Quantitative flow cytometry: inter-laboratory variation

Optimizing interpretation of survival studies of fresh and aged transfused biotin-labeled RBCs

BACKGROUND

Ex vivo labeling with 51 chromium represents the standard method to determine red blood cell (RBC) survival after transfusion. Limitations and safety concerns spurred the development of alternative methods, including biotinylated red blood cells (BioRBC).

STUDY DESIGN AND METHODS

Autologous units of whole blood were divided equally into two bags and stored under standard blood bank conditions at 2 to 6°C (N = 4 healthy adult volunteers). One bag was biotinylated (15 μg/ml) on storage days 5 to 7 (fresh) and the other was biotinylated (3 μg/ml) on days 35 to 42 (aged). The proportion of circulating BioRBC was measured serially, and cell-surface biotin was quantified with reference to molecules of equivalent soluble fluorochrome. Clearance kinetics were modeled by RBC age distribution at infusion (Gaussian vs. uniform) and decay over time (constant vs. exponential).

RESULTS

Data were consistent with biphasic exponential clearance of cells of uniform age. Our best estimate of BioRBC clearance (half-life [T1/2 ]) was 49.7 ± 1.2 days initially, followed by more rapid clearance 82 days after transfusion (T1/2  = 15.6 ± 0.6 days). As BioRBC aged in vivo, molecules of equivalent soluble fluorochrome declined with a T1/2 of 122 ± 9 days, suggesting gradual biotin cleavage. There were no significant differences between the clearance of fresh and aged BioRBC.

CONCLUSION

Similar clearance kinetics of fresh and aged BioRBC may be due to the extensive washing required during biotinylation. Survival kinetics consistent with cells with uniform rather than Gaussian or other non-uniform age distributions suggest that washing, and potentially RBC culling, may extend the storage life of RBC products.

Exposing cryptic epitopes on the Venezuelan equine encephalitis virus E1 glycoprotein prior to treatment with alphavirus cross-reactive monoclonal antibody allows blockage of replication early in infection

Eastern equine encephalitis virus (EEEV), western equine encephalitis virus (WEEV) and Venezuelan equine encephalitis virus (VEEV) can cause fatal encephalitis in humans and equids. Some MAbs to the E1 glycoprotein are known to be cross-reactive, weakly neutralizing in vitro but can protect from disease in animal models. We investigated the mechanism of neutralization of VEEV infection by the broadly cross-reactive E1-specific MAb 1A4B-6. 1A4B-6 protected 3-week-old Swiss Webster mice prophylactically from lethal VEEV challenge. Likewise, 1A4B-6 inhibited virus growth in vitro at a pre-attachment step after virions were incubated at 37 °C and inhibited virus-mediated cell fusion. Amino acid residue N100 in the fusion loop of E1 protein was identified as critical for binding. The potential to elicit broadly cross-reactive MAbs with limited virus neutralizing activity in vitro but that can inhibit virus entry and protect animals from infection merits further exploration for vaccine and therapeutic developmental research.

Evaluation of Exosome Proteins by on-Bead Flow Cytometry

Exosomes, recently re-named "small extracellular vesicles" or "sEV," are emerging as an intercellular communication system. Quantification of the molecular cargo exosomes carry by on-bead flow cytometry is needed for defining their role in information transfer and in human disease. Exosomes (sEV) isolated from cell supernatants or plasma of cancer patients by size-exclusion chromatography were captured by biotinylated antibodies specific for antigens in the exosome cargo (e.g., tetraspanins) and placed on streptavidin-labeled beads. Detection was performed with pretitered fluorochrome-labeled antibodies of desired specificity. The data were acquired in a conventional cytometer, and molecules of equivalent soluble fluorochrome (MESF) beads were used to quantify the number of fluorescent molecules bound per bead. Isotype antibody controls were obligatory. The mean fluorescence intensity (MFI) value of each sample was converted into MESF units, and the separation index (SI), which quantifies separation of stained and isotype control beads, was determined. Various proteins identified by labeled antibodies were quantified on the surface of tumor cell-derived exosomes. To identify intravesicular cargo, such as cytokines or chemokines, exosomes were lysed with 0.3% Triton-100, and the proteins in lysates were loaded on aldehyde/sulfate latex beads for flow cytometry. Examples of quantitative surface and/or intravesicular on-bead flow cytometry for exosomes produced by various cells or present in body fluids of cancer patients are provided. On-bead flow cytometry standardized for use with conventional cytometers is a useful method for protein detection and quantitation in exosomes isolated from supernatants of cell lines or plasma of patients with cancer. © 2020 International Society for Advancement of Cytometry.

Development of microfluidic platform capable of high-throughput absolute quantification of single-cell multiple intracellular proteins from tumor cell lines and patient tumor samples

Quantification of single-cell proteins plays key roles in cell heterogeneity while due to technical limitations absolute numbers of multiple intracellular proteins from large populations of single cells were still missing, leading to compromised results in cell-type classifications. This paper presents a microfluidic platform capable of high-throughput absolute quantification of single-cell multiple types of intracellular proteins where cells stained with fluorescent labelled antibodies are aspirated into the constriction microchannels with excited fluorescent signals detected and translated into numbers of binding sites of targeted proteins based on calibration curves formed by flushing gradient solutions of fluorescent labelled antibodies directly into constriction microchannels. Based on this approach, single-cell numbers of binding sites of β-actin, α-tubulin and β-tubulin from tens of thousands of five representative tumor cell lines were first quantified, reporting cell-type classification rates of 83.0 ± 7.1%. Then single-cell numbers of binding sites of β-actin, biotin and RhoA from thousands of five tumor cell lines with varieties in malignant levels were quantified, reporting cell-type classification rates of 93.7 ± 2.8%. Furthermore, single-cell numbers of binding sites of Ras, c-Myc and p53 from thousands of cells derived from two oral tumor lines of CAL 27, WSU-HN6 and two oral tumor patient samples were quantified, contributing to high classifications of both tumor cell lines (98.6%) and tumor patient samples (83.4%). In conclusion, the developed microfluidic platform was capable of quantifying multiple intracellular proteins from large populations of single cells, and the collected data of protein expressions enabled effective cell-type classifications.

Improved quantitative detection of biotin-labeled red blood cells by flow cytometry

BACKGROUND

Biotin-labeled red blood cells (BioRBC) can be tracked after transfusion, providing a convenient and safe way to measure RBC survival in vivo. RBC survival is of interest for determining optimal blood storage conditions and for assessing the impact of genetic and biologic variants in blood donors on the survival of transfused RBCs. Here we present an improved, platform-independent assay for quantifying biotin on BioRBC. This approach is also useful for detecting BioRBC in peripheral blood samples as rare events.

STUDY DESIGN AND METHODS

We optimized the signal-to-noise ratio of the detecting reagent (phycoerythrin-conjugated streptavidin [SA-PE]) by determining the SA-PE concentration yielding the greatest separation index between BioRBC and unlabeled RBCs. We calibrated the fluorescence intensity measurements to molecules of equivalent soluble fluorochrome (MESF), a quantitative metric of fluorochrome binding and therefore of biotin bound per RBC. We then characterized the limit of blank and limit of quantification (LoQ) for BioRBC labeled at different densities.

