the wild-type T7 RNA. They include the five mutations previously described using different approaches and four novel mutations. One improves T7 RNA catalytic activity and one has no positive effect on the natural enzyme but increases the activity of some combined mutants. Additive effects of mutations amount to an increase of as much as 10°C in T1/2 compared with the wild-type enzyme and up to a 2-fold activity enhancement.

Towards proteome scale antibody selections using phage display

In vitro antibody generation by panning a large universal gene library with phage display was employed to generate antibodies to more than 60 different antigens. Of particular interest was a comparison of pannings on 20 different SH2 domains provided by the Structural Genomics Consortium (SGC). Streamlined methods for high throughput antibody generation developed within the 'Antibody Factory' of the German National Genome Research Network (NGFN) were demonstrated to minimise effort and provide a reliable and robust source for antibodies. For the SH2 domains, in two successive series of selections, 2668 clones were analysed, resulting in 347 primary hits in ELISA. Half of these hits were further analysed, and more than 90 different scFv antibodies to all antigens were identified. The validation of selected antibodies by cross-reactivity ELISA, western blot and on protein microarrays demonstrated the versatility of the in vitro antibody selection pipeline to generate a renewable resource of highly specific monoclonal binders in proteome scale numbers with substantially reduced effort and time.

Efficient production of single-chain Fv antibody possessing rare codon linkers in fed-batch fermentation

Single-chain Fv antibody (scFv) having 2 types of polypeptide linkers with or without rare codons, namely scFv (G(4)S)(3)(R) and scFv No.10 (with rare codons) and scFv (G(4)S)(3) and scFv No.10(NR) (without rare codon), were expressed under controllable conditions in batch and fed-batch fermentation, in order to compare volumetric productivity and specific productivity levels of scFvs as a soluble form. In batch fermentation, volumetric productivity levels of scFv (G(4)S)(3)(R) and scFv No.10, namely scFvs having the rare codon linkers were 3-5 times higher than those of scFvs that had linkers without the rare codon. In fed-batch fermentation controlled by an exponential feeding system, the cell concentrations of the transformants increased with similar specific growth rates (0.1 h(-1)), while the specific productivity levels of scFvs with the rare codon linkers were 1.6 times higher than those of scFvs without the rare codon linkers. These results indicate that the presence of several rare codons in the gene of a polypeptide linker increases soluble amount of scFvs. This might be caused by a temporary decrease in translation speed at the position of the polypeptide linker allowing time for the folding of the V(H) domain and avoiding unfavorable interactions between amino acid residues at the unfolded V(H) and V(L) domains. Higher specific productivity levels of both scFv No. 10 and scFv No. 10(NR) than those of scFv (G(4)S)(3)(R) and (G(4)S)(3) might be caused by difference in stability of the polypeptide linkers on the basis of amino acid sequences. Thus, the rare codon linkers tested in this study will be considerably useful for large-scale production of soluble and active scFvs in fed-batch or continuous fermentations, in which high cell activity can be maintained.

Recombinant single-chain Fv antibody fragment–alkaline phosphatase conjugate: A novel in vitro tool to estimate rabies viral glycoprotein antigen in vaccine manufacture

The purpose of this study was to design a novel in vitro tool by using recombinant protein technology to qualify the whole reagent preparation procedure, to be used to quantify rabies viral antigen preparation in a simple and rapid format for potency control of rabies vaccines. 50AD1 is a neutralizing monoclonal antibody directed against the rabies virus glycoprotein that binds to native conformational antigenic site III. In the present study, the DNA fragments encoding the variable domains of 50AD1 were inserted into a prokaryotic expression vector so as to produce a single-chain Fv antibody fragment (scFv) genetically fused to the bacterial alkaline phosphatase (AP). The recombinant fusion protein preserved both the AP enzymatic activity and the antigen-binding activity against the rabies virus glycoprotein nearly identical to the parental antibody, and was used successfully in different assays including ELISA, dot-blot and cell culture tests. The present study shows that the genetic fusion protein provides a new tool for one-step rabies virus immunodetection, which can be produced in homogeneous bifunctional reagent, easily, quickly and reproducibly. In addition, this recombinant immunoconjugate is a promising alternative reagent for applications involving immunodetection, it presents a similar sensitivity and specificity to that obtained with classical reagents.

