Recombinant fusion proteins for haemagglutination-based rapid detection of antibodies to HIV in whole blood

Recombinant fusion proteins, consisting of a monovalent anti-human RBC monoclonal antibody B6, and conserved immunodominant peptide of HIV-1 envelope glycoprotein gp41 or HIV-2 envelope glycoprotein gp36, have been designed and purified after over-expression in E. coli. These fusion proteins are Fab-based and were obtained by assembling the light chain with Fd (variable domain and the first constant domain of the heavy chain) or Fd fusions containing HIV-derived peptide, and following a protocol of in vitro denaturation of inclusion bodies and subsequent renaturation to assemble functional Fab. Using a multistep column chromatographic procedure, monomeric Fab and Fab fusion proteins containing HIV-derived peptide were purified to high degree, free of aggregates. The yield of various proteins on the laboratory scale (1-2 l of shake flask culture) was in the range of tens of milligram. Purified anti-human RBC Fab fusion proteins containing sequences derived from HIV-1 gp41 and HIV-2 gp36 were highly specific for detection of antibodies to HIV-1 and HIV-2, respectively. The described design, expression and purification protocols will make it possible to produce specific recombinant reagents in large quantities for agglutination-based rapid detection of antibodies to HIV in whole blood.

Effects of antivenom on Buthus occitanus tunetanus (Bot) scorpion venom pharmacokinetics: towards an optimization of antivenom immunotherapy in a rabbit model

The pharmacokinetic parameters of Bot venom were determined in a rabbit model using a specific sandwich type ELISA. After intravenous injection, Bot venom seems to follow a three-compartment pharmacokinetic open model. However, after subcutaneous injection, the distribution and elimination kinetics of Bot venom are best characterized by a bi-compartment pharmacokinetic open model. Bot venom is completely absorbed from its SC injection site, since the absolute bioavailability is higher than 95%; the maximum plasma venom concentration is reached between 30 and 60 min after venom injection. Bot venom diffuses rapidly to tissues and is distributed in a high body volume. The total body clearance of Bot venom is relatively high in agreement with a low mean residence time. Antivenom immunotherapy experiments were carried out in the rabbit model, in order to select the most appropriate strategy for the adequate use of this treatment. The effects of the route, the dose and the delay of antivenom injection on Bot venom pharmacokinetic parameters and on the antivenom immunotherapy efficacy were then studied. These studies indicated in particular that: (1) the injection of a minimal neutralizing antivenom dose is required for a complete and permanent neutralization of circulating venom antigens; this dose is named minimal (threshold) efficacious antivenom dose; (2) the intramuscular route is not the most appropriate way for antivenom injection; and (3) a delayed antivenom immunotherapy remains efficacious especially on the neutralization of the remaining circulating venom. In short, these experimental studies show that early intravenous injection of an appropriate antivenom dose (at least the threshold efficacious dose) is the indicated way for a rapid and permanent neutralization of circulating scorpion venom toxins.

Purification, crystallization and preliminary X-ray analysis of the Fab fragment from MNAC13, a novel antagonistic anti-tyrosine kinase A receptor monoclonal antibody

The monoclonal antibody MNAC13 is a potent antagonist that prevents the binding of nerve-growth factor (NGF) to its tyrosine kinase A receptor (TrkA) in a variety of systems. Structural studies of the FabMNAC13 fragment were performed to gain insights into the mechanism of action of this potentially therapeutic monoclonal antibody. The optimal conditions for crystallization of FabMNAC13 were determined. Crystals appeared as prismatic bundles, displayed P2(1)2(1)2(1) space-group symmetry and diffracted to a resolution of 1.8 A. The unit-cell parameters were determined to be a = 52.73, b = 67.55, c = 111.43 A. The data set was 99.5% complete. Molecular replacement was performed, resulting in a correlation coefficient of 0.55 and an R value of 0.40. The structure refinement is now in progress.

The preparation and crystallization of Fab fragments of a family of mouse esterolytic catalytic antibodies and their complexes with a transition-state analogue

The Fab fragments of a family of mouse esterolytic monoclonal antibodies MS6-12, MS6-126 and MS6-164 have been obtained by digestion of whole antibodies with papain, purified and crystallized in a range of different forms either alone or in complex with a transition-state analogue. The crystals diffract X-rays to resolutions between 2.1 and 1.2 A and are suitable for structural studies. The determination of these structures could be important in understanding the different catalytic power of each of these related catalytic antibodies.

