Name: preparation of bead-supported lipid bilayers to study the particulate output of t cell immune synapses
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Description: Watch this Scientific Journal Video about Preparation of Bead-supported Lipid Bilayers to Study the Particulate Output of T Cell Immune Synapses at JoVE.com

We are grateful to our laboratory members and the Kennedy Institute of Rheumatology community for constructive scientific discussions, especially our flow cytometry facility manager Jonathan Webber. This work was funded by Wellcome Trust Principal Research Fellowship 100262Z/12/Z, the ERC Advanced Grant (SYNECT AdG 670930), and the Kennedy Trust for Rheumatology Research (KTRR) (all three to MLD). PFCD was supported by EMBO Long-Term Fellowship (ALTF 1420-2015, in conjunction with the European Commission (LTFCOFUND2013, GA-2013-609409) and Marie Sklodowska-Curie Actions) and an Oxford-Bristol Myers Squibb Fellowship.

1. Measurement of cell surface protein densities with quantitative flow cytometry
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	<li>Prepare 0.22 &#181;m-filtered human flow cytometry buffer (hFCB) by adding EDTA (to a final 2 mM concentration) and human AB serum (to a final 10%) to sterile phosphate-buffered saline (PBS), pH 7.4 (see Table 1). Filter the solution using a 0.22 &#181;m pore filter unit to remove serum impurities and store at 4 &#176;C.</li>
	<li>Recover the cells and sediment them by centrifugation at 300 &#21...

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BSLBs are versatile tools for studying the particulate output of T cells stimulated with model APC membranes. The flexibility of the method allows the reconstitution of complex and reductionist membrane compositions to study the effects of ligands and their signals on the secretion of tSVs and supramolecular attack particles and their components. We have tested this technology on various T cells, including preactivated TH, CTL, Tregs, and CART15. This protocol also works for the measurement of syn...

The immunological synapse (IS) is a pivotal molecular structure formed at the interface of cells engaged in physical contact that facilitates the regulated exchange of juxtracrine information. Different ISs have been described in the literature, and a growing body of evidence suggests these molecular hubs are a conserved feature of cellular networks. Various immune cells, including B cells, natural killer cells, dendritic cells, macrophages, and T cells, exchange information via the assembly of short-lived contacts1. Multiomic studies are advancing the understanding of novel subsets of leukocytes and stromal cells driving pathogenic cellular networks and expressing surface proteins with unknown functions. As synthetic APCs, BSLBs allow the direct investigation of the functional role of individual proteins in the integration of activating signals, namely antigens and costimulation/corepression, by T cells and the resulting release of effector particles referred to as signal four.
This paper describes the protocols and critical technical points to consider while using BSLBs to mimic the surface composition of model APCs. The protocols for the quantitative measurement of immune receptors and other surface proteins on APCs are presented along with the protocol for the reconstitution of synthetic APCs containing these measured quantities. Then, the steps required for coculturing T cells and BSLB are presented along with the protocol for the quantitative measurement of trans-synaptic particle transfer using flow cytometry. Most remarkably, BSLBs facilitate studying a plasma membrane-derived population of tSVs termed synaptic ectosomes (SEs). T-cell antigen receptor-enriched (TCR+) SEs are shed in response to TCR triggering2 and efficiently captured by BSLBs3, representing an excellent readout to assess the agonistic properties of antigens and the modelled membrane composition. CD63+ exosomes and supramolecular attack particles (SMAPs) are also released by stimulated T cells and captured by BSLBs. They can be used as additional readouts of activation and the resulting exocytic and lytic granule secretion by T cells. The mobilization of exocytic vesicles to the interacting pole of the T cell also facilitates the directional release of cytokines, such as IL-2, IFN-&#947;, and IL-10 in response to activation4,5,6,7,8. Although T-cell released cytokines can also be detected on BSLBs, a more dedicated study is currently under development to validate the quantitative analysis of cytokine release at the immunological synapse.
To interrogate how specific membrane compositions influence T cells&#39; synaptic output requires defining the physiological density of the target membrane component. Flow cytometry-based quantifications of cell surface proteins are an essential step in this protocol and require: 1) the use of antibodies with known numbers of fluorochromes per antibody (F/P), and 2) benchmark beads providing a standard reference for interpolating fluorochrome molecules from measured mean fluorescence intensities (MFIs).
These benchmark standards consist of five bead populations, each containing an increasing number of equivalent soluble fluorochromes (MESFs), which span the dynamic range of arbitrary fluorescence detection. These standard populations yield discrete fluorescence peaks, facilitating the conversion of arbitrary fluorescence units into MESFs by simple linear regression. The resulting MESFs are then used alongside antibody F/P values to calculate the average number of bound molecules per cell (or BSLB in later steps). The application of estimated cell surface areas to the average number of detected molecules then enables the calculation of physiological densities as molecules/&#181;m2. This quantification protocol can also be adapted to the measurement of protein densities on T cells and the biochemical reconstitution of membrane compositions mediating the formation of homotypic T cell synapses (i.e., T-T synapses9). If needed, the valency of antibody binding can be further estimated by using recombinant targets labeled with known numbers of fluorochromes per molecule. Then, the antibody-binding valency can be calculated for the same BSLB population by simultaneously comparing the number of bound fluorescent proteins and quantification antibodies (using two different quantification fluorochromes and MESF standards).
The reconstitution of APC membranes requires the assembly of supported lipid bilayers (SLBs) on silica beads1. Liposome stocks containing different phospholipid species can be harnessed to form a versatile lipid-bilayer matrix, enabling the anchoring of recombinant proteins with different binding chemistry (the preparation of liposomes is detailed in 10). Once the physiological density (or densities) of the relevant ligand &#34;on cells&#34; is defined, the same flow cytometry protocol is adapted to estimate the concentration of recombinant protein needed to coat BSLBs with the target physiological density. Two different anchoring systems can be used either in combination or separately.
First, SLB containing a final 12.5 mol% of Ni2+-containing phospholipids is sufficient to provide approximately 10,000 His-tag binding sites per square micron10 and works well to decorate BSLBs with most commercially available proteins whose physiological densities do not exceed this maximum loading capacity. The second loading system harnesses biotin-containing phospholipids (as mol%) to load biotinylated anti-CD3e Fab (or HLA/MHC monomers) via streptavidin bridges. The combination of these two BSLB decoration methods then enables the flexible tailoring of BSLBs as synthetic APCs. For highly complex APC surface compositions, the mol% of phospholipids and proteins can be increased to load as many proteins as the question at hand requires. Once the working concentrations of proteins and mol% of biotinylated phospholipids are defined, BSLBs can be assembled to interrogate the synaptic output of T cells with multiparametric flow cytometry.

