The role of p21ras in CD28 signal transduction: triggering of CD28 with antibodies, but not the ligand B7-1, activates p21ras

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.

The putative role of the C1858T polymorphism of protein tyrosine phosphatase PTPN22 gene in autoimmunity

Autoimmune diseases represent a heterogeneous group of conditions whose incidence is increasing worldwide. This has stimulated studies on their etiopathogenesis, derived from a complex interaction between genetic and environmental factors, in order to improve prevention and treatment of these diseases. An increasing amount of epidemiologic investigations has associated the presence of the C1858T polymorphism in the protein tyrosine phosphatase non-receptor type 22 (PTPN22) gene to the onset of several autoimmune diseases including insulin-dependent diabetes mellitus (Type 1 diabetes). PTPN22 encodes for the lymphoid tyrosine phosphatase Lyp. This belongs to non-receptor-type protein tyrosine phosphatases involved in lymphocyte activation and differentiation. In humans, Lyp may have a role in the negative regulation of T cell receptor signaling. The single nucleotide polymorphism C1858T encodes for a more active phosphatase Lyp R620W. This has the ability to induce a higher negative regulation of T cell receptor signaling. Thus, C1858T could play an important role at the level of thymocyte polarization and escape of autoreactive T lymphocytes, through the positive selection of otherwise negatively selected autoimmune T cells. In this review we discuss the physiological role exerted by the PTPN22 gene and its encoded Lyp product in lymphocyte processes. We highlight the pathogenic significance of the C1858T PTPN22 polymorphism in human autoimmunity with special reference to Type 1 diabetes. Recently the genetic variation in PTPN22 was shown to induce altered function of T and B-lymphocytes. In particular BCR signaling defects and alterations in the B cell compartment were reported in T1D patients. We finally speculate on the possible development of novel therapeutic treatments in human autoimmunity aiming to selectively target the variant Lyp protein in autoreactive T and B lymphocytes.

Human neonatal naive CD4+ T cells have enhanced activation-dependent signaling regulated by the microRNA miR-181a

Compared with older children and adults, human neonates have reduced and delayed CD4(+) T cell immunity to certain pathogens, but the mechanisms for these developmental differences in immune function remain poorly understood. We investigated the hypothesis that impaired human neonatal CD4(+) T cell immunity was due to reduced signaling by naive CD4(+) T cells following engagement of the αβ-TCR/CD3 complex and CD28. Surprisingly, calcium flux following engagement of CD3 was significantly higher in neonatal naive CD4(+) T cells from umbilical cord blood (CB) compared with naive CD4(+) T cells from adult peripheral blood. Enhanced calcium flux was also observed in adult CD4(+) recent thymic emigrants. Neonatal naive CD4(+) T cells also had higher activation-induced Erk phosphorylation. The microRNA miR-181a, which enhances activation-induced calcium flux in murine thymocytes, was expressed at significantly higher levels in CB naive CD4(+) T cells compared with adult cells. Overexpression of miR-181a in adult naive CD4(+) T cells increased activation-induced calcium flux, implying that the increased miR-181a levels of CB naive CD4(+) T cells contributed to their enhanced signaling. In contrast, AP-1-dependent transcription, which is downstream of Erk and required for full T cell activation, was decreased in CB naive CD4(+) T cells compared with adult cells. Thus, CB naive CD4(+) T cells have enhanced activation-dependent calcium flux, indicative of the retention of a thymocyte-like phenotype. Enhanced calcium signaling and Erk phosphorylation are decoupled from downstream AP-1-dependent transcription, which is reduced and likely contributes to limitations of human fetal and neonatal CD4(+) T cell immunity.

Evidence implicating the Ras pathway in multiple CD28 costimulatory functions in CD4+ T cells

