Competitive assay formats for high-throughput affinity arrays

Fast Protocols for Characterizing Antibody-Peptide Binding

Microarray assay formats gained popularity in the 1990s, first implemented in DNA-based arrays but later adopted for use with proteins, namely antibodies, peptides, low molecular weight (LMW) molecules, such as lipids, and even tissues. In nucleic acid-based affinity assays and arrays, but not in protein or peptide arrays, the specificity and affinity of complementary strand interactions can be deduced from or adjusted through modifications to the nucleotide sequence. Arrays of LMW molecules are characterized by largely uniform but low binding affinities. Multiplexed protein-based affinity assays, such as microarrays, might present an additional challenge due to heterogeneity of antigen properties and of their binding affinities. The use of peptides instead of proteins reduces physical heterogeneity of these reagents through either the widened peptide selection options or rational sequence engineering. However, rational engineering of binding affinities remains an unmet need, and peptide-binding affinities to the respective antipeptide antibodies could vary by orders of magnitude. Hence, multiplexing of such assays by using a microarray format and data analysis and interpretation requires some knowledge of their binding affinities. Low-throughput binding assays to characterize such peptide-antipeptide antibodies interactions are widely available, but scaling-up of traditional protein- and peptide-binding assays might present practical challenges. Here, we describe fast label-free practical approach especially suitable for estimating peptide-binding affinities. The method in question relies on commercially available biolayer interferometry-based equipment with a protocol which can be easily scaled-up, subject to user needs and equipment availability.

Peptides and Anti-peptide Antibodies for Small- and Medium-Scale Peptide and Anti-peptide Affinity Microarrays

This chapter describes the principles for selection of antigenic peptides for the development of anti-peptide antibodies suitable for microarray-based multiplex affinity assays and optional mass spectrometry detection. The methods described here are mostly applicable to small- and medium-scale multiplex affinity assay and microarrays. Although the same principles of peptide selection may also be applied to larger-scale arrays (with 100+ features), informatics software and printing methods may well differ. Due to the sheer number of proteins/peptides to be processed and analyzed, dedicated software with high processing capacity and enterprise-level array robotics may be required for larger-scale efforts. This report aims to provide practical advice to those seeking to develop or use arrays with up to ~100 different peptide or protein features.

Antibody microarray analysis of serum inflammatory cytokines in patients with calcific aortic valve disease

Background

Calcific aortic valve disease (CAVD) is a slowly progressive pathologic process associated with significant morbidity and mortality, CAVD is the most common valve heart disease in the elderly and a leading cause of aortic valve stenosis. Multiple steps characterize the process: inflammation, cell apoptosis, lipid deposition, renin-angiotensin system activation, extracellular matrix remodeling, and bone formation. This paper focuses on detecting and analyzing the expression of serum inflammatory factors in CAVD by antibody microarray techniques.

Methods

In this study, a total of 258 patients were included at Tianjin Chest Hospital between January 2017 and December 2018, subjects were divided into three groups: control, coronary artery disease (CAD), and CAVD. Blood samples were collected, and adipokine/cytokine/chemokine serum profiles were measured by antibody arrays.

Results

These data suggest that B-Lymphocyte Chemoattractant (BLC), Interleukin (IL)-12p40, monokine inducible by γ interferon (MIG), and Macrophage inflammatory protein (MIP)-1delta were significantly increased in CAVD compared to control or CAD. Furthermore, Real-time quantified PCR, Western blot assay, and Flow cytometer detection showed that these four cytokines/chemokines were from peripheral blood mononuclear cells.

Conclusions

These findings suggest that BLC, IL-12p40, MIG, and MIP-1delta can be used as a marker to assess CAVD, which could have significant clinical implications.

Peptides and Anti-peptide Antibodies for Small and Medium Scale Peptide and Anti-peptide Affinity Microarrays: Antigenic Peptide Selection, Immobilization, and Processing

This chapter describes the principles of selection of antigenic peptides for the development of anti-peptide antibodies for use in microarray-based multiplex affinity assays and also with mass-spectrometry detection. The methods described here are mostly applicable to small to medium scale arrays. Although the same principles of peptide selection would be suitable for larger scale arrays (with 100+ features) the actual informatics software and printing methods may well be different. Because of the sheer number of proteins/peptides to be processed and analyzed dedicated software capable of processing all the proteins and an enterprise level array robotics may be necessary for larger scale efforts. This report aims to provide practical advice to those who develop or use arrays with up to ~100 different peptide or protein features.

