Stimulation of CD28 triggers an association between CD28 and phosphatidylinositol 3-kinase in Jurkat T cells

PD-1 and BTLA regulate T cell signaling differentially and only partially through SHP1 and SHP2

Blockade antibodies of the immunoinhibitory receptor PD-1 can stimulate the anti-tumor activity of T cells, but clinical benefit is limited to a fraction of patients. Evidence suggests that BTLA, a receptor structurally related to PD-1, may contribute to resistance to PD-1 targeted therapy, but how BTLA and PD-1 differ in their mechanisms is debated. Here, we compared the abilities of BTLA and PD-1 to recruit effector molecules and to regulate T cell signaling. While PD-1 selectively recruited SHP2 over the stronger phosphatase SHP1, BTLA preferentially recruited SHP1 to more efficiently suppress T cell signaling. Contrary to the dominant view that PD-1 and BTLA signal exclusively through SHP1/2, we found that in SHP1/2 double-deficient primary T cells, PD-1 and BTLA still potently inhibited cell proliferation and cytokine production, albeit more transiently than in wild type T cells. Thus, PD-1 and BTLA can suppress T cell signaling through a mechanism independent of both SHP1 and SHP2.

CD28 Costimulatory Domain-Targeted Mutations Enhance Chimeric Antigen Receptor T-cell Function

An obstacle to the development of chimeric antigen receptor (CAR) T cells is the limited understanding of CAR T-cell biology and the mechanisms behind their antitumor activity. We and others have shown that CARs with a CD28 costimulatory domain drive high T-cell activation, which leads to exhaustion and shortened persistence. This work led us to hypothesize that by incorporating null mutations of CD28 subdomains (YMNM, PRRP, or PYAP), we could optimize CAR T-cell costimulation and enhance function. In vivo, we found that mice given CAR T cells with only a PYAP CD28 endodomain had a significant survival advantage, with 100% of mice alive after 62 days compared with 50% for mice with an unmutated endodomain. We observed that mutant CAR T cells remained more sensitive to antigen after ex vivo antigen and PD-L1 stimulation, as demonstrated by increased cytokine production. The mutant CAR T cells also had a reduction of exhaustion-related transcription factors and genes such as Nfatc1, Nr42a, and Pdcd1 Our results demonstrated that CAR T cells with a mutant CD28 endodomain have better survival and function. This work allows for the development of enhanced CAR T-cell therapies by optimizing CAR T-cell costimulation.

SAP interacts with CD28 to inhibit PD-1 signaling in T lymphocytes

T cell co-stimulation is important for the maintenance of immunologic tolerance. Co-inhibitory receptors including programmed cell death-1 (PD-1) confer peripheral tolerance to prevent autoimmunity. SAP (SH2D1A) is an adaptor molecule that is important in T cell signaling and has been shown to interact with signaling lymphocytic activation molecule (SLAM) family receptors also in the context of self-tolerance. We recently reported that SAP interferes with PD-1 function. In the current study, we investigated the levels of SAP and PD-1 in patients with rheumatoid arthritis (RA) to further understand what role they play in disease activity. We observed increased SAP levels in lymphocytes of RA patients and found that PD-1 levels correlated positively with RA disease activity. Additionally, we found that SAP interacts with CD28 to inhibit T cell signaling in vitro. This work demonstrates a putative molecular mechanism for SAP mediated PD-1 inhibition.

CTLA-4: From mechanism to autoimmune therapy

CD28 and CTLA-4 are both important stimulatory receptors for the regulation of T cell activation. Because receptors share common ligands, B7.1 and B7.2, the expression and biological function of CTLA-4 is important for the negative regulation of T cell responses. Therefore, elimination of CTLA-4 can result in the breakdown of immune tolerance and the development of several diseases such as autoimmunity. Inhibitory signals of CTLA-4 suppress T cell responses and protect against autoimmune diseases in many ways. In this review, we summarize the structure, expression and signaling pathway of CTLA-4. We also highlight how CTLA-4 defends against potentially self-reactive T cells. Finally, we discuss how the CTLA-4 regulates a number of autoimmune diseases that indicate manipulation of this inhibitory molecule is a promise as a strategy for the immunotherapy of autoimmune diseases.

CD45RO regulates the HIV-1 gp120-mediated apoptosis of T cells by activating Lck

CD45 has been reported to regulate the HIV-1 gp120-induced apoptosis of Jurkat cells. Here, we demonstrate that the extracellular domain of CD45 plays an important role in this function. We observed that CD45RO-transfected cells, but not cells transfected with other CD45 isoforms, underwent significant apoptosis induced by gp120. However, a CD45RA-transfected cell line treated with an O-glycan inhibitor was able to undergo apoptosis. The role of the extracellular domain of CD45 was further confirmed using CD45 isoform-transfected cell lines by analyzing the phosphorylation of Lck, which is a direct substrate of CD45 tyrosine phosphatase, and by using an Lck inhibitor. These results suggest that CD45RO modulates HIV-1 gp120-induced apoptosis by regulating the activity of Lck.

Emerging Concepts for Immune Checkpoint Blockade-Based Combination Therapies

Checkpoint blockade has formally demonstrated that reactivating anti-tumor immune responses can regress tumors. However, this only occurs in a fraction of patients. Incorporating these therapies in more powerful combinations is thus a logical next step. Here, we review functional roles of immune checkpoints and molecular determinants of checkpoint-blockade clinical activity. Limited-size T cell-infiltrated tumors, differing substantially from "self," generally respond to checkpoint blockade. Therefore, we propose that reducing tumor burden and increasing tumor immunogenicity are key factors to improve immunotherapy. Lastly, we outline criteria to select proper immunotherapy combination partners and highlight the importance of activity biomarkers for timely treatment optimization.

ISA-2011B, a Phosphatidylinositol 4-Phosphate 5-Kinase α Inhibitor, Impairs CD28-Dependent Costimulatory and Pro-inflammatory Signals in Human T Lymphocytes

Phosphatidylinositol 4,5-biphosphate (PIP2) is a membrane phospholipid that controls the activity of several proteins regulating cytoskeleton reorganization, cytokine gene expression, T cell survival, proliferation, and differentiation. Phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) are the main enzymes involved in PIP2 biosynthesis by phosphorylating phosphatidylinositol 4-monophosphate (PI4P) at the D5 position of the inositol ring. In human T lymphocytes, we recently found that CD28 costimulatory molecule is pivotal for PIP2 turnover by recruiting and activating PIP5Kα. We also found that PIP5Kα is the main regulator of both CD28 costimulatory signals integrating those delivered by TCR as well as CD28 autonomous signals regulating the expression of pro-inflammatory genes. Given emerging studies linking alterations of PIP2 metabolism to immune-based diseases, PIP5Kα may represent a promising target to modulate immunity and inflammation. Herewith, we characterized a recently discovered inhibitor of PIP5Kα, ISA-2011B, for its inhibitory effects on T lymphocyte functions. We found that the inhibition of PIP5Kα lipid-kinase activity by ISA-2011B significantly impaired CD28 costimulatory signals necessary for TCR-mediated Ca²⁺ influx, NF-AT transcriptional activity, and IL-2 gene expression as well as CD28 autonomous signals regulating the activation of NF-κB and the transcription of pro-inflammatory cytokine and chemokine genes. Moreover, our data on the inhibitory effects of ISA-2011B on CD28-mediated upregulation of inflammatory cytokines related to Th17 cell phenotype in type 1 diabetes patients suggest ISA-2011B as a promising anti-inflammatory drug.

