Use of genetically prepared enzyme conjugates in enzyme immunoassay

Enhancing the detection of Toxoplasma gondii via an anti-SAG1 scFv-alkaline phosphatase immunoconjugate

The purpose of this study was to implement a fluorometric method for enhancing the detection sensitivity of Toxoplasma gondii in biological fluids. To address this challenge, we designed and produced a recombinant immunoconjugate tool based on a single-chain antibody fragment anti-T. gondii SAG1 antigen (scFvSG15) genetically fused to the bacterial alkaline phosphatase (AP) using 4-methyl-umbelliferyl-phosphate as fluorogenic substrate. The anti-SAG1 scFv-AP conjugate was fully bifunctional and was used successfully in different assays including immunoblot, fluorometric ELISA and direct immunofluorescence. The fluorometric immunoassay afforded an extremely low detection limit (1 tachyzoite/well), which was in agreement with the real-time PCR control test. The immunofluorescence imaging has provided captivating visual evidence of T. gondii detection. These results strongly suggest that the recombinant anti-SAG1-AP conjugate generated here might serve as useful and highly sensitive immunoassay probe to direct detect T. gondii in a one-step procedure, opening up new perspectives for diagnosis of toxoplasmosis.

A Simple and Specific Noncompetitive ELISA Method for HT-2 Toxin Detection

We developed an HT-2 toxin-specific simple ELISA format with a positive read-out. The assay is based on an anti-immune complex (IC) scFv antibody fragment, which is genetically fused with alkaline phosphatase (AP). The anti-IC antibody specifically recognizes the IC between a primary anti-HT-2 toxin Fab fragment and an HT-2 toxin molecule. In the IC ELISA format, the sample is added together with the scFv-AP antibody to the ELISA plate coated with the primary antibody. After 15 min of incubation and a washing step, the ELISA response is read. A competitive ELISA including only the primary antibody recognizes both HT-2 and T-2 toxins. The anti-IC antibody makes the assay specific for HT-2 toxin, and the IC ELISA is over 10 times more sensitive compared to the competitive assay. Three different naturally contaminated matrices: wheat, barley and oats, were used to evaluate the assay performance with real samples. The corresponding limits of detection were 0.3 ng/mL (13 µg/kg), 0.1 ng/mL (4 µg/kg) and 0.3 ng/mL (16 µg/kg), respectively. The IC ELISA can be used for screening HT-2 toxin specifically and in relevant concentration ranges from all three tested grain matrices.

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.

A semi-quantitative serological method to assess the potency of inactivated rabies vaccine for veterinary use

Potency is one of the most important indexes of inactivated vaccines. A number of methods have been established to assay the potency, of which the NIH test and single-dose mouse protection test are the "prescribed methods". Here, we report a method to semi-quantitatively assay the potency of an inactivated rabies vaccine, which uses fewer animals and takes less time to complete. Depending on the quality requirements of a vaccine (e.g. minimum potency), a rabies reference vaccine is, for example, diluted to the minimum potency, and 50 μL of the dilution is taken to inoculate 10 mice. The same amount of the test rabies vaccine is inoculated into another 10 mice. After two weeks, all mice are bled and serum samples are assayed for viral neutralizing antibody by the fluorescent antibody virus neutralization (FAVN) test. By comparing the median and interquartile range of antibody titers of the reference vaccine with those of the test vaccine, the test vaccine potency can be semi-quantitatively judged as to whether it is in accord with the required quality. The reliability of this method was also confirmed in dogs. The procedure can be recommended for batch potency testing during inactivated rabies vaccine production.

Recombinant diabody-based immunocapture enzyme-linked immunosorbent assay for quantification of rabies virus glycoprotein

The potency of rabies vaccines, determined using the NIH mouse protection test, can be directly correlated to the amount of rabies virus glycoprotein (RV GP) present in the vaccine. In an effort to develop a simple and sensitive enzyme-linked immunosorbent assay (ELISA) using recombinant diabody for quantification of RV GP, the variable heavy (V(H)) and light chain (V(L)) domains of an RV GP-specific human monoclonal antibody (MAb) secreted by a human x mouse heterohybridoma (human MAb R16E5) was amplified, linked using splicing by overlap extension PCR (SOE PCR), and expressed as a recombinant diabody (D06) in the pET28a bacterial expression system. The diabody D06 was purified by immobilized metal affinity chromatography on a nickel-nitrilotriacetic acid (NTA) agarose column and characterized. The purified diabody was used in combination with a well-characterized RV GP-specific mouse MAb, M5B4, to develop an immunocapture ELISA (IC-ELISA) for the quantification of RV GP in human rabies vaccine preparations. The maximum detection limit of the IC-ELISA using the M5B4-D06 combination was up to 31.25 ng/ml of RV GP. The specificity of the diabody was established by its nonreactivity toward other human viral antigens as determined by ELISA and toward RV GP as determined by immunoblot transfer assay and competitive ELISA with the parent human MAb R16E5 and MAb M5B4. The adjusted r(2) value obtained by the regression through the origin model was 0.902, and the equation for predicted potency values for M5B4-D06-based IC-ELISA and MAb M5B4 IC-ELISA were 0.5651x and 0.8044x, respectively, where x is the estimate of RV GP from the IC-ELISA in micrograms. Analysis of variance (ANOVA) results showed the estimates of the two methods differed significantly (P < 0.001), while the predicted potencies by the two tests did not differ significantly (P > 0.05). The IC-ELISA can be readily adapted to measure the RV GP content in purified antigen, and a vaccine can be formulated based on the estimated GP.

