Surface CD4 density remains constant on lymphocytes of HIV-infected patients in the progression of disease

Tuning cellular uptake of nanoparticles via ligand density: Contribution of configurational entropy

The bioactivity of nanoparticles (NPs) crucially depends on their ability to cross biological membranes. A fundamental understanding of cell-NP interaction is hence essential to improve the performance of the NP-based biomedical applications. Although extensive studies of cellular uptake have converged upon the idea that the uptake process is mainly regulated by the elastic deformation of the cell membrane or NP, recent experimental observations indicate the ligand density as another critical factor in modulating NP uptake into cells. In this study, we propose a theoretical model of the wrapping of an elastic vesicle NP by a finite lipid membrane to depict the relevant energetic and morphological evolutions during the wrapping process driven by forming receptor-ligand bonds. In this model, the deformations of the membrane and the vesicle NP are assumed to follow the continuum Canham-Helfrich framework, whereas the change of configurational entropy of receptors is described from statistical thermodynamics. Results show that the ligand density strongly affects the binding energy and configurational entropy of free receptors, thereby altering the morphology of the vesicle-membrane system in the steady wrapping state. For the wrapping process by the finite lipid membrane, we also find that there exists optimal ligand density for the maximum wrapping degree. These predictions are consistent with relevant experimental observations reported in the literature. We have further observed that there are transitions of various wrapping phases (no wrapping, partial wrapping, and full wrapping) in terms of ligand density, membrane tension, and molecular binding energy. In particular, the ligand and receptor shortage regimes for the small and high ligand density are, respectively, identified. These results may provide guidelines for the rational design of nanocarriers for drug delivery.

A biophysical perspective on receptor-mediated virus entry with a focus on HIV

As part of their entry and infection strategy, viruses interact with specific receptor molecules expressed on the surface of target cells. The efficiency and kinetics of the virus-receptor interactions required for a virus to productively infect a cell is determined by the biophysical properties of the receptors, which are in turn influenced by the receptors' plasma membrane (PM) environments. Currently, little is known about the biophysical properties of these receptor molecules or their engagement during virus binding and entry. Here we review virus-receptor interactions focusing on the human immunodeficiency virus type 1 (HIV), the etiological agent of acquired immunodeficiency syndrome (AIDS), as a model system. HIV is one of the best characterised enveloped viruses, with the identity, roles and structure of the key molecules required for infection well established. We review current knowledge of receptor-mediated HIV entry, addressing the properties of the HIV cell-surface receptors, the techniques used to measure these properties, and the macromolecular interactions and events required for virus entry. We discuss some of the key biophysical principles underlying receptor-mediated virus entry and attempt to interpret the available data in the context of biophysical mechanisms. We also highlight crucial outstanding questions and consider how new tools might be applied to advance understanding of the biophysical properties of viral receptors and the dynamic events leading to virus entry.

Mechanobiological Mimicry of Helper T Lymphocytes to Evaluate Cell-Biomaterials Crosstalk

The unique properties of immune cells have inspired many efforts in engineering advanced biomaterials capable of mimicking their behaviors. However, an inclusive model capable of mimicking immune cells in different situations remains lacking. Such models can provide invaluable data for understanding immune-biomaterial crosstalk. Inspired by CD4+ T cells, polymeric microparticles with physicochemical properties similar to naïve and active T cells are engineered. A lipid coating is applied to enhance their resemblance and provide a platform for conjugation of desired antibodies. A novel dual gelation approach is used to tune the elastic modulus and flexibility of particles, which also leads to elongated circulation times. Furthermore, the model is enriched with magnetic particles so that magnetotaxis resembles the chemotaxis of cells. Also, interleukin-2, a proliferation booster, and interferon-γ cytokines are loaded into the particles to manipulate the fates of killer T cells and mesenchymal stem cells, respectively. The penetration of these particles into 3D environments is studied to provide in vitro models of immune-biomaterials crosstalk. This biomimicry model enables optimization of design parameters required for engineering more efficient drug carriers and serves as a potent replica for understanding the mechanical behavior of immune cells.

