The CD28 and CTLA-4 receptors associate with the serine/threonine phosphatase PP2A

CTLA-4 activation of phosphatidylinositol 3-kinase (PI 3-K) and protein kinase B (PKB/AKT) sustains T-cell anergy without cell death

The balance of T-cell proliferation, anergy and apoptosis is central to immune function. In this regard, co-receptor CTLA-4 is needed for the induction of anergy and tolerance. One central question concerns the mechanism by which CTLA-4 can induce T-cell non-responsiveness without a concurrent induction of antigen induced cell death (AICD). In this study, we show that CTLA-4 activation of the phosphatidylinositol 3-kinase (PI 3-K) and protein kinase B (PKB/AKT) sustains T-cell anergy without cell death. CTLA-4 ligation induced PI 3K activation as evidenced by the phosphorylation of PKB/AKT that in turn inactivated GSK-3. The level of activation was similar to that observed with CD28. CTLA-4 induced PI 3K and AKT activation also led to phosphorylation of the pro-apoptotic factor BAD as well as the up-regulation of BcL-XL. In keeping with this, CD3/CTLA-4 co-ligation prevented apoptosis under the same conditions where T-cell non-responsiveness was induced. This effect was PI 3K and PKB/AKT dependent since inhibition of these enzymes under conditions of anti-CD3/CTLA-4 co-ligation resulted in cell death. Our findings therefore define a mechanism by which CTLA-4 can induce anergy (and possibly peripheral tolerance) by preventing the induction of cell death.

Calcium/calmodulin-dependent kinase IV in immune and inflammatory responses: novel routes for an ancient traveller

Ca(2+) is a pivotal second messenger controlling the activation of lymphocytes. Crucial events in the social life of immunocytes are regulated by the calcium/calmodulin complex (Ca(2+)/CaM), which controls the activation status of many enzymes, including the Ca(2+)/CaM-dependent Ser-Thr kinases (CaMK) I, II and IV. Although CaMKI and CaMKII are expressed ubiquitously, CaMKIV is found predominately in cells of the nervous and immune systems. To be active, CaMKIV requires binding of Ca(2+)/CaM and phosphorylation by CaMKKalpha or beta. The requirement of two CaM kinases in the same signalling pathway led to the concept of a CaM kinase cascade. In this review, we focus on the roles of CaMKK and CaMKIV cascades in immune and inflammatory 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.

CTLA-4 trafficking and surface expression

The T-cell co-receptor cytotoxic T-cell antigen 4 (CTLA-4) has a strong inhibitory role as shown by the lymphoproliferative phenotype of CTLA-4-deficient mice. Despite its potent effects on T-cell function, CTLA-4 is primarily an intracellular antigen whose surface expression is tightly regulated by restricted trafficking to the cell surface and rapid internalisation. Recently, several signalling molecules such as Trim, PLD, ARF-1 and TIRC7 have been described to be involved in the transport of CTLA-4 to the cell surface. Minor changes in surface expression levels have major effects on the outcome of T-cell activation. Optimal regulation of CTLA-4 surface expression is crucial for the balance of stimulatory and inhibitory signals to maximize protective immune responses while maintaining immunological tolerance and preventing autoimmunity.

CTLA-4 disrupts ZAP70 microcluster formation with reduced T cell/APC dwell times and calcium mobilization

CTLA-4 is a co-receptor that modulates the threshold of T cell activation and autoimmunity. We previously showed that CTLA-4 reverses the TCR-mediated stop signal needed for T cell/APC interactions [Schneider et al., Science 2006, 313: 1972]. In this study, using a different T cell system, we show that CTLA-4 expression changed the behavior of T8.1 T cells by reducing the contact time between T cell and APC, preventing re-inforced contacts, and reducing the contact area at the immunological synapse. This led to a major reduction in Ca(2+) influx/mobilization and interleukin-2 production. Further, anti-CD3/CTLA-4 increased T cell motility on antibody-coated glass slides, concurrent with an abrogation of ZAP70 microcluster formation. Our findings further support a role for CTLA-4 in limiting the interaction between T cell and APC that is needed for optimal activation.

The reverse stop-signal model for CTLA4 function

Activation of the T-cell co-receptor cytotoxic T-lymphocyte antigen 4 (CTLA4) has a pivotal role in adjusting the threshold for T-cell activation and in preventing autoimmunity and massive tissue infiltration by T cells. Although many mechanistic models have been postulated, no single model has yet accounted for its overall function. In this Opinion article, I outline the strengths and weaknesses of the current models, and present a new 'reverse stop-signal model' to account for CTLA4 function.

Tpl2 and ERK transduce antiproliferative T cell receptor signals and inhibit transformation of chronically stimulated T cells

The protein kinase encoded by the Tpl2 protooncogene plays an obligatory role in the transduction of Toll-like receptor and death receptor signals in macrophages, B cells, mouse embryo fibroblasts, and epithelial cells in culture and promotes inflammatory responses in animals. To address its role in T cell activation, we crossed the T cell receptor (TCR) transgene 2C, which recognizes class I MHC presented peptides, into the Tpl2(-/-) genetic background. Surprisingly, the TCR2C(tg/tg)/Tpl2(-/-) mice developed T cell lymphomas with a latency of 4-6 months. The tumor cells were consistently TCR2C(+)CD8(+)CD4(-), suggesting that they were derived either from chronically stimulated mature T cells or from immature single positive (ISP) cells. Further studies showed that the population of CD8(+) ISP cells was not expanded in the thymus of TCR2C(tg/tg)/Tpl2(-/-) mice, making the latter hypothesis unlikely. Mature peripheral T cells of Tpl2(-/-) mice were defective in ERK activation and exhibited enhanced proliferation after TCR stimulation. The same cells were defective in the induction of CTLA4, a negative regulator of the T cell response, which is induced by TCR signals via ERK. These findings suggest that Tpl2 functions normally in a feedback loop that switches off the T cell response to TCR stimulation. As a result, Tpl2, a potent oncogene, functions as a tumor suppressor gene in chronically stimulated T cells.

