Receptor clustering and transmembrane signaling in T cells

An extracellular driving force of cell-shape changes

The cellular capacity to internalise objects, involving attachment, engulfment and uptake, exists in virtually all organisms. Many uptake reactions are associated with cell signalling. However, the mechanical forces that form endocytotic vesicles are not known. We propose a 'leverage-mediated' uptake mechanism involving lateral cross-linking processes on the cell surface that can generate the configurational energy to create an inverse curvature of the membrane.

Exosomes as a tumor vaccine: enhancing potency through direct loading of antigenic peptides

Exosomes secreted by dendritic cells (DCs) contain MHC-I, MHC-II, and other accessory molecules required for antigen presentation to T cells. Previous studies have shown that exosome MHC-I "indirectly" loaded by adding peptides to DC cultures are immunogenic. However, analysis of peptide binding was not performed to link T-cell-stimulating activity with the amount of MHC-I/peptide complexes on the exosomes. In this study, we measured peptide binding to MHC-I under different loading conditions and tested the exosomes' potencies in T-cell activation assays. We demonstrate that MHC-I on purified exosomes can be directly loaded with peptide at much greater levels than indirect loading. The direct loading method performed in mildly acidic conditions was effective even in the absence of exogenous beta2m. This increase in peptide binding greatly enhanced exosome potency, allowing us to further study the biologic activity of exosomes in vitro. In the presence of antigen-presenting cells (APC), exosomes directly loaded with the HLA-A2 restricted MART1 tumor peptide stimulated an HLA-A2/MART1 specific T-cell line. The T cells responded to exosomes using HLA-A2neg APC, demonstrating transfer of functional MHC-I/peptide complexes and not peptide alone to APC. MHC-II molecules, which are abundantly expressed on DC exosomes, were also functionally loaded under the same conditions as MHC-I. This feature allows for delivery of multiple peptide antigens that can stimulate both CD8+ cytotoxic T cells as well CD4+ T helper cells critical for an effective antitumor response. The optimized loading conditions and the ability to transfer both MHC-I and MHC-II antigens to APC have led to the development of exosomes as an "acellular" immunotherapy approach currently being tested in clinical trials.

A complicated relationship: fulfilling the interactive needs of the T lymphocyte and the dendritic cell

T cells recognize antigenic peptides displayed on the surface of MHC-bearing antigen-presenting cells (APCs), and with sufficient costimulation become activated. However, the ability of an APC (even bearing the correct peptide) to initiate and fulfill the requirements for T cell activation is not easily achieved. Naive T cells use multiple copies of a single receptor to survey the vast array of peptides presented on an APC, and require multiple receptor engagements to initiate T cell activation. Dendritic cells (DCs) are specialized cells with optimal capabilities for priming naive CD4+ T cells. Activation occurs, after initial antigen recognition by T cells, followed by a rapid dialogue between the T cells and the DCs. The resulting changes in both the cytoskeleton and the expression or regulation of cell-surface molecules on both cell types act to further strengthen engagement. In this report, we review the fundamentals of CD4+ T helper cell : DC interactions and discuss recent data concerning the molecular characteristics of this engagement.

Tandem gramicidin channels cross-linked by streptavidin

The interaction of biotin-binding proteins with biotinylated gramicidin (gA5XB) was studied by monitoring single-channel activity and sensitized photoinactivation kinetics. It was discovered that the addition of streptavidin or avidin to the bathing solutions of a bilayer lipid membrane (BLM) with incorporated gA5XB induced the opening of a channel characterized by approximately doubled single-channel conductance and extremely long open-state duration. We believe that the deceleration of the photoinactivation kinetics observed here with streptavidin and previously (Rokitskaya, T.I., Y.N. Antonenko, E.A. Kotova, A. Anastasiadis, and F. Separovic. 2000. Biochemistry. 39:13053-13058) with avidin reflects the formation of long-lived channels of this type. Both opening and closing of the double-conductance channels occurred via a transient sub-state of the conductance coinciding with that of the usual single-channel transition. The appearance of the double-conductance channels after the addition of streptavidin was preceded by bursts of fast fluctuations of the current with the open state duration of the individual events of 60 ms. The streptavidin-induced double-conductance channels appeared to be inherent only to the gramicidin analogue with a biotin group linked to the COOH terminus through a long linker arm. Including biotinylated phosphatidylethanolamine into the BLM prevented the formation of the double-conductance channels even with the excess streptavidin. In view of the results obtained here, it is suggested that the double-conductance channel represents a tandem of two neighboring gA5XB channels with their COOH termini being cross-linked by the bound streptavidin at both sides of the BLM. The finding that streptavidin induces the formation of the tandem gramicidin channel comprising two channels functioning in concert is considered to be relevant to the physiologically important phenomenon of ligand-induced receptor oligomerization.

