Distribution function approach to the study of the kinetics of IgM antibody binding to FcγRIIIb (CD16b) receptors on neutrophils by flow cytometry

Kinetic titration method in flow cytometry for quantification of cell receptors

For the quantitative determination of cell receptors by fluorescence flow cytometry, we proposed a new method, which takes into account the reaction kinetics. The binding reaction of the ligand with receptors begins after placing the cells in the ligand solution. In the proposed method, there are several samples with the same concentration of cells and different initial concentrations of fluorescently labeled ligand, and each sample is measured by a flow cytometer once at the time when the following condition is met: the product of the incubation time (cells with ligand) and the initial concentration of ligand is the same for all samples. The proposed approach eliminates disadvantages and combines advantages of both kinetic and titration methods for quantification of receptors on single cells without the use of traditional calibration fluorescent beads. Practical application of the method was demonstrated in quantification of CD8 and CD14 on peripheral blood human leukocytes. Particularly, we found decreased (by a factor of two) mean number of CD14 on monocytes and granulocytes in patients with atherosclerosis (treated in the hospital) compared to conditionally healthy donors, whereas no difference was found in the mean CD8 expression on leukocytes between the same patient and donor groups.

A quantitative view on multivalent nanomedicine targeting

Although the concept of selective delivery has been postulated over 100 years ago, no targeted nanomedicine has been clinically approved so far. Nanoparticles modified with targeting ligands to promote the selective delivery of therapeutics towards a specific cell population have been extensively reported. However, the rational design of selective particles is still challenging. One of the main reasons for this is the lack of quantitative theoretical and experimental understanding of the interactions involved in cell targeting. In this review, we discuss new theoretical models and experimental methods that provide a quantitative view of targeting. We show the new advancements in multivalency theory enabling the rational design of super-selective nanoparticles. Furthermore, we present the innovative approaches to obtain key targeting parameters at the single-cell and single molecule level and their role in the design of targeting nanoparticles. We believe that the combination of new theoretical multivalent design and experimental methods to quantify receptors and ligands aids in the rational design and clinical translation of targeted nanomedicines.

Calibration-free quantitative immunoassay by flow cytometry: Theoretical consideration and practical implementation for IgG antibody binding to CD14 receptors on human leukocytes

We propose a calibration-free method to determine the number of receptors per cell, as well as the direct and the reverse reaction rate constants for a single receptor. The method is based on the analysis of the temporal evolution of the cells mean fluorescent intensity measured by a flow cytometer during the ligand-receptor (antigen-antibody) binding under the conditions of their comparable concentrations. We developed the kinetic approach accounting both for the delay between the dilution and the measurement and for the practical duration of the measurement itself. The method was applied to determine thenumber of CD14 receptors on human blood mononuclear (granulocytes, monocytes, lymphocytes) cells of several donors. We also obtained the direct ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>k</mml:mi><mml:mo>+</mml:mo></mml:msub><mml:mo>=</mml:mo></mml:mrow></mml:math> (5.6 ± 0.2) × 10⁷ M^-1 min^-1 ) and reverse ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>k</mml:mi><mml:mo>-</mml:mo></mml:msub><mml:mo>=</mml:mo></mml:mrow></mml:math> (1.3 ± 0.2) × 10^-2 min^-1 ) rate constants of ligand-receptor interaction, and estimated the size of the binding site as b = 0.5 nm. The latter allows one to recalculate the rate constants for a different ligand, fluorescent label, medium viscosity, and/or temperature. The knowledge of the rate constants is essential for the calibration-free determination of the number of receptors per cell from a single kinetic curve of the cells mean fluorescence intensity.