RESULTS

Biotin-labeled RBCs at sulfo-NHS-biotin concentrations of 3 to 30 μg/mL (27-271 nmol/mL RBCs) ranged from approximately 32,000 to 200,000 MESF/RBC. The LoQ ranged from one in 274,000 to one in 649,000, depending on biotin-labeling density.

CONCLUSION

Increased sensitivity to detect BioRBC may facilitate tracking over longer periods and/or reduction of the BioRBC dose. Total RBC-bound biotin dose has been shown to correlate with the likelihood of developing antibodies to BioRBC. Lowering the dose of labeled cells may help avoid this eventuality.

Microfluidic Analyzer Enabling Quantitative Measurements of Specific Intracellular Proteins at the Single-Cell Level

This paper presents a microfluidic instrument capable of quantifying single-cell specific intracellular proteins, which are composed of three functioning modules and two software platforms. Under the control of a LabVIEW platform, a pressure module flushed cells stained with fluorescent antibodies through a microfluidic module with fluorescent intensities quantified by a fluorescent module and translated into the numbers of specific intracellular proteins at the single-cell level using a MATLAB platform. Detection ranges and resolutions of the analyzer were characterized as 896.78–6.78 × 10⁵ and 334.60 nM for Alexa 488, 314.60–2.11 × 10⁵ and 153.98 nM for FITC, and 77.03–5.24 × 10⁴ and 37.17 nM for FITC-labelled anti-beta-actin antibodies. As a demonstration, the numbers of single-cell beta-actins of two paired oral tumor cell types and two oral patient samples were quantified as: 1.12 ± 0.77 × 10⁶/cell (salivary adenoid cystic carcinoma parental cell line (SACC-83), n_cell = 13,689) vs. 0.90 ± 0.58 × 10⁵/cell (salivary adenoid cystic carcinoma lung metastasis cell line (SACC-LM), n_cell = 15,341); 0.89 ± 0.69 × 10⁶/cell (oral carcinoma cell line (CAL 27), n_cell = 7357) vs. 0.93 ± 0.69 × 10⁶/cell (oral carcinoma lymphatic metastasis cell line (CAL 27-LN2), n_cell = 6276); and 0.86 ± 0.52 × 10⁶/cell (patient I) vs. 0.85 ± 0.58 × 10⁶/cell (patient II). These results (1) validated the developed analyzer with a throughput of 10 cells/s and a processing capability of ~10,000 cells for each cell type, and (2) revealed that as an internal control in cell analysis, the expressions of beta-actins remained stable in oral tumors with different malignant levels.

A Microfluidic Fluorescent Flow Cytometry Capable of Quantifying Cell Sizes and Numbers of Specific Cytosolic Proteins

This study presents a microfluidics based cytometry capable of characterizing cell sizes and counting numbers of specific cytosolic proteins where cells were first bound by antibodies labelled with fluorescence and then aspirated into a constriction microchannel in which fluorescent levels were measured. These raw fluorescent pulses were further divided into a rising domain, a stable domain and a declining domain. In addition, antibody solutions with labelled fluorescence were aspirated through the constriction microchannel, yielding curves to translate raw fluorescent levels to protein concentrations. By using key parameters of three domains as well as the calibration curves, cell diameters and the absolute number of β-actins at the single-cell level were quantified as 14.2 ± 1.7 μm and 9.62 ± 4.29 × 10⁵ (A549, n_cell = 14 242), 13.0 ± 2.0 μm and 6.46 ± 3.34 × 10⁵ (Hep G2, n_cell = 35 932), 13.8 ± 1.9 μm and 1.58 ± 0.90 × 10⁶ (MCF 10 A, n_cell = 16 650), and 12.7 ± 1.5 μm and 1.09 ± 0.49 × 10⁶ (HeLa, n_cell = 26 246). This platform could be further adopted to measure numbers of various cytosolic proteins, providing key insights in proteomics at the single-cell level.

Single-Cell Protein Assays: A Review

Quantification of single-cell proteomics provides key insights in the field of cellular heterogeneity. This chapter discusses the emerging techniques that are being used to measure the protein copy numbers at the single-cell level, which includes flow cytometry, mass cytometry, droplet cytometry, microengraving, and single-cell barcoding microchip. The advantages and limitations of each technique are compared, and future research opportunities are highlighted.

A microfluidic flow cytometer enabling absolute quantification of single-cell intracellular proteins

Quantification of single-cell proteomics provides key insights into cellular heterogeneity while conventional flow cytometry cannot provide absolute quantification of intracellular proteins of single cells due to the lack of calibration approaches. This paper presents a constriction channel (with a cross sectional area smaller than cells) based microfluidic flow cytometer, capable of collecting copy numbers of specific intracellular proteins. In this platform, single cells stained with fluorescence labelled antibodies were forced to squeeze through the constriction channel with the fluorescence intensities quantified and since cells fully filled the constriction channel during the squeezing process, solutions with fluorescence labelled antibodies were flushed into the constriction channel to obtain calibration curves. By combining raw fluorescence data and calibration curves, absolute quantification of intracellular proteins was realized. As a demonstration, copy numbers of beta-actin of single tumour cells were quantified to be 0.90 ± 0.30 μM (A549, n_cell = 14 228), 2.34 ± 0.70 μM (MCF 10A, n_cell = 2455), and 0.98 ± 0.65 μM (Hep G2, n_cell = 6945). The travelling time for individual cells was quantified to be roughly 10 ms and thus a throughput of 100 cells per s can be achieved. This microfluidic system can be used to quantify the copy numbers of intracellular proteins in a high-throughput manner, which may function as an enabling technique in the field of single-cell proteomics.

Two-colour fluorescence fluorimetric analysis for direct quantification of bacteria and its application in monitoring bacterial growth in cellulose degradation systems

Monitoring bacterial growth is an important technique required for many applications such as testing bacteria against compounds (e.g. drugs), evaluating bacterial composition in the environment (e.g. sewage and wastewater or food suspensions) and testing engineered bacteria for various functions (e.g. cellulose degradation). T?=1,^FigItem(1) ^ReloadFigure=Yesraditionally, rapid estimation of bacterial growth is performed using spectrophotometric measurement at 600nm (OD600) but this estimation does not differentiate live and dead cells or other debris. Colony counting enumerates live cells but the process is laborious and not suitable for large numbers of samples. Enumeration of live bacteria by flow cytometry is a more suitable rapid method with the use of dual staining with SYBR I Green nucleic acid gel stain and Propidium Iodide (SYBR-I/PI). Flow cytometry equipment and maintenance costs however are relatively high and this technique is unavailable in many laboratories that may require a rapid method for evaluating bacteria growth. We therefore sought to adapt and evaluate the SYBR-I/PI technique of enumerating live bacterial cells for a cheaper platform, a fluorimeter. The fluorimetry adapted SYBR-I/PI enumeration of bacteria in turbid growth media had direct correlations with OD600 (p>0.001). To enable comparison of fluorescence results across labs and instruments, a fluorescence intensity standard unit, the equivalent fluorescent DNA (EFD) was proposed, evaluated and found useful. The technique was further evaluated for its usefulness in enumerating bacteria in turbid media containing insoluble particles. Reproducible results were obtained which OD600 could not give. An alternative method based on the assessment of total protein using the Pierce Coomassie Plus (Bradford) Assay was also evaluated and compared. In all, the SYBR-I/PI method was found to be the quickest and most reliable. The protocol is potentially useful for high-throughput applications such as monitoring of growth of live bacterial cells in 96-well microplates and in assessing in vivo activity of cellulose degrading enzyme systems.

