The immunological synapse: a molecular machine controlling T cell activation

The immunological synapse and the actin cytoskeleton: molecular hardware for T cell signaling

The actin cytoskeleton seems to play two critical roles in the activation of T cells. One of these roles is T cell shape development and movement, including formation of the immunological synapse. The other is the formation of a scaffold for signaling components. This review focuses on the recent convergence of cell biology and immunology studies to explain the role of the actin cytoskeleton in creating the molecular basis for immunological synapse formation and T cell signaling.

Genetic analysis of integrin function in man: LAD-1 and other syndromes

The integrins are cell membrane receptors composed of alpha and beta subunits which orchestrate adhesive events in all tissues of the body (Hynes, R.O., 1992. Integrins: versatility, modulation, and signalling in cell adhesion. Cell 69, 11-25; and Hynes, R.O., 1999. Cell adhesion: old and new questions. Trends Cell Biol. 9, M33-37). At present 18 alpha subunits and 8 beta subunits have been identified which are loosely organised into families. There are three inherited autosomal recessive diseases in man which involve germline mutations in genes coding for integrins. Leukocyte adhesion deficiency-1 (LAD-1) is the result of mutations in the beta2 subunit of the CD11/CD18 integrins, LFA-1, Mac-1, p150,95 and alphadbeta2. The bleeding disorder Glanzmann thrombasthenia is caused by mutations in either the alpha or beta subunit of the platelet integrin, alphaIIbbeta3. Thirdly, it is now recognised than one of the variants of the usually lethal skin blistering disorder, epidermolysis bullosa (JEB-PA), is caused by mutation in either the alpha or beta subunit of the epithelial hemidesmosome integrin, alpha6beta4. Many of the mutations cause defective alphabeta heterodimer formation. The majority of the beta subunit mutations are in the conserved N-terminal region known as the betaI domain. It is suggested that this region participates in alphabeta heterodimer formation.

Altered peptide ligand-mediated TCR antagonism can be modulated by a change in a single amino acid residue within the CDR3 beta of an MHC class I-restricted TCR

The Ag receptor of cytotoxic CD8+ T lymphocytes recognizes peptides of 8-10 aa bound to MHC class I molecules. This Ag recognition event leads to the activation of the CD8+ lymphocyte and subsequent lysis of the target cell. Altered peptide ligands are analogues derived from the original antigenic peptide that commonly carry amino acid substitutions at TCR contact residues. TCR engagement by these altered peptide ligands usually impairs normal T cell function. Some of these altered peptide ligands (antagonists) are able to specifically antagonize and inhibit T cell activation induced by the wild-type antigenic peptide. Despite significant advances made in understanding TCR antagonism, the molecular interactions between the TCR and the MHC/peptide complex responsible for the inhibitory activity of antagonist peptides remain elusive. To approach this question, we have identified altered peptide ligands derived from the vesicular stomatitis virus peptide (RGYVYQGL) that specifically antagonize an H-2Kb/vesicular stomatitis virus-specific TCR. Furthermore, by site-directed mutagenesis, we altered single amino acid residues of the complementarity-determining region 3 of the beta-chain of this TCR and tested the effect of these point mutations on Ag recognition and TCR antagonism. Here we show that a single amino acid change on the TCR CDR3 beta loop can modulate the TCR-antagonistic properties of an altered peptide ligand. Our results highlight the role of the TCR complementarity-determining region 3 loops for controlling the nature of the T cell response to TCR/altered peptide ligand interactions, including those leading to TCR antagonism.

Tyrosine-phosphorylated Vav1 as a point of integration for T-cell receptor- and CD28-mediated activation of JNK, p38, and interleukin-2 transcription

In this study we identified tyrosine-phosphorylated Vav1 as an early point of integration between the signaling routes triggered by the T-cell receptor and CD28 in human T-cell leukemia cells. Costimulation resulted in a prolonged and sustained phosphorylation and membrane localization of Vav1 in comparison to T-cell receptor activation alone. T-cell stimulation induced the recruitment of Vav1 to an inducible multiprotein T-cell activation signaling complex at the plasma membrane. Vav1 activated the mitogen-activated protein kinases JNK and p38. The Vav1-mediated activation of JNK employed a pathway involving Rac, HPK1, MLK3, and MKK7. The costimulation-induced activation of p38 was inhibited by dominant negative forms of Vav1, Rac, and MKK6. Here we show that Vav1 also induces transcription factors that bind to the CD28RE/AP element contained in the interleukin-2 promoter. A detailed mutational analysis of Vav1 revealed a series of constitutively active and nonfunctional forms of Vav1. Almost all inactive versions were mutated in their Dbl homology domain and behaved as dominant negative mutants that impaired costimulation-induced activation of JNK, p38, and CD28RE/AP-dependent transcription. In contrast to NF-AT-dependent transcription, Vav1-mediated transcriptional induction of the CD28RE/AP element in the interleukin-2 promoter could only partially be inhibited by cyclosporin A, suggesting a dual role of Vav1 for controlling Ca(2+)-dependent and -independent events.

Signal transduction by the TCR for antigen

The past several years have seen the beginning of a shift in the way that TCR signal transduction is studied. Although many investigators continue to identify new molecules, particularly adaptor proteins, others have attempted to look at signaling events in a larger cellular context. Thus the identification of distinct formations of signaling molecules at junctions between T cells and antigen-presenting cells, the role of the cytoskeleton and the partitioning of molecules into specialized lipid subdomains have been the subjects of many publications. Such concepts are helping to assemble a blueprint of how the myriad adaptors and kinases fit together to effect T cell activation.