A stochastic spatio-temporal (SST) model to study cell-to-cell variability in HIV-1 infection

Although HIV viremia in infected patients proceeds in a manner that may be accounted for by deterministic mathematical models, single virus-cell encounters following initial HIV exposure result in a variety of outcomes, only one of which results in a productive infection. The development of single molecule tracking techniques in living cells allows studies of intracellular transport of HIV, but it remains less clear what its impact may be on viral integration efficiency. Here, we present a stochastic intracellular mathematical model of HIV replication that incorporates microtubule transport of viral components. Using this model, we could study single round infections and observe how viruses entering cells reach one of three potential fates - degradation of the viral RNA genome, formation of LTR circles, or successful integration and establishment of a provirus. Our model predicts global trafficking properties, such as the probability and the mean time for a HIV viral particle to reach the nuclear pore. Interestingly, our model predicts that trafficking determines neither the probability or time of provirus establishment - instead, they are a function of vRNA degradation and reverse transcription reactions. Thus, our spatio-temporal model provides novel insights into the HIV infection process and may constitute a useful tool for the identification of promising drug targets.

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.

The coreceptor CD4 is expressed in distinct nanoclusters and does not colocalize with T-cell receptor and active protein tyrosine kinase p56lck

CD4 molecules on the surface of T lymphocytes greatly augment the sensitivity and activation process of these cells, but how it functions is not fully understood. Here we studied the spatial organization of CD4, and its relationship to T-cell antigen receptor (TCR) and the active form of Src kinase p56lck (Lck) using single and dual-color photoactivated localization microscopy (PALM) and direct stochastic optical reconstruction microscopy (dSTORM). In nonactivated T cells, CD4 molecules are clustered in small protein islands, as are TCR and Lck. By dual-color imaging, we find that CD4, TCR, and Lck are localized in their separate clusters with limited interactions in the interfaces between them. Upon T-cell activation, the TCR and CD4 begin clustering together, developing into microclusters, and undergo a larger scale redistribution to form supramolecluar activation clusters (SMACs). CD4 and Lck localize in the inner TCR region of the SMAC, but this redistribution of disparate cluster structures results in enhanced segregation from each other. In nonactivated cells these preclustered structures and the limited interactions between them may serve to limit spontaneous and random activation events. However, the small sizes of these island structures also ensure large interfacial surfaces for potential interactions and signal amplification when activation is initiated. In the later activation stages, the increasingly larger clusters and their segregation from each other reduce the interfacial surfaces and could have a dampening effect. These highly differentiated spatial distributions of TCR, CD4, and Lck and their changes during activation suggest that there is a more complex hierarchy than previously thought.

Quantifying CD4 receptor protein in two human CD4+ lymphocyte preparations for quantitative flow cytometry

BACKGROUND

In our previous study that characterized different human CD4+ lymphocyte preparations, it was found that both commercially available cryopreserved peripheral blood mononuclear cells (PBMC) and a commercially available lyophilized PBMC (Cyto-Trol™) preparation fulfilled a set of criteria for serving as biological calibrators for quantitative flow cytometry. However, the biomarker CD4 protein expression level measured for T helper cells from Cyto-Trol was about 16% lower than those for cryopreserved PBMC and fresh whole blood using flow cytometry and mass cytometry. A primary reason was hypothesized to be due to steric interference in anti- CD4 antibody binding to the smaller sized lyophilized control cells.

METHOD

Targeted multiple reaction monitoring (MRM) mass spectrometry (MS) is used to quantify the copy number of CD4 receptor protein per CD4+ lymphocyte. Scanning electron microscopy (SEM) is utilized to assist searching the underlying reasons for the observed difference in CD4 receptor copy number per cell determined by MRM MS and CD4 expression measured previously by flow cytometry.

RESULTS

The copy number of CD4 receptor proteins on the surface of the CD4+ lymphocyte in cryopreserved PBMCs and in lyophilized control cells is determined to be (1.45 ± 0.09) × 10(5) and (0.85 ± 0.11) × 10(5), respectively, averaged over four signature peptides using MRM MS. In comparison with cryopreserved PBMCs, there are more variations in the CD4 copy number in lyophilized control cells determined based on each signature peptide. SEM images of CD4+ lymphocytes from lyophilized control cells are very different when compared to the CD4+ T cells from whole blood and cryopreserved PBMC.

CONCLUSION

Because of the lyophilization process applied to Cyto-Trol control cells, a lower CD4 density value, defined as the copy number of CD4 receptors per CD4+ lymphocyte, averaged over three different production lots is most likely explained by the loss of the CD4 receptors on damaged and/or broken microvilli where CD4 receptors reside. Steric hindrance of antibody binding and the association of CD4 receptors with other biomolecules likely contribute significantly to the nearly 50% lower CD4 receptor density value for cryopreserved PBMC determined from flow cytometry compared to the value obtained from MRM MS.

