New views of the immunological synapse: variations in assembly and function

Periodic Membrane Potential and Ca2+ Oscillations in T Cells Forming an Immune Synapse

The immunological synapse (IS) is a specialized contact area formed between a T cell and an antigen presenting cell (APC). Besides molecules directly involved in antigen recognition such as the TCR/CD3 complex, ion channels important in the membrane potential and intracellular free Ca²⁺ concentration control of T cells are also recruited into the IS. These are the voltage-gated Kv1.3 and Ca²⁺-activated KCa3.1 K⁺ channels and the calcium release-activated Ca²⁺ channel (CRAC). However, the consequence of this recruitment on membrane potential and Ca²⁺ level control is not known. Here we demonstrate that the membrane potential (MP) of murine T cells conjugated with APCs in an IS shows characteristic oscillations. We found that depolarization of the membrane by current injection or by increased extracellular K⁺ concentration produced membrane potential oscillations (MPO) significantly more frequently in conjugated T cells than in lone T cells. Furthermore, oscillation of the free intracellular Ca²⁺ concentration could also be observed more frequently in cells forming an IS than in lone cells. We suggest that in the IS the special arrangement of channels and the constrained space between the interacting cells creates a favorable environment for these oscillations, which may enhance the signaling process leading to T cell activation.

Direct cell-cell communication with three-dimensional cell morphology on wrinkled microposts

Cell-cell communication plays a critical role in a myriad of processes, such as homeostasis, angiogenesis, and carcinogenesis, in multi-cellular organisms. Monolayer cell models have notably improved our understanding of cellular interactions. However, the cultured cells on the planar surfaces adopt a two-dimensional morphology, which poorly imitates cellular organization in vivo, providing physiologically-irrelevant cell responses. Non-planar surfaces comprising various patterns have demonstrated great abilities in directing cellular growth and producing different cell morphologies. In recent years, a few topographical substrates have provided valuable information about cell-cell signalling, however, none of these studies have reported a three-dimensional (3D) cell morphology. Here, we introduce a structurally tunable topographical platform that can maintain cell coupling while inducing a true 3D cell morphology. Optical imaging and fluorescence recovery after photobleaching are used to illustrate these capabilities. Our analyses suggest that the intercellular signalling on the present platform, which we propose is mainly through gap junctions, is comparable to that in natural tissue.

STATEMENT OF SIGNIFICANCE

A better understanding of direct cellular communication can help treating neurological diseases and cancers, which may be caused by dysfunctional intercellular signaling. To investigate cell-cell contact, cells are conventionally plated onto planar surfaces, where they flatten and adopt a two-dimensional cell morphology. These unrealistic models are physiologically-irrelevant since cells exhibit a three-dimensional (3D) shape in the body. Therefore, porous scaffolds and topographical surfaces, capable of inducing various cell morphologies, have been introduced, in which the latter is more desirable for sample imaging and screening. However, the few non-planar substrates used to study cell coupling have not produced a 3D cell shape. Here, we present a tunable culture platform that can control direct cell-cell communication while maintaining true 3D cell morphologies.

What is the evidence for the role of TRP channels in inflammatory and immune cells?

A complex network of many interacting mechanisms orchestrates immune and inflammatory responses. Among these, the cation channels of the transient receptor potential (TRP) family expressed by resident tissue cells, inflammatory and immune cells and distinct subsets of primary sensory neurons, have emerged as a novel and interrelated system to detect and respond to harmful agents. TRP channels, by means of their direct effect on the intracellular levels of cations and/or through the indirect modulation of a large series of intracellular pathways, orchestrate a range of cellular processes, such as cytokine production, cell differentiation and cytotoxicity. The contribution of TRP channels to the transition of inflammation and immune responses from a defensive early response to a chronic and pathological condition is also emerging as a possible underlying mechanism in various diseases. This review discusses the roles of TRP channels in inflammatory and immune cell function and provides an overview of the effects of inflammatory and immune TRP channels on the pathogenesis of human diseases.

Calcium-dependent permeabilization of erythrocytes by a perforin-like protein during egress of malaria parasites

Clinical malaria is associated with proliferation of blood-stage parasites. During the blood stage, Plasmodium parasites invade host red blood cells, multiply, egress and reinvade uninfected red blood cells to continue the life cycle. Here we demonstrate that calcium-dependent permeabilization of host red blood cells is critical for egress of Plasmodium falciparum merozoites. Although perforin-like proteins have been predicted to mediate membrane perforation during egress, the expression, activity and mechanism of action of these proteins have not been demonstrated. Here, we show that two perforin-like proteins, perforin-like protein 1 and perforin-like protein 2, are expressed in the blood stage. Perforin-like protein 1 localizes to the red blood cell membrane and parasitophorous vacuolar membrane in mature schizonts following its Ca(2+)-dependent discharge from micronemes. Furthermore, perforin-like protein 1 shows Ca(2+)-dependent permeabilization and membranolytic activities suggesting that it may be one of the effector proteins that mediate Ca(2+)-dependent membrane perforation during egress.

