Selecting the right fluorophores and flow cytometer for fluorescence resonance energy transfer measurements

Phenylboronic Acid-Functionalized Polyplexes Tailored to Oral CRISPR Delivery

Effective delivery of the CRISPR-Cas9 components is crucial to realizing the therapeutic potential. Although many delivery approaches have been developed for this application, oral delivery has not been explored due to the degradative nature of the gastrointestinal tract. For this issue, we developed a series of novel phenylboronic acid (PBA)-functionalized chitosan-polyethylenimine (CS-PEI) polymers for oral CRISPR delivery. PBA functionalization equipped the polyplex with higher stability, smooth transport across the mucus, and efficient endosomal escape and cytosolic unpackaging in the cells. From a library of 12 PBA-functionalized CS-PEI polyplexes, we identified a formulation that showed the most effective penetration in the intestinal mucosa after oral gavage to mice. The optimized formulation performed feasible CRISPR-mediated downregulation of the target protein and reduction in the downstream cholesterol. As the first oral CRISPR carrier, this study suggests the potential of addressing the needs of both local and systemic editing in a patient-compliant manner.

Advanced FRET normalization allows quantitative analysis of protein interactions including stoichiometries and relative affinities in living cells

FRET (Fluorescence Resonance Energy Transfer) measurements are commonly applied to proof protein-protein interactions. However, standard methods of live cell FRET microscopy and signal normalization only allow a principle assessment of mutual binding and are unable to deduce quantitative information of the interaction. We present an evaluation and normalization procedure for 3-filter FRET measurements, which reflects the process of complex formation by plotting FRET-saturation curves. The advantage of this approach relative to traditional signal normalizations is demonstrated by mathematical simulations. Thereby, we also identify the contribution of critical parameters such as the total amount of donor and acceptor molecules and their molar ratio. When combined with a fitting procedure, this normalization facilitates the extraction of key properties of protein complexes such as the interaction stoichiometry or the apparent affinity of the binding partners. Finally, the feasibility of our method is verified by investigating three exemplary protein complexes. Altogether, our approach offers a novel method for a quantitative analysis of protein interactions by 3-filter FRET microscopy, as well as flow cytometry. To facilitate the application of this method, we created macros and routines for the programs ImageJ, R and MS-Excel, which we make publicly available.

Flow Cytometric FRET Analysis of Protein Interactions

In the past decades, investigation of protein-protein interactions in situ in living or intact cells has gained expanding importance as structure/function relationships proposed from bulk biochemistry and molecular modeling experiments required confirmation at the cellular level. Förster (fluorescence) resonance energy transfer (FRET)-based methods are excellent tools for determining proximity and supramolecular organization of biomolecules at the cell surface or inside the cell. This could well be the basis for the increasing popularity of FRET. In fact, the number of publications exploiting FRET has exploded since the turn of the millennium. Interestingly, most applications are microscope-based, and only a fraction employs flow cytometry, even though the latter offers great statistical power owed to the potentially huge number of individually measured cells. However, with the increased availability of multi-laser flow cytometers, strategies to obtain absolute FRET efficiencies can now be relatively facilely implemented. In this chapter, we intend to provide generally useable protocols for measuring FRET in flow cytometry. After a concise theoretical introduction, recipes are provided for successful labeling techniques and measurement approaches. The simple, quenching-based population-level measurement, the classic ratiometric, intensity-based technique providing cell-by-cell actual FRET efficiencies, and a more advanced version of the latter, allowing for cell-by-cell autofluorescence correction are described. An Excel macro pre-loaded with spectral data of the most commonly used fluorophores is also provided for easy calculation of average FRET efficiencies. Finally, points of caution are given to help design proper experiments and critically interpret the results.