Dynamic quantification of antigen molecules with flow cytometry

Traditional methods for estimating the number of expressed molecules, based on the detection of target antigens bound with fluorescently labeled antibodies, assume that the antigen-antibody reaction reaches equilibrium. A calibration procedure is used to convert the intensity of the fluorescence signal to the number of target molecules. Along with the different limitations of every calibration system, this substantially limits the applicability of the traditional approaches especially in the case of low affinity antibodies. We address this problem here with studies in which we demonstrate a new approach to the antigen molecule quantification problem. Instead of using a static calibration system, we analyzed mean fluorescence values over time by flow cytometry during antibody-antigen binding. Experimental data obtained with an LSRII cytometer were fitted by a diffusion-reaction mathematical model using the Levenberg-Marquardt nonlinear least squares curve-fitting algorithm in order to obtain the number of target antigen molecules per cell. Results were compared with the Quanti-BRITE calibration system. We conclude that, instead of using experiment-specific calibration, the value of the binding rate constant for each particular antibody-antigen reaction can be used to quantify antigen molecules with flow cytometry. The radius of CD8 antibody molecule binding site was found, that allows recalculating the binding rate constant for other conditions (different sizes of reagent molecules, fluorescent label, medium viscosity and temperature). This approach is independent of specially prepared calibration beads, antibody reagents and the specific dye and can be applied to both low and high affinity antibodies, under both saturating and non-saturating binding conditions. The method was demonstrated on a human blood sample dataset investigating CD8α antigen on T cells in stable binding conditions.

Optimized flow cytometry protocol for analysis of surface expression of interleukin-1 receptor types I and II

The biological effects of interleukin (IL)-1 are realized through binding to specific membrane-bound receptors. The efficiency of IL-1 action depends on the number of receptors on the cell. We determined the percentage of cells that express IL-1 receptor type I (IL-1RI) and IL-1 receptor type II (IL-1RII) by flow cytometry using phycoerythrin (PE)-labelled antibodies to the IL-1Rs, and the mean absolute number of membrane-bound IL-1Rs per cell using QuantiBRITE PE calibration beads. We showed that different subpopulations of immunocompetent cells expressed different numbers of molecules of membrane-bound IL-1RI and IL-1RII. We also established that when cells were stimulated with bacterial lipopolysaccharide, there was a significant increase in the number of IL-1RI expressed, and a significant decrease in the mean number of IL-1RII molecules per cell. Determination of the mean number of membrane-bound IL-1R molecules using this protocol enables us to obtain precise and reproducible data that are necessary for full evaluation of expression levels.

Quantitative flow cytometric analysis of expression of tumor necrosis factor receptor types I and II on mononuclear cells

BACKGROUND

Tumor necrosis factor (TNF)-α is an inflammatory cytokine, the biological effects of which are mediated by the interaction with specific membrane-bound receptors. To assess TNF-α receptor (TNFR) expression, it is important to estimate both the number of cells that carry these receptors and the number of receptors per cell, because the cell fate depends on the balance between TNFRI and TNFRII signaling.

OBJECTIVE

The aim of the present study was to develop an optimized protocol to estimate the level of expression of membrane-bound TNFRI and TNFRII, using QuantiBRITE PE calibration beads.

MATERIALS AND METHODS

The percentage of cells that expressed membrane-bound TNFRI and TNFRII and the mean number of receptors per cell were determined by flow cytometry using PE-labeled antibodies against TNFR. To create a calibration curve and convert cell fluorescence intensity values to absolute numbers of receptors, we used QuantiBRITE PE beads.

RESULTS

CD19(+) B lymphocytes had the least percentage of cells expressing TNFRI and the greatest number of receptor molecules per cell, whereas CD3(+) T lymphocytes had the greatest percentage of cells expressing TNFRII and the lowest density of these receptors. We also established that stimulation of peripheral blood mononuclear cells (PBMCs) with the lipopolysaccharide (LPS) significantly increased the number of TNFRI and TNFRII on CD14(+) monocytes.

CONCLUSION

Application of the protocol-identified differences in the percentage of cells that expressed TNFRs, as well as the absolute number of receptors per cell, among different subpopulations of PBMCs, and between PBMCs cultured with and without LPS.