Validation of cell-based fluorescence assays: practice guidelines from the ICSH and ICCS - part III - analytical issues

Clinical diagnostic assays, may be classified as quantitative, quasi-quantitative or qualitative. The assay's description should state what the assay needs to accomplish (intended use or purpose) and what it is not intended to achieve. The type(s) of samples (whole blood, peripheral blood mononuclear cells (PBMC), bone marrow, bone marrow mononuclear cells (BMMC), tissue, fine needle aspirate, fluid, etc.), instrument platform for use and anticoagulant restrictions should be fully validated for stability requirements and specified. When applicable, assay sensitivity and specificity should be fully validated and reported; these performance criteria will dictate the number and complexity of specimen samples required for validation. Assay processing and staining conditions (lyse/wash/fix/perm, stain pre or post, time and temperature, sample stability, etc.) should be described in detail and fully validated.

C-C chemokine receptor type five (CCR5): An emerging target for the control of HIV infection

When HIV was initially discovered as the causative agent of AIDS, many expected to find a vaccine within a few years. This has however proven to be elusive; it has been approximately 30 years since HIV was first discovered, and a suitable vaccine is still not in effect. In 2009, a paper published by Hutter et al. reported on a bone marrow transplant performed on an HIV positive individual using stem cells that were derived from a donor who was homozygous for a mutation in the CCR5 gene known as CCR5 delta-32 (Δ32) (Hütter et al., 2009). The HIV positive individual became HIV negative and remained free of viral detection after transplantation despite having halted anti-retroviral (ARV) treatment. This review will focus on CCR5 as a key component in HIV immunity and will discuss the role of CCR5 in the control of HIV infection.

Quantifying the cluster of differentiation 4 receptor density on human T lymphocytes using multiple reaction monitoring mass spectrometry

Cluster of differentiation 4 (CD4) is an important glycoprotein containing four extracellular domains, a transmembrane portion and a short intracellular tail. It locates on the surface of various types of immune cells and performs a critical role in multiple cellular functions such as signal amplification and activation of T cells. It is well-known as a clinical cell surface protein marker for study of HIV progression and for defining the T helper cell population in immunological applications. Moreover, CD4 protein has been used as a biological calibrator for quantification of other surface and intracellular proteins. However, flow cytometry, the conventional method of quantification of the CD4 density on the T cell surface depends on antibodies and has suffered from variables such as antibody clones, the fluorophore and conjugation chemistries, the fixation conditions, and the flow cytometric quantification methods used. In this study, we report the development of a highly reproducible nano liquid chromatography-multiple reaction monitoring mass spectrometry-based quantitative method to quantify the CD4 receptor density in units of copy number per cell on human CD4+ T cells. The method utilizes stable isotope-labeled full-length standard CD4 as an internal standard to measure endogenous CD4 directly, without the use of antibodies. The development of the mass spectrometry-based approach of CD4 protein quantification is important as a complementary strategy to validate the analysis from the cytometry-based conventional method. It also provides new support for quantitative understanding and advanced characterization of CD4 on CD4+ T cells.

Human CD4+ lymphocytes for antigen quantification: characterization using conventional flow cytometry and mass cytometry

To transform the linear fluorescence intensity scale obtained with fluorescent microspheres to an antibody bound per cell (ABC) scale, a biological cell reference material is needed. Optimally, this material should have a reproducible and tight ABC value for the expression of a known clinical reference biomarker. In this study, we characterized commercially available cryopreserved peripheral blood mononuclear cells (PBMCs) and two lyophilized PBMC preparations, Cyto-Trol and PBMC-National Institute for Biological Standard and Control (NIBSC) relative to freshly prepared PBMC and whole blood samples. It was found that the ABC values for CD4 expression on cryopreserved PBMC were consistent with those of freshly obtained PBMC and whole blood samples. By comparison, the ABC value for CD4 expression on Cyto-Trol is lower and the value on PBMC-NIBSC is much lower than those of freshly prepared cell samples using both conventional flow cytometry and CyTOF™ mass cytometry. By performing simultaneous surface and intracellular staining measurements on these two cell samples, we found that both cell membranes are mostly intact. Moreover, CD4(+) cell diameters from both lyophilized cell preparations are smaller than those of PBMC and whole blood. This could result in steric interference in antibody binding to the lyophilized cells. Further investigation of the fixation effect on the detected CD4 expression suggests that the very low ABC value obtained for CD4(+) cells from lyophilized PBMC-NIBSC is largely due to paraformaldehyde fixation; this significantly decreases available antibody binding sites. This study provides confirmation that the results obtained from the newly developed mass cytometry are directly comparable to the results from conventional flow cytometry when both methods are standardized using the same ABC approach.