preparation of bead-supported lipid bilayers to study the particulate output of t cell immune synapses

Antigen-presenting cells (APCs) present three activating signals to T cells engaged in physical contact: 1) antigen, 2) costimulation/corepression, and 3) soluble cytokines. T cells release two kinds of effector particles in response to activation: trans-synaptic vesicles (tSVs) and supramolecular attack particles, which transfer intercellular messengers and mediate cytotoxicity, respectively. These entities are quickly internalized by APCs engaged in physical contact with T cells, making their characterization daunting. This paper presents the protocol to fabricate and use Bead-Supported Lipid Bilayers (BSLBs) as antigen-presenting cell (APC) mimetics to capture and analyze these trans-synaptic particles. Also described are the protocols for the absolute measurements of protein densities on cell surfaces, the reconstitution of BSLBs with such physiological levels, and the flow cytometry procedure for tracking synaptic particle release by T cells. This protocol can be adapted to study the effects of individual proteins, complex ligand mixtures, pathogen virulence determinants, and drugs on the effector output of T cells, including helper T cells, cytotoxic T lymphocytes, regulatory T cells, and chimeric antigen receptor-expressing T cells (CART).

FCM for absolute protein quantification on the cell surface 
The reconstitution of BSLBs presenting physiological densities of ligands requires the estimation of total protein densities on the modeled cell subset. To reconstitute BSLBs, include any relevant ligand expected to play a role in the signaling axis of interest alongside proteins supporting the adhesion and functional interaction between BSLB and cells, such as ICAM-1 and costimulatory molecules, e.g., CD40, CD58, and B7 receptors (CD80 an...

Watch this Scientific Journal Video about Preparation of Bead-supported Lipid Bilayers to Study the Particulate Output of T Cell Immune Synapses at JoVE.com

Preparation of Bead-supported Lipid Bilayers to Study the Particulate Output of T Cell Immune Synapses

Here we present the protocol for the stepwise reconstitution of synthetic antigen-presenting cells using Bead-Supported Lipid Bilayers and their use to interrogate the synaptic output from activated T cells.

Antigen-presenting cells (APCs) present three activating signals to T cells engaged in physical contact: 1) antigen, 2) costimulation/corepression, and 3) ...

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