Reevaluating multicolor flow cytometry to assess microbial viability

Flow cytometry is a rapid and quantitative method to determine bacterial viability. Although different stains can be used to establish viability, staining protocols are inconsistent and lack a general optimization approach. Very few "true" multicolor protocols, where dyes are combined in one sample, have been developed for microbiological applications. In this mini-review, the discrepancy between protocols for cell-permeant nucleic acid and functional stains are discussed as well as their use as viability dyes. Furthermore, optimization of staining protocols for a specific setup are described. Original data using the red-excitable SYTO dyes SYTO 59 to 64 and SYTO 17, combined with functional stains, for double and triple staining applications is also included. As each dye and dye combination behaves differently within a certain combination of medium matrix, microorganism, and instrument, protocols need to be tuned to obtain reproducible results. Therefore, single, double, and triple stains are reviewed, including the different parameters that influence staining such as stain kinetics, optimal stain concentration, and the effect of the chelator EDTA as membrane permeabilizer. In the last section, we highlight the need to investigate the stability of multicolor assays to ensure correct results as multiwell autoloaders are now commonly used.

Automated nanoscale flow cytometry for assessing protein-protein interactions

Despite their importance for signalling events, protein-protein interactions cannot easily be analyzed on a single cell level. We developed a robust automated FRET measurement system implemented on a commercial flow cytometer allowing for rapid profiling of molecular associations in living cells. We used this method to measure the most proximal signaling events on human T lymphocyte activation, which preceded calcium influx, and could automatically detect T cell receptor/CD3 complex clustering defects in immunocompromised patients. © 2016 International Society for Advancement of Cytometry.

Toll-Like Receptor Interactions Measured by Microscopic and Flow Cytometric FRET

Protein-protein interactions regulate biological networks. The most proximal events that initiate signal transduction frequently are receptor dimerization or conformational changes in receptor complexes. Toll-like receptors (TLRs) are transmembrane receptors that are activated by a number of exogenous and endogenous ligands. Most TLRs can respond to multiple ligands and the different TLRs recognize structurally diverse molecules ranging from proteins, sugars, lipids, and nucleic acids. TLRs can be expressed on the plasma membrane or in endosomal compartments and ligand recognition thus proceeds in different microenvironments. Not surprisingly, distinctive mechanisms of TLR receptor activation have evolved. A detailed understanding of the mechanisms of TLR activation is important for the development of novel synthetic TLR activators or pharmacological inhibitors of TLRs. Confocal laser scanning microscopy combined with GFP technology allows the direct visualization of TLR expression in living cells. Fluorescence resonance energy transfer (FRET) measurements between two differentially tagged proteins permit the study of TLR interaction, and distances between receptors in the range of molecular interactions can be measured and visualized. Additionally, FRET measurements combined with confocal microscopy provide detailed information about molecular interactions in different subcellular localizations. These techniques permit the dynamic visualization of early signaling events in living cells and can be utilized in pharmacological or genetic screens.

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.

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.

An activatable, polarity dependent, dual-luminescent imaging agent with a long luminescence lifetime

A combination of two quenching interactions was incorporated in a new activatable imaging agent. Partial and total activation of luminescence can be achieved, as well as luminescence lifetime imaging.

Förster resonance energy transfer microscopy and spectroscopy for localizing protein-protein interactions in living cells

The fundamental theory of Förster resonance energy transfer (FRET) was established in the 1940s. Its great power was only realized in the past 20 years after different techniques were developed and applied to biological experiments. This success was made possible by the availability of suitable fluorescent probes, advanced optics, detectors, microscopy instrumentation, and analytical tools. Combined with state-of-the-art microscopy and spectroscopy, FRET imaging allows scientists to study a variety of phenomena that produce changes in molecular proximity, thereby leading to many significant findings in the life sciences. In this review, we outline various FRET imaging techniques and their strengths and limitations; we also provide a biological model to demonstrate how to investigate protein-protein interactions in living cells using both intensity- and fluorescence lifetime-based FRET microscopy methods.