TCR/CD3 mediated stop-signal is decoupled in T-cells from Ctla4 deficient mice

CTLA-4 is a co-receptor that plays a pivotal role in regulating the threshold for T-cell activation. We recently reported that CTLA-4 ligation can over-ride the stop-signal induced by anti-CD3 ligation [Schneider H, Downey J, Smith A, Zinselmeyer BH, Rush C, Brewer JM, et al. Reversal of the TCR stop-signal by CTLA-4. Science 2006;313:1972]. While these studies compared CTLA-4 positive and negative T-cells from normal mice, little is known regarding the behaviour of T-cells from diseased Ctla4 deficient mice with auto-proliferative disease. In this study, we show that while activated wild-type and Ctla-4-/- T-cells have similar rates of motility, Ctla-4-/- T-cells show a marked resistance to the induction of a stop-signal by anti-CD3 ligation. By contrast, T-cells from normal mice and CD28 deficient mice underwent a normal slowing of motility in response to anti-CD3 ligation. Our findings identify a fundamental difference between normal versus CTLA-4-/- T-cells from diseased mice in the regulation of motility by anti-CD3 ligation. This dysregulation of motility may contribute to the tissue infiltration and the autoimmune disorder observed in Ctla-4-/- mice.

Cytotoxic T-Lymphocyte–Associated Antigen-4: Fig. 1

Previously, the development of immune-based therapies has primarily focused on vaccines and cytokines, yielding benefit in a small percentage of patients. Recent advances in our understanding of the function of costimulatory molecules have revitalized enthusiasm in the development of immune therapies for cancer. This family of proteins possesses properties involved in both lymphocyte activation and immune-inhibitory functions. The costimulatory molecule with the greatest translation into the clinic thus far is CTL-associated antigen-4 (CTLA-4). CTLA-4 engagement leads to T-cell inhibition by two principle mechanisms. The first involves competitive binding with CD28 for B7 on the antigen-presenting cell. The second is direct intracellular inhibitory signals mediated by the CTLA-4 cytoplasmic tail. Numerous clinical trials testing the blockade of CTLA-4 signaling with fully human monoclonal antibodies have treated a variety of cancers, with the most experience in the treatment of metastatic melanoma. Significant antitumor activity as well as potential autoimmune-related toxicities have been observed. Further clinical investigation with CTLA-4 blockade, planned clinical trials testing manipulation of other costimulatory molecules, and continued improvement in understanding of costimulatory pathways present a new era of immune therapies for cancer patients.

The PP2A-associated protein alpha4 plays a critical role in the regulation of cell spreading and migration

Compared with kinases, the role of protein phosphatases in regulating biological functions is less well understood. Here we show that alpha4, a non-catalytic subunit of the protein phosphatase 2A, plays a major role in the control of cell spreading, migration, and cytoskeletal architecture. Fibroblasts lacking alpha4 were impaired in their ability to spread and migrate compared with wild-type cells, whereas enforced expression of alpha4 promoted cell spreading and migration. These effects were not restricted to fibroblasts. Using a T cell-specific alpha4 transgenic mouse model, increased alpha4 expression was found to increase lymphocyte motility and chemotaxis. Elevated alpha4 expression results in an increase in the GTP-bound state of Rac1, and GTP-bound Rac1 was dramatically reduced in alpha4-deficient cells. A constitutively active mutant of Rac1 rescued the defects of cell spreading and migration caused by alpha4 deletion, while inhibition of Rac1 blocked the ability of alpha4 to promote cell migration. Together, these data define a novel role for the protein phosphatase 2A regulatory subunit alpha4 in the regulation of cell spreading and migration.

A role for mammalian target of rapamycin in regulating T cell activation versus anergy

Whether TCR engagement leads to activation or tolerance is determined by the concomitant delivery of multiple accessory signals, cytokines, and environmental cues. In this study, we demonstrate that the mammalian target of rapamycin (mTOR) integrates these signals and determines the outcome of TCR engagement with regard to activation or anergy. In vitro, Ag recognition in the setting of mTOR activation leads to full immune responses, whereas recognition in the setting of mTOR inhibition results in anergy. Full T cell activation is associated with an increase in the phosphorylation of the downstream mTOR target S6 kinase 1 at Thr(421)/Ser(424) and an increase in the mTOR-dependent cell surface expression of transferrin receptor (CD71). Alternatively, the induction of anergy results in markedly less S6 kinase 1 Thr(421)/Ser(424) phosphorylation and CD71 surface expression. Likewise, the reversal of anergy is associated not with proliferation, but rather the specific activation of mTOR. Importantly, T cells engineered to express a rapamycin-resistant mTOR construct are resistant to anergy induction caused by rapamycin. In vivo, mTOR inhibition promotes T cell anergy under conditions that would normally induce priming. Furthermore, by examining CD71 surface expression, we are able to distinguish and differentially isolate anergic and activated T cells in vivo. Overall, our data suggest that by integrating environmental cues, mTOR plays a central role in determining the outcome of Ag recognition.