Structure of a factor VIII C2 domain-immunoglobulin G4kappa Fab complex: identification of an inhibitory antibody epitope on the surface of factor VIII

The development of an immune response to infused factor VIII is a complication affecting many patients with hemophilia A. Inhibitor antibodies bind to antigenic determinants on the factor VIII molecule and block its procoagulant activity. A patient-derived inhibitory immunoglobulin G4kappa antibody (BO2C11) produced by an immortalized memory B-lymphocyte cell line interferes with the binding of factor VIII to phospholipid surfaces and to von Willebrand factor. The structure of a Fab fragment derived from this antibody complexed with the factor VIII C2 domain was determined at 2.0 A resolution. The Fab interacts with solvent-exposed basic and hydrophobic side chains that form a membrane-association surface of factor VIII. This atomic resolution structure suggests a variety of amino acid substitutions in the C2 domain of factor VIII that might prevent the binding of anti-C2 inhibitor antibodies without significantly compromising the procoagulant functions of factor VIII.

Rapid two-step purification of a recombinant mouse Fab fragment expressed in Escherichia coli

We report a rapid, large-scale process for the purification of a recombinant Fab fragment specific for the tobacco mosaic virus coat protein (Fab57P). The fragment is expressed periplasmically in Escherichia coli. The expression level was optimized in 0.3-L fermentors. The highest levels were obtained using the following conditions: (1) low postinduction temperature (21 degrees C), (2) combined use of two beta-lactam antibiotics (carbenicillin and ampicillin), (3) IPTG concentration 0.1 mM, (4) regulated pH 7.2, (5) 17-h induction time, and (6) conditions that reduce mechanical stress. Optimized large-scale fermentations were done in 15- and 300-L capacity fermentors. The recombinant Fab fragment was purified by two chromatographic steps. After disruption of the bacteria using an APV Gaulin homogenizer, the crude E. coli homogenate was directly applied, without centrifugation, to an SP Sepharose Big Beads column. The recombinant Fab fragment was eluted as a single peak in a sodium chloride gradient. The fragment was further purified by affinity adsorption to a column packed with Epoxy-activated Sepharose 6B to which the antigen peptide NH(2)-CGS YNR GSF SQS SGLV-CONH(2) had been coupled through its N-terminal cysteine. The purified Fab57P fragment showed one band in SDS-PAGE. The overall purification yield was 35%.

A dual-mode approach to the selective separation of antibodies and their fragments

A novel chromatography method for the separation of antibodies is described. The adsorption of antibodies on the solid phase involves interaction with a ligand that combines mild hydrophobic characteristics and some degree of molecular recognition with a derivative of pyridine. This combined effect results in the adsorption of antibodies in the absence of lyotropic salts. When environmental pH is changed, the ligand becomes ionically charged, allowing the desorption of antibodies. The mechanism of adsorption, involving hydrophobic associations and ionic related interaction, is here qualified as dual-mode. Studies on the determination of the apparent dissociation constant for immunoglobulins G are presented. Adsorption of antibodies from crude feedstocks typically occurs without adjustment of pH or ionic strength. The sorbent is then washed with a buffer to eliminate protein impurities and, when lowering the environmental pH, antibodies are desorbed. The solid-phase material is used for the separation of antibodies from an ascites fluid and from a cell culture supernatant, followed by a polishing step on an hydroxyapatite column. Preliminary studies, related to the ability of the solid phase to separate antibody fragments, are also reported. In these studies, it has been demonstrated that both Fab and Fc fragments from polyclonal IgG are adsorbed to the solid phase under typical binding conditions. Under other defined physico-chemical conditions (ionic strength and pH), separation of both fragments in a single step has been achieved.