Validating pharmacological disruption of protein-protein interactions by acceptor photobleaching FRET imaging

Proteins are the major targets of drug discovery and many of the new drugs are designed to exert their effect by disrupting protein-protein interactions. Validation of the inhibition of molecular interactions is generally done by biochemical methods, however, these are often not feasible when the interaction is not stable enough. Fluorescence resonance energy transfer (FRET) is an excellent tool for determining direct molecular interactions between two molecules in the cell membrane or inside cells in their natural state. Although originally established as a flow cytometric approach, FRET has been adapted for microscopy, allowing for analysis of sub-cellular co-localization at the single cell level. In this chapter, we provide theoretical introduction to the phenomenon of FRET, and a protocol - including labeling techniques, measurement, and evaluation of microscopy images - of the simplest microscopic FRET approach, acceptor photobleaching FRET. This technique is generally usable for studying protein interactions and requires only a standard confocal laser scanning microscope. To demonstrate the value of image based FRET for testing pharmacological disruption of protein-protein interactions, we show how inhibition of the hetero-dimerization of ErbB2 and ErbB1 by the humanized monoclonal antibody pertuzumab can be validated using this technique.

Fluorescence in nanobiotechnology: sophisticated fluorophores for novel applications

Nanobiotechnology is one of the fastest growing and broadest-ranged interdisciplinary subfields of the nanosciences. Countless hybrid bio-inorganic composites are currently being pursued for various uses, including sensors for medical and diagnostic applications, light- and energy-harvesting devices, along with multifunctional architectures for electronics and advanced drug-delivery. Although many disparate biological and nanoscale materials will ultimately be utilized as the functional building blocks to create these devices, a common element found among a large proportion is that they exert or interact with light. Clearly continuing development will rely heavily on incorporating many different types of fluorophores into these composite materials. This review covers the growing utility of different classes of fluorophores in nanobiotechnology, from both a photophysical and a chemical perspective. For each major structural or functional class of fluorescent probe, several representative applications are provided, and the necessary technological background for acquiring the desired nano-bioanalytical information are presented.

Photon budget analysis for fluorescence lifetime imaging microscopy

We have constructed a mathematical model to analyze the photon efficiency of frequency-domain fluorescence lifetime imaging microscopy (FLIM). The power of the light source needed for illumination in a FLIM system and the signal-to-noise ratio of the detector have led us to a photon "budget." These measures are relevant to many fluorescence microscope users and the results are not restricted to FLIM but applicable to widefield fluorescence microscopy in general. Limitations in photon numbers, however, are more of an issue with FLIM compared to other less quantitative types of imaging. By modeling a typical experimental configuration, examples are given for fluorophores whose absorption peaks span the visible spectrum from Fura-2 to Cy5. We have performed experiments to validate the assumptions and parameters used in our mathematical model. The influence of fluorophore concentration on the intensity of the fluorescence emission light and the Poisson distribution assumption of the detected fluorescence emission light have been validated. The experimental results agree well with the mathematical model. This photon budget is important in order to characterize the constraints involved in current fluorescent microscope systems that are used for lifetime as well as intensity measurements and to design and fabricate new systems.

Flow cytometric FRET analysis of protein interaction

Investigation of protein-protein interactions in situ in living or intact cells gains expanding importance as structure/function relationships proposed from bulk biochemistry and molecular modeling experiments require demonstration at the cellular level. Fluorescence resonance energy transfer (FRET)-based methods are excellent tools for determining proximity and supramolecular organization of biomolecules at the cell surface or inside the cell. This could well be the basis for the increasing popularity of FRET; in fact, the number of publications exploiting FRET has doubled in the past 5 years. In this chapter, we intend to provide a generally useable protocol for measuring FRET in flow cytometry. After a concise theoretical introduction, recipes are provided for successful labeling techniques and measurement approaches. The simple, quenching-based population-level measurement; the classic ratiometric, intensity-based technique providing cell-by-cell actual FRET efficiencies, and a more advanced version of the latter, allowing for cell-by-cell autofluorescence correction, are described. Finally, points of caution are given to help design proper experiments and critically interpret the results.

