Factors affecting the specific activity of immobilized antibodies and their biologically active fragments

Progress in affinity ligand-functionalized bacterial magnetosome nanoparticles for bio-immunomagnetic separation of HBsAg protein

Efficient Bio-immunomagnetic separation (BIMS) of recombinant hepatitis B surface antigen (rHBsAg) with high binding capacity was studied using affinity ligand immobilized bacterial magnetosome nanoparticles (Magnetospirillum gryphiswaldense strain MSR-1 bacteria) as an immunomagnetic sorbent. Our results showed immunomagnetic adsorption, acted by affinity interactions with the immobilized monoclonal antibody, offered higher antigen adsorption and desorption capacities as compared with the commercially available immunoaffinity sorbents. Four different ligand densities of the Hep-1 monoclonal antibody were examined during covalent immobilization on Pyridyl Disulfide-functionalized magnetosome nanoparticles for HBsAg immunomagnetic separation. The average of adsorption capacity was measured as 3 mg/ml in optimized immunomagnetic sorbent (1.056 mg rHBsAg/ml immunomagneticsorbent/5.5 mg of total purified protein) and 5mg/ml in immunoaffinity sorbent (0.876 mg rHBsAg/ml immunosorbent/5.5 mg total purified protein during 8 runs. Immunomagnetic sorbent demonstrated ligand leakage levels below 3 ng Mab/Ag rHBsAg during 12 consecutive cycles of immunomagnetic separation (IMS). The results suggest that an immunomagnetic sorbent with a lower ligand density (LD = 3 mg Mab/ml matrix) could be the best substitute for the immunosorbent used in affinity purification of r-HBsAg there are significant differences in the ligand density (98.59% (p-value = 0.0182)), adsorption capacity (97.051% (p-value = 0.01834)), desorption capacity (96.06% (p-value = 0.036)) and recovery (98.97% (p-value = 0.0231)). This study indicates that the immunosorbent approach reduces the cost of purification of Hep-1 protein up to 50% as compared with 5 mg Mab/ml immunoaffinity sorbent, which is currently used in large-scale production. As well, these results demonstrate that bacterial magnetosome nanoparticles (BMs) represent a promising alternative product for the economical and efficient immobilization of proteins and the immunomagnetic separation of Biomolecules, promoting innovation in downstream processing.

Interfacing Live Cells with Surfaces: A Concurrent Control Technique for Quantifying Surface Ligand Activity

Surface ligand activity is a key design parameter for successfully interfacing surfaces with cells─whether in the context of in vitro investigations for understanding cellular signaling pathways or more applied applications in drug delivery and medical implants. Unlike other crucial surface parameters, such as stiffness and roughness, surface ligand activity is typically based on a set of assumptions rather than directly measured, giving rise to interpretations of cell adhesion that can vary with the assumptions made. To fill this void, we have developed a concurrent control technique for directly characterizing in vitro ligand surface activity. Pairs of gold-coated glass chips were biofunctionalized with RGD ligand in a parallel workflow: one chip for in vitro applications and the other for surface plasmon resonance (SPR)-based RGD activity characterization. Recombinant α_Vβ₃ integrins were injected over the SPR chip surface as mimics of the cellular-membrane-bound receptors and the resulting binding kinetics parameterized to quantify surface ligand activity. These activity measurements were correlated with cell morphological features, measured by interfacing MDA-MB-231 cells with the in vitro chip surfaces on the live cell microscope. We demonstrate how the interpretation of a cell phenotype based on direct activity measurements can vary markedly from interpretations based on assumed activity. The SPR concurrent control approach has multiple advantages due to the fact that SPR is a standardized technique and has the sensitivity to measure ligand activity across the most relevant range of extracellular surface densities, while the in vitro chip design can be used with all commonly used light microscopy modalities (e.g., phase contrast, DIC, and fluorescence) so that a wide range of phenotypic and molecular markers can be correlated to the ligand surface activity.

Problem of Diminished cRGD Surface Activity and What Can Be Done about It

RGD peptides play a pivotal role in growing and diverse areas of biological research, ranging from in vitro experiments probing fundamental molecular mechanisms of cell adhesion to more applied in vivo strategies in medical imaging and cancer therapeutics. To better understand the outcomes of RGD-based approaches, we quantified the degree to which cyclic RGD (cRGD) activity is blocked by nonspecific binding of commonly used medium constituents. First, we show that recombinant α_Vβ₃ integrins can be used as a highly sensitive cell-free sensor to quantitatively and reliably characterize the activity of cRGD-functionalized surfaces via surface plasmon resonance (SPR). Next, SPR experiments were utilized to measure the extent of blocking of cRGD-functionalized surfaces by the commonly used agents BSA, PLL-g-PEG, and fetal calf serum (FCS)-supplemented media, using recombinant α_Vβ₃ integrin as a probe for cRGD binding activity in the presence of blocking agents. All three additives were highly efficient blockers of cRGD activity, as exemplified by cell culture media containing 1% FCS which reduced the cRGD activity by 33-fold. We then developed a strategy to combat these deleterious effects by employing the recombinant integrins as a protective cap. We show that the unblocked cRGD activity can be preserved in the presence of PLL-g-PEG by employing the α_Vβ₃ integrin as a removable protective cap, both in cell-free and in vitro experiments. In vitro studies with MDA-MB-231 cells cultured atop cRGD-functionalized surfaces found that cell adhesion and migration prevented by PLL-g-PEG were restored when this protective cap approach was used.