Cross-linking of cell surface receptors enhances cooperativity of molecular adhesion

Cooperativity of molecular adhesion has been proposed as a mechanism for enhanced binding strength of adhesion molecules on the cell surface. Direct evidence for its mechanism, however, has been lacking until now. Atomic force microscopy (AFM) was used to measure the adhesive strength between concanavalin A (Con A) coupled to an AFM tip and Con A receptors on the surface of NIH3T3 fibroblast cells. Cross-linking of receptors with either glutaraldehyde or 3, 3'-dithio-bis(sulfosuccinimidylproprionate) (DTSSP) led to an increase in adhesion that could be attributed to enhanced cooperativity among adhesion complexes. An increase in loading rate due to greater stiffness of fixed cells also contributed to the twofold increase in binding strength. These results show that receptor cross-linking can greatly contribute to a total increase in cell adhesion by creating a shift toward cooperative binding of receptors.

Lck protein tyrosine kinase is a key regulator of T-cell activation and a target for signal intervention by Herpesvirus saimiri and other viral gene products

Protein tyrosine kinases (PTKs) are critically involved in signaling pathways that regulate cell growth, differentiation, activation, and transformation. It is not surprising, therefore, that viruses acquire effector molecules targeting these kinases to ensure their own replication and/or persistence. This review summarizes our current knowledge on Lck, a member of the Src family of PTK, and its viral interaction partners. Lck plays a key role in T lymphocyte activation and differentiation. It is associated with a variety of cell surface receptors and is critical for signal transduction from the T-cell antigen receptor (TCR). Consequently, Lck is targeted by regulatory proteins of T-lymphotropic viruses, especially by the Herpesvirus saimiri (HVS) tyrosine kinase interacting protein (Tip). This oncoprotein physically interacts with Lck in HVS transformed T cells and has an impact on its catalytic activity. However, while Tip inhibits Lck activity in stably expressing cell lines, opposite effects were observed in several in vitro systems. At least in part, this complex situation may be related to the bipartite nature of the interaction surface of the two proteins. Studies on the interrelationships between Lck and its viral partners contribute to the understanding of the mechanisms of T-cell growth regulation, in general, and of viral pathogenicity in particular. In addition, understanding the regulation of Lck activity by viral proteins may serve as a basis for the development of new drugs capable of modifying Lck activity in different pathological situations.

Membrane rafts and signaling by the multichain immune recognition receptors

The recent recognition of the presence of rafts in the plasma membrane and of their involvement in cell signaling has strongly stimulated the search for their function in receptor-mediated signal transduction in lymphocytes. Recent progress suggests that a general feature of membrane rafts is to serve as platforms wherein the signaling cascades triggered through different multichain immune recognition receptors (e.g. the TCR, BCR and FcepsilonRI) are initiated and organized.

Dynamic regulation of activated leukocyte cell adhesion molecule-mediated homotypic cell adhesion through the actin cytoskeleton

Restricted expression of activated leukocyte cell adhesion molecule (ALCAM) by hematopoietic cells suggests an important role in the immune system and hematopoiesis. To get insight into the mechanisms that control ALCAM-mediated adhesion we have investigated homotypic ALCAM-ALCAM interactions. Here, we demonstrate that the cytoskeleton regulates ALCAM-mediated cell adhesion because inhibition of actin polymerization by cytochalasin D (CytD) strongly induces homotypic ALCAM-ALCAM interactions. This induction of cell adhesion is likely due to clustering of ALCAM at the cell surface, which is observed after CytD treatment. Single-particle tracking demonstrated that the lateral mobility of ALCAM in the cell membrane is increased 30-fold after CytD treatment. In contrast, both surface distribution and adhesion of a glycosylphosphatidylinositol (GPI)-anchored ALCAM mutant are insensitive to CytD, despite the increase in lateral mobility of GPI-ALCAM upon CytD treatment. This demonstrates that clustering of ALCAM is essential for cell adhesion, whereas enhanced diffusion of ALCAM alone is not sufficient for cluster formation. In addition, upon ligand binding, both free diffusion and the freely dragged distance of wild-type ALCAM, but not of GPI-ALCAM, are reduced over time, suggesting strengthening of the cytoskeleton linkage. From these findings we conclude that activation of ALCAM-mediated adhesion is dynamically regulated through actin cytoskeleton-dependent clustering.

Induction of T cell anergy by liposomes with incorporated major histocompatibility complex (MHC) II/peptide complexes

PURPOSE

The aim of this study was to use small unilamellar liposomes with incorporated MHC II/peptide complexes as a carrier system for multivalent antigen presentation to CD4 + T cells.

METHODS

Purified peptide pre-loaded MHC II molecules were incorporated into small unilamellar liposomes and tested for their ability to activate A2b T cells. The outcome of T cell activation by such liposomes in the absence of accessory cells was tested via flow cytometry and a T cell anergy assay.

RESULTS

Provided the presence of external co-stimulation, MHC II/ peptide liposomes were able to induce proliferation of the A2b T cell clone. More importantly incubation of these T cells with MHC II/ peptide liposomes in the absence of co-stimulation did not induce proliferation, however, a MHC/peptide ligand-density dependent down-regulation of the TCR was observed. Interestingly, when T cells after incubation with the MHC II/peptide liposomes were restimulated with their specific antigen in the presence of professional APC, these cells were anergic.

CONCLUSIONS

We propose MHC II/peptide liposomes as a novel means to induce T cell anergy. The possibility to prepare 'tailor-made' liposomal formulations may provide liposomes with an important advantage for applications in immunotherapy.

