The immunochemistry of sandwich ELISAs--VI. Greater than 90% of monoclonal and 75% of polyclonal anti-fluorescyl capture antibodies (CAbs) are denatured by passive adsorption

Surface Modification of Polystyrene with Boronic Acid for Immunoaffinity-Based Cell Enrichment

Obtaining an enriched and phenotypically pure cell population from heterogeneous cell mixtures is important for diagnostics and biosensing. Existing techniques such as fluorescent-activated cell sorting (FACS) and magnetic-activated cell sorting (MACS) require preincubation with antibodies (Ab) and specialized equipment. Cell immunopanning removes the need for preincubation and can be done with no specialized equipment. The majority of the available antibody-mediated analyte capture techniques require a modification to the Abs for binding. In this work, no antibody modification is used because we take advantage of the carbohydrate chain in the Fc region of Ab. We use boronic acid as a cross-linker to bind the Ab to a modified surface. The process allows for functional orientation and cleavable binding of the Ab. In this study, we created an immunoaffinity matrix on polystyrene (PS), an inexpensive and ubiquitous plastic. We observed a 37% increase in Ab binding compared with that of a passive adsorption approach. The method also displayed a more consistent antibody binding with 17 times less variation in Ab loading among replicates than did the passive adsorption approach. Surface topography analysis revealed that a dextran coating reduced nonspecific antibody binding. Elemental analysis (XPS) was used to characterize the surface at different stages and showed that APBA molecules can bind upside-down on the surface. While upside-down antibodies likely remain functional, their elution behavior might differ from those bound in the desired way. Cell capture experiments show that the new surface has 43% better selectivity and 2.4-fold higher capture efficiency compared to a control surface of passively adsorbed Abs. This specific surface chemistry modification will allow the targeted capture of cells or analytes with the option of chemical detachment for further research and characterization.

Plasma Activation of Microplates Optimized for One-Step Reagent-Free Immobilization of DNA and Protein

Activated microplates are widely used in biological assays and cell culture to immobilize biomolecules, either through passive physical adsorption or covalent cross-linking. Covalent attachment gives greater stability in complex biological mixtures. However, current multistep chemical activation methods add complexity and cost, require specific functional groups, and can introduce cytotoxic chemicals that affect downstream cellular applications. Here, we show a method for one-step linker-free activation of microplates by energetic ions from plasma for covalent immobilization of DNA and protein. Two types of energetic ion plasma treatment were shown to be effective: plasma immersion ion implantation (PIII) and plasma-activated coating (PAC). This is the first time that PIII and PAC have been reported in microwell plates with nonflat geometry. We confirm that the plasma treatment generates radical-activated surfaces at the bottom of wells despite potential shadowing from the walls. Comprehensive surface characterization studies were used to compare the PIII and PAC microplate surface composition, wettability, radical density, optical properties, stability, and biomolecule immobilization density. PAC plates were found to have more nitrogen and lower radical density and were more hydrophobic and more stable over 3 months than PIII plates. Optimal conditions were obtained for high-density DNA (PAC, 0 or 21% nitrogen, pH 3-4) and streptavidin (PAC, 21% nitrogen, pH 5-7) binding while retaining optical properties required for typical high-throughput biochemical microplate assays, such as low autofluorescence and high transparency. DNA hybridization and protein activity of immobilized molecules were confirmed. We show that PAC activation allows for high-density covalent immobilization of functional DNA and protein in a single step on both 96- and 384-well plates without specific linker chemistry. These microplates could be used in the future to bind other user-selected ligands in a wide range of applications, for example, for solid phase polymerase chain reaction and stem cell culture and differentiation.

Bioaffinity-based surface immobilization of antibodies to capture endothelial colony-forming cells

Maximizing the re-endothelialization of vascular implants such as prostheses or stents has the potential to significantly improve their long-term performance. Endothelial progenitor cell capture stents with surface-immobilized antibodies show significantly improved endothelialization in the clinic. However, most current antibody-based stent surface modification strategies rely on antibody adsorption or direct conjugation via amino or carboxyl groups which leads to poor control over antibody surface concentration and/or molecular orientation, and ultimately bioavailability for cell capture. Here, we assess the utility of a bioaffinity-based surface modification strategy to immobilize antibodies targeting endothelial cell surface antigens. A cysteine-tagged truncated protein G polypeptide containing three Fc-binding domains was conjugated onto aminated polystyrene substrates via a bi-functional linking arm, followed by antibody immobilization. Different IgG antibodies were successfully immobilized on the protein G-modified surfaces. Covalent grafting of the protein G polypeptide was more effective than surface adsorption in immobilizing antibodies at high density based on fluorophore-labeled secondary antibody detection, as well as endothelial colony-forming cell capture through anti-CD144 antibodies. This work presents a potential avenue for enhancing the performance of cell capture strategies by using covalent grafting of protein G polypeptides to immobilize IgG antibodies.

The effect of charged residue substitutions on the thermodynamics of protein-surface interactions

The interactions of proteins with surfaces are important in both biological processes and biotechnologies. In contrast to decades of study regarding the biophysics of proteins in bulk solution, however, our mechanistic understanding of the biophysics of proteins interacting with surfaces remains largely qualitative. In response, we have set to explore quantitatively the thermodynamics of protein-surface interactions. In this work, we explore systematically the role of electrostatics in modulating the interaction between proteins and charged surfaces. In particular, we use electrochemistry to explore the extent to which a macroscopic, hydroxyl-coated surface held at a slightly negative potential affects the folding thermodynamics of surface-attached protein variants with different composition of charged amino acids. Doing so, we find that attachment to the surface generally leads to a net stabilization, presumably due to excluded volume effects that reduce the entropy of the unfolded state. The magnitude of this stabilization, however, is strongly correlated with the charged-residue content of the protein. In particular, we find statistically significant correlations with both the net charge of the protein, with greater negative charge leading to less stabilization by the surface, and with the number of arginines, with more arginines leading to greater stabilization. Such findings refine our understanding of protein-surface interactions, providing in turn a guiding rationale to achieve the functional deposition of proteins on artificial surfaces for implementation in, for example, protein-based biotechnologies.