Development of Microfluidic Systems Enabling High-Throughput Single-Cell Protein Characterization

This article reviews recent developments in microfluidic systems enabling high-throughput characterization of single-cell proteins. Four key perspectives of microfluidic platforms are included in this review: (1) microfluidic fluorescent flow cytometry; (2) droplet based microfluidic flow cytometry; (3) large-array micro wells (microengraving); and (4) large-array micro chambers (barcode microchips). We examine the advantages and limitations of each technique and discuss future research opportunities by focusing on three key performance parameters (absolute quantification, sensitivity, and throughput).

High sensitivity flow cytometry of membrane vesicles

Extracellular vesicles (EVs) are attracting attention as vehicles for inter-cellular signaling that may have value as diagnostic or therapeutic targets. EVs are released by many cell types and by different mechanisms, resulting in phenotypic heterogeneity that makes them a challenge to study. Flow cytometry is a popular tool for characterizing heterogeneous mixtures of particles such as cell types within blood, but the small size of EVs makes them difficult to measure using conventional flow cytometry. To address this limitation, a high sensitivity flow cytometer was constructed and EV measurement approaches that allowed them to enumerate and estimate the size of individual EVs, as well as measure the presence of surface markers to identify phenotypic subsets of EVs. Several fluorescent membrane probes were evaluated and it was found that the voltage sensing dye di-8-ANEPPS could produce vesicle fluorescence in proportion to vesicle surface area, allowing for accurate measurements of EV number and size. Fluorescence-labeled annexin V and anti-CD61 antibody was used to measure the abundance of these surface markers on EVs in rat plasma. It was shown that treatment of platelet rich plasma with calcium ionophore resulted in an increase in the fraction of annexin V and CD61-positive EVs. Vesicle flow cytometry using fluorescence-based detection of EVs has the potential to realize the potential of cell-derived membrane vesicles as functional biomarkers for a variety of applications.

Impact of Cell-surface Antigen Expression on Target Engagement and Function of an Epidermal Growth Factor Receptor × c-MET Bispecific Antibody

The efficacy of engaging multiple drug targets using bispecific antibodies (BsAbs) is affected by the relative cell-surface protein levels of the respective targets. In this work, the receptor density values were correlated to the in vitro activity of a BsAb (JNJ-61186372) targeting epidermal growth factor receptor (EGFR) and hepatocyte growth factor receptor (c-MET). Simultaneous binding of the BsAb to both receptors was confirmed in vitro. By using controlled Fab-arm exchange, a set of BsAbs targeting EGFR and c-MET was generated to establish an accurate receptor quantitation of a panel of lung and gastric cancer cell lines expressing heterogeneous levels of EGFR and c-MET. EGFR and c-MET receptor density levels were correlated to the respective gene expression levels as well as to the respective receptor phosphorylation inhibition values. We observed a bias in BsAb binding toward the more highly expressed of the two receptors, EGFR or c-MET, which resulted in the enhanced in vitro potency of JNJ-61186372 against the less highly expressed target. On the basis of these observations, we propose an avidity model of how JNJ-61186372 engages EGFR and c-MET with potentially broad implications for bispecific drug efficacy and design.

Development of a consensus protocol to quantify primate anti-non-Gal xenoreactive antibodies using pig aortic endothelial cells

BACKGROUND

Scientists working in the field of xenotransplantation do not employ a uniform method to measure and report natural and induced antibody responses to non-Galα(1,3)Gal (non-Gal) epitopes. Such humoral responses are thought to be particularly pathogenic after transplantation of vascularized GalTKO pig organs and having a more uniform assay and reporting format would greatly facilitate comparisons between laboratories.

METHODS

Flow cytometry allows examination of antibody reactivity to intact antigens in their natural location and conformation on cell membranes. We have established a simple and reproducible flow cytometric assay to detect antibodies specific for non-Gal pig antigens using primary porcine aortic endothelial cells (pAECs) and cell culture-adapted pAEC cell lines generated from wild type and α1,3galactosyl transferase knockout (GalTKO) swine.

RESULTS

The consensus protocol we propose here is based on procedures routinely used in four xenotransplantation centers and was independently evaluated at three sites using shared cells and serum samples. Our observation support use of the cell culture-adapted GalTKO pAEC KO:15502 cells as a routine method to determine the reactivity of anti-non-Gal antibodies in human and baboon serum.

CONCLUSIONS

We have developed an assay that allows the detection of natural and induced non-Gal xenoreactive antibodies present in human or baboon serum in a reliable and consistent manner. This consensus assay and format for reporting the data should be accessible to laboratories and will be useful for assessing experimental results between multiple research centers. Adopting this assay and format for reporting the data should facilitate the detection, monitoring, and detailed characterization of non-Gal antibody responses.

DNA-directed assembly of antibody-fluorophore conjugates for quantitative multiparametric flow cytometry

Multiparametric flow cytometry offers a powerful approach to single-cell analysis with broad applications in research and diagnostics. Despite advances in instrumentation, progress in methodology has lagged. Currently there is no simple and efficient method for antibody labeling or quantifying the number of antibodies bound per cell. Herein, we describe a DNA-directed assembly approach to fluorescent labeling that overcomes these barriers. Oligonucleotide-tagged antibodies and microparticles can be annealed to complementary oligonucleotides bearing fluorophores to create assay-specific labeling probes and controls, respectively. The ratio of the fluorescence intensity of labeled cells to the control particles allows direct conversion of qualitative data to quantitative units of antibody binding per cell. Importantly, a single antibody can be labeled with any fluorophore by using a simple mix-and-match labeling strategy. Thus, any antibody can provide a quantitative probe in any fluorescent channel, thus overcoming major barriers to the use of flow cytometry as a technique for systems biology and clinical diagnostics.