Expression of the Fabs of human auto-antibodies in Escherichia coli: optimization and determination of their fine binding characteristics and cross-reactivity

The Fabs of three human auto-antibodies (B3/33H11, anti-DNA; UK4, anti-phospholipid) and six related hybrids have been cloned and expressed in Escherichia coli, and their relative binding to single-stranded or double-stranded DNA or to cardiolipin has been assessed in the presence of modulators (salts and serum). We describe optimized conditions that have led to significant improvement in the quality and quantity of the purified auto-antibodies. Protein expression of the assembled and functionally active Fabs was achievable with a yield of up to 5 to 9 mg/l of culture. The comparative DNA/cardiolipin-binding analyses of the nine Fabs in the presence of modulators demonstrated that B3 and 33H11 L chains possess both anti-DNA and anti-cardiolipin activities. This is the first report of the demonstration that both anti-DNA and anti-cardiolipin activities may lie on the same light chain of a human auto-antibody. We provide evidence that the auto-antibodies that appeared to be similar, in that they bound DNA or cardiolipin in conventional ELISA immunoassays, exhibited significant difference in their cross-reactivity and binding to the antigen in the presence of modulators. Such auto- antigen specificity and/or cross-reactivity may dictate the potential of an auto-antibody to cause pathogenicity and may provide an explanation as to why apparently similar auto-antibodies behave differently in vivo.

Production of humanized antibody against human high-affinity IgE receptor in a serum-free culture of CHO cells, and purification of the Fab fragments

We describe the preparation of Fab fragments of a humanized anti-human high-affinity IgE receptor (Fc epsilonRIalpha) antibody potentially useful for treatment of IgE-mediated allergic diseases. IgE-binding capacities of sixteen combinations of light and heavy chains of four recombinant anti-Fc epsilonRIalpha antibodies, chimeric CRA2, humanized CRA2, chimeric CRA4, and humanized CRA4, were compared. A combination in which both chains were of humanized CRA2 had the highest activity. Stable transfectant clones of four kinds of host cells expressing recombinant antibodies were established. CHO-K1 cells were the most productive. Serum-free media suitable for culture of the stable CHO-transfectant clones were screened. The concentration of the humanized CRA2, which the most productive clone secreted into the chosen serum-free medium, was approximately 100 microg/ml. A procedure for the purification of the antibody, papain-digestion, and purification of Fab fragments was established. The highly purified humanized Fab fragments are suitable for use to examine their in vivo activity and immunogenicity in primates.

Catalytic heterogeneity of polyclonal DNA-hydrolyzing antibodies from the sera of patients with multiple sclerosis

Various catalytic antibodies or abzymes have been detected recently in the sera of patients with several autoimmune pathologies, where their presence is most probably associated with autoimmunization. Recently we have shown that DNase activity is associated with IgGs from the sera of patients with multiple sclerosis (MS) but not with those from the sera of normal humans. Here we present evidence showing that MS IgG, its F(ab) fragments, and separated L-chains catalyze DNA hydrolysis. The properties of the DNase activity of these polyclonal IgGs distinguish them from other known human DNases. In addition, their specific activities with different oligonucleotide substrates and the range of optimal pHs, apparent K(M) values and substrate specificities varied widely for different patients. The findings speak in favor of the generation by the immune systems of individual patients of a variety of polyclonal catalytic IgG pools, from relatively small to extremely large ones.

Isolation of an IgG anti-B from a human Fab-phage display library

BACKGROUND

ABO incompatibility is a common cause for mild hemolysis in the newborn, ranging from 1 in 30 to 1 in 150 births. Fortunately, hemolysis requiring transfusion is rare and restricted to blood group O mothers, because blood group A and B individuals make poor IgG anti-B and anti-A responses. No human IgG ABO antibody sequences have been reported, in part because of the difficulty in obtaining human IgG hybridomas. Phage-display technology may be able to circumvent these difficulties, but its application to carbohydrate antigens is poorly studied.

STUDY DESIGN AND METHODS

A human IgG1 phage-display Fab library was constructed from splenocytes derived from a nonhyperimmunized blood group O person, and panned against group B RBCs.

RESULTS

After five rounds of panning, essentially all phage bound to group B RBCs. Nucleotide sequence analysis of a single monoclonal IgG1lambda phage, FB5.7, revealed a highly mutated VH4 family heavy chain, and a nearly germline VL7 family lambda light chain. The Fab agglutinated group B, but not group A, random-donor RBCs. However, group B ELISA reactivity could be inhibited by soluble B-trisaccharide, soluble A-trisaccharide, galactose, and N-acetyl galactosamine. Similarly, galactose and N-acetyl galactosamine were able to inhibit group B RBC agglutination.

