Quantum dot fluorescence characterizes the nanoscale organization of T cell receptors for antigen

Genetically Stable and Scalable Nanoengineering of Human Primary T Cells via Cell Mechanoporation

Effective tumor regression has been observed with chimeric antigen receptor (CAR) T cells; however, the development of an affordable, safe, and effective CAR-T cell treatment remains a challenge. One of the major obstacles is that the suboptimal genetic modification of T cells reduces their yield and antitumor activity, necessitating the development of a next-generation T cell engineering approach. In this study, we developed a nonviral T cell nanoengineering system that allows highly efficient delivery of diverse functional nanomaterials into primary human T cells in a genetically stable and scalable manner. Our platform leverages the unique cell deformation and restoration process induced by the intrinsic inertial flow in a microchannel to create nanopores in the cellular membrane for macromolecule internalization, leading to effective transfection with high scalability and viability. The proposed approach demonstrates considerable potential as a practical alternative technique for improving the current CAR-T cell manufacturing process.

Transient protein accumulation at the center of the T cell antigen-presenting cell interface drives efficient IL-2 secretion

Supramolecular signaling assemblies are of interest for their unique signaling properties. A µm scale signaling assembly, the central supramolecular signaling cluster (cSMAC), forms at the center of the interface of T cells activated by antigen-presenting cells. We have determined that it is composed of multiple complexes of a supramolecular volume of up to 0.5 µm3 and associated with extensive membrane undulations. To determine cSMAC function, we have systematically manipulated the localization of three adaptor proteins, LAT, SLP-76, and Grb2. cSMAC localization varied between the adaptors and was diminished upon blockade of the costimulatory receptor CD28 and deficiency of the signal amplifying kinase Itk. Reconstitution of cSMAC localization restored IL-2 secretion which is a key T cell effector function as dependent on reconstitution dynamics. Our data suggest that the cSMAC enhances early signaling by facilitating signaling interactions and attenuates signaling thereafter through sequestration of a more limited set of signaling intermediates.

A High-Throughput Image Correlation Method for Rapid Analysis of Fluorophore Photoblinking and Photobleaching Rates

Super-resolution fluorescence imaging based on localization microscopy requires tuning the photoblinking properties of fluorescent dyes employed. Missing is a rapid way to analyze the blinking rates of the fluorophore probes. Herein we present an ensemble autocorrelation technique for rapidly and simultaneously measuring photoblinking and bleaching rate constants from a microscopy image time series of fluorescent probes that is significantly faster than individual single-molecule trajectory analysis approaches. Our method is accurate for probe densities typically encountered in single-molecule studies as well as for higher density systems which cannot be analyzed by standard single-molecule techniques. We also show that we can resolve characteristic blinking times that are faster than camera detector exposure times, which cannot be accessed by threshold-based single-molecule approaches due to aliasing. We confirm this through computer simulation and single-molecule imaging data of DNA-Cy5 complexes. Finally, we demonstrate that with sufficient sampling our technique can accurately recover rates from stochastic optical reconstruction microscopy super-resolution data.

T cell immunotherapy enhanced by designer biomaterials

Cancer immunotherapy has recently burst onto the center stage of cancer treatment and research. T lymphocyte adoptive cellular transfer (ACT), a form of cancer immunotherapy, has spawned unprecedented complete remissions for terminal patients with certain leukemias and lymphomas. Unfortunately, the successes have been overshadowed by the disappointing clinical results of ACT administered to treat solid tumors, in addition to the toxicities associated with the treatment, a lack of efficacy in a significant proportion of the patient population, and cancer relapse following the treatment. Biomaterials hold the promise of addressing these shortcomings. ACT consists of two main stages - T lymphocyte ex vivo expansion followed by reinfusion into the patient - and biomaterials can improve the efficacy of ACT at both stages. In this review, we highlight recent advances in the use of biomaterials for T lymphocyte adoptive cellular cancer immunotherapy and discuss the challenges at each stage.

A high fat diet containing saturated but not unsaturated fatty acids enhances T cell receptor clustering on the nanoscale

Cell culture studies show that the nanoscale lateral organization of surface receptors, their clustering or dispersion, can be altered by changing the lipid composition of the membrane bilayer. However, little is known about similar changes in vivo, which can be effected by changing dietary lipids. We describe the use of a newly developed method, k-space image correlation spectroscopy, kICS, for analysis of quantum dot fluorescence to show that a high fat diet can alter the nanometer-scale clustering of the murine T cell receptor, TCR, on the surface of naive CD4(+) T cells. We found that diets enriched primarily in saturated fatty acids increased TCR nanoscale clustering to a level usually seen only on activated cells. Diets enriched in monounsaturated or n-3 polyunsaturated fatty acids had no effect on TCR clustering. Also none of the high fat diets affected TCR clustering on the micrometer scale. Furthermore, the effect of the diets was similar in young and middle aged mice. Our data establish proof-of-principle that TCR nanoscale clustering is sensitive to the composition of dietary fat.