Phosphatidylinositol 4-Phosphate 5-Kinases in the Regulation of T Cell Activation

Phosphatidylinositol 4,5-biphosphate kinases (PIP5Ks) are critical regulators of T cell activation being the main enzymes involved in the synthesis of phosphatidylinositol 4,5-biphosphate (PIP2). PIP2 is indeed a pivotal regulator of the actin cytoskeleton, thus controlling T cell polarization and migration, stable adhesion to antigen-presenting cells, spatial organization of the immunological synapse, and co-stimulation. Moreover, PIP2 also serves as a precursor for the second messengers inositol triphosphate, diacylglycerol, and phosphatidylinositol 3,4,5-triphosphate, which are essential for the activation of signaling pathways regulating cytokine production, cell cycle progression, survival, metabolism, and differentiation. Here, we discuss the impact of PIP5Ks on several T lymphocyte functions with a specific focus on the role of CD28 co-stimulation in PIP5K compartimentalization and activation.

Cell Type-Specific Regulation of Immunological Synapse Dynamics by B7 Ligand Recognition

B7 proteins CD80 (B7-1) and CD86 (B7-2) are expressed on most antigen-presenting cells and provide critical co-stimulatory or inhibitory input to T cells via their T-cell-expressed receptors: CD28 and CTLA-4. CD28 is expressed on effector T cells and regulatory T cells (Tregs), and CD28-dependent signals are required for optimum activation of effector T cell functions. CD28 ligation on effector T cells leads to formation of distinct molecular patterns and induction of cytoskeletal rearrangements at the immunological synapse (IS). CD28 plays a critical role in recruitment of protein kinase C (PKC)-θ to the effector T cell IS. CTLA-4 is constitutively expressed on the surface of Tregs, but it is expressed on effector T cells only after activation. As CTLA-4 binds to B7 proteins with significantly higher affinity than CD28, B7 ligand recognition by cells expressing both receptors leads to displacement of CD28 and PKC-θ from the IS. In Tregs, B7 ligand recognition leads to recruitment of CTLA-4 and PKC-η to the IS. CTLA-4 plays a role in regulation of T effector and Treg IS stability and cell motility. Due to their important roles in regulating T-cell-mediated responses, B7 receptors are emerging as important drug targets in oncology. In this review, we present an integrated summary of current knowledge about the role of B7 family receptor–ligand interactions in the regulation of spatial and temporal IS dynamics in effector and Tregs.

Phosphatidylinositol 4–Phosphate 5–Kinase α and Vav1 Mutual Cooperation in CD28-Mediated Actin Remodeling and Signaling Functions

Phosphatidylinositol 4,5–biphosphate (PIP2) is a cell membrane phosphoinositide crucial for cell signaling and activation. Indeed, PIP2 is a pivotal source for second messenger generation and controlling the activity of several proteins regulating cytoskeleton reorganization. Despite its critical role in T cell activation, the molecular mechanisms regulating PIP2 turnover remain largely unknown. In human primary CD4+ T lymphocytes, we have recently demonstrated that CD28 costimulatory receptor is crucial for regulating PIP2 turnover by allowing the recruitment and activation of the lipid kinase phosphatidylinositol 4–phosphate 5–kinase (PIP5Kα). We also identified PIP5Kα as a key modulator of CD28 costimulatory signals leading to the efficient T cell activation. In this study, we extend these data by demonstrating that PIP5Kα recruitment and activation is essential for CD28-mediated cytoskeleton rearrangement necessary for organizing a complete signaling compartment leading to downstream signaling functions. We also identified Vav1 as the linker molecule that couples the C-terminal proline-rich motif of CD28 to the recruitment and activation of PIP5Kα, which in turn cooperates with Vav1 in regulating actin polymerization and CD28 signaling functions.

Cutting edge: A double-mutant knockin of the CD28 YMNM and PYAP motifs reveals a critical role for the YMNM motif in regulation of T cell proliferation and Bcl-xL expression

CD28 is a critical regulator of T cell function, augmenting proliferation, cytokine secretion, and cell survival. Our previous work using knockin mice expressing point mutations in CD28 demonstrated that the distal proline motif was primarily responsible for much of CD28 function, whereas in marked contrast to prior studies, mutation of the PI3K-binding motif had little discernible effect. In this study, we examined the phenotype of mice in which both motifs are simultaneously mutated. We found that mutation of the PYAP motif unmasks a critical role for the proximal tyrosine motif in regulating T cell proliferation and expression of Bcl-xL but not cytokine secretion. In addition, we demonstrated that, although function is more severely impaired in the double mutant than in either single mutant, there remained residual CD28-dependent responses, definitively establishing that additional motifs can partially mediate CD28 function.

Molecular mechanisms of T-cell anergy

Anergy is a long-term stable state of T-lymphocyte unresponsiveness to antigenic stimulation associated with the blockade of IL-2 production and proliferation. Anergy is a pathway of peripheral tolerance formation. In this review, mechanisms underlying T-cell tolerization are considered in a classical in vitro model of clonal anergy, and these mechanisms are compared with different pathways of anergy induction in vivo. Special attention is given to regulatory T-lymphocytes because, on one hand, anergy is a specific feature of these cells, and on the other hand anergy is also a mechanism of their action on target cells - effector T-lymphocytes. The role of this phenomenon in the differentiation of regulatory T-cells and also in the development of activation-induced apoptosis in effector T-lymphocytes is discussed.