Improving biosensing activity to carcinoembryonic antigen with orientated single domain antibodies

Carcinoembryonic antigen (CEA), also referred as CEACAM5, is integral to the adhesion process during cancer invasion and metastasis and is one of the most widely used tumor markers for assisting the diagnosis of cancer recurrence and cancer metastasis. Antibodies against CEA molecules have been developed for detection and diagnostic applications following tumor removal. Single domain antibodies (sdAbs) against CEA isolated from dromedary and llama exhibited high specificity in binding to tumor cells. However, because these CEA sdAbs were not designed to be orientated when conjugated to surface sensors, there is potential for significant improvements in their activity and limit of detection. Herein we modified the CEA sdAbs with two different C-terminal fusions designed to aid with orientation by way of the tail’s charge and biotin binding. A fusion which incorporated the C-terminus addition of a positively charged tail (B5-GS3K) improved biosensor sensitivity to CEA while also retaining the sub-nanomolar binding affinity and thermal stability of the unmodified sdAb. Using our fabricated surfaces on bare gold chips and a multiplexed surface plasmon resonance imager (SPRi), we quantified the specific binding activities, defined as the percentage of bound epitopes to the total immobilized, of the sdAb fusions and anti-CEA mAb. Our results demonstrate that monovalent B5-GS3K exhibited significantly improved binding activity, approximately 3-fold higher than bivalent mAb.

Immunoaffinity Chromatography: Concepts and Applications

Antibody-based separation methods, such as immunoaffinity chromatography (IAC), are powerful purification and isolation techniques. Antibodies isolated using these techniques have proven highly efficient in applications ranging from clinical diagnostics to environmental monitoring. Immunoaffinity chromatography is an efficient antibody separation method which exploits the binding efficiency of a ligand to an antibody. Essential to the successful design of any IAC platform is the optimization of critical experimental parameters such as (a) the biological affinity pair, (b) the matrix support, (c) the immobilization coupling chemistry, and (d) the effective elution conditions. These elements and the practicalities of their use are discussed in detail in this review. At the core of all IAC platforms is the high affinity interactions between antibodies and their related ligands; hence, this review entails a brief introduction to the generation of antibodies for use in immunoaffinity chromatography and also provides specific examples of their potential applications.

Oriented Immobilization of Fab Fragments by Site-Specific Biotinylation at the Conserved Nucleotide Binding Site for Enhanced Antigen Detection

Oriented immobilization of antibodies and antibody fragments has become increasingly important as a result of the efforts to reduce the size of diagnostic and sensor devices to miniaturized dimensions for improved accessibility to the end-user. Reduced dimensions of sensor devices necessitate the immobilized antibodies to conserve their antigen binding activity for proper operation. Fab fragments are becoming more commonly used in small-scaled diagnostic devices due to their small size and ease of manufacture. In this study, we used the previously described UV-NBS(Biotin) method to functionalize Fab fragments with IBA-EG11-Biotin linker utilizing UV energy to initiate a photo-cross-linking reaction between the nucleotide binding site (NBS) on the Fab fragment and IBA-Biotin molecule. Our results demonstrate that immobilization of biotinylated Fab fragments via UV-NBS(Biotin) method generated the highest level of immobilized Fab on surfaces when compared to other typical immobilization methods while preserving antigen binding activity. UV-NBS(Biotin) method provided 432-fold, 114-fold, and 29-fold improved antigen detection sensitivity than physical adsorption, NHS-Biotin, and ε-NH3(+), methods, respectively. Additionally, the limit of detection (LOD) for PSA utilizing Fab fragments immobilized via UV-NBS(Biotin) method was significantly lower than that of the other immobilization methods, with an LOD of 0.4 pM PSA. In summary, site-specific biotinylation of Fab fragments without structural damage or loss in antigen binding activity provides a wide range of application potential for UV-NBS immobilization technique across numerous diagnostic devices and nanotechnologies.

Optimizing Nanoplasmonic Biosensor Sensitivity with Orientated Single Domain Antibodies

Localized surface plasmon resonance (LSPR) spectroscopy and imaging are emerging biosensor technologies which tout label-free biomolecule detection at the nanoscale and ease of integration with standard microscopy setups. The applicability of these techniques can be limited by the restrictions that surface-conjugated ligands must be both sufficiently small and orientated to meet analyte sensitivity requirements. We demonstrate that orientated single domain antibodies (sdAb) can optimize nanoplasmonic sensitivity by comparing three anti-ricin sdAb constructs to biotin-neutravidin, a model system for small and highly orientated ligand studies. LSPR imaging of electrostatically orientated sdAb exhibited a ricin sensitivity equivalent to that of the biotinylated LSPR biosensors for neutravidin. These results, combined with the facts that sdAb are highly stable and readily produced in bacteria and yeast, build a compelling case for the increased utilization of sdAbs in nanoplasmonic applications.