Quantification of cell surface proteins with bispecific antibodies

Flow cytometry is an established method for fast and accurate quantitation of cellular protein levels and requires fluorescently labeled antibodies as well as calibration standards. A critical step for quantitation remains the production of suitable detection antibodies with a precisely defined ratio of antigen-binding sites to fluorophores. Problems often arise as a consequence of inefficient and unspecific labeling which can influence antibody properties. In addition, the number of incorporated fluorophores necessitates a special normalization step for quantitation. To address these problems, we constructed different mono- and bivalent bispecific antibodies with binding site(s) for the cell surface antigens, cMET, EGFR1/HER1, ErbB2/HER2 or ErbB3/HER3 and with an additional digoxigenin-binding single-chain Fv fusion. The fluorophore Cy5 was covalently coupled to digoxigenin and quantitatively bound by the bispecific antibody. A panel of tumor cell lines was assessed under different culture conditions for absolute receptor expression levels of the indicated antigens and the data were set in relation to mRNA, gene count and immunoblot data. We could reproducibly quantify these receptors, omit the otherwise required normalization step and demonstrate the superiority of a 1 + 1 bispecific antibody. The same antibodies were also used to quantify the number of proteins in intracellular vesicles in confocal microscopy. The antibodies can be stored like regular antibodies and can be coupled with different digoxigenin-labeled fluorophores which makes them excellent tools for FACS and imaging-based experiments.

Deconstructing stem cell population heterogeneity: single-cell analysis and modeling approaches

Isogenic stem cell populations display cell-to-cell variations in a multitude of attributes including gene or protein expression, epigenetic state, morphology, proliferation and proclivity for differentiation. The origins of the observed heterogeneity and its roles in the maintenance of pluripotency and the lineage specification of stem cells remain unclear. Addressing pertinent questions will require the employment of single-cell analysis methods as traditional cell biochemical and biomolecular assays yield mostly population-average data. In addition to time-lapse microscopy and flow cytometry, recent advances in single-cell genomic, transcriptomic and proteomic profiling are reviewed. The application of multiple displacement amplification, next generation sequencing, mass cytometry and spectrometry to stem cell systems is expected to provide a wealth of information affording unprecedented levels of multiparametric characterization of cell ensembles under defined conditions promoting pluripotency or commitment. Establishing connections between single-cell analysis information and the observed phenotypes will also require suitable mathematical models. Stem cell self-renewal and differentiation are orchestrated by the coordinated regulation of subcellular, intercellular and niche-wide processes spanning multiple time scales. Here, we discuss different modeling approaches and challenges arising from their application to stem cell populations. Integrating single-cell analysis with computational methods will fill gaps in our knowledge about the functions of heterogeneity in stem cell physiology. This combination will also aid the rational design of efficient differentiation and reprogramming strategies as well as bioprocesses for the production of clinically valuable stem cell derivatives.

Spectral flow cytometry

Interest in measuring the complete fluorescence spectra of individual cells in flow can be traced to the earliest days of flow cytometry. Recent advances in detectors, optics, and computation have made it possible to make full spectral measurements in the sub-millisecond time frame in which flow cytometry measurements typically occur. This opens up new possibilities for applying spectroscopy to the analysis of individual cells. This unit reviews historical and contemporary approaches to spectral flow cytometry, as well as instrument design, calibration, and data analysis for spectral flow cytometry applications.

A quantitative flow cytometric assay for determining binding characteristics of chimeric, humanized and human antibodies in whole blood: proof of principle with rituximab and ofatumumab

Clinical successes of antibody-based drugs has led to extensive (pre-) clinical development of human(ized) monoclonal antibodies in a great number of diseases. The high specificity of targeted therapy with antibodies makes it ideally suited for personalized medicine approaches in which treatments needs are tailored to individual patients. One aspect of patient stratification pertains to the accurate determination of target occupancy and target expression to determine individual pharmacodynamic properties as well as the therapeutic window. The availability of reliable tools to measure target occupancy and expression on diseased and normal cells is therefore essential. Here, we evaluate a novel human antibody detection assay (Human-IgG Calibrator assay), which allows the flow cytometric quantification of therapeutic antibodies bound to the surface of cells circulating in whole blood. This assay not only permits the determination of the number of specific antibody bound per cell (sABC), but, when combined with quantification of exogenously added mouse antibody, also provides information on binding kinetics and antigen modulation. Our data indicate that the calibrator assay has all properties required for a pharmacodynamic tool to quantify target occupancy of chimeric, humanized and human therapeutic antibodies during therapy, as well as to collect valuable information on both antibody and antigen kinetics.

Validation of a quantitative flow cytometer assay for monitoring HER-2/neu expression level in cell-based cancer immunotherapy products

GVAX immunotherapy for prostate cancer is comprised of two genetically modified prostate cancer cell lines, CG1940 and CG8711, engineered to secrete granulocyte macrophage-colony-stimulating factor. As part of the matrix of potency assays, CG1940 and CG8711 are tested for the expression level of cell surface HER-2/neu using a quantitative flow cytometer assay. This assay reports the antibody binding capacity value of the cells as a measure of HER-2/neu expression using cells immediately after thawing from cryogenic storage. With optimized cell handling and staining procedure and appropriate system suitability controls, the assay was validated as a quantitative assay. The validation results showed that assay accuracy, specificity, precision, linearity, and range were suitable for the intended use of ensuring lot-to-lot consistency of HER-2/neu expression. Assay robustness was demonstrated using design of experiments that evaluated critical assay parameters. Finally, the assay was successfully transferred to a current good manufacturing practice Quality Control laboratory in a separate facility. Since the overall precision of this assay is better than that of ELISA methods and it can be performed with ease and high throughput, quantitative flow cytometer-based assays may be an appropriate immunological assay platform for Quality Control laboratories for characterization and release of cell-based therapies.

Quantitative measurement of multifunctional quantum dot binding to cellular targets using flow cytometry

Semiconductor nanocrystals such as quantum dots (QDs) are a potentially powerful resource in the fields of flow cytometry and fluorescence microscopy. QD size and fluorescence characteristics offer attractive features for use in targeted delivery systems and detection by flow cytometry. While quantitative measurements of a variety of fluorescent molecules are routinely performed, fluorophores for which no calibration standards exist, such as QDs, pose a problem for quantitation in flow cytometry. Our goal was to develop a targeted nanoparticle delivery platform as well as a corresponding method to accurately and quantitatively assess the performance of this system. We synthesized surface-modified QD probes targeted to cellular surface receptors and measured the MFI of the resulting cell-probe conjugates by flow cytometry. MFI was converted to mean equivalent R-PE intensity (MEPE) using standard calibration microspheres. Known concentrations of both R-PE and QD probes were measured by fluorometry to relate R-PE and QD fluorescence. Fluorometry results were then used to translate MEPE measurements to the number of bound QD probes. The targeted probes exhibited superior binding characteristics over unmodified and untargeted particles. This binding interaction was shown to be specific and mediated by the NGR targeting peptide tethered to the QD surface. The calibration method developed to assess this system proved successful at converting raw fluorescence data to quantitative probe binding values. We demonstrate the synthesis and performance of a highly modular nanoparticle system capable of targeted binding and fluorescent imaging. The calibration method implemented to quantify the performance of this system represents a potentially powerful tool to utilize truly quantitative flow cytometry measurements with an array of fluorescent molecules, including QDs.