The optimal antigen response of chimeric antigen receptors harboring the CD3zeta transmembrane domain is dependent upon incorporation of the receptor into the endogenous TCR/CD3 complex

Chimeric Ag receptors (CARs) expressed in T cells permit the redirected lysis of tumor cells in an MHC-unrestricted manner. In the Jurkat T cell model system, expression of a carcinoembryonic Ag-specific CD3zeta CAR (MFEzeta) resulted in an increased sensitivity of the transduced Jurkat cell to generate cytokines when stimulated through the endogenous TCR complex. This effect was driven through two key characteristics of the MFEzeta CAR: 1) receptor dimerization and 2) the interaction of the CAR with the endogenous TCR complex. Mutations of the CAR transmembrane domain that abrogated these interactions resulted in a reduced functional capacity of the MFEzeta CAR to respond to carcinoembryonic Ag protein Ag. Taken together, these results indicate that CARs containing the CD3zeta transmembrane domain can form a complex with the endogenous TCR that may be beneficial for optimal T cell activation. This observation has potential implications for the future design of CARs for cancer therapy.

Single-pair fluorescence resonance energy transfer (spFRET) for the high sensitivity analysis of low-abundance proteins using aptamers as molecular recognition elements

We have developed a strategy for the detection of single protein molecules, which uses single-pair fluorescence resonance energy transfer (spFRET) as the readout modality and provides exquisite analytical sensitivity and reduced assay turn-around-time by eliminating various sample pre-processing steps. The single-protein detection assay uses two independent aptamer recognition events to form an assembly conducive to intramolecular hybridization of oligonucleotide complements that are tethered to the aptamers. This hybridization brings a donor-acceptor pair within the Förster distance to create a fluorescence signature indicative of the presence of the protein-aptamer(s) association complex. As an example of spFRET, we demonstrate the technique for the analysis of serum thrombin. The assay requires co-association of two distinct epitope-binding aptamers, each of which is labeled with a donor or acceptor fluorescent dye (Cy3 or Cy5, respectively) to produce a FRET response. The FRET response between Cy3 and Cy5 was monitored by single-molecule photon-burst detection, which provides high analytical sensitivity when the number of single-molecule events is plotted versus the target concentration. We are able to identify thrombin with high efficiency based on photon burst events transduced in the Cy5 detection channel. We also demonstrate that the technique can discriminate thrombin molecules from its analogue prothrombin. The analytical sensitivity was >200-fold better than an ensemble measurement.

Evaluation of intensity-based ratiometric FRET in image cytometry--approaches and a software solution

The intensity-based ratiometric FRET (fluorescence resonance energy transfer) method is a powerful technique for following molecular interactions in living cells. Since it is not based on irreversibly destroying the donor or the acceptor fluorophores, the time course of changes in FRET efficiency values can be monitored by this method. ImageJ, a sophisticated software tool for many types of image processing allows users to extend it with programs for various purposes. Implementing intensity-based ratiometric FRET with ImageJ vastly enhances the applicability of the FRET method. We developed an efficient ImageJ plugin, RiFRET, which calculates FRET efficiency on a pixel-by-pixel basis from ratiometric FRET images. It allows the user to correct for channel cross-talk (bleed-through) and to calculate FRET from image stacks, i.e., from 3D data sets. Semiautomatic processing for larger datasets is also included in the program. Furthermore, several options for calibrating FRET efficiency calculations were tested and their applicability to various expression systems is discussed. Although the ratiometric FRET method is widely applied, our plugin is the first freely available software for evaluating such FRET data. The program is user friendly and provides reliable, standardized results.

