Topology of the CD2-CD48 cell-adhesion molecule complex: implications for antigen recognition by T cells

HSPA4 upregulation induces immune evasion via ALKBH5/CD58 axis in gastric cancer

INTRODUCTION

Gastric cancer (GC) is one of the leading causes of cancer-related death worldwide. Recently, targeted therapies including PD1 (programmed cell death 1) antibodies have been used in advanced GC patients. However, identifying new biomarker for immunotherapy is still urgently needed. The objective of this study is to unveil the immune evasion mechanism of GC cells and identify new biomarkers for immune checkpoint blockade therapy in patients with GC.

METHODS

Coimmunoprecipitation and meRIP were performed to investigate the mechanism of immune evasion of GC cells. Cocuture system was established to evaluate the cytotoxicity of cocultured CD8+ T cells. The clinical significance of HSPA4 upregulation was analyzed by multiplex fluorescent immunohistochemistry staining in GC tumor tissues.

RESULTS

Histone acetylation causes HSPA4 upregulation in GC tumor tissues. HSPA4 upregulation increases the protein stability of m6A demethylase ALKBH5. ALKBH5 decreases CD58 in GC cells through m6A methylation regulation. The cytotoxicity of CD8+ T cells are impaired and PD1/PDL1 axis is activated when CD8+ T cells are cocultured with HSPA4 overexpressed GC cells. HSPA4 upregulation is associated with worse 5-year overall survival of GC patients receiving only surgery. It is an independent prognosis factor for worse survival of GC patients. In GC patients receiving the combined chemotherapy with anti-PD1 immunotherapy, HSPA4 upregulation is observed in responders compared with non-responders.

CONCLUSION

HSPA4 upregulation causes the decrease of CD58 in GC cells via HSPA4/ALKBH5/CD58 axis, followed by PD1/PDL1 activation and impairment of CD8+ T cell's cytotoxicity, finally induces immune evasion of GC cells. HSPA4 upregulation is associated with worse overall survival of GC patients with only surgery. Meanwhile, HSPA4 upregulation predicts for better response in GC patients receiving the combined immunotherapy.

The CD2-CD58 axis: A novel marker predicting poor prognosis in patients with low-grade gliomas and potential therapeutic approaches

INTRODUCTION

Low-grade gliomas (LGGs) are currently considered a premalignant condition for high-grade gliomas (HGGs) and are characterized by a relatively intact immune system. Immunotherapeutic modalities may offer a safe and effective treatment option for these patients. However, the CD2-CD58 axis, an important component of the immunological synapse, remains unknown in LGG.

METHODS

RNA-seq data from TCGA databases were analyzed. Immune cell infiltration was determined using a single-sample gene set enrichment analysis (ssGSEA) based on integrated immune gene sets from published studies. Kaplan-Meier survival analysis, univariate and multivariate logistic analysis, and the ESTIMATE algorithm were employed to evaluate the impact of the CD2-CD58 axis on adult LGG patients.

RESULTS

The expression of the CD2-CD58 axis was found to be elevated with increasing of WHO grade (p < .05). Uni- and multi-variable logistic analysis demonstrated that age, WHO grade, and CD58 levels were associated with poor prognosis in LGG patients with (p < .01). MetaSape pathways analysis revealed the involvement of CD58 in regulating T cell activation, leukocyte-mediated immunity, and the positive regulation of cell activation in WHO grade II and III. CD58 expression correlated with infiltrations of CD4+ lymphocytes, NK cells, and macrophages cells. The ESTIMATE algorithm indicated that patients with high CD58 expression had significantly higher immune scores compared with low CD58 expression in WHO grade II/III, but no statistical difference was observed in WHO grade IV (p < .05). Furthermore, correlation analysis demonstrated the significant association between CD58 and CD274 (r = 0.581, p < .001), HAVCR2 (r = 0.58i7, p < .001), and LGALS9 (r = 0.566, p < .001). Immunohistochemical staining further confirmed the relationship of CD58, HAVCR2, WHO grade, and prognosis in grade II and III patients.

CONCLUSION

Overall, our findings highlight the significant association between the CD2-CD58 axis and poor survival in LGG patients. High CD58 expression is implicated in T cell-mediated immune responses as an immunosuppressive factor and affect inhibitory immune checkpoint genes.

Antigen discrimination by T cells relies on size-constrained microvillar contact

T cells use finger-like protrusions called 'microvilli' to interrogate their targets, but why they do so is unknown. To form contacts, T cells must overcome the highly charged, barrier-like layer of large molecules forming a target cell's glycocalyx. Here, T cells are observed to use microvilli to breach a model glycocalyx barrier, forming numerous small (<0.5 μm diameter) contacts each of which is stabilized by the small adhesive protein CD2 expressed by the T cell, and excludes large proteins including CD45, allowing sensitive, antigen dependent TCR signaling. In the absence of the glycocalyx or when microvillar contact-size is increased by enhancing CD2 expression, strong signaling occurs that is no longer antigen dependent. Our observations suggest that, modulated by the opposing effects of the target cell glycocalyx and small adhesive proteins, the use of microvilli equips T cells with the ability to effect discriminatory receptor signaling.

Trapping or slowing the diffusion of T cell receptors at close contacts initiates T cell signaling

T cell activation is initiated by T cell receptor (TCR) phosphorylation. This requires the local depletion of large receptor-type phosphatases from "close contacts" formed when T cells interact with surfaces presenting agonistic TCR ligands, but exactly how the ligands potentiate signaling is unclear. It has been proposed that TCR ligands could enhance receptor phosphorylation and signaling just by holding TCRs in phosphatase-depleted close contacts, but this has not been directly tested. We devised simple methods to move the TCR in and out of close contacts formed by T cells interacting with supported lipid bilayers (SLBs) and to slow the receptor's diffusion in the contacts, using a series of anti-CD3ε Fab- and ligand-based adducts of the receptor. TCRs engaging a Fab extended with the large extracellular region of CD45 were excluded from contacts and produced no signaling. Conversely, allowing the extended Fab to become tethered to the SLB trapped the TCR in the close contacts, leading to very strong signaling. Importantly, attaching untethered anti-CD3ε Fab or peptide/MHC ligands, each of which were largely inactive in solution but both of which reduced TCR diffusion in close contacts approximately fivefold, also initiated signaling during cell/SLB contact. Our findings indicate that holding TCRs in close contacts or simply slowing their diffusion in phosphatase-depleted regions of the cell surface suffices to initiate signaling, effects we could reproduce in single-particle stochastic simulations. Our study shows that the TCR is preconfigured for signaling in a way that allows it to be triggered by ligands acting simply as receptor "traps."