Immunoenzymometric Assay for a Small Molecule,11-Deoxycortisol, with Attomole-Range Sensitivity Employing an scFv−Enzyme Fusion Protein and Anti-Idiotype Antibodies

To overcome the sensitivity limit in immunoassays for small molecules (haptens), we established a noncompetitive immunoenzymometric assay (IEMA) format that can detect attomole-range hapten molecules. We selected 11-deoxycortisol (11-DC; Mr 346.5), a corticosteroid serving a diagnostic index for pituitary-adrenal function, as a model target hapten. A fusion of a single-chain Fv fragment (scFv) specific for 11-DC and alkaline phosphatase (ALP) was generated for use as an enzyme-labeled antibody, instead of the conventional chemically linked enzyme-antibody conjugates. After binding reaction of 11-DC and fixed amounts of the fusion protein (scFv-ALP), the unbound fusion protein was removed by incubation with a mouse beta-type anti-idiotype antibody recognizing the scFv paratope. These complexes were captured by magnetic separation using anti-mouse IgG antibody-coated magnetic beads. Following magnetic sedimentation of the beads, immune complexes of scFv-ALP and 11-DC remained in the supernatant were further purified by capture on microtiter plates with immobilized alpha-type anti-idiotype antibody. As measured fluorometrically, ALP activity from bound immune complexes on the plates increased with increasing 11-DC, which is characteristic of a noncompetitive relationship. This IEMA afforded an extremely low detection limit (20 amol/assay), a very wide measurable range, and practical specificity. The plasma 11-DC levels determined for healthy subjects were validated as reliable.

Expression System for Enhanced Green Fluorescence Protein Conjugated Recombinant Antibody Fragment

Recent development of recombinant antibody technology has enabled fusion of recombinant antibody fragment with fluorescent proteins for various applications such as flow cytometry, fluorescence immunoassay, and fluorescent microscopy. In this study, we generated various forms of green fluorescence protein (EGFP)-fused anti-c-Met antibody fragment. Among these fusion proteins, EGFP fusion to the light chain showed high expression in a soluble form of protein in E. coli, and high binding activity to c-Met. A feasibility of the constructs was further examined by replacing the Fab gene by a Fab library of catalytic subunit of protein kinase A (PKA) to construct the Fab library in EGFP fused form. We also constructed the conventional Fab library. After a series of biopanning, we found that the binding capability of EGFP-anti-PKA Fab was comparable with anti-PKA Fab. Sequence analysis of the selected clones showed > or =99% identity in amino acid sequence and shared the same CDR sequence. These results demonstrate that EGFP fusion to the light chain using our vector system does not influence the selection of reactive Fab and that this vector system is useful for EGFP fusion to Fab to develop a one-step detection system.

Binding proteins from alternative scaffolds

The use of so-called protein scaffolds for the generation of novel binding proteins via combinatorial engineering has recently emerged as a powerful alternative to natural or recombinant antibodies. This concept requires an extraordinary stable protein architecture tolerating multiple substitutions or insertions at the primary structural level. With respect to broader applicability it should involve a type of polypeptide fold which is observed in differing natural contexts and with distinct biochemical functions, so that it is likely to be adaptable to novel molecular recognition purposes. The quickly growing number of approaches can be classified into three groups: carrier proteins for the display of single variegated loops, scaffolds providing rigid elements of secondary structure, and protein frameworks supporting a group of conformationally variable loops in a fixed spatial arrangement. Generally, such artificial receptor proteins should be based on monomeric and small polypeptides that are robust, easily engineered, and efficiently produced in inexpensive prokaryotic expression systems. Today, progress in protein library technology allows for the parallel development of immunoglobulin (Ig) as well as scaffold-based affinity reagents. Both biomolecular tools have the potential to complement each other, thus expanding the possibility to find an affinity reagent suitable for a given application. The repertoire of protein scaffolds hitherto recruited for combinatorial protein engineering purposes will probably be further expanded in the future, including both additional natural proteins and de novo designed proteins, contributing to the collection of libraries available at present. In this review both the structural features and the practical use of scaffold proteins will be discussed and exemplified.