CONCLUSION

FB5.7 is the first human IgG ABO MoAb described. Alhough it behaves serologically like a group B-specific antibody, it demonstrates interaction with both the A and B epitopes. Phage-display technology can be used to better define the relationship between antibody genotype and phenotype in anti-carbohydrate responses in nonhyperimmunized hosts, and thus to improve our understanding of the composition of the antibody repertoire.

Crystallization and preliminary X-ray analysis of a recombinant Fab fragment in complex with 17beta-oestradiol

The recombinant Fab fragment of the anti-17beta-oestradiol antibody 57-2 has been a target for several protein-engineering experiments. A method for production, purification and crystallization of the Fab fragment alone (apo form) and in complex with the major female sex hormone 17beta-oestradiol is reported here. Diffracting apo-form crystals were only obtained with microseeding; crystals of the Fab-steroid complex were produced by co-crystallization in the presence of oestradiol and cross-seeding with the apo-form crystals. The crystals were grown using vapour-diffusion methods with reservoir solutions containing 10-14% PEG 4000 or 8-12% PEG 8000 and Tris-HCl buffer at high pH (9.0-9.5). Both the apo and complex crystals belong to space group P2(1)2(1)2(1) and diffract to 2.0 A resolution. High-resolution X-ray data sets suitable for structure determination were collected from flash-cooled crystals using 25% glycerol as the cryoprotectant.

Generation and characterization of a recombinant human CCR5-specific antibody. A phage display approach for rabbit antibody humanization

We describe the isolation of a CCR5-specific antibody, ST6, from an antibody phage display library generated from an immune rabbit. ST6 was previously shown to efficiently prevent the surface expression of CCR5 when expressed intracellularly (Steinberger, P., Andris-Widhopf, J., Buhler, B., Torbett, B. E., and Barbas, C. F., III (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 805-810). Because ST6 has therapeutic potential in human immunodeficiency virus, type 1 disease, its humanization was desired to minimize the potential for immunogenicity. ST6 was humanized using a phage display-based approach. Like the parental rabbit clone, the humanized version ST6/34 efficiently prevented the surface expression of CCR5. The conserved linear peptide epitope bound by these antibodies was mapped using phage display. Both ST6 as well as the humanized anti-CCR5 antibody ST6/34 were produced as complete IgG antibodies and shown to bind to cell surface CCR5.

Phycobiliprotein-Fab conjugates as probes for single particle fluorescence imaging

BACKGROUND AND METHODS

Single particle fluorescence imaging (SPFI) is a recently developed method that has provided a powerful approach to observing receptor movement and associations at high spatial resolution. It provides a noninvasive alternative to the existing biochemical techniques. It can also quantify and resolve molecular interactions at the cell surface at a nanometer scale. Probes that have been used in the past to study mobility and associations of cell surface receptors have many limitations. These include concerns about the specificity of the probes, the possibility that their size interferes with the receptor once bound to it, the nonuniform fluorescence, and the questionable stoichiometry.

RESULTS

In this study, we have generated phycobiliprotein-Fab conjugates, and have shown that they are a significant advance on existing probes for SPFI studies. They are small in size, highly specific, highly fluorescent, of known stoichiometry, photostable, emit uniform fluorescence, and are generally well defined.

CONCLUSIONS

It is highly important that when studying receptor mobility or associations, fully characterized probes are used. Phycoerythrin(PE)-Fab probes provide us with the perfect tool for SPFI, and a system with a wide range of applicability to study any cell surface receptor against which a monoclonal antibody exists.

CHO expression of a novel human recombinant IgG1 anti-RhD antibody isolated by phage display