Dynamics of BSA adsorption onto a photoablated polymer surface in a dielectric microchip

Impedance sensorgrams of adsorbed proteins on a microchannel obtained using contactless microelectrodes in a dielectric microchip.

Immunoaffinity chromatography

Antibody-based separation methods, such as immunoaffinity chromatography (IAC), are powerful purification and isolation techniques. Antibodies isolated using these techniques have proven highly efficient in applications ranging from clinical diagnostics to environmental monitoring. IAC is an efficient antibody separation method which exploits the binding efficiency of a ligand to an antibody. Essential to the successful design of any IAC platform is the optimisation of critical experimental parameters such as: (a) the biological affinity pair, (b) the matrix support, (c) the immobilisation coupling chemistry, and (d) the effective elution conditions. These elements and the practicalities of their use are discussed in detail in this review. At the core of all IAC platforms is the high-affinity interactions between antibodies and their related ligands; hence, this review entails a brief introduction to the generation of antibodies for use in IAC and also provides specific examples of their potential applications.

Multivalent anchoring and oriented display of single-domain antibodies on cellulose

Antibody engineering has allowed for the rapid generation of binding agents against virtually any antigen of interest, predominantly for therapeutic applications. Considerably less attention has been given to the development of diagnostic reagents and biosensors using engineered antibodies. Recently, we produced a novel pentavalent bispecific antibody (i.e., decabody) by pentamerizing two single-domain antibodies (sdAbs) through the verotoxin B subunit (VTB) and found both fusion partners to be functional. Using a similar approach, we have engineered a bispecific pentameric fusion protein consisting of five sdAbs and five cellulose-binding modules (CBMs) linked via VTB. To find an optimal design format, we constructed six bispecific pentamers consisting of three different CBMs, fused to the Staphylococcus aureus-specific human sdAb HVHP428, in both orientations. One bispecific pentamer, containing an N-terminal CBM9 and C-terminal HVHP428, was soluble, non-aggregating, and did not degrade upon storage at 4 °C for over six months. This molecule was dually functional as it bound to cellulose-based filters as well as S. aureus cells. When impregnated in cellulose filters, the bispecific pentamer recognized S. aureus cells in a flow-through detection assay. The ability of pentamerized CBMs to bind cellulose may form the basis of an immobilization platform for multivalent display of high-avidity binding reagents on cellulosic filters for sensing of pathogens, biomarkers and environmental pollutants.

Improving immunobinding using oriented immobilization of an oxidized antibody

Recent technical advances in biorecognition engineering and microparticle fabrication enabled us to develop a single-step purification process using magnetic particles (MPs). The process is simple, efficacious, easy to automate, and economical. The method immobilizes the ligand molecule in a particular orientation on commercial MPs that have surface carboxyl groups. Mouse IgG and anti-mouse IgG antibody were the model capture and ligand molecules for this study. The immunobinding efficacy of anti-mouse IgG antibody using "oriented immobilization" was compared with the efficacy of a conventional amine-coupling system that results in random orientation and of another standard method, the biotin-streptavidin system. The oriented immobilization was accomplished by oxidizing the sugar moiety in the CH(2) domain of the antibody's Fc and covalently conjugating the moiety to the hydrazine-coated MP. The specific binding affinity of the oriented immobilization process was about 2.5 times that of the amine-coupling system, and selectivity from a binary mixture was about 2 times greater for the oriented immobilization method. Results were nearly identical for the biotin-streptavidin system and the oriented immobilization system, matching the calculated binding stoichiometry between mouse IgG and anti-mouse IgG antibody. The binding improvement over the amine-coupling system shown by assay was confirmed by a separate surface plasmon resonance experiment. In summary, the oriented immobilization method was as effective as the streptavidin-biotin system, yet simpler and cost-effective.

Chimeric protein for selective cell attachment onto cellulosic substrates

We have developed a fusion protein (CBD-LG) incorporating a cellulose-binding domain and an antibody binding domain, protein LG, to provide an adaptor molecule for cell separation with regenerated cellulose hollow fiber arrays. A single hollow fiber cell adhesion assay utilizing a CD34+ cell line, KG1a, was used to investigate whether ligand affinity interactions were strong enough for cell attachment and separation. CBD-LG efficiently captured CD34+ cells labeled with the mouse IgG2a monoclonal antibody MHCD3400. However, it was not possible to bind CD34+ cells labeled with an IgG1 antibody (HPCA-2). The low affinity of HPCA-2 for LG was overcome by secondary antibodies: KG1a cells that were dual labeled with HPCA-2 followed by rat anti-mouse IgG1 adhered inside hollow fibers coated with CBD-LG. Alternatively, immobilized rabbit polyclonal anti-mouse IgG1 captured cells labeled with HPCA-2. The development of an adaptor molecule to display recombinant domains at the surface of hollow fibers will be an effective tool to investigate cellular ligand-receptor interactions, a necessary step in the development of hollow fiber bioreactors for manufacture of human cellular products.

