A photobleaching energy transfer analysis of CD8/MHC-I and LFA-1/ICAM-1 interactions in CTL-target cell conjugates

Dynamic P-glycoprotein expression in early and late memory states of human CD8 + T cells and the protective role of ruxolitinib

ABCB1/MDR-1/P-glycoprotein (Pgp) is an ABC transporter responsible for cancer cell multi-drug resistance. It is expressed in cytotoxic T lymphocytes (CTL). Eliminating sensitive cancer cells during high-dose chemotherapy can also damage immune cells. Our study aimed to assess which maturing human CD8 + CTL memory subsets may be affected based on their Pgp protein expression. In an in vitro CTL differentiation model system, we tracked the maturation of naive, effector, and memory cells and the expression of Pgp. This system involves co-culturing blood lymphocytes with proliferation-inhibited JY antigen-presenting B-lymphoblastoid cells expressing HLA-I A2. These JY-primed maturing CTLs were TCR-activated using beads, and the effect of the maturation-modifying JAK1/2 inhibitor ruxolitinib was examined. Multidimensional analysis identified six major CTL subsets: naive, young memory (Tym), stem cell memory (Tscm), central memory (Tcm), effector memory (Tem), and effectors (Te). These subsets were further divided into thirteen specific subsets: TymCD127 + , TymCD127-, Tscm, TcmCD95 + , TcmCD73 +CD95 + , TcmCD95+CD127 + , TcmPD1 + , TemCD95 + , TemraCD127 + , TemraCD127-, TeCD95 + , and TeCD73 +CD95 + . Pgp expression was detectable in naïve cells and dynamically changed across the thirteen identified subsets. Increased Pgp was detected in young memory T cells and in Tscm, TcmCD95 + , and TcmPD1 + human CTL subsets. Unlike other transiently appearing memory cells, the number of cells in these core Pgp-expressing memory subsets stabilized by the end of the contraction phase. Ruxolitinib treatment downregulated effector T-cell polarization while upregulating small memory subsets expressing Pgp. In conclusion, activation increased Pgp expression, whereas ruxolitinib treatment preserved small early and late memory subset core that primarily expressed Pgp.

IL-15 Trans-Presentation Is an Autonomous, Antigen-Independent Process

IL-15 plays a pivotal role in the long-term survival of T cells and immunological memory. Its receptor consists of three subunits (IL-15Rα, IL-2/15Rβ, and γ_c). IL-15 functions mainly via trans-presentation (TP), during which an APC expressing IL-15 bound to IL-15Rα presents the ligand to the βγ_c receptor-heterodimer on a neighboring T/NK cell. To date, no direct biophysical evidence for the intercellular assembly of the IL-15R heterotrimer exists. Ag presentation (AP), the initial step of T cell activation, is also based on APC-T cell interaction. We were compelled to ask whether AP has any effect on IL-15 TP or whether they are independent processes. In our human Raji B cell-Jurkat T cell model system, we monitored inter-/intracellular protein interactions upon formation of IL-15 TP and AP receptor complexes by Förster resonance energy transfer measurements. We detected enrichment of IL-15Rα and IL-2/15Rβ at the synapse and positive Förster resonance energy transfer efficiency if Raji cells were pretreated with IL-15, giving direct biophysical evidence for IL-15 TP. IL-15Rα and MHC class II interacted and translocated jointly to the immunological synapse when either ligand was present, whereas IL-2/15Rβ and CD3 moved independently of each other. IL-15 TP initiated STAT5 phosphorylation in Jurkat cells, which was not further enhanced by AP. Conversely, IL-15 treatment slightly attenuated Ag-induced phosphorylation of the CD3ζ chain. Our studies prove that in our model system, IL-15 TP and AP can occur independently, and although AP enhances IL-15R assembly, it has no significant effect on IL-15 signaling during TP. Thus, IL-15 TP can be considered an autonomous, Ag-independent process.