Quantitative fluorescence measurements with multicolor flow cytometry

Multicolor flow cytometer assays are routinely used in clinical laboratories for immunophenotyping, -monitoring disease and treatment, and determining prognostic factors. However, existing methods for quantitative measurements have not yet produced satisfactory results. This chapter details a procedure for quantifying surface and intracellular protein biomarkers by calibrating the output of a multicolor flow cytometer in units of antibodies bound per cell (ABC). The procedure includes the following critical steps (a) quality control (QC) of the multicolor flow cytometer, (b) fluorescence calibration using hard dyed microspheres assigned with fluorescence intensity values in equivalent number of reference fluorophores (ERF), (c) compensation for correction of fluorescence spillover, and (d) application of a biological reference standard for translating the ERF scale to the ABC scale. The chapter also points out current efforts for implementing quantification of biomarkers in a manner which is independent of instrument platforms and reagent differences.

Low postseroconversion CD4 count and rapid decrease of CD4 density identify HIV+ fast progressors

CD4 expression in HIV replication is paradoxical: HIV entry requires high cell-surface CD4 densities, but replication requires CD4 down-modulation. However, is CD4 density in HIV+ patients affected over time? Do changes in CD4 density correlate with disease progression? Here, we examined the role of CD4 density for HIV disease progression by longitudinally quantifying CD4 densities on CD4+ T cells and monocytes of ART-naive HIV+ patients with different disease progression rates. This was a retrospective study. We defined three groups of HIV+ patients by their rate of CD4+ T cell loss, calculated by the time between infection and reaching a CD4 level of 200 cells/microl: fast (<7.5 years), intermediate (7.5-12 years), and slow progressors (>12 years). Mathematical modeling permitted us to determine the maximum CD4+ T cell count after HIV seroconversion (defined as "postseroconversion CD4 count") and longitudinal profiles of CD4 count and density. CD4 densities were quantified on CD4+ T cells and monocytes from these patients and from healthy individuals by flow cytometry. Fast progressors had significantly lower postseroconversion CD4 counts than other progressors. CD4 density on T cells was lower in HIV+ patients than in healthy individuals and decreased more rapidly in fast than in slow progressors. Antiretroviral therapy (ART) did not normalize CD4 density. Thus, postseroconversion CD4 counts define individual HIV disease progression rates that may help to identify patients who might benefit most from early ART. Early discrimination of slow and fast progressors suggests that critical events during primary infection define long-term outcome. A more rapid CD4 density decrease in fast progressors might contribute to progressive functional impairments of the immune response in advanced HIV infection. The lack of an effect of ART on CD4 density implies a persistent dysfunctional immune response by uncontrolled HIV infection.

Toward quantitative fluorescence measurements with multicolor flow cytometry

A procedure is presented for calibrating the output of a multicolor flow cytometer in units of antibodies bound per cell (ABC). The procedure involves two steps. First, each of the fluorescence channels of the flow cytometer is calibrated using Ultra Rainbow beads with assigned values of equivalent number of reference fluorophores (ERF). The objective of this step is to establish a linear relation between the fluorescence signal in a given fluorescence channel of multicolor flow cytometers and the value of ERF. The second step involves a biological standard such as a lymphocyte with a known number of antibody binding sites (e.g., CD4 binding sites). The biological standard is incubated with antibodies labeled with one type of fluorophores for a particular fluorescence channel and serves to translate the ERF scale to an ABC scale. A significant part of the two-step calibration procedure involves the assignment of ERF values to the different populations of Ultra Rainbow beads. The assignment of ERF values quantifies the relative amount of embedded fluorophore mixture in each bead population. It is crucial to insure that the fluorescence signal in a given range of fluorescence emission wavelengths is related linearly to the assigned values of ERF. The biological standard has to poses a known number of binding sites for a given antibody. In addition, this antibody has to be amenable to labeling with different types of fluorophores associated with various fluorescence channels. The present work suggests that all of the requirements for a successful calibration of a multicolor flow cytometer in terms of ABC values can be fulfilled. The calibration procedure is based on firm scientific foundations so that it is easy to envision future improvements in accuracy and ease of implementation.

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.

Comparison of fluorescein and phycoerythrin conjugates for quantifying CD20 expression on normal and leukemic B-cells

BACKGROUND

Numerous methods for quantitative fluorescence calibration (QFC) have been developed to quantify receptor expression on lymphocytes as potential disease biomarkers. CD20 expression in B-cell chronic lymphocytic leukemia (B-CLL) is one of the best examples of such a biomarker, but results from the use of different QFC methods vary considerably.