Comparison of random and oriented immobilisation of antibody fragments on mixed self-assembled monolayers

The sensitivity of immunosensors is strongly dependent on the amount of immobilised antibodies and their remaining antigen binding properties. The use of smaller and well-oriented antibody fragments as bioreceptor molecules influences the final immunosensor signal. The aim of this study was to compare the immunosensor responses of different immobilised antibody fragments, such as F(ab')2 and Fab', with their parental IgG. In addition, we evaluated the oriented versus the random covalent immobilisation method of the Fab' fragments. First, an optimisation of cleavage protocol to generate these F(ab')2 and Fab' fragments was performed. Subsequently, we pursued a study with limited denaturation effects during immobilisation of the bioreceptor molecules and with reduced steric hindrance during antigen binding using mixed self-assembled monolayers (SAM) of thiols as the chemical linking layer. The Surface Plasmon Resonance technique was used to evaluate the degree of immobilisation of the antibody fragments and their parental IgGs on the mixed SAMs and the binding signals of their specific antigens. In this study, we demonstrate that for a particular antibody/antigen system (anti-hIgG/hIgG), the optimised fragmentation protocol in combination with an oriented immobilisation of Fab' fragments on mixed SAMs leads to a >2-fold increase of the antigen binding signals compared to randomly covalent immobilised full-length antibodies.

Development of Specific Adsorbents for Human Tumor Necrosis Factor-α: Influence of Antibody Immobilization on Performance and Biocompatibility

To develop adsorbents for the specific removal of tumor necrosis factor-alpha (TNF) in extracorporeal blood purification, cellulose microparticles were functionalized either with a monoclonal anti-TNF antibody (mAb) or with recombinant human antibody fragments (Fab). The TNF binding capacity of the adsorbents was determined with in vitro batch experiments using spiked human plasma (spike: 1200 pg TNF/mL; 1 mg particles in 250 muL plasma). Random immobilization of the full-sized monoclonal antibody to periodate-activated cellulose yielded particles with excellent adsorption capacity (258.1 +/- 48.6 pg TNF per mg adsorbent wet weight). No leaching of antibody was detectable, and the adsorbents retained their activity for at least 12 months at 4 degrees C. We found that the conditions used during immobilization of the antibody (pH, nature of the reducing agent) profoundly influenced the biocompatibility of the resulting adsorbents, especially with respect to activation of the complement system. Particles obtained by random immobilization of the monovalent Fab fragments on periodate-activated cellulose using the same conditions as for immobilization of the mAb exhibited only low adsorption capacity (44 +/- 7 pg/mg adsorbent wet weight). Oriented coupling of the Fab fragments on chelate-epoxy cellulose via a C-terminal histidine tag, however, increased the adsorption capacity to 178.3 +/- 8.6 pg TNF/mg adsorbent wet weight. Thus, in the case of small, monovalent ligands, the orientation on the carrier is critical to retain full binding activity.

Influence of antibody immobilization strategy on molecular recognition force microscopy measurements

A systematic evaluation of the effects of antibody immobilization strategy on the binding efficiency and selectivity (e.g., ability to distinguish between specific and nonspecific interactions) of immunosurfaces prepared with F(ab') antibody fragments of rabbit Immunoglobulin G (IgG) is described. F(ab') was attached to gold surfaces either (1) directly via the formation of a gold-thiolate bond or (2) indirectly through a series of a bifunctional linkers containing an alkane chain or ethylene glycol spacer. Immobilization of F(ab') via the sulfhydryl reactive group located opposite the antigen binding site ensured optimum orientation of the antigen binding site. X-ray photoelectron spectroscopy (XPS) and surface plasmon resonance (SPR) were used to confirm surface modification with the bifunctional linkers and antibody immobilization, respectively. Binding efficiency assays performed with SPR indicated that increasing the length of the linker increased the antigen binding efficiency. Atomic force microscopy (AFM) adhesion force measurements indicated that AFM probes functionalized with directly immobilized F(ab') more effectively discriminated between specific and nonspecific surface-bound proteins than probes modified indirectly via linker-immobilized F(ab'). In addition, a greater number of antibody-antigen binding events were observed with directly immobilized F(ab')-functionalized probes.