CD58 Immunobiology at a Glance

The glycoprotein CD58, also known as lymphocyte-function antigen 3 (LFA-3), is a costimulatory receptor distributed on a broad range of human tissue cells. Its natural ligand CD2 is primarily expressed on the surface of T/NK cells. The CD2-CD58 interaction is an important component of the immunological synapse (IS) that induces activation and proliferation of T/NK cells and triggers a series of intracellular signaling in T/NK cells and target cells, respectively, in addition to promoting cell adhesion and recognition. Furthermore, a soluble form of CD58 (sCD58) is also present in cellular supernatant in vitro and in local tissues in vivo. The sCD58 is involved in T/NK cell-mediated immune responses as an immunosuppressive factor by affecting CD2-CD58 interaction. Altered accumulation of sCD58 may lead to immunosuppression of T/NK cells in the tumor microenvironment, allowing sCD58 as a novel immunotherapeutic target. Recently, the crucial roles of costimulatory molecule CD58 in immunomodulation seem to be reattracting the interests of investigators. In particular, the CD2-CD58 interaction is involved in the regulation of antiviral responses, inflammatory responses in autoimmune diseases, immune rejection of transplantation, and immune evasion of tumor cells. In this review, we provide a comprehensive summary of CD58 immunobiology.

CD2 Immunobiology

The glycoprotein CD2 is a costimulatory receptor expressed mainly on T and NK cells that binds to LFA3, a cell surface protein expressed on e.g., antigen-presenting cells. CD2 has an important role in the formation and organization of the immunological synapse that is formed between T cells and antigen-presenting cells upon cell-cell conjugation and associated intracellular signaling. CD2 expression is upregulated on memory T cells as well as activated T cells and plays an important role in activation of memory T cells despite the coexistence of several other costimulatory pathways. Anti-CD2 monoclonal antibodies have been shown to induce immune modulatory effects in vitro and clinical studies have proven the safety and efficacy of CD2-targeting biologics. Investigators have highlighted that the lack of attention to the CD2/LFA3 costimulatory pathway is a missed opportunity. Overall, CD2 is an attractive target for monoclonal antibodies intended for treatment of pathologies characterized by undesired T cell activation and offers an avenue to more selectively target memory T cells while favoring immune regulation.

The Costs of Close Contacts: Visualizing the Energy Landscape of Cell Contacts at the Nanoscale

Cell-cell contacts often underpin signaling between cells. For immunology, the binding of a T cell receptor to an antigen-presenting pMHC initiates downstream signaling and an immune response. Although this contact is mediated by proteins on both cells creating interfaces with gap sizes typically around 14 nm, many, often contradictory observations have been made regarding the influence of the contact on parameters such as the binding kinetics, spatial distribution, and diffusion of signaling proteins within the contact. Understanding the basic physical constraints on probes inside this crowded environment will help inform studies on binding kinetics and dynamics of signaling of relevant proteins in the synapse. By tracking quantum dots of different dimensions for extended periods of time, we have shown that it is possible to obtain the probability of a molecule entering the contact, the change in its diffusion upon entry, and the impact of spatial heterogeneity of adhesion protein density in the contact. By analyzing the contacts formed by a T cell interacting with adhesion proteins anchored to a supported lipid bilayer, we find that probes are excluded from contact entry in a size-dependent manner for gap-to-probe differences of 4.1 nm. We also observed probes being trapped inside the contact and a decrease in diffusion of up to 85% in dense adhesion protein contacts. This approach provides new, to our knowledge, insights into the nature of cell-cell contacts, revealing that cell contacts are highly heterogeneous because of topography- and protein-density-related processes. These effects are likely to profoundly influence signaling between cells.

Multiple actin networks coordinate mechanotransduction at the immunological synapse

Activation of naive T cells by antigen-presenting cells (APCs) is an essential step in mounting an adaptive immune response. It is known that antigen recognition and T cell receptor (TCR) signaling depend on forces applied by the T cell actin cytoskeleton, but until recently, the underlying mechanisms have been poorly defined. Here, we review recent advances in the field, which show that specific actin-dependent structures contribute to the process in distinct ways. In essence, T cell priming involves a tug-of-war between the cytoskeletons of the T cell and the APC, where the actin cytoskeleton serves as a mechanical intermediate that integrates force-dependent signals. We consider each of the relevant actin-rich T cell structures separately and address how they work together at the topologically and temporally complex cell-cell interface. In addition, we address how this mechanobiology can be incorporated into canonical immunological models to improve how these models explain T cell sensitivity and antigenic specificity.

OX40: Structure and function - What questions remain?

OX40 is a type 1 transmembrane glycoprotein, reported nearly 30 years ago as a cell surface antigen expressed on activated T cells. Since its discovery, it has been validated as a bone fide costimulatory molecule for T cells and member of the TNF receptor family. However, many questions still remain relating to its function on different T cell sub-sets and with recent interest in its utility as a target for antibody-mediated immunotherapy, there is a growing need to gain a better understanding of its biology. Here, we review the expression pattern of OX40 and its ligand, discuss the structure of the receptor:ligand interaction, the downstream signalling it can elicit, its function on different T cell subsets and how antibodies might engage with it to provide effective immunotherapy.

Initiation of T cell signaling by CD45 segregation at 'close contacts'

It has been proposed that the local segregation of kinases and the tyrosine phosphatase CD45 underpins T cell antigen receptor (TCR) triggering, but how such segregation occurs and whether it can initiate signaling is unclear. Using structural and biophysical analysis, we show that the extracellular region of CD45 is rigid and extends beyond the distance spanned by TCR-ligand complexes, implying that sites of TCR-ligand engagement would sterically exclude CD45. We also show that the formation of 'close contacts', new structures characterized by spontaneous CD45 and kinase segregation at the submicron-scale, initiates signaling even when TCR ligands are absent. Our work reveals the structural basis for, and the potent signaling effects of, local CD45 and kinase segregation. TCR ligands have the potential to heighten signaling simply by holding receptors in close contacts.