Standardized method to minimize variability in a functional P2X(7) flow cytometric assay for a multi-center clinical trial

BACKGROUND

Flow cytometric analysis of human P2X(7) pore activity segregates variant from common P2RX7 genotypes and may serve as a biomarker for cancer, pain, inflammation, and immune responses to infection. Standardization is needed to accommodate variable sample age and instrumentation differences in a multicenter clinical trial.

METHODS

CD14-PE-stained whole blood samples were treated with YO-PRO-1 combined with a P2X(7) agonist (BzATP) or control, followed by the addition of PI after closure of the P2X(7) pore. Recalled instrument settings from previous publications were used to adapt a standardized fluorescent particle-adjusted set-up method. Experiments were performed to compare the two methods while evaluating components of systematic variability and facilitating reliable processing of samples with varied ages.

RESULTS

The median YO-PRO-1 fluorescence of BzATP-treated samples had less variability when collected by the bead-adjusted method and was less influenced by the compensation strategy used. The average day-to-day coefficient of variance for assessments of P2X(7) pore activity by this method was 0.11 +/- 0.04, and the exclusion of nonviable cells was found to accommodate samples aged up to 4 days after phlebotomy. The bead-adjusted set-up method produced measurements differing by only 2.0% +/- 1.5% on two analog cytometers, and within similar decades when comparing analog to digital instruments.

CONCLUSIONS

These results provide a standardized method for quantitative flow cytometric analysis of P2X(7) receptor phenotypes in blood monocytes with minimal intralaboratory variation and potential for interlaboratory comparisons that can greatly facilitate multicenter functional genomic clinical studies.

Transcriptome-wide noise controls lineage choice in mammalian progenitor cells

Phenotypic cell-to-cell variability within clonal populations may be a manifestation of 'gene expression noise', or it may reflect stable phenotypic variants. Such 'non-genetic cell individuality' can arise from the slow fluctuations of protein levels in mammalian cells. These fluctuations produce persistent cell individuality, thereby rendering a clonal population heterogeneous. However, it remains unknown whether this heterogeneity may account for the stochasticity of cell fate decisions in stem cells. Here we show that in clonal populations of mouse haematopoietic progenitor cells, spontaneous 'outlier' cells with either extremely high or low expression levels of the stem cell marker Sca-1 (also known as Ly6a; ref. 9) reconstitute the parental distribution of Sca-1 but do so only after more than one week. This slow relaxation is described by a gaussian mixture model that incorporates noise-driven transitions between discrete subpopulations, suggesting hidden multi-stability within one cell type. Despite clonality, the Sca-1 outliers had distinct transcriptomes. Although their unique gene expression profiles eventually reverted to that of the median cells, revealing an attractor state, they lasted long enough to confer a greatly different proclivity for choosing either the erythroid or the myeloid lineage. Preference in lineage choice was associated with increased expression of lineage-specific transcription factors, such as a >200-fold increase in Gata1 (ref. 10) among the erythroid-prone cells, or a >15-fold increased PU.1 (Sfpi1) (ref. 11) expression among myeloid-prone cells. Thus, clonal heterogeneity of gene expression level is not due to independent noise in the expression of individual genes, but reflects metastable states of a slowly fluctuating transcriptome that is distinct in individual cells and may govern the reversible, stochastic priming of multipotent progenitor cells in cell fate decision.

Discrepancy in measuring CD4 expression on T-lymphocytes using fluorescein conjugates in comparison with unimolar CD4-phycoerythrin conjugates

BACKGROUND

Numerous methods for quantitative fluorescence calibration (QFC) have been developed to quantify receptor expression on lymphocytes. However, the results from the use of these different QFC methods vary considerably in the literature. To better identify the causes of these discrepancies, we measured CD4 expression using FITC and phycoerythrin (PE) conjugates to stain CYTO-TROL Control Cells and T-lymphocytes in whole blood and isolated cell preparations. We further examined pH of the cellular microenvironment as a cause of discordant results obtained with the FITC conjugate.

METHODS

Calibration with Quantibrite PE-labeled microspheres and the use of unimolar CD4-PE conjugates provided direct measurement of the antibody bound per cell value (ABC) for CD4 expression on normal T-lymphocytes. Calibration for CD4-FITC monoclonal antibody (Mab) labeled CYTO-TROL Control Cells and normal T-lymphocytes was based on molecules of equivalent soluble fluorochrome (MESF) as determined by FITC-labeled microspheres traceable to NIST RM 8640. The MESF value for CD4-FITC Mab was determined that enabled the conversion of the MESF values obtained for CYTO-TROL cells to ABC. We investigated the likely pH change in the fluorescein microenvironments within FITC-labeled Mab and cells stained with FITC-labeled Mab using a pH sensitive indicator.

RESULTS

The mean ABC value for T-lymphocytes prepared from fresh whole blood using CD4-PE conjugate (48,321) was consistent with previous results, and it was much higher than the mean ABC using CD4-FITC Mab (22,156). The mean ABC value for CYTO-TROL cells using CD4-PE conjugate (43,090) was also higher than that using CD4-FITC conjugate (34,734), although the discrepancy was not as great. Further studies suggested the discrepancy in CYTO-TROL results may be accounted for by the low pH of the membrane microenvironment, but the greater discrepancy in T-lymphocytes could not be fully explained.

CONCLUSION

CD4 expression on fresh normal whole blood samples and CYTO-TROL cells can be consistently quantified in ABC units using Quantibrite PE quantification beads and unimolar CD4-PE conjugates. Quantification with CD4-FITC conjugate is not as consistent, but may be improved by the use of CD4 T-cells as biological calibrators. This approximation is valid only for surface receptors with consensus ABC values measured by different QFC methods serving as biological standards.

Quantitative flow cytometry of ZAP-70 levels in chronic lymphocytic leukemia using molecules of equivalent soluble fluorochrome

BACKGROUND

ZAP-70 has emerged as a potential pivotal prognostic marker for patients with chronic lymphocytic leukemia (CLL), which could replace immunoglobulin heavy chain mutation status. Although several flow cytometry assays have been described for assessing ZAP-70 in CLL, certain technical and scientific issues remain unsolved, which have prevented results of this crucial test from being reported, even in the best routine flow cytometry laboratories. In this report, we aimed to solve some of these issues by providing a computerized quantitative flow cytometric assay for ZAP-70 within the entire CLL population, which would be easy to perform and enable standardization between laboratories.

METHODS

Intracellular ZAP-70 levels in CLL and normal B cells were assessed by molecules of equivalent soluble fluorochrome (MESF), employing Quantum FITC MESF calibration beads to establish a standard curve relating channel value to fluorescence intensity in MESF units and the QuickCal v. 2.2 program (www.bangslabs.com) and clinical relevance of the data was determined.

RESULTS

The average ZAP-70 expression value in the CD19(+)/CD5(+) cells from 35 CLL patients was 103,701 MESF when compared with 12,621 MESF in B cells from 20 normal blood samples. "Low" and "high" ZAP-70 CLL subgroups were defined. Patients with "high ZAP-70 MESF" CLL had a shorter time to disease progression (P = 0.0005) and a more advanced clinical stage (P = 0.0018) when compared with patients in the "low ZAP-70 MESF" CLL subgroup.