Initiation of TCR signaling: regulation within CD3 dimers

The number of possible T cell activation outcomes resulting from T cell receptor (TCR) engagement suggests that the TCR is able to differentially activate a myriad of signaling pathways depending on the nature of the stimulus. The complex structural organization of the TCR itself could underlie this diversity of responses. Assembly and stoichiometric studies have helped us to shed some light on the initiation of TCR signaling. The TCR is composed of TCR and CD3 dimers. Changes in the interaction between CD3 subunits within the CD3 dimers and in the interaction of these dimers with the TCR heterodimer could be the triggering mechanism that initiates the first activation events. One of the hallmarks of these early changes in TCR conformation is the induced recruitment of the adapter protein Nck to a proline-rich sequence of the cytoplasmic tail of CD3epsilon, but there may be others. According to our most recent observations, the TCR is organized in pre-existing clusters within plasma membrane microdomains, exhibiting a complexity above and beyond that of dimer composition complexity. How the presence of TCR in clusters influences TCR avidity and propagation of TCR signals is something that has yet to be investigated.

Labeling antigen-specific CD4(+) T cells with class II MHC oligomers

Class I MHC-peptide oligomers (MHC tetramers) have become popular reagents for the detection and characterization of antigen-specific CD8(+) T cells. Class II MHC proteins can be produced by expression in Escherichia coli followed by in vitro folding, or by native expression in insect cells; biotin can be introduced by site-specific chemical modification of cysteine, or by enzymatic modification of a peptide tag; and a variety of fluorescent streptavidin preparations can be used for oligomerization. Here we review methodologies for production of fluorescent oligomers of soluble class II MHC proteins and discuss their use in analysis of antigen-specific CD4(+) T cells. We explore the experimental conditions necessary for efficient staining of CD4(+) T cells using oligomers of class II MHC proteins, and we establish a standard protocol. Finally, we consider complications and challenges associated with these reagents, discuss the interpretation of staining results, and suggest future directions for investigation, in particular the use of MHC oligomers for the study of T cell avidity modulation.

Disabling receptor ensembles with rationally designed interface peptidomimetics

Members of the erbB family receptor tyrosine kinases (erbB1, erbB2, erbB3, and erbB4) are overexpressed in a variety of human cancers and represent important targets for the structure-based drug design. Homo- and heterodimerization (oligomerization) of the erbB receptors are known to be critical events for receptor signaling. To block receptor self-associations, we have designed a series of peptides derived from potential dimerization surfaces in the extracellular subdomain IV of the erbB receptors (erbB peptides). In surface plasmon resonance (BIAcore) studies, the designed peptides have been shown to selectively bind to the erbB receptor ectodomains and isolated subdomain IV of erbB2 with submicromolar affinities and to inhibit heregulin-induced interactions of erbB3 with different erbB receptors. A dose-dependent inhibition of native erbB receptor dimerization by the erbB peptides has been observed in 32D cell lines transfected with different combinations of erbB receptors. The peptides effectively inhibited growth of two types of transformed cells overexpressing different erbB receptors, T6-17 and 32D, in standard MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) and cell viability assays. The study identifies distinct loops within the membrane-proximal part of the subdomain IV as potential receptor-receptor interaction sites for the erbB receptors and demonstrates the possibility of disabling receptor activity by structure-based targeting of the dimerization interfaces. Molecular models for possible arrangement of the erbB1.EGF complex, consistent with the involvement of subdomain IV in inter-receptor interactions, are proposed. Small dimerization inhibitors described herein can be useful as probes to elucidate different erbB signaling pathways and may be developed as therapeutic agents.