Signalling to suit function: tailoring phosphoinositide 3-kinase during T-cell activation

Members of the CD28 family of co-receptors are crucial determinants of the outcome of T-cell activation. These receptors interact with ligands in the B7 family and either costimulate or co-inhibit signals through antigen-specific receptors. The T-cell-costimulatory molecules CD28 and inducible costimulator recruit and activate class 1A phosphoinositide 3-kinase (PI3K). Interestingly, the co-inhibitory molecules cytotoxic T lymphocyte antigen-4 and B and T lymphocyte attenuator also interact with class 1A PI3K. However, all co-inhibitory receptors share an ability to oppose activation of the key PI3K effector protein kinase B (also known as Akt). Recent evidence suggests that distinct mechanisms exist to limit Akt activation by different co-inhibitory receptors. This article examines how differential positive or negative regulation of the PI3K-Akt signalling pathway by CD28 family receptors enables functional differences between the receptors.

Mitogenic CD28 Signals Require the Exchange Factor Vav1 to Enhance TCR Signaling at the SLP-76-Vav-Itk Signalosome

Almost all physiological T cell responses require costimulation-engagement of the clonotypic TCR with MHC/Ag and CD28 by its ligands CD80/86. Whether CD28 provides signals that are qualitatively unique or quantitatively amplify TCR signaling is poorly understood. In this study, we use superagonistic CD28 Abs, which induce T cell proliferation without TCR coligation, to determine how CD28 contributes to mitogenic responses. We show that mitogenic CD28 signals require but do not activate the proximal TCR components TCRzeta and Zap-70 kinase. In cell lines lacking proximal TCR signaling, an early defect in the CD28 pathway is in phosphorylation of the adaptor molecule SLP-76, which we show is essential for recruitment of the exchange factor Vav leading to Ca(2+) flux and IL-2 production. Point mutations in CD28 that result in diminished Vav phosphorylation also result in defective Ca(2+) flux, IL-2 production, and Tec-kinase phosphorylation. Using Vav1-deficient mice, we further demonstrate the importance of Vav1 for efficient proliferation, IL-2 production, and Ca(2+) flux. Our results indicate that CD28 signals feed into the TCR signaling pathway at the level of the SLP-76 signalosome.

IMGT Colliers de Perles and IgSF domain standardization for T cell costimulatory activatory (CD28, ICOS) and inhibitory (CTLA4, PDCD1 and BTLA) receptors

T cell activation depends on the specific recognition by their T cell receptors (TR) of antigenic peptides bound to major histocompatibility complex (pMHC). Optimal T cell responses occur when T cells not only receive antigen-specific signals through the TR but also non-antigen-specific costimulatory activatory or inhibitory signals through costimulatory receptors. The activatory CD28/B7-1 (or B7-2), inhibitory CTLA4/B7-1 (or B7-2), activatory ICOS/B7H2 and inhibitory PDCD1/B7H1 (or B7DC) pathways involve the interaction of the V-LIKE-DOMAIN of the receptor with a B7 family member. The BTLA/HVEM pathway involves the interaction of the BTLA receptor C-LIKE-DOMAIN with HVEM, a TNFR family member. The human and mouse CD28, CTLA4, ICOS, PDCD1 and BTLA genes, alleles and alternative transcripts and the IMGT Colliers de Perles of the IgSF domains, based on the IMGT unique numbering, are described according to the IMGT-ONTOLOGY concepts of IMGT, the international ImMunoGeneTics information system, http://imgt.cines.fr).

CD28 ligation costimulates cell death but not maturation of double-positive thymocytes due to defective ERK MAPK signaling

The differentiation of double-positive (DP) CD4(+)CD8(+) thymocytes to single-positive CD4(+) or CD8(+) T cells is regulated by signals that are initiated by coengagement of the Ag (TCR) and costimulatory receptors. CD28 costimulatory receptors, which augment differentiation and antiapoptotic responses in mature T lymphocytes, have been reported to stimulate both differentiation and apoptotic responses in TCR-activated DP thymocytes. We have used artificial APCs that express ligands for TCR and CD28 to show that CD28 signals increase expression of CD69, Bim, and cell death in TCR-activated DP thymocytes but do not costimulate DP thymocytes to initiate the differentiation program. The lack of a differentiation response is not due to defects in CD28-initiated TCR proximal signaling events but by a selective defect in the activation of ERK MAPK. To characterize signals needed to initiate the death response, a mutational analysis was performed on the CD28 cytoplasmic domain. Although mutation of all of CD28 cytoplasmic domain signaling motifs blocks cell death, the presence of any single motif is able to signal a death response. Thus, there is functional redundancy in the CD28 cytoplasmic domain signaling motifs that initiate the thymocyte death response. In contrast, immobilized Abs can initiate differentiation responses and cell death in DP thymocytes. However, because Ab-mediated differentiation occurs through CD28 receptors with no cytoplasmic domain, the response may be mediated by increased adhesion to immobilized anti-TCR Abs.