Bacterial expression of a human monoclonal antibody-alkaline phosphatase conjugate specific for Entamoeba histolytica

We previously produced human monoclonal antibody Fab fragments specific to Entamoeba histolytica in Escherichia coli. In order to use these Fab fragments for diagnostic purposes, an expression vector to produce a fusion protein of Fab and alkaline phosphatase (PhoA) in E. coli was designed and constructed. The E. coli PhoA gene was fused to the 3' terminus of the gene encoding the heavy-chain Fd region. The kappa and Fd genes from a previously prepared antibody clone, CP33, which is specific for the 260-kDa lectin of E. histolytica, were used as human antibody genes. When the fusion protein of CP33 and PhoA was incubated with paraformaldehyde-fixed trophozoites of E. histolytica and developed with a substrate, the trophozoites appeared to be stained. These results demonstrate the feasibility of bacterial expression of a human monoclonal antibody-PhoA conjugate specific for E. histolytica and that the antibody can be used to detect E. histolytica antigen without the use of chemically conjugated secondary antibodies.

Cloning, expression, and characterization of a bi-functional disintegrin/alkaline phosphatase hybrid protein

Integrins are transmembrane heterodimeric glycoproteins responsible for cellular communication; therefore, they play an essential role in many physiological events. Viper snake venoms contain integrin antagonists called disintegrins which bind and inhibit integrin function. They present a loop containing an RGD motif responsible for integrin binding. The engineering of disintegrins fused to a reporter enzyme will be an interesting approach to build integrin markers. Even more, the disintegrin scaffold could be used to present other protein binding motifs. In this work, we have obtained alkaline phosphatase (APv) tagged eristostatin (Er) by cloning and expressing eristostatin DNA into the pLIP6-GN vector. Eristostatin, a 49 residue disintegrin, binds selectively to alphaIIbbeta3 integrin, inhibiting its binding to fibrinogen. The resulting fusion protein Er/APv was identified by SDS-PAGE and by Western blotting using both anti-Er and anti-AP antibodies. This fusion protein showed enzymatic AP activity similar to that of wild APv and its potential use for an alphaIIbbeta3 integrin assay was tested in a one-step dot blot using immobilized cells incubated with the marker and developed by AP substrate. Er/APv showed selectivity towards platelets and alphaIIbbeta3 integrin transfected cells and reacted with the same region as unlabeled Er, as analyzed in competition assays. Our data present a novel tool, Er/APv, with potential use as molecular marker in processes where the alphaIIbbeta3 integrin is involved.

A recombinant scFv/streptavidin-binding peptide fusion protein for the quantitative determination of the scorpion venom neurotoxin AahI

We created a construct encoding a peptide known to mimic the binding properties of biotin fused to the carboxy-terminus of a scFv fragment that binds a scorpion toxin (AahI). This fusion protein was produced in the periplasm of bacteria and purified to homogeneity by single-step affinity chromatography on streptavidin-agarose with a yield close to 1 mg/l. DNA sequencing, dot blot and mass spectrometric analyses demonstrated the integrity of the soluble immunoconjugate. Fusion to the streptavidin-binding peptide did not affect the ability of the scFv to recognize its antigen with a high affinity (Kd = 2.3 x 10(-10) M). Similarly, the streptavidin-binding property was not impaired in the fusion protein. Thus, the immunoconjugate was bifunctional and had a low molecular mass of 28 kDa. This enabled us to develop rapid and sensitive immunoassays for the specific detection of the toxin AahI accurately to 0.6 ng/ml, opening up new perspectives for the diagnosis of envenomations.