Antigen-specific blocking of CD4-specific immunological synapse formation using BPI and current therapies for autoimmune diseases

In this review, we discuss T-cell activation, etiology, and the current therapies of autoimmune diseases (i.e., MS, T1D, and RA). T-cells are activated upon interaction with antigen-presenting cells (APC) followed by a "bull's eye"-like formation of the immunological synapse (IS) at the T-cell-APC interface. Although the various disease-modifying therapies developed so far have been shown to modulate the IS and thus help in the management of these diseases, they are also known to present some undesirable side effects. In this study, we describe a novel and selective way to suppress autoimmunity by using a bifunctional peptide inhibitor (BPI). BPI uses an intercellular adhesion molecule-1 (ICAM-1)-binding peptide to target antigenic peptides (e.g., proteolipid peptide, glutamic acid decarboxylase, and type II collagen) to the APC and therefore modulate the immune response. The central hypothesis is that BPI blocks the IS formation by simultaneously binding to major histocompatibility complex-II and ICAM-1 on the APC and selectively alters the activation of T cells from T(H)1 to T(reg) and/or T(H)2 phenotypes, leading to tolerance.

Mechanistic insight into pertussis toxin and lectin signaling using T cells engineered to express a CD8α/CD3ζ chimeric receptor

Mammalian cell-surface receptors typically display N- or O-linked glycans added post-translationally. Plant lectins such as phytohemagluttinin (PHA) can activate the T cell receptor (TCR) and other cell-surface receptors by binding to glycans and initiating receptor cross-linking. Pathogenic microorganisms such as Bordetella pertussis also express proteins with lectin-like activities. Similar to plant lectins, pertussis toxin (PTx) can activate the TCR and bind to a variety of glycans. However, whether the lectin-like activity of PTx is responsible for its ability to activate TCR signaling has not been formally proven. Here we examined the ability of PTx and a panel of lectins to activate the TCR or a CD8α/CD3ζ chimeric receptor (termed CD8ζ). We demonstrate that CD8ζ rescues PTx-induced signaling events lacking in TCR null cells. This result indicates that CD8ζ can substitute for TCR and supports the hypothesis that PTxB (functioning as a lectin) stimulates signaling via receptor cross-linking rather than by binding to a specific epitope on the TCR. Moreover, PTx is able to activate signaling by binding either N-linked or O-linked glycan-modified receptors as the TCR displays N-linked glycans while CD8ζ displays O-linked glycans. Finally, studies with a diverse panel of lectins indicate that the signaling activity of the lectins does not always correlate with the biochemical reports of ligand preferences. Comparison of lectin signaling through TCR or CD8ζ allows us to better define the structural and functional properties of lectin-glycan interactions using a biologically based signaling readout.

Apicomplexan perforin-like proteins

Numerous perforin-like proteins are encoded in the genomes of apicomplexan parasites, where they are expressed in various life-cycle stages and play critical roles in pathogenesis and lifecycle progression. These ApiPLPs are characterized by the presence of a MACPF domain, responsible for pore-formation in target membranes in a number of systems, including many bacterial pathogens and effector cells of the immune response. ApiPLP MACPF domains maintain the critical structural elements but are often present in new and intriguing domain arrangements. Recent work in Toxoplasma and Plasmodium has shown that ApiPLPs are important for breaching membranes during parasite egress and cell traversal. Here we present an overview of this important protein family from a structural, functional and phylogenetic perspective across the Apicomplexa.

Docking of lytic granules at the immunological synapse in human CTL requires Vti1b-dependent pairing with CD3 endosomes

Lytic granule (LG)-mediated apoptosis is the main mechanism by which CTL kill virus-infected and tumorigenic target cells. CTL form a tight junction with the target cells, which is called the immunological synapse (IS). To avoid unwanted killing of neighboring cells, exocytosis of lytic granules (LG) is tightly controlled and restricted to the IS. In this study, we show that in activated human primary CD8(+) T cells, docking of LG at the IS requires tethering LG with CD3-containing endosomes (CD3-endo). Combining total internal reflection fluorescence microscopy and fast deconvolution microscopy (both in living cells) with confocal microscopy (in fixed cells), we found that LG and CD3-endo tether and are cotransported to the IS. Paired but not single LG are accumulated at the IS. The dwell time of LG at the IS is substantially enhanced by tethering with CD3-endo, resulting in a preferential release of paired LG over single LG. The SNARE protein Vti1b is required for tethering of LG and CD3-endo. Downregulation of Vti1b reduces tethering of LG with CD3-endo. This leads to an impaired accumulation and docking of LG at the IS and a reduction of target cell killing. Therefore, Vti1b-dependent tethering of LG and CD3-endo determines accumulation, docking, and efficient lytic granule secretion at the IS.