CD28 costimulation is a critical event in the full activation of CD4⁺ T cells that augments cytokine gene transcription, promotes cytokine mRNA stability, prevents induction of anergy, increases cellular metabolism, and increases cell survival. However, despite extensive biochemical analysis of the signaling events downstream of CD28, molecular pathways sufficient to functionally replace the diverse aspects of CD28-mediated costimulation in normal T cells have not been identified. Ras/MAPK signaling is a critical pathway downstream of T cell receptor stimulation, but its role in CD28-mediated costimulation has been controversial. We observed that physiologic CD28 costimulation caused a relocalization of the RasGEF RasGRP to the T cell-APC interface by confocal microscopy. In whole cell biochemical analysis, CD28 cross-linking with either anti-CD28 antibody or B7.1-Ig augmented TCR-induced Ras activation. To determine whether Ras signaling was sufficient to functionally mimic CD28 costimulation, we utilized an adenoviral vector encoding constitutively active H-Ras (61L) to transduce normal, Coxsackie-Adenovirus Receptor (CAR) transgenic CD4⁺ T cells. Like costimulation via CD28, active Ras induced AKT, JNK and ERK phosphorylation. In addition, constitutive Ras signaling mimicked the ability of CD28 to costimulate IL-2 protein secretion, prevent anergy induction, increase glucose uptake, and promote cell survival. Importantly, we also found that active Ras mimicked the mechanism by which CD28 costimulates IL-2 production: by increasing IL-2 gene transcription, and promoting IL-2 mRNA stability. Finally, active Ras was able to induce IL-2 production when combined with ionomycin stimulation in a MEK-1-dependent fashion. Our results are consistent with a central role for Ras signaling in CD28-mediated costimulation.

Artificial antigen-presenting cells for use in adoptive immunotherapy

The observation that T cells can recognize and specifically eliminate cancer cells has spurred interest in the development of efficient methods to generate large numbers of T cells with specificity for tumor antigens that can be harnessed for use in cancer therapy. Recent studies have demonstrated that during encounter with tumor antigen, the signals delivered to T cells by professional antigen-presenting cells can affect T-cell programming and their subsequent therapeutic efficacy. This has stimulated efforts to develop artificial antigen-presenting cells that allow optimal control over the signals provided to T cells. In this review, we will discuss the advantages and disadvantages of cellular and acellular artificial antigen-presenting cell systems and their use in T-cell adoptive immunotherapy for cancer.

Protein kinase Ctheta focusing at the cSMAC is a consequence rather than cause of TCR signaling and is dependent on the MEK/ERK pathway

Correlation between protein kinase Ctheta focusing within the central supramolecular activation cluster (cSMAC) of the immunological synapse and optimal TCR/costimulatory receptor ligation was interpreted to imply that PKCtheta focusing is required for productive signaling. However, this notion has been called into question and competing data suggest that the cSMAC contributes to receptor down-modulation. The observation that PKCtheta focusing at the cSMAC is promoted by CD28 coligation, and also that it occurs late after proximal tyrosine phosphorylation events have been initiated, has led us to investigate an alternative possibility that PKCtheta focusing might be a consequence rather than a cause of productive integrated signaling. Indeed, we found that inhibition of the downstream signaling molecules MEK and PI3K (but not of calcineurin, NF-kappaB, JNK, or p38 MAPK) significantly prevented the focusing of PKCtheta at the cSMAC. It recently has been suggested that the cSMAC may be associated with TCR degradation and signal termination. Using MEK inhibition as a tool, we observed that absence of detectable PKCtheta focusing had no significant effect on TCR down-modulation or duration of CD3zeta phosphorylation. Our results suggest that PKCtheta focusing at the cSMAC occurs as a consequence of productive integrated downstream signaling at least at the level of MEK. If PKCtheta focusing accurately reflects the cSMAC as a whole, then our data also argue against the cSMAC as being required for proximal TCR signal termination.

Two pathways of costimulation through CD28

CD28 is recognized as the primary costimulatory molecule involved in the activation of naïve T cells. However, the biochemical signaling pathways that are activated by CD28 and how these pathways are integrated with TCR signaling are still not understood. We have recently shown that there are at least two independent activation pathways induced by CD28 costimulation. One is integrated with TCR signaling in the context of the immunological synapse and is mediated through transcriptional enhancement and the second is mediated through the induction of mRNA stability. Here, we review the immunological consequences and biochemical mechanisms associated with CD28 costimulation and discuss the major questions that need to be resolved to understand the molecular mechanisms that transduce CD28 costimulation.