Improving Escherichia coli alkaline phosphatase efficacy by additional mutations inside and outside the catalytic pocket

We describe a strategy that allowed us to confer on a bacterial (E. coli) alkaline phosphatase (AP) the high catalytic activity of the mammalian enzyme while maintaining its high thermostability. First, we identified mutations, at positions other than those occupied by essential catalytic residues, which inactivate the bacterial enzyme without destroying its overall conformation. We transferred concomitantly into the bacterial enzyme four residues of the mammalian enzyme, two being in the catalytic pocket and two being outside. Second, the gene encoding the inactive mutant was submitted to random mutagenesis. Enzyme activity was restored upon the single mutation D330N, at a position that is 12 A away from the center of the catalytic pocket. Third, this mutation was combined with other mutations previously reported to increase AP activity slightly in the presence of magnesium. As a result, at pH 10.0 the phosphatase activity of both mutants D330N/D153H and D330N/D153G was 17-fold higher than that of the wild-type AP. Strikingly, although the two individual mutations D153H and D153G destabilize the enzyme, the double mutant D330N/D153G remained highly stable (T(m)=87 degrees C). Moreover, when combining the phosphatase and transferase activities, the catalytic activity of the mutant D330N/D153G increased 40-fold (k(cat)=3200 s-1) relative to that of the wild-type enzyme (k(cat)=80 s-1). Due to the simultaneous increase in K(m), the resulting k(cat)/K(m) value was only increased by a factor of two. Therefore, a single mutation occurring outside a catalytic pocket can dramatically control not only the activity of an enzyme, but also its thermostability. Preliminary crystallographic data of a covalent D330N/D153G enzyme-phosphate complex show that the phosphate group has significantly moved away from the catalytic pocket, relative to its position in the structure of another mutant previously reported.

Functional characterization of two anti-estradiol antibodies as deduced from modelling and site-directed mutagenesis experiments

Monoclonal antibodies are now widely used to measure the concentration of steroid hormones in human serum samples. The great development of molecular engineering techniques over the past 10 years has made possible the improvement of specificity and/or sensitivity of selected antibodies. We have obtained two monoclonal antibodies, 17E12E5 and 10G6D6, using estradiol-6-ethyl methoxy carbonyl (EMC)-bovine serum albumin (BSA) as immunogen. To tentatively improve their affinities for natural estradiol, we have initiated their structural and functional studies. For this purpose, we have cloned and sequenced the genes encoding the variable fragments of each antibody. Single chain variable fragments (scFv) were produced into the periplasmic space of E. coli using the pLIP6 expression vector. Mapping of the functional structures of both antibodies was obtained by combination of modelling and mutational analyses together with cross-reaction studies. The two binding pockets are described and models of estradiol complexed to 17E12E5 and 10G6D6 are proposed.

Construction and functional evaluation of a single-chain antibody fragment that neutralizes toxin AahI from the venom of the scorpion Androctonus australis hector

9C2 is a murine monoclonal IgG that participates in the neutralization of Androctonus australis hector scorpion venom. It recognizes AahI and AahIII, two of the three main neurotoxins responsible for almost all the toxicity of the venom when injected into mammals. Using PCR we cloned the antibody variable region coding genes from 9C2 hybridoma cells and constructed a gene encoding a single-chain antibody variable fragment molecule (scFv). This scFv was produced in the periplasm of Escherichia coli in a soluble and functional form and purified in a single step using protein L-agarose beads yielding 1-2 mg.L(-1) of bacterial culture. scFv9C2 was predominantly monomeric but also tended to form dimeric and oligomeric structures, all capable of binding toxin AahI. The affinity of scFv and the parental mAb for toxin AahI and homologous toxin AahIII was of the same magnitude, in the nanomolar range. Similarly, purified forms of scFv9C2 completely inhibited the binding of toxin AahI to rat brain synaptosomes. Finally, scFv9C2 was efficient in protecting mice against the toxic effects of AahI after injection of the toxin and scFv to mice by the intracerebroventricular route in a molar ratio as low as 0.36 : 1. Thus, we produced a recombinant scFv that reproduces the recognition properties of the parent antibody and neutralizes the scorpion neurotoxin AahI, thereby opening new prospects for the treatment of envenomation.