Linking form to function: Biophysical aspects of artificial antigen presenting cell design

Artificial antigen presenting cells (aAPCs) are engineered platforms for T cell activation and expansion, synthesized by coupling T cell activating proteins to the surface of cell lines or biocompatible particles. They can serve both as model systems to study the basic aspects of T cell signaling and translationally as novel approaches for either active or adoptive immunotherapy. Historically, these reductionist systems have not been designed to mimic the temporally and spatially complex interactions observed during endogenous T cell-APC contact, which include receptor organization at both micro- and nanoscales and dynamic changes in cell and membrane morphologies. Here, we review how particle size and shape, as well as heterogenous distribution of T cell activating proteins on the particle surface, are critical aspects of aAPC design. In doing so, we demonstrate how insights derived from endogenous T cell activation can be applied to optimize aAPC, and in turn how aAPC platforms can be used to better understand endogenous T cell stimulation. This article is part of a Special Issue entitled: Nanoscale membrane organisation and signalling.

Quantum dots: bright and versatile in vitro and in vivo fluorescence imaging biosensors

Colourful cells and tissues: semiconductor quantum dots and their versatile applications in multiplexed bioimaging research.

TCR Microclusters pre-exist and contain molecules necessary for TCR signal transduction

TCR-dependent signaling events have been observed to occur in TCR microclusters. We found that some TCR microclusters are present in unstimulated murine T cells, indicating that the mechanisms leading to microcluster formation do not require ligand binding. These pre-existing microclusters increase in absolute number following engagement by low-potency ligands. This increase is accompanied by an increase in cell spreading, with the result that the density of TCR microclusters on the surface of the T cell is not a strong function of ligand potency. In characterizing their composition, we observed a constant number of TCRs in a microcluster, constitutive exclusion of the phosphatase CD45, and preassociation with the signaling adapters linker for activation of T cells and growth factor receptor-bound protein 2. The existence of TCR microclusters prior to ligand binding in a state that is conducive for the initiation of downstream signaling could explain, in part, the rapid kinetics with which TCR signal transduction occurs.

Magnetic field-induced T cell receptor clustering by nanoparticles enhances T cell activation and stimulates antitumor activity

Iron-dextran nanoparticles functionalized with T cell activating proteins have been used to study T cell receptor (TCR) signaling. However, nanoparticle triggering of membrane receptors is poorly understood and may be sensitive to physiologically regulated changes in TCR clustering that occur after T cell activation. Nano-aAPC bound 2-fold more TCR on activated T cells, which have clustered TCR, than on naive T cells, resulting in a lower threshold for activation. To enhance T cell activation, a magnetic field was used to drive aggregation of paramagnetic nano-aAPC, resulting in a doubling of TCR cluster size and increased T cell expansion in vitro and after adoptive transfer in vivo. T cells activated by nano-aAPC in a magnetic field inhibited growth of B16 melanoma, showing that this novel approach, using magnetic field-enhanced nano-aAPC stimulation, can generate large numbers of activated antigen-specific T cells and has clinically relevant applications for adoptive immunotherapy.

An induced rebinding model of antigen discrimination

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We propose that pMHC binding to TCR can increase (induce) pMHC rebinding.

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Published studies are consistent with an induced rebinding model.

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Induced rebinding improves the ability of T cells to discriminate antigens.

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Induced rebinding relates 3D to 2D TCR–pMHC binding parameters.

T cells have to detect rare high-affinity ‘foreign’ peptide MHC (pMHC) ligands among abundant low-affinity ‘self’-peptide MHC ligands. It remains unclear how this remarkable discrimination is achieved. Kinetic proofreading mechanisms can provide the required specificity but only at the expense of much reduced sensitivity. A number of recent observations suggest that pMHC engagement of T cell receptors (TCRs) induces changes such as clustering and/or conformational alterations that enhance subsequent rebinding. We show that inclusion of induced rebinding to the same pMHC in kinetic proofreading models enhances the sensitivity of TCR recognition while retaining specificity. Moreover, induced rebinding is able to reproduce the striking, and hitherto unexplained, 2D membrane-binding properties recently reported for the TCR.