T Cell Receptor-Interacting Molecule Acts as a Chaperone to Modulate Surface Expression of the CTLA-4 Coreceptor

The costimulatory molecule CTLA-4 is a potent downregulator of T cell responses. Although localized mostly in intracellular compartments, little is understood regarding the mechanism that regulates its transport to the cell surface. In this study, we demonstrated that the adaptor TRIM (T cell receptor-interacting molecule) bound to CTLA-4 in the trans Golgi network (TGN) and promoted transport of CTLA-4 to the surface of T cells. Increased TRIM expression augmented surface CTLA-4 expression, and pulse-chase analysis showed a more rapid transport of CTLA-4 to the cell surface. A reduction of TRIM expression by small hairpin RNAs reduced the expression of surface CTLA-4. This resulted in a more localized pattern of CTLA-4 in the TGN. Altered CTLA-4 expression by TRIM was accompanied by corresponding changes in coreceptor-mediated effects on cytokine production and proliferation. Our findings identify a role for TRIM as a chaperone in regulating CTLA-4 expression and function by enhancing CTLA-4 transport to the surface of T cells.

Balancing co-stimulation and inhibition with BTLA and HVEM

The interaction between B- and T-lymphocyte attenuator (BTLA), an inhibitory receptor whose extracellular domain belongs to the immunoglobulin superfamily, and herpesvirus-entry mediator (HVEM), a co-stimulatory tumour-necrosis factor receptor, is unique in that it is the only receptor-ligand interaction that directly bridges these two families of receptors. This interaction has raised many questions about how receptors from two different families could interact and what downstream signalling events might occur as a result of receptor ligation. As we discuss, recent studies show that engagement of HVEM with its endogenous ligand (LIGHT) from the tumour-necrosis factor family induces a powerful immune response, whereas HVEM interactions with BTLA negatively regulate T-cell responses.

A genetic library screen for signaling proteins that interact with phosphorylated T cell costimulatory receptors

In the past decade, the fundamental importance and therapeutic potential of costimulatory signals for lymphocyte activation have spurred a large amount of work in immunology, infection, cancer, autoimmune diseases, etc. However, the mechanisms behind T cell costimulation remain unclear, partly due to the lack of suitable techniques. There is an urgent need for functional genomic research to develop comprehensive approaches to direct identification of protein-protein interactions that are dependent on the posttranslational modification of one component of the complex, particularly in the field of T cell immunology. Using inducible costimulator (ICOS) as a model, we failed to find any proteins that associated with the cytoplasmic tail of ICOS by the yeast two-hybrid approach. Therefore, we have developed a new yeast three-hybrid system that facilitates the rapid screening of cDNA libraries to find signaling molecules that interact with phosphorylated T cell costimulatory receptors. We demonstrate the utility of this technique to detect the interaction between ICOS and the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3K). The p85 unit of PI3K is the only signaling molecule identified so far that interacts with ICOS. This system may be of great help in dissecting the mechanisms of T cell costimulation and could be applied to other receptors.

Reversal of the TCR stop signal by CTLA-4

The coreceptor cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) is pivotal in regulating the threshold of signals during T cell activation, although the underlying mechanism is still not fully understood. Using in vitro migration assays and in vivo two-photon laser scanning microscopy, we showed that CTLA-4 increases T cell motility and overrides the T cell receptor (TCR)-induced stop signal required for stable conjugate formation between T cells and antigen-presenting cells. This event led to reduced contact periods between T cells and antigen-presenting cells that in turn decreased cytokine production and proliferation. These results suggest a fundamentally different model of reverse stop signaling, by which CTLA-4 modulates the threshold for T cell activation and protects against autoimmunity.

A molecular perspective of CTLA-4 function

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

CTLA-4 lacking the cytoplasmic domain costimulates IL-2 production in T-cell hybridomas

Optimal T-cell activation depends on the antigen-specific signal mediated by the TCR and engagement of costimulatory receptors such as CD28. CTLA-4, a homologous counterpart of CD28, is considered to be a crucial inhibitory receptor. To test its function separately from CD28 in an antigen-driven and ligand-specific model, we stably transfected the T-cell hybridomas A1.1 and DO11.10, which lack significant endogenous CD28 or CTLA-4 expression, with wild-type CTLA-4 (CTLA-4 WT) and a construct lacking the cytoplasmic tail (tailless [TL]). Functional studies were carried out by co-incubation with APC expressing the B7 ligands for CTLA-4 and appropriate MHC molecules loaded with their cognate antigens. IL-2 production on costimulation of CTLA-4WT and TCR did not differ significantly from untransfected controls. However, coligation of TCR and CTLA-4TL resulted in a vigorous IL-2 response specific for the interaction of CTLA-4 with B7. Thus, lack of the cytoplasmic tail converted CTLA-4 into a costimulatory receptor. This indicates that the CTLA-4 inhibitory function may not be attributable to sequestration of the common B7 ligands when competing with CD28. Rather, ligation of B7 by the CTLA-4 extracellular domain can enhance TCR activation, whereas in the full-length receptor, inhibitory signals mediated by the cytoplasmic domain may override this activation.

CD28 and ICOS: Similar or separate costimulators of T cells?