Development of a functional monoclonal single-chain variable fragment antibody against Venezuelan equine encephalitis virus

We have generated a single-chain variable fragment (ScFv) antibody, from a previously well-characterized monoclonal antibody (MAb) to Venezuelan equine encephalitis (VEE) virus, 5B4D-6. The variable regions of the heavy (V(H)) and light (V(L)) chain antibody genes, were connected by a DNA linker and cloned in the phagemid vector pCANTAB5E. The ScFv clone in Escherichia coli strain TG-1, 5B4D-6-6, was expressed as a approximately 30 kDa ScFv protein and higher molecular weight fusion products which were functional in recognizing VEE virus by enzyme-linked immunosorbent assay (ELISA). Results were reproduced in Escherichia coli strain HB2151, where clone D66 was expressed mainly as soluble periplasmic protein. The D66 ScFv antibody bound VEE virus strongly as determined by ELISA. Nucleotide sequence analysis of 5B4D-6-6 ScFv indicated that the Vkappa gene belonged to family XVI, subgroup V, while the V(H) gene was unique in its sequence, though its amino acid sequence could be subgrouped as IA. The deduced protein sequence of D66 was highly homologous to published murine ScFv protein sequences. This work demonstrates, for the first time, cloning of a functional ScFv antibody against VEE virus.

Identification of mimotope peptides which bind to the mycotoxin deoxynivalenol-specific monoclonal antibody

Monoclonal antibody 6F5 (mAb 6F5), which recognizes the mycotoxin deoxynivalenol (DON) (vomitoxin), was used to select for peptides that mimic the mycotoxin by employing a library of filamentous phages that have random 7-mer peptides on their surfaces. Two phage clones selected from the random peptide phage-displayed library coded for the amino acid sequences SWGPFPF and SWGPLPF. These clones were designated DONPEP.2 and DONPEP.12, respectively. The results of a competitive enzyme-linked immunosorbent assay (ELISA) suggested that the two phage displayed peptides bound to mAb 6F5 specifically at the DON binding site. The amino acid sequence of DONPEP.2 plus a structurally flexible linker at the C terminus (SWGPFPFGGGSC) was synthesized and tested to determine its ability to bind to mAb 6F5. This synthetic peptide (designated peptide C430) and DON competed with each other for mAb 6F5 binding. When translationally fused with bacterial alkaline phosphatase, DONPEP.2 bound specifically to mAb 6F5, while the fusion protein retained alkaline phosphatase activity. The potential of using DONPEP.2 as an immunochemical reagent in a DON immunoassay was evaluated with a DON-spiked wheat extract. When peptide C430 was conjugated to bovine serum albumin, it elicited antibody specific to peptide C430 but not to DON in both mice and rabbits. In an in vitro translation system containing rabbit reticulocyte lysate, synthetic peptide C430 did not inhibit protein synthesis but did show antagonism toward DON-induced protein synthesis inhibition. These data suggest that the peptides selected in this study bind to mAb 6F5 and that peptide C430 binds to ribosomes at the same sites as DON.

Recombinant single-chain Fv antibody fragment-alkaline phosphatase conjugate for one-step immunodetection in molecular hybridization

Using phage-display technology, a recombinant single-chain Fv antibody fragment (scFv) was rapidly generated from the K16-16 hybridoma secreting mouse monoclonal antibody (MAb) that binds to acetylaminofluorene-labeled DNA (AAF-DNA). The selected A4 phage-scFv specifically bound to AAF-DNA. The anti-AAF scFv gene was then recloned into a fusion vector for the production of a hybrid protein comprising the antibody fragment fused to a potent bacterial alkaline phosphatase variant (PhoAv). The anti-AAF scFv-PhoAv hybrid protein was bifunctional and possessed both antigen binding capacity and PhoA activity. The recombinant conjugate was directly used, without further purification, for one-step immunodetection in dot-blot hybridization. The detection limit was identical and the test was quicker than the conventional two-step procedure with the purified anti-AAF MAb revealed with a secondary enzyme-labeled antibody. To assess the value of this new reagent for the immunodetection of genomic nucleic acids, genomic DNAs of Campylobacter jejuni and Campylobacter coli were then one-step immunodetected with non-purified recombinant scFv-PhoAv conjugate in a Southern-blot hybridization experiment. The present study shows that the genetic fusion with PhoAv provides a new tool for immunodetection which presents easier and quicker production and use with the same sensitivity and specificity as classical reagents. The recombinant anti-AAF scFv-PhoAv conjugate is a promising alternative reagent for applications involving the immunodetection of specific DNA or RNA sequences, such as the detection and characterization of microorganisms.

