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

Solving technical issues in flow cytometry to characterize porcine CD8α/β expressing lymphocytes

The CD8 molecule is a cell surface receptor and well described as co-receptor on T cells, binding directly to the major histocompatibility complex class I on antigen presenting cells. CD8 antigens are comprised of two distinct polypeptide chains, the α and the β chain. In the pig, the CD8 receptor is expressed by several lymphocyte subsets, including Natural Killer cells, γδ T cells and antigen experienced CD4+ αβ T cells. On these cell populations CD8 is expressed as αα homodimers. Porcine cytolytic T cells on the other hand exclusively express CD8 αβ heterodimers. Several monoclonal antibodies (mAbs) for either of the two chains are available and are frequently used in flow cytometry. We observed that distinct combinations of mAb clones for CD8α and CD8β chains can cause troubles in multi-color staining panels. Therefore, we aimed for an in-depth study of the usage of different CD8-specific mAb clones and optimizing co-staining strategies for flow cytometry. We tested mAb clones 11/295/33 and 76-2-11 for the detection of CD8α and mAb clones PPT23 and PG164A for the detection of CD8β. The results indicate that the CD8α clone 11/295/33 should not be used together with either of the two CD8β clones in the same incubation step, as co-staining led to a highly reduced ability of CD8β mAb binding and loss in signal in flow cytometry. This can lead to potential false results in detecting CD8αβ cytolytic T cells. In case of the CD8α mAb clone 76-2-11, no inhibition in binding of either CD8β mAb clones was observed, making it the preferred choice in multi-color staining panels. The obtained data will help in future panel designs for flow cytometry in the pig and therefore improving studies of porcine immune cells.

Standardization of flow cytometric detection of antigen expression

Since response to antigen-based immunotherapy relies upon the level of tumor antigen expression we developed an antigen quantification assay using ABC values. Antigen quantification as a clinical assay requires methods for quality control and for interlaboratory and inter-cytometer platform standardization. A single lot of Cytotrol™ Lyophilized Control Cells (Beckman Coulter) used for all studies. The variability in antigen quantification across 4 different instrument platforms in 2 separate laboratories was evaluated. The effect of the antibody clone utilized, importance of custom 1:1 molar ratio (fluorophore to protein, F/P) verses off-the-shelf antibodies, and QuantiBrite PE calibration verses linearity calibration combined with a single point scale transformation with CD4 as reference were determined. Use of single lot control cells allowed validation of reproducibility between flow cytometer platforms and laboratories and allowed assessment of different antibody lots, cocktail preparation, and different antibody clones. Off the shelf antibody preparations provide reproducible estimates of antigen density, however custom 1:1 unimolar antibody preparations should be utilized for definitive measurement of antigen expression.Geometric Mean fluorescent Intensity (GeoMFI) was not comparable across instruments and inter-laboratory. The use of CD4 as the reference marker can minimize variability in ABC values. Comparable antigen quantification is vital in managing patients receiving antigen-based immunotherapy. If this assay is to be utilized in a clinical setting, quality control methods have to be instituted to assure reproducibility and allow validation across laboratories. We have demonstrated that use of a lyophilized cell control is highly valuable in achieveing these goals.

Agent-based computational modeling of glioblastoma predicts that stromal density is central to oncolytic virus efficacy

Oncolytic viruses (OVs) are emerging cancer immunotherapy. Despite notable successes in the treatment of some tumors, OV therapy for central nervous system cancers has failed to show efficacy. We used an ex vivo tumor model developed from human glioblastoma tissue to evaluate the infiltration of herpes simplex OV rQNestin (oHSV-1) into glioblastoma tumors. We next leveraged our data to develop a computational, model of glioblastoma dynamics that accounts for cellular interactions within the tumor. Using our computational model, we found that low stromal density was highly predictive of oHSV-1 therapeutic success, suggesting that the efficacy of oHSV-1 in glioblastoma may be determined by stromal-to-tumor cell regional density. We validated these findings in heterogenous patient samples from brain metastatic adenocarcinoma. Our integrated modeling strategy can be applied to suggest mechanisms of therapeutic responses for central nervous system cancers and to facilitate the successful translation of OVs into the clinic.