Ptpn22 and Cd2 Variations Are Associated with Altered Protein Expression and Susceptibility to Type 1 Diabetes in Nonobese Diabetic Mice

By congenic strain mapping using autoimmune NOD.C57BL/6J congenic mice, we demonstrated previously that the type 1 diabetes (T1D) protection associated with the insulin-dependent diabetes (Idd)10 locus on chromosome 3, originally identified by linkage analysis, was in fact due to three closely linked Idd loci: Idd10, Idd18.1, and Idd18.3. In this study, we define two additional Idd loci—Idd18.2 and Idd18.4—within the boundaries of this cluster of disease-associated genes. Idd18.2 is 1.31 Mb and contains 18 genes, including Ptpn22, which encodes a phosphatase that negatively regulates T and B cell signaling. The human ortholog of Ptpn22, PTPN22, is associated with numerous autoimmune diseases, including T1D. We, therefore, assessed Ptpn22 as a candidate for Idd18.2; resequencing of the NOD Ptpn22 allele revealed 183 single nucleotide polymorphisms with the C57BL/6J (B6) allele—6 exonic and 177 intronic. Functional studies showed higher expression of full-length Ptpn22 RNA and protein, and decreased TCR signaling in congenic strains with B6-derived Idd18.2 susceptibility alleles. The 953-kb Idd18.4 locus contains eight genes, including the candidate Cd2. The CD2 pathway is associated with the human autoimmune disease, multiple sclerosis, and mice with NOD-derived susceptibility alleles at Idd18.4 have lower CD2 expression on B cells. Furthermore, we observed that susceptibility alleles at Idd18.2 can mask the protection provided by Idd10/Cd101 or Idd18.1/Vav3 and Idd18.3. In summary, we describe two new T1D loci, Idd18.2 and Idd18.4, candidate genes within each region, and demonstrate the complex nature of genetic interactions underlying the development of T1D in the NOD mouse model.

Force and affinity in ligand discrimination by the TCR

T cell recognition of antigen is a physical process that requires formation of a cell-cell junction that is rich in active force generation. Recently a biomolecular force probe was used to examine how the T cell receptor (TCR)-pMHC interaction responds to force and the consequences of force-dependent interactions for T cell activation. While adhesion and costimulatory molecules in the immunological synapse impact upon the overall force of the interaction, these results suggest that the TCR uses a force-dependent bond - a catch bond - and that it may therefore be important to consider the TCR-pMHC interaction in isolation in the early phases of the decision process. We discuss here these findings in the context of other work on the impact of forces on the TCR and the quantification of interaction in interfaces.

Flow cytometric immunophenotypic analysis of systemic mastocytosis involving bone marrow

CONTEXT

Mast cells of systemic mastocytosis (SM) have aberrant immunophenotypes that are useful for their detection by flow cytometry immunophenotyping.

OBJECTIVES

To assess the usefulness of CD2, CD25, and other antigens for establishing the diagnosis of SM in bone marrow using flow cytometry immunophenotyping.

DESIGN

We studied 50 bone marrow aspirates of patients with SM using flow cytometry immunophenotyping. The bone marrow aspirates were stained with antibodies specific for CD2, CD25, CD35, CD59, CD63, and CD69. For the detection of CD2 and CD25, antibodies conjugated with phycoerythrin (PE) or fluorescein isothiocyanate (FITC) were compared. CD45-PerCP and CD117-APC were used for gating. Data were acquired on FACS Calibur cytometers and analyzed using CellQuest software.

RESULTS

CD2 and CD25 were positive in 41 of 50 (82%) and 45 of 50 (90%) SM cases, respectively. For CD2, the PE-conjugated antibody yielded better sensitivity than the FITC-conjugated antibody (31 of 40 [78%] versus 28 of 40 [70%]). For CD25, PE-conjugated and FITC-conjugated antibodies showed similar detection sensitivity, although the intensity of expression was brighter with CD25-PE. Compared with immunohistochemistry, flow cytometry immunophenotyping was superior for detecting CD2 (14 of 23 [61%] versus 9 of 23 [39%]). Other antigens frequently overexpressed in SM were CD35 (43 of 50 [86%]), CD59 (46 of 50 [92%]), CD63 (43 of 49 [88%]), and CD69 (39 of 48 [81%]).

CONCLUSIONS

Flow cytometry immunophenotyping is a rapid and sensitive technique for characterizing mast cells in bone marrow aspirate specimens. The use of PE-conjugated antibodies for CD2 and CD25 improves the detection rate (CD2) or facilitates analysis (CD25); therefore, PE-conjugated antibodies are suggested. Antibodies reactive with CD35, CD59, CD63, and CD69 are also helpful in detecting SM in bone marrow.

The interdisciplinary science of T-cell recognition

The recognition of peptide/MHC antigens by T-cells has continued to challenge the imagination of immunologists, biochemists, and cell biologists alike. This is at least in part because T-cell recognition connects a diversity of issues and transcends many scientific disciplines. A fundamental unsolved issue is how T-cells manage to detect even a single molecule of an agonist pMHC complex, which is vastly outnumbered by endogenous pMHCs, many of which involve the same MHC molecule. They do so although TCRs are cross-reactive and typically low in affinity when measured in isolation. Importantly, T-cell antigen recognition takes place within the contact zone between a T-cell and the antigen-presenting cell, termed the immunological synapse. This bimembrane structure sets the stage for the antigen-binding events and all subsequent molecular recognition events. There is increasing evidence that the molecular dynamics of receptor-ligand interactions are not only dependent on the intrinsic properties of the binding partners but also become transformed by cell biological parameters such as the geometrical constraints within the immune synapse, mechanical forces, and local molecular crowding. To appreciate the complete picture, we think a multidisciplinary approach is imperative, which includes genetics, biochemistry, and structure determination and also biophysical analyses and the latest molecular imaging techniques. Here, we review earlier pioneering work and also recent developments in the fascinating and interdisciplinary science of T-cell antigen recognition. In many ways, this work may present a useful "roadmap" for work in other systems of cell-cell recognition, which underlie many fundamental biological phenomenons of interest.

Evolving immune circuits are generated by flexible, motile, and sequential immunological synapses

The immune system is made up of a diverse collection of cells, each of which has distinct sets of triggers that elicit unique and overlapping responses. It is correctly described as a 'system' because its overall properties (e.g. 'tolerance', 'allergy') emerge from multiple interactions of its components cells. To mobilize a response where needed, the majority of the cells of the system are obligatorily highly motile and so must communicate with one another over both time and space. Here, we discuss the flexibility of the primary immunological synapse (IS) with respect to motility. We then consider the primary IS as an initiating module that licenses 'immunological circuits': the latter consisting of two or more cell-cell synaptic interactions. We discuss how two or three component immunological circuits interact might with one another in sequence and how the timing, stoichiometry, milieu, and duration of assembly of immunological circuits are likely to be key determinants in the emergent outcome and thus the system-wide immune response. An evolving consideration of immunological circuits, with an emphasis on the cell-cell modules that complement T-antigen-presenting cell interaction, provides a fundamental starting point for systems analysis of the immune response.