Numerous studies have revealed that the B7.1/B7.2-CD28 and B7RP-1-ICOS (Inducible COStimulator) pathways provide crucial costimulatory signals to T cells. We have compared the contribution of these pathways during primary and effector responses, in vitro and in vivo, molecularly as well as functionally. This comparison between CD28 an ICOS after initiation of T cell activation demonstrates that both CD28 and ICOS function similarly during expansion, survival and differentiation of T cells and that both CD28 and ICOS are necessary for proper IgG responses. The major differences between CD28 and ICOS are differences in expression of both receptors and ligands, and the fact that CD28 induces IL-2 production, whereas ICOS does not. In addition, ICOS is more potent in the induction of IL-10 production, a cytokine important for suppressive function of T regulatory cells. All data available at present indicate that both molecules are very suitable candidates for immunotherapy, each in their own unique way.

Low surface expression of B7-1 (CD80) is an immunoescape mechanism of colon carcinoma

Artificially enforced expression of CD80 (B7-1) and CD86 (B7-2) on tumor cells renders them more immunogenic by triggering the CD28 receptor on T cells. The enigma is that such B7s interact with much higher affinity with CTLA-4 (CD152), an inhibitory receptor expressed by activated T cells. We show that unmutated CD80 is spontaneously expressed at low levels by mouse colon carcinoma cell lines and other transplantable tumor cell lines of various tissue origins. Silencing of CD80 by interfering RNA led to loss of tumorigenicity of CT26 colon carcinoma in immunocompetent mice, but not in immunodeficient Rag-/- mice. CT26 tumor cells bind CTLA-4Ig, but much more faintly with a similar CD28Ig chimeric protein, thus providing an explanation for the dominant inhibitory effects on tumor immunity displayed by CD80 at that expression level. Interestingly, CD80-negative tumor cell lines such as MC38 colon carcinoma and B16 melanoma express CD80 at dim levels during in vivo growth in syngeneic mice. Therefore, low CD80 surface expression seems to give an advantage to cancer cells against the immune system. Our findings are similar with the inhibitory role described for the dim CD80 expression on immature dendritic cells, providing an explanation for the low levels of CD80 expression described in various human malignancies.

CTLA-4 interacts with STAT5 and inhibits STAT5-mediated transcription

Cytotoxic T-lymphocyte antigen-4 (CTLA-4; CD152) is a member of the immunoglobulin gene superfamily with strong homology to the receptor CD28 with which it shares the ligands CD80 and CD86. Unlike CD28, a potent costimulator of T-cell responses, CTLA-4 is transiently expressed on the cell surface of activated T cells and appears to operate predominantly as a negative regulator of T-cell proliferation. Signal transduction mechanisms utilized by CTLA-4 remain unclear although several mechanisms have been implicated. In this study, we show that the cytoplasmic domain of CTLA-4, but not of CD28, binds to STAT5 in yeast two-hybrid assay and in coimmunoprecipitation assays. Mutations of Tyr165 and Tyr182 in CTLA-4 did not abrogate the interaction of STAT5 with CTLA-4. Finally, the overexpression of CTLA-4 in Jurkat T cells inhibits STAT-mediated activation of STAT5 responsive elements. These results suggest that CTLA-4 and STAT5 interact in T cells and that this interaction is important for CTLA-4 signalling.

Costimulatory Receptors in a Teleost Fish: Typical CD28, Elusive CTLA4

T cell activation requires both specific recognition of the peptide-MHC complex by the TCR and additional signals delivered by costimulatory receptors. We have identified rainbow trout sequences similar to CD28 (rbtCD28) and CTLA4 (rbtCTLA4). rbtCD28 and rbtCTLA4 are composed of an extracellular Ig-superfamily V domain, a transmembrane region, and a cytoplasmic tail. The presence of a conserved ligand binding site within the V domain of both molecules suggests that these receptors likely recognize the fish homologues of the B7 family. The mRNA expression pattern of rbtCD28 and rbtCTLA4 in naive trout is reminiscent to that reported in humans and mice, because rbtCTLA4 expression within trout leukocytes was quickly up-regulated following PHA stimulation and virus infection. The cytoplasmic tail of rbtCD28 possesses a typical motif that is conserved in mammalian costimulatory receptors for signaling purposes. A chimeric receptor made of the extracellular domain of human CD28 fused to the cytoplasmic tail of rbtCD28 promoted TCR-induced IL-2 production in a human T cell line, indicating that rbtCD28 is indeed a positive costimulator. The cytoplasmic tail of rbtCTLA4 lacked obvious signaling motifs and accordingly failed to signal when fused to the huCD28 extracellular domain. Interestingly, rbtCTLA4 and rbtCD28 are not positioned on the same chromosome and thus do not belong to a unique costimulatory cluster as in mammals. Finally, our results raise questions about the origin and evolution of positive and negative costimulation in vertebrate immune systems.

SHP2 forecast for the immune system: fog gradually clearing

The src homology 2 (SH2) domain containing tyrosine phosphatase SHP2 (also referred to as SHP-2) is ubiquitously expressed in mammalian tissues and has been shown to be essential for embryonic development, haematopoiesis and signalling downstream of a variety of growth factors. Dysregulation of SHP2 function or expression has recently been implicated in the pathogenesis of human diseases involving haematopoietic cell lineages. New findings also demonstrate the involvement of SHP2 in the regulation of immune responses through its effects on cytokine and inhibitory receptor signalling pathways, and novel transgenic models are providing valuable insights into the role of SHP2 in T cells.