Observing FcepsilonRI signaling from the inside of the mast cell membrane

We have determined the membrane topography of the high-affinity IgE receptor, FcstraightepsilonRI, and its associated tyrosine kinases, Lyn and Syk, by immunogold labeling and transmission electron microscopic (TEM) analysis of membrane sheets prepared from RBL-2H3 mast cells. The method of Sanan and Anderson (Sanan, D.A., and R.G.W. Anderson. 1991. J. Histochem. Cytochem. 39:1017-1024) was modified to generate membrane sheets from the dorsal surface of RBL-2H3 cells. Signaling molecules were localized on the cytoplasmic face of these native membranes by immunogold labeling and high-resolution TEM analysis. In unstimulated cells, the majority of gold particles marking both FcepsilonRI and Lyn are distributed as small clusters (2-9 gold particles) that do not associate with clathrin-coated membrane. Approximately 25% of FcepsilonRI clusters contain Lyn. In contrast, there is essentially no FcepsilonRI-Syk colocalization in resting cells. 2 min after FcepsilonRI cross-linking, approximately 10% of Lyn colocalizes with small and medium-sized FcepsilonRI clusters (up to 20 gold particles), whereas approximately 16% of Lyn is found in distinctive strings and clusters at the periphery of large receptor clusters (20-100 gold particles) that form on characteristically osmiophilic membrane patches. While Lyn is excluded, Syk is dramatically recruited into these larger aggregates. The clathrin-coated pits that internalize cross-linked receptors bud from membrane adjacent to the Syk-containing receptor complexes. The sequential association of FcstraightepsilonRI with Lyn, Syk, and coated pits in topographically distinct membrane domains implicates membrane segregation in the regulation of FcstraightepsilonRI signaling.

Incomplete CD8(+) T lymphocyte differentiation as a mechanism for subdominant cytotoxic T lymphocyte responses to a viral antigen

CD8(+) cytotoxic T lymphocytes (CTLs) recognize antigen in the context of major histocompatibility complex (MHC) class I molecules. Class I epitopes have been classified as dominant or subdominant depending on the magnitude of the CTL response to the epitope. In this report, we have examined the in vitro memory CTL response of H-2(d) haplotype murine CD8(+) T lymphocytes specific for a dominant and subdominant epitope of influenza hemagglutinin using activation marker expression and staining with soluble tetrameric MHC-peptide complexes. Immune CD8(+) T lymphocytes specific for the dominant HA204-210 epitope give rise to CTL effectors that display activation markers, stain with the HA204 tetramer, and exhibit effector functions (i.e., cytolytic activity and cytokine synthesis). In contrast, stimulation of memory CD8(+) T lymphocytes directed to the subdominant HA210-219 epitope results in the generation of a large population of activated CD8(+) T cells that exhibit weak cytolytic activity and fail to stain with the HA210 tetramer. After additional rounds of restimulation with antigen, the HA210-219-specific subdominant CD8(+) T lymphocytes give rise to daughter cells that acquire antigen-specific CTL effector activity and transition from a HA210 tetramer-negative to a tetramer-positive phenotype. These results suggest a novel mechanism to account for weak CD8(+) CTL responses to subdominant epitopes at the level of CD8(+) T lymphocyte differentiation into effector CTL. The implications of these findings for CD8(+) T lymphocyte activation are discussed.

Searching for significance in TCR-cytoskeleton interactions

Two T-cell receptor (TCR) populations are expressed on T cells; one is linked to the cytoskeleton via its zeta chain. These cytoskeleton-linked receptors (30-40% of the total number of TCRs) might be important in TCR-mediated signaling and/or concurrent events. Here, differences between the two populations are summarized, and new data are examined to speculate on the functional significance of cytoskeleton-linked TCRs.

Intercellular adhesion molecule-1 (ICAM-1) expression and cell signaling cascades

The collective interaction between cells is, in part, mediated by different families of adhesion molecules. Intercellular adhesion molecules (ICAMs) are structurally related members of the immunoglobulin supergene family and are ligands for the beta2 integrin molecules present on leukocytes. Of the five ICAMs identified, ICAM-1 is the most extensively studied. Although ICAM-1 is expressed constitutively at low levels on endothelial cells and on some lymphocytes and monocytes, its expression can be significantly increased in the presence of cytokines (TNFalpha, IL-1, IFNgamma) and reactive oxygen species. Depending upon cell type, ICAM-1 participates in trafficking of inflammatory cells, in cell:cell interactions during antigen presentation, in microbial pathogenesis, and in signal transduction through outside-in signaling events. Again, depending upon cell type examined, ICAM-1 engagement has been documented to activate specific kinases through phosphorylation, resulting in transcription factor activation and increased cytokine production, increased cell membrane protein expression, reactive oxygen species production, and cell proliferation.

Transport of peptide-MHC class II complexes in developing dendritic cells

Major histocompatibility complex class II (MHC II) molecules capture peptides within the endocytic pathway to generate T cell receptor (TCR) ligands. Immature dendritic cells (DCs) sequester intact antigens in lysosomes, processing and converting antigens into peptide-MHC II complexes upon induction of DC maturation. The complexes then accumulate in distinctive, nonlysosomal MHC II+ vesicles that appear to migrate to the cell surface. Although the vesicles exclude soluble lysosomal contents and antigen-processing machinery, many contain MHC I and B7 costimulatory molecules. After arrival at the cell surface, the MHC and costimulatory molecules remain clustered. Thus, transport of peptide-MHC II complexes by DCs not only accomplishes transfer from late endocytic compartments to the plasma membrane, but does so in a manner that selectively concentrates TCR ligands and costimulatory molecules for T cell contact.