T cell receptors are structures capable of initiating signaling in the absence of large conformational rearrangements

Native and non-native ligands of the T cell receptor (TCR), including antibodies, have been proposed to induce signaling in T cells via intra- or intersubunit conformational rearrangements within the extracellular regions of TCR complexes. We have investigated whether any signatures can be found for such postulated structural changes during TCR triggering induced by antibodies, using crystallographic and mutagenesis-based approaches. The crystal structure of murine CD3ε complexed with the mitogenic anti-CD3ε antibody 2C11 enabled the first direct structural comparisons of antibody-liganded and unliganded forms of CD3ε from a single species, which revealed that antibody binding does not induce any substantial rearrangements within CD3ε. Saturation mutagenesis of surface-exposed CD3ε residues, coupled with assays of antibody-induced signaling by the mutated complexes, suggests a new configuration for the complex within which CD3ε is highly exposed and reveals that no large new CD3ε interfaces are required to form during antibody-induced signaling. The TCR complex therefore appears to be a structure that is capable of initiating intracellular signaling in T cells without substantial structural rearrangements within or between the component subunits. Our findings raise the possibility that signaling by native ligands might also be initiated in the absence of large structural rearrangements in the receptor.

Line tension and stability of domains in cell-adhesion zones mediated by long and short receptor-ligand complexes

Submicron scale domains of membrane-anchored receptors play an important role in cell signaling. Central questions concern the stability of these microdomains, and the mechanisms leading to the domain formation. In immune-cell adhesion zones, microdomains of short receptor-ligand complexes form next to domains of significantly longer receptor-ligand complexes. The length mismatch between the receptor-ligand complexes leads to membrane deformations and has been suggested as a possible cause of the domain formation. The domain formation is a nucleation and growth process that depends on the line tension and free energy of the domains. Using a combination of analytical calculations and Monte Carlo simulations, we derive here general expressions for the line tension between domains of long and short receptor-ligand complexes and for the adhesion free energy of the domains. We argue that the length mismatch of receptor-ligand complexes alone is sufficient to drive the domain formation, and obtain submicron-scale minimum sizes for stable domains that are consistent with the domain sizes observed during immune-cell adhesion.

Boltzmann energy-based image analysis demonstrates that extracellular domain size differences explain protein segregation at immune synapses

Immune synapses formed by T and NK cells both show segregation of the integrin ICAM1 from other proteins such as CD2 (T cell) or KIR (NK cell). However, the mechanism by which these proteins segregate remains unclear; one key hypothesis is a redistribution based on protein size. Simulations of this mechanism qualitatively reproduce observed segregation patterns, but only in certain parameter regimes. Verifying that these parameter constraints in fact hold has not been possible to date, this requiring a quantitative coupling of theory to experimental data. Here, we address this challenge, developing a new methodology for analysing and quantifying image data and its integration with biophysical models. Specifically we fit a binding kinetics model to 2 colour fluorescence data for cytoskeleton independent synapses (2 and 3D) and test whether the observed inverse correlation between fluorophores conforms to size dependent exclusion, and further, whether patterned states are predicted when model parameters are estimated on individual synapses. All synapses analysed satisfy these conditions demonstrating that the mechanisms of protein redistribution have identifiable signatures in their spatial patterns. We conclude that energy processes implicit in protein size based segregation can drive the patternation observed in individual synapses, at least for the specific examples tested, such that no additional processes need to be invoked. This implies that biophysical processes within the membrane interface have a crucial impact on cell:cell communication and cell signalling, governing protein interactions and protein aggregation.

Accelerated and adaptive evolution of yeast sexual adhesins

There is a recent emergence of interest in the genes involved in gametic recognition as drivers of reproductive isolation. The recent population genomic sequencing of two species of sexually primitive yeasts (Liti G, Carter DM, Moses AM, Warringer J, Parts L, James SA, Davey RP, Roberts IN, Burt A, Koufopanou V et al. [23 co-authors]. 2009. Population genomics of domestic and wild yeasts. Nature 458:337-341.) has provided data for systematic study of the roles these genes play in the early evolution of sex and speciation. Here, we discovered that among genes encoding cell surface proteins, the sexual adhesin genes have evolved significantly more rapidly than others, both within and between Saccharomyces cerevisiae and its closest relative S. paradoxus. This result was supported by analyses using the PAML pairwise model, a modified McDonald-Kreitman test, and the PAML branch model. Moreover, using a combination of a new statistic of neutrality, an information theory-based measure of evolutionary variability, and functional characterization of amino acid changes, we found that a higher proportion of amino acid changes are fixed in the sexual adhesins than in other proteins and a greater proportion of the fixed amino acid changes either between the two species or the two subgroups of S. paradoxus are functionally dissimilar or radically different. These results suggest that the accelerated evolution of sexual adhesin genes may facilitate speciation, or incipient speciation, and promote sexual selection in general.

Molecular basis for LLT1 protein recognition by human CD161 protein (NKRP1A/KLRB1)

Human Th17 cells express high levels of CD161, a member of the killer cell lectin-like receptor (KLR) family (also referred to as NK receptor-P1A (NKRP1A) or KLRB1), as a representative marker. CD161 is also expressed on natural killer (NK) cells and NKT cells. Lectin-like transcript 1 (LLT1), another KLR family member, was recently identified as a ligand for CD161. This interaction may play pivotal roles in the immunomodulatory functions of Th17 cells as well as those of NK and NKT cells. However, the molecular basis for the interaction is poorly understood. Here we show that the extracellular domain of CD161 bound directly to LLT1 with a K(d) of 48 μM and with the fast kinetics typical of cell-cell recognition receptors. Mutagenesis revealed that the similar membrane-distal β-sheet and loop regions of both CD161 and LLT1 were utilized for the binding, and notably, these regions correspond to the ligand-binding sites for major histocompatibility complex (MHC)-recognizing KLRs. Furthermore, we found a pair of detrimental mutations for both molecules that restored the binding. These results reveal a new template model for the recognition mode between the KLR family members and provide insights into the molecular mechanism underlying Th17/NK/NKT-mediated immune responses.

Lck and the nature of the T cell receptor trigger

Exactly how ligand binding 'triggers' T cell receptor (TCR) phosphorylation is unclear. It has been proposed that ligand engagement by the TCR somehow activates the Src kinase Lck, which in turn phosphorylates the receptor. Recent data, however, suggest instead that a significant fraction of the Lck in resting T cells is already activated and that the proportion of active Lck does not change during the early stages of T cell activation. We argue that, caveats notwithstanding, these new observations offer support for the 'kinetic-segregation' model of TCR triggering, which involves spatial reorganization of signalling proteins upon ligand binding and requires a fraction of Lck to be active in resting T cells.

Is human blood a good surrogate for brain tissue in transcriptional studies?

BACKGROUND

Since human brain tissue is often unavailable for transcriptional profiling studies, blood expression data is frequently used as a substitute. The underlying hypothesis in such studies is that genes expressed in brain tissue leave a transcriptional footprint in blood. We tested this hypothesis by relating three human brain expression data sets (from cortex, cerebellum and caudate nucleus) to two large human blood expression data sets (comprised of 1463 individuals).