CONCLUSIONS

This quantitative analysis method can be employed to obtain a more specific and highly accurate assessment of ZAP-70 levels in CLL cells. The method can easily be standardized, in any routine flow laboratory, thereby improving reproducibility and reliability of ZAP-70 analysis.

Long-term effects of idiotype vaccination on the specific T-cell response in peripheral blood and bone marrow of multiple myeloma patients

OBJECTIVES

To elucidate long-term effects of idiotype (Id) vaccination on Id-specific T cells of multiple myeloma (MM) patients and compare Id-specific T-cell responses of peripheral blood with those of bone marrow (BM).

MATERIALS AND METHODS

Id-specific T-cell responses of peripheral blood mononuclear cells (PBMC) were compared with those of BM mononuclear cells (BMMC) in 10 MM patients vaccinated with the Id protein at a median time of 41 months since the last immunization. The PBMC responses at late follow-up were also compared with those during active immunization. The responses were assessed by a proliferation assay, enzyme-linked immunospot (ELISPOT) (gamma-interferon), cytometric bead array (CBA) for secreted cytokines and quantitative real-time polymerase chain reaction (QRT-PCR) for cytokine gene expression.

RESULTS

At the late testing time, an Id-specific response was detected in PBMC of five patients (ELISPOT, CBA, QRT-PCR) and in BMMC of four patients (CBA, QRT-PCR). A response in both compartments was noted only in three patients. The cytokines gene profile was consistent with a predominance of Th(2) cells [interleukin (IL)-4, IL-5, IL-10]. Comparison of the Id-specific responses of PBMC during active immunization with those at the late follow-up showed that the frequency and magnitude of the responses had decreased significantly by time (proliferation/ELISPOT) (P < 0.02) and shifted at the gene level from a Th(1) to a Th(2) profile (P < 0.05).

CONCLUSION

Id-specific T-cells may decline overtime and shift toward a Th(2) response and may be found at a similar frequency of patients in blood and BM.

Quality assurance for polychromatic flow cytometry

This protocol outlines a three-part quality assurance program to optimize, calibrate and monitor flow cytometers used to measure cells labeled with five or more fluorochromes (a practice known as polychromatic flow cytometry). The initial steps of this program (system optimization) ensure that the instrument's lasers, mirrors and filters are optimally configured for the generation and transmission of multiple fluorescent signals. To determine the sensitivity and dynamic range of each fluorescence detector, the system is then calibrated by measuring fluorescence over a range of photomultiplier tube (PMT) voltages by determining the PMT voltage range and linearity (Steps 2-10) and validating the PMT voltage (Steps 11-17). Finally, to ensure consistent performance, we provide procedures to monitor the precision, accuracy and sensitivity of fluorescence measurements over time. All three aspects of this program should be performed upon installation, or whenever changes occur along the flow cytometer's optical path. However, only a few of these procedures need to be carried out on a routine basis.

A novel quantitative flow cytometric method for measuring glucocorticoid receptor (GR) in cell lines: correlation with the biochemical determination of GR

Currently, a time consuming biochemical method is used for GR quantification. Here we compare the biochemical approach with a newly developed flow cytometric method of measuring GR in cell lines, which is less time consuming and does not requires the use of radioactive materials. The biochemical assay is easy to apply but the cells need to be grown in media free of endogenous glucocorticoids, in order to prevent them from interfering with radiolabelled hormone binding to the receptor. The presence of endogenous GR ligands is known to reduce receptor levels and to often produce false negative results. The immunofluorescent method is free of such limitations, as it depends entirely on detecting the receptor using a highly specific monoclonal antibody. Additionally, the biochemical assay cannot measure heterogeneity in individual cells, in contrast the flow cytometric one enables the enumeration of the receptor on a per cell basis, allowing exact description of differences in receptor levels amongst intact cells. Our results demonstrate that the flow cytometric method is of similar accuracy but of higher precision compared to the biochemical one. Also, the data we obtained using the immunofluorescent method correlated well with the biochemical one (R(2)=0.9712). In conclusion, flow cytometric method requires small cell numbers, is more accurate and lesser time consuming than the biochemical one. Thus, it could be useful for the quantification of GR in lymphocyte subpopulations, in lymphoproliferative disorders and in tumor cells from cancer patients, in order to design more efficient clinical treatment protocols.

The 2005 Italian Quality Control Study for the evaluation of CD4 cells in centers involved in the treatment of HIV type 1 patients

We report the results of an external quality control program, including 17 Italian centers involved in the care of patients infected by HIV, to evaluate CD4 T cell count proficiency and reproducibility. The centers received two commercial stabilized blood preparations, one with "normal" and one with "low" CD4 T cell content. The centers were asked to process the samples two times, 1 week apart, with the same procedure used for samples from HIV patients. Most centers showed a good performance of CD4 frequency and absolute count determinations. In particular, the "low" sample was correctly analyzed by all centers; only two underestimated the "normal" sample CD4 frequency, and only one underestimated the CD4 absolute count by >100 CD4 cells/microl. Overall, our data suggest that most Italian laboratories provide reliable and reproducible results in evaluating CD4 T cells in HIV(+) samples.

Identification of a novel chromosome region, 13q21.33-q22.2, for susceptibility genes in familial chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is the most prevalent form of leukemia in adults in western countries. A genome scan of CLL-prone families revealed a lod score of one in band 13q22.1. To investigate this finding, we selected 6 CLL families consisting of 63 individuals (CLL affected, n = 19; unaffected, n = 44) for fine mapping of a 23-megabase region in 13q14.2-q22.2. Interphase fluorescence in situ hybridization (FISH) revealed 13q14 deletion in 85% (11/13) of CLL patients. Four CLL families shared a 3.68-Mb minimal region in 13q21.33-q22.2. Two asymptomatic siblings who shared the 13q21.33-q22.2 at-risk haplotype exhibited CD5+ monoclonal B-cell lymphocytosis (MBL) on flow cytometry. One of these individuals also had a 13q14 deletion by FISH. These 2 individuals with MBL shared the at-risk haplotype with their CLL-affected relatives, providing further evidence of the relationship between CLL and MBL, as well as of the biologic significance of this novel region. Using direct DNA sequencing analysis, we screened 13 genes for mutations, but no frameshift or nonsense mutations were detected. Our studies revealed that 11 of the 13 genes in the candidate region were expressed in immune tissues, supporting their functional relevance in investigations of familial CLL. In conclusion, we identified a novel candidate region that may predispose to familial CLL.