Entry of B cell receptor into signaling domains is inhibited in tolerant B cells

Signal transduction through the B cell antigen receptor (BCR) is altered in B cells that express a receptor that recognizes self-antigen. To understand the molecular basis for the change in signaling in autoreactive B cells, a transgenic model was used to isolate a homogeneous population of tolerant B lymphocytes. These cells were compared with a similar population of naive B lymphocytes. We show that the BCR from naive B cells enters a detergent-insoluble domain of the cell within 6 s after antigen binding, before a detectable increase in BCR phosphorylation. This fraction appears to be important for signaling because it is enriched for lyn kinase but lacks CD45 tyrosine phosphatase and because the BCR that moves into this domain becomes more highly phosphorylated. Partitioning of the BCR into this fraction is unaffected by src family kinase inhibition. Tolerant B cells do not efficiently partition the BCR into the detergent-insoluble domain, providing an explanation for their reduced tyrosine kinase activation and calcium flux in response to antigen. These results identify an early, regulated step in antigen receptor signaling and self-tolerance.

T cells can use either T cell receptor or CD28 receptors to absorb and internalize cell surface molecules derived from antigen-presenting cells

At the site of contact between T cells and antigen-presenting cells (APCs), T cell receptor (TCR)-peptide-major histocompatibility complex (MHC) interaction is intensified by interactions between other molecules, notably by CD28 and lymphocyte function-associated antigen 1 (LFA-1) on T cells interacting with B7 (B7-1 and B7-2), and intracellular adhesion molecule 1 (ICAM-1), respectively, on APCs. Here, we show that during T cell-APC interaction, T cells rapidly absorb various molecules from APCs onto the cell membrane and then internalize these molecules. This process is dictated by at least two receptors on T cells, namely CD28 and TCR molecules. The biological significance of T cell uptake of molecules from APCs is unclear. One possibility is that this process may allow activated T cells to move freely from one APC to another and eventually gain entry into the circulation.

Induction of human cytotoxic T lymphocytes by artificial antigen-presenting cells

The adoptive transfer of antigen-specific cytotoxic T lymphocytes (CTLs) is a promising therapeutic approach for a number of diseases. To overcome the difficulty in generating specific CTLs, we established stable artificial antigen-presenting cells (AAPCs) that can be used to stimulate T cells of any patient of a given human leukocyte antigen (HLA) type. Mouse fibroblasts were retrovirally transduced with a single HLA-peptide complex along with the human accessory molecules B7.1, ICAM-1, and LFA-3. These AAPCs consistently elicit strong stimulation and expansion of HLA-restricted CTLs. Owing to the high efficiency of retrovirus-mediated gene transfer, stable AAPCs can be readily engineered for any HLA molecule and any specific peptide.

Signaling pathways engaged by NK cell receptors: double concerto for activating receptors, inhibitory receptors and NK cells

Despite the absence of antigen-specific receptors at their surface, NK cells can selectively eliminate virus-infected cells, tumor cells and allogenic cells. A dynamic and precisely coordinated balance between activating and inhibitory receptors governs NK cell activation programs. Multiple activating and inhibitory NK cell surface molecules have been described, a group of them acting as receptors for MHC class I molecules. In spite of their heterogeneity, activating NK cell receptors present remarkable structural and functional homologies with T cell- and B cell-antigen receptors. Inhibitory NK cell receptors operate at early stages of activating cascades by recruiting protein tyrosine phosphatases via intra- cytoplasmic motifs (ITIM), a strategy which is widely conserved in hematopoietic and non-hematopoietic cells.

Regulation of ICAM-1 expression in mouse macrophages

In a mouse model of silica (SI) induced lung injury, SI exposure increases expression of intercellular adhesion molecule-1 (ICAM-1) on lung (alveolar/interstitial) macrophages and alveolar type II epithelial cells. To investigate the regulation of SI induced ICAM-1 expression on mouse macrophages, freshly isolated macrophages (alveolar, peritoneal) and macrophage cell lines (MH-S, RAW 264.7) were evaluated for ICAM-1 expression elicited by the particle silica (alpha quartz; 20 microg/ml; 6 microg/cm2) or the inflammatory cytokine, TNFalpha (20 ng/ml). TNFalpha significantly increased ICAM-1 expression in all cell types whereas SI elicited an increase in peritoneal macrophages (PM) and the cell line, MH-S. This pattern of increased expression was confirmed by immunocytochemistry. To investigate the regulation of ICAM-1 expression, PM were incubated with SI, TNFalpha or media concomitantly with anti-TNFalpha antibody, the antioxidant, NAC, or the iNOS synthase inhibitor, L-NAME. Both anti-TNFalpha and NAC, but not L-NAME, inhibited elicited (TNFalpha, SI) as well as constitutive (media) ICAM-1 expression. These data demonstrate that both inflammatory cytokines and inorganic particles can increase ICAM-1 expression on mouse macrophages and that this expression is mediated, in part, by TNFalpha and reactive oxygen species.