Directional immobilization of antibody onto magnetic nanoparticles by Fc-binding protein-assisted photo-conjugation for high sensitivity detection of antigen

Immobilized antibodies with site-specific, oriented, and covalent pattern are of great significance to improve the sensitivity of solid-phase immunoassay. Here, we developed a novel antibody conjugation strategy that can immobilize antibodies in a directional and covalent manner. In this study, an IgG-Fc binding protein (Z domain) carrying a site-specific photo-crosslinker, p-benzoyl-L-phenylalanine, and a single C-terminal cysteine (Cys) handle was genetically engineered. Upon UV irradiation, the chimeric protein enables the Cys handle to couple with the native antibody in Fc-specific and covalent conjugation pattern, resulting in a novel thiolated antibody. Thus, an approach for the covalent, directional immobilization of antibodies to maleimide-modified magnetic nanoparticles (MNPs) was developed on the basis of the crosslinking between sulfhydryl and maleimide groups. The antibody-conjugated MNPs were applied in MNP-based enzyme-linked immunosorbent assay (ELISA) for the detection of carcinoembryonic antigen. The MNP-based ELISA presented a quantification linear range of 0.1-100 ng mL^-1 and detection limit of 0.02 ng mL^-1, which was approximately 100 times more sensitive than the traditional microplate ELISA (2.0 ng mL^-1). Thus, the proposed antibody immobilization approach can be used in surface functionalization for the sensitive detection of various biomarkers.

Highly enhanced ELISA sensitivity using acetylated chitosan surfaces

Background

The enzyme-linked immunosorbent assay (ELISA), is the most widely used and reliable clinical routine method for the detection of important protein markers in healthcare. Improving ELISAs is crucial for detecting biomolecules relates to health disorders and facilitating diagnosis at the early diseases stages. Several methods have been developed to improve the ELISA sensitivity through immobilization of antibodies on the microtiter plates. We have developed a highly sensitive ELISA strategy based on the preparation of acetylated chitosan surfaces in order to improve the antibodies orientation.

Results

Chitin surfaces were obtained by mixing small quantities of chitosan and acetic anhydride in each well of a microtiter plate. Anti-c-myc 9E10 low affinity antibody fused to ChBD was cloned and expressed in CHO cells obtaining the anti-c-myc-ChBD antibody. We found that anti c-myc-ChBD binds specifically to the chitin surfaces in comparison with anti-c-myc 9E10, which did not. Chitin surface was used to develop a sandwich ELISA to detect the chimeric human protein c-myc-GST-IL8 cloned and expressed in Escherichia coli. The ELISA assays developed on chitin surfaces were 6-fold more sensitive than those performed on standard surface with significant differences (p<0,0001).

Conclusions

As shown here, acetylated chitosan surfaces improve the antibody orientation on the substrate and constitute a suitable method to replace the standard surfaces given the stability over time and the low cost of its preparation.

Endothelialization of cardiovascular devices

Blood-contacting surfaces of cardiovascular devices are not biocompatible for creating an endothelial layer on them. Numerous research studies have mainly sought to modify these surfaces through physical, chemical and biological means to ease early endothelial cell (EC) adhesion, migration and proliferation, and eventually to build an endothelial layer on the surfaces. The first priority for surface modification is inhibition of protein adsorption that leads to inhibition of platelet adhesion to the device surfaces, which may favor EC adhesion. Surface modification through surface texturing, if applicable, can bring some hopeful outcomes in this regard. Surface modifications through chemical and/or biological means may play a significant role in easy endothelialization of cardiovascular devices and inhibit smooth muscle cell proliferation. Cellular engineering of cells relevant to endothelialization can boost the positive outcomes obtained through surface engineering. This review briefly summarizes recent developments and research in early endothelialization of cardiovascular devices. STATEMENT OF SIGNIFICANCE: Endothelialization of cardiovascular implants, including heart valves, vascular stents and vascular grafts is crucial to solve many problems in our health care system. Numerous research efforts have been made to improve endothelialization on the surfaces of cardiovascular implants, mainly through surface modifications in three ways - physically, chemically and biologically. This review is intended to highlight comprehensive research studies to date on surface modifications aiming for early endothelialization on the blood-contacting surfaces of cardiovascular implants. It also discusses future perspectives to help guide endothelialization strategies and inspire further innovations.

Characterization of canine anti-mouse antibodies highlights that multiple strategies are needed to combat immunoassay interference

Immunoassays are widely used for detection and quantification of analytes in biological samples, but are vulnerable to analytical errors caused by interfering sample substances. Of particular interest are endogenous anti-animal antibodies that may bind to the immunoassay antibodies and cause erroneous test results. This phenomenon is a hazard to patient safety in both human and veterinary medicine. Here, we demonstrate that anti-mouse antibodies in dogs bind selectively to different regions of the murine IgG molecule, cross-react with IgG from different species, and consist of all major antibody classes present in canine serum (IgA, IgG and IgM). The antibody characteristics varied among individuals and their prevalence differed between two dog breeds. The selective binding to different IgG regions suggests that the antibodies might not originate from immunization through exposure to mice or other species. These findings show that canine anti-mouse antibodies are highly heterogeneous in nature and therefore require a combination of strategies to be counteracted.