Transcriptional repression of interleukin-2 in human systemic lupus erythematosus

T cells from patients with SLE produce decreased amounts of interleukin-2 (IL-2), a central cytokine in the regulation of the immune response. We discuss herein the abnormalities underlying IL-2 deficiency in SLE T cells.

Controlling NF-κB activation in T cells by costimulatory receptors

Full and productive activation of T lymphocytes relies on the simultaneous delivery of T cell receptor (TCR)- and coreceptor-derived signals. In naïve T cells engagement of the TCR alone causes anergy, while TCR triggering of preactivated T cells results in activation-induced cell death. Costimulatory signals are prominently mirrored by the activation of NF-kappaB, which needs input from the TCR as well as from coreceptors in order to be fully activated and to fulfil its crucial function in the immune response. Coreceptor-generated signals tightly control the duration and amplitude of the NF-kappaB response. The activation of IkappaB kinase (IKK) complex at the contact zone between a T cell and an antigen-presenting cell offers the unique opportunity to study the spatial organization of IKK activation. Recent studies indicate that coreceptor pathways influence the threshold activities of many signalling mediators and thus act on multiple layers of the NF-kappaB pathway.

The src homology 2 domain-containing tyrosine phosphatase 2 regulates primary T-dependent immune responses and Th cell differentiation

The src homology 2 domain-containing tyrosine phosphatase 2 (SHP2) plays an important role in development and in growth factor receptor signaling pathways, yet little is known of its role in the immune system. We generated mice expressing a dominant-negative version of the protein, SHP2(CS), specifically in T cells. In SHP2(CS) mice, T cell development appears normal with regard to both negative and positive selection. However, SHP2(CS) T cells express higher levels of activation markers, and aged mice have elevated serum Abs. This is associated with a marked increase in IL-4, IL-5, and IL-10 secretion by SHP2(CS) T cells in vitro. In addition, primary thymus-dependent B cell responses are deficient in SHP2(CS) mice. We show that whereas TCR-induced linker for activation of T cells phosphorylation is defective, CTLA-4 and programmed death-1 signaling are not affected by SHP2(CS) expression. Our results suggest that a key action of wild-type SHP2 is to suppress differentiation of T cells to the Th2 phenotype.

BTLA and HVEM Cross Talk Regulates Inhibition and Costimulation

Recently a new inhibitory immunoglobulin domain-containing lymphocyte receptor was identified on the basis of its T helper 1 (T(H)1)-selective expression in murine T cell lines, which was named B and T lymphocyte attenuator (BTLA). Several groups have confirmed the initial characterization of BTLA as an inhibitory receptor, which was initially inferred from the mild increases in several parameters of BTLA-deficient mice. The initial expectation that BTLA would interact with a B7 family ligand, such as the B7x protein, was surprisingly overturned with the functional cloning of the actual BTLA ligand as herpesvirus entry mediator (HVEM). This was unexpected largely due to the fact that this interaction represents the convergence of two very different, although each quite extensive, families of receptors and ligands. The interaction of BTLA, which belongs to the CD28 family of the immunoglobulin superfamily, and HVEM, a costimulatory tumor-necrosis factor (TNF) receptor (TNFR), is quite unique in that it is the only receptor-ligand interaction that directly bridges these two families of receptors. This interaction has raised many questions about how receptors from two different families could interact and which are the signaling events downstream of receptor ligation. As we discuss here and recently demonstrated, HVEM interaction with BTLA serves to negatively regulate T cell responses, in contrast to the strong activation observed when HVEM engages its endogenous ligand from the TNF family. Finally, as studies of BTLA are just now beginning to extend beyond the initial characterizations, it is becoming clear that there are many complex issues remaining to be resolved, particularly potential polymorphisms that may engender disease susceptibility in the human.

Use of penetrating peptides interacting with PP1/PP2A proteins as a general approach for a drug phosphatase technology

Protein phosphatase types 1 (PP1) and 2A (PP2A) represent two major families of serine/threonine protein phosphatases that have been implicated in the regulation of many cellular processes, including cell growth and apoptosis in mammalian cells. PP1 and PP2A proteins are composed of oligomeric complexes comprising a catalytic structure (PP1c or PP2AC) containing the enzymatic activity and at least one more interacting subunit. The binding of different subunits to a catalytic structure generates a broad variety of holoenzymes. We showed here that casein kinase 2alpha (Ck2alpha) and simian virus 40 small t antigen share a putative common beta-strand structure required for PP2A1 trimeric holoenzyme binding. We have also characterized DPT-sh1, a short basic peptide from Ck2alpha that interacted only in vitro with the PP2A-A subunit and behaves as a nontoxic penetrating shuttle in several cultivated human cell lines and chick embryos. In addition, DPT-sh1 specifically accumulated in human red cells infected with Plasmodium falciparum malaria parasites. We therefore designed bipartite peptides containing DPT-sh1 and PP1- or PP2A-interacting sequences. We found that DPT-5, a DPT-sh1-derived peptide containing a short sequence identified in CD28 antigen, interacts with PP2A-Balpha, and DPT-7, another DPT-sh1-derived peptide containing a short sequence identified in Bad as a PP1 catalytic consensus docking motif, induce apoptosis in cultivated cell lines. These results clearly indicate that the rational design of PP1/PP2A interacting peptides is a pertinent strategy to deregulate intracellular survival pathways.