Peptidomics: divide et impera

The term "peptidomics" can be defined as the systematic analysis of the peptide content within a cell, organelle, tissue or organism. The science of peptidomics usually refers to the studies of naturally occurring peptides. Another meaning refers to the peptidomics approach to protein analysis. An ancient Roman strategy divide et impera (divide and conquer) reflects the essence of peptidomics. Most effort in this field is spent purifying and dividing the peptidomes, which consist of tens, hundreds or sometimes thousands of functional peptides, followed by their structural and functional characterisation. This chapter introduces the concept of peptidomics, outlines the range of methodologies employed and describes key targets - the peptide groups which are often sought after in such studies.

Affinity peptidomics: Peptide selection and affinity capture on hydrogels and microarrays

Affinity peptidomics relies on the successfully proven approach used widely in mass-spectrometry-based protein analysis, where protein samples are proteolytically digested prior to the analysis. Unlike traditional proteomic analyses, affinity peptidomics employs affinity detection instead of, or in addition to, the mass-spectrometry detection. Affinity peptidomics, therefore, bridges the gap between protein microarrays and mass spectrometry and can be used for the detection, identification and quantification of endogenous or proteolytic peptides on microarrays and by MALDI-MS. Phage display technology is a widely applicable generic molecular display method suitable for studying protein-protein or protein-peptide interactions and the development of recombinant affinity reagents. Phage display complements the affinity peptidomics approach when the latter is used, e.g. to characterise a repertoire of antigenic determinants of polyclonal, monoclonal antibodies or other recombinantly obtained affinity reagents or in studying protein-protein interactions. 3D materials such as membrane-based porous substrates and acrylamide hydrogels provide convenient alternatives and are superior to many 2D surfaces in maintaining protein conformation and minimising non-specific interactions. Hydrogels have been found to be advantageous in performing antibody affinity assays and peptide-binding assays. Here we report a range of peptide selection and peptide-binding assays used for the detection, quantification or validation of peptide targets using array-based techniques and fluorescent or MS detection.

Protein Microarrays for the Detection of Biothreats

Although protein microarrays have proven to be an important tool in proteomics research, the technology is emerging as useful for public health and defense applications. Recent progress in the measurement and characterization of biothreat agents is reviewed in this chapter. Details concerning validation of various protein microarray formats, from contact-printed sandwich assays to supported lipid bilayers, are presented. The reviewed technologies have important implications for in vitro characterization of toxin–ligand interactions, serotyping of bacteria, screening of potential biothreat inhibitors, and as core components of biosensors, among others, research and engineering applications.

Expanding Assay Dynamics: A Combined Competitive and Direct Assay System for the Quantification of Proteins in Multiplexed Immunoassays

Background: The concurrent detection and quantification of analytes that vary widely in concentration present a principal problem in multiplexed assay systems. Combining competitive and sandwich immunoassays permits coverage of a wide concentration range, and both highly abundant molecules and analytes present in low concentration can be quantified within the same assay.

Methods: The use of different fluorescence readout channels allows the parallel use of a competitive system and a sandwich configuration. The 2 generated assay signals are combined and used to calculate the amount of analyte. The measurement range can be adjusted by varying the competitor concentration, and an extension of the assay system’s dynamic range is possible.

Results: We implemented the method in a planar protein microarray–based autoimmune assay to detect autoantibodies against 13 autoantigens and to measure the concentration of a highly abundant protein, total human IgG, in one assay. Our results for autoantibody detection and IgG quantification agreed with results obtained with commercially available assays. The use of 2 readout channels in the protein microarray–based system reduced spot-to-spot variation and intraassay variation.

Conclusions: By combining a direct immunoassay with a competitive system, analytes present in widely varying concentrations can be quantified within a single multiplex assay. Introducing a second readout channel for analyte quantification is an effective tool for spot-to-spot normalization and helps to lower intraassay variation.

Dissecting cancer serum protein profiles using antibody arrays

Antibody arrays represent one of the high-throughput techniques enabling detection of multiple proteins simultaneously. One of the main advantages of the technology over other proteomic approaches resides on that the identities of the measured proteins are known at front of the experimental design or can be readily characterized, facilitating a biological interpretation of the obtained results. This chapter overviews the technical issues of the main antibody array formats as well as various applications using serum specimens in the context of neoplastic diseases. Clinical applications of antibody arrays vary from biomarker discovery for diagnosis, prognosis, and drug response to characterization of s protein pathways and modification changes associated with disease development and progression. As a high-throughput tool addressing protein levels and post-translational modifications, it improves the functional characterization of molecular bases for cancer. Furthermore, the identification and validation of protein expression patterns characteristic of cancer progression and tumor subtypes may enable tailored therapeutic intervention and improvement in the clinical management of cancer patients. Technical requirements such as lower sample volume, antibody concentration, format versatility, and high reproducibility support their increasing impact in cancer research.