Iterative community-driven development of a SARS-CoV-2 tissue simulator

The 2019 novel coronavirus, SARS-CoV-2, is a pathogen of critical significance to international public health. Knowledge of the interplay between molecular-scale virus-receptor interactions, single-cell viral replication, intracellular-scale viral transport, and emergent tissue-scale viral propagation is limited. Moreover, little is known about immune system-virus-tissue interactions and how these can result in low-level (asymptomatic) infections in some cases and acute respiratory distress syndrome (ARDS) in others, particularly with respect to presentation in different age groups or pre-existing inflammatory risk factors. Given the nonlinear interactions within and among each of these processes, multiscale simulation models can shed light on the emergent dynamics that lead to divergent outcomes, identify actionable "choke points" for pharmacologic interventions, screen potential therapies, and identify potential biomarkers that differentiate patient outcomes. Given the complexity of the problem and the acute need for an actionable model to guide therapy discovery and optimization, we introduce and iteratively refine a prototype of a multiscale model of SARS-CoV-2 dynamics in lung tissue. The first prototype model was built and shared internationally as open source code and an online interactive model in under 12 hours, and community domain expertise is driving regular refinements. In a sustained community effort, this consortium is integrating data and expertise across virology, immunology, mathematical biology, quantitative systems physiology, cloud and high performance computing, and other domains to accelerate our response to this critical threat to international health. More broadly, this effort is creating a reusable, modular framework for studying viral replication and immune response in tissues, which can also potentially be adapted to related problems in immunology and immunotherapy.

Establishing CD19 B-cell reference control materials for comparable and quantitative cytometric expression analysis

In the field of cell-based therapeutics, there is a great need for high-quality, robust, and validated measurements for cell characterization. Flow cytometry has emerged as a critically important platform due to its high-throughput capability and its ability to simultaneously measure multiple parameters in the same sample. However, to assure the confidence in measurement, well characterized biological reference materials are needed for standardizing clinical assays and harmonizing flow cytometric results between laboratories. To date, the lack of adequate reference materials, and the complexity of the cytometer instrumentation have resulted in few standards. This study was designed to evaluate CD19 expression in three potential biological cell reference materials and provide a preliminary assessment of their suitability to support future development of CD19 reference standards. Three commercially available human peripheral blood mononuclear cells (PBMCs) obtained from three different manufacturers were tested. Variables that could potentially contribute to the differences in the CD19 expression, such as PBMCs manufacturing process, number of healthy donors used in manufacturing each PBMC lot, antibody reagent, operators, and experimental days were included in our evaluation. CD19 antibodies bound per cell (ABC) values were measured using two flow cytometry-based quantification schemes with two independent calibration methods, a single point calibration using a CD4 reference cell and QuantiBrite PE bead calibration. Three lots of PBMC from three different manufacturers were obtained. Each lot of PBMC was tested on three different experimental days by three operators using three different lots of unimolar anti-CD19PE conjugates. CD19 ABC values were obtained in parallel on a selected lot of the PBMC samples using mass spectrometry (CyTOF) with two independent calibration methods, EQ4 and bead-based calibration were evaluated with CyTOF-technology. Including all studied variabilities such as PBMC lot, antibody reagent lot, and operator, the averaged mean values of CD19 ABC for the three PBMC manufacturers (A,B, and C) obtained by flow cytometry were found to be: 7953 with a %CV of 9.0 for PBMC-A, 10535 with a %CV of 7.8 for PBMC-B, and 12384 with a %CV of 16 for PBMC-C. These CD19 ABC values agree closely with the findings using CyTOF. The averaged mean values of CD19 ABC for the tested PBMCs is 9295 using flow cytometry-based method and 9699 using CyTOF. The relative contributions from various sources of uncertainty in CD19 ABC values were quantified for the flow cytometry-based measurement scheme. This uncertainty analysis suggests that the number of antigens or ligand binding sites per cell in each PBMC preparation is the largest source of variability. On the other hand, the calibration method does not add significant uncertainty to the expression estimates. Our preliminary assessment showed the suitability of the tested materials to serve as PBMC-based CD19+ reference control materials for use in quantifying relevant B cell markers in B cell lymphoproliferative disorders and immunotherapy. However, users should consider the variabilities resulting from different lots of PBMC and antibody reagent when utilizing cell-based reference materials for quantification purposes and perform bridging studies to ensure harmonization between the results before switching to a new lot.

