Chip-based immunoaffinity CE: application to the measurement of brain-derived neurotrophic factor in skin biopsies

A Two-Dimensional Affinity Capture and Separation Mini-Platform for the Isolation, Enrichment, and Quantification of Biomarkers and Its Potential Use for Liquid Biopsy

Biomarker detection for disease diagnosis, prognosis, and therapeutic response is becoming increasingly reliable and accessible. Particularly, the identification of circulating cell-free chemical and biochemical substances, cellular and subcellular entities, and extracellular vesicles has demonstrated promising applications in understanding the physiologic and pathologic conditions of an individual. Traditionally, tissue biopsy has been the gold standard for the diagnosis of many diseases, especially cancer. More recently, liquid biopsy for biomarker detection has emerged as a non-invasive or minimally invasive and less costly method for diagnosis of both cancerous and non-cancerous diseases, while also offering information on the progression or improvement of disease. Unfortunately, the standardization of analytical methods to isolate and quantify circulating cells and extracellular vesicles, as well as their extracted biochemical constituents, is still cumbersome, time-consuming, and expensive. To address these limitations, we have developed a prototype of a portable, miniaturized instrument that uses immunoaffinity capillary electrophoresis (IACE) to isolate, concentrate, and analyze cell-free biomarkers and/or tissue or cell extracts present in biological fluids. Isolation and concentration of analytes is accomplished through binding to one or more biorecognition affinity ligands immobilized to a solid support, while separation and analysis are achieved by high-resolution capillary electrophoresis (CE) coupled to one or more detectors. When compared to other existing methods, the process of this affinity capture, enrichment, release, and separation of one or a panel of biomarkers can be carried out on-line with the advantages of being rapid, automated, and cost-effective. Additionally, it has the potential to demonstrate high analytical sensitivity, specificity, and selectivity. As the potential of liquid biopsy grows, so too does the demand for technical advances. In this review, we therefore discuss applications and limitations of liquid biopsy and hope to introduce the idea that our affinity capture-separation device could be used as a form of point-of-care (POC) diagnostic technology to isolate, concentrate, and analyze circulating cells, extracellular vesicles, and viruses.

Capillary electrophoresis-based immunoassay and aptamer assay: A review

Over the last two decades, the group of techniques called affinity probe CE has been widely used for the detection and the determination of several types of biomolecules with high sensitivity. These techniques combine the low sample consumption and high separation power of CE with the selectivity of the probe to the target molecule. The assays can be defined according to the type of probe used: CE immunoassays, with an antibody as the probe, or aptamer-based CE, with an aptamer as the probe. Immunoassays are generally divided into homogeneous and heterogeneous groups, and homogeneous variant can be further performed in competitive or noncompetitive formats. Interacting partners are free in solution at homogeneous assay, as opposed to heterogeneous analyses, where one of them is immobilized onto a solid support. Highly sensitive fluorescence, chemiluminescence or electrochemical detections were typically used in this type of study. The use of the aptamers as probes has several advantages over antibodies such as shorter generation time, higher thermal stability, lower price, and lower variability. The aptamer-based CE technique was in practice utilized for the determination of proteins in biological fluids and environmentally or clinically important small molecules. Both techniques were also transferred to microchip. This review is focused on theoretical principles of these techniques and a summary of their applications in research.

Application of Capillary Electrophoresis with Laser-Induced Fluorescence to Immunoassays and Enzyme Assays

Capillary electrophoresis using laser-induced fluorescence detection (CE-LIF) is one of the most sensitive separation tools among electrical separation methods. The use of CE-LIF in immunoassays and enzyme assays has gained a reputation in recent years for its high detection sensitivity, short analysis time, and accurate quantification. Immunoassays are bioassay platforms that rely on binding reactions between an antigen (analyte) and a specific antibody. Enzyme assays measure enzymatic activity through quantitative analysis of substrates and products by the reaction of enzymes in purified enzyme or cell systems. These two category analyses play an important role in the context of biopharmaceutical analysis, clinical therapy, drug discovery, and diagnosis analysis. This review discusses the expanding portfolio of immune and enzyme assays using CE-LIF and focuses on the advantages and disadvantages of these methods over the ten years of existing technology since 2008.