Antibody-based Proteomics: From bench to bedside

Over the past 75 years, antibodies have gone from being recognized as disease biomarkers to being used as very powerful therapeutic tools. This evolution has been accelerated by the identification of mAb and the extensive use of immunological tools both at fundamental and clinical levels. In this review, we evaluate how antibodies can be used to assess the proteome of cells or tissues and their relevance for clinical applications. These antibody-based proteomics approaches also require analytical and technological pipelines as well as specific enabling tools which are described. Our first objective was to establish how large-scale datasets (provided by high-throughput studies such as proteomics and transcriptomics) can be integrated with literature searches and clinical data to identify potentially relevant markers against which antibodies should be raised. Then based on an extensive literature review and our experience, we compare the methodologies developed to produce specific antibodies either in vivo or in vitro. This is followed by the description of the validation tools currently available and it also includes the use of antibody-based approaches in the establishment of molecular signatures utilized at the bench and soon available for bedside use.

Antibody Arrays: Technical Considerations and Clinical Applications in Cancer

Antibody arrays represent one of the high-throughput techniques that are able to detect multiple proteins simultaneously. One of the main advantages of this technology over other proteomic approaches is that the identities of the measured proteins are known or can be readily characterized, allowing a biological interpretation of the results. Features such as lower sample volume and antibody concentration requirements, higher format versatility, and reproducibility support the increasing use of antibody arrays in cancer research. Clinical applications include disease marker discovery for diagnosis, prognosis, and drug response, characterization of signaling and protein pathways, and modifications associated with disease development and progression. This report presents an overview of technical issues of the main antibody array formats and various applications in cancer research. Antibody arrays are high-throughput tools that improve the functional characterization of molecular bases for disease. Furthermore, identification and validation of protein expression patterns, characteristic of cancer progression, and tumor subtypes may intervene and improve tailored therapies in the clinical management of cancer patients.

Applications of antibody array platforms

Antibody arrays are valuable for the parallel analysis of multiple proteins in small sample volumes. The earliest and most widely used application of antibody arrays has been to measure multiple protein abundances, using sandwich assays and label-based assays, for biomarker discovery and biological studies. Modifications to these assays have led to studies profiling specific protein post-translational modifications. Additional novel uses include profiling enzyme activities and protein cell-surface expression. Finally, array-based antibody platforms are being used to assist the development and characterization of antibodies. Continued progress in the technology will surely lead to extensions of these applications and the development of new ways of using the methods.

Antibody microarrays for native toxin detection

We have developed antibody-based microarray techniques for the multiplexed detection of cholera toxin beta-subunit, diphtheria toxin, anthrax lethal factor and protective antigen, Staphylococcus aureus enterotoxin B, and tetanus toxin C fragment in spiked samples. Two detection schemes were investigated: (i) a direct assay in which fluorescently labeled toxins were captured directly by the antibody array and (ii) a competition assay that employed unlabeled toxins as reporters for the quantification of native toxin in solution. In the direct assay, fluorescence measured at each array element is correlated with labeled toxin concentration to yield baseline binding information (Langmuir isotherms and affinity constants). Extending from the direct assay, the competition assay yields information on the presence, identity, and concentration of toxins. A significant advantage of the competition assay over reported profiling assays is the minimal sample preparation required prior to analysis because the competition assay obviates the need to fluorescently label native proteins in the sample of interest. Sigmoidal calibration curves and detection limits were established for both assay formats. Although the sensitivity of the direct assay is superior to that of the competition assay, detection limits for unmodified toxins in the competition assay are comparable to values reported previously for sandwich-format immunoassays of antibodies arrayed on planar substrates. As a demonstration of the potential of the competition assay for unlabeled toxin detection, we conclude with a straightforward multiplexed assay for the differentiation and identification of both native S. aureus enterotoxin B and tetanus toxin C fragment in spiked dilute serum samples.