Simplified Volumetric Flow Cytometry Allows Feasible and Accurate Determination of Cd4 T Lymphocytes in Immunodeficient Patients Worldwide

The determination of CD4 cells is of crucial clinical importance for patients with AIDS. However, the high costs involved represent limitations for CD4 cell counting in developing countries. In order to provide an affordable technique, we introduced a simplified volumetric counting (SVC) technique without sample manipulations and investigated it in a multicentre study. Blood samples from 434 healthy donors and immunodeficient patients were tested in eight hospital laboratories in Europe, Africa and Asia. CD4 cell counts were compared using inhouse flow cytometric methods and the SVC technique. The SVC method was performed on a low-cost flow cytometer (CyFlow SL, Partec, Münster, Germany) after 15 min antibody incubation without pre-analytic manipulations, such as washing or erythrocyte lysing procedures. Linear regression analysis demonstrated a correlation of r=0.942 (Europe), r=0.952 (Africa) and r=0.989 (Asia) between the SVC technique and the in-house methods. Bland Altman plot analysis of all patient data showed a mean bias between the two methods of +26 CD4 cells in favour of the SVC technique (measured range: 6–1905 cells/μl; median CD4 cell count: 388/μl). Three centres used the FACS-count technique (Becton-Dickinson, San José, Calif., USA) as an in-house method dispensing with pre-analytic manipulations. The comparison of SVC and FACS-count method revealed a mean bias of +32 CD4 cells/μl (median CD4 cell count: 349/μl). The accuracy of the SVC was tested on standards with known CD4 cell counts ( n=6) and was shown to be 95.2%. The low-cost device and the simplified no-lyse, no-wash test procedure reduces the costs per determination and facilitates the use of flow cytometry in developing countries.

Pan B-cell markers are not redundant in analysis of chronic lymphocytic leukemia (CLL)

BACKGROUND

The classic immunophenotype for chronic lymphocytic leukemia (CLL) is CD19(+), restricted dim surface expression of kappa or lambda light chain, CD5(+), CD23(+), dim CD20(+), negative FMC7, and negative CD79b. However, the necessity of assaying for all 3 pan B-cell markers (CD20, FMC7, and CD79b) by flow cytometry has not been definitively documented for CLL.

METHODS

Qualitative patterns and semi-quantitative assessment of staining intensity for CD20, FMC7 and CD79b were performed in 70 cases with a current or prior diagnosis of CLL or CLL with increased prolymphocytes leukemia (CLL/PL). The concurrent morphology in 66 of 70 specimens was classified as typical CLL in 53 cases, CLL/PL in 10 cases, and large cell lymphoma in 3 cases.

RESULTS

Forty percent of the cases varied from the characteristic immunophenotype by having moderate or bright staining of CD20 (36%), FMC7 (7%), and/or CD79b (18%). Discrepant qualitative staining patterns were found between FMC7 and CD20 (21%), CD20 and CD79b (15%), and CD79b and FMC7 (10%). Semiquantitative measurement of staining intensity showed little correlation between CD79b and CD20 or FMC7. Moderate correlation was seen between CD20 and FMC7. No correlation was observed between morphology and intensity of marker expression.

CONCLUSIONS

Variable patterns and intensity of staining were seen for FMC7, CD20, and CD79b in this cohort of CLL samples. Dim or negative staining was most consistently seen for FMC7 (93% of specimens). Although FMC7 staining intensity was moderately correlated with CD20, CD79b intensity was poorly correlated with either CD20 or FMC7, and thus, may provide some independent information.

Associating changes in the immune system with clinical diseases for interpretation in risk assessment

This overview unit discusses the relationship between immunosuppression, a potential consequence of immunotoxicity, and disease progression. It also discusses other factors, such as stress and age, that affect disease susceptibility. These factors play an important role in risk assessment for exposures to environmental factors.

Quantitative fluorescence cytometric measurement of estrogen and progesterone receptors: correlation with the hormone binding assay

We describe, here, a rapid flow cytometry technique for the detection and quantification of estrogen (ER) and progesterone (PgR) receptors in several human cell lines and in clinical samples obtained from breast cancer tumors. ER and PgR quantitation can be very useful in patients with breast cancer as their role in diagnosis and prognosis is well established. However ligand binding assays and immunohistochemical assays are difficult to measure heterogeneity in individual cells. On the other hand, flow cytometry is a convenient tool for quantification in individual cells. Flow cytometric results with breast cancer cell lines and clinical samples were compared to those obtained by quantitative biochemical ER and PgR performed by the standard dextran-coated charcoal biochemical assay. The latter assay is affected by the level of endogenous steroids. This is also the case in the routine measurement of ER/PgR in patient's tumor cells whereby estradiol molecules in patient's serum produced negative or low values in the biochemical assay. The mAbs used in our flow cytometric method bind to their specific ER or PgR independently of whether they are preoccupied by their ligands and they produce reliable results. With the use of beads calibrated in MESF (Molecules of Equivalent Soluble Fluorochrome) units, the ER and PgR can be measured on a per cell basis. The flow cytometric method showed a strong correlation with biochemical receptor assessments of either ER alpha (ER alphaDCC, r = 0.918, p = 0.073) or PgR (PgRDCC, r = 0.75, p = 0.001). This study demonstrates that ER alpha and PgR can be detected by flow cytometry on a per cell basis in intact cells, and can be quantitated reliably in terms of MESF without the limitations of competition with serum's estradiol molecules.

B-cell monoclonal lymphocytosis and B-cell abnormalities in the setting of familial B-cell chronic lymphocytic leukemia

BACKGROUND

Among all hematologic malignancies, B-cell chronic lymphocytic leukemia (BCLL) has the highest familial clustering (three- to sevenfold increase), strongly suggesting a genetic component to its etiology. Familial BCLL can be used as a model to study the early pathogenesis of this disease.

METHODS

We examined nine kindreds from the National Cancer Institute's Familial BCLL Registry, consisting of 19 affected members with BCLL and 33 clinically unaffected first-degree relatives. Flow cytometric immunophenotyping to detect a B-cell monoclonal lymphocytosis (BCML) was performed. Monoclonality was confirmed by polymerase chain reaction analysis of whole blood DNA. Cell cycle analysis for aneuploidy was conducted.

RESULTS

In all affected individuals, we observed the classic BCLL CD5/CD19/CD20/CD23 immunophenotypic patterns. Six of the 33 unaffected individuals (18%) had evidence of BCML. Additional individuals (13/33, 39%) showed some other abnormality, whereas 14 individuals (42%) were normal. Based on an estimated prevalence of 0.7% for BCML in the general population, the finding of six subjects (18%) with clonal abnormalities in this relatively modest sample was significantly greater than expected (i.e., 18% vs. 0.7%, P < 5.7 x 10(-9)).

CONCLUSIONS

Individual components of BCML and other B-cell abnormalities were observed in almost half of the apparently unaffected individuals. Our findings suggested that BCML may be an early detectable abnormality in BCLL. The spectrum of some of these observed abnormalities suggested the involvement of different B-cell subpopulations or different pathways in clonal evolution. Population-based, longitudinal studies will be required to determine the incidence of BCML and other B-cell abnormalities and their relation to disease progression in BCLL and other closely related B-cell lymphoproliferative disorders.