PKC-theta-mediated signal delivery from the TCR/CD28 surface receptors

Protein kinase C-theta (PKCθ) is a key enzyme in T lymphocytes, where it plays an important role in signal transduction downstream of the activated T cell antigen receptor (TCR) and the CD28 costimulatory receptor. Interest in PKCθ as a potential drug target has increased following recent findings that PKCθ is essential for harmful inflammatory responses mediated by Th2 (allergies) and Th17 (autoimmunity) cells as well as for graft-versus-host disease (GvHD) and allograft rejection, but is dispensable for beneficial responses such as antiviral immunity and graft-versus-leukemia (GvL) response. TCR/CD28 engagement triggers the translocation of the cytosolic PKCθ to the plasma membrane (PM), where it localizes at the center of the immunological synapse (IS), which forms at the contact site between an antigen-specific T cell and antigen-presenting cells (APC). However, the molecular basis for this unique localization, and whether it is required for its proper function have remained unresolved issues until recently. Our recent study resolved these questions by demonstrating that the unique V3 (hinge) domain of PKCθ and, more specifically, a proline-rich motif within this domain, is essential and sufficient for its localization at the IS, where it is anchored to the cytoplasmic tail of CD28 via an indirect mechanism involving Lck protein tyrosine kinase (PTK) as an intermediate. Importantly, the association of PKCθ with CD28 is essential not only for IS localization, but also for PKCθ-mediated activation of downstream signaling pathways, including the transcription factors NF-κB and NF-AT, which are essential for productive T cell activation. Hence, interference with formation of the PKCθ-Lck-CD28 complex provides a promising basis for the design of novel, clinically useful allosteric PKCθ inhibitors. An additional recent study demonstrated that TCR triggering activates the germinal center kinase (GSK)-like kinase (GLK) and induces its association with the SLP-76 adaptor at the IS, where GLK phosphorylates the activation loop of PKCθ, converting it into an active enzyme. This recent progress, coupled with the need to study the biology of PKCθ in human T cells, is likely to facilitate the development of PKCθ-based therapeutic modalities for T cell-mediated diseases.

Rigid-body ligand recognition drives cytotoxic T-lymphocyte antigen 4 (CTLA-4) receptor triggering

The inhibitory T-cell surface-expressed receptor, cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), which belongs to the class of cell surface proteins phosphorylated by extrinsic tyrosine kinases that also includes antigen receptors, binds the related ligands, B7-1 and B7-2, expressed on antigen-presenting cells. Conformational changes are commonly invoked to explain ligand-induced "triggering" of this class of receptors. Crystal structures of ligand-bound CTLA-4 have been reported, but not the apo form, precluding analysis of the structural changes accompanying ligand binding. The 1.8-Å resolution structure of an apo human CTLA-4 homodimer emphasizes the shared evolutionary history of the CTLA-4/CD28 subgroup of the immunoglobulin superfamily and the antigen receptors. The ligand-bound and unbound forms of both CTLA-4 and B7-1 are remarkably similar, in marked contrast to B7-2, whose binding to CTLA-4 has elements of induced fit. Isothermal titration calorimetry reveals that ligand binding by CTLA-4 is enthalpically driven and accompanied by unfavorable entropic changes. The similarity of the thermodynamic parameters determined for the interactions of CTLA-4 with B7-1 and B7-2 suggests that the binding is not highly specific, but the conformational changes observed for B7-2 binding suggest some level of selectivity. The new structure establishes that rigid-body ligand interactions are capable of triggering CTLA-4 phosphorylation by extrinsic kinase(s).

CD28 co-signaling in the adaptive immune response

T-cell proliferation and function depends on signals from the antigen-receptor complex (TCR/CD3) and by various co-receptors such as CD28 and CTLA-4. The balance of positive and negative signals determines the outcome of the T-cell response to foreign and self-antigen. CD28 is a prominent co-receptor in naïve and memory T-cell responses. Its blockade has been exploited clinically to dampen T-cell responses to self-antigen. Current evidence shows that CD28 both potentiates TCR signaling and engages a unique array of mediators (PI3K, Grb2, FLNa) in the regulation of aspects of T-cell signaling including the transcription factor NFkB. In this mini-review, we provide an up-to-date overview of our understanding of the signaling mechanisms that underlie CD28 function and its potential application to the modulation of reactivity to autoimmunity.

An enigmatic tail of CD28 signaling

CD28 costimulation regulates a wide range of cellular processes, from proliferation and survival to promoting the differentiation of specialized T-cell subsets. Since first being identified over 20 years ago, CD28 has remained a subject of intense study because of its profound consequences on T cell function and its potential for therapeutic manipulation. In this review we highlight the signaling cascades initiated by the major signaling motifs in CD28, focusing on PI-3 kinase-dependent and -independent pathways and how these are linked to specific cellular outcomes. Recent studies using gene targeted knockin mice have clarified the relative importance of these motifs on in vivo immune responses; however, much remains to be elucidated. Understanding the mechanism behind costimulation holds great potential for development of new clinically relevant reagents, a fact beginning to be realized with the advent of drugs that prevent CD28 ligation and signaling.

Dynamic regulation of T-cell costimulation through TCR-CD28 microclusters

SUMMARY

T-cell activation requires contact between T cells and antigen-presenting cells (APCs) to bring T-cell receptors (TCRs) and major histocompatibility complex peptide (MHCp) together to the same complex. These complexes rearrange to form a concentric circular structure, the immunological synapse (IS). After the discovery of the IS, dynamic imaging technologies have revealed the details of the IS and provided important insights for T-cell activation. We have redefined a minimal unit of T-cell activation, the 'TCR microcluster', which recognizes MHCp, triggers an assembly of assorted molecules downstream of the TCR, and induces effective signaling from TCRs. The relationship between TCR signaling and costimulatory signaling was analyzed in terms of the TCR microcluster. CD28, the most valuable costimulatory receptor, forms TCR-CD28 microclusters in cooperation with TCRs, associates with protein kinase C theta, and effectively induces initial T-cell activation. After mature IS formation, CD28 microclusters accumulate at a particular subregion of the IS, where they continuously assemble with the kinases and not TCRs, and generate sustained T-cell signaling. We propose here a 'TCR-CD28 microcluster' model in which TCR and costimulatory microclusters are spatiotemporally formed at the IS and exhibit fine-tuning of T-cell responses by assembling with specific players downstream of the TCR and CD28.