METHODS

We measured CD20 expression on normal and B-CLL B-cells, using FITC and PE conjugates from the same monoclonal antibody (Mab). As a biological control and calibrator, we also measured CD4 expression on T-cells with FITC and PE Mab. Calibration curves were constructed using the CLSI (formerly NCCLS) consensus guidelines for QFC. Calibration with QuantiBRITE PE-labeled microspheres and the use of unimolar PE conjugates provided direct measurement of antibody bound per cell (ABC) for CD4 and CD20. Calibration for FITC conjugates was based on molecules of equivalent soluble fluorochrome (MESF), as determined by NIST RM 8640 microsphere standards. These MESF values were then converted to ABC, using the CD4 T-cell as a biologic calibrator, to normalize FITC and PE results for CD20 expression.

RESULTS

On normal B cells, the mean ABC value for unimolar CD20-PE conjugate was 143,500 (CV +/- 19.1%). The mean ABC value for B-CLL B-cells stained with the same conjugate was 21,700 (CV +/- 42.0%). Using the CD4 T-cell as a biologic calibrator for FITC conjugate, the mean ABC value for CD20-FITC on normal B-cells was 199,300. CD20-FITC staining on B-CLL cells was generally too weak for accurate quantification. On normal T-cells, the mean ABC value for CD4 unimolar PE conjugate was (36,800 +/- 10.4)%, and it did not differ significantly in CLL samples.

CONCLUSION

The expression of CD20 on normal and B-CLL lymphocytes can be quantified in ABC units using unimolar CD20-PE conjugates. In addition, CD4 expression on T-cells can be used as a biological calibrator to quantify CD20-FITC ABC, with reasonable agreement between the two conjugates with different fluorochromes. Issues regarding the accuracy of MESF microsphere calibrators and effective F/P ratios for FITC conjugates will require additional laboratory studies.

Mobility measurements of immunomagnetically labeled cells allow quantitation of secondary antibody binding amplification

Magnetic cell separation methods commonly utilize paramagnetic materials conjugated to antibodies that target specific cell surface molecules. The amount of magnetic material bound to a cell is directly proportional to the magnetophoretic mobility of that cell. A mathematical model has been developed which characterizes the fundamental parameters controlling the amount of magnetic material bound, and thus, the magnetophoretic mobility of an immunomagnetically labeled cell. In characterization of the paramagnetic labeling, one of the parameters of interest is the increase in magnetophoretic mobility due to the secondary antibody binding to multiple epitopes on the primary antibody, referred to as the "secondary antibody binding amplification," Psi. Secondary antibody-binding amplification has been investigated and quantitated by comparing the mobilities of lymphocytes directly labeled with anti-CD4 MACS (Miltenyi Biotec, Auburn, CA) magnetic nanoparticle antibody with the mobilities of lymphocytes from the same sample labeled with two different indirect antibody-labeling schemes. Each indirect labeling scheme incorporated a primary mouse anti-CD4 FITC antibody that provides both FITC and mouse-specific binding sites for two different secondary antibody-magnetic nanoparticle conjugates: either anti-FITC MACS magnetic nanoparticle antibody or anti-mouse MACS magnetic nanoparticle antibody. The magnetophoretic mobilities of the immunomagnetically labeled cells were obtained using Cell Tracking Velocimetry (CTV). The results indicate that an average of 3.4 anti-FITC MACS magnetic nanoparticle antibodies bind to each primary CD4 FITC antibody, Psi(1,2f) = 3.4 +/- 0.33, and that approximately one, Psi(1,2m) = 0.98 +/- 0.081, anti-mouse MACS magnetic nanoparticle antibody binds to each primary mouse CD4 FITC antibody on a CD4 positive lymphocyte. These results have provided a better understanding of the antibody-binding mechanisms used in paramagnetic cell labeling for magnetic cell separation.

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.