Optimisation of the coupled monoclonal antibody density for recombinant hepatitis B virus surface antigen immunopurification

Using immunosorbents based upon cyanogen bromide-Sepharose CL-4B, we have examined different ligand densities in coupling of monoclonal antibody (MAb) to find the best performance, for recombinant hepatitis B surface antigen (rHBsAg) purification. Three replicates of 5 and 15 cycles of densities ranges: 2.17-2.19, 3.18-3.62, 4.06-4.17, and 5.13-5.40 mg/ml (control); or 1.81-2.47, 3.17-3.41, 4.16-4.28, and 5.16-5.19 mg/ml (control), respectively were evaluated in terms of binding capacity, antigen recovery, ligand leakage and purity of antigen, and compared to the control. Adsorption and antigen recovery of immunosorbents manufactured were not different statistically, eventhough increased 8.08 and 9.90% at a range of 3.17-3.41 mg/ml. At this range, efficiency expressed as productivity and MAb saving was optimal. Ligand leakage and purity of antigen showed similar behaviour among all densities. Aspects related to ligand density in antigen immunoaffinity purification are discussed.

Hyaluronic acid hydrogel as Nogo-66 receptor antibody delivery system for the repairing of injured rat brain: in vitro

Nogo-66 and NgR are important receptors inhibiting neuronal regeneration and therefore are targets for treating CNS injury. Antagonists of this receptor including blocking antibodies are potential therapeutic agents for CNS axonal injuries such as spinal cord and brain trauma. A new antibody (IgG) releasing system has been developed by covalently attaching IgG to the biodegradable hyaluronic acid (HA) hydrogel via the hydrolytically unstable hydrazone linkage, aiming to deliver the antibody of CNS regeneration inhibitors to the injured brain. In this paper we describe the synthesis, physico-chemical characteristics and test results of biological activity of antibody released from hyluronic acid hydrogel. To form the conjugates the antibody is attached to the polymer backbone using a condensation reaction between aldehyde group of the antibody and hydrazide group of the HA hydrogel. Furthermore, pH sensitive linkage-hydrozone has been formed between hydrogel and antibody. The amount of conjugated antibodies can reach 135 microg antibody/mg hydrogel in the dry state. At low pH, the antibodies released quite fast. However, the antibodies released much slower in neutral and alkaline environment. The bioactivity of antibody released from hydrogel was retained as demonstrated by indirect immunofluorescence technique.

Influence of surfactants and antibody immobilization strategy on reducing nonspecific protein interactions for molecular recognition force microscopy

Specific and nonspecific interactions between antibody-modified probes and substrate-immobilized proteins were monitored by atomic force microscopy (AFM). Probes were modified with anti-ovalbumin IgG antibodies immobilized in either an oriented or a random manner. The oriented immobilization of whole IgG was accomplished through the use of Protein A, and random immobilization was carried out with glutaraldehyde. Nonspecific interactions may lead to false detection of antibody-antigen binding events even when the antigen binding sites are properly positioned by an oriented immobilization strategy. Thus, nonionic and zwitterionic surfactants, including Tween 20, Tween 80, Triton X-100, and CHAPS, were evaluated to determine if nonspecific binding events could be reduced without compromising the desired specific antibody-antigen binding. Enzyme-linked immunosorbent assay and surface plasmon resonance assays were also employed to study antibody-antigen binding as a function of immobilization strategy and surfactant concentration. The data from these studies indicate that Protein A can be used to immobilize whole IgG onto AFM probes for force measurement experiments and that a surfactant is useful for improving the selectivity for such measurements.

Glow-discharge treated piezoelectric quartz crystals as immunosensors for HSA detection

The aim of this study is to develop an immunoaffinity sensor based on piezoelectric crystals for human serum albumin (HSA) detection in aqueous media. Quartz crystals were treated with ethylene diamine (EDA) plasma in a glow-discharge apparatus in order to substitute amino groups on their surfaces. Then anti-HSA antibodies were immobilized via these amino groups by using glutaraldehyde (GA) as cross-linker. Immobilization of the antibody on the quartz crystal was examined for different pH, antibody concentration and treatment time. The optimum conditions for anti-HSA immobilization were evaluated by the measurements of the activity of the surface against HSA. The optimum values of pH, antibody concentration and treatment time were found 6.2, 0.15 mg/ml and 2 h, respectively. For detection of HSA into the solution, two methods were used. In the first (dip and dry) method, the frequency shifts were measured in air after the 1 h interaction of the anti-HSA immobilized crystals with HSA solution. In the other (direct) method, the frequency shifts were followed continuously for 60 min. while the probe was immersed in the HSA solution. An increase for the frequency shifts was observed with increasing of HSA concentration of 16-128 microg/ml. Both the immobilization and antibody-interaction conditions were found important on the extend of these specific interaction. The relations between the HSA concentrations and frequency shifts were exponential in both methods.