CD28 stimulation triggers NF-kappaB activation through the CARMA1-PKCtheta-Grb2/Gads axis

CD28 stimulation contributes to activation of the IL-2 promoter by up-regulating the activity of several transcription factors, including nuclear factor kappaB (NF-kappaB)/Rel family members. However, the signal-transducing cascades linking the CD28 molecule and activation of NF-kappaB remain unclear. Protein kinase C (PKC) , CARMA1 and Bcl10 have recently been reported to integrate TCR-mediated NF-kappaB activation. However, since the data in these studies were drawn from experiments in which T cells were usually stimulated with both TCR and CD28, the relative contributions of TCR- and CD28-mediated signals to initiation of the NF-kappaB pathway remain elusive. To examine the role of these molecules in NF-kappaB activation through CD28-mediated stimulation, Bcl10 was over-expressed in Jurkat cells and their NF-kappaB activation by CD28- or TCR-cross-linking was evaluated. We found that CD28 stimulation alone can induce NF-kappaB activation in Bcl10-over-expressing Jurkat cells, whereas TCR stimulation alone has only little effect. In addition, we found that Bcl10-induced NF-kappaB activation through CD28-mediated stimulation could be blocked by the dominant-negative form of PKC or CARMA1. Furthermore, genetic studies revealed that Grb2/Gads binding, but not phosphatidylinositol 3-kinase binding, is important in CD28-mediated NF-kappaB activation. These findings indicate that the PKC-CARMA1-Bcl10 signaling pathway participates in the CD28 co-stimulatory signal independently of the TCR-signaling pathway, which leads us to propose that the activation of the NF-kappaB-signaling pathway via PKC-CARMA1-Bcl10 may be markedly dependent on CD28 stimulation rather than TCR stimulation.

Flesh and blood: The story of Vav1, a gene that signals in hematopoietic cells but can be transforming in human malignancies

Cancer results from the interaction of multiple aberrations including activation of dominant oncogenes and upregulation of signal transduction pathways. Identification of the genes involved in malignant transformation is a pre-requisite for understanding cancer and improving its diagnosis and treatment. Quite a few of the genes that have been implicated in cancer are mutant or aberrantly expressed versions of genes that are important mediators of the normal growth that occurs during development. An important example of this is Vav1, a cytoplasmic signal transducer protein initially identified as an oncogene. Physiological expression of Vav1 is restricted to the hematopoietic system, where its best-known function is as a GDP/GTP nucleotide exchange factor for Rho/Rac GTPases, an activity strictly controlled by tyrosine phosphorylation. Vav1 was shown to regulate cytoskeletal rearrangement during activation of hematopoietic cells. Vav1 can also mediate other cellular functions including activation of the JNK, ERK, Ras, NF-kB, and NFAT pathways, in addition to association with numerous adapter proteins such as Shc, NCK, SLP-76, GRB2, and Crk. Although the oncogenic form of Vav1 has not been detected in clinical human tumors, its wild-type form has recently been implicated in mammalian malignancies such as neuroblastoma, melanoma, pancreatic tumors and B-cell chronic lymphocytic leukemia. This review addresses the physiological function of wild-type Vav1, its mode of activation as an oncogene, and its emerging role as a transforming protein in human cancer.

Enhanced cardiac allograft survival by Vav1-Rac signaling blockade in a mouse model

BACKGROUND

Vav1-Rac signaling plays a pivotal role in TCR/antigen and CD28 signals for T cell activation. However, pharmacological interference of this signaling has not been tested in the prevention of alloimmune-mediated allograft rejection. It has been demonstrated that 6-thio-GTP, a metabolite of azathioprine, specifically inhibits Vav1-Rac activity in T lymphocytes. Here we show the immunosuppressive efficacy of 6-thio-GTP in the prevention of cardiac allograft rejection.

METHODS

T cell proliferations were measured by (3)H-thymidine uptake. The immunosuppressive activities of 6-thio-GTP were tested in the cardiac allograft model of C57BL/6 (H-2(b)) to Balb/c (H-2(d)) mice.

RESULTS

6-Thio-GTP inhibited TCR/alloantigen stimulated T cell proliferation and CD28-dependent T cell survival. Administration of 6-thio-GTP (0.5 mg/kg) prolonged graft survival to 13.8+/-2.39 days compared to 8.3+/-0.48 days in PBS controls (p<0.0001). Combination of 6-thio-GTP (0.5 mg/kg) with CsA (15 mg/kg) enhanced graft survival from 15.0+/-1.61 days in CsA treated recipients to 36.8+/-2.17 days in those received 20 days of combination therapy of CsA and 6-thio-GTP (p<0.0001), or to 42.7+/-16.63 days in the group treated with 20 days of CsA and 60 days of 6-thio-GTP (p<0.0001). Lymphocytes from 6-thio-GTP treated recipients with long-term surviving grafts (>60 days) displayed reduced proliferative response to alloantigen and higher frequencies of regulatory T cells (Treg).