Analysis of Inflammatory Mediators in Newborn Dried Blood Spot Samples by Chip-Based Immunoaffinity Capillary Electrophoresis

A chip-based immunoaffinity capillary electrophoresis (ICE) system has been developed for measuring inflammatory mediators in dried blood samples routinely taken from newborn babies. A defined area of each dried blood spot was removed from the sample card and its contents eluted. The recovered eluates were injected into the chip and the analytes of interest isolated by the immunoaffinity disk within the chip. The captured analytes were labeled in-situ with a red light-emitting laser dye and electro-eluted into the chip separation channel. Electrophoretic separation of all of the analytes was achieved within 2.0 min with quantification of each peak being performed by online LIF detection and integration of each peak area. The degree of accuracy and precision achieved by the chip-based system is comparable to conventional immunoassays and the system is robust enough to be applied to the analysis of clinical samples.

C-reactive protein and interleukin 6 microfluidic immunoassays with on-chip pre-stored reagents and centrifugo-pneumatic liquid control

We present a fully automated centrifugal microfluidic method for particle based protein immunoassays. Stick-pack technology is employed for pre-storage and release of liquid reagents. Quantitative layout of centrifugo-pneumatic particle handling, including timed valving, switching and pumping is assisted by network simulations. The automation is exclusively controlled by the spinning frequency and does not require any additional means. New centrifugal microfluidic process chains are developed in order to sequentially supply wash buffer based on frequency dependent stick-pack opening and pneumatic pumping to perform two washing steps from one stored wash buffer; pre-store and re-suspend functionalized microparticles on a disk; and switch between the path of the waste fluid and the path of the substrate reaction product with 100% efficiency. The automated immunoassay concept is composed of on demand ligand binding, two washing steps, the substrate reaction, timed separation of the reaction products, and termination of the substrate reaction. We demonstrated separation of particles from three different liquids with particle loss below 4% and residual liquid remaining within particles below 3%. The automated immunoassay concept was demonstrated by means of detecting C-reactive protein (CRP) in the range of 1-81 ng ml^-1 and interleukin 6 (IL-6) in the range of 64-13 500 pg ml^-1. The limit of detection and quantification were 1.0 ng ml^-1 and 2.1 ng ml^-1 for CRP and 64 pg ml^-1 and 205 pg ml^-1 for IL-6, respectively.

Measurement of Inflammatory Chemokines in Micro-dissected Tissue Biopsy Samples by Chip-Based Immunoaffinity Capillary Electrophoresis

To aid in the biochemical analysis of human skin biopsies, a chip-based immunoaffinity capillary electrophoresis (ICE) system has been developed for measuring inflammatory chemokines in micro-dissected areas of the biopsy. Following isolation of the areas of interest, the tissue was solubilized and the analytes of interest were isolated by the immunoaffinity disk within the chip. The captured analytes were labeled in situ with a 635 nm light-emitting laser dye and electro-eluted into the chip separation channel. Electrophoretic separation of all of the analytes was achieved in 2.5 min with quantification of each peak being performed by online LIF detection and integration of each peak area. The degree of accuracy and precision achieved by the chip-based system is comparable to conventional immunoassays and the system is robust enough to be applied to the analysis of clinical samples. Further, with the expanding array of antibodies that are commercially available, this chip-based system can be applied to a wide variety of different biomedical and clinical analyses.

Clinical applications of capillary electrophoresis based immunoassays

Immunoassays have long been an important set of tools in clinical laboratories for the detection, diagnosis, and treatment of disease. Over the last two decades, there has been growing interest in utilizing CE as a means for conducting immunoassays with clinical samples. The resulting method is known as a CE immunoassay. This approach makes use of the selective and strong binding of antibodies for their targets, as is employed in a traditional immunoassay, and combines this with the speed, efficiency, and small sample requirements of CE. This review discusses the variety of ways in which CE immunoassays have been employed with clinical samples. An overview of the formats and detection modes that have been employed in these applications is first presented. A more detailed discussion is then given on the type of clinical targets and samples that have been measured or studied by using CE immunoassays. Particular attention is given to the use of this method in the fields of endocrinology, pharmaceutical measurements, protein and peptide analysis, immunology, infectious disease detection, and oncology. Representative applications in each of these areas are described, with these examples involving work with both traditional and microanalytical CE systems.