Structural and thermodynamic correlates of T cell signaling

The first crystal structures of intact T cell receptors (TCRs) bound to class I peptide-MHC (pMHCs) antigens were determined in 1996. Since then, further structures of class I TCR/pMHC complexes have explored the degree of structural variability in the TCR-pMHC system and the structural basis for positive and negative selection. The recent determination of class II and allogeneic class I TCR/pMHC structures, as well as those of accessory molecules (e.g., CD3), has pushed our knowledge of TCR/pMHC interactions into new realms, shedding light on clinical pathologies, such as graft rejection and graft-versus-host disease. Furthermore, the determination of coreceptor structures lays the foundation for a more comprehensive structural description of the supramolecular TCR signaling events and those assemblies that arise in the immunological synapse. While these telling photodocumentaries of the TCR/pMHC interaction are composed mainly from static crystal structures, a full description of the biological snapshots in T cell signaling requires additional analytical methods that record the dynamics of the process. To this end, surface plasmon resonance (SPR), isothermal titration calorimetry (ITC), and ultracentrifugation (UC) have furnished both affinities and kinetics of the TCR/pMHC association. In the past year, structural, biochemical, and molecular biological data describing TCR/pMHC interactions have sublimely coalesced into a burgeoning well of understanding that promises to deliver further insights into T cell recognition. The coming years will, through a more intimate union of structural and kinetic data, allow many pressing questions to be addressed, such as how TCR/pMHC ligation is affected by coreceptor binding and what is the mechanism of TCR signaling in both early and late stages of T cell engagement with antigen-presenting cells.

Recruitment of Nck by CD3 epsilon reveals a ligand-induced conformational change essential for T cell receptor signaling and synapse formation

How membrane receptors initiate signal transduction upon ligand binding is a matter of intense scrutiny. The T cell receptor complex (TCR-CD3) is composed of TCR alpha/beta ligand binding subunits bound to the CD3 subunits responsible for signal transduction. Although it has long been speculated that TCR-CD3 may undergo a conformational change, confirmation is still lacking. We present strong evidence that ligand engagement of TCR-CD3 induces a conformational change that exposes a proline-rich sequence in CD3 epsilon and results in recruitment of the adaptor protein Nck. This occurs earlier than and independently of tyrosine kinase activation. Finally, by interfering with Nck-CD3 epsilon association in vivo, we demonstrate that TCR-CD3 recruitment of Nck is critical for maturation of the immune synapse and for T cell activation.

The biogenesis and functions of exosomes

Exosomes are membrane vesicles with a diameter of 40-100 nm that are secreted by many cell types into the extracellular milieu. They correspond to the internal vesicles of an endosomal compartment, the multivesicular body and are released upon exocytic fusion of this organelle with the plasma membrane. Intracellularly, they are formed by inward budding of the endosomal membrane in a process that sequesters particular proteins and lipids. The unique composition of exosomes may confer specific functions on them upon secretion. Although their physiological role in vivo is far from being unraveled, it is apparent that they function in a multitude of processes, including intercellular communication during the immune response. Exosomes may have evolved early in the evolution of multicellular organisms and also seem to be important for tissue developmental processes.

T-cell activation by soluble MHC oligomers can be described by a two-parameter binding model

T-cell activation is essential for initiation and control of immune system function. T cells are activated by interaction of cell-surface antigen receptors with major histocompatibility complex (MHC) proteins on the surface of other cells. Studies using soluble oligomers of MHC-peptide complexes and other types of receptor cross-linking agents have supported an activation mechanism that involves T cell receptor clustering. Receptor clustering induced by incubation of T cells with MHC-peptide oligomers leads to the induction of T-cell activation processes, including downregulation of engaged receptors and upregulation of the cell-surface proteins CD69 and CD25. Dose-response curves for these T-cell activation markers are bell-shaped, with different maxima and midpoints, depending on the valency of the soluble oligomer used. In this study, we have analyzed the activation behavior using a mathematical model that describes the binding of multivalent ligands to cell-surface receptors. We show that a simple equilibrium binding model accurately describes the activation data for CD4(+) T cells treated with MHC-peptide oligomers of varying valency. The model can be used to predict activation and binding behavior for T cells and MHC oligomers with different properties.

Receptor proximity, not intermolecular orientation, is critical for triggering T-cell activation

Engagement of antigen receptors on the surface of T-cells with peptides bound to major histocompatibility complex (MHC) proteins triggers T-cell activation in a mechanism involving receptor oligomerization. Receptor dimerization by soluble MHC oligomers is sufficient to induce several characteristic activation processes in T-cells including internalization of engaged receptors and up-regulation of cell surface proteins. In this work, the influence of intermolecular orientation within the activating receptor dimer was studied. Dimers of class II MHC proteins coupled in a variety of orientations and topologies each were able to activate CD4+ T-cells, indicating that triggering was not dependent on a particular receptor orientation. In contrast to the minimal influence of receptor orientation, T-cell triggering was affected by the inter-molecular distance between MHC molecules, and MHC dimers coupled through shorter cross-linkers were consistently more potent than those coupled through longer cross-linkers. These results are consistent with a mechanism in which intermolecular receptor proximity, but not intermolecular orientation, is the key determinant for antigen-induced CD4+ T-cell activation.