Antigen presenting and effector cell cluster formation in BALB/c mice during mousepox: model studies*

AIMS

The objective of this study was to access APC-effector cell cluster formation in genetically susceptible BALB/c (H-2(d) ) mice infected with highly virulent Moscow strain of ectromelia virus (ECTV-MOS) and estimate of lymphocyte activation based upon expression of CD62L and CD44 molecules.

METHODS AND RESULTS

APC-effector cell clusters were obtained by enzymatic digestion from draining lymph nodes (DLNs) and spleens of BALB/c mice. We found that APCs infected with ECTV-MOS form unstable clusters with effector cells, and thus may diminish T-cell activation at the early stage of mousepox. Different types of effector cells including T-cell subsets (CD4(+) and CD8(+) ), B cells and polymorphonuclear cells colocalize within individual clusters. Increase in CD19(+) B cells within APC-effector cell clusters during severe clinical mousepox may reflect B-cell activation.

CONCLUSIONS

Our studies indicated vigorous changes in APC-effector cell cluster formation in genetically susceptible BALB/c mice during mousepox (up to 2 weeks). ECTV-MOS can modulate APC interactions with effector cells and consequently may impair T-cell activation probably owing to unstable cluster formation and/or subsequent weak stimulation by infected APCs at the early stages of mousepox.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report of APC-effector cell cluster formation in BALB/c mice during mousepox. It gives us a new light on the mutual cell-cell interactions and development of the immune response during ECTV-MOS infection.

What is the function of the dendritic cell side of the immunological synapse?

The adaptive immune response requires the formation of a specialized interface called the immunological synapse (IS), which is formed between a mature dendritic cell (DC) and a CD4(+) T cell in the lymph node. The IS involves organized motifs formed by cell-surface and cytoplasmic molecules at both the DC side (IS-DC) and the T cell side (IS-T) of the IS. Most studies of the functions of the IS have focused on the IS-T; however, to understand the function(s) of the entire IS, it is also necessary to gain insight into the role(s) of the IS-DC. Unlike T cells, which upon their activation leave the lymph node and return to the circulation, DCs largely become apoptotic and die in the node region. This latter observation and the known stability of the IS, which may last for hours, is consistent with the hypothesis that one of the functions of the IS-DC could be the temporal inhibition of the apoptosis of DCs, which would enable the activation of clonal T cells in the lymph nodes. Here, we discuss experimental data supporting the latter hypothesis, as well as the concept that the IS-DC is a signaling region that contributes to the functions of the IS.

Efficiency of T-cell costimulation by CD80 and CD86 cross-linking correlates with calcium entry

Costimulation is a fundamental principle of T-cell activation. In addition to T-cell receptor engagement, the interaction between CD80 and/or CD86 with CD28 and/or cytotoxic T-lymphocyte antigen 4 (CTLA-4) receptors is required to regulate T-cell activation and tolerance. While the importance of costimulation is clearly established, the exact molecular mechanism is unknown. We demonstrate that T-cell proliferation and the ability of CD8(+) T-effector cells to kill were enhanced slightly by CD80 but dramatically by CD86 costimulation. To further analyse the cellular process of costimulation, we developed a single-cell assay to analyse Ca(2+) signals following costimulation with bi-specific antibodies. We found that this stimulation method worked in every human T-cell that was analysed, making it one of the most efficient T-cell activation methods to date for primary human T cells. The enhanced proliferation and killing by costimulation was paralleled by an increase of Ca(2+) influx following CD86 costimulation and it was dependent on CD28/CTLA-4 expression. The enhanced Ca(2+) influx following CD86 costimulation was abrogated by an antibody that interfered with CD28 function. The differences in Ca(2+) influx between CD80 and CD86 costimulation were not dependent on the depletion of Ca(2+) stores but were eliminated by the application of 10 mum 2-aminoethyldiphenyl borate which has recently been shown to enhance stromal interaction molecule 2 (STIM2)-dependent Ca(2+) entry while reducing STIM1-dependent Ca(2+) entry. Our data indicate that differences in the efficiency of costimulation are linked to differences in Ca(2+) entry.

Spatiotemporal patterning during T cell activation is highly diverse

Temporal and spatial variations in the concentrations of signaling intermediates in a living cell are important for signaling in complex networks because they modulate the probabilities that signaling intermediates will interact with each other. We have studied 30 signaling sensors, ranging from receptors to transcription factors, in the physiological activation of murine ex vivo T cells by antigen-presenting cells. Spatiotemporal patterning of these molecules was highly diverse and varied with specific T cell receptors and T cell activation conditions. The diversity and variability observed suggest that spatiotemporal patterning controls signaling interactions during T cell activation in a physiologically important and discriminating manner. In support of this, the effective clustering of a group of ligand-engaged receptors and signaling intermediates in a joint pattern consistently correlated with efficient T cell activation at the level of the whole cell.