Confocal microscopic dual-laser dual-polarization FRET (2polFRET) at the acceptor side for correlating rotations at different distances on the cell surface

Relationship of donor and acceptor fluorescence anisotropies as well as efficiency of fluorescence resonance energy transfer (FRET) has been investigated in a confocal microscope in the context of FRET systems comprised of donor and acceptor-labeled MHCI and MHCII receptors on the surface of Kit-225 K6 human T-cells. The measurements have been carried out in a 2-laser, 5-signal platform where the total donor fluorescence intensity and 2 acceptor fluorescence intensities with their anisotropies - one at the donor's excitation wavelength, the other at the acceptor's excitation wavelength - have been detected. This configuration enabled the determination of FRET efficiency and correlating it with the two acceptor fluorescence anisotropies as a kind of calibration. Estimations for the FRET-enhanced donor fluorescence anisotropy, the directly excited acceptor fluorescence anisotropy, and the fluorescence anisotropy of sensitized emission have been obtained. Procedures for determining FRET by measuring only the total donor intensity and the acceptor intensity and its anisotropy, or two acceptor intensities and their anisotropies have been elaborated, the errors of which have been estimated based on the fluorescence anisotropy values obtained in the calibration with the method of flow cytometric energy transfer (FCET). The combined detection of the donor and acceptor fluorescence anisotropies enabled also the determination of the lower and upper limits of the orientation factor for FRET (κ²). An increase in range for κ² with increasing FRET efficiency has been observed, with average κ² values different from the dynamic random average of 2/3. These observations call for the need of κ² determination in proximity measurements, where the donor and acceptor orientations are not predictable. An increasing range of κ² with increasing intermolecular proximity of the MHCI and MHCII receptors has been observed. This indicates that molecular flexibility in the clusters of the MHCI and MHCII receptors reduces with increasing cluster density, i.e. a "fluidity gradient" exists in the clusters. More specifically, the local density dependent flexibility can also be taken as a direct proof for that the association of these receptors is non-random, but mediated by some type of physical interaction, a finding as a benefit of FRET detection by polarization spectroscopy. Two new quantities - the quenched donor fluorescence anisotropy and a fluorescence anisotropy analogue, the "dissymmetry index" of the polarized FRET efficiency components - have also been introduced for the characterization of the orientational dynamics of the excited state during FRET.

Understanding FRET as a research tool for cellular studies

Communication of molecular species through dynamic association and/or dissociation at various cellular sites governs biological functions. Understanding these physiological processes require delineation of molecular events occurring at the level of individual complexes in a living cell. Among the few non-invasive approaches with nanometer resolution are methods based on Förster Resonance Energy Transfer (FRET). FRET is effective at a distance of 1-10 nm which is equivalent to the size of macromolecules, thus providing an unprecedented level of detail on molecular interactions. The emergence of fluorescent proteins and SNAP- and CLIP- tag proteins provided FRET with the capability to monitor changes in a molecular complex in real-time making it possible to establish the functional significance of the studied molecules in a native environment. Now, FRET is widely used in biological sciences, including the field of proteomics, signal transduction, diagnostics and drug development to address questions almost unimaginable with biochemical methods and conventional microscopies. However, the underlying physics of FRET often scares biologists. Therefore, in this review, our goal is to introduce FRET to non-physicists in a lucid manner. We will also discuss our contributions to various FRET methodologies based on microscopy and flow cytometry, while describing its application for determining the molecular heterogeneity of the plasma membrane in various cell types.

FRET Imaging by Laser Scanning Cytometry on Large Populations of Adherent Cells

The application of FRET (fluorescence resonance energy transfer) sensors for monitoring protein-protein interactions under vital conditions is attracting increasing attention in molecular and cell biology. Laser-scanning cytometry (LSC), a slide-based sister procedure to flow cytometry, provides an opportunity to analyze large populations of adherent cells or 2-D solid tissues in their undisturbed physiological settings. Here we provide an LSC-based three-laser protocol for high-throughput ratiometric FRET measurements utilizing cyan and yellow fluorescent proteins as a FRET pair. Membrane labeling with Cy5 dye is used for cell identification and contouring. Pixel-by-pixel and single-cell FRET efficiencies are calculated to estimate the extent of the molecular interactions and their distribution in the cell populations examined. We also present a non-high-throughput donor photobleaching FRET application, for obtaining the required instrument parameters for ratiometric FRET. In the biological model presented, HeLa cells are transfected with the ECFP- or EYFP-tagged Fos and Jun nuclear proteins, which heterodimerize to form active AP1 transcription factor.