Open sandwich ELISA with V(H)-/V(L)-alkaline phosphatase fusion proteins

The Sandwich ELISA is a widely used technique to measure antigen concentration. Recently, a novel ELISA based on the interchain interaction of separated V(H) and V(L) chains from a single antibody variable region (Fv) was proposed (Open Sandwich ELISA). Since it employs a single antibody recognizing one epitope, the assay requires, in essence, only one cycle of incubation and washing steps. To demonstrate this directly, we have constructed a recombinant gene fusion encoding the V(H) chain of an anti-hen egg lysozyme (HEL) antibody HyHEL-10 and Escherichia coli alkaline phosphatase (V(H)-PhoA). The same type of gene fusion using V(L) chain instead of V(H) chain (V(L)-PhoA) was also constructed and the proteins were obtained with an E. coli expression/secretion system. Open Sandwich ELISAs were performed using microtiter plates with immobilized V(L) or V(H) fragment, and V(H)-PhoA or V(L)-PhoA, respectively, as the detection reagent which was simultaneously added to each well with samples. As a result, HEL concentrations in the samples were determined after one round of incubation and washing steps, with a signal generated in a direct relationship to the concentration of HEL added to the reaction mixture. The minimum detectable HEL concentration was approximately 10 ng/ml, which was almost equal to the value previously obtained with plate-immobilized V(L) and V(H) fragment displayed on M13 phage. When the active-site mutant V(H)-PhoA(D101S) was employed instead of V(H)-PhoA and reacted at an optimum pH of 10, a significant enhancement in signal was attained.

Development of a streptavidin-conjugated single-chain antibody that binds Bacillus cereus spores

Control of microorganisms such as Bacillus cereus spores is critical to ensure the safety and a long shelf life of foods. A bifunctional single chain antibody has been developed for detection and binding of B. cereus T spores. The genes that encode B. cereus T spore single-chain antibody and streptavidin were connected for use in immunoassays and immobilization of the recombinant antibodies. A truncated streptavidin, which is smaller than but has biotin binding ability similar to that of streptavidin, was used as the affinity domain because of its high and specific affinity with biotin. The fusion protein gene was expressed in Escherichia coli BL21 (DE3) with the T7 RNA polymerase-T7 promoter expression system. Immunoblotting revealed an antigen specificity similar to that of its parent native monoclonal antibody. The single-chain antibody-streptavidin fusion protein can be used in an immunoassay of B. cereus spores by applying a biotinylated enzyme detection system. The recombinant antibodies were immobilized on biotinylated magnetic beads by taking advantage of the strong biotin-streptavidin affinity. Various liquids were artificially contaminated with 5 x 10(4) B. cereus spores per ml. Greater than 90% of the B. cereus spores in phosphate buffer or 37% of the spores in whole milk were tightly bound and removed from the liquid phase by the immunomagnetic beads.

Construction and expression of a bifunctional single-chain antibody against Bacillus cereus p6ores

The variable-region genes of monoclonal antibody against Bacillus cereus spores were cloned from mouse hybridoma cells by reverse transcription-PCR. The heavy- and light-chain variable-region genes were connected by a 45-base linker DNA to allow folding of the fusion protein into a functional tertiary structure. For detection of protein expression, a 10-amino-acid strep tag (biotin-like peptide) was attached to the C terminus of recombinant antibody as the reporter peptide. The single-chain antibody construct was inserted into the expression vector and expressed in Escherichia coli under the control of the T7 RNA polymerase-T7 promoter expression system. The expressed single-chain antibody was detected on Western blots by using a streptavidin-conjugated enzyme system. This small recombinant antibody fragment (ca. 28,000 Da by calculation) had B. cereus spore binding ability and antigen specificity similar to those of its parent native monoclonal antibody.