Influence of antibody immobilization strategy on carbon electrode immunoarrays

We report here the influence of antibody immobilization strategy for protein immunosensors on screen printed carbon electrode arrays in terms of antibody binding activity, analytical sensitivity, limit of detection, and stability. Horseradish peroxidase (HRP) was the model analyte with anti-HRP immobilized on the sensors, and HRP activity was used for detection. Covalently immobilized anti-HRP antibodies on electrodes coated with chitosan, electrochemically reduced graphene oxide (rGO), and dense gold nanoparticle (AuNP) films had only 20-30% of the total immobilized antibodies active for binding. Active antibodies increased to 60% with passively adsorbed antibodies on bare electrodes, to 85% with oriented antibodies using protein A covalently immobilized on AuNP-coated carbon electrode, and to 98% when attached to protein A passively adsorbed onto bare electrodes. Passively adsorbed antibodies on bare electrodes lost activity in 1-2 days, but antibodies immobilized using other strategies remained relatively stable after 5 days. Covalent immobilization gave limits of detection (LOD) of 40 fg mL-1, while passively adsorbed antibodies or protein A on carbon electrodes had LODs 4-8 fg mL-1, but were unstable. Sensitivity was highest for antibodies covalently attached to AuNP electrodes (2.40 nA per log pg per mL) that also had highest antibody coverage, and decreased slightly when protein A on AuNP was used to orient antibodies. Passively adsorbed antibodies and oriented antibodies on protein A gave slightly lower sensitivities. Immobilization strategy or antibody orientation did not have a significant effect on LOD, but dynamic range increased as the number of active antibodies on sensor surfaces increased.

Quantitation of monoclonal antibody by capture ELISA based on initial enzyme activity rate

We developed a noncompetitive two-site sandwich ELISA to quantitate monoclonal antibodies in culture supernatant. This assay measures the initial enzyme activity rate during the first minute of the reaction, which ensures linear velocity relative to time and a progress curve slope proportional to analyte concentration. During this period, the enzyme substrate is in large excess relative to the analyte/antibody-enzyme complex, and enzyme catalysis proceeds in steady-state conditions. Analyses of repeatability gave coefficients of variation between 4.4 and 9.7 (interassay) and 4.4 and 6.4 (intra-assay), and analyte detectability ranged from 5.8 to 12 ng/ml. The Z-factor calculated for analyte samples at their end dilution yielded mean values from 0.57 to 0.87, which confirmed assay robustness. This initial velocity-based sandwich ELISA is a simple, sensitive, reproducible method to quantitate bi-epitopic antigens.

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Antibody quantitation by measuring initial velocity of enzyme product formation

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Initial rate measurements ensure ELISA velocity proportional to analyte concentration.

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A mAb isotype-matched standard curve could reduce antiglobulin heterogenous reactivity.

Accessible Telemedicine Diagnostics with ELISA in a 3D Printed Pipette Tip

We report herein a novel pipet-based "ELISA in a tip" as a new versatile diagnostic tool featuring better sensitivity, shorter incubation time, accessibility, and low sample and reagent volumes compared to traditional ELISA. Capture and analysis of data by a cell phone facilitates electronic delivery of results to health care providers. Pipette tips were designed and 3D printed as adapters to fit most commercial 50-200 μL pipettes. Capture antibodies (Ab1) are immobilized on the inner walls of the pipet tip, which serves as the assay compartment where samples and reagents are moved in and out by pipetting. Signals are generated using colorimetric or chemiluminescent (CL) reagents and can be quantified using a cell phone, CCD camera, or plate reader. We utilized pipet-tip ELISA to detect four cancer biomarker proteins with detection limits similar to or lower than microplate ELISAs at 25% assay cost and time. Recoveries of these proteins from spiked human serum were 85-115% or better, depending slightly on detection mode. Using CCD camera quantification of CL with femto-luminol reagent gave limits of detection (LOD) as low as 0.5 pg/mL. Patient samples (13) were assayed for 3 biomarker proteins with results well correlated to conventional ELISA and an established microfluidic electrochemical immunoassay.

Fabrication of a polypropylene immunoassay platform by photografting reaction

The technology of an immunoassay detection platform is critical to clinical disease diagnoses, especially for developing a medical diagnostic system. A polymer-based immunoassay platform was fabricated on nonwoven fabric polypropylene (PP) using a photografting reaction to graft 2-hydroxyethyl methacrylate (HEMA) and sulfobetaine (SBMA). The antifouling properties of PP-g-P(HEMA-co-SBMA) were investigated by fibrinogen adsorption and platelet adhesion. Carbonyldiimidazole was employed to activate the pendant hydroxyl groups in HEMA moieties and covalently coupled antibody molecules. The detection of the limit of the immunoassay platform was as low as 10 pg/mL. Antibody amount and bioactivity affected the availability of antibody and the sensitivity of immunoassay. The immune efficiency was dependent on the strategies of antibody immobilization. The immune efficiency of Au-g-P(SBMA-co-HEMA) and Au-SH surfaces measured by QCM-D was 165% and 35.7%, respectively. The covalently binding antibody via hydrophilic polymer chains as spacers could retain fragment antigen-binding up orientation, maintain the bioactivity of antibody, and mainly improve the accessibility of antibody molecules via adjusting the conformations of polymer chains when the antibodies recognized the antigens. Therefore, grafting hydrophilic polymers, such as zwitterionic PSBMA and reactive PHEMA onto nonwoven fabric PP, and binding antibody by covalent strategy had the potential to be developed as a commercial immunoassay platform.

Inhibitory Anti-Peroxidasin Antibodies in Pulmonary-Renal Syndromes

BACKGROUND

Goodpasture syndrome (GP) is a pulmonary-renal syndrome characterized by autoantibodies directed against the NC1 domains of collagen IV in the glomerular and alveolar basement membranes. Exposure of the cryptic epitope is thought to occur via disruption of sulfilimine crosslinks in the NC1 domain that are formed by peroxidasin-dependent production of hypobromous acid. Peroxidasin, a heme peroxidase, has significant structural overlap with myeloperoxidase (MPO), and MPO-ANCA is present both before and at GP diagnosis in some patients. We determined whether autoantibodies directed against peroxidasin are also detected in GP.