Toll-like receptor interactions imaged by FRET microscopy and GFP fragment reconstitution

Protein-protein interactions regulate biological networks. The most proximal events that initiate signal transduction frequently are receptor dimerization or conformational changes in receptor complexes. Toll-like receptors (TLRs) are transmembrane receptors that are activated by a number of exogenous and endogenous ligands. Most TLRs can respond to multiple ligands and the different TLRs recognize structurally diverse molecules ranging from proteins, sugars, lipids, and nucleic acids. TLRs can be expressed on the plasma membrane or in endosomal compartments, and ligand recognition thus proceeds in different microenvironments. Not surprisingly, distinctive mechanisms of TLR receptor activation have evolved. A detailed understanding of the mechanisms of TLR activation is important for the development of novel synthetic TLR activators or pharmacological inhibitors of TLRs. Confocal laser scanning microscopy (LSM) combined with green fluorescent protein (GFP) technology allows the direct visualization of TLR expression in living cells. Fluorescence resonance energy transfer (FRET) measurements between two differentially tagged proteins permit the study of TLR interaction and distances between receptors in the range of molecular interactions can be measured and visualized. Additionally, FRET measurements combined with confocal microscopy provide detailed information about molecular interactions in different subcellular localizations. Bimolecular complementation using split fluorescent proteins (BiFC) represents an additional valuable method to study mechanisms of receptor activation in living cells. These techniques permit the dynamic visualization of early signaling events in living cells and can be utilized in pharmacological or genetic screens.

Flow cytometric FRET analysis of ErbB receptor tyrosine kinase interaction

The homologous and heterologous interaction of members of the epidermal growth factor (EGF)-related receptor tyrosine kinase (RTK) family (ErbB or HER family receptors) upon ligand binding is the initial key event in signal transduction by these receptors. In addition to the availability of their respective ligands, the relative expression level of the four ErbB receptors triggers receptor cross-activation, which determines signal diversification and the cells' biological response. However, the function of ErbB receptors and their ligands appears highly complex, and its impact on cell growth and proliferation of normal and tumor cells is incompletely understood. Flow cytometric fluorescence resonance energy transfer (FRET) measurements facilitate the quantitative analysis of receptor interaction. This unit delineates the cell-by-cell analysis of epidermal growth factor receptor (EGFR, ErbB1, HER1) and ErbB2 (HER2) receptor interaction in ErbB2-overexpressing BT474 and SK-BR-3 breast cancer cell lines, using a dual-laser flow cytometer. ReFlex software-based quantification of energy transfer efficiency (E) directly reflects the amount of receptor interaction.

Cytometry of raft and caveola membrane microdomains: from flow and imaging techniques to high throughput screening assays

The evolutionarily developed microdomain structure of biological membranes has gained more and more attention in the past decade. The caveolin-free "membrane rafts," the caveolin-expressing rafts (caveolae), as well as other membrane microdomains seem to play an essential role in controlling and coordinating cell-surface molecular recognition, internalization/endocytosis of the bound molecules or pathogenic organisms and in regulation of transmembrane signal transduction processes. Therefore, in many research fields (e.g. neurobiology and immunology), there is an ongoing need to understand the nature of these microdomains and to quantitatively characterize their lipid and protein composition under various physiological and pathological conditions. Flow and image cytometry offer many sophisticated and routine tools to study these questions. In this review, we give an overview of the past efforts to detect and characterize these membrane microdomains by the use of classical cytometric technologies, and finally we will discuss the results and perspectives of a new line of raft cytometry, the "high throughput screening assays of membrane microdomains," based on "lipidomic" and "proteomic" approaches.

Quantitative characterization of the large-scale association of ErbB1 and ErbB2 by flow cytometric homo-FRET measurements