CD28 and CTLA-4 coreceptor expression and signal transduction

SUMMARY

T-cell activation is mediated by antigen-specific signals from the TCRzeta/CD3 and CD4-CD8-p56lck complexes in combination with additional co-signals provided by coreceptors such as CD28, inducible costimulator (ICOS), cytotoxic T-lymphocyte antigen-4 (CTLA-4), programmed death (PD-1), and others. CD28 and ICOS provide positive signals that promote and sustain T-cell responses, while CTLA-4 and PD-1 limit responses. The balance between stimulatory and inhibitory co-signals determines the ultimate nature of T-cell responses where response to foreign pathogen is achieved without excess inflammation and autoimmunity. In this review, we outline the current knowledge of the CD28 and CTLA-4 signaling mechanisms [involving phosphatidylinositol 3 kinase (PI3K), growth factor receptor-bound protein 2 (Grb2), Filamin A, protein kinase C theta (PKCtheta), and phosphatases] that control T-cell immunity. We also present recent findings on T-cell receptor-interacting molecule (TRIM) regulation of CTLA-4 surface expression, and a signaling pathway involving CTLA-4 activation of PI3K and protein kinase B (PKB)/AKT by which cell survival is ensured under conditions of anergy induction.

p85beta phosphoinositide 3-kinase regulates CD28 coreceptor function

CD28 is a receptor expressed on T cells that regulates their differentiation after antigen stimulation to long-term-survival memory T cells. CD28 enhances T-cell receptor signals and reduces expression of CBL ubiquitin ligases, which negatively control T-cell activation. In the absence of CD28 ligation during the primary stimulation, CBL levels remain high and T cells fail to mount an efficient secondary response. CD28 associates with p85alpha, one of the regulatory subunits of phosphoinositide-3-kinase (PI3K), but the relevance of this interaction is debated. We examined here the contribution of the other ubiquitous PI3K regulatory subunit, p85beta, in CD28 function. We describe that p85beta bound to CD28 and to CBL with greater affinity than p85alpha. Moreover, deletion of p85beta impaired CD28-induced intracellular events, including c-CBL and CBL-b down-regulation as well as PI3K pathway activation. This resulted in defective differentiation of activated T cells, which failed to exhibit an efficient secondary immune response. Considering that p85beta-deficient T cells fail in recall responses and that p85beta binds to and regulates CD28 signals, the presented observations suggest the involvement of p85beta in CD28-mediated activation and differentiation of antigen-stimulated T cells.

Antigen-independent adhesion and cell spreading by inducible costimulator engagement inhibits T cell migration in a PI-3K-dependent manner

Engagement of the costimulatory protein ICOS activates effector/memory T cells in tissue by enhancing TCR-mediated proliferation and cytokine production. We now report that in an antigen-independent manner, ICOS also induces adhesion and spreading in human effector/memory T cells, consequently inhibiting cell migration. T cell spreading and elongation after ICOS ligation are accompanied by the formation of two types of actin-rich membrane protrusions: thin, finger-like structures similar to filopodia and short, discrete microspikes. Although filopodia/microspike formation occurs independently of the PI-3K signaling cascade, ICOS-mediated T cell elongation depends on PI-3K activity, which inhibits the accumulation of GTP-bound RhoA. Further inhibition of RhoA activation exacerbates the ICOS-mediated, elongated phenotype. We propose that in inflamed tissue, ICOS engagement by ICOS ligand on a professional or nonprofessional APC prevents the forward motility of the T cell by inhibiting RhoA-dependent uropod retraction. The resulting ICOS-induced T cell spreading and filopodia/microspike formation may promote antigen recognition by enhancing a T cell's scanning potential of an adherent APC surface.

Regulation of T-cell responses by PTEN

The phosphoinositide 3-kinase (PI3K) signaling pathway plays a critical role in the development, activation, and homeostasis of T cells by modulating the expression of survival and mitogenic factors in response to a variety of stimuli. Ligation of the antigen receptor, costimulatory molecules, and cytokine receptors activate PI3K, resulting in the production of the lipid second messenger phosphatidylinositol-3,4,5-triphosphate (PIP(3)). A number of molecules help to regulate the activity of this pathway, including the lipid phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10). By limiting the amount of PIP(3) available within the cell, PTEN directly opposes PI3K activity and influences the selection of developing thymocytes as well as the activation requirements of mature T cells. T cells with unchecked PI3K activity, as a result of PTEN deficiency, contribute to the development of both autoimmune disease and lymphoma. This review dissects our current understanding of PI3K and PTEN and discusses why appropriate balance of these molecules is necessary to maintain normal T-cell responses.

Spatiotemporal regulation of T cell costimulation by TCR-CD28 microclusters and protein kinase C theta translocation

T cell activation is mediated by microclusters (MCs) containing T cell receptors (TCRs), kinases, and adaptors. Although TCR MCs translocate to form a central supramolecular activation cluster (cSMAC) of the immunological synapse at the interface of a T cell and an antigen-presenting cell, the role of MC translocation in T cell signaling remains unclear. Here, we found that the accumulation of MCs at cSMAC was important for T cell costimulation. Costimulatory receptor CD28 was initially recruited coordinately with TCR to MCs, and its signals were mediated through the assembly with the kinase PKCtheta. The accumulation of MCs at the cSMAC was accompanied by the segregation of CD28 from the TCR, which resulted in the translocation of both CD28 and PKCtheta to a spatially unique subregion of cSMAC. Thus, costimulation is mediated by the generation of a unique costimulatory compartment in the cSMAC via the dynamic regulation of MC translocation.

CD28 and Grb-2, relative to Gads or Grap, preferentially co-operate with Vav1 in the activation of NFAT/AP-1 transcription

The co-receptor CD28 binds to several intracellular proteins including PI3 kinase, Grb-2, Gads and ITK. Grb-2 and PI3 kinase binding has been mapped to the pYMNM motif within the cytoplasmic tail of CD28 and has been shown to play a role in co-stimulation. In this study, we demonstrate that amongst the Grb-2 family adapter proteins, CD28 precipitated Grb-2 and specifically co-operated in the up-regulation of NFAT/AP-1 transcription. By contrast, Gads and Grap either failed or only weakly collaborated with CD28 ligation. Further, the loss of Grb-2 binding interferes with the ability of Vav1 to co-operate with CD28. Anti-CD28 ligation alone was capable for co-operating with Grb-2 or Grb-2-Vav1. Our findings define a pathway involving CD28 binding to Grb-2 and its co-operativity with Vav1 in the regulation of T-cell co-stimulation.

Proliferating CD4+ T cells undergo immediate growth arrest upon cessation of TCR signaling in vivo

To investigate the role of TCR signaling in the exit of CD4+ T cells from cell cycle, we took advantage of a low frequency TEa T cell adoptive transfer technique as well as the Y-Ae mAb to interrupt Ag/MHC recognition before the completion of clonal expansion. Termination of TCR signaling after 36 h of Ag exposure caused an immediate reduction in cell size and deceleration of G1->SG2M phase cell cycle progression. As a consequence, clonal expansion in the absence of durable TCR signaling decreased by two-thirds. Thus, CD4+ T cells scan for the presence Ag throughout their clonal expansion response, and continuously adjust their rate of cell growth and G1->S phase transition to match their intensity of TCR signaling.