CD44 mediates successful interstitial navigation by killer T cells and enables efficient antitumor immunity

Although T lymphocytes are constitutively nonadherent cells, they undergo facultative polarity during migration and upon interaction with cells presenting cognate antigen, suggesting that cell polarity might be critical for target cell destruction. Using two-photon imaging of tumor-infiltrating T lymphocytes, we found that CD44, a receptor for extracellular matrix proteins and glycosaminoglycans, was crucial for interstitial T cell navigation and, consequently, efficient tumor cell screening. CD44 functioned as a critical regulator of intratumoral movement by stabilizing cell polarity in migrating T cells, but not during target cell interactions. Stable anterior-posterior asymmetry was maintained by CD44 independently of its extracellular domain. Instead, migratory polarity depended on the recruitment of ezrin, radixin, moesin (ERM) proteins by the intracellular domain of CD44 to the posterior cellular protrusion. Our results formally demonstrate that CD44-dependent T lymphocyte locomotion within target sites represents an essential immunologic checkpoint that determines the potency of T cell effector functions.

Dendritic cell-based immunotherapy in myeloid leukaemia: translating fundamental mechanisms into clinical applications

Immunotherapy for leukaemia patients, aiming at the generation of anti-leukaemic T cell responses, could provide a new therapeutic approach to eliminate minimal residual disease (MRD) cells in acute myeloid leukaemia (AML). Leukaemic blasts harbour several ways to escape the immune system including deficient MHC class II expression, low levels of co-stimulatory molecules and suppressive cytokines. Therapeutic vaccination with dendritic cells (DC) is now recognized as an important investigational therapy. Due to their unique antigen presenting capacity, immunosuppressive features of the leukaemic blasts can be circumvented. DC can be successfully cultured from leukaemic blasts in 60-70% of patients and show functional potential in vivo. Alternatively, monocyte derived DC obtained at time of complete remission loaded with leukaemia-specific antigens can be used as vaccine. Several sources of leukaemia-associated antigen and different methods of loading antigen onto DC have been used in an attempt to optimize antitumour responses including apoptotic cells, necrotic cell lysates and tumour-associated pep-tides. Currently, the AML-derived cell line MUTZ-3, an immortalized equivalent of CD34(+) DC precursor cells, is under investigation for vaccination purposes. For effective DC vaccination the intrinsic tolerant state of the patient must be overcome. Therefore, the development of efficient and safe adjuvants in antigen specific immunotherapeutic programs should be encouraged.

Morphological changes of T cells following formation of the immunological synapse modulate intracellular calcium signals

Sustained Ca(2+) influx through plasma membrane Ca(2+) released-activated Ca(2+) (CRAC) channels is essential for T cell activation. Since inflowing Ca(2+) inactivates CRAC channels, T cell activation is only possible if Ca(2+)-dependent inactivation is prevented. We have previously reported that sustained Ca(2+) influx through CRAC channels requires both mitochondrial Ca(2+) uptake and mitochondrial translocation towards the plasma membrane in order to prevent Ca(2+)-dependent channel inactivation. Here, we show that morphological changes following formation of the immunological synapse (IS) modulate Ca(2+) influx through CRAC channels. Cell shape changes were dependent on the actin cytoskeleton, and they sustained Ca(2+) entry by bringing mitochondria and the plasma membrane in closer proximity. The increased percentage of mitochondria beneath the plasma membrane following shape changes occurred in all 3 dimensions and correlated with an increase in the amplitude of Ca(2+) signals. The shape change-dependent mitochondrial localization close to the plasma membrane prevented CRAC channel inactivation even in T cells in which dynein motor protein-dependent mitochondria movements towards the plasma membrane were completely abolished, highlighting the importance of the shape change-dependent control of Ca(2+) influx. Our results suggest that morphological changes do not only facilitate an efficient contact with antigen presenting cells but also strongly modulate Ca(2+) dependent T cell activation.

The actin cytoskeleton in T cell activation

T cell cytoarchitecture differs dramatically depending on whether the cell is circulating within the bloodstream, migrating through tissues, or interacting with antigen-presenting cells. The transition between these states requires important signaling-dependent changes in actin cytoskeletal dynamics. Recently, analysis of actin-regulatory proteins associated with T cell activation has provided new insights into how T cells control actin dynamics in response to external stimuli and how actin facilitates downstream signaling events and effector functions. Among the actin-regulatory proteins that have been identified are nucleation-promoting factors such as WASp, WAVE2, and HS1; severing proteins such as cofilin; motor proteins such as myosin II; and linker proteins such as ezrin and moesin. We review the current literature on how signaling pathways leading from diverse cell surface receptors regulate the coordinated activity of these and other actin-regulatory proteins and how these proteins control T cell function.