The association of receptor tyrosine kinases is a key step in the initiation of growth factor-mediated signaling. Although the ligand-induced dimerization of inactive, monomeric receptors was the central dogma of receptor tyrosine kinase activation for decades, the existence of larger oligomers is now accepted. Both homoassociations and heteroassociations are of extreme importance in the epidermal growth factor (EGF) receptor family, leading to diverse and robust signaling. We present a statistically reliable, flow-cytometric homo-fluorescence resonance energy transfer method for the quantitative characterization of large-scale receptor clusters. We assumed that a fraction of a certain protein species is monomeric, whereas the rest are present in homoclusters of N-mers. We measured fluorescence anisotropy as a function of the saturation of fluorescent antibody binding, and fitted the model to the anisotropy data yielding the fraction of monomers and the cluster size. We found that ErbB2 formed larger homoclusters than ErbB1. Stimulation with EGF and heregulin led to a decrease in ErbB2 homocluster size, whereas ErbB1 homoclusters became larger after EGF stimulation. The activation level of ErbB2 was inversely proportional to its homocluster size. We conclude that homoclusters of ErbB1 and ErbB2 behave in a fundamentally different way. Whereas huge ErbB2 clusters serve as a reservoir of inactive coreceptors and dissociate upon stimulation, small ErbB1 homoclusters form higher-order oligomers after ligand binding.

Hyaluronan-induced masking of ErbB2 and CD44-enhanced trastuzumab internalisation in trastuzumab resistant breast cancer

Although trastuzumab, a recombinant humanised anti-ErbB2 antibody, is widely used in the treatment of breast cancer, neither its mechanism of action, nor the factors leading to resistance are fully understood. We have previously shown that antibody-dependent cellular cytotoxicity is pivotal in the in vivo effect of trastuzumab against JIMT-1, a cell line showing in vitro resistance to the antibody, and suggested that masking of the trastuzumab-binding epitope by MUC-4, a cell surface mucin, took place. Here, we further explored the role of masking of ErbB2 in connection with CD44 expression and synthesis of its ligand, hyaluronan. We show that high expression of CD44 observed in JIMT-1 cells correlates with ErbB2 downregulation in vivo, while siRNA-mediated inhibition of CD44 expression leads to decreased rate of trastuzumab internalisation and low cell proliferation in vitro. An inhibitor of hyaluronan synthesis, 4-methylumbelliferon (4-MU) significantly reduced the hyaluronan level of JIMT-1 cells both in vivo and in vitro leading to enhanced binding of trastuzumab to ErbB2 and increased ErbB2 down-regulation. Furthermore, the inhibitory effect of trastuzumab on the growth of JIMT-1 xenografts was significantly increased by 4-MU treatment. Our results point to the importance of the CD44-hyaluronan pathway in the escape of tumour cells from receptor-oriented therapy.

Cis binding between inhibitory receptors and MHC class I can regulate mast cell activation

Allergy is caused by immune effector cells, including mast cells and basophils. Cellular signaling that activates these effector cells is regulated by different inhibitory receptors on their surface. We show that human leukocyte immunoglobulin (Ig)-like receptor (LILR) B2 and its mouse orthologue, paired Ig-like receptor (PIR)–B, constitutively associate to major histocompatibility complex (MHC) class I on the same cell surface (in cis). The IgE-mediated effector responses were augmented in β₂-microglobulin (β₂m) and PIR-B–deficient mast cells. In addition, the increased cytokine production of β₂m-deficient mast cells was not affected by the co-culture with MHC class I–positive mast cells, showing that less cis interaction between PIR-B and MHC class I on mast cells led to the increased cytokine release. Thus, the constitutive cis binding between LILRB2 or PIR-B and MHC class I has an essential role in regulating allergic responses.

Increased immunofluorescence sensitivity using 532 nm laser excitation

OBJECTIVE

We evaluated the use of a high power, diode pulsed solid-state laser emitting 532 nm light for immunofluorescence applications. We compared the sensitivity and utility of this laser with the standard 488 nm excitation.

METHODS

A flow cytometer was equipped with both a 488 nm and a 532 nm laser; fluorescence emissions from each laser were collected using the same filters and the same detector system. Cells or compensation beads (e.g. latex beads coated with anti-kappa antibodies) were stained with monoclonal antibodies conjugated to phycoerythrin (PE) as well as the PE tandem dyes TRPE, Cy5PE, Cy5.5PE, and Cy7PE. The sensitivity of detection of these reagents as well as those in heavily compensated channels was quantified by measuring the spreading error for a primary detector into a secondary detector.