RESULTS

We found mean expression levels were weakly correlated between the brain and blood data (r range: [0.24,0.32]). Further, we tested whether co-expression relationships were preserved between the three brain regions and blood. Only a handful of brain co-expression modules showed strong evidence of preservation and these modules could be combined into a single large blood module. We also identified highly connected intramodular "hub" genes inside preserved modules. These preserved intramodular hub genes had the following properties: first, their expression levels tended to be significantly more heritable than those from non-preserved intramodular hub genes (p < 10⁻⁹⁰); second, they had highly significant positive correlations with the following cluster of differentiation genes: CD58, CD47, CD48, CD53 and CD164; third, a significant number of them were known to be involved in infection mechanisms, post-transcriptional and post-translational modification and other basic processes.

CONCLUSIONS

Overall, we find transcriptome organization is poorly preserved between brain and blood. However, the subset of preserved co-expression relationships characterized here may aid future efforts to identify blood biomarkers for neurological and neuropsychiatric diseases when brain tissue samples are unavailable.

TCR-peptide-MHC interactions in situ show accelerated kinetics and increased affinity

The recognition of foreign antigens by T lymphocytes is essential to most adaptive immune responses. It is driven by specific T-cell antigen receptors (TCRs) binding to antigenic peptide-major histocompatibility complex (pMHC) molecules on other cells. If productive, these interactions promote the formation of an immunological synapse. Here we show that synaptic TCR-pMHC binding dynamics differ significantly from TCR-pMHC binding in solution. We used single-molecule microscopy and fluorescence resonance energy transfer (FRET) between fluorescently tagged TCRs and their cognate pMHC ligands to measure the kinetics of TCR-pMHC binding in situ. When compared with solution measurements, the dissociation of this complex was increased significantly (4-12-fold). Disruption of actin polymers reversed this effect, indicating that cytoskeletal dynamics destabilize this interaction directly or indirectly. Nevertheless, TCR affinity for pMHC was significantly elevated as the result of a large (about 100-fold) increase in the association rate, a likely consequence of complementary molecular orientation and clustering. In helper T cells, the CD4 molecule has been proposed to bind cooperatively with the TCR to the same pMHC complex. However, CD4 blockade had no effect on the synaptic TCR affinity, nor did it destabilize TCR-pMHC complexes, indicating that the TCR binds pMHC independently of CD4.

Peptide-major histocompatibility complex dimensions control proximal kinase-phosphatase balance during T cell activation

T cell antigen recognition requires binding of the T cell receptor (TCR) to a complex between peptide antigen and major histocompatibility complex molecules (pMHC), and this recognition occurs at the interface between the T cell and the antigen-presenting cell. The TCR and pMHC molecules are small compared with other abundant cell surface molecules, and it has been suggested that small size is functionally important. We show here that elongation of both mouse and human MHC class I molecules abrogates T cell antigen recognition as measured by cytokine production and target cell killing. This elongation disrupted tyrosine phosphorylation and Zap70 recruitment at the contact region without affecting TCR or coreceptor binding. Contact areas with elongated forms of pMHC showed an increase in intermembrane distance and less efficient segregation of CD3 from the large tyrosine phosphatase CD45. These findings demonstrate that T cell antigen recognition is strongly dependent on pMHC size and are consistent with models of TCR triggering requiring segregation or mechanical pulling of the TCR.

New assay to detect low-affinity interactions and characterization of leukocyte receptors for collagen including leukocyte-associated Ig-like receptor-1 (LAIR-1)

Leukocyte activity is controlled by numerous interactions between membrane receptors and ligands on the cell surface. These interactions are of low affinity making detection difficult. We developed a sensitive assay that could readily detect extremely weak interactions such as that between CD200 and the activating receptor CD200RLa (K(d)>500 microM) at the protein level. We used the new technology to screen for interactions of inhibitory receptors for collagens. We confirmed that both human and mouse leukocyte-associated Ig-like receptor-1, and in addition the related inhibitory leukocyte Ig-like receptor subfamily B member 4 (CD85K, Gp49B), bound collagen specifically, whereas other cell surface proteins gave no binding. The monomeric affinities of the interactions were then determined to allow comparison with other leukocyte interactions and indicate conditions when these interactions might lead to inhibitory signals.

Multiscale analysis of T cell activation: correlating in vitro and in vivo analysis of the immunological synapse

Recently implemented fluorescence imaging techniques, such as total internal reflection fluorescence microscopy and two-photon laser scanning microscopy, have made possible multiscale analysis of the immune response from single molecules in an interface to cells moving in lymphoid tissues and tumors. In this review, we consider components of T cell sensitivity: the immunological synapse, the coordination of migration, and antigen recognition in vivo. Potency, dose, and detection threshold for peptide-MHC determine T cell sensitivity. The immunological synapse incorporates T cell receptor microclusters that initiate and sustain signaling, and it also determines the positional stability of the T cells through symmetry and symmetry breaking. In vivo decisions by T cells on stopping or migration are based on antigen stop signals and environmental go signals that can sometimes prevent arrest of T cells altogether, and thus can change the outcome of antigen encounters.

Nanoscale increases in CD2-CD48-mediated intermembrane spacing decrease adhesion and reorganize the immunological synapse

The relationship between intermembrane spacing, adhesion efficiency, and lateral organization of adhesion receptors has not been established for any adhesion system. We have utilized the CD2 ligand CD48 with two (wild type CD48 (CD48-WT)), four (CD48-CD2), or five (CD48-CD22) Ig-like domains. CD48-WT was 10-fold more efficient in mediating adhesion than CD48-CD2 or CD48-CD22. Electron tomography of contact areas with planar bilayers demonstrated average intermembrane spacing of 12.8 nm with CD48-WT, 14.7 nm with CD48-CD2, and 15.6 nm with CD48-CD22. Both CD48-CD2 and CD48-CD22 chimeras segregated completely from CD48-WT in mixed contact areas. In contrast, CD48-CD2 and CD48-CD22 co-localized when mixed contacts were formed. Confocal imaging of immunological synapses formed between primary T lymphocytes and Chinese hamster ovary cells presenting major histocompatibility complex-peptide complexes, and different forms of CD48 demonstrated that CD48-CD2 and CD48-CD22 induce an eccentric CD2/T cell antigen receptor cluster. We propose that this reorganization of the immunological synapse sequesters the T cell antigen receptor in a location where it cannot interact with its ligand and dramatically reduces T cell sensitivity.