Anti-digoxin scFv fragments expressed in bacteria and in insect cells have different antigen binding properties

A gene encoding a single-chain antibody fragment directed against digoxin (named 1C10 scFv) was cloned in two expression systems. For this purpose, a new baculovirus transfer cassette fully compatible with the procaryotic pHEN vector was constructed. Baculovirus production led to higher yield than did Escherichia coli expression. The procaryotic fragment showed variations in the fine specificity profile but an affinity constant nearly identical to that of the 1C10 Fab, whereas the eucaryotic scFv fragment had a lower affinity with a specificity profile identical to original mAb. The half-lives of the digoxin:scFv complexes and the global specificity are compatible with therapeutic use of this antibody fragment.

The functional architecture of an acetylcholine receptor-mimicking antibody

Malpha2-3 is a monoclonal antibody that partially mimics the nicotinic acetylcholine receptor (AChR). Its three-dimensional structure has been previously predicted by molecular modeling, suggesting that 29 complementarity determining region (CDR) residues and 2 framework residues are exposed to solvent. To identify the antibody residues that bind to the antigen, i.e. snake toxin that binds specifically to AChR, we (i) produced the scFv form of Malpha2-3 fused to alkaline phosphatase, in the periplasmic space of Escherichia coli; (ii) submitted approximately 75% of exposed residues of the fused scFv to individual or combined mutations, and (iii) identified the residues whose mutations affect scFv binding to the toxin, using a sensitive enzyme-linked immunosorbent assay. 11 critical residues were identified, including 8 heavy chain residues, 2 framework residues, and 1 light chain residue. They cover a surface of approximately 800 A2, with a subset of most critical residues (VHD31, VHY32, and VHG101) and several aromatic residues. This functional architecture not only constitutes a plausible complementary binding surface for the snake toxin but also offers a structural basis to ultimately understand the capacity of the antibody to partially mimic AChR.

Enzyme immunoassays using bispecific diabodies

BACKGROUND

Bispecific antibodies with a first binding specificity to a target antigen and a second to an enzyme have great potential in enzyme immunoassays. As bispecific antibodies are difficult to make, the use of recombinant bispecific antibody fragments may provide a breakthrough.

OBJECTIVES

To make bispecific antibody fragments directed against an enzyme and to demonstrate their application in enzyme immunoassays.

STUDY DESIGN

Bispecific antibody fragments were assembled as diabodies (Holliger P., Prospero T., Winter G. Proc. Natl. Acad. Sci. USA 90, 1993, 6444-6448) directed to an enzyme, E. coli beta-galactosidase, and to each of three target antigens, hen-egg lysozyme (HEL), carcinoembryonic antigen (CEA), and HIV gpl20 (HIV). The diabodies were then evaluated in immunoassays.

RESULTS

The HEL diabody was shown to recruit beta-galactosidase in a microtiter plate immunoassay in which diabody and enzyme were co-incubated with antigen, washed and enzyme substrate added. The CEA diabody was shown to detect CEA by immunocytochemical staining of transfected, CEA-expressing HeLa cells and of adenocarcinoma colon tissue sections, and the HIV diabody to detect gpl20 in immunoblots of total cell extracts.

CONCLUSION

The results illustrate the diagnostic potential of diabodies in enzyme immunoassays.

Examining the specificity of Src homology 3 domain--ligand interactions with alkaline phosphatase fusion proteins

Sixteen-amino-acid-long peptides, corresponding to the optimal ligand preferences of the Src homology 3 (SH3) domains of Abl, Cortactin, Crk, p53BP2, and Src, were fused to the N-terminus of Escherichia coli alkaline phosphatase (AP). These secreted fusion proteins have been used as one-step detection probes of peptide ligand-SH3 domain interactions on microtiter plates and membranes. The binding of both the class I and II SH3 ligand-AP fusion proteins to their targets is robust and specific in comparison to chemically synthesized biotinylated peptides, used either in monovalent or tetravalent formats. p53BP2 and Cortactin SH3 ligand-AP fusions have been used to screen a mouse embryo lambda cDNA expression library and resulted in the cloning of p53BP2 and several known proteins with SH3 domains similar to that of Cortactin, respectively. In addition, the approximately 60-amino-acid-long SH3 domains of Src and Abl were fused to AP and the resulting fusion proteins were found to bind specifically to their respective peptide ligands in microtiter plates and proteins containing proline-rich regions in screens of a lambda cDNA expression library. Thus, SH3 peptide ligand- and SH3 domain-AP fusion proteins are convenient and sensitive reagents for examining the specificity of SH3 domain-ligand interactions, identifying potentially interacting proteins, and establishing high-throughput screens of combinatorial chemical libraries.