Single-laser polarization FRET (polFRET) on the cell surface

A new method for the simultaneous detection of rotational mobility and proximity of cell surface receptors is presented based on cell-by-cell basis measurement of polarized fluorescence intensity components of the donor and acceptor of a FRET system. In addition to the FRET efficiency and the donor and acceptor concentrations, the method makes also possible the determination of the rotational characteristics and the associated fraction of the donors (FRET-fraction). The method is illustrated with flow cytometric and rFLIM measurements on donor-acceptor systems comprising fluorescently labeled whole antibodies and their Fab fragments against epitopes of the MHCI and MHCII cell surface receptors on human lymphoblast cells. Fluorescence anisotropy of donor and acceptor and FRET efficiency were measured for samples of different acceptor-to-donor concentration ratios. Acceptor anisotropy proved to be more sensitive than the donor anisotropy for sensing FRET. After determining the rotational constants of the donor-conjugated antibodies by measurements of FRET in the steady state, and by rFLIM as a reference, the associated fractions of the MHCI and MHCII molecules in their clusters were determined. Besides the flow cytometer and the wide-field rFLIM used in this study, the method can be applied also in other devices capable of dual-anisotropy detection.

Intensity correlation-based calibration of FRET

Dual-laser flow cytometric resonance energy transfer (FCET) is a statistically efficient and accurate way of determining proximity relationships for molecules of cells even under living conditions. In the framework of this algorithm, absolute fluorescence resonance energy transfer (FRET) efficiency is determined by the simultaneous measurement of donor-quenching and sensitized emission. A crucial point is the determination of the scaling factor α responsible for balancing the different sensitivities of the donor and acceptor signal channels. The determination of α is not simple, requiring preparation of special samples that are generally different from a double-labeled FRET sample, or by the use of sophisticated statistical estimation (least-squares) procedures. We present an alternative, free-from-spectral-constants approach for the determination of α and the absolute FRET efficiency, by an extension of the presented framework of the FCET algorithm with an analysis of the second moments (variances and covariances) of the detected intensity distributions. A quadratic equation for α is formulated with the intensity fluctuations, which is proved sufficiently robust to give accurate α-values on a cell-by-cell basis in a wide system of conditions using the same double-labeled sample from which the FRET efficiency itself is determined. This seemingly new approach is illustrated by FRET measurements between epitopes of the MHCI receptor on the cell surface of two cell lines, FT and LS174T. The figures show that whereas the common way of α determination fails at large dye-per-protein labeling ratios of mAbs, this presented-as-new approach has sufficient ability to give accurate results. Although introduced in a flow cytometer, the new approach can also be straightforwardly used with fluorescence microscopes.

Visualization of Protein Interactions in Living Cells

Ligand binding to cell membrane receptors sets off a series of protein interactions that convey the nuances of ligand identity to the cell interior. The information may be encoded in conformational changes, the interaction kinetics and, in the case of multichain immunoreceptors, by chain rearrangements. The signals may be modulated by dynamic compartmentalization of the cell membrane, cellular architecture, motility, and activation-all of which are difficult to reconstitute for studies of receptor signaling in vitro. In this paper, we will discuss how protein interactions in general and receptor signaling in particular can be studied in living cells by different fluorescence imaging techniques. Particularly versatile are methods that exploit Förster resonance energy transfer (FRET), which is exquisitely sensitive to the nanometer-range proximity and orientation between fluorophores. Fluorescence correlation microscopy (FCM) can provide complementary information about the stoichiometry and diffusion kinetics of large complexes, while bimolecular fluorescence complementation (BiFC) and other complementation techniques can capture transient interactions. A continuing challenge is extracting from the imaging data the quantitative information that is necessary to verify different models of signal transduction.