CTLA-4 and PD-1 receptors inhibit T-cell activation by distinct mechanisms

CTLA-4 and PD-1 are receptors that negatively regulate T-cell activation. Ligation of both CTLA-4 and PD-1 blocked CD3/CD28-mediated upregulation of glucose metabolism and Akt activity, but each accomplished this regulation using separate mechanisms. CTLA-4-mediated inhibition of Akt phosphorylation is sensitive to okadaic acid, providing direct evidence that PP2A plays a prominent role in mediating CTLA-4 suppression of T-cell activation. In contrast, PD-1 signaling inhibits Akt phosphorylation by preventing CD28-mediated activation of phosphatidylinositol 3-kinase (PI3K). The ability of PD-1 to suppress PI3K/AKT activation was dependent upon the immunoreceptor tyrosine-based switch motif located in its cytoplasmic tail, adding further importance to this domain in mediating PD-1 signal transduction. Lastly, PD-1 ligation is more effective in suppressing CD3/CD28-induced changes in the T-cell transcriptional profile, suggesting that differential regulation of PI3K activation by PD-1 and CTLA-4 ligation results in distinct cellular phenotypes. Together, these data suggest that CTLA-4 and PD-1 inhibit T-cell activation through distinct and potentially synergistic mechanisms.

Protein phosphatase 2A is a negative regulator of IL-2 production in patients with systemic lupus erythematosus

Decreased IL-2 production in systemic lupus erythematosus (SLE) represents a central component of the disease immunopathology. We report that the message, protein, and enzymatic activity of the catalytic subunit of protein phosphatase 2A (PP2Ac), but not PP1, are increased in patients with SLE regardless of disease activity and treatment and in a disease-specific manner. Treatment of SLE T cells with PP2Ac-siRNA decreased the protein levels and activity of PP2Ac in a specific manner and increased the levels of phosphorylated cAMP response element-binding protein and its binding to the IL2 and c-fos promoters, as well as increased activator protein 1 activity, causing normalization of IL-2 production. Our data document increased activity of PP2A as a novel SLE disease-specific abnormality and define a distinct mechanism whereby it represses IL-2 production. We propose the use of PP2Ac-siRNA as a novel tool to correct T cell IL-2 production in SLE patients.

CTLA-4 up-regulation of lymphocyte function-associated antigen 1 adhesion and clustering as an alternate basis for coreceptor function

Although cytotoxic T lymphocyte antigen-4 (CTLA-4) negatively regulates T cell activation, the full range of functions mediated by this coreceptor has yet to be established. In this study, we report the surprising finding that CTLA-4 engagement by soluble antibody or CD80 potently up-regulates lymphocyte function-associated antigen 1 (LFA-1) adhesion to intercellular adhesion molecule-1 (ICAM-1) and receptor clustering concurrent with IL-2 inhibition. This effect was also observed with CTLA-4 ligation and not with other coreceptors. T cell antigen receptor (TcR)-induced lymphocyte function-associated antigen 1 function was also dependent on CTLA-4 expression as observed with reduced adhesion/clustering on CTLA-4(-/-) primary T cells. CTLA-4 up-regulated adhesion was mediated by regulator for cell adhesion and polarization type 1 (Rap-1) as shown by anti-CTLA-4-induced Rap-1 activation as well as Rap-1-N17 blockade and Rap-1-V12 mimicry of adhesion/clustering. Our findings identify a potent role for CTLA-4 in directing integrin adhesion and provide an alternate mechanism to account for aspects of CTLA-4 function in T cell immunity.

In vivo overexpression of CTLA-4 suppresses lymphoproliferative diseases and thymic negative selection

Cytotoxic T lymphocyte antigen-4 (CTLA-4) induces major inhibitory signals for T cell activation. From analyses of TCR-transgenic (Tg) CTLA-4-deficient mice, it has been believed that CTLA-4 does not affect thymocyte development. To focus upon the in vivo function of CTLA-4 in thymocyte development from a different aspect, we have established Tg mice expressing either full-length CTLA-4 (FL-Tg) or a mutant CTLA-4 lacking the cytoplasmic region (truncated, TR-Tg), and analyzed thymocyte development. TR-T cells express much higher CTLA-4 on the cell surface than FL-T cells, in which most CTLA-4 was localized in intracellular vesicles. While CTLA-4-/- mice exhibit lymphoproliferative disease, neither of the Tg mice with CTLA-4-/- background developed the disorder. Although the development of thymocytes appeared normal in both Tg mice, in vivo depletion of double-positive thymocytes by injection of anti-CD3 Ab as well as the elimination of minor lymphocyte-stimulating antigen-reactive thymocytes were impaired in FL-Tg mice but not in TR-Tg mice. Functionally, cross-linking of CTLA-4 on thymocytes from FL-Tg mice, but not from TR-Tg mice, inhibited proliferation. These results reveal a potential role of CTLA-4, through its cytoplasmic domain, in the negative selection of thymocytes and in the prevention of lymphoproliferative disease.