T cell activation requires mitochondrial translocation to the immunological synapse

T helper (Th) cell activation is required for the adaptive immune response. Formation of the immunological synapse (IS) between Th cells and antigen-presenting cells is essential for Th cell activation. IS formation induces the polarization and redistribution of many signaling molecules; however, very little is known about organelle redistribution during IS formation in Th cells. We show that formation of the IS induced cytoskeleton-dependent mitochondrial redistribution to the immediate vicinity of the IS. Using total internal reflection microscopy, we found that upon stimulation, the distance between the IS and mitochondria was decreased to values<200 nm. Consequently, mitochondria close to the IS took up more Ca2+ than the ones farther away from the IS. The redistribution of mitochondria to the IS was necessary to maintain Ca2+ influx across the plasma membrane and Ca2+-dependent Th cell activation. Our results suggest that mitochondria are part of the signaling complex at the IS and that their localization close to the IS is required for Th cell activation.

T-cell-receptor-dependent actin regulatory mechanisms

Following stimulation, T cells undergo marked changes in actin architecture that are required for productive immune responses. T-cell-receptor-dependent reorganization of the actin cytoskeleton is necessary for the formation of the immunological synapse at the T-cell-antigen-presenting-cell contact site and the distal pole complex at the opposite face of the T cell. Convergence of specific signaling molecules within these two plasma membrane domains facilitates downstream signaling events leading to full T-cell activation. Recent studies have identified many of the relevant actin-regulatory proteins, and significant progress has been made in our understanding of how these proteins choreograph molecular movements associated with T-cell activation. Proteins such as WASp, WAVE2, HS1 and cofilin direct the formation of a cortical actin scaffold at the immune synapse, while actin-binding proteins such as ezrin and moesin direct binding of signaling molecules to actin filaments within the distal pole complex.

B and T lymphocyte attenuator interacts with CD3zeta and inhibits tyrosine phosphorylation of TCRzeta complex during T-cell activation

B and T lymphocyte attenuator (BTLA) is an important negative regulator of T-cell activation. T-cell activation involves partitioning of receptors into discrete membrane compartments known as lipid rafts and the formation of an immunological synapse (IS) between the T cell and antigen-presenting cell (APC). Here we show that after T-cell stimulation, BTLA co-clusters with the CD3zeta and is then involved in IS, as determined by a two-photon microscope. BTLA can interact with the phosphorylated form of T-cell receptor (TCR) within the lipid raft, which is associated with the T-cell signaling complex. Coligation of BTLA with the TCR significantly decreased the amount of phosphorylated TCR-related signal accumulation in the lipid raft during T-cell activation. These results suggest that BTLA functions to regulate T-cell signaling by controlling the phosphorylated form of TCRzeta accumulation in the lipid raft.

CTLs Target Th Cells That Acquire Bystander MHC Class I-Peptide Complex from APCs

CTLs can acquire MHC class I-peptide complexes from their target cells, whereas CD4(+) T cells obtain MHC class II-peptide complexes from APCs in a TCR-specific manner. As a consequence, Ag-specific CTL can kill each other (fratricide) or CD4(+) T cells become APCs themselves. The purpose of the acquisition is not fully understood and may be either inhibition or prolongation of an immunological response. In this study, we demonstrate that human CD4(+) Th cells are able to capture membrane fragments from APC during the process of immunological synapse formation. The fragments contain not only MHC class II-peptide complexes but also MHC class I-peptide complexes, rendering these cells susceptible to CTL killing in an Ag-specific manner. The control of CD4(+) Th cells by Ag-specific CTL, therefore, maybe another mechanism to regulate CD4(+) T cell expansion in normal immune responses or cause immunopathology during the course of viral infections such as HIV.

Global analysis of dynamic changes in lipid raft proteins during T-cell activation

Lipid rafts are considered as specialized microdomains within the plasma membrane with unique lipid compositions different from surrounding membranes. Following T-cell receptor (TCR) stimulation, lipid rafts assemble in T-cell/antigen-presenting cell (APC) contact site known as the immunological synapse, inner leaflets of which serve as activation or docking sites for downstream signaling components. To understand the signaling events occurring in lipid rafts, we globally analyzed dynamic changes in lipid raft proteins during TCR/CD28 costimulation using 2-D fluorescence difference gel electrophoresis. We detected multiple spots whose intensities were enhanced after costimulation, and identified proteins in these spots by PMF. Identified proteins include Src family tyrosine kinases, tyrosine phosphatase, phosphatidylinositol 3-kinase (PI3-kinase), actin-binding proteins, and regulators for small GTPases. Of particular interest, a number of pleckstrin homology (PH) domain-containing proteins were identified. Biochemical and histochemical analyses confirmed the translocation of these proteins from cytosol to lipid rafts. We also demonstrated that these proteins assembled at the T-cell/APC interface. These results indicate the efficacy of our system to systematically analyze dynamics of lipid raft proteins during extracellular stimulation.