Visualization of protein interactions in living cells

Ligand binding to cell membrane receptors sets off a series of protein interactions that convey the nuances ofligand identity to the cell interior. The information may be encoded in conformational changes, the interaction kinetics and, in the case of multichain immunoreceptors, by chain rearrangements. The signals may be modulated by dynamic compartmentalization of the cell membrane, cellular architecture, motility, and activation--all of which are difficult to reconstitute for studies of receptor signaling in vitro. In this chapter, we will discuss how protein interactions in general and receptor signaling in particular can be studied in living cells by different fluorescence imaging techniques. Particularly versatile are methods that exploit Förster resonance energy transfer (FRET), which is exquisitely sensitive to the nanometer-range proximity and orientation between fluorophores. Fluorescence correlation microscopy (FCM) can provide complementary information about the stoichiometry and diffusion kinetics of large complexes, while bimolecular fluorescence complementation (BiFC) and other complementation techniques can capture transient interactions. A continuing challenge is extracting from the imaging data the quantitative information that is necessary to verify different models of signal transduction.

Two-dimensional receptor patterns in the plasma membrane of cells. A critical evaluation of their identification, origin and information content

A concise review is presented on the nature, possible origin and functional significance of cell surface receptor patterns in the plasma membrane of lymphoid cells. A special emphasize has been laid on the available methodological approaches, their individual virtues and sources of errors. Fluorescence energy transfer is one of the oldest available means for studying non-randomized co-distribution patterns of cell surface receptors. A detailed and critical description is given on the generation of two-dimensional cell surface receptor patterns based on pair-wise energy transfer measurements. A second hierarchical-level of receptor clusters have been described by electron and scanning force microscopies after immuno-gold-labeling of distinct receptor kinds. The origin of these receptor islands at a nanometer scale and island groups at a higher hierarchical (mum) level, has been explained mostly by detergent insoluble glycolipid-enriched complexes known as rafts, or detergent insoluble glycolipids (DIGs). These rafts are the most-likely organizational forces behind at least some kind of receptor clustering [K. Simons et al., Nature 387 (1997) 569]. These models, which have great significance in trans-membrane signaling and intra-membrane and intracellular trafficking, are accentuating the necessity to revisit the Singer-Nicolson fluid mosaic membrane model and substitute the free protein diffusion with a restricted diffusion concept [S.J. Singer et al., Science 175 (1972) 720].

Raft and cytoskeleton associations of an ABC transporter: P-glycoprotein

BACKGROUND

A novel flow cytometric assay has been described in an accompanying report (Gombos et al.,

METHODS

The kinetics of the decrease in immunofluorescence intensity was analyzed after the addition of the raft-preserving Triton X-100 or Nonidet P-40, both of which disrupt the entire membrane. Mild treatments by both detergents leave cells attached to only those proteins that are anchored to the cytoskeleton by rafts or independent of rafts. Agents that affect microfilaments and modulate membrane levels of cholesterol by cyclodextrin were used to distinguish between the raft-mediated and non-raft-related associations of the Pgp. Confocal microscopy and flow cytometric fluorescence energy transfer measurements were used to confirm colocalization of Pgp with raft constituents.

RESULTS

The assay was proved to be sensitive enough to resolve differences between the resistance of UIC2-labeled cell-surface Pgps to Triton X-100 versus Nonidet P-40. Approximately 34% of the UIC2 Fab-labeled Pgp molecules were associated with the cytoskeleton through detergent-resistant, cholesterol-sensitive microdomains or directly, whereas approximately 15% were found to be directly linked to the cytoskeleton. Accordingly, confocal microscopy showed that Pgps colocalize with raft markers, mainly in microvilli. Fluorescence resonance energy transfer efficiency data indicating molecular proximity between Pgp and the raft markers CD44, CD59, and G(M1)-gangliosides also suggested that a significant fraction of Pgps resides in raft microdomains. Raft association of Pgp appears to be of functional significance because its modulation markedly affected drug pumping.