Identification of P-selectin glycoprotein ligand-1 as a useful marker in acute myeloid leukaemias

Immunophenotyping is considered to be less valuable in the diagnosis of acute myeloid leukaemias (AML) compared with acute lymphoid leukaemias. Here, we present data on the use of quantitative flow cytometry (QFC) of P-selectin glycoprotein ligand 1 (PSGL-1, CD162) and three-colour immunophenotyping including CD162 staining in the identification of myeloid precursors in AML. Analysis of normal peripheral blood (n = 20) and normal bone marrow (n = 5) samples and on 20 samples from de novo M1, M2, M4 and M5 AML patients demonstrated that PSGL-1 is differentially expressed on various mature and immature leucocyte subsets. It was found by QFC that neutrophils expressed 26500 +/- 4500 and monocytes 47200 +/- 9900 copies of PSGL-1 on their surface, whereas AML blasts from M1 and M2 AML patients expressed significantly less PSGL-1 (12 000 +/- 5300) than mature neutrophils (P < 0.001). In M4 and M5 leukaemias, however, the amount of PSGL-1 on monocytic precursors is displayed in a fairly broad range that is not significantly different from that of mature monocytes (P = 0.084). Using three-colour immunophenotyping PSGL-1-dim staining was co-expressed with CD7 and C34 positivity and PSGL-1 staining intensity on immature myeloid cells paralleled with CD45 expression. This would imply a differential expression of PSGL-1 during myeloid haematopoietic development and suggests that quantification of surface PSGL-1 may aid in differentiating myeloblasts from monoblasts by immunophenotyping in different AML subsets.

Performance evaluation of QuantiBRITE phycoerythrin beads

BACKGROUND

The performance of QuantiBRITE phycoerythrin (PE) beads to standardize quantitation in terms of antibodies bound per cell (ABC) was evaluated by measuring precision, variation across multiple instruments, and variation across time.

METHODS

For CD4 quantitation, whole blood was stained with a two-color CD4 reagent using a no-wash/no-lyse format. For CD69 quantitation, whole blood was activated with either phorbol myristate acetate (PMA) or CD3 beads and then stained with a three-color CD69 reagent using a lyse-no-wash format.

RESULTS

Across 20 normal donors, the mean CD4 ABC was 51,000. Within-assay precision on quantitation of CD4 ABC on T cells had a coefficient of variance (CV) of <1.0%. Across multiple flow cytometers, quantitation of CD4 ABC had a CV of <5.0%. Within-donor CV on CD4 ABC on 20 donors across 2 months ranged from 1.3% to 3.2%. Within-assay precision on quantitation of CD69 on T cells activated with either PMA or CD3 beads had a CV of <3.0%. Within-donor CV of CD69 ABC across 1 month ranged from 2% to 18% on PMA-activated samples and from 7% to 24% on CD3 bead-activated samples.

CONCLUSIONS

Our results indicate that the QuantiBRITE PE beads provide a useful tool for standardized analysis across labs. When used in conjunction with 1:1 conjugates of PE-to-monoclonal antibody, the QuantiBRITE PE beads provide a simple yet robust means of quantitating expression levels in terms of ABC.

Flow cytometry: an 'old' tool for novel applications in medical genetics

Flow cytometry was originally established as an automated method for measuring optical or fluorescence characteristics of cells or particles in suspension. In the meantime, flow cytometers have become user-friendlier, less expensive instruments with an increasing importance in clinical diagnostics. Besides the classical fields of application, such as immunophenotyping blood cells or analyzing the cell cycle status by measuring the DNA content, novel flow cytometric methods have been developed to identify and to quantify disease-related gene sequences. Here we give an overview of current and future applications, including the detection of viral sequences via microsphere-based PCR assays and the analysis of single nucleotide polymorphisms, reflecting individual phenotypic traits. Furthermore, flow cytometry allows the quantification of gene expression changes as well as the isolation of differentially expressed gene sequences. Flow cytometry is also convenient for multiplex analyses, e.g. when hybridizing DNA samples to a mixture of various microsphere populations each coated with different DNA probes. Last but not least, the use of magnetic beads in combination with flow cytometers coupled with automated devices enables molecular diagnostics on a large scale. Overall, this review demonstrates flow cytometry as a rapid, sensitive, and reproducible tool applicable to a wide range of medical genetic approaches.

Polarization of scatter and fluorescence signals in flow cytometry

BACKGROUND

The pulses of light scatter and fluorescence measured in flow cytometers exhibit varying degrees of polarization. Flow cytometers are heterogeneously sensitive to this polarization, depending on the light source(s), the optical layout, and the types of mirrors and filters used. Therefore, fluorescence polarization can affect apparent intensity ratios between particles and interfere with schemes for interlaboratory standardization.

METHODS

We investigate the degree to which polarization affects common flow cytometry measurements. Our technique for determining polarization differs from previous methods because complete distributions of intensity versus polarization angle are measured, rather than intensities at just two orthogonal polarization angles. Theoretical models for scatter and fluorescence are presented and verified by making polarization measurements of calibration beads.

RESULTS

Measurements of cells stained with a variety of dyes illustrate that fluorescence polarization occurs frequently in flow cytometry.

CONCLUSIONS

Consequences for quantitative cytometry are discussed, and the use of the "magic angle" to make a flow cytometer insensitive to fluorescence polarization is proposed.

Molecular quantification of cell cycle-related gene expression at the protein level

BACKGROUND

Immunofluorescence cytometry of antigen and DNA content provides relative measurements of the cell cycle phase distribution of a specific epitope. Measurement of correlated expression of epitopes on signaling and regulatory proteins will be useful in the study of the complex pathways involved in cell cycle regulation and carcinogenesis. However, to formulate regulatory pathway models, measurements of molecules per cell would be more useful than relative measurements of intensity. Here, we report on a system in which the relationship between molecules and fluorescence is determined for a reference set of cell lines that are then used to directly calculate the number of molecules for unknowns. To demonstrate the process, we calculated the cell cycle phase distribution of SV40 large T antigen (Tag) in the reference cells.

METHODS

A set of cell line clones expressing different levels of Tag were isolated. Quantitative Western blots of these cells and purified, recombinant Tag were performed. Cells from the same sample were stained and analyzed by flow cytometry for Tag and DNA. The relationship between molecules and fluorescence was established and calculations were performed for the phase distributions of Tag.

RESULTS

The five cell lines had 0.11, 0.27, 1.06, 2.44, and 2.63 x 10(6) molecules of Tag per cell, determined by Western blot. The average coefficient of variation was 10.6%. The relationship of molecules to fluorescence fit a linear equation (r(2) = 0.96) over the range, 0.11 - 2.63 x 10(6) molecules, however, the same equation did not fit the relationship between 0 molecules, defined by isotype staining controls, and the lowest expressing cell line. To calculate the phase distributions of molecules in the lowest cell line, a second linear equation from 0 to 110,000 molecules was used.

CONCLUSIONS

This work describes a system where fixed cells expressing various levels of a target antigen quantified by Western blots can be used to standardize flow cytometric measurements of gene expression in absolute terms.