METHODS

We used ELISA and competitive binding assays to assess the presence and specificity of autoantibodies in serum from patients with GP and healthy controls. Peroxidasin activity was fluorometrically measured in the presence of partially purified IgG from patients or controls. Clinical disease severity was gauged by Birmingham Vasculitis Activity Score.

RESULTS

We detected anti-peroxidasin autoantibodies in the serum of patients with GP before and at clinical presentation. Enriched anti-peroxidasin antibodies inhibited peroxidasin-mediated hypobromous acid production in vitro. The anti-peroxidasin antibodies recognized peroxidasin but not soluble MPO. However, these antibodies did crossreact with MPO coated on the polystyrene plates used for ELISAs. Finally, peroxidasin-specific antibodies were also found in serum from patients with anti-MPO vasculitis and were associated with significantly more active clinical disease.

CONCLUSIONS

Anti-peroxidasin antibodies, which would previously have been mischaracterized, are associated with pulmonary-renal syndromes, both before and during active disease, and may be involved in disease activity and pathogenesis in some patients.

Generating FN3-Based Affinity Reagents Through Phage Display

Antibodies are useful tools for detecting individual proteins in complex samples and for learning about their location, amount, binding partners, and function in cells. Unfortunately, generating antibodies is time consuming and laborious, and their affinity and/or specificity is often limited. This protocol offers a fast and inexpensive alternative to generate antibody surrogates through phage display of a library of fibronectin type III (FN3) monobody variants and affinity selection for binders. © 2018 by John Wiley & Sons, Inc.

A Sensitive and Robust Assay for Urokinase and Tissue-Type Plasminogen Activators (Upa and Tpa) and Their Inhibitor Type I (Pai-1) in Breast Tumor Cytosols

uPA and PAI-1 are becoming established as amongst the most effective markers of poor prognosis for patients with node-negative breast cancer; tPA is an index of longer survival. This paper describes a sensitive ELISA for the measurement of uPA, tPA and PAI-1 in breast cancer cytosols. The structure of the assay involves coating Ab (sheep α-Chicken IgY), catching Ab (chicken α-analyte), tagging Ab (rabbit α-analyte) and detecting Ab (goat α-rabbit IgG) labelled with HRP. The assay has a high degree of accuracy and specificity. Comparison with the American Diagnostica kits shows the results’ equivalence for PAI-1 and tPA. For uPA the results of the assay were twice as high. The assay is sensitive and relatively inexpensive. It is the first published assay to yield strictly comparative values for uPA, tPA and PAI-1 in tissue extracts and is readily subject to external quality control.

Human neonatal Fc receptor as a new potential antibody binding protein for antibody immobilization

A critical challenge in producing an antibody-based assay with the highest reproducibility and sensitivity is the strategy to immobilize antibodies to solid phase. To date, numerous methods of antibody immobilization were reported but each was subjected to its advantages and limitations. The current study proposes a new potential antibody binding protein, the human neonatal fragment crystallizable (Fc) receptor. This protein has shown its high affinity to the Fc of antibody either in vivo or in vitro. Human neonatal Fc receptor is a heterodimer constructed by p51 α-heavy chain and β2-microglobulin light chain; however, the binding sites toward the antibody are located in the p51 α-heavy chain. Hence, vector cloning and recombinant protein expression were carried out to express the p51 α-heavy chain of the human neonatal Fc receptor (hFcRn-α). The recombinant protein expressed, hFcRn-α, was adopted to pin rabbit IgG against hepatitis B virus surface antigen to a solid phase. A sandwich enzyme-linked immunosorbent assay was further developed to evaluate the efficiency of hFcRn-α-directed immobilization in antigen detection. The result was compared with the conventional physical adsorption method. The findings demonstrated that human neonatal Fc receptor was efficient in pinning antibodies and generating higher signals compared with the physical adsorption of antibody.

Materials and microfluidics: enabling the efficient isolation and analysis of circulating tumour cells

We present a critical review of microfluidic technologies and material effects on the analyses of circulating tumour cells (CTCs) selected from the peripheral blood of cancer patients. CTCs are a minimally invasive source of clinical information that can be used to prognose patient outcome, monitor minimal residual disease, assess tumour resistance to therapeutic agents, and potentially screen individuals for the early diagnosis of cancer. The performance of CTC isolation technologies depends on microfluidic architectures, the underlying principles of isolation, and the choice of materials. We present a critical review of the fundamental principles used in these technologies and discuss their performance. We also give context to how CTC isolation technologies enable downstream analysis of selected CTCs in terms of detecting genetic mutations and gene expression that could be used to gain information that may affect patient outcome.

Surface immobilized antibody orientation determined using ToF-SIMS and multivariate analysis

Antibody orientation at solid phase interfaces plays a critical role in the sensitive detection of biomolecules during immunoassays. Correctly oriented antibodies with solution-facing antigen binding regions have improved antigen capture as compared to their randomly oriented counterparts. Direct characterization of oriented proteins with surface analysis methods still remains a challenge however surface sensitive techniques such as Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) provide information-rich data that can be used to probe antibody orientation. Diethylene glycol dimethyl ether plasma polymers (DGpp) functionalized with chromium (DGpp+Cr) have improved immunoassay performance that is indicative of preferential antibody orientation. Herein, ToF-SIMS data from proteolytic fragments of anti-EGFR antibody bound to DGpp and DGpp+Cr are used to construct artificial neural network (ANN) and principal component analysis (PCA) models indicative of correctly oriented systems. Whole antibody samples (IgG) test against each of the models indicated preferential antibody orientation on DGpp+Cr. Cross-reference between ANN and PCA models yield 20 mass fragments associated with F(ab')₂ region representing correct orientation, and 23 mass fragments associated with the Fc region representing incorrect orientation. Mass fragments were then compared to amino acid fragments and amino acid composition in F(ab')₂ and Fc regions. A ratio of the sum of the ToF-SIMS ion intensities from the F(ab')₂ fragments to the Fc fragments demonstrated a 50% increase in intensity for IgG on DGpp+Cr as compared to DGpp. The systematic data analysis methodology employed herein offers a new approach for the investigation of antibody orientation applicable to a range of substrates.