Comparison of different ligand densities for the manufacture of CB Hep-1 immunosorbents

Different ligand densities of monoclonal antibody (Mab) CB.Hep-1 were studied during covalent coupling on Sepharose CL-4B for recombinant hepatitis B surface antigen (rHBsAg) immunoaffinity purification. Ligand densities of 2.2, 3.2, 4.2 and 5.2 mg Mab/ml immunosorbents, respectively, were assayed during five cycles of immunoaffinity chromatography (IAC). Adsorption capacities averaged either 3.2 mg/ml (0.57 mg rHBsAg/ml immunosorbent/5.42 mg of total purified protein) or 5.2 mg/ml (0.56 mg rHBsAg/ml immunosorbent/5.05 mg total purified protein). Immunosorbents showed ligand leakage levels below 3 ng Mab/microg rHBsAg. Antigen purity was higher than 95% in all cases. The results suggest that a ligand density (LD) of 3.2 mg Mab/ml immunosorbent should be used for immunoaffinity chromatography because no significant differences were found in the ligand densities studied (P-value=0.012), which saves 40% of CB.Hep-1 immunosorbent manufacturing cost in comparison with 5 mg Mab/ml immunosorbent, which is currently used in large-scale production.

Affinity protection chromatography for efficient labeling of antibodies for use in affinity capillary electrophoresis

Capillary electrophoresis immunoassay (CEIA) is shown to be substantially more sensitive to the antibody (Ab) reagent quality than are immunosorbent methods such as enzyme-linked immunosorbent assays (ELISA). Cyanine 5 (Cy5)-labeled monoclonal anti-ovalbumin (mAb*) was inactive for CEIA of ovalbumin (Ov), yet was functional in ELISA for Ov. ELISA showed the mAb* was at least ten times less active, accounting for the poor CEIA performance. Labeled polyclonal Ab was inactive for a dye to protein ratio greater than 1.6. An affinity protection chromatography procedure (APC) was developed for Ab labeling, which avoided degradation of the Ab binding site. Ov was covalently bound to cyanogen bromide activated cellulose gel in a column, and used to capture the Ab. The coupling efficiency for Ov to the gel was 74-97%, Ab could then be bound with 95-100% efficiency, and Ab* was recovered in 50% yield following labeling on the column. This procedure was performed successfully in three different laboratories, indicating the robustness of the optimized APC synthetic method. No inactive Ab* could be detected in the APC product. The CEIA detection limit for ovalbumin using APC labeled mAb was 173 nM, when [Ab*] was fixed at 163 nM. The association constants of mAb and mAb* were determined by CEIA.

The solid phase in affinity chromatography: strategies for antibody attachment

Antibodies (Ab) are commonly used in affinity chromatography (AC) as a versatile and specific means of isolating target molecules from complex mixtures. A number of procedures have been developed to immobilize antibodies on the solid matrix. Some of these methods couple the antibody via chemical groups that may be important for specific recognition of antigen, resulting in loss of functionality in a proportion of the antibodies. In other methods, the outcome of immobilization is coupling via unique sites in the Fc region of the antibody molecule, ensuring orientation of the antibody combining sites (Fab) towards the mobile phase. This review discusses the advantages and disadvantages of the various methods available for immobilization and outlines protocols for site-directed, covalent coupling of the antibody to the solid phase that essentially retains the activity of the antibody.

Stirrer tank: an appropriate technology to immobilize the CB.Hep-1 monoclonal antibody for immunoaffinity purification

The CB.Hep-1 monoclonal antibody was coupled to CNBr-activated Sepharose CL 4B at three different immobilization scales for purification of recombinant hepatitis B surface antigen. Standard laboratory apparatus to obtain immunosorbents of 1 l (scale I) and 3 l (scale II) as well as a stirrer tank to prepare 6 l immunosorbents (scale III) were used. The binding capacity at scale III was 2- and 1.5-fold higher with respect to the scales II and I, while a reduction in the ligand leakage of 5- and 2-folds was observed. Immunosorbents from scale II showed a significantly reduced adsorption, and an increased ligand leakage. Differences in the coupling efficiency were not observed. Antigen purity eluted from the immunosorbents was always above 85%.

Immunosensing platforms using spontaneously adsorbed antibody fragments on gold

This paper describes the construction and characterization of miniaturized antigenic immunosurfaces composed of spontaneously adsorbed Fab'-SH fragments on gold. Rabbit Fab'-SH fragments contain a free sulfhydryl group that forms a thiolate bond with a gold substrate as detailed by X-ray photoelectron spectroscopy. This approach creates surfaces of higher epitope density, a factor critical to the early detection of disease, than surfaces composed of adsorbed whole molecule IgG on gold. The viability and specificity of antigenic Fab'-SH immunosurfaces is demonstrated using atomic force microscopy and confocal fluorescence microscopy, and possible explanations for the larger epitope density are discussed.

Immobilization of antibodies onto glass wool

The immobilization of antibodies onto solid phases in an efficient and activity-retaining form is an important goal for both research and industry. Methods have been developed for the site-directed attachment of antibodies to agarose by oxidation of the carbohydrate moieties in their Fc region. Similar attachment to silianized supports have not been as successful. Here we describe a novel combination protocol for the site-directed attachment of periodate oxidized, goat polyclonal antibodies to glass wool fibers activated with 3-aminopropyltriethoxysilane. The study demonstrates that this procedure results in effective immobilization of polyclonal antibodies that retain their antigen-binding capacity. This protocol should prove useful in the development of more efficient and effective glass-based immunosupports.