Detection of cerebral spinal fluid-associated chemokines in birth traumatized premature babies by chip-based immunoaffinity CE

A major concern in treating premature infants with birth-associated head trauma is the rapid determination of reliable biomarkers of neuroinflammation. To this end a chip-based immunoaffinity CE device has been applied to determine the concentrations of inflammation-associated chemokines in samples of cerebral spinal fluid collected from such subjects. The chip utilizes replaceable immunoaffinity disks, to which reactive antibody fragments (FAb) of six antichemokine-specific antibodies were immobilized. Following injection of a sample into the device, the analytes were captured by the immobilized FAbs, labeled in situ with a red laser dye, chemically released and separated by CE. Each resolved peak was measured on-line by LIF detection and the results compared to standard curves produced by running known chemokine standards through the immunoaffinity system. The complete processing of a sample took 10 min with separation of all six analytes being achieved in less than 2 min. The system compared well to commercial ELISA, analysis of the results by linear regression demonstrating r(2) values in the range of 0.903-0.978, and intra and interassay CV of the migration times and the measured peak areas being less than 2.3 and 5%, respectively. Application of the system to analysis of cerebrospinal fluid from head traumatized babies clearly indicated the group with mild trauma versus those with severe injury. Additionally, CE analysis demonstrated that the severe trauma group could be divided into individuals with good and poor prognosis, which correlated with the clinical finding for each patient.

Miniaturized immunoassays: moving beyond the microplate

After more than 40 years, immunoassays are still the backbone of protein biomarker analysis in clinical diagnostics and drug development. They have come a long way since their inception, incorporating technical developments including monoclonal antibodies, novel labels and lately microfluidics. A number of microfluidic platforms have been tested, such as centrifugational compact disc assays, lab-on-a-chip, arrays and digital electrochemical assays. This review focuses on commercial applications of microfluidic immunoassays with reference to some applied academic examples of interest. Advantages and disadvantages of the platform technologies are discussed in general.

Assessment of chemokine profiles in human skin biopsies by an immunoaffinity capillary electrophoresis chip

Atopic dermatitis is a skin condition resulting in a skin rash from exposure to environmental factors. Skin biopsies taken from patients suffering from atopic dermatitis were micro-dissected and analyzed using a microchip-based immunoaffinity CE system for the presence of CXCL1, CXCL5 and CXCL8 and CCL1, CCL3 and CCL5 chemokines. Disposable immunoaffinity disks with immobilized antibodies were used to capture the CXC and CC chemokines from the homogenized skin samples. The captured analytes were then labeled with AlexaFluor 633, eluted from the disk and separated by CE. The labeled chemokines were identified and quantified by laser induced fluorescence. The total analysis time was less than 40min, including the biopsy microdissection, pre-analysis preparation of the sample and the ICE-CHIP analysis, which took less than 10min with inter- and intra-assay CV's below 6.4%. Microchip-based immunoaffinity CE could distinguish between normal skin biopsies and those with inflammation. Patients with neutrophil cellular infiltrates by histopathology showed increased concentrations of CXCL1, CXCL5 and CXCL8 while increases of CCL1, CCL3 and CCL5 corresponded to the patient group demonstrating monocytic and T-lymphocyte infiltration by histopathology. This system demonstrates the ability to identify and quantify immunochemical analytes in frozen sections taken from clinical histopathology samples.

Microdevices integrating affinity columns and capillary electrophoresis for multibiomarker analysis in human serum

Biomarkers in human body fluids have great potential for use in screening for diseases such as cancer and diabetes, diagnosis, determining the effectiveness of treatments, and detecting recurrence. Present 96-well immunoassay technology effectively analyzes large numbers of samples; however, this approach is more expensive and less time effective on single or a few samples. In contrast, microfluidic systems are well suited for assaying small numbers of specimens in a point-of-care setting, provided suitable procedures are developed to work within peak capacity constraints when analyzing complex mixtures like human blood serum. Here, we developed integrated microdevices with an affinity column and capillary electrophoresis channels to isolate and quantitate a panel of proteins in complex matrices. To form an affinity column, a thin film of a reactive polymer was photopolymerized in a microchannel, and four antibodies were covalently immobilized to it. The retained protein amounts were consistent from chip to chip, demonstrating reproducibility. Furthermore, the signals from four fluorescently labeled proteins captured on-column were in the same range after rinsing, indicating the column has little bias toward any of the four antibodies or their antigens. These affinity columns have been integrated with capillary electrophoresis separation, enabling us to simultaneously quantify four protein biomarkers in human blood serum in the low ng mL(-1) range using either a calibration curve or standard addition. Our systems provide a fast, integrated and automated platform for multiple biomarker quantitation in complex media such as human blood serum.