Replacement of the hyperimmune anti-Rhesus (Rh) D immunoglobulin, currently used to prevent haemolytic disease of the newborn, by fully recombinant human anti-RhD antibodies would solve the current logistic problems associated with supply and demand. The combination of phage display repertoire cloning with precise selection procedures enables isolation of specific genes that can then be inserted into mammalian expression systems allowing production of large quantities of recombinant human proteins. With the aim of selecting high-affinity anti-RhD antibodies, two human Fab libraries were constructed from a hyperimmune donor. Use of a new phage panning procedure involving bromelin-treated red blood cells enabled the isolation of two high-affinity Fab-expressing phage clones. LD-6-3 and LD-6-33, specific for RhD. These showed a novel reaction pattern by recognizing the D variants D(III), D(IVa), D(IVb), D(Va), D(VI) types I and II. D(VII), Rh33 and DFR. Full-length immunoglobulin molecules were constructed by cloning the variable regions into expression vectors containing genomic DNA encoding the immunoglobulin constant regions. We describe the first, stable, suspension growth-adapted Chinese hamster ovary (CHO) cell line producing a high affinity recombinant human IgG1 anti-RhD antibody adapted to pilot-scale production. Evaluation of the Fc region of this recombinant antibody by either chemiluminescence or antibody-dependent cell cytotoxicity (ADCC) assays demonstrated macrophage activation and lysis of red blood cells by human lymphocytes. A consistent source of recombinant human anti-RhD immunoglobulin produced by CHO cells is expected to meet the stringent safety and regulatory requirements for prophylactic application.

The unfolding/denaturation of immunogammaglobulin of isotype 2b and its F(ab) and F(c) fragments

The unfolding and further denaturation of IgG and its F(ab) and F(c) fragments were studied both on a macroscopic and molecular level, using differential scanning calorimetry and circular dichroism spectroscopy, respectively. It was shown that the structural integrity of the F(ab) and F(c) units was retained after fragmentation of the IgG. The F(ab) fragment denatured at approximately 61 degrees C and the F(c) fragment at 71 degrees C. The structural transitions observed in the whole IgG is the sum effect of those determined for the isolated F(ab) and F(c) fragments.

Synthesis of bifunctional antibodies for immunoassays

The synthesis of bifunctional antibodies using the principle of solid-phase synthesis is described. Two Fab' fragments were chemically linked together via a bismaleimide crosslinking reagent. The F(ab')(2) fragments from intact immunoglobulin G (IgG) were prepared using an immobilized pepsin column. Goat, mouse, and human antibodies were digested completely within 4 h. The F(ab')(2) fragments thus produced did not contain any IgG impurities. Fab' fragments were produced by reducing the heavy interchain disulfide bonds using 2-mercaptoethylamine. Use of the solid-phase reactor in the preparation of the bifunctional antibodies eliminated many of the time-consuming separation steps between the fragmentation and conjugation steps. This procedure facilitates the automation of bifunctional antibody preparation and the rapid optimization of reaction conditions.

Site-specific photobiotinylation of antibodies, light chains, and immunoglobulin fragments

The high affinity of biotin for avidin has been exploited for many antibody-based assays. This requires that biotin is covalently conjugated to the antibody molecule. Several chemically reactive biotinylation reagents are commercially available. Except for the attachment via sulfhydryl groups in the immunoglobulin (Ig) molecule, these reagents attach biotin randomly to various amino acid side chains. Although non-site-specific modification of antibodies does not interfere in most immunoassays, specific application and sensitive antibodies would benefit from site-specific biotinylation. Here we describe an affinity biotinylation technique based on a photoreactive biotin reagent. The design of this reaction was possible from the discovery of a conserved binding site in the variable Ig domain for nucleotides and nucleosides. The described photoaffinity biotinylation offers the advantages of ease, convenience, and production of a reproducible and defined biotinylated antibody preparation.

Targeting a recombinant adenovirus vector to HCC cells using a bifunctional Fab-antibody conjugate

We developed a specific adenoviral gene delivery system with monoclonal antibody (mAb) AF-20 that binds to a 180 kDa antigen highly expressed on human hepatocellular carcinoma (HCC) cells. A bifunctional Fab-antibody conjugate (2Hx-2-AF-20) was generated through AF-20 mAb crosslinkage to an anti-hexon antibody Fab fragment. Uptake of adenoviral particles and gene expression was examined in FOCUS HCC and NIH 3T3 cells by immunofluorescence; beta-galactosidase expression levels were determined following competitive inhibition of adenoviral CAR receptor by excess fibre knob protein. The chimeric complex was rapidly internalized at 37 degrees C, and enhanced levels of reporter gene expression was observed in AF-20 antigen positive HCC cells, but not in AF-20 antigen negative NIH 3T3 control cells. Targeting of recombinant adenoviral vectors to a tumor associated antigen by a bifunctional Fab-antibody conjugate is a promising approach to enhance specificity and efficiency of gene delivery to HCC.