Design and expression of a stable bispecific scFv dimer with affinity for both glycophorin and N9 neuraminidase

We have designed and produced a stable bispecific scFv dimer (bisFv) by non-covalent association of two hybrid VH-VL pairs derived from an anti-neuraminidase antibody (NC10) and an anti-glycophorin antibody (1C3). The bisFv dimer was demonstrated to have binding activity to the two respective target antigens and was evaluated as a reagent for rapid whole blood agglutination assays. The bisFv was expressed in the periplasm of Escherichia coli, from a secretion vector which comprised two cistrons in tandem under the control of a single lac promoter, inducible with IPTG. Each cistron encoded one of the hybrid VH-VL pairs, with V domains separated by a linker region encoding the five amino acids, Gly4Ser. The short linker region was designed to prevent association of VH and VL regions of the same molecule and favour the formation of dimers. The protein synthesized from each hybrid scFv cistron was directed to the E. coli periplasm by the inclusion of distinctive signal secretion sequences preceding each hybrid gene; from pel B of Erwinia cartovora and from gene III of fd phage. The bisFv was affinity-purified from culture supernatants via the C-terminal tag epitope FLAG and was shown, by FPLC on a Superose 6 column, to be consistent in size with that of a scFv dimer. The bisFv was stable for more than 4 months at 4 degrees C and was shown by BIAcore analysis to bind to either target antigen, human glycophorin, or tern N9 neuraminidase. Simultaneous binding to both target antigens was demonstrated when a pre-formed bisFv-neuraminidase complex was shown to bind to immobilized glycophorin. In whole blood agglutination assays, the bisFv dimer was able to agglutinate red blood cells when crosslinked with an anti-idiotype antibody (3-2G12) binding to the NC10 combining site, but no agglutination occurred on binding the antigen neuraminidase. These results are a function of the topology of the epitopes on neuraminidase and have implications for the use of relatively rigid bifunctional molecules (as bisFv dimers) to cross link two large membrane-anchored moieties, in this case, red blood cell glycophorin and neuraminidase, an M(r) 190,000 tetramer.

Engineering of a recombinant colorimetric fusion protein for immunodiagnosis of insulin

A synthetic DNA encoding human proinsulin was inserted in frame in the bacterial alkaline phosphatase gene. A homogeneous recombinant human proinsulin-alkaline phosphatase conjugate was obtained directly from the periplasm of Escherichia coli transformed with a plasmid carrying the hybrid gene. The recombinant conjugate was stable and could be produced in the bacteria. The immunological properties of the recombinant conjugate and those of the human insulin and human proinsulin were compared using a panel of six different human insulin-specific monoclonal antibodies. Three immunological groups were thus distinguished and one of them indiscriminately recognized all of the insulin-like molecules. One monoclonal antibody from this group was used in combination with the recombinant conjugate to develop successfully a competitive immunoenzymatic assay for detecting insulin.

Advances in immunoassay technology

Major developments continue to be reported in key areas of immunoassay technology. Following the development of excellent signal generation methods, attention has shifted to the development of immunochemical methods and instrumentation to provide convenient systems of high performance. Important advances have been made in the design of immunochemical approaches that permit the replacement of competitive format assays for small molecules, such as drugs, metabolites and pollutants, with non-competitive formats, bringing advantages previously seen only with large molecular analytes. Bispecific antibodies and recombinant proteins are also beginning to impact immunodiagnostics, with the promise of even more highly specified reagents. Improvements in automation have brought the facility of homogeneous systems to high-throughput and high-performance heterogeneous systems. Similarly, 'point of need' testing continues to progress. Through all of these advances, systems are evolving according to the needs of users in terms of operator convenience, accuracy, specificity, speed, robustness, and sensitivity.