Modulation of immunological synapse by membrane-bound and soluble ligands

An efficient adaptive immune response should prevent pathogen infections and tumor growth without causing significant damage to host constituents. A crucial event determining the balance between tolerance and immunity is antigen recognition by T cells on the surface of antigen presenting cells (APC). Several molecular contacts at the interface between T cells and APCs contribute to define the nature of the adaptive immune response against a particular antigen. Upon TCR engagement by a peptide-MHC complex (pMHC) on the surface of an APC, a specialized supra-molecular structure known as immunological synapse (IS) assembles at the interface between these two cells. This structure involves massive re-distribution of membrane proteins, including TCR and pMHC complexes, as well as co-stimulatory and adhesion molecules. Furthermore, IS assembly leads to several important intracellular events necessary for T cell activation, such as recruitment of signaling molecules and cytoskeleton rearrangements. Because IS assembly leads to major consequences on the function of T cells, several studies have attempted to identify both soluble and membrane-bound molecules that could contribute to modulate the IS function. Here we describe recent literature on the regulation of IS assembly and modulation by TCR/pMHC binding kinetics, chemokines and cytokines focusing on their role at controlling the balance between adaptive immunity and tolerance.

T-cell avidity and tuning: the flexible connection between tolerance and autoimmunity

Thymic T-cell selection mechanisms generate a cross-reactive, self-MHC restricted peripheral T-cell pool. Affinity and avidity are of profound influence on this selection and the generation of immunity. Autoreactive T cells can escape thymic deletion by lowering their avidity and retain this "tuned" state in the periphery. Upon activation, tuned T cells can cause autoimmunity, while immunotherapeutic strategies may be hampered by existing T-cell tolerance. The regulation of T-cell avidity and tuning therefore determines the balance between tolerance and autoimmunity and should be taken into account in the design of therapeutic strategies aimed at T-cell reactivity.

A molecular perspective of CTLA-4 function

Within the paradigm of the two-signal model of lymphocyte activation, the interest in costimulation has witnessed a remarkable emergence in the past few years with the discovery of a large array of molecules that can serve this role, including some with an inhibitory function. Interest has been further enhanced by the realization of these molecules' potential as targets to modulate clinical immune responses. Although the therapeutic translation of mechanistic knowledge in costimulatory molecules has been relatively straightforward, the capacity to target their inhibitory counterparts has remained limited. This limited capacity is particularly apparent in the case of the cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), a major negative regulator of T cell responses. Because there have been several previous comprehensive reviews on the function of this molecule, we focus here on the physiological implications of its structural features. Such an exercise may ultimately help us to design immunotherapeutic agents that target CTLA-4.

Mechanisms and Role of HLA and non-HLA Alloantibodies

The role of alloantibodies against HLA and non-HLA targets is becoming increasingly recognized as critical in the pathogenesis of acute and chronic renal allograft outcomes. This review discusses the antigenic targets, the mechanisms of T and B cell activation that result in the production of antibody, the complement cascade, methods of antibody detection, and the evidence that alloantibody-mediated mechanisms are active in acute and chronic rejection.

Engulfment-regulated proteolysis of SpoIIQ: evidence that dual checkpoints control sigma activity

During Bacillus subtilis sporulation, the engulfment checkpoint is thought to directly regulate late forespore transcription but to indirectly regulate late mother cell transcription, via the sigmaG-produced protease SpoIVB. We here demonstrate that SpoIIQ is subject to sigmaG-independent, but engulfment-dependent, proteolysis that depends on SpoIVB. Thus, SpoIVB produced before engulfment supports some SpoIVB-dependent events, suggesting that its activity or access to substrates must be regulated by engulfment. Furthermore, a mutation (bofA) that allows sigmaK to be active without sigmaG does not allow sigmaK activity in engulfment mutants, although the pro-sigmaK processing enzyme (SpoIVFB) is localized to the septum in engulfment mutants, suggesting that engulfment comprises a second checkpoint for sigmaK Finally, we find that SpoIIQ and another protein required for sigmaG activity (SpoIIIAH), which directly interact and assemble helical structures around the forespore, recruit the sigmaK-processing enzyme SpoIVFB to the forespore and these structures. We suggest that these foci serve a synapse-like role, allowing engulfment to simultaneously control both sigmaG and sigmaK, and integrating multiple checkpoints and signalling pathways.

The role of supramolecular protein complexes and membrane potential in transmembrane signaling processes of lymphocytes

The formation of protein patterns in lymphocyte plasma membranes is analyzed in the light of past and, also, very recent experiments. The analysis surveys the lateral organization of major histocompatibility complex glycoproteins, intercellular adhesion molecule-1, interleukin-2 and -15 receptors, Kv1.3 K+ ion channels and the T-cell receptor as well as their behavior under different conditions. These molecules form small- and large-scale clusters in the membrane of human lymphocytes. Many of the association motifs occur in other investigated cell types. The conclusions point toward a possible role for ion channel activities, membrane potential changes and alterations of the lateral organization of proteins in transmembrane signaling and cytotoxic interactions. In our outlook new factors that potentially affect membrane protein cluster formation and interactions are discussed. A role for MHC glycoproteins in concentrating membrane proteins and organizing protein patterns is suggested, and the possibility that the membrane potential may modulate protein conformation and, thereby, affect protein-protein interactions is pointed out. A well-defined role for the presence of ion channels in the immune synapse is offered, which could explain the significance of ion channel accumulation in the immune synapse together with the T-cell receptor.