STATEMENT OF SIGNIFICANCE

Controlled orientation of antibodies at solid phases is critical for maximizing antigen detection in biosensors and immunoassays. Surface-sensitive techniques (such as ToF-SIMS), capable of direct characterization of surface immobilized and oriented antibodies, are under-utilized in current practice. Selection of a small number of mass fragments for analysis, typically pertaining to amino acids, is commonplace in literature, leaving the majority of the information-rich spectra unanalyzed. The novelty of this work is the utilization of a comprehensive, unbiased mass fragment list and the employment of principal component analysis (PCA) and artificial neural network (ANN) models in a unique methodology to prove antibody orientation. This methodology is of significant and broad interest to the scientific community as it is applicable to a range of substrates and allows for direct, label-free characterization of surface bound proteins.

One-step preparation of surface modified electrospun microfibers as suitable supports for protein immobilization

Surface modified microfibers were prepared in a one-step process, and were prone to retain the activity and improve the stability of immobilized enzymes.

Cross-reactive and pre-existing antibodies to therapeutic antibodies--Effects on treatment and immunogenicity

The potential for immunogenicity is an ever-present concern during the development of biopharmaceuticals. Therapeutic antibodies occasionally elicit an antibody response in patients, which can result in loss of response or adverse effects. However, antibodies that bind a drug are sometimes found in pre-treatment serum samples, with the amount depending on drug, assay, and patient population. This review summarizes published data on pre-existing antibodies to therapeutic antibodies, including rheumatoid factors, anti-allotype antibodies, anti-hinge antibodies, and anti-glycan antibodies. Unlike anti-idiotype antibodies elicited by the drug, pre-formed antibodies in general appear to have little consequences during treatment. In the few cases where (potential) clinical consequences were encountered, antibodies were characterized and found to bind a distinct, unusual epitope of the therapeutic. Immunogenicity testing strategies should therefore always include a proper level of antibody characterization, especially when pre-formed antibodies are present. This minimizes false-positives, particularly due to rheumatoid factors, and helps to judge the potential threat in case a genuine pre-dose antibody reactivity is identified.

Ultrasensitive Antibody Detection by Agglutination-PCR (ADAP)

Antibodies are widely used biomarkers for the diagnosis of many diseases. Assays based on solid-phase immobilization of antigens comprise the majority of clinical platforms for antibody detection, but can be undermined by antigen denaturation and epitope masking. These technological hurdles are especially troublesome in detecting antibodies that bind nonlinear or conformational epitopes, such as anti-insulin antibodies in type 1 diabetes patients and anti-thyroglobulin antibodies associated with thyroid cancers. Radioimmunoassay remains the gold standard for these challenging antibody biomarkers, but the limited multiplexability and reliance on hazardous radioactive reagents have prevented their use outside specialized testing facilities. Here we present an ultrasensitive solution-phase method for detecting antibodies, termed antibody detection by agglutination-PCR (ADAP). Antibodies bind to and agglutinate synthetic antigen-DNA conjugates, enabling ligation of the DNA strands and subsequent quantification by qPCR. ADAP detects zepto- to attomoles of antibodies in 2 μL of sample with a dynamic range spanning 5-6 orders of magnitude. Using ADAP, we detected anti-thyroglobulin autoantibodies from human patient plasma with a 1000-fold increased sensitivity over an FDA-approved radioimmunoassay. Finally, we demonstrate the multiplexability of ADAP by simultaneously detecting multiple antibodies in one experiment. ADAP's combination of simplicity, sensitivity, broad dynamic range, multiplexability, and use of standard PCR protocols creates new opportunities for the discovery and detection of antibody biomarkers.

Label-free electrochemical impedance biosensor to detect human interleukin-8 in serum with sub-pg/ml sensitivity

Biosensors with high sensitivity and short time-to-result that are capable of detecting biomarkers in body fluids such as serum are an important prerequisite for early diagnostics in modern healthcare provision. Here, we report the development of an electrochemical impedance-based sensor for the detection in serum of human interleukin-8 (IL-8), a pro-angiogenic chemokine implicated in a wide range of inflammatory diseases. The sensor employs a small and robust synthetic non-antibody capture protein based on a cystatin scaffold that displays high affinity for human IL-8 with a K_D of 35±10 nM and excellent ligand specificity. The change in the phase of the electrochemical impedance from the serum baseline, ∆θ(ƒ), measured at 0.1 Hz, was used as the measure for quantifying IL-8 concentration in the fluid. Optimal sensor signal was observed after 15 min incubation, and the sensor exhibited a linear response versus logarithm of IL-8 concentration from 900 fg/ml to 900 ng/ml. A detection limit of around 90 fg/ml, which is significantly lower than the basal clinical levels of 5–10 pg/ml, was observed. Our results are significant for the development of point-of-care and early diagnostics where high sensitivity and short time-to-results are essential.

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A label-free electrochemical impedance-based sensor for the detection of human interleukin-8 (IL-8) in full serum was developed.

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Detection limit of 90 fg/ml and time-to-result of 15 min was found.

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A large dynamic range of the sensor was observed, with sensor response linear vs logarithm of IL-8 concentration from 900 fg/ml to 900 ng/ml.

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The sensor employs a small and robust synthetic non-antibody capture protein, with high stability and excellent ligand specificity.

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Findings are particularly relevant for the development of point-of-care and early diagnosis sensors where high sensitivity and short time-to-results are essential.