RESULTS

Measurement of the fluorescence emission of PE and PE-tandem dyes was considerably more sensitive when using 532 nm excitation (150 mW) as compared with 488 nm excitation (20 mW). In addition, as the absolute number of photoelectrons collected was greater, there was less measurement-error-induced spread into the compensated channels. As an example, when comparing the spreading error of PE labeled cells into the TRPE detector, the green laser was found to be 15-fold more sensitive as compared with the blue laser. In addition, the blue laser produced more autofluoresent signal from cells as compared with the green laser. Together, these advantages of the 532 nm excitation line provides for a significantly improved detection of immunofluorescence staining.

Materials for Fluorescence Resonance Energy Transfer Analysis: Beyond Traditional Donor–Acceptor Combinations

The use of Förster or fluorescence resonance energy transfer (FRET) as a spectroscopic technique has been in practice for over 50 years. A search of ISI Web of Science with just the acronym "FRET" returns more than 2300 citations from various areas such as structural elucidation of biological molecules and their interactions, in vitro assays, in vivo monitoring in cellular research, nucleic acid analysis, signal transduction, light harvesting and metallic nanomaterials. The advent of new classes of fluorophores including nanocrystals, nanoparticles, polymers, and genetically encoded proteins, in conjunction with ever more sophisticated equipment, has been vital in this development. This review gives a critical overview of the major classes of fluorophore materials that may act as donor, acceptor, or both in a FRET configuration. We focus in particular on the benefits and limitations of these materials and their combinations, as well as the available methods of bioconjugation.

Systems biology and clinical cytomics: The 10th Leipziger Workshop and the 3rd International Workshop on Slide-Based Cytometry, Leipzig, Germany, April 2005

Despite very significant technical and software improvements in flow cytometry (FCM) since the 1980's, the demand for a cytometric technology combining both quantitative cell analysis and morphological documentation in Cytomics became evident. Improvements in microtechnology and computing permit nowadays similar quantitative and stoichiometric single cell-based high-throughput analyses by microscopic instruments, like Slide-Based Cytometry (SBC). SBC and related techniques offer unique tools to perform complex immunophenotyping, thereby enabling diagnostic procedures during early disease stages. Multicolor or polychromatic analysis of cells by SBC is of special importance not only as a cytomics technology platform but also because of low quantities of required reagents and biological material. The exact knowledge of the location of each cell on the slide permits repetitive restaining and reanalysis of specimens. Various separate measurements of the same specimen can be ultimately fused to one database increasing the information obtained per cell. Relocation and optical evaluation of cells as typical SBC feature, can be of integral importance for cytometric analysis, since artifacts can be excluded and morphology of measured cells can be documented. Progress in cell analytic: In the SBC, new horizons can be opened by the new techniques of structural and functional analysis with the high resolution from intracellular and membrane (confocal microscopy, nanoscopy, total internal fluorescence microscopy (TIRFM), and tissue level (tissomics), to organ and organism level (in vivo cytometry, optical whole body imaging). Predictive medicine aims at the detection of changes in patient's state prior to the manifestation of the disease or the complication. Such instances concern immune consequences of surgeries or noninfectious posttraumatic shock in intensive care patients or the pretherapeutic identification of high risk patients in cancer cytostatic therapy. Preventive anti-infectious or anti-shock therapy as well as curative chemotherapy in combination with stem cell transplantation may provide better survival chances for patient at concomitant cost containment. Predictive medicine-guided optimization of therapy could lead to individualized medicine that gives significant therapeutic effect and may lower or abrogate potential therapeutic side effects. The 10th Leipziger Workshop combined with the 3rd International Workshop on SBC aimed to offer new methods in Image- and Slide-Based Cytometry for solutions in clinical research. It moved towards practical applications in the clinics and the clinical laboratory. This development will be continued in 2006 at the upcoming Leipziger Workshop and the International Workshop on Slide-Based Cytometry.