µFBI: a microfluidic bead-based immunoassay for multiplexed detection of proteins from a µL sample volume

Background

Over the last ten years, miniaturized multiplexed immunoassays have become robust, reliable research tools that enable researchers to simultaneously determine a multitude of parameters. Among the numerous analytical protein arrays available, bead-based assay systems have evolved into a key technology that enables the quantitative protein profiling of biological samples whilst requiring only a minimal amount of sample material.

Methodology/Principal Findings

A microfluidic bead-based immunoassay, µFBI, was developed to perform bead-based multiplexed sandwich immunoassays in a capillary. This setup allows the simultaneous detection of several parameters and only requires 200 ng of tissue lysate in a 1 µL assay volume. In addition, only 1 µL of detection antibodies and 1 µL of the reporter molecule Streptavidin-Phycoerythrin were required. The µFBI was used to compare the expression of seven receptor tyrosine kinases and their degree of tyrosine phosphorylation in breast cancer tissue and in normal tissue lysates. The total amount of HER-2, as well the degree of tyrosine phosphorylation was much higher in breast cancer tissue than in normal tissue. µFBI and a standard bead-based assay led to identical protein expression data. Moreover, it was possible to reduce the quantity of sample material required by a factor of 100 and the quantity of reagents by a factor of 30.

Conclusions/Significance

The µFBI, microfluidic bead-based immunoassay, allows the analysis of multiple parameters from a very small amount of sample material, such as tumor biopsies or tissue sections.

Integrated Multi-process Microfluidic Systems for Automating Analysis

Microfluidic technologies have been applied extensively in rapid sample analysis. Some current challenges for standard microfluidic systems are relatively high detection limits, and reduced resolving power and peak capacity compared to conventional approaches. The integration of multiple functions and components onto a single platform can overcome these separation and detection limitations of microfluidics. Multiplexed systems can greatly increase peak capacity in multidimensional separations and can increase sample throughput by analyzing many samples simultaneously. On-chip sample preparation, including labeling, preconcentration, cleanup and amplification, can all serve to speed up and automate processes in integrated microfluidic systems. This paper summarizes advances in integrated multi-process microfluidic systems for automated analysis, their benefits and areas for needed improvement.

Receptor affinity CE for measuring bioactive inflammatory cytokines in human skin biopsies

A chip-based receptor affinity CE system has been employed to measure the concentrations of bioactive pro-inflammatory cytokines in biopsy materials obtained from human atopic skin lesions. The device employs a replaceable affinity disk to which recombinant cytokine receptors have been chemically immobilized. Homogenates obtained from micro-dissected human skin samples were injected into the system where the bioactive cytokines were captured in the receptor affinity port and labeled in situ with a laser dye. The captured cytokines were released and separated by CE, the resolved peaks being detected and measured by laser-induced fluorescence. When compared with conventional cell-based bioassays, the affinity receptor chip showed reasonable correlation with r(2) values of 0.998 for interferon gamma, 0.994 for IL-6 and 0.991 for tumor necrosis factor alpha. The complete process including cytokine capture, labeling, and analysis took approximately 12.5 min with intra- and inter-assay CVs below 5.3% and recoveries of 84.9-98.4% at the 100 pg/mL concentration in buffer solutions and 84.5-95% in normal human tissue extract. The system could indicate clear differences between the various clinical stages of atopic dermatitis in human patients and could run 4-6 samples per hour. This system, like previous chip-based systems designed in our laboratory, holds the potential for being modified to be a portable unit that could be used in clinics and other biomedical screening studies.

Microfluidic, bead-based assay: Theory and experiments

Microbeads are frequently used as a solid support for biomolecules such as proteins and nucleic acids in heterogeneous microfluidic assays. However, relatively few studies investigate the binding kinetics on modified bead surfaces in a microfluidics context. In this study, a customized hot embossing technique is used to stamp microwells in a thin plastic substrate where streptavidin-coated agarose beads are selectively placed and subsequently immobilized within a conduit. Biotinylated quantum dots are used as a label to monitor target analyte binding to the bead's surface. Three-dimensional finite element simulations are carried out to model the binding kinetics on the bead's surface. The model accounts for surface exclusion effects resulting from a single quantum dot occluding multiple receptor sites. The theoretical predictions are compared and favorably agree with experimental observations. The theoretical simulations provide a useful tool to predict how varying parameters affect microbead reaction kinetics and sensor performance. This study enhances our understanding of bead-based microfluidic assays and provides a design tool for developers of point-of-care, lab-on-chip devices for medical diagnosis, food and water quality inspection, and environmental monitoring.