A Highly Sensitive Immunosorbent Assay Based on Biotinylated Graphene Oxide and the Quartz Crystal Microbalance

A high-sensitivity flow-based immunoassay is reported based on a gold-coated quartz crystal microbalance (QCM) chip functionalized directly in the QCM without requiring covalent conjugation steps. Specifically, the irreversible adsorption of a biotinylated graphene oxide-avidin complex followed by loading of a biotinylated capture antibody is applied to avoid more complex conventional surface modification chemistries and enable chip functionalization and sensing all within the QCM instrument. The resulting immunosensors exhibit significantly lower nonspecific protein adsorption and stronger signal for antigen sensing relative to simple avidin-coated sensors. Reproducible quantification of rabbit IgG concentrations ranging from 0.1 ng/mL to 10 μg/mL (6 orders of magnitude) can be achieved depending on the approach used to quantify the binding with simple mass changes used to detect higher concentrations and a horseradish peroxidase-linked detection antibody that converts its substrate to a measurable precipitate used to detect very low analyte concentrations. Sensor fabrication and assay performance take ∼5 h in total, which is on par with or faster than other techniques. Quantitative sensing is possible in the presence of complex protein mixtures, such as human plasma. Given the broad availability of biotinylated capture antibodies, this method offers both an easy and flexible platform for the quantitative sensing of a variety of biomolecule targets.

The use of a proteinaceous "cushion" with a polystyrene-binding peptide tag to control the orientation and function of a target peptide adsorbed to a hydrophilic polystyrene surface

In immobilizing target biomolecules on a solid surface, it is essential (i) to orient the target moiety in a preferred direction and (ii) to avoid unwanted interactions of the target moiety including with the solid surface. The preferred orientation of the target moiety can be achieved by genetic conjugation of an affinity peptide tag specific to the immobilization surface. Herein, we report on a strategy for reducing the extent of direct interaction between the target moiety and surface in the immobilization of hexahistidine peptide (6His) and green fluorescent protein (GFP) on a hydrophilic polystyrene (PS) surface: Ribonuclease HII from Thermococcus kodakaraensis (cHII) was genetically inserted as a "cushion" between the PS-affinity peptide tag and target moiety. The insertion of a cushion protein resulted in a considerably stronger immobilization of target biomolecules compared to conjugation with only a PS affinity peptide tag, resulting in a substantially enhanced accessibility of the detection antibody to the target 6His peptide. The fluorescent intensity of the GFP moiety was decreased by approximately 30% as the result of fusion with cHII and the PS-affinity peptide tag but was fully retained in the immobilization on the PS surface irrespective of the increased binding force. Furthermore, the fusion of cHII did not impair the stability of the target GFP moiety. Accordingly, the use of a proteinaceous cushion appears to be promising for the immobilization of functional biomolecules on a solid surface. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:527-534, 2016.

Genetic fusion of single-chain variable fragments to partial spider silk improves target detection in micro- and nanoarrays

Immobilizing biomolecules with retained functionality and stability on solid supports is crucial for generation of sensitive immunoassays. However, upon use of conventional immobilization strategies, a major portion of the biomolecules (e.g. antibodies) frequently tends to lose their bioactivity. In this study, we describe a procedure to immobilize human single-chain variable fragment (scFv) via genetic fusion to partial spider silk, which have a high tendency to adhere to solid supports. Two scFvs, directed towards serum proteins, were genetically fused to partial spider silk proteins and expressed as silk fusion proteins in E. coli. Antigen binding ability of scFvs attached to a partial silk protein denoted RC was investigated using microarray analysis, whereas scFvs fused to the NC silk variant were examined using nanoarrays. Results from micro- and nanoarrays confirmed the functionality of scFvs attached to both RC and NC silk, and also for binding of targets in crude serum. Furthermore, the same amount of added scFv gives higher signal intensity when immobilized via partial spider silk compared to when immobilized alone. Together, the results suggest that usage of scFv-silk fusion proteins in immunoassays could improve target detection, in the long run enabling novel biomarkers to be detected in crude serum proteomes.

Heavy chain single-domain antibodies to detect native human soluble epoxide hydrolase

The soluble epoxide hydrolase (sEH) is a potential pharmacological target for treating hypertension, vascular inflammation, pain, cancer, and other diseases. However, there is not a simple, inexpensive, and reliable method to estimate levels of active sEH in tissues. Toward developing such an assay, a polyclonal variable domain of heavy chain antibody (VHH) sandwich immunoassay was developed. Ten VHHs, which are highly selective for native human sEH, were isolated from a phage-displayed library. The ten VHHs have no significant cross-reactivity with human microsomal epoxide hydrolase, rat and mouse sEH, and denatured human sEH. There is a high correlation between protein levels of the sEH determined by the enzyme-linked immunosorbent assay (ELISA) and the catalytic activity of the enzyme in S9 fractions of human tissues (liver, kidney, and lung). The VHH-based ELISA appears to be a new reliable method for monitoring the sEH and may be useful as a diagnostic tool for diseases influenced by sEH. This study also demonstrates the broad utility of VHH in biochemical and pharmacological research.

Site-specific covalent attachment of an engineered Z-domain onto a solid matrix: an efficient platform for 3D IgG immobilization

Immobilized antibodies with oriented and homogeneous patterns are crucial to solid-phase molecular recognition assay. Antibody binding protein-based immobilization can effectively present the desired antibodies. However, steadily installing the stromatoid protein with site-specific attachment manner onto a matrix surface remains to be elucidated. In this study, we present an optimal protocol to tightly attach an immunoglobulin G (IgG)-binding protein (Z-domain) through covalent incorporation of Cys-tag and maleimide group onto polystyrene surface to guarantee site-specific, oriented, and irreversible attachment, resulting in a highly efficient platform for three-dimensional IgG immobilization. The actual IgG-binding characteristic of immobilized Z-Cys was investigated by employing affinity chromatography and size exclusion chromatography. And the efficacy and potential of this platform was demonstrated by applying it to the analysis of interaction between rabbit anti-HRP IgG and its binding partner HRP. The proposed approach may be an attractive strategy to construct high performance antibody arrays and biosensors given that the antibody is compatible with the Z-domain.