Competition on Nitrocellulose-immobilized Antibody Arrays

Large scale comparative evaluation of protein expression requires miniaturized techniques to provide sensitive and accurate measurements of the abundance of molecules present as individual and/or assembled protein complexes in cells. The principle of competition between target molecules for binding to arrayed antibodies has recently been proposed to assess differential expression of numerous proteins with one-color or two-color fluorescence detection methods. To establish the limiting factors and to validate the use of alternative detection for protein profiling, we performed competitive binding assays under different conditions. A model experimental protocol was developed whereby the competitive displacement of multi-subunit bacterial RNA polymerase and/or its subunits was evaluated through binding to subunit-specific immobilized monoclonal antibodies. We show that the difference in physico-chemical properties of unlabeled and labeled molecules significantly affects the performance of one-color detection, whereas epitope inaccessibility in the protein complex can prohibit the assessment of competition by both detection methods. Our data also demonstrate that antibody cross-reactivity, target protein truncation and abundance, as well as the cellular compartment of origin are major factors that affect protein profiling on antibody arrays. The experimental conditions established for prokaryotic proteins were adopted to compare protein profiles in the breast tumor-derived cell lines MDA MB-231 and SKBR3. Competitive displacement was detected and confirmed for a number of proteins using both detection methods; however, we show that overall the two-color method is better suited for accurate expression profile evaluation of a large, complex set of proteins. Antibody array data confirm the functional linkage between the ErbB2 receptor and AP-2 transcription factors in these cell lines and highlight unexpected differences in G1 cyclin expression.

Antibody arrays in cancer research

Antibody arrays have valuable applications in cancer research. Many different antibody array technologies have been developed, each with particular advantages, disadvantages, and optimal applications. The methods have been demonstrated on various sample types, such as serum, plasma, and other bodily fluids; cell culture supernatants; tissue culture lysates; and resected tumor specimens. The applications to cancer research have included profiling proteins to identify candidate biomarkers, characterizing signaling pathways, and the measurement of changes in modification or expression level of cancer-related proteins. Further innovations in the methods and experimental strategies are broadening the scope of the applications and the type of information that can be gathered. These alternate formats and uses of antibody arrays include arrays to measure whole cells, arrays to measure enzyme activities, reverse phase arrays, and bead-based arrays. This article reviews the various types of antibody array methods and their applications to cancer research.

Quantitative protein profiling using antibody arrays

Traditional approaches to microarrays rely on direct binding assays where the extent of hybridisation and the signal detected are a measure of the analyte concentration in the experimental sample. This approach, directly imported from the nucleic acid field, may fail if applied to antibody-antigen interactions due to the shortage of characterised antibodies, the significant heterogeneity of antibody affinities, their dependence on the extent of protein modification during labelling and the inherent antibody cross-reactivity. These problems can potentially limit the multiplexing capabilities of protein affinity assays and in many cases rule out quantitative protein profiling using antibody microarrays. A number of approaches aimed at achieving quantitative protein profiling in a multiplex format have been reported recently. Of those reported, the three most promising routes include signal amplification, multicolour detection and competitive displacement approaches to multiplex affinity assays. One in particular, competitive displacement, also overcomes the problems associated with quantitation of affinity interactions and provides the most generic approach to highly parallel affinity assays, including antibody arrays.

Microfluidics in biotechnology

Microfluidics enables biotechnological processes to proceed on a scale (microns) at which physical processes such as osmotic movement, electrophoretic-motility and surface interactions become enhanced. At the microscale sample volumes and assay times are reduced, and procedural costs are lowered. The versatility of microfluidic devices allows interfacing with current methods and technologies. Microfluidics has been applied to DNA analysis methods and shown to accelerate DNA microarray assay hybridisation times. The linking of microfluidics to protein analysis techologies, e.g. mass spectrometry, enables picomole amounts of peptide to be analysed within a controlled micro-environment. The flexibility of microfluidics will facilitate its exploitation in assay development across multiple biotechnological disciplines.

Combinatorial peptidomics: a generic approach for protein expression profiling

Traditional approaches to protein profiling were built around the concept of investigating one protein at a time and have long since reached their limits of throughput. Here we present a completely new approach for comprehensive compositional analysis of complex protein mixtures, capable of overcoming the deficiencies of current proteomics techniques. The Combinatorial methodology utilises the peptidomics approach, in which protein samples are proteolytically digested using one or a combination of proteases prior to any assay being carried out. The second fundamental principle is the combinatorial depletion of the crude protein digest (i.e. of the peptide pool) by chemical crosslinking through amino acid side chains. Our approach relies on the chemical reactivities of the amino acids and therefore the amino acid content of the peptides (i.e. their information content) rather than their physical properties. Combinatorial peptidomics does not use affinity reagents and relies on neither chromatography nor electrophoretic separation techniques. It is the first generic methodology applicable to protein expression profiling, that is independent of the physical properties of proteins and does not require any prior knowledge of the proteins. Alternatively, a specific combinatorial strategy may be designed to analyse a particular known protein on the basis of that protein sequence alone or, in the absence of reliable protein sequence, even the predicted amino acid translation of an EST sequence. Combinatorial peptidomics is especially suitable for use with high throughput micro- and nano-fluidic platforms capable of running multiple depletion reactions in a single disposable chip.