Capture of human Fab fragments by expanded bed adsorption with a mixed mode adsorbent

A novel group of mixed mode adsorbents has been developed for purification of monoclonal and polyclonal antibodies from a broad range of raw materials such as hybridoma cell culture, ascites fluid, animal sera, milk, whey and egg yolk. The aim of this study was to determine whether such mixed mode adsorbents were also useful for the recovery of recombinant proteins from microbial feedstocks. This paper describes the performance of one of these adsorbents for expanded bed capture of a human Fab fragment from recombinant E. Coli cell extracts. It is concluded that the mixed mode adsorbent binds the Fab fragment efficiently from crude extracts without any requirement for preconditioning the extract by for example de-salting or dilution. The capacity of the mixed mode adsorbent is approx. 12 mg Fab/ml matrix. The novel mixed mode adsorbent can be useful during production of highly purified Fab fragments as the first step in a purification scheme. In this respect the mixed mode adsorbent is advantageous over alternative commercially available ion-exchange materials which require pre-conditioning of cell extract for Fab' capture. Together with the concentration and clarification effect a significant enrichment of the Fab fragment is obtained in one single high yield operation.

A neutralizing recombinant human antibody Fab fragment against Puumala hantavirus

A combinatorial human antibody Fab pComb3H library, generated from splenic lymphocytes of a Puumala hantavirus (PUUV) immune individual, was selected against PUUV using the phage display technique. Panning was carried out with antigens immobilized by MAbs directed to the two PUUV envelope glycoproteins G1 and G2. Thirteen Fabs, with reactivity directed to PUUV and specifically the G2 protein, as assessed by immunofluorescence and ELISA respectively, were isolated in crude preparations. By a focus reduction neutralization test (FRNT), four of the 13 crude Fab preparations exhibited type-specific neutralization of PUUV (strain Sotkamo) with 44-54% reduction in the number of foci. After affinity purification, the four Fab clones exhibited 50% focus reduction of PUUV at concentrations below 2 microg/ml. Sequencing of the heavy and light chain complementarity determining regions (CDR) 1-3 showed that the four selected clones were identical within the antibody binding regions. In inhibition tests with the PUUV G2-specific MAbs, 4G2 and 1C9, a new epitope important for neutralization, designated as G2-a3, was defined. This epitope, overlapping partially the neutralizing epitope recognized by the human MAb 1C9, seems to be unique for the PUUV serotype since none of the Fab clones neutralized any of the other hantaviruses tested.

High-performance sodium dodecyl sulfate-capillary gel electrophoresis of antibodies and antibody fragments

High-performance sodium dodecyl sulfate-capillary gel electrophoresis (SDS-CGE) has been used to separate antibodies and their fragments according to size. Under non-reduced conditions, murine monoclonal antibodies generally show a predominant peak with five to six apparent fragment peaks. The magnitude of the apparent fragmentation is temperature-dependent and is more pronounced with rabbit, sheep and bovine immunoglobulin G's than murine monoclonal antibodies. In addition to temperature, pH and buffer also affect the fragmentation. Without heat treatment during the preparation of the SDS-antibody complexes, the observed fragments become nearly absent; however, some murine monoclonal antibodies exhibit several peaks that group near the expected migration time of an immunoglobulin G, presumably due to their anomalous interaction with sodium dodecyl sulfate. The method can also be used to monitor the progress of peptic digestions to generate murine F(ab')(2) antibody, to perform "gel-shift" assays, and to separate mouse ascitic fluid. This high-performance electrophoretic technique is suitable for quality control as well as the characterization of the antibodies under experimental conditions.

Photoreduction of monoclonal antibodies for conjugation and fragmentation

The conjugation of enzymes, fluorescent or radioactive labels, cross-linkers and other moieties to antibodies is a commonly performed procedure in biochemical research. Using reduced disulphides, conjugation can be an inconvenient, multistep, time- and material-consuming process. We have developed a reduction technique based on UV irradiation, which lacks these drawbacks. Antibodies are irradiated in a sealed vial for a few minutes by a common laboratory UV source in the presence of stannous ions, following the depletion of atmospheric oxygen. The preparation may subsequently be conjugated with thiol-reactive probes such as maleimide derivatives, with no need for any prior purification or concentration. This simple, rapid and effective reduction and conjugation process results in a fully functional immunoglobulin conjugate that can be used for a variety of biochemical applications.