Purification and quantitation of tumor necrosis factor receptor immunoadhesin using a combination of immunoaffinity and reversed-phase chromatography

The development of an automated, dual column assay to quantitate and recover the glycoprotein, tumor necrosis factor receptor immunoadhesin (TNFr-IgG) from monkey plasma, human serum, cell culture fluid and buffer samples is described. A combination of immunoaffinity and reversed-phase chromatographies are used. The targeted protein was captured using an anti-TNFr-1 monoclonal antibody immobilized on POROS resin. After non-specific adsorption had been reduced, the affinity column was placed in-line with a reversed-phase column and eluted with dilute acid. The reversed-phase column was subsequently eluted with an acetonitrile gradient and the TNFr-IgG collected and quantitated by comparison with peak areas of similarly treated standards. Detection was performed by measurement of absorbance at 214 nm. The dynamic range is from 0.5-15 microg total sample. Samples were quantitated and recovered from monkey and human pharmacokinetics samples, as well as from cell culture fluid and buffers. The lowest concentrations assayed were 100 ng ml(-1). Quantitation is reproducible, with a coefficient of variation of 2%. The procedure was used to develop a pharmacokinetic profile for the clearance of TNFr-IgG in humans and cynomolgus monkeys. Sufficient material was recovered such that the glycoforms could be identified. Additionally it has been used for process monitoring. The results compared favorably with data generated by ELISA. Optimization of the method and results are presented.

Microminiaturized immunoassays using atomic force microscopy and compositionally patterned antigen arrays

This paper combines the topographic imaging capability of the atomic force microscope (AFM) with a compositionally patterned array of immobilized antigenic rabbit IgG on gold as an approach to performing immunoassays. The substrates are composed of micrometer-sized domains of IgG that are covalently linked to a photolithographically patterned array of a monolayer-based coupling agent. The immobilized coupling agent, which is prepared by the chemisorption of dithiobis(succinimidyl undecanoate) on gold, is separated by micrometer-sized grids of a monolayer formed from octadecanethiol (ODT). The strong hydrophobicity of the ODT adlayer, combined with the addition of the surfactant Tween 80 to the buffer solution that is used in forming the antibody-antigen pairs, minimizes the nonspecific adsorption of proteinaceous materials to the grid regions. This minimization allows the grids to function as a reference plane for the AFM detection of the height increase when a complementary antibody-antigen pair is formed. The advantageous features of this strategy, which include ease of sample preparation, an internal reference plane for the detection of topographic changes, and the potential for regeneration and reuse, are demonstrated using rabbit IgG as an immobilized antigen and goat anti-rabbit IgG as the complementary antibody. The prospects for further miniaturization are discussed.

Capillary electrophoresis based immunoassays: a critical review

The purpose of this article is to review and evaluate the uses and potentials of capillary electrophoresis (CE) in immunoassay analysis (IA). This review will be divided into four sections. First, a brief summary of the fundamentals, applications and requirements of immunoassays in both research and clinical diagnostics will be given. This section will also cover the rationale behind the current research to combine CE and IA. Also, the specific needs to CE in this field will be addressed. Second, the modes of use of CE in IA will be discussed and typical applications for each will be given. Third, a separate section will be devoted to the investigation of performing immunoassays on micro fabricated devices, an interesting alternative to the conventional capillary-based approach. Finally, a critical assessment of the current status and merits of this technology will be presented.

An analysis of transport resistances in the operation of BIAcore; implications for kinetic studies of biospecific interactions

The mass-transfer processes that affect kinetic measurements of biospecific interactions between one species in a flowing solution and another species immobilized in a thin hydrogel instrument were analysed by means of convection-diffusion reaction models. The specific purpose was to identify experimental design considerations for kinetic measurements using the BIAcore instrument. Numerical solutions identified three different regimes of operation. A kinetic regime exists at low values of Damkohler number (Da), when the intrinsic kinetics are slow and the diffusion is relatively fast. This allows for the accurate determination of kinetic constants. A limiting value of Da, above which mass-transfer limitations appear, is presented as a function of Peclet number, Pe. At higher Da values, the reaction occurs in the mass-transfer-controlled regime where the reaction-rate is independent of the intrinsic kinetics. It was observed that, frequently, the reaction occured in an intermediate regime where, although the mass-transfer rate was not strictly limiting, substantial concentration gradients were present. Analysing the data in this regime by direct application of kinetic equations underestimates the association rate constant. Even when the reaction was not limited by mass-transfer in the flow channel, it may have been affected by steric hindrance to transport in the hydrogel, if a large concentration of capturing antibody or ligand was immobilized. The primary effect of the hindrance was to lower the soluble-species (analyte) concentration in the hydrogel when compared to the bulk solution. Non-uniformity of conditions within the hydrogel in the presence of steric hindrance had a significant effect on the observed reaction. The effect was most prominent at higher analyte concentration, when the rate constant showed an apparent reduction as the reaction progressed.