Nanoscale artificial antigen presenting cells for T cell immunotherapy

Artificial antigen presenting cells (aAPC), which deliver stimulatory signals to cytotoxic lymphocytes, are a powerful tool for both adoptive and active immunotherapy. Thus far, aAPC have been synthesized by coupling T cell activating proteins such as CD3 or MHC-peptide to micron-sized beads. Nanoscale platforms have different trafficking and biophysical interaction properties and may allow development of new immunotherapeutic strategies. We therefore manufactured aAPC based on two types of nanoscale particle platforms: biocompatible iron-dextran paramagnetic particles (50-100 nm in diameter) and avidin-coated quantum dot nanocrystals (~30 nm). Nanoscale aAPC induced antigen-specific T cell proliferation from mouse splenocytes and human peripheral blood T cells. When injected in vivo, both iron-dextran particles and quantum dot nanocrystals enhanced tumor rejection in a subcutaneous mouse melanoma model. This is the first description of nanoscale aAPC that induce antigen-specific T cell proliferation in vitro and lead to effective T cell stimulation and inhibition of tumor growth in vivo.

FROM THE CLINICAL EDITOR

Artifical antigen presenting cells could revolutionize the field of cancer-directed immunotherapy. This team of investigators have manufactured two types of nanoscale particle platform-based aAPCs and demonstrates that both iron-dextran particles and quantum dot nanocrystals enhance tumor rejection in a melanoma model, providing the first description of nanoscale aAPCs that lead to effective T cell stimulation and inhibition of tumor growth.

Selective activation of antigen-experienced T cells by anti-CD3 constrained on nanoparticles

Activation of T cells through the TCR is mediated by the TCR-CD3 signaling complex. Cross linking of this complex with Abs directed against CD3 leads to potent activation of T cells. However, such activation is not Ag-specific. We exploited the observation that the TCR-CD3 complex is clustered on T cells that have been activated by Ag by using anti-CD3 nanoparticles to selectively activate Ag-experienced mouse T cells. We find that constraining anti-CD3 on the surface of a nanoparticle markedly and selectively enhances proliferation and cytokine production of Ag-experienced T cells but does not activate naive T cells. This effect was recapitulated in heterogeneous cultures containing mixtures of Ag-specific CD4(+) or CD8(+) T cells and bystander T cells. Furthermore, in vivo anti-CD3-coated nanoparticles increased the expansion of Ag-specific T cells following vaccination. Overall, these findings indicate that anti-CD3-coated nanoparticles could be use to enhance the efficacy of vaccines and immunotherapy. The results also suggest constraining a ligand on the surface of a nanoparticle might as general strategy for selectively targeting clustered receptors.

Modulation of MHC binding by lateral association of TCR and coreceptor

The structure of a T cell receptor (TCR) and its affinity for cognate antigen are fixed, but T cells regulate binding sensitivity through changes in lateral membrane organization. TCR microclusters formed upon antigen engagement participate in downstream signaling. Microclusters are also found 3-4 days after activation, leading to enhanced antigen binding upon rechallenge. However, others have found an almost complete loss of antigen binding four days after T cell activation, when TCR clusters are present. To resolve these contradictory results, we compared binding of soluble MHC-Ig dimers by transgenic T cells stimulated with a high (100 μM) or low (100 fM) dose of cognate antigen. Cells activated by a high dose of peptide bound sixfold lower amounts of CD8-dependent ligand K(b)-SIY than cells activated by a low dose of MHC/peptide. In contrast, both cell populations bound a CD8-independent ligand L(d)-QL9 equally well. Consistent with the differences between binding of CD8-dependent and CD8-independent peptide/MHC, Förster resonance energy transfer (FRET) measurements of molecular proximity reported little nanoscale association of TCR with CD8 (16 FRET units) compared to their association on cells stimulated by low antigen dose (62 FRET units). Loss of binding induced by changes in lateral organization of TCR and CD8 may serve as a regulatory mechanism to avoid excessive inflammation and immunopathology in response to aggressive infection.

Ultrabright fluorescent mesoporous silica nanoparticles for prescreening of cervical cancer

We report on the first functional use of recently introduced ultrabright fluorescent mesoporous silica nanoparticles, which are functionalized with folic acid, to distinguish cancerous and precancerous cervical epithelial cells from normal cells. The high brightness of the particles is advantageous for fast and reliable identification of both precancerous and cancerous cells. Normal and cancer cells were isolated from three healthy women and three cancer patients. Three precancerous cell lines were derived by immortalization of primary cultures of normal cells with human papillomavirus type-16 (HPV-16) DNA. We observed substantially different particle internalization by normal and cancerous/precancerous cells after a short incubation time of 15 minutes. Compared to HPV-DNA and cell pathology tests, which are currently used for prescreening of cervical cancer, we demonstrated that the specificity of our method was similar (94-95%), whereas its sensitivity was significantly better (95-97%) than the sensitivity of those currently used tests (30-80%).