Antibody mimetic receptor proteins for label-free biosensors

Small synthetic antibody mimetic receptor proteins which offer high stability, specificity and affinity are presented as capture molecules in solid-state electro-chemical biosensors.

High efficiency acetylcholinesterase immobilization on DNA aptamer modified surfaces

We report here the in vitro selection of DNA aptamers for electric eel acetylcholinesterase (AChE). One selected aptamer sequence (R15/19) has a high affinity towards the enzyme (Kd=157±42 pM). Characterization of the aptamer showed its binding is not affected by low ionic strength (~20 mM), however significant reduction in affinity occurred at high ionic strength (~1.2 M). In addition, this aptamer does not inhibit the catalytic activity of AChE that we exploit through immobilization of the DNA on a streptavidin-coated surface. Subsequent immobilization of AChE by the aptamer results in a 4-fold higher catalytic activity when compared to adsorption directly on to plastic.

Cytokine release assays: current practices and future directions

As a result of the CD28 superagonist biotherapeutic monoclonal antibody (TGN 1412) "cytokine storm" incident, cytokine release assays (CRA) have become hazard identification and prospective risk assessment tools for screening novel biotherapeutics directed against targets having a potential risk for eliciting adverse pro-inflammatory clinical infusion reactions. Different laboratories may have different strategies, assay formats, and approaches to the reporting, interpretation, and use of data for either decision making or risk assessment. Additionally, many independent contract research organizations (CROs), academic and government laboratories are involved in some aspect of CRA work. As a result, while some pharmaceutical companies are providing CRA data as part of the regulatory submissions when necessary, technical and regulatory practices are still evolving to provide data predictive of cytokine release in humans and that are relevant to safety. This manuscript provides an overview of different approaches employed by the pharmaceutical industry and CROs, for the use and application of CRA based upon a survey and post survey follow up conducted by ILSI-Health and Environmental Sciences Institute (HESI) Immunotoxicology Committee CRA Working Group. Also discussed is ongoing research in the academic sector, the regulatory environment, current limitations of the assays, and future directions and recommendations for cytokine release assays.

UV activation of polymeric high aspect ratio microstructures: ramifications in antibody surface loading for circulating tumor cell selection

The need to activate thermoplastic surfaces using robust and efficient methods has been driven by the fact that replication techniques can be used to produce microfluidic devices in a high production mode and at low cost, making polymer microfluidics invaluable for in vitro diagnostics, such as circulating tumor cell (CTC) analysis, where device disposability is critical to mitigate artifacts associated with sample carryover. Modifying the surface chemistry of thermoplastic devices through activation techniques can be used to increase the wettability of the surface or to produce functional scaffolds to allow for the covalent attachment of biologics, such as antibodies for CTC recognition. Extensive surface characterization tools were used to investigate UV activation of various surfaces to produce uniform and high surface coverage of functional groups, such as carboxylic acids in microchannels of different aspect ratios. We found that the efficiency of the UV activation process is highly dependent on the microchannel aspect ratio and the identity of the thermoplastic substrate. Colorimetric assays and fluorescence imaging of UV-activated microchannels following EDC/NHS coupling of Cy3-labeled oligonucleotides indicated that UV-activation of a PMMA microchannel with an aspect ratio of ~3 was significantly less efficient toward the bottom of the channel compared to the upper sections. This effect was a consequence of the bulk polymer's damping of the modifying UV radiation due to absorption artifacts. In contrast, this effect was less pronounced for COC. Moreover, we observed that after thermal fusion bonding of the device's cover plate to the substrate, many of the generated functional groups buried into the bulk rendering them inaccessible. The propensity of this surface reorganization was found to be higher for PMMA compared to COC. As an example of the effects of material and microchannel aspect ratios on device functionality, thermoplastic devices for the selection of CTCs from whole blood were evaluated, which required the immobilization of monoclonal antibodies to channel walls. From our results, we concluded the CTC yield and purity of isolated CTCs were dependent on the substrate material with COC producing the highest clinical yields for CTCs as well as better purities compared to PMMA.

Surface design of antibody-immobilized thermoresponsive cell culture dishes for recovering intact cells by low-temperature treatment

Antibody-immobilized thermoresponsive poly(N-isopropylacrylamide-co-2-carboxyisopropylacrylamide) [poly(IPAAm-co-CIPAAm)]-grafted cell culture surfaces were designed to enhance both the initial adhesion of weakly adhering cells and the ability of cells to detach in response to low temperature through the regulation of affinity binding between immobilized antibodies and antigens on the cellular surface. Ty-82 cells and neonatal normal human dermal fibroblasts (NHDFs), which express CD90 on the cell surface, adhered to anti-CD90 antibody-immobilized thermoresponsive surfaces at 37°C, a condition at which the grafted thermoresponsive polymer chains shrank. Adherent Ty-82 cells were detached from the surfaces by lowering the temperature to 20°C and applying external forces, such as pipetting, whereas cultured NHDF sheets spontaneously detached themselves from the surface in response to reduced temperature alone. When the temperature was decreased to 20°C, the swelling of grafted thermoresponsive polymer chains weakened the affinity binding between immobilized antibody and antigen on the cells due to the increasing steric hindrance of the polymer chains around the antigen-recognition site of the immobilized antibodies. No contamination was detected on cells harvested from covalently immobilized antibodies on the culture surfaces by low-temperature treatment, whereas a carryover of the antibody and avidin from the avidin-biotin binding surface was observed. Furthermore, the initial adhesion of adipose tissue-derived cells, which adhere weakly to PIPAAm-grafted surfaces, was enhanced on the antibody-immobilized thermoresponsive surfaces.