Standardisation of a procedure for quantifying surface antigens by indirect immunofluorescence

Quantitative indirect immunofluorescence (QIIF) methods used to measure absolute numbers of surface-expressed antigens have produced conflicting results [Marchant, A., Duchow, J., Delville, J., Goldman, M., 1992. Lipopolysaccharide induces up-regulation of CD14 molecule on monocytes in human whole blood. European Journal of Immunology 22, 1663-1665; Antal-Szalmas, P., van Strijp, J.A.G., Weersink, A.J.L., Verhoef, J., van Kessel, K.P.M., 1997. Quantitation of surface CD14 on human monocytes and neutrophils. Journal of Leukocyte Biology 61, 721-728.]. The aim of this study was to standardise a flow cytometric method using the quantitative indirect immunofluorescence kit (QIFIkit, Dako, Denmark) for quantifying surface-expressed bovine classical major histocompatibility complex (MHC) class I molecules. The importance of accurately titrating antibodies in this procedure and using live cell gates is already accepted. However, little work has been carried out in optimising cell washes to remove excess antibody, or to study the influence of cell numbers used in the assay. In addition, information on the binding properties of each antibody is required in order to make accurate measurements. This study demonstrates that a number of critical parameters must be established prior to using this method for accurate numerical assessment of cell surface-expressed molecules.

Cell surface marker evaluation of infant Macaca monkey leukocytes in peripheral whole blood using simultaneous dual-color immunophenotypic analysis

Cross-reactivity between several commercially available mouse antihuman monoclonal antibodies (mAbs), conjugated to phycoerythrin (PE) and fluorescein isothiocyanate (FITC) fluorochromes, and peripheral blood leukocyte surface antigens, has been established in infant cynomolgus (Macaca fascicularis) monkeys using whole blood lysis, and two-color, PE and FITC flow cytometric analysis. With the exception of the CD8 marker, the bivariate dot-plot patterns for all other markers were similar in infant monkeys and in humans. For the CD8 marker, however, a CD8+CD2- population of cells was observed in the majority of monkeys tested (10 out of 12). The number of CD8+CD2- cells was higher (13%) in infant monkeys compared to the 3% reported for adult human blood. The mean percentage and absolute numbers for the cell surface markers identified with the human mAbs CD2 (FITC, Ortho, Paritan, NJ), CD4 (PE, B-D, Mountain View, CA), and CD8 (PE, B-D) when these were combined with a series of PE- or FITC-labelled human mAbs were similar across all combinations tested. Statistically significant differences were observed between male and female monkeys for the mean percentage levels of CD4 (females > males) and for the CD4/CD8 ratio (females > males). Such gender differences need to be taken into consideration when infant cynomolgus monkeys are used as models for chemical-induced immunotoxicity studies. Measurement of the interleukin-2 (IL-2) and transferrin proved to be useful in monitoring in vitro cellular activation in infant cynomolgus and possibly in rhesus (M. mulatta) monkeys.

Comparison of CD4 cell count by a simple enzyme-linked immunosorbent assay using the TRAx CD4 test kit and by flow cytometry and hematology

Measurement of CD4 T-lymphocyte levels is clinically useful in monitoring immune status in a number of conditions, including human immunodeficiency virus (HIV) infection, in which the absolute CD4 count is used to guide therapy. The absolute CD4 count is obtained by multiplying the results of the leukocyte count and the differential with a hematology cell counter and the percentage of CD4+ T lymphocytes determined by flow cytometry. These techniques require expensive, complex instrumentation, and interlaboratory results are difficult to standardize and reproduce. The rapid growth of HIV infection worldwide has increased the need for more-reproducible and cost-effective methods for CD4 T-cell monitoring. The TRAx CD4 test kit is based on a novel adaptation of conventional enzyme-linked immunosorbent assay (ELISA) and permits the simple quantitation of total CD4 protein from whole-blood lysates. In this study, the relationship between total CD4 protein measured in units per milliliter (TRAx) and in cells per microliter (flow cytometry and hematology) was defined in a multisite clinical study using linear regression analysis. Data from 230 HIV-seronegative and 321 HIV-seropositive specimens were used to calibrate the TRAx assay recombinant CD4 standards and controls in equivalent CD4 T lymphocytes per microliter (cells per microliter). The calibration of the TRAx CD4 assay in cells per microliter was validated with a second group of specimens from 17 healthy volunteers and 20 HIV-seropositive patients which were collected and tested under strictly controlled conditions intended to minimize the effects of specimen aging on the results of the reference method. These data were also used to estimate the variability of absolute CD4 count by cytometric methods as well as the precision of the TRAx CD4 result after it was calibrated in cells per microliter. Overall, correlations between the two methods ranged from 0.87 to 0.95. Additional studies demonstrated that the contribution of CD4 protein from monocytes and any soluble CD4 in sera are negligible in the TRAx assay and do not significantly affect results. This new method represents a promising alternative to absolute CD4 T-cell enumeration by flow cytometry and hematology.