CONCLUSIONS

By using the flow cytometric detergent resistance assay in kinetic mode, we were able to assess the extent of raft association and actin cytoskeleton anchorage of Pgp expressed at physiologically relevant levels. We demonstrated that a significant fraction of Pgp is raft associated on LS-174-T human colon carcinoma cells and that this localization may influence its transporter function. The kinetic flow cytometric detergent resistance assay presented in this report is considered to be generally applicable for the analysis of molecular interactions of membrane proteins expressed at low levels.

Kv1.3 potassium channels are localized in the immunological synapse formed between cytotoxic and target cells

Membrane proteins of cytotoxic T cells specifically reorganize to form an immunological synapse (IS) on interaction with their specific target. In this paper, we investigated the redistribution of Kv1.3 channels, which are the dominant voltage-gated potassium channels, in the plasma membrane of allogen-activated human cytotoxic T lymphocytes (CTLs) on interacting with their specific target cells. Kv1.3 channels bearing a FLAG epitope were expressed in the CTLs and the cell-surface distribution of fluorescently labeled ion channels was determined from confocal laser-scanning microscopy images. FLAG epitope-tagged Kv1.3 channels showed a patchy distribution in CTLs not engaged with target cells, whereas the channels were accumulated in the IS formed between CTLs and specific target lymphocytes. Localization of Kv1.3 channels in the IS might open an unrevealed possibility in the regulation of ion channel activity by signaling molecules accumulated in the IS.

INF-gamma rearranges membrane topography of MHC-I and ICAM-1 in colon carcinoma cells

Flow-cytometric fluorescence energy transfer (FCET) measurements between fluorescently labeled cell surface MHC-I and ICAM-1 molecules indicated similar receptor patterns in the plasma membrane of interferon-gamma (INF-gamma)-treated colon carcinoma cells as those observed earlier at the surface of lymphoid cells. INF-gamma activation significantly increased the density of MHC-I and ICAM-1 proteins in the membrane. This increase in receptor density was accompanied by decreased proximity level of the homo-associated MHC-I receptors. Hetero-association of MHC-I and ICAM-1 molecules was increased by INF-gamma treatment. INF-gamma changed neither hetero- nor homo-association of transferrin receptors. By staining the sphingomyelin/cholesterol-enriched lipid microdomains with fluorescently labeled cholera toxin B subunit, we found an increase in the amount of lipid-raft associated G(M1)-gangliosides due to INF-gamma treatment. Confocal microscopic results and FCET measurements show that MHC-I and ICAM-1 are components of G(M1)-ganglioside containing lipid-rafts and also support an increase in the size of these lipid-rafts upon INF-gamma treatment.

AV.TK-mediated killing of subcutaneous tumors in situ results in effective immunization against established secondary intracranial tumor deposits

Gene transfer vectors expressing herpes simplex thymidine kinase (HSVtk), in addition to direct killing of tumor cells, often have an associated local "bystander effect" mediated by metabolic coupling of tumor cells. A systemic antitumor effect mediated by the immune system, termed the distant bystander effect, has also been reported. We have observed the development of cytotoxic T-lymphocyte (CTL) populations and long-lasting antitumor immunity following treatment of subcutaneous tumors with an adenoviral vector expressing HSVtk (AV.TK) and ganciclovir (GCV) in rat glioma model. This vaccination effect seen with AV.TK/GCV treatment of subcutaneous tumor could even abrogate or retard growth of previously established secondary intracranial tumors.