PIP3 pathway in regulatory T cells and autoimmunity

Regulatory T cells (Tregs) play an important role in preventing both autoimmune and inflammatory diseases. Many recent studies have focused on defining the signal transduction pathways essential for the development and the function of Tregs. Increasing evidence suggest that T-cell receptor (TCR), interleukin-2 (IL-2) receptor (IL-2R), and co-stimulatory receptor signaling are important in the early development, peripheral homeostasis, and function of Tregs. The phosphoinositide-3 kinase (PI3K)-regulated pathway (PIP3 pathway) is one of the major signaling pathways activated upon TCR, IL-2R, and CD28 stimulation, leading to T-cell activation, proliferation, and cell survival. Activation of the PIP3 pathway is also negatively regulated by two phosphatidylinositol phosphatases SHIP and PTEN. Several mouse models deficient for the molecules involved in PIP3 pathway suggest that impairment of PIP3 signaling leads to dysregulation of immune responses and, in some cases, autoimmunity. This review will summarize the current understanding of the importance of the PIP3 pathway in T-cell signaling and the possible roles this pathway performs in the development and the function of Tregs.

Ras/PI3kinase/cofilin-independent activation of human CD45RA+ and CD45RO+ T cells by superagonistic CD28 stimulation

T cell activation requires costimulation of TCR/CD3 plus accessory receptors (e.g. CD28). A hallmark of costimulation is the dynamic reorganization of the actin cytoskeleton, important for receptor polarization in the immunological synapse. The classical model of T cell costimulation was challenged by the detection of superagonistic anti-CD28 antibodies. These induce T cell proliferation and--as demonstrated here--production of IFN-gamma, CD25 and CD69 even in the absence of TCR/CD3 coligation. Here, we analyzed whether superagonistic CD28 stimulation induces costimulatory signaling events. Costimulation leads to phosphorylation of the actin-bundling protein L-plastin and dephosphorylation of the actin-reorganizing protein cofilin. Cofilin binds to F-actin only in its dephosphorylated form. Binding of cofilin to F-actin leads to depolymerization or severing of F-actin. The latter ends up in smaller F-actin fragments, which can be elongated at the free barbed ends. This results in enhanced actin polymerization. Dephosphorylation of cofilin requires activation of Ras and PI3Kinase. Interestingly, superagonistic CD28 stimulation activates human peripheral blood T cells independently of Ras and PI3Kinase. Accordingly, it does not lead to cofilin dephosphorylation and receptor polarization. Likewise, L-plastin is not phosphorylated. Thus, superagonistic CD28 stimulation does not mimic costimulation. Instead, it leads to a Ras/PI3Kinase/cofilin-independent state of "unpolarized T cell activation".

Cloning, sequencing, and cell surface expression pattern of bovine immunoreceptor NKG2D and adaptor molecules DAP10 and DAP12

NKG2D is an activating lectin-like receptor that initiates natural killer (NK) cell responses against transformed tumor cells expressing its ligands, i.e., molecules related to major histocompatibility complex (MHC) class I molecules. NKG2D lacks signaling elements in its cytoplasmic domain and can deliver stimulatory signals only in association with transmembrane adaptor proteins DAP10 or DAP12. The complementary DNAs (cDNAs) encoding the bovine homologues of NKG2D and the adaptor proteins DAP10 and DAP12 were cloned by reverse transcriptase-polymerase chain reaction (RT-PCR) from resting bovine peripheral blood mononuclear cells (PBMC) and sequenced. Comparison with human, pig, and mouse sequences showed that bovine NKG2D is most similar to pig NKG2D and short mouse NKG2D (NKG2D-S). Similar to its human, mouse, and pig homologues, the cDNA for bovine DAP10 codes for a phosphatidyl-inositol-3 (PI-3) kinase-binding site (YxxM) in its cytoplasmic region. Finally, similar to its human, mouse, and pig homologues, the cDNA encoding bovine DAP12 demonstrates one tyrosine-based activated motif (ITAM) in its cytoplasmic domain. Bovine NKG2D cell surface expression was analyzed by flow cytometry on HEK 293 cells transiently transfected with cDNA expression vectors encoding COOH-terminal polyhistidine-tagged NKG2D and NH(2)-terminal Flag-tagged DAP10 and DAP12. Confirming previous findings for short mouse NKG2D-S, bovine NKG2D immunoreceptor could associate with either DAP10 or DAP12 adaptor protein for its cell surface expression.

mTOR at the crossroads of T cell proliferation and tolerance

Several events control the activation, proliferation, and the continued Ag responsiveness of naïve and memory T lymphocytes. Here we review the individual contributions of TCR, CD28, and IL-2-driven signaling to T cell proliferation and anergy avoidance. The role of mTOR as a rheostat capable of integrating extracellular, plasma membrane-associated, and intracellular signals with relevance to T cell priming and tolerance is discussed.

Phosphoinositide-dependent kinase 1 targets protein kinase A in a pathway that regulates interleukin 4

CD28 plays a critical role in T cell immune responses. Although the kinase Akt has been shown to act downstream of CD28 in T helper (Th)1 cytokine induction, it does not induce Th2 cytokines such as interleukin 4 (IL-4). We recently reported that phosphoinositide-dependent kinase 1 (PDK1) partially corrects the defect in IL-4 production present in CD28-deficient T cells, suggesting that PDK1 regulates IL-4 independently of Akt. We now describe a signaling pathway in which PDK1 targets IL-4 in the murine Th2 cell line D10. PDK1-mediated activation of this pathway is dependent on protein kinase A (PKA) and the nuclear factor of activated T cells (NFAT) P1 transcriptional element in the IL-4 promoter. PDK1 localizes to the immune synapse in a phosphatidylinositol 3-kinase–dependent manner, partially colocalizes with PKA at the synapse, and physically interacts with PKA. In RNA interference knockdown experiments, PDK1 is necessary for phosphorylation of PKA in T cells, as well as for activation of the IL-4 NFAT P1 element by the T cell receptor (TCR) and CD28. Phosphorylation of the critical PKA threonine residue is stimulated by engagement of TCR/CD28 via a PDK1-dependent mechanism. These findings together define a pathway linking the kinases PDK1 and PKA in the induction of the Th2 cytokine IL-4.

Association of Grb-2 and PI3K p85 with phosphotyrosile peptides derived from BTLA

B and T lymphocyte attenuator (BTLA) is a recently identified inhibitory receptor expressed by B and T cells. We previously identified two tyrosine-containing signaling motifs in the cytoplasmic domain of BTLA that interact with the SHP-1 and SHP-2 phosphatases. BTLA has a third conserved tyrosine-containing motif within the cytoplasmic domain, similar in sequence to a Grb-2 recruitment site. To identify specific interacting proteins that would be recruited to this motif, we carried out an unbiased screen by using synthetic peptides in active (e.g., phosphotyrosil-containing) or control (e.g., non-phosphorylated) forms as baits. Using mass spectrometry, we identified two specific interacting proteins, Grb-2 and the p85 subunit of PI3K. Further, we demonstrate that the interaction with Grb-2 is direct, whereas the recruitment of the p85 subunit by BTLA phosphotyrosile-containing peptides may be indirect via its association with Grb-2. These findings may provide biochemical basis for previously unexplained actions of BTLA.