CONCLUSION

Vav1-Rac inhibitor 6-thio-GTP prolongs allograft survival alone or in combination with CsA by suppression of alloreactive T cell activation. Our findings suggest the therapeutic potential of pharmacological interference of Vav1-Rac signaling for transplantation.

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.

Manipulation of Regulatory T‐Cell Number and Function with CD28‐Specific Monoclonal Antibodies

Suppressor or "regulatory" CD4 T cells play a key role in the control of autoimmunity and overshooting immune responses to foreign antigens, but can also obstruct effective anticancer therapies. The homeostasis and activation of these regulatory T cells (Treg cells) is tightly connected to that of effector CD4 T cells via the costimulatory receptor CD28 and the cytokine IL-2: Both subsets require costimulation to be activated by antigen, and Treg cells additionally depend on IL-2 produced by effector CD4 T cells in a costimulation-dependent fashion. Depending on the therapeutic aim, blockade, or stimulation of CD28 with monoclonal antibodies (mAb) can therefore profoundly affect the size and activity of the Treg compartment. In this chapter, experiments performed in rodents with distinct types of CD28-specific mAb, and the recent failure to translate CD28-driven Treg activation into humans, are discussed.

The p110delta isoform of phosphoinositide 3-kinase controls clonal expansion and differentiation of Th cells

The role of PI3K in T cell activation and costimulation has been controversial. We previously reported that a kinase-inactivating mutation (D910A) in the p110delta isoform of PI3K results in normal T cell development, but impaired TCR-stimulated cell proliferation in vitro. This proliferative defect can be overcome by providing CD28 costimulation, which raises the question as to whether p110delta activity plays a role in T cell activation in vivo, which occurs primarily in the context of costimulation. In this study, we show that the PI3K signaling pathway in CD28-costimulated p110delta D910A/D910A T cells is impaired, but that ERK phosphorylation and NF-kappaB nuclear translocation are unaffected. Under in vitro conditions of physiological Ag presentation and costimulation, p110delta D910A/D910A T cells showed normal survival, but underwent fewer divisions. Differentiation along the Th1 and Th2 lineages was impaired in p110delta D910A/D910A T cells and could not be rescued by exogenous cytokines in vitro. Adoptive transfer and immunization experiments in mice revealed that clonal expansion and differentiation in response to Ag and physiological costimulation were also compromised. Thus, p110delta contributes significantly to Th cell expansion and differentiation in vitro and in vivo, also in the context of CD28 costimulation.

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.

Ligation of CD28 Stimulates the Formation of a Multimeric Signaling Complex Involving Grb-2-Associated Binder 2 (Gab2), Src Homology Phosphatase-2, and Phosphatidylinositol 3-Kinase: Evidence That Negative Regulation of CD28 Signaling Requires the Gab2 Pleckstrin Homology Domain

Grb-2-associated binder (Gab)2 is a scaffolding adaptor protein that has been reported to promote growth factor and cytokine receptor signal transduction, but inhibit TCR-mediated signaling events. In this study, we show that ligation of CD28 by its natural ligand B7-1/CD80, induces tyrosine phosphorylation of Gab2 and its coassociation with Src homology phosphatase (SHP)-2 and class IA PI3K in Jurkat cells. Overexpression of wild-type Gab2 revealed a negative role in regulation of CD3/CD28 induction of the transcription factors NF-kappaB and AP-1. To characterize this inhibitory function further, we used Gab2 mutants unable to bind either PI3K or SHP-2 and a PH domain deletion mutant. Although PI3K has previously been implicated as necessary for Gab2-mediated inhibition of TCR signaling, Gab2 mutants defective in their ability to bind PI3K or SHP-2 retained their inhibitory function, whereas deletion of the PH domain ablated the inhibitory effect of Gab2. Together, these data demonstrate that CD28 stimulation of T cells is sufficient to induce an inhibitory multimeric signaling complex involving Gab2, SHP-2, and PI3K. Furthermore, the inhibitory capacity of Gab2 is strictly dependent upon the integrity of its PH domain, suggesting phosphoinositide-mediated membrane recruitment is important to Gab2 function in T cells.

CD28 superagonists: mode of action and therapeutic potential

Some CD28-specific monoclonal antibodies (mAbs) have the surprising property to activate T-cells without the need for TCR ligation. This review summarizes the differences between these "superagonistic" and conventional CD28-specific mAbs with regard to binding specificity and signalling properties. Furthermore, the dramatic effects of in vivo application of CD28 superagonists with regard to the induction of regulatory T-cells and polyclonal T-cell expansion in lymphopenic settings are discussed under the aspect of potential therapeutic applications.