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.

Barnase-barstar high affinity interaction phenomenon as the base for the heterogenous bioluminescence pseudorabies virus' immunoassay

The effective new variant of "sandwich" bioluminescent enzyme immunoassay (BEIA) for the sensitive detection of glycoprotein B (gB) of pseudorabies virus (PrV) was presently developed. The high affinity interaction of barnase-barstar protein pair and photoprotein obelin as bioluminescent marker were for the first time successfully applied to BEIA development. Preliminary the two monoclonal antibodies, 11/5 and 34/2, were raised against gB for ELISA PrV detection. Presently we used the same immuno-"sandwich" principle for BEIA. To do this the two different bioconjugates were elaborated. Recombinant barnase was chemically conjugated with monoclonal anti-PrV's gB IgG, and also barstar was fused in frame to obelin. The characteristics of BEIA method have been compared to ELISA PrV detection. We have shown the proposed here gB-BEIA was 40-fold more sensitive as opposed to gB-ELISA test. The construction might have a broad promise in multiple potential immunological applications.

Affibody-beta-galactosidase immunoconjugates produced as soluble fusion proteins in the Escherichia coli cytosol

Recombinant immunoconjugates constitute a novel class of immunoassay reagents produced by genetic fusion between an antigen recognizing moiety and a reporter enzyme or fluorescent protein, obviating the need for chemical coupling. In this work, we describe the construction, Escherichia coli production and characterization of recombinant beta-galactosidase (beta-gal)-based immunoconjugates directed to human immunoglobulin A (IgA). As the antigen recognizing moieties, either monovalent or dimeric (head-to-tail) versions of an IgA-specific affibody (Z(IgA1)) were used, previously selected in vitro from a protein library based on combinatorial engineering of a single staphylococcal protein A domain. To increase the likelihood of proper presentation on the assembled homotetrameric enzyme surface, the affibody moieties were linked to the N-terminus of the enzyme subunits via a heptapeptide linker sequence. The two resulting immunoconjugates Z(IgA1)-beta-gal and (Z(IgA1))(2)-beta-gal, containing four and eight affibody moieties per enzyme, respectively, could be expressed as soluble and proteolytically stable proteins intracellularly in E. coli from where they were purified to high purity by a single anion exchange chromatography step. The yields of immunoconjugates were in the range 200-400 mg/l culture. Biosensor-binding studies showed that both the Z(IgA1)-beta-gal and (Z(IgA1))(2)-beta-gal immunoconjugates were capable of selective IgA-recognition, but with an apparent higher binding affinity for the variant containing divalent affibody moieties, presumably due to avidity effects. The applicability of this class of recombinant immunoconjugates was demonstrated by IgA detection in enzyme-linked immunosorbent assay (ELISA) and dot-blot analyses. In addition, using human kidney biopsy samples from a nephropathy patient, IgA depositions in glomeruli could be detected by immunohistochemistry with low background staining of tissue.

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.

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.

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.

A bifunctional murine::human chimeric antibody with one antigen-binding arm replaced by bacterial beta-lactamase

We here report the genetic engineering of a murine::human chimeric antibody--directed against the tumor marker human placental alkaline phosphatase--in which one antigen-binding arm (Fab) has been replaced by Escherichia coli beta-lactamase (Bla). A mutated Bla gene in which the termination codon had been replaced by GAG, was fused in-phase to the cDNA sequence encoding the hinge region, CH2 and CH3 of the human IgG3 heavy chain. The resulting BlaHG3f fusion gene was placed under control of the Simian Virus 40 late promoter, and transiently expressed in COS-1 cells together with the genes encoding the murine light and murine::human chimeric heavy chains. Approximately 200 ng/ml of correctly assembled bifunctional antibody-Bla immunoconjugates were detected in the culture supernatant. This observation indicates that Bla (with its own leader peptide) can efficiently direct secretion into the culture medium of adventitious sequences fused at its C-terminus. Furthermore, the assembly in the Fc region was not affected by steric hindrance due to a Bla moiety and an Fab arm in close proximity. The antibody-Bla immunoconjugate could be of therapeutic value for the activation of cephalosporin-based anti-cancer prodrugs at the tumor site. Moreover, the expression strategy adopted here is particularly suitable for a quick and convenient analysis of newly designed gene products in which the Bla moiety has been replaced by other enzymes or by antigen-binding fragments in order to engineer bispecific antibodies.