Imaging protein-protein interactions using fluorescence resonance energy transfer microscopy

Fluorescence resonance energy transfer (FRET) detects the proximity of fluorescently labeled molecules over distances >100 A. When performed in a fluorescence microscope, FRET can be used to map protein-protein interactions in vivo. We here describe a FRET microscopy method that can be used to determine whether proteins that are colocalized at the level of light microscopy interact with one another. This method can be implemented using digital microscopy systems such as a confocal microscope or a wide-field fluorescence microscope coupled to a charge-coupled device (CCD) camera. It is readily applied to samples prepared with standard immunofluorescence techniques using antibodies labeled with fluorescent dyes that act as a donor and acceptor pair for FRET. Energy transfer efficiencies are quantified based on the release of quenching of donor fluorescence due to FRET, measured by comparing the intensity of donor fluorescence before and after complete photobleaching of the acceptor. As described, this method uses Cy3 and Cy5 as the donor and acceptor fluorophores, but can be adapted for other FRET pairs including cyan fluorescent protein and yellow fluorescent protein.

Studying protein dynamics in living cells

Since the advent of the green fluorescent protein, the subcellular localization, mobility, transport routes and binding interactions of proteins can be studied in living cells. Live cell imaging, in combination with photobleaching, energy transfer or fluorescence correlation spectroscopy are providing unprecedented insights into the movement of proteins and their interactions with cellular components. Remarkably, these powerful techniques are accessible to non-specialists using commercially available microscope systems.

Clustering of class I HLA oligomers with CD8 and TCR: three-dimensional models based on fluorescence resonance energy transfer and crystallographic data

Fluorescence resonance energy transfer (FRET) data, in accordance with lateral mobility measurements, suggested the existence of class I HLA dimers and oligomers at the surface of live human cells, including the B lymphoblast cell line (JY) used in the present study. Intra- and intermolecular class I HLA epitope distances were measured on JY B cells by FRET using fluorophore-conjugated Ag-binding fragments of mAbs W6/32 and L368 directed against structurally well-characterized heavy and light chain epitopes, respectively. Out-of-plane location of these epitopes relative to the membrane-bound BODIPY-PC (2-(4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-pentanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine) was also determined by FRET. Computer-simulated docking of crystallographic structures of class I HLA and epitope-specific Ag-binding fragments, with experimentally determined interepitope and epitope to cell surface distances as constraints, revealed several sterically allowed and FRET-compatible class I HLA dimeric and tetrameric arrangements. Extension of the tetrameric class I HLA model with interacting TCR and CD8 resulted in a model of a supramolecular cluster that may exist physiologically and serve as a functionally significant unit for a network of CD8-HLA-I complexes providing enhanced signaling efficiency even at low MHC-peptide concentrations at the interface of effector and APCs.

Anomalous diffusion of major histocompatibility complex class I molecules on HeLa cells determined by single particle tracking

Single-particle tracking (SPT) was used to determine the mobility characteristics of MHC (major histocompatibility complex) class I molecules at the surface of HeLa cells at 22 degrees C and on different time scales. MHC class I was labeled using the Fab fragment of a monoclonal antibody (W6/32), covalently bound to either R-phycoerythrin or fluorescent microspheres, and the particles were tracked using high-sensitivity fluorescence imaging. Analysis of the data for a fixed time interval suggests a reasonable fit to a random diffusion model. The best fit values of the diffusion coefficient D decreased markedly, however, with increasing time interval, demonstrating the existence of anomalous diffusion. Further analysis of the data shows that the diffusion is anomalous over the complete time range investigated, 4-300 s. Fitting the results obtained with the R-phycoerythrin probe to D = D0talpha-1, where Do is a constant and t is the time, gave D0 = (6.7 +/- 4.5) x 10(-11) cm2 s-1 and alpha = 0.49 +/- 0.16. Experiments with fluorescent microspheres were less reproducible and gave slower anomalous diffusion. The R-phycoerythrin probe is considered more reliable for fluorescent SPT because it is small (11 x 8 nm) and monovalent. The type of motion exhibited by the class I molecules will greatly affect their ability to migrate in the plane of the membrane. Anomalous diffusion, in particular, greatly reduces the distance a class I molecule can travel on the time scale of minutes. The present data are discussed in relation to the possible role of diffusion and clustering in T-cell activation.