T-cell triggering by CD3- and CD28-binding molecules linked to a human virus-modified tumor cell vaccine

The aim was to develop T cell costimulatory molecules that are broadly applicable to augment anti-tumor immune responses upon application of a virus-modified tumor vaccine to cancer patients. We generated recombinant bispecific single-chain antibodies with one specificity directed against the CD3 or the CD28 antigen on human T cells and the other against the viral target molecule hemagglutinin-neuraminidase (HN) of Newcastle Disease Virus (NDV). By re-directing unstimulated primary human T cells against HN-expressing NDV-infected tumor cells, the bispecific molecule bsHN-CD3 cross-linked effector and target cells and rapidly induced cytotoxicity at nanomolar concentrations. The bsHN-CD28 molecule exerted T cell co-stimulatory function. Maximal T cell activation was achieved with tumor cells infected by NDV and modified with both new stimulatory molecules. This was revealed by T cell proliferation, upregulation of CD69 and CD25 and by release of cytokines, interferons and chemokines. The new molecules combine high-effectivity with specificity and safety.

Vav proteins, masters of the world of cytoskeleton organization

Vav proteins are evolutionarily conserved from nematodes to mammals and play a pivotal role in many aspects of cellular signaling, coupling cell surface receptors to various effectors functions. In mammals, there are three family members; Vav1 is specifically expressed in the hematopoietic system, whereas Vav2 and Vav3 are more ubiquitously expressed. Vav proteins contain multiple domains that enable their function in various fashions. The participation of the Vav proteins in several processes that require cytoskeletal reorganization, such as the formation of the immunological synapse (IS), phagocytosis, platelet aggregation, spreading, and transformation will be discussed in this review. We will also cover how the Vav proteins succeed in controlling these processes by their function as guanine nucleotide exchange factors (GEFs) for the Rho/Rac family of GTPases. The contribution of the Vav proteins in a GEF-independent manner to the organization of the cytoskeleton will also be deliberated. The scope of this review is to highlight the numerous roles of the Vav signal transducer proteins in actin organization.

Inducible-costimulator-mediated suppression of human immunodeficiency virus type 1 replication in CD4(+) T lymphocytes

We investigated the effects of signaling through CD28 family molecules on human immunodeficiency virus type 1 (HIV-1) replication in vitro. A monoclonal antibody (mAb) specific for inducible costimulator (ICOS) suppressed both X4 and R5 HIV-1 replication in CD4(+) peripheral blood mononuclear cells (PBMC). This suppression was not attributable to reduced cell growth or viability. CD28 mAb showed variable effects and also suppressed HIV-1 replication when immobilized. Replication of pseudotype viruses with HIV-1-but not with vesicular stomatitis virus G-envelope was efficiently suppressed in CD4(+) PBMC treated with ICOS or CD28 mAbs. However, CD4, CXCR4, and CCR5 expression on the surface was not down-regulated. Moreover, HIV-1 replication in CD4(+) PBMC was suppressed by a soluble form of human B7-H2, a ligand of ICOS, but was enhanced by soluble B7-1, a ligand for CD28. These findings suggest that natural or artificial ligands for ICOS potentially suppress HIV-1 replication mainly at the entry stages.

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.

Distinct mechanisms determine the patterns of differential activation of H-Ras, N-Ras, K-Ras 4B, and M-Ras by receptors for growth factors or antigen

Although GTPases of the Ras family have been implicated in many aspects of the regulation of cells, little is known about the roles of individual family members. Here, we analyzed the mechanisms of activation of H-Ras, N-Ras, K-Ras 4B, and M-Ras by two types of external stimuli, growth factors and ligation of the antigen receptors of B or T lymphocytes (BCRs and TCRs). The growth factors interleukin-3, colony-stimulating factor 1, and epidermal growth factor all preferentially activated M-Ras and K-Ras 4B over H-Ras or N-Ras. Preferential activation of M-Ras and K-Ras 4B depended on the presence of their polybasic carboxy termini, which directed them into high-buoyant-density membrane domains where the activated receptors, adapters, and mSos were also present. In contrast, ligation of the BCR or TCR resulted in activation of H-Ras, N-Ras, and K-Ras 4B, but not M-Ras. This pattern of activation was not influenced by localization of the Ras proteins to membrane domains. Activation of H-Ras, N-Ras, and K-Ras 4B instead depended on the presence of phospholipase C-gamma and RasGRP. Thus, the molecular mechanisms leading to activation of Ras proteins vary with the stimulus and can be influenced by either colocalization with activated receptors or differential sensitivity to the exchange factors activated by a stimulus.