Purification of trypsin by affinity chromatography with sulphamethoxazolum ligand

Pharmaceuticals have already been studied comprehensively both in their physico-chemical properties and their biological effect. Most of these compounds are chemically synthesized and less susceptible to degradation by micro-organism or suffering from solvent effect compared with the bio-active substances. Affinity chromatographic columns composed of pharmaceutical compounds as the ligand should have advantages such as long life, low cost and low toxicity. This paper describes the preparation of sulphamethoxazolum immobilized on silica as the column packing and the study of its interactions with proteins and enzymes. It was found that this material showed an affinity specific to trypsin with a dissociation constant of around 10(-6) M. The enzymatic activity of commercial trypsin can be increased by a factor of ten after purification with such a column.

Selective preconcentration for capillary zone electrophoresis using protein G immunoaffinity capillary chromatography

Capillaries with 150 microns inner diameter were packed with a perfused protein G chromatographic support and used as immunoaffinity preconcentrators for capillary zone electrophoresis. Antibody was loaded onto the protein G support to form an immunoaffinity stationary phase. Injection of samples onto the column caused selective retention and preconcentration of antigen. Injection of appropriate buffers onto the column caused desorption of the antibody and antigen which were then separated by capillary zone electrophoresis. The combination was used on-line and off-line. For on-line combination, a flow-gated interface coupled the two columns and allowed injection of desorbed zones onto the electrophoresis system. Off-line coupling required collection of desorbed fractions and then injection onto the electrophoresis system. Flow rates as high as 100 microL/min were used to load sample onto the affinity column. Desorbing flow rates had to be 1 microL/min or less to prevent excessive dilution during desorption. Using the system, 1 mL insulin samples could be loaded onto the affinity column and desorbed in volumes as small as 1 microL for 1000-fold preconcentration. The use of the preconcentrator with serum samples spiked with insulin was demonstrated.

Comparison of methods for immobilization to carboxymethyl dextran sensor surfaces by analysis of the specific activity of monoclonal antibodies

The authors have recently described the development of a carboxymethyl dextran-based sensor surface for biospecific interaction analysis by surface plasmon resonance. Ligands are immobilized via primary amine groups after activation of the carboxymethyl groups on the sensor surface with a mixture of N-hydroxysuccinimide and N-ethyl-N'-(dimethylaminopropyl) carbodiimide. Methods have now been developed for efficient immobilization via thiol/disulfide exchange, aldehyde coupling and biotin-avidin coupling. The specific activity of monoclonal antibodies immobilized by the four different methods was investigated by altering the immobilization conditions, e.g., activation time, protein concentration, ionic strength and the degree of modification, etc. Investigations have also been made concerning possible differences in the specific activity for antibodies immobilized using optimized conditions with respect to the four different chemistries. These studies show that, with the flexible carboxymethyl dextran matrix used here, the immobilization methods give rise to only minor differences in specific activity. Thus, with this solid support, a 'site directed' immobilization strategy for monoclonal antibodies has no advantage. In general the specific activity for optimized systems was approximately 75% for the binding of beta 2 mu-globulin to an immobilized monoclonal antibody directed against beta 2 mu-globulin. Reduced specific activities of immobilized antibodies induced by variation of the coupling conditions could be attributed to the deterioration of the active site of the antibody.

Development of enzyme-linked immunosorbent assays (ELISAs) for the detection of monoclonal antibody leakage from immunoaffinity chromatography matrices

Antibodies can leak from immunoaffinity chromatography (IAC) matrices, reducing the working life of the IAC matrix and/or compromising the purity of the product, purified by IAC, for therapeutic use. There is therefore a need to monitor the leakage of antibody from IAC matrices. Antibody leakage from a model IAC system was measured using two-site 'non-competitive' ELISAs. Two assays were developed to measure the leakage of intact and fragmented antibody from the IAC matrix. By measuring the leakage of intact and fragmented antibody the mechanisms underlying antibody leakage from solid-supports could be elucidated.

Immunoaffinity chromatography

The basic procedure of immunoaffinity chromatography (IAC) is described. The insoluble support matrices available for IAC and their activation chemistries, including some of the most recently introduced, are reviewed. Means of selecting the most appropriate monoclonal antibody (MAb) are described, although an empirical approach is still required for the final choice of antibody. Precise methods of running IAC columns are surveyed including the binding, washing, and elution stages, although no precise recommendations can be made particularly for the elution step since this is unique to a particular MAb and antigen. All IAC sorbents lose activity with time through a combination of MAb inactivation and ligand leakage. The relative importance of the two phenomena is discussed, and suggestions are made to minimize the problem along with an indication of the relative stabilities of a range of coupling chemistries. A sample of the proteins purified by IAC is given together with pointers to the future of the technique.