The duration of antigen receptor signalling determines CD4+ versus CD8+ T-cell lineage fate

Signals elicited by binding of the T-cell antigen receptor and the CD4/CD8 co-receptor to major histocompatibility complex (MHC) molecules control the generation of CD4+ (helper) or CD8+ (cytotoxic) T cells from thymic precursors that initially express both co-receptor proteins. These precursors have unique, clonally distributed T-cell receptors with unpredictable specificity for the self-MHC molecules involved in this differentiation process. However, the mature T cells that emerge express only the CD4 (MHC class II-binding) or CD8 (MHC class I-binding) co-receptor that complements the MHC class-specificity of the T-cell receptor. How this matching of co-receptor-defined lineage and T-cell-receptor specificity is achieved remains unknown, as does whether signalling by the T-cell receptors, co-receptors and/or general cell-fate regulators such as Notch-1 contributes to initial lineage choice, to subsequent differentiation processes or to both. Here we show that the CD4 versus CD8 lineage fate of immature thymocytes is controlled by the co-receptor-influenced duration of initial T-cell receptor-dependent signalling. Notch-1 does not appear to be essential for this fate determination, but it is selectively required for CD8+ T-cell maturation after commitment directed by T-cell receptors. This indicates that the signals constraining CD4 versus CD8 lineage decisions are distinct from those that support subsequent differentiation events such as silencing of co-receptor loci.

PKC-theta is required for TCR-induced NF-kappaB activation in mature but not immature T lymphocytes

Productive interaction of a T lymphocyte with an antigen-presenting cell results in the clustering of the T-cell antigen receptor (TCR) and the recruitment of a large signalling complex to the site of cell-cell contact. Subsequent signal transduction resulting in cytokine gene expression requires the activation of one or more of the multiple isoenzymes of serine/threonine-specific protein kinase C (PKC). Among the several PKC isoenzymes expressed in T cells, PKC-theta is unique in being rapidly recruited to the site of TCR clustering. Here we show that PKC-theta is essential for TCR-mediated T-cell activation, but is dispensable during TCR-dependent thymocyte development. TCR-initiated NF-kappaB activation was absent from PKC-theta(-/-) mature T lymphocytes, but was intact in thymocytes. Activation of NF-kappaB by tumour-necrosis factor alpha and interleukin-1 was unaffected in the mutant mice. Although studies in T-cell lines had suggested that PKC-theta regulates activation of the JNK signalling pathway, induction of JNK was normal in T cells from mutant mice. These results indicate that PKC-theta functions in a unique pathway that links the TCR signalling complex to the activation of NF-kappaB in mature T lymphocytes.

CD28, Ox-40, LFA-1, and CD4 modulation of Th1/Th2 differentiation is directly dependent on the dose of antigen

The involvement of specific accessory/costimulatory molecules in differentiation to Th1 and Th2 phenotypes is controversial. Reports suggest that molecules such as CD4, CD28, and Ox-40 support Th2 differentiation and suppress Th1 differentiation, whereas others such as LFA-1 support Th1 responses and suppress Th2 responses. We have previously defined an in vitro model of differentiation that is absolutely dependent on the initial dose and affinity of peptide presented to a naive CD4 cell. The dose and affinity of Ag regulate autocrine production of IL-2, IL-4, and IFN-gamma, which in turn govern differentiation to Th1 and Th2 phenotypes. We have used this system to confirm that CD4, CD28, and Ox-40 interactions can promote, and LFA-1 interactions can suppress, differentiation of cells secreting the Th2 cytokines IL-5 and IL-13. However, for CD4 and LFA-1, this is only seen over a certain range of peptide doses. In addition, CD28 and Ox-40 interactions also promote Th1 differentiation. In general, agonist Abs to accessory molecules shifted the response curves for IFN-gamma, IL-5, and IL-13 to lower doses, whereas antagonist reagents resulted in similar curves shifted toward the higher doses. We conclude that ligation of cell surface accessory receptors enables low doses of Ag to promote responses normally induced only by higher doses. Individual receptors do not intrinsically regulate one cytokine phenotype or another, suggesting that differentiation is controlled by the level of expression of multiple accessory molecule pairs integrated with the number and affinity of peptide/MHC complexes.

Exclusion of CD45 from the T-cell receptor signaling area in antigen-stimulated T lymphocytes

T lymphocytes are activated by the engagement of their antigen receptors (TCRs) with complexes of peptide and major histocompatibility complex (MHC) molecules displayed on the cell surface of antigen-presenting cells (APCs) [1]. An unresolved question of antigen recognition by T cells is how TCR triggering actually occurs at the cell-cell contact area. We visualized T-cell-APC contact sites using confocal microscopy and three-dimensional reconstruction of z-sections. We show the rapid formation of a specialized signaling domain at the T-cell-APC contact site that is characterized by a broad and sustained area of tyrosine phosphorylation. The T-lymphocyte cell-surface molecule CD2 is rapidly recruited into this signaling domain, whereas TCRs progressively percolate from the entire T-cell surface into the phosphorylation area. Remarkably, the highly expressed phosphatase CD45 is excluded from the signaling domain. Our results indicate that physiological TCR triggering at the T-cell-APC contact site is the result of a localized alteration in the balance between cellular kinases and phosphatases. We therefore provide experimental evidence to support current models of T-cell activation based on CD45 exclusion from the TCR signaling area [2] [3] [4].

Identification of DC-SIGN, a novel dendritic cell-specific ICAM-3 receptor that supports primary immune responses

Contact between dendritic cells (DC) and resting T cells is essential to initiate a primary immune response. Here, we demonstrate that ICAM-3 expressed by resting T cells is important in this first contact with DC. We discovered that instead of the common ICAM-3 receptors LFA-1 and alphaDbeta2, a novel DC-specific C-type lectin, DC-SIGN, binds ICAM-3 with high affinity. DC-SIGN, which is abundantly expressed by DC both in vitro and in vivo, mediates transient adhesion with T cells. Since antibodies against DC-SIGN inhibit DC-induced proliferation of resting T cells, our findings predict that DC-SIGN enables T cell receptor engagement by stabilization of the DC-T cell contact zone.