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.

T cell anergy and costimulation

T lymphocytes play a key role in immunity by distinguishing self from nonself peptide antigens and regulating both the cellular and humoral arms of the immune system. Acquired, antigen-specific unresponsiveness is an important mechanism by which T cell responses to antigen are regulated in vivo. Clonal anergy is the term that describes T cell unresponsiveness at the cellular level. Anergic T cells do not proliferate or secrete interleukin (IL)-2 in response to appropriate antigenic stimulation. However, anergic T cells express the IL-2 receptor, and anergy can be broken by exogenous IL-2. Anergy can be induced by submitogenic exposure to peptide antigen in the absence of a costimulatory signal provided by soluble cytokines or by interactions between costimulatory receptors on T cells and counter-receptors on antigen-presenting cells. The molecular events that mediate the induction and maintenance of T cell anergy are the focus of this review. The molecular consequences of CD28-B7 interaction are discussed as a model for the costimulatory signal that leads to T cell activation rather than the induction of anergy.

GTPases and T cell activation

Guanine nucleotide binding proteins rapidly cycle between a guanosine diphosphate (GDP)-bound and guanosine triphosphate (GTP)-bound state, and they operate as binary switches that control cell activation in response to environmental cues. GTPases adopt different conformations when binding GTP vs. GDP. The GTP-bound state is generally considered to be the active conformation that allows GTPases to interact with downstream effectors and thereby initiate downstream signaling pathways, which regulate many important biological processes. Many members of the Ras family of GTPases, notably Ras and Rap1A, and the Rho family GTPases, Cdc42Hs, Rac1, Rac2 and RhoA, are important components of signal transduction pathways used by antigen receptors, costimulatory, cytokine and chemokine receptors to regulate the immune response. This review discusses current knowledge and ideas about the regulation and function of these GTPases in lymphocytes.

Actin cytoskeletal dynamics in T lymphocyte activation and migration

Dynamic rearrangements of the actin cytoskeleton are crucial for the function of numerous cellular elements including T lymphocytes. They are required for migration of T lymphocytes through the body to scan for the presence of antigens, as well as for the formation and stabilization of the immunological synapse at the interface between antigen-presenting cells and T lymphocytes. Supramolecular activation clusters within the immunological synapse play an important role for the initiation of T cell responses and for the execution of T cell effector functions. In addition to the T cell receptor/CD3 induced actin nucleation via Wasp/Arp2/3-activation, signals through accessory receptors of the T cell (i.e., costimulation) regulate actin cytoskeletal dynamics. In this regard, the actin-binding proteins cofilin and L-plastin represent prominent candidates linking accessory receptor stimulation to the rearrangement of the actin cytoskeleton. Cofilin enhances actin polymerization via its actin-severing activity, and as a long-lasting effect, cofilin generates novel actin monomers through F-actin depolymerization. L-plastin stabilizes actin filament structures by means of its actin-bundling activity.

Sustained and dynamic inositol lipid metabolism inside and outside the immunological synapse

T cell activation is triggered by several hours of contact with peptide-major histocompatibility (MHC) complexes on the surface of antigen-presenting cells (APCs). The nature and location of the sustained signal transduction pathways required for T cell activation are unknown. We show here that the production of phosphatidylinositol(3,4,5)triphosphate (PIP3) was dynamically sustained for hours as T cells responded to antigen. In addition, sustained elevation of PIP3 was essential for T cell proliferation. There was PIP3 accumulation in the T cell-APC contact zone and at the antipodal pole of the cell. The immune synapse is thus not the sole site of sustained signal transduction in activated T cells.