Rational design of a novel calcium-binding site adjacent to the ligand-binding site on CD2 increases its CD48 affinity

Electrostatic interactions are important for molecular recognition processes including Ca2+-binding and cell adhesion. To understand these processes, we have successfully introduced a novel Ca2+-binding site into the non-Ca2+-dependent cell adhesion protein CD2 using our criteria that are specifically tailored to the structural and functional properties of the protein environment and charged adhesion surface. This designed site with ligand residues exclusively from the beta-sheets selectively binds to Ca2+ and Ln3+ over other mono- and divalent cations. While Ca2+ and Ln3+ binding specifically alters the local environment of the designed Ca2+-binding site, the designed protein undergoes a significantly smaller conformation change compared with those observed in naturally occurring Ca2+-binding sites that are composed of at least part of the flexible loop and helical regions. In addition, the CD2-CD48-binding affinity increased approximately threefold after protein engineering, suggesting that the cell adhesion of CD2 can be modulated by altering the local electrostatic environment. The study provides site-specific information for regulating cell adhesion within CD2 and gives insight into the structural factors required for Ca2+-modulated biological processes.

The structure of the macrophage signal regulatory protein alpha (SIRPalpha) inhibitory receptor reveals a binding face reminiscent of that used by T cell receptors

Signal regulatory protein (SIRP) alpha is a membrane receptor that sends inhibitory signals to myeloid cells by engagement of CD47. The high resolution x-ray structure of the N-terminal ligand binding domain shows it to have a distinctive immunoglobulin superfamily V-like fold. Site-directed mutagenesis suggests that CD47 is bound at a surface involving the BC, FG, and DE loops, which distinguishes it from other immunoglobulin superfamily surface proteins that use the faces of the fold, but resembles antigen receptors. The SIRP interaction is confined to a single domain, and its use of an extended DE loop strengthens the similarity with T cell receptor binding and the suggestion that they are closely related in evolution. The employment of loops to form the CD47-binding surface provides a mechanism for small sequence changes to modulate binding specificity, explaining the different binding properties of SIRP family members.

Impact of salt bridges on the equilibrium binding and adhesion of human CD2 and CD58

This study describes quantitative investigations of the impact of single charge mutations on equilibrium binding, kinetics, and the adhesion strength of the CD2-CD58 interaction. Previously steered molecular dynamics simulations guided the selection of the charge mutants investigated, which include the CD2 mutants D31A, K41A, K51A, and K91A. This set includes mutations in which the previous cell aggregation and binding data either agreed or disagreed with the steered molecular dynamics predictions. Surface plasmon resonance measurements quantified the solution binding properties. Adhesion was quantified with the surface force apparatus, which was used previously to study the closely related CD2-CD48 interaction. The results reveal roles that these salt bridges play in equilibrium binding and adhesion. We discuss both the molecular basis of this behavior and its implications for cell adhesion.

The kinetic-segregation model: TCR triggering and beyond

How the T cell receptor engages antigen is known, but not how that 'triggers' intracellular signaling. The first direct support for a mechanism based on the spatial reorganization of signaling proteins, proposed 10 years ago and referred to as the 'kinetic-segregation' model, is now beginning to emerge, along with indications that it may also apply to the triggering of nonclonotypic receptors. We describe here the development of the model, review new data and suggest how the model fits a broader conceptual framework for receptor triggering. We also consider the capacity of the model, versus that of other proposals, to account for the established features of TCR triggering.

Regulation of FasL expression: A SH3 domain containing protein family involved in the lysosomal association of FasL

As a death factor of T cells and Natural Killer (NK) cells, Fas Ligand (FasL) is stored in association with secretory lysosomes. Upon stimulation, these cytotoxic granules are transported to the cell membrane where FasL is exposed on the cell surface, shed or secreted. It has been noted before that the proline-rich domain within the cytosolic part of FasL is required for its vesicular association. However, the molecular interactions involved in targeting FasL to secretory lysosomes or to the plasma membrane have not been elucidated. We now identified a family of structurally related proteins that upon co-expression with FasL reallocate the death factor from a membrane to an intracellular localization. Members of this protein family are characterized by a similar domain structure and include FBP17, PACSIN1-3, CD2BP1, CIP4, Rho-GAP C1 and several hypothetical proteins. We show that all tested members of this "FCH/SH3-family" co-precipitate FasL from transfectants. The interactions strictly depend on functional SH3 domains within the FCH/SH3 proteins. Since co-expression of FasL with individual FCH/SH3 proteins dramatically alters the intracellular localization of FasL especially in non-hematopoietic cells, our data suggest that FCH/SH3 proteins might play an important role for the subcellular distribution and lysosomal association of FasL.

Crystal structure and binding properties of the CD2 and CD244 (2B4)-binding protein, CD48

The structural analysis of surface proteins belonging to the CD2 subset of the immunoglobulin superfamily has yielded important insights into transient cellular interactions. In mice and rats, CD2 and CD244 (2B4), which are expressed predominantly on T cells and natural killer cells, respectively, bind the same, broadly expressed ligand, CD48. Structures of CD2 and CD244 have been solved previously, and we now present the structure of the receptor-binding domain of rat CD48. The receptor-binding surface of CD48 is unusually flat, as in the case of rat CD2, and shares a high degree of electrostatic complementarity with the equivalent surface of CD2. The relatively simple arrangement of charged residues and this flat topology explain why CD48 cross-reacts with CD2 and CD244 and, in rats, with the CD244-related protein, 2B4R. Comparisons of modeled complexes of CD2 and CD48 with the complex of human CD2 and CD58 are suggestive of there being substantial plasticity in the topology of ligand binding by CD2. Thermodynamic analysis of the native CD48-CD2 interaction indicates that binding is driven by equivalent, weak enthalpic and entropic effects, in contrast to the human CD2-CD58 interaction, for which there is a large entropic barrier. Overall, the structural and biophysical comparisons of the CD2 homologues suggest that the evolutionary diversification of interacting cell surface proteins is rapid and constrained only by the requirement that binding remains weak and specific.

A genomics approach to the detection of positive selection in cattle: adaptive evolution of the T-cell and natural killer cell-surface protein CD2

The detection of adaptive evolution at the molecular level is of interest not only as an insight into the process of evolution but also because of its functional implications for genes of interest. Here, we present the first genomics approach to detecting positive selection operating on the Bos taurus lineage, an important domestic species. This analysis led to the identification of the T-cell and natural killer (NK) cell receptor cluster of differentiation 2 (CD2) as having a strong signal of selection. Further detailed investigation of CD2 revealed that this gene was subject to positive selection during the evolution of a number of mammalian lineages. Moreover, we show that selection has operated primarily on the extracellular domain of CD2 and discuss the implications of this for an important regulator of the adaptive immune response.