A functional hot spot for antigen recognition in a superagonist TCR/MHC complex

A longstanding question in T cell receptor signaling is how structurally similar ligands, with similar affinities, can have substantially different biological activity. The crystal structure of the 2C TCR complex of H-2Kb with superagonist peptide SIYR at 2.8 A elucidates a structural basis for TCR discrimination of altered peptide ligands. The difference in antigen potency is modulated by two cavities in the TCR combining site, formed mainly by CDRs 3alpha, 3beta, and 1beta, that complement centrally located peptide residues. This "functional hot spot" allows the TCR to finely discriminate amongst energetically similar interactions within different ligands for those in which the peptide appropriately stabilizes the TCR/pMHC complex and provides a new structural perspective for understanding differential signaling resulting from T cell cross-reactivity.

Rap1 is a potent activation signal for leukocyte function-associated antigen 1 distinct from protein kinase C and phosphatidylinositol-3-OH kinase

To identify the intracellular signals which increase the adhesiveness of leukocyte function-associated antigen 1 (LFA-1), we established an assay system for activation-dependent adhesion through LFA-1/intercellular adhesion molecule 1 ICAM-1 using mouse lymphoid cells reconstituted with human LFA-1 and then introduced constitutively active forms of signaling molecules. We found that the phorbol myristate acetate (PMA)-responsive protein kinase C (PKC) isotypes (alpha, betaI, betaII, and delta) or phosphatidylinositol-3-OH kinase (PI 3-kinase) itself activated LFA-1 to bind ICAM-1. H-Ras and Rac activated LFA-1 in a PI 3-kinase-dependent manner, whereas Rho and R-Ras had little effect. Unexpectedly, Rap1 was demonstrated to function as the most potent activator of LFA-1. Distinct from H-Ras and Rac, Rap1 increased the adhesiveness independently of PI 3-kinase, indicating that Rap1 is a novel activation signal for the integrins. Rap1 induced changes in the conformation and affinity of LFA-1 and, interestingly, caused marked LFA-1/ICAM-1-mediated cell aggregation. Furthermore, a dominant negative form of Rap1 (Rap1N17) inhibited T-cell receptor-mediated LFA-1 activation in Jurkat T cells and LFA-1/ICAM-1-dependent cell aggregation upon differentiation of HL-60 cells into macrophages, suggesting that Rap1 is critically involved in physiological processes. These unique functions of Rap1 in controlling cellular adhesion through LFA-1 suggest a pivotal role as an immunological regulator.

The relationship of MHC-peptide binding and T cell activation probed using chemically defined MHC class II oligomers

A series of novel chemically defined soluble oligomers of the human MHC class II protein HLA-DR1 was constructed to probe the molecular requirements for initiation of T cell activation. MHC dimers, trimers, and tetramers stimulated T cells, as measured by upregulation of the activation markers CD69 and CD25, and by internalization of activated T cell receptor subunits. Monomeric MHC-peptide complexes engaged T cell receptors but did not induce activation. For a given amount of receptor engagement, the extent of activation was equivalent for each of the oligomers and correlated with the number of T cell receptor cross-links induced. These results suggest that formation or rearrangement of a T cell receptor dimer is necessary and sufficient for initiation of T cell signaling.

Single-cytokine-producing CD4 memory cells predominate in type 1 and type 2 immunity

The patterns of Ag-induced cytokine coexpression in normal, in vivo-primed CD4 memory T cells has remained controversial because the low frequency at which these cells occur has effectively prevented direct ex vivo measurements. We have overcome this limitation by using two-color cytokine enzyme-linked immunospot assays and computer-assisted image analysis. We found CD4 memory cells that simultaneously expressed IL-2, IL-3, IL-4, IL-5, and IFN-gamma to be rare (0-10%). This cytokine segregation was seen in adjuvant-induced type 1, type 2, and mixed immunity to OVA, in Leishmania infection regardless of the Ag dose used or how long after immunization the assay was performed. The data suggest that type 1 and type 2 immunity in vivo is not mediated by classic Th1 or Th2 cells but by single-cytokine-producing memory cells.

Pharmacological regulation of network kinetics by protein kinase C localization

Protein kinase C (PKC) is a conserved family of 11 serine/threonine kinases. Most cell types express multiple members of the family. Because the catalytic sites are homologous, and able to accommodate a broad range of substrates in vitro, specificity in function is dependent on subcellular localization of each isozyme in each cell type. Physiological stimulation can result in major changes in localization of individual PKC isozymes, mediated through binding to specific anchoring proteins. We describe data demonstrating that disruption of such translocations of PKC isozymes by pharmacological agents, peptides, or antibodies, causes profound effects on T cell functions. The pharmacological opportunity provided by distinct kinetic properties of complex assembly is also discussed.

From synapses to immunological memory: the role of sustained T cell stimulation

T cell activation is a sustained process driven by antigen and cytokines, which results in the generation of large numbers of effector and memory cells. Recent experiments from different fields have shed light on the mechanisms that maintain the signaling process at the level of a single synapse between a T cell and an antigen-presenting cell, as well as at the level of a secondary lymphoid organ, in the course of the immune response. These findings explain the unique capacity of the immune system to discriminate between antigens from infectious and noninfectious agents.