POLKADOTS are foci of functional interactions in T-Cell receptor-mediated signaling to NF-kappaB

Stimulation of the T-cell receptor (TCR) results in the activation of several transcription factors, including NF-kappaB, that are crucial for T-cell proliferation and gain of effector functions. On TCR engagement, several proteins within the TCR-directed NF-kappaB signaling pathway undergo dynamic spatial redistribution, but the significance of these redistribution events is largely unknown. We have previously described TCR-induced cytoplasmic structures called POLKADOTS (punctate and oligomeric killing or activating domains transducing signals) that are enriched in the NF-kappaB signaling intermediate, Bcl10. We now show that these structures are formed only under conditions that promote efficient NF-kappaB activation. Furthermore, POLKADOTS formation is dependent on functional domains of specific NF-kappaB signal transducers. Through use of a photoactivatable GFP, we demonstrate that POLKADOTS contain both a highly stable and a rapidly equilibrating protein component. FRET analyses show that POLKADOTS are sites of enriched interactions between Bcl10 and partner signaling proteins. These observations strongly suggest that POLKADOTS are focal sites of dynamic information exchange between cytosolic intermediates in the process of TCR activation of NF-kappaB.

Yeast surface display of a noncovalent MHC class II heterodimer complexed with antigenic peptide

Microbial protein display technologies have enabled directed molecular evolution of binding and stability properties in numerous protein systems. In particular, dramatic improvements to antibody binding affinity and kinetics have been accomplished using these tools in recent years. Examples of successful application of display technologies to other immunological proteins have been limited to date. Herein, we describe the expression of human class II major histocompatibility complex allele (MHCII) HLA-DR4 on the surface of Saccharomyces cerevisiae as a noncovalently associated heterodimer. The yeast-displayed MHCII is fully native as assessed by binding of conformationally specific monoclonal antibodies; failure of antibodies specific for empty HLA-DR4 to bind yeast-displayed protein indicates antigenic peptide is bound. This report represents the first example of a noncovalent protein dimer displayed on yeast and of successful display of wild-type MHCII. Results further point to the potential for using yeast surface display for engineering and analyzing the antigen binding properties of MHCII.

Immunology of the central nervous system

Immunology of the central nervous system (CNS) is a growing field of study. Until recently the brain was considered an 'immunologically privileged' site. It is increasingly apparent that the CNS has a significant but tightly regulated capability to mount an inflammatory and immune response. This article serves as an introduction to the special section at the start of this issue on neuroimmunology. We also focus on several immunological concepts that are particularly relevant in the context of neuroimmunology-cross-reactivity, the immunological synapse and the nature of the immune response to transplantation in the CNS. We conclude that the fundamental concepts are common to all branches of immunology. Better understanding of the basic mechanisms will blur the borders between the different areas of immunology.

The connective connection: interstitial cells of Cajal (ICC) and ICC-like cells establish synapses with immunoreactive cells. Electron microscope study in situ

We present transmission electron microscope (TEM) evidence that ICC and ICC-like cells frequently establish close contacts (synapses) with several types of immunoreactive cells (IRC): lymphocytes, plasma cells, eosinophils, basophils, macrophages and mast cells. Such synapses were found in various organs: human mammary gland and myometrium, as well as rat stomach, gut, bladder and uterus. Specimens were observed by conventional TEM on ultrathin sections. Based on morphometric analyses and computer-aided 3-D reconstructions from serial sections, we propose an operational definition of ICC-IRC synapses: cell-to-cell close contacts where the two cells are separated by only approximately 15 nm, equivalent to twice the plasmalemmal thickness. Two types of such synapses were found: (i) uniform ('plain') synapses (PS). close contact extending for >200 nm, and (ii) multi-contact ('kiss and run') synapses (MS)--with multiple, focal, close-contact points alternating with regions of wider intermembrane distance. For instance, a typical PS between a rat bladder ICC-like cell and an eosinophil was 2.48 microm long and 11+/-4 nm wide. By contrast, a MS synapse in rat myometrium (between an ICC-like cell and an eosinophil) was 8.64 microm long and had 13 contact points. The synaptic cleft measured 15+/-8 nm at contact points and approximately 100 nm or more in wider areas. These synapses are different from gap junctions usually seen between ICC and between ICC and smooth muscle cells. We previously proposed that ICC-like cells might represent stromal progenitor cells, participate in juxtacrine/paracrine signaling and play a role in immune surveillance. The nanoscopic distances between the two contiguous membranes suggest a juxtacrine cell-to-cell signaling (chemical synapse), via juxtacrinins, a specific case of phenomenins. However, the (micro)vesicles found in the synaptic cleft may correspond to an exosome-based mechanism.