Impaired Activation and Localization of LAT in Anergic T Cells as a Consequence of a Selective Palmitoylation Defect

The molecular basis of T cell anergy is not completely understood. We show that in antigen-primed anergic murine CD4(+) T cells the linker for activation of T cells (LAT) is hypophosphorylated upon CD3/CD28 restimulation. Signaling events downstream of LAT (PLCgamma1 phosphorylation and p85 [PI3-K] association) were impaired, whereas upstream events (CD3zeta and ZAP-70 phosphorylation) remained intact. LAT recruitment to the immunological synapse and its localization in detergent-resistant membrane (DRM) fractions were defective in anergic T cells. These defects resulted from impaired palmitoylation of LAT and were selective since the DRM localization and palmitoylation of Fyn were intact. This LAT defect was independent of Cbl-b and did not reflect enhanced LAT degradation. These results identify LAT as the most upstream target of anergy induction; moreover, they suggest that regulation of the amount of LAT in the immunological synapse and DRM by posttranslational palmitoylation contributes to the induction of T cell anergy.

NKG2D-mediated signaling requires a DAP10-bound Grb2-Vav1 intermediate and phosphatidylinositol-3-kinase in human natural killer cells

NKG2D is an important immunosurveillance receptor that responds to stress-induced ligand expression on tumors and virus-infected cells. Human natural killer cells express NKG2D and require the transmembrane adaptor DAP10 to initiate their full cytotoxic activation. However, DAP10 has no immunoreceptor tyrosine-based activation motif and thus the mechanism of recruiting 'downstream' effector proteins is unclear. We show here that binding of the p85 subunit of phosphatidylinositol-3- kinase to DAP10 could not by itself trigger cell-mediated cytotoxicity and that binding of an intermediate consisting of the DAP10 binding partner Grb2 and the effector molecule Vav1 (Grb2-Vav1) to DAP10 was sufficient to initiate tyrosine-phosphorylation events. For full calcium release and cytotoxicity to occur, both Grb2-Vav1 and p85 had to bind to DAP10. These findings identify a previously unknown mechanism by which NKG2D-DAP10 mediates cytotoxicity and provides a framework for evaluating activation by other receptor complexes that lack immunoreceptor tyrosine-based activation motifs.

Phosphatidylinositol 3-Kinase Functions as a Ras Effector in the Signaling Cascade That Regulates Dephosphorylation of the Actin-Remodeling Protein Cofilin after Costimulation of Untransformed Human T Lymphocytes

The activity of cofilin, an actin-remodeling protein, is required for T lymphocyte activation with regard to formation of the immunological synapse, cytokine production, and proliferation. In unstimulated T PBL (PB-T), cofilin is present in its Ser3-phosphorylated inactive form. Costimulation of TCR/CD3 and CD28 induces dephosphorylation and, thus, activation of cofilin. In this study we characterized the signaling cascades leading to cofilin activation in untransformed human PB-T. We show that a Ras-PI3K cascade regulates dephosphorylation of cofilin in PB-T. The GTPase Ras is a central mediator of this pathway; transient expression of an activated form of H-Ras in PB-T triggered the dephosphorylation of cofilin. Inhibition of either MAPK/ERK kinase or PI3K blocked both Ras-induced and costimulation-induced cofilin dephosphorylation in PB-T, showing that the combined activities of both signaling proteins are required to activate cofilin. That Ras functions as a central regulator of cofilin dephosphorylation after costimulation through CD3 x CD28 was finally proven by transient expression of a dominant negative form of H-Ras in primary human PB-T. It clearly inhibited costimulation-induced cofilin dephosphorylation, and likewise, activation of PI3K was diminished. Our data, in addition, demonstrate that regarding the downstream effectors of Ras, a clear difference exists between untransformed human PB-T and the T lymphoma line Jurkat. Thus, in PB-T the Ras signaling cascade is able to activate PI3K, whereas in Jurkat cells this is not the case. In addition to the insights into the regulation of cofilin, this finding discloses a to date unrecognized possibility of PI3K activation in T lymphocytes.

Se-methylselenocysteine inhibits phosphatidylinositol 3-kinase activity of mouse mammary epithelial tumor cells in vitro

Introduction

Se-methylselenocysteine (MSC), a naturally occurring selenium compound, is a promising chemopreventive agent against in vivo and in vitro models of carcinogen-induced mouse and rat mammary tumorigenesis. We have demonstrated previously that MSC induces apoptosis after a cell growth arrest in S phase in a mouse mammary epithelial tumor cell model (TM6 cells) in vitro. The present study was designed to examine the involvement of the phosphatidylinositol 3-kinase (PI3-K) pathway in TM6 tumor model in vitro after treatment with MSC.

Methods

Synchronized TM6 cells treated with MSC and collected at different time points were examined for PI3-K activity and Akt phosphorylation along with phosphorylations of Raf, MAP kinase/ERK kinase (MEK), extracellular signal-related kinase (ERK) and p38 mitogen-activated protein kinase (MAPK). The growth inhibition was determined with a [³H]thymidine incorporation assay. Immunoblotting and a kinase assay were used to examine the molecules of the survival pathway.

Results

PI3-K activity was inhibited by MSC followed by dephosphorylation of Akt. The phosphorylation of p38 MAPK was also downregulated after these cells were treated with MSC. In parallel experiments MSC inhibited the Raf–MEK–ERK signaling pathway.

Conclusion

These studies suggest that MSC blocks multiple signaling pathways in mouse mammary tumor cells. MSC inhibits cell growth by inhibiting the activity of PI3-K and its downstream effector molecules in mouse mammary tumor cells in vitro.

CD28 regulates the translation of Bcl-xL via the phosphatidylinositol 3-kinase/mammalian target of rapamycin pathway

In concert with the TCR, CD28 promotes T cell survival by regulating the expression of the antiapoptotic protein Bcl-x(L). The mechanism by which CD28 mediates the induction of Bcl-x(L) remains unknown. We show that although signaling through the TCR is sufficient to stimulate transcription of Bcl-x(L) mRNA, CD28, by activating PI3K and mammalian target of rapamycin, provides a critical signal that regulates the translation of Bcl-x(L) transcripts. We observe that CD28 induced 4E-binding protein-1 phosphorylation, an inhibitor of the translational machinery, and that CD28 costimulation directly augmented the translation of a Bcl-x(L) 5'-untranslated region reporter construct. Lastly, costimulation by CD28 shifted the distribution of Bcl-x(L) mRNA transcripts from the pretranslation complex to the translationally active polyribosomes. These results demonstrate that CD28 relieves the translational inhibition of Bcl-x(L) in a PI3K/mammalian target of rapamycin-dependent manner.