Cytoskeletal polarization and redistribution of cell-surface molecules during T cell antigen recognition

T cell antigen recognition is accompanied by cytoskeletal polarization towards the APC and large-scale redistribution of cell surface molecules into 'supramolecular activation clusters' (SMACs), forming an organized contact interface termed the 'immunological synapse' (IS). Molecules are arranged in the IS in a micrometer scale bull's eye pattern with a central accumulation of TCR/peptide-MHC (the cSMAC) surrounded by a peripheral ring of adhesion molecules (the pSMAC). We propose that segregation of cell surface molecules on a much smaller scale initiates TCR triggering, which drives the formation of the IS by active transport processes. IS formation may function as a checkpoint for full T cell activation, integrating information on the presence and quality of TCR ligands and the nature and activation state of the APC.

A novel functional interaction between Vav and PKCtheta is required for TCR-induced T cell activation

Vav and PKCtheta play an early and important role in the TCR/CD28-induced stimulation of MAP kinases and activation of the IL-2 gene. Vav is also essential for actin cytoskeleton reorganization and TCR capping. Here, we report that PKCtheta function was selectively required in a Vav signaling pathway that mediates the TCR/CD28-induced activation of JNK and the IL-2 gene and the upregulation of CD69 expression. Vav also promoted PKCtheta translocation from the cytosol to the membrane and cytoskeleton and induced its enzymatic activation in a CD3/CD28-initiated pathway that was dependent on Rac and on actin cytoskeleton reorganization. These findings reveal that the Vav/Rac pathway promotes the recruitment of PKCtheta to the T cell synapse and its activation, essential processes for T cell activation and IL-2 production.

The role of lipid rafts in T cell antigen receptor (TCR) signalling

Plasma membranes of many cell types contain domains enriched in specific lipids and cholesterol, called lipid rafts. In T lymphocytes, key T cell antigen receptor (TCR) signalling molecules associate with rafts, and disrupting raft-association of certain of these abrogates TCR signalling. The TCR itself associates with lipid rafts, and TCR cross-linking causes aggregation of raft-associated proteins. Furthermore, raft aggregation promotes tyrosine phosphorylation and recruitment of signalling proteins, but excludes the tyrosine phosphatase CD45. Together the data suggest that lipid rafts are important in controlling appropriate protein interactions in resting and activated T cells, and that aggregation of rafts following receptor ligation may be a general mechanism for promoting immune cell signalling.

Kinetics of T-cell receptor binding by bivalent HLA-DR. Peptide complexes that activate antigen-specific human T-cells

Monovalent major histocompatibility complex-peptide complexes dissociate within seconds from the T-cell receptor (TCR), indicating that dimerization/multimerization may be important during early stages of T-cell activation. Soluble bivalent HLA-DR2.myelin basic protein (MBP) peptide complexes were expressed by replacing the F(ab) arms of an IgG2a antibody with HLA-DR2.MBP peptide complexes. The binding of bivalent HLA-DR2.peptide complexes to recombinant TCR was examined by surface plasmon resonance. The bivalent nature greatly enhanced TCR binding and slowed dissociation from the TCR, with a t((1)/(2)) of 2.1 to 4.6 min. Soluble bivalent HLA-DR2.MBP peptide complexes activated antigen-specific T-cells in the absence of antigen presenting cells. In contrast, soluble antibodies to the TCR.CD3 complex were ineffective, indicating that they failed to induce an active TCR dimer. TCR/CD3 antibodies induced T-cell proliferation when bound by antigen presenting cells that expressed Fc receptors. In the presence of dendritic cells, bivalent HLA-DR2. MBP peptide complexes induced T-cell activation at >100-fold lower concentrations than TCR/CD3 antibodies and were also superior to peptide or antigen. These results demonstrate that bivalent HLA-DR. peptide complexes represent effective ligands for activation of the TCR. The data support a role for TCR dimerization in early TCR signaling and kinetic proofreading.

Structure and dimerization of a soluble form of B7-1

B7-1 (CD80) and B7-2 (CD86) are glycoproteins expressed on antigen-presenting cells. The binding of these molecules to the T cell homodimers CD28 and CTLA-4 (CD152) generates costimulatory and inhibitory signals in T cells, respectively. The crystal structure of the extracellular region of B7-1 (sB7-1), solved to 3 A resolution, consists of a novel combination of two Ig-like domains, one characteristic of adhesion molecules and the other previously seen only in antigen receptors. In the crystal lattice, sB7-1 unexpectedly forms parallel, 2-fold rotationally symmetric homodimers. Analytical ultracentrifugation reveals that sB7-1 also dimerizes in solution. The structural data suggest a mechanism whereby the avidity-enhanced binding of B7-1 and CTLA-4 homodimers, along with the relatively high affinity of these interactions, favors the formation of very stable inhibitory signaling complexes.

The human natural killer cell immune synapse

Inhibitory killer Ig-like receptors (KIR) at the surface of natural killer (NK) cells induced clustering of HLA-C at the contacting surface of target cells. In this manner, inhibitory immune synapses were formed as human NK cells surveyed target cells. At target/NK cell synapses, HLA-C/KIR distributed into rings around central patches of intercellular adhesion molecule-1/lymphocyte function-associated antigen-1, the opposite orientation to mature murine T cell-activating synapses. This organization of protein was stable for at least 20 min. Cells could support multiple synapses simultaneously, and clusters of HLA-C moved as NK cells crawled over target cells. Clustering required a divalent metal cation, explaining how metal chelators inhibit KIR function. Surprisingly, however, formation of inhibitory synapses was unaffected by ATP depletion and the cytoskeletal inhibitors, colchicine and cytochalsins B and D. Clearly, supramolecular organization within plasma membranes is critical for NK cell immunosurveillance.