Tuning immune responses: diversity and adaptation of the immunological synapse

The onset and regulation of a specific immune response results from communication between T cells and antigen-presenting cells (APCs), which form molecular interactions at the site of cell-cell contact--and this is known as the immunological synapse. Initially, the immunological synapse was viewed as a stereotypical adhesion and signalling device with a defined molecular structure and signalling processes. However, as we discuss here, T-cell-APC interactions comprise a diverse range of contact modes and distinct molecular arrangements. These diverse interaction modes might define a molecular code, in which the differences in timing, spacing and molecular composition of the signalling platform determine the outcome of T-cell-APC interactions.

Employing the immunological synapse in AML: Development of leukemic dendritic cells for active specific immunization

Cytotoxic T cells directed against leukemic blasts have been observed in patients with acute myeloid leukemia (AML). However, generation of efficient T-cell responses is hampered due to several factors that enable AML blasts to protect themselves from the patients immune system. Improved immune responses can be established by the differentiation of AML blasts into AML-derived dendritic cells (DC) thereby conserving their intrinsic leukemia specific antigens and obtaining full capacity to present these antigens to naive T cells. This review discusses increased immunogenicity of AML blasts by differentiation into AML-DC and describes ways to augment the AML-DC vaccination approach.

Dynamic recruitment of PAK1 to the immunological synapse is mediated by PIX independently of SLP-76 and Vav1

T cell receptor engagement activates p21-activated kinase 1 (PAK1) through a LAT-SLP-76-Nck-Vav-Rac-dependent pathway. A second independent pathway involving a GIT-PIX-PAK1 trimolecular complex is also activated by T cell receptor ligation. Here we show a Vav-independent pathway exists that leads to PAK1 activation. In addition, PAK1, PIX and GIT1 were recruited to the T cell-antigen-presenting cell contact site independently of SLP-76 and Vav1. PAK1 recruitment to the T cell-antigen-presenting cell interface required interaction with PIX, which also led to optimal PLC-gamma1 activation and T cell receptor-dependent transcriptional responses. These data indicate that a pathway involving the GIT-PIX-PAK1 complex has a crucial function in PAK1 activation by recruiting PAK1 to the immunological synapse.

Effects of the geometry of the immunological synapse on the delivery of effector molecules

Recent experiments focusing on the function of the immunological synapse formed between a T cell and an antigen-presenting cell raise many questions about its purpose. We examine the proposal that the close apposition of the cell membranes in the central region of the synapse acts to focus T-cell secretions on the target cell, thus reducing the effect on nearby cells. We show that the efficiency of targeted T-cell responses to closely apposed cells is only weakly dependent on the distance between the cells. We also calculate effective (diffusion-limited) rates of binding and unbinding for molecules secreted within the synapse. We apply our model to the stimulation of B cells by secreted interleukin-4 (IL-4), and find that very few molecules of IL-4 need be released to essentially saturate the IL-4 receptors on the B-cell surface.

T and B Cell Cooperation: A Dance of Life and Death

In order for the body to develop a good antibody response, B cells need to react intimately with antigen specific T cells. Experimental evidence using hapten-carriers revealed that T and B cells do not recognize the same epitope and this led to the view that the physical contact is mediated by the antigen. Although the modern concept of antigen presentation has changed our perception on how the antigen can bridge both cells, the basic virtues of earlier bridging models remain. Over the past few years, a number of surface ligand-receptor pathways have been described, most of them belonging either to the CD28/B7 Ig or to the TNF/TNFR-like families. These act in concert, whether they are agonist or antagonist, in a timely and spatially organized manner. They form cascades of successive induction and recruitment to ensure that T-B cooperation is closely controlled at all stages of antibody induction.

Imaging the single cell dynamics of CD4+ T cell activation by dendritic cells in lymph nodes

The adaptive immune response is initiated in secondary lymphoid organs by contact between antigen-bearing dendritic cells (DCs) and antigen-specific CD4+ T cells. However, there is scant information regarding the single cell dynamics of this process in vivo. Using two-photon microscopy, we imaged the real-time behavior of naive CD4+ T cells and in vivo-labeled DCs in lymph nodes during a robust T cell response. In the first 2 h after entry into lymph nodes, T cells made short-lived contacts with antigen-bearing DCs, each contact lasting an average of 11-12 min and occurring mainly on dendrites. Altered patterns of T cell motility during this early stage of antigen recognition promoted serial engagement with several adjacent DCs. Subsequently, T cell behavior progressed through additional distinct stages, including long-lived clusters, dynamic swarms, and finally autonomous migration punctuated by cell division. These observations suggest that the immunological synapse in native tissues is remarkably fluid, and that stable synapses form only at specific stages of antigen presentation to T cells. Furthermore, the serial nature of these interactions implies that T cells activate by way of multiple antigen recognition events.