Pleiotropic defects in TCR signaling in a Vav-1-null Jurkat T-cell line

The Rac/Rho-specific guanine nucleotide exchange factor, Vav-1, is a key component of the T-cell antigen receptor (TCR)-linked signaling machinery. Here we have used somatic cell gene-targeting technology to generate a Vav-1-deficient Jurkat T-cell line. The J.Vav1 cell line exhibits dramatic defects in TCR-dependent interleukin (IL)-2 promoter activation, accompanied by significant reductions in the activities of the NFAT(IL-2), NFkappaB, AP-1 and REAP transcription factors that bind to the IL-2 promoter region. In contrast, loss of Vav-1 had variable effects on early TCR-stimulated signaling events. J.Vav1 cells display a selective defect in sustained Ca(2+) signaling during TCR stimulation, and complementation of this abnormality by exogenously introduced Vav-1 is dependent on the Vav-1 calponin homology domain. While JNK activation was severely impaired, the stimulation of Ras, ERK and protein kinase C-theta activities, as well as the mobilization of lipid rafts, appeared normal in the J.Vav1 cells. Finally, evidence is presented to suggest that the alternative Vav family members, Vav-2 and Vav-3, are activated during TCR ligation, and partially compensate for the loss of Vav-1 in Jurkat T cells.

TCR-independent CD30 signaling selectively induces IL-13 production via a TNF receptor-associated factor/p38 mitogen-activated protein kinase-dependent mechanism

Initiation of T lymphocyte responses to most Ags requires concurrent stimulation through the TCR and costimulatory receptors such as CD28. Following initial activation, secondary receptors are up-regulated that can costimulate T cells in concert with TCR engagement. One such receptor is the TNFR family member CD30. In this study, we report that unlike CD28, ligation of CD30 on normal effector T cells induces IL-13 production in the absence of concurrent TCR engagement. TCR-independent CD30-mediated IL-13 release correlated with activation of c-Jun N-terminal kinase, p38 mitogen-activated protein kinase (MAPK), and NF-kappaB, and was completely inhibited by the expression of a TNFR-associated factor 2 (TRAF2) dominant-negative transgene (TRAF2.DN-Tg), but not by that of an I-kappaBalpha dominant-negative transgene. In parallel, expression of the TRAF2.DN-Tg selectively prevented the induction of c-Jun N-terminal kinase and p38 MAPK, but not that of NF-kappaB. Furthermore, IL-13 production was reduced in a dose-dependent manner by the p38 MAPK inhibitor SB203580. Together, these results suggest that TCR-independent CD30-mediated production of IL-13 is triggered by association of CD30 with TRAF family members and subsequent activation of p38 MAPK. Inasmuch as IL-13 can promote airway inflammation and cancer progression, production of IL-13 in a TCR-independent manner has important pathological implications in vivo.

VAV proteins as signal integrators for multi-subunit immune-recognition receptors

In recent years, substantial progress has been made towards the identification of intracellular signalling molecules that couple multi-subunit immune-recognition receptors (MIRRs) to their various effector functions. Among these, the VAV proteins have been observed to have a crucial role in regulating some of the earliest events in receptor signalling. VAV proteins function, in part, as guanine-nucleotide exchange factors (GEFs) for the RHO/RAC family of GTPases. This review focuses on the role of VAV proteins in the regulation of lymphocyte development and function, and emphasizes the regulatory roles that these proteins have through both GEF-dependent and -independent mechanisms.

T-cell tolerance and autoimmune diabetes

Herein we describe the major signaling events that occur in T-cells upon T-cell receptor (TCR) engagement, and the mechanisms responsible for the induction of T-cell anergy that may ultimately lead to the development of immunospecific therapies in T-cell mediated autoimmune diseases. A new type of antigen presenting molecule (dimeric MHC class-II/peptide, DEF) endowed with antigen-specific immunomodulatory effects such as induction of Th2 polarization and T-cell anergy is also described as a potential antidiabetogenic agent. According to our preliminary results, the MHC II/peptide-based approach may provide rational grounds for further development of antigen-specific immunotherapeutic agents such as human-like MHC lI/peptide chimeras endowed with efficient down-regulatory effects in CD4 T-cell-mediated autoimmune diseases such as Type 1 diabetes, multiple sclerosis, primary biliary cirrhosis, and rheumatoid arthritis.

Simian immunodeficiency virus Nef protein delays the progression of CD4+ T cells through G1/S phase of the cell cycle