Involvement of the Wiskott-Aldrich syndrome protein and other actin regulatory adaptors in T cell activation

The actin cytoskeleton is a dynamic structure recognized for many years as integral to the coupling of external stimuli to cell activation and ensuing changes in morphology and movement. It is only recently, however, that a molecular understanding of actin involvement in these activities has emerged coincident with the identification of cytosolic signaling effectors that couple extracellular stimuli to induction of actin nucleation. Notable among these actin regulatory effectors are members of the Wiskott-Aldrich syndrome protein (WASp) family, a group of cytoskeletal adaptors imbued with the capacity to connect various signal transduction pathways to the Arp 2/3 complex and Arp 2/3-mediated actin polymerization. In T cells, the functional characterization of WASp and other actin-modulatory adaptors has proved instrumental in delineating the molecular interactions evoking actin cytoskeletal reorganization downstream of antigen receptor engagement and in clarifying the influence of actin-based processes on T cell activation. In this review, the structural and functional properties of the major actin regulatory cytoskeletal adaptors in T cells are described with an emphasis on the roles of these proteins in fostering the TCR actin cytoskeletal interplay required for induction of T cell activation and expression of dynamic effector responses.

Molecular interactions mediating T cell antigen recognition

Over the past decade, key protein interactions contributing to T cell antigen recognition have been characterized in molecular detail. These have included interactions involving the T cell antigen receptor (TCR) itself, its coreceptors CD4 and CD8, the accessory molecule CD2, and the costimulatory receptors CD28 and CTLA-4. A clear view is emerging of how these molecules interact with their ligands at the cell-cell interface. Structural and binding studies have confirmed that the proteins span small but comparable distances and that, overall, they interact very weakly. However, there have been important surprises as well: that TCR interactions with peptide-MHC are topologically constrained and characterized by considerable conformational flexibility at the binding interface; that coreceptors engage peptide-MHC with extraordinarily fast kinetics and at angles apparently precluding direct interactions with the TCR bound to the same peptide-MHC; that the structural mechanisms allowing recognition by costimulatory and accessory molecules to be weak and yet specific are very heterogeneous; and that because of differences in both binding affinity and stoichiometry, there is enormous variation in the stability of the various costimulatory receptor/ligand complexes. These studies provide the necessary framework for exploring how these molecular interactions initiate T cell activation.

HMG-CoA reductase inhibitors as immunomodulators: potential use in transplant rejection

The benefit of HMG-CoA reductase inhibitors (statins) to the cardiovascular system is now well established and these drugs are being used extensively to treat hypercholesterolaemia clinically. However, as clinical outcomes become available it appears that statins are proving more beneficial than expected and thus it is being proposed that the actions of statins go beyond their ability to lower serum cholesterol levels. The report that statins can interact directly with lymphocyte function-associated antigen (LFA)-1 and prevent it engaging with the intracellular adhesion molecule (ICAM)-1 receptor on T cells is a novel mechanism of statin action and provides convincing evidence that these compounds can regulate biological systems other than by the cholesterol synthesis pathway. Immunosuppression to prevent organ transplant rejection is one application for which statins are currently being assessed. The clinical evidence is conflicting and does not convincingly reflect whether statins are beneficial as immunomodulators. However, in vivo studies investigating the cellular actions of statins have identified two mechanisms by which statins can potentially modulate an in vivo immune response. Firstly, statins regulate inducible class II major histocompatibility complex (MHC) expression on macrophages and endothelial cells. Secondly, statins can inhibit LFA-1 adhesion to ICAM-1 and thus regulate T cell activation. These findings suggest that statins have the potential to regulate an immune response in vivo and that more investigation is essential in order to explain the opposing clinical data.

The Wiskott-Aldrich syndrome protein: forging the link between actin and cell activation

The Wiskott-Aldrich syndrome protein (WASp) has emerged as a central player in the regulation of actin remodeling in T cells. The unique domain structure of WASp and other WASp family members enables these proteins to associate with a myriad of signaling effectors and to thereby regulate the coupling of T cell antigen receptor (TCR) engagement to both cytoskeletal rearrangement and transcriptional activation. This review focuses on these biochemical properties of WASp and also on the mechanisms whereby WASp interactions with its cognate ligands influence T cell activation. Because of its capacity to shift intracellular location and thereby dictate both the timing and the spatial distribution of actin polymerization following cell stimulation, WASp is well positioned to play major regulatory roles in directing a wide range of cellular processes and signaling pathways. Further dissection of the functional and biochemical properties of WASp therefore represents a promising avenue towards defining the molecular mechanisms that convey TCR stimulatory signals to the actin cytoskeleton and integrate cytoskeletal and other signaling systems so as to evoke a biological response.

The nature of molecular recognition by T cells

Considerable progress has been made in characterizing four key sets of interactions controlling antigen responsiveness in T cells, involving the following: the T cell antigen receptor, its coreceptors CD4 and CD8, the costimulatory receptors CD28 and CTLA-4, and the accessory molecule CD2. Complementary work has defined the general biophysical properties of interactions between cell surface molecules. Among the major conclusions are that these interactions are structurally heterogeneous, often reflecting clear-cut functional constraints, and that, although they all interact relatively weakly, hierarchical differences in the stabilities of the signaling complexes formed by these molecules may influence the sequence of steps leading to T cell activation. Here we review these developments and highlight the major challenges remaining as the field moves toward formulating quantitative models of T cell recognition.

The Wiskott-Aldrich syndrome protein acts downstream of CD2 and the CD2AP and PSTPIP1 adaptors to promote formation of the immunological synapse

The Wiskott-Aldrich syndrome protein (WASp) couples actin cytoskeletal rearrangement to T cell activation, but the mechanisms involved are unknown. Here, we show that antigen-induced formation of T cell:APC conjugates and synapses is abrogated in WASp-deficient T cells and that CD2 engagement evokes interactions between the proline-rich region required for WASp translocation to the synapse and the PSTPIP1 adaptor SH3 domain and between the PSTPIp1 coiled-coil domain and both CD2 and another CD2-binding adaptor, CD2AP. The induced colocalization of these proteins at the synapse is disrupted by expression of coiled-coil domain-deleted PSTPIP1. These data, together with the impairment in CD2-induced actin polymerization observed in WASp-deficient cells, suggest that PSTPIP1 acts downstream of CD2/CD2AP to link CD2 engagement to the WASp-evoked actin polymerization required for synapse formation and T cell activation.

Combining conformational flexibility and continuum electrostatics for calculating pK(a)s in proteins

Protein stability and function relies on residues being in their appropriate ionization states at physiological pH. In situ residue pK(a)s also provides a sensitive measure of the local protein environment. Multiconformation continuum electrostatics (MCCE) combines continuum electrostatics and molecular mechanics force fields in Monte Carlo sampling to simultaneously calculate side chain ionization and conformation. The response of protein to charges is incorporated both in the protein dielectric constant (epsilon(prot)) of four and by explicit conformational changes. The pK(a) of 166 residues in 12 proteins was determined. The root mean square error is 0.83 pH units, and >90% have errors of <1 pH units whereas only 3% have errors >2 pH units. Similar results are found with crystal and solution structures, showing that the method's explicit conformational sampling reduces sensitivity to the initial structure. The outcome also changes little with protein dielectric constant (epsilon(prot) 4-20). Multiconformation continuum electrostatics titrations show coupling of conformational flexibility and changes in ionization state. Examples are provided where ionizable side chain position (protein G), Asn orientation (lysozyme), His tautomer distribution (RNase A), and phosphate ion binding (RNase A and H) change with pH. Disallowing these motions changes the calculated pK(a).