Regioselective covalent immobilization of recombinant antibody-binding proteins A, G, and L for construction of antibody arrays

Immobilized antibodies are useful for the detection of antigens in highly sensitive microarray diagnostic applications. Arrays with the antibodies attached regioselectively in a uniform orientation are typically more sensitive than those with random orientations. Direct regioselective immobilization of antibodies on a solid support typically requires a modified form of the protein. We now report a general approach for the regioselective attachment of antibodies to a surface using truncated forms of antibody-binding proteins A, G, and L that retain the structural motifs required for antibody binding. The recombinant proteins have a C-terminal CVIX protein farnesyltransferase recognition motif that allows us to append a bioorthogonal azide or alkyne moiety and use the Cu(I)-catalyzed Huisgen cycloaddition to attach the binding proteins to a suitably modified glass surface. This approach offers several advantages. The recombinant antibody-binding proteins are produced in Escherichia coli, chemoselectively modified posttranslationally in the cell-free homogenate, and directly attached to the glass surface without the need for purification at any stage of the process. Complexes between immobilized recombinant proteins A, G, and L and their respective strongly bound antibodies were stable to repeated washing with PBST buffer at pH 7.2. However, the antibodies could be stripped from the slides by treatment with 0.1 M glycine·HCl buffer, pH 2.6, for 30 min and regenerated by shaking with PBS buffer, pH 7.2, at 4 °C overnight. The recombinant forms of proteins A, G, and L can be used separately or in combination to give glass surfaces capable of binding a wide variety of antibodies.

Oriented antibody immobilization by site-specific UV photocrosslinking of biotin at the conserved nucleotide binding site for enhanced antigen detection

The nucleotide binding site (NBS) is an under-utilized, highly conserved binding site found within the variable region of nearly all antibody Fab arms. Here, we describe an NBS specific UV photocrosslinking biotinylation method (UV-NBS(Biotin)) for the oriented immobilization of antibodies to streptavidin-coated surfaces, such that the antigen binding activity remains unaffected. An optimal UV exposure of 1J/cm(2) yielded an average conjugation efficiency of ≈ 1 biotin per antibody resulting in significant immobilization efficiency while maintaining maximal antigen binding activity. With the continued push for miniaturization of medical diagnostics to reduce cost and increase patient accessibility the ever shrinking on chip detection areas necessitate the highest level of immobilized antibody activity to maximize assay detection capabilities. The UV-NBS(Biotin) method yielded surfaces with significantly enhanced antigen detection capabilities, improved antigen detection sensitivity and the highest amount of active surface immobilized antibody when compared to other common immobilization methods including: ε-NH3(+) surface conjugation, NHS-Biotin, and direct physical adsorption. Taken together, the UV-NBS(Biotin) method provides a universal, site-specific immobilization method that is amenable to any available assay detection modality with potential significant implications in the development of miniaturized medical diagnostics and lab on a chip technologies.

Well-oriented ZZ-PS-tag with high Fc-binding onto polystyrene surface for controlled immobilization of capture antibodies

The site specificity and bioactivity retention of antibodies immobilized on a solid substrate are crucial requirements for solid phase immunoassays. A fusion protein between an immunoglobulin G (IgG)-binding protein (ZZ protein) and a polystyrene-binding peptide (PS-tag) was constructed, and then used to develop a simple method for the oriented immobilization of the ZZ protein onto a PS support by the specific attachment of the PS-tag onto a hydrophilic PS. The orientation of intact IgG was achieved via the interaction of the ZZ protein and the constant fragment (Fc), thereby displayed the Fab fragment for binding antigen. The interaction between rabbit IgG anti-horseradish peroxidase (anti-HRP) and its binding partner HRP was analyzed. Results showed that the oriented ZZ-PS-tag yielded an IgG-binding activity that is fivefold higher than that produced by the passive immobilization of the ZZ protein. The advantage of the proposed immunoassay strategy was demonstrated through an enzyme-linked immunosorbent assay, in which monoclonal mouse anti-goat IgG and HRP-conjugated rabbit F(ab')2 anti-goat IgG were used to detect goat IgG. The ZZ-PS-tag presented a tenfold higher sensitivity and a wider linear range than did the passively immobilized ZZ protein. The proposed approach may be an attractive strategy for a broad range of applications involving the oriented immobilization of intact IgGs onto PS supports, in which only one type of phi-PS (ZZ-PS-tag) surface is used.

An extracellular matrix–like surface modification on titanium improves implant endothelialization through the reduction of platelet adhesion and the capture of endothelial progenitor cells

To address the problem of surface-induced thrombosis and restenosis, an extracellular matrix–like biological membrane was constructed from collagen, heparin, vascular endothelial growth factor, and an anti-CD34 antibody. This membrane was assembled on a titanium surface using a layer-by-layer self-assembly technique and induced the spontaneous endothelialization of synthetic cardiovascular implants in vivo. The multilayer growth process was carried out by first depositing a single layer of positively charged poly-L-lysine on the negatively charged NaOH-treated titanium substrate. This was followed by alternating depositions of negatively charged heparin, containing vascular endothelial growth factor and an anti-CD34 antibody and positively charged collagen, terminating with an outermost layer of heparin containing vascular endothelial growth factor and the anti-CD34 antibody. The uncoated and coated titanium samples were exposed to platelet-rich plasma and endothelial progenitor cells, respectively, under static and flow conditions in vitro. Then, the samples were implanted into dog femoral arteries. The results suggest that the multilayering process led to reduced platelet adhesion and activation, promoted the attachment and growth of endothelial progenitor cells in vitro, and induced the rapid and complete endothelialization of the lumenal surface of the implant. Thus, the approach described here may be used in the fabrication of titanium-based vascular implant surfaces to induce endothelialization in vivo.