The PP2A-associated protein alpha4 is an essential inhibitor of apoptosis

Despite evidence that protein kinases are regulators of apoptosis, a specific role for phosphatases in regulating cell survival has not been established. Here we show that alpha4, a noncatalytic subunit of protein phosphatase 2A (PP2A), is required to repress apoptosis in murine cells. alpha4 is a nonredundant regulator of the dephosphorylation of the transcription factors c-Jun and p53. As a result of alpha4 deletion, multiple proapoptotic genes were transcribed. Either inhibition of new protein synthesis or Bcl-xL overexpression suppressed apoptosis initiated by alpha4 deletion. Thus, mammalian cell viability depends on repression of transcription-initiated apoptosis mediated by a component of PP2A.

The CD28 family: a T-cell rheostat for therapeutic control of T-cell activation

The CD28 family of receptors (CD28, cytotoxic T-lymphocyte-associated antigen 4 [CTLA-4], inducible costimulator [ICOS], program death-1 [PD-1], and B- and T-lymphocyte attenuator [BTLA]) plays a critical role in controlling the adaptive arm of the immune response. While considerable information is available regarding CD28 and CTLA-4, the function of the more recently discovered members of the CD28 family is less well understood. This review will highlight recent findings regarding the CD28 family with special emphasis on effects the CD28 family has on immunopathology, the discovery of costimulatory antibodies with superagonist function, and the status of clinical trials using various strategies to augment or block T-cell costimulation.

Conversion of CTLA-4 from inhibitor to activator of T cells with a bispecific tandem single-chain Fv ligand

Abs or their recombinant fragments against surface receptors of the Ig superfamily can induce or block the receptors' native function depending on whether they induce or prevent the assembly of signalosomes on their cytoplasmic tails. In this study, we introduce a novel paradigm based on the observation that a bispecific tandem single-chain variable region fragment ligand of CTLA-4 by itself converts this inhibitory receptor into an activating receptor for primary human T lymphocytes. This reversal of function results from increased recruitment of the serine/threonine phosphatase 2A to the cytoplasmic tail of CTLA-4, consistent with a role of this phosphatase in the regulation of CTLA-4 function, and assembly of a distinct signalosome that activates an lck-dependent signaling cascade and induces IL-2 production. Our data demonstrate that the cytoplasmic domain of CTLA-4 has an inherent plasticity for signaling that can be exploited therapeutically with recombinant ligands for this receptor.

Overlapping Binding Sites in Protein Phosphatase 2A for Association with Regulatory A and α-4 (mTap42) Subunits

Diverse functions of protein Ser/Thr phosphatases depend on the distribution of the catalytic subunits among multiple regulatory subunits. In cells protein phosphatase 2A catalytic subunit (PP2Ac) mostly binds to a scaffold subunit (A subunit or PR65); however, PP2Ac alternatively binds to alpha-4, a subunit related to yeast Tap42 protein, which also associates with phosphatases PP4 or PP6. We mapped alpha-4 binding to PP2Ac to the helical domain, residues 19-165. We mutated selected residues and transiently expressed epitope-tagged PP2Ac to assay for association with A and alpha-4 subunits by co-precipitation. The disabling H118N mutation at the active site or the presence of the active site inhibitor microcystin-LR did not interfere with binding of PP2Ac to either the A subunit or alpha-4, showing that these are allosteric regulators. Positively charged side chains Lys(41), Arg(49), and Lys(74) on the back surface of PP2Ac are unique to PP2Ac, compared with phosphatases PP4, PP6, and PP1. Substitution of one, two, or three of these residues with Ala produced a progressive loss of binding to the A subunit, with a corresponding increase in binding to alpha-4. Conversely, mutation of Glu(42) in PP2Ac essentially eliminated PP2Ac binding to alpha-4, with an increase in binding to the A subunit. Reciprocal changes in binding because of mutations indicate competitive distribution of PP2Ac between these regulatory subunits and demonstrate that the mutated catalytic subunits retained a native conformation. Furthermore, neither the Lys(41)-Arg(49)-Lys(74) nor Glu(42) mutations affected the phosphatase-specific activity or binding to microcystin-agarose. Binding of PP2Ac to microcystin and to alpha-4 increased with temperature, consistent with an activation energy barrier for these interactions. Our results reveal that the A subunit and alpha-4 (mTap42) require charged residues in separate but overlapping surface regions to associate with the back side of PP2Ac and modulate phosphatase activity.

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.

Regulation of T Lymphocyte Metabolism

Upon stimulation, lymphocytes develop from small resting cells into highly proliferative and secretory cells. Although a great deal of study has focused on the genetic program induced by Ag receptor signals, lymphocytes must also regulate their metabolic function to meet the energetic demands of activation. In this review, we discuss the changes in cellular metabolism that accompany lymphocyte activation, with a particular emphasis on glucose metabolism, a major source of both energy and biosynthetic building blocks. We will also cover the signaling pathways that positively and negatively regulate these changes to maintain metabolic homeostasis in cells that are rapidly growing, dividing, and differentiating.

Genetic analysis of CD28 signaling

T cell activation is central to initiating an immune response. Two signals are required: an antigen-specific signal through the T cell receptor (TCR) and an antigen-independent costimulatory signal, primarily through CD28 in naïve T cells. Although many of the molecules involved in TCR signal transduction have been identified, the signaling pathways downstream of CD28 involved in costimulation are not well-defined. Through mutagenesis, we have generated a panel of Jurkat T cell lines in which CD28 costimulation fails to upregulate the RE/AP composite element of the IL-2 promoter. Biochemical analysis and genetic rescue of the defects in these cell lines will lead to a better understanding of CD28 signal transduction.

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.