The interaction properties of costimulatory molecules revisited

B7-1 and B7-2 are generally thought to have comparable structures and affinities for their receptors, CD28 and CTLA-4, each of which is assumed to be bivalent. We show instead (1) that B7-2 binds the two receptors more weakly than B7-1, (2) that, relative to its CTLA-4 binding affinity, B7-2 binds CD28 2- to 3-fold more effectively than B7-1, (3) that, unlike B7-1, B7-2 does not self-associate, and (4) that, in contrast to CTLA-4 homodimers, which are bivalent, CD28 homodimers are monovalent. Our results indicate that B7-1 markedly favors CTLA-4 over CD28 engagement, whereas B7-2 exhibits much less bias. We propose that the distinct structures and binding properties of B7-1 and B7-2 account for their overlapping but distinct effects on T cell responses.

CD43 polarization in unprimed T cells can be dissociated from raft coalescence by inhibition of HMG CoA reductase

Movement of T-lymphocyte cell surface CD43 is associated with both antigen activation of T-cell clones and chemokine induction of T-lymphocyte motility. Here, we demonstrate that CD43 movement away from the site of T-cell receptor ligation occurs in unprimed CD4(+) T cells as well as T-cell clones. The T-cell receptor (TCR)-dependent movement of CD43 in unprimed T cells is associated with a polarized morphology and CD43 accumulation at the uropods of the cells, unlike that reported for primed T cells. The polarization of CD43 has a requirement for Src kinases and occurs in conjunction with lipid raft coalescence. Thymocytes and T-cell hybridomas, cells that have altered responses to TCR activation and lack lipid raft coalescence, do not polarize CD43 as readily as unprimed T cells. The movement of CD43 depends on the cholesterol biosynthetic pathway enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase. Blockade of this enzyme can specifically prevent CD43 redistribution without affecting cell shape polarization. The likely mechanism of this alteration in CD43 redistribution is through decreased protein prenylation because the cholesterol-dependent lipid rafts still coalesce on activation. These findings suggest that the polarization of cell shape, lipid raft coalescence, and CD43 redistribution on T-cell activation have signaling pathway distinctions. Dissecting out the relationships between various stages of molecular redistribution and lymphocyte activation may facilitate fine-tuning of immunologic responses.

A response calculus for immobilized T cell receptor ligands

To address the molecular mechanism of T cell receptor (TCR) signaling, we have formulated a model for T cell activation, termed the 2D-affinity model, in which the density of TCR on the T cell surface, the density of ligand on the presenting surface, and their corresponding two-dimensional affinity determine the level of T cell activation. When fitted to T cell responses against purified ligands immobilized on plastic surfaces, the 2D-affinity model adequately simulated changes in cellular activation as a result of varying ligand affinity and ligand density. These observations further demonstrated the importance of receptor cross-linking density in determining TCR signaling. Moreover, it was found that the functional two-dimensional affinity of TCR ligands was affected by the chemical composition of the ligand-presenting surface. This makes it possible that cell-bound TCR ligands, despite their low affinity in solution, are of optimal two-dimensional affinity thereby allowing effective TCR binding under physiological conditions, i.e. at low ligand densities in cellular interfaces.

The immunological synapse and CD28-CD80 interactions

According to the two-signal model of T cell activation, costimulatory molecules augment T cell receptor (TCR) signaling, whereas adhesion molecules enhance TCR-MHC-peptide recognition. The structure and binding properties of CD28 imply that it may perform both functions, blurring the distinction between adhesion and costimulatory molecules. Our results show that CD28 on naïve T cells does not support adhesion and has little or no capacity for directly enhancing TCR-MHC-peptide interactions. Instead of being dependent on costimulatory signaling, we propose that a key function of the immunological synapse is to generate a cellular microenvironment that favors the interactions of potent secondary signaling molecules, such as CD28.

CD26: a multifunctional integral membrane and secreted protein of activated lymphocytes

CD26 has proved interesting in the fields of immunology, endocrinology, cancer biology and nutrition owing to its ubiquitous and unusual enzyme activity. This dipeptidyl aminopeptidase (DPP IV) activity generally inactivates but sometimes alters or enhances the biological activities of its peptide substrates, which include several chemokines. CD26 costimulates both the CD3 and the CD2 dependent T-cell activation and tyrosine phosphorylation of TCR/CD3 signal transduction pathway proteins. CD26 in vivo has integral membrane protein and soluble forms. Soluble CD26 is at significant levels in serum, these levels alter in many diseases and soluble CD26 can modulate in vitro T-cell proliferation. CD26, being an adenosine deaminase binding protein (ADAbp), functions as a receptor for ADA on lymphocytes. The focus of this review is the structure and function of CD26 and the influence of its ligand binding activity on T-cell proliferation and the T cell costimulatory activity of CD26.

Glycosylphosphatidylinositol-anchored receptor-mediated bacterial endocytosis

An increasing number of pathogens or their toxins appear to utilize glycosylphosphatidylinositol(GPI)-anchored receptors to trigger entry into immune and other host cells. Since these receptors have no transmembrane and intracellular moieties, how endocytosis is initiated is unclear. Recently, CD48 on mast cell membranes was shown to trigger endocytosis of bacteria via a route that avoids fusion with lysosomes and by a mechanism involving discrete cellular entities called caveolae. The localization of CD48 within caveolae appears to be a prerequisite for caveolae-mediated bacterial entry.

Co-stimulation and counter-stimulation: lipid raft clustering controls TCR signaling and functional outcomes

T cell receptor (TCR) antigen recognition induces the formation of a specialized 'immunological synapse' at the T cell : antigen presenting cell (APC) junction. This junction is generated by the recruitment and exclusion of particular proteins from the contact area and is required for T cell activation. We and others have hypothesized that lipid raft/non-raft partitioning provides a molecular basis for protein sorting which organizes the TCR, co-stimulators, signal transducers and the actin cytoskeleton at the T cell : APC interface. Here we discuss the emerging paradigm that co-stimulators induce the directional transport and clustering of lipid rafts at the T cell : APC interface, thus generating platform(s) specialized for processive and sustained TCR signal transduction and T cell activation. We also discuss recent data implicating the involvement of 'counter-stimulators' and other negative regulators which prevent optimal raft clustering at the TCR contact site and, thus, facilitate T cell inactivation and tolerance induction.