Dynamic interactions of macrophages with T cells during antigen presentation

We have established a method for real-time video analysis of the interaction of antigen-presenting cells (APCs) with T cells. Green fluorescent protein expression controlled by a nuclear factor of activated T cells (NFAT)-responsive promoter permits the visualization of productive antigen presentation in single T cells. The readout is rapid (within 2 h) and semiquantitative and allows analysis by video microscopy and flow cytometry. Using this approach, we demonstrate that macrophages have the capacity to simultaneously activate multiple T cells. In addition, the interaction of T cells with macrophages is extraordinarily dynamic: after initial stable contact, the T cells migrate continuously on the surface of the macrophage and from APC to APC during productive antigen presentation. Thus, T cells sum up signals from multiple interactions with macrophages during stimulation.

Restimulation of resting autoreactive T cells by Schwann cells in vitro

This study demonstrates that rat Schwann cells can reactivate resting experimental allergic neuritis generating P(2) and P(2) peptide specific CD4(+) T cell lines. T cell proliferation was significantly greater to P(2) than to P(2) peptide (SP-26) or ovalbumin (OA). Four-level analysis of variance showed that T cell proliferation with endogenous or exogenous P(2) was not significantly different for Schwann cells plus cytokine IFN-gamma (P = 0.5071) unlike P(2) peptide or OA specific T cells (P = 0.0056 and 0.0003, respectively). Untreated Schwann cells were more effective inducers than irradiated or fixed Schwann cells. As stimulated CD4(+) P(2) T cells produce IFN-gamma and TNF-alpha, this could exacerbate blood nerve barrier breakdown that has been increasingly implicated in inflammatory demyelinating neuropathies (IDNs). This would permit entry of antibodies and complement, thereby contributing to the demyelination process. Schwann cell induced reactivation of CD4(+) T cells may therefore play a role in IDNs.

MHC superfamily structure and the immune system

During the past year, a plethora of structural information has provided detailed insights into the interactions between classical MHC class I molecules and their cognate receptors on T cells. Likewise, there have been major advances in our knowledge of the structures and functions of five nonclassical MHC-like molecules: HLA-DM (murine H2-M), HLA-E, HFE, ZAG and MIC-A.

Differential Requirements for CD4 in TCR-Ligand Interactions

The coreceptor molecule, CD4, plays an integral part in T cell activation; it is involved in both extracellular Ag recognition and intracellular signaling. We wanted to examine the functional role of CD4 in the recognition of agonist and altered peptide ligands (APLs). We generated two CD4-deficient T cell lines expressing well-characterized TCRs specific for Hb(64–76)/I-Ek. Although the responsiveness of the T cell lines to the agonist peptide was differently affected by the loss of CD4 expression, the recognition of APLs was in both cases dramatically reduced. Nearly full responsiveness to the agonist peptide was achieved by expression of a CD4 variant that did not associate with p56lck; however, the stimulation by APLs was only partially restored. Importantly, the expression of a CD4 variant in which domains interacting with MHC class II molecules have been mutated failed to restore the reactivity to all ligands. CD4-deficient T cells were able to be antagonized by APLs, indicating that CD4 was not required for antagonism. Overall, these findings support the concepts that CD4 is an integral part of the initial formation of the immunological synapse, and that the requirement for different CD4 functions in T cell activation varies depending upon the potency of the ligand.

TCR-Mediated internalization of peptide-MHC complexes acquired by T cells

Peptide-major histocompatibility complex protein complexes (pMHCs) on antigen-presenting cells (APCs) are central to T cell activation. Within minutes of peptide-specific T cells interacting with APCs, pMHCs on APCs formed clusters at the site of T cell contact. Thereafter, these clusters were acquired by T cells and internalized through T cell receptor-mediated endocytosis. During this process, T cells became sensitive to peptide-specific lysis by neighboring T cells (fratricide). This form of immunoregulation could explain the "exhaustion" of T cell responses that is induced by high viral loads and may serve to down-regulate immune responses.

Roles for glycosylation of cell surface receptors involved in cellular immune recognition

The majority of cell surface receptors involved in antigen recognition by T cells and in the orchestration of the subsequent cell signalling events are glycoproteins. The length of a typical N-linked sugar is comparable with that of an immunoglobulin domain (30 A). Thus, by virtue of their size alone, oligosaccharides may be expected to play a significant role in the functions and properties of the cell surface proteins to which they are attached. A databank of oligosaccharide structures has been constructed from NMR and crystallographic data to aid in the interpretation of crystal structures of glycoproteins. As unambiguous electron density can usually only be assigned to the glycan cores, the remainder of the sugar is then modelled into the crystal lattice by superimposing the appropriate oligosaccharide from the database. This approach provides insights into the roles that glycosylation might play in cell surface receptors, by providing models that delineate potential close packing interactions on the cell surface. It has been proposed that the specific recognition of antigen by T cells results in the formation of an immunological synapse between the T cell and the antigen-presenting cell. The cell adhesion glycoproteins, such as CD2 and CD48, help to form a cell junction, providing a molecular spacer between opposing cells. The oligosaccharides located on the membrane proximal domains of CD2 and CD48 provide a scaffold to orient the binding faces, which leads to increased affinity. In the next step, recruitment of the peptide major histocompatibility complex (pMHC) by the T-cell receptors (TCRs) requires mobility on the membrane surface. The TCR sugars are located such that they could prevent non-specific aggregation. Importantly, the sugars limit the possible geometry and spacing of TCR/MHC clusters which precede cell signalling. We postulate that, in the final stage, the sugars could play a general role in controlling the assembly and stabilisation of the complexes in the synapse and in protecting them from proteolysis during prolonged T-cell engagement.