Stochastic Reaction-Diffusion Model of the Binding of Monoclonal Antibodies to CD4 Receptors on the Surface of T Cells

A stochastic reaction-diffusion model was developed to describe the binding of labeled monoclonal antibodies (mAbs) to CD4 receptors on the surface of T cells. The mAbs diffused to, adsorbed on, and underwent monovalent and bivalent binding to CD4 receptors on the cell surface. The model predicted the time-dependent nature of all populations involved in the labeling process. At large time, the populations reached equilibrium values, giving the number of antibodies bound to the T cell (ABC) defined as the sum of monovalently and bivalently bound mAbs. The predicted coefficient of variation (CV%) of the (ABC) values translated directly to a corresponding CV% of the measured mean fluorescence intensity (MFI). The predicted CV% was about 0.2% from the intrinsic fluctuations of the stochastic reaction process, about 5% after inclusion of the known fluctuations in the number of available CD4 receptors, and about 11% when fluctuations in bivalent binding affinity were included. The fluorescence detection process is expected to contribute approximately 7%. The abovementioned contributions to CV% sum up to approximately 13%. Work is underway to reconcile the predicted values and the measured values of 17% to 22%.

A Model for the Binding of Fluorescently Labeled Anti-Human CD4 Monoclonal Antibodies to CD4 Receptors on Human Lymphocytes

The CD4 glycoprotein is a component of the T cell receptor complex which plays an important role in the human immune response. This manuscript describes the measurement and modeling of the binding of fluorescently labeled anti-human CD4 monoclonal antibodies (mAb; SK3 clone) to CD4 receptors on the surface of human peripheral blood mononuclear cells (PBMC). CD4 mAb fluorescein isothiocyanate (FITC) and CD4 mAb allophycoerythrin (APC) conjugates were obtained from commercial sources. Four binding conditions were performed, each with the same PBMC sample and different CD4 mAb conjugate. Each binding condition consisted of the PBMC sample incubated for 30 min in labeling solutions containing progressively larger concentrations of the CD4 mAb-label conjugate. After the incubation period, the cells were re-suspended in PBS-based buffer and analyzed using a flow cytometer to measure the mean fluorescence intensity (MFI) of the labeled cell populations. A model was developed to estimate the equilibrium concentration of bound CD4 mAb-label conjugates to CD4 receptors on PBMC. A set of parameters was obtained from the best fit of the model to the measured MFI data and the known number of CD4 receptors on PBMC surface. Divalent and monovalent binding had to be invoked for the APC and FITC CD4 mAb conjugates, respectively. This suggests that the mAb binding depends on the size of the label, which has significant implications for quantitative flow cytometry. The study supports the National Institute of Standards and Technology program to develop quantitative flow cytometry measurements.

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.

Physiologically-based modeling to predict the clinical behavior of monoclonal antibodies directed against lymphocyte antigens

Many clinically approved and investigational monoclonal antibody (mAb)-based therapeutics are directed against proteins located in the systemic circulation, including cytokines, growth factors, lymphocyte proteins, and shed antigens. Interaction between mAb and target may lead to non-linear pharmacokinetics (PK), characterized by rapid, target-mediated elimination. Several groups have reported that determinants of target-mediated elimination could include mAb-target binding, target expression, and target turnover. Recently, we scaled a physiologically-based pharmacokinetic model for mAb disposition to man and used it to predict the non-linear PK of mAbs directed against tumor epithelial proteins. In this work, we extended the previously described model to account for the influence of lymphocyte proteins on mAb PK in man. To account for the dynamic behavior of lymphocytes in the circulation, lymphocyte cycling between blood and lymphoid organs was described using first-order transfer rate constants. Use of lymphocyte cycling and reported target turnover rates in the model allowed the accurate prediction of the pharmacokinetics and pharmacodynamics (PD) of 4 mAbs (TRX1, MTRX1011a, rituximab, daclizumab) directed against 3 lymphocyte targets (CD4, CD20, CD25). The results described here suggest that the proposed model structure may be useful in the a priori prediction of the PK/PD properties of mAbs directed against antigens in the circulation.