Ras initiates phosphatidyl-inositol-3-kinase (PI3K)/PKB mediated signalling pathways in untransformed human peripheral blood T lymphocytes

Activation of T lymphocytes through costimulation of the T cell receptor/CD3 complex (TCR/CD3) and coreceptors (e.g. CD2 or CD28) leads to production of the growth factor interleukin-2 (IL-2) and subsequent proliferation. For these activation processes, remodelling of the actin cytoskeleton plays an important functional role. We have shown that the activity of the actin-remodelling protein cofilin is crucially involved in T lymphocyte activation processes. In unstimulated human peripheral blood T lymphocytes (PB-T) cofilin exists in its inactive ser-3-phosphorylated form. T lymphocyte activation through costimulation of TCR plus the coreceptors CD28 or CD2, respectively, induces the dephosphorylation of cofilin. Concomitantly, cofilin associates with the actin cytoskeleton. The functional importance of cofilin for T lymphocyte activation was shown employing cell permeable peptides which block binding of cofilin to actin. In human PB-T these peptides impair the formation of the immunological synapse and inhibit the induction of T lymphocyte proliferation and cytokine production. The serine phosphatases PP1 and PP2A dephosphorylate cofilin in T lymphocytes. Importantly, a PKC-Ras-MEK/PI3K-cascade links costimulation of PB-T through TCR/CD3 and CD28 to activation of cofilin through dephosphorylation. Notably, the induction of cofilin dephosphorylation requires the combined activities of two Ras-effectors, namely MEK and PI3K. With respect to PI3K, this result was unexpected since so far it was generally assumed that-unlike in other cell types-Ras is not able to activate PI3K in T lymphocytes, as concluded from experiments performed with the human T-lymphoma line Jurkat. This discrepancy implied that the signalling events upstream of PI3K differ between PB-T and Jurkat cells. In line with this, we found that in PB-T the PI3K-inhibitors wortmannin and LY294002 block activation induced cofilin dephosphorylation and its association with the actin cytoskeleton. In Jurkat cells, however, where cofilin is present mainly in its non-phosphorylated form and permanently associated with the actin cytoskeleton, wortmannin and LY294002 do not block these events. Studies by others employing these PI3K-inhibitors have also led to such contradictory results: While in stimulated PB-T these inhibitors repress expression of IL-2, they even enhance IL-2 expression in Jurkat cells. These findings show that signalling events in Jurkat cells are not representative for signalling processes in untransformed human T lymphocytes. Importantly, our data demonstrate that-rebutting a persistent dogma-a T-cell specific uncoupling of PI3K from Ras does not exist.

SHP-1 and SHP-2 associate with immunoreceptor tyrosine-based switch motif of programmed death 1 upon primary human T cell stimulation, but only receptor ligation prevents T cell activation

To study the cis- and trans-acting factors that mediate programmed death 1 (PD-1) signaling in primary human CD4 T cells, we constructed a chimeric molecule consisting of the murine CD28 extracellular domain and human PD-1 cytoplasmic tail. When introduced into CD4 T cells, this construct mimics the activity of endogenous PD-1 in terms of its ability to suppress T cell expansion and cytokine production. The cytoplasmic tail of PD-1 contains two structural motifs, an ITIM and an immunoreceptor tyrosine-based switch motif (ITSM). Mutation of the ITIM had little effect on PD-1 signaling or functional activity. In contrast, mutation of the ITSM abrogated the ability of PD-1 to block cytokine synthesis and to limit T cell expansion. Further biochemical analyses revealed that the ability of PD-1 to block T cell activation correlated with recruitment of Src homology region 2 domain-containing phosphatase-1 (SHP-1) and SHP-2, and not the adaptor Src homology 2 domain-containing molecule 1A, to the ITSM domain. In TCR-stimulated T cells, SHP-2 associated with PD-1, even in the absence of PD-1 engagement. Despite this interaction, the ability of PD-1 to block T cell activation required receptor ligation, suggesting that colocalization of PD-1 with CD3 and/or CD28 may be necessary for inhibition of T cell activation.

Translocation of CD28 to lipid rafts and costimulation of IL-2

Stimulation of the CD28 costimulatory receptor can lead to an increased surface lipid raft expression in T lymphocytes. Here, we demonstrate that CD28 itself is recruited to lipid rafts in both Jurkat and peripheral blood T lymphocytes. This recruitment of CD28 is triggered by engagement with either anti-CD28 mAbs or a natural ligand of CD28, B7.2 (CD86). All detectable tyrosine-phosphorylated CD28 is in the lipid raft fractions, as is all of the CD28 associated with phosphatidylinositol 3-kinase, which is recruited to CD28 by tyrosine phosphorylation. Targeting the CD28 cytoplasmic domain to lipid rafts results in its tyrosine phosphorylation, indicating that tyrosine phosphorylation of CD28 may occur after translocation to lipid rafts. Studies with Jurkat cells deficient in Lck and CD45 demonstrate that movement of CD28 into lipid rafts does not require Lck and CD45 and can occur despite reduction of CD28 tyrosine phosphorylation to below the levels of detection. Analysis of murine CD28 mutants reveals a correlation between translocation to lipid rafts and costimulation of IL-2 production. Taken together with the known importance of lipid rafts in T cell activation, these observations suggest that translocation to lipid rafts may play an important role in CD28 signaling.

Distinct regions in the CD28 cytoplasmic domain are required for T helper type 2 differentiation

CD28 costimulation is essential for CD4(+) T cell proliferation, survival, interleukin 2 (IL-2) production and T helper type 2 development. To define the nature of the signals that may drive different T cell responses, we have done a structure-function analysis of the CD28 cytoplasmic tail in primary T cells. CD28-mediated T cell proliferation and IL-2 production did not require a particular cytoplasmic domain. In contrast, IL-4 production was driven by the cooperative activity of specific motifs within the CD28 cytoplasmic tail. Using a gene-complementation approach, we provide evidence that one component of this T helper type 2 differentiation signal was mediated by 3-phosphoinositide-dependent protein kinase 1. Thus, different mechanisms underlie the induction of distinct T cell functional responses by CD28.