FROM THE CLINICAL EDITOR

This team of investigators reports on the development of a new screening test for cervical cancer using ultrabright fluorescent mesoporous silica nanoparticles functionalized with folic acid, enabling significantly better sensitivity (95-97% vs. 30-80%) and maintained specificity (94-95%) compared with current clinical tests. This test should find a way to clinical use in the near future.

Quantum dots in bioanalysis: a review of applications across various platforms for fluorescence spectroscopy and imaging

Semiconductor quantum dots (QDs) are brightly luminescent nanoparticles that have found numerous applications in bioanalysis and bioimaging. In this review, we highlight recent developments in these areas in the context of specific methods for fluorescence spectroscopy and imaging. Following a primer on the structure, properties, and biofunctionalization of QDs, we describe select examples of how QDs have been used in combination with steady-state or time-resolved spectroscopic techniques to develop a variety of assays, bioprobes, and biosensors that function via changes in QD photoluminescence intensity, polarization, or lifetime. Some special attention is paid to the use of Förster resonance energy transfer-type methods in bioanalysis, including those based on bioluminescence and chemiluminescence. Direct chemiluminescence, electrochemiluminescence, and charge transfer quenching are similarly discussed. We further describe the combination of QDs and flow cytometry, including traditional cellular analyses and spectrally encoded barcode-based assay technologies, before turning our attention to enhanced fluorescence techniques based on photonic crystals or plasmon coupling. Finally, we survey the use of QDs across different platforms for biological fluorescence imaging, including epifluorescence, confocal, and two-photon excitation microscopy; single particle tracking and fluorescence correlation spectroscopy; super-resolution imaging; near-field scanning optical microscopy; and fluorescence lifetime imaging microscopy. In each of the above-mentioned platforms, QDs provide the brightness needed for highly sensitive detection, the photostability needed for tracking dynamic processes, or the multiplexing capacity needed to elucidate complex systems. There is a clear synergy between advances in QD materials and spectroscopy and imaging techniques, as both must be applied in concert to achieve their full potential.

Fluorescence fluctuation spectroscopy approaches to the study of receptors in live cells

Communication between cells and their environment, including other cells, is often mediated by cell surface receptors. Fluorescence methodologies are among the most important techniques used to study receptors and their interactions, and in the past decade, fluorescence fluctuation spectroscopy (FFS) approaches have been increasingly utilized. In this overview, we illustrate how diverse FFS approaches have been used to elucidate important aspects of receptor systems, including interactions of receptors with their ligands and receptor oligomerization and clustering. We also describe the most popular methods used to introduce fluorescent moieties into the biological systems. Finally, specific attention will be given to cell maintenance and transfection strategies especially as related to microscopy studies.

Characterization of differential Toll-like receptor responses below the optical diffraction limit

Many membrane receptors are recruited to specific cell surface domains to form nanoscale clusters upon ligand activation. This step appears to be necessary to initiate cell signaling, including pathways in innate immune system activation. However, virulent pathogens such as Yersinia pestis (the causative agent of plague) are known to evade innate immune detection, in contrast to similar microbes (such as Escherichia coli) that elicit a robust response. This disparity has been partly attributed to the structure of lipopolysaccharides (LPS) on the bacterial cell wall, which are recognized by the innate immune receptor TLR4. It is hypothesized that nanoscale differences exist between the spatial clustering of TLR4 upon binding of LPS derived from Y. pestis and E. coli. Although optical imaging can provide exquisite details of the spatial organization of biomolecules, there is a mismatch between the scale at which receptor clustering occurs (<300 nm) and the optical diffraction limit (>400 nm). The last decade has seen the emergence of super-resolution imaging methods that effectively break the optical diffraction barrier to yield truly nanoscale information in intact biological samples. This study reports the first visualizations of TLR4 distributions on intact cells at image resolutions of <30 nm using a novel, dual-color stochastic optical reconstruction microscopy (STORM) technique. This methodology permits distinction between receptors containing bound LPS from those without at the nanoscale. Importantly, it is also shown that LPS derived from immunostimulatory bacteria result in significantly higher LPS-TLR4 cluster sizes and a nearly twofold greater ligand/receptor colocalization as compared to immunoevading LPS.