PI3-kinase and MAP-kinase signaling cascades in AILIM/ICOS- and CD28-costimulated T-cells have distinct functions between cell proliferation and IL-10 production

Both AILIM/ICOS and CD28 provide positive costimulatory signals for T-cell activation, resulting in proliferation and cytokine production. In this study, we attempted to clarify the key signaling molecules in T-cell proliferation, and also IL-2 and IL-10 production, during T-cell activation by CD3 induced by costimulation with either AILIM/ICOS or CD28. We examined the role of both the PI3-kinase/Akt pathway and MAP kinase family members such as ERK1/2, JNK, and p38 kinase in this process. PI3-kinase and Erk1/2 were shown to potentially regulate primary T-cell activation and subsequent proliferation via both AILIM/ICOS- or CD28-mediated costimulation and the Erk signaling cascade was essential for this proliferation induction and also for IL-2 production. The JAK inhibitor, AG490, inhibited this induction. Our studies indicate that IL-2 is necessary for induction of T-cell proliferation and that the quantities of IL-2 produced by AILIM/ICOS ligation are also sufficient for T-cells to proliferate. In contrast, inhibition of Akt and p38, that are phosphorylated by both AILIM/ICOS and CD28-ligation, could downregulate IL-10 production but not T-cell proliferation. These data raise the interesting possibility that the signaling cascades between T-cell proliferation and IL-10 production are regulated by different molecules in AILIM/ICOS- and CD28-costimulated T-cells.

Regulation of sustained actin dynamics by the TCR and costimulation as a mechanism of receptor localization

The localization of receptors, signaling intermediates, and cytoskeletal components at the T cell/APC interface is thought to be a major determinant of efficient T cell activation. However, important questions remain open. What are the dynamics of the T cell cytoskeleton as a potential mediator of such localization? How are they regulated by the TCR and costimulatory receptors? Do they actually mediate receptor localization? In this study, we have addressed these questions. Even under limiting T cell activation conditions, actin accumulated immediately and transiently at the T cell/APC interface, the microtubule organizing center reoriented toward it. In contrast, sustained (>5 min) actin accumulation in highly dynamic patterns depended on an optimal T cell stimulus: high concentrations of the strong TCR ligand agonist peptide/MHC and engagement of the costimulatory receptors CD28 and LFA-1 were required in an overlapping, yet distinct, fashion. Intact sustained actin dynamics were required for interface accumulation of TCR/MHC in a central pattern and for efficient T cell proliferation, as established using a novel approach to selectively block only the sustained actin dynamics. These data suggest that control of specific elements of actin dynamics by TCR and costimulatory receptors is a mechanism to regulate the efficiency of T cell activation.

Molecular characterization of CD28 and cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) of woodchuck (Marmota monax)

Eastern woodchuck (Marmota monax) became an important animal model to study the immunological processes in hepatitis B virus infection. To facilitate further study of T-cell responses in this model, we cloned and sequenced the cDNAs of Woodchuck CD28 and cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), which play important roles for the regulation of T-cell activation by delivering the costimulation signals. According to the deduced amino-acid sequences, Woodchuck CD28 showed a similarity of 70% to 80% to its mammalian homologues. Woodchuck CTLA-4 has a higher similarity of 74% to 85% to corresponding mammalian CTLA-4 molecules. The strict conservation of critical amino-acid residues like cystein and asparagine residues in Woodchuck CD28 and CTLA-4 suggests that both molecules may structurally resemble their human or mouse homologues. A hexapeptide motif, MYPPPY, which has been supposed to be essential for the interaction with CD80, is present in both Woodchuck CD28 and CTLA-4. The cloned cDNAs of Woodchuck CD28 and CTLA-4 were placed under the control of the cytomegalovirus (CMV) promoter of the mammalian expression vector pcDNA3. Both proteins were expressed and detected by respective crossreactive antibodies in transiently transfected mammalian cells. By immunohistochemical staining with these antibodies, CD28 and CTLA-4 were also detected on cultured woodchuck peripheral blood lymphocytes. The molecular characterization of Woodchuck CD28 and CTLA-4 will facilitate studies on the T-cell response to hepadnavirus in the woodchuck model.

PI3K in lymphocyte development, differentiation and activation

Phosphoinositide 3-kinases (PI3Ks) regulate numerous biological processes, including cell growth, differentiation, survival, proliferation, migration and metabolism. In the immune system, impaired PI3K signalling leads to immunodeficiency, whereas unrestrained PI3K signalling contributes to autoimmunity and leukaemia. New insights into the role of PI3Ks in lymphocyte biology have been derived from gene-targeting studies, which have identified the PI3K subunits that are involved in B-cell and T-cell signalling. In particular, the catalytic subunit p110delta seems to be adapted to transmit antigen-receptor signalling in B and T cells. Additional recent work has provided new insights into the molecular interactions that lead to PI3K activation and the signalling pathways that are regulated by PI3K.

The PI-3 kinase/Akt pathway and T cell activation: pleiotropic pathways downstream of PIP3

Ligation of the T cell receptor for antigen (TCR) and/or costimulatory receptor CD28 results in rapid activation of phosphoinositide-3 kinase (PI-3 kinase). It remains unclear, however, precisely how this activation occurs and also how the newly generated phospholipid products trigger the various events associated with T cell activation. Here we discuss the current understanding of how PI-3 kinase is activated by the TCR and CD28 and what roles its products play in T cell activation. We also review recent advances in understanding the function of Akt in particular, especially its role in CD28 costimulation. Several functional targets of Akt are discussed in this regard: inducible transcription, cell survival, glucose metabolism, and the cellular translational machinery. These pathways have been associated with TCR/CD28 costimulation, and they have also been implicated as targets of Akt.

Lipid phosphatases in the regulation of T cell activation: living up to their PTEN-tial

The initiating events associated with T activation in response to stimulation of the T cell antigen receptor (TCR) and costimulatory receptors, such as CD28, are intimately associated with the enzymatically catalyzed addition of phosphate not only to key tyrosine, threonine and serine residues in proteins but also to the D3 position of the myo-inositol ring of phosphatidylinositol (PtdIns). This latter event is catalyzed by the lipid kinase phosphoinositide 3-kinase (PI3K). The consequent production of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 serves both to recruit signaling proteins to the plasma membrane and to induce activating conformational changes in proteins that contain specialized domains for the binding of these phospholipids. The TCR signaling proteins that are subject to regulation by PI3K include Akt, phospholipase Cgamma1 (PLCgamma1), protein kinase C zeta (PKC-zeta), Itk, Tec and Vav, all of which play critical roles in T cell activation. As is the case for phosphorylation of protein substrates, the phosphorylation of PtdIns is under dynamic regulation, with the D3 phosphate being subject to hydrolysis by the 3-phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10), thereby placing PTEN in direct opposition to PI3K. In this review we consider recent data concerning how PTEN may act in regulating the process of T cell activation.