Human and simian immunodeficiency virus (HIV and SIV, respectively) infections are characterized by gradual depletion of CD4+ T cells. The underlying mechanisms of CD4+ T-cell depletion and HIV and SIV persistence are not fully determined. The Nef protein is expressed early in infection and is necessary for pathogenesis. Nef can cause T-cell activation and downmodulates cell surface signaling molecules. However, the effect of Nef on the cell cycle has not been well characterized. To determine the role of Nef in the cell cycle, we investigated whether the SIV Nef protein can modulate cell proliferation and apoptosis in CD4+ Jurkat T cells. We developed a CD4+ Jurkat T-cell line that stably expresses SIV Nef under the control of an inducible promoter. Alterations in cell proliferation were determined by flow cytometry using stable intracytoplasmic fluorescent dye 5- and 6-carboxyfluorescein diacetate succinimidyl ester and bromodeoxyuridine incorporation. Apoptotic cell death was measured by annexin V and propidium iodide staining. Our results demonstrated that SIV Nef inhibited Fas-induced apoptosis in these cells and that the mechanism involved upregulation of the Bcl-2 protein. SIV Nef suppressed CD4+ T-cell proliferation by inhibiting the progression of cells into S phase of the cell cycle. Suppression involved an upregulation of cyclin-dependent kinase inhibitors p21 and p27 and the downregulation of cyclin D1 and cyclin A. In summary, inhibition of apoptosis by Nef can lead to persistence of infected cells and can support viral replication. In addition, a Nef-mediated delay in cell cycle progression may contribute to CD4+ T-cell anergy/depletion seen in HIV and SIV disease.

CD28 costimulation mediates down-regulation of p27kip1 and cell cycle progression by activation of the PI3K/PKB signaling pathway in primary human T cells

CD28 provides a costimulatory signal that cooperates with the TCR/CD3 complex to induce T cell activation, cytokine production, and clonal expansion. We have recently shown that CD28 directly regulates progression of T lymphocytes through the cell cycle. Although a number of signaling pathways have been linked to the TCR/CD3 and to CD28, it is not known how these two receptors cooperate to induce cell cycle progression. Here, using cell-permeable pharmacologic inhibitors of phosphatidylinositol 3-hydroxykinase (PI3K) and mitogen-activated protein kinase kinase (MEK1/2), we show that cell cycle progression of primary T lymphocytes requires simultaneous activation of PI3K- and MEK1/2-dependent pathways. Decreased abundance of cyclin-dependent kinase inhibitor p27(kip1), which requires simultaneous TCR/CD3 and CD28 ligation, was dependent upon both MEK and PI3K activity. Ligation of TCR/CD3, but not CD28 alone, resulted in activation of MEK targets extracellular signal-related kinase 1/2, whereas ligation of CD28 alone was sufficient for activation of PI3K target protein kinase B (PKB; c-Akt). CD28 ligation alone was also sufficient to mediate inactivating phosphorylation of PKB target glycogen synthase kinase-3 (GSK-3). Moreover, direct inactivation of GSK-3 by LiCl in the presence of anti-CD3, but not in the presence of anti-CD28, resulted in down-regulation of p27(kip1), hyperphosphorylation of retinoblastoma tumor suppressor gene product, and cellular proliferation. Thus, inactivation of the PI3K-PKB target GSK-3 could substitute for CD28 but not for CD3 signals. These results show that the PI3K-PKB pathway links CD28 to cell cycle progression and suggest that p27(kip1) integrates mitogenic MEK- and PI3K-dependent signals from TCR and CD28 in primary T lymphocytes.

Negative effect of CTLA-4 on induction of T-cell immunity in vivo to B7-1+, but not B7-2+, murine myelogenous leukemia

B7 molecules provide important costimulatory signals to T cells, and B7 genes have been introduced into B7-negative tumor cells to enhance their immunogenicity. However, the role of B7 molecules in inducing tumor immunity is controversial because of conflicting results and reports of differential signaling through the B7 molecules and their ligands CD28 and CTLA-4. In this study, we compared the effect of B7-1 (CD80) and B7-2 (CD86) on the induction of T-cell immunity to C1498, a murine myelogenous leukemia. When cultured with exogenous cytokines in vitro, C1498/B7-1 and C1498/B7-2 induced syngeneic CD8+ T cells to kill parental C1498. In vivo, C1498/B7-1 grew progressively after subcutaneous injection, whereas C1498/B7-2 completely regressed after transient growth in naive mice. Spontaneous rejection of C1498/B7-2 resulted in immunity to challenge doses of C1498 and C1498/B7-1. Antibody-depletion studies in vivo showed that CD8+ T cells rejected C1498/B7-2, whereas only natural killer cells affected the growth of C1498/B7-1. Two approaches were used to determine whether preferential interaction of B7-1 with CTLA-4 contributed to the failure of C1498/B7-1 to activate CD8+ T cells in vivo. First, CTLA-4 specific monoclonal antibody was used to block B7-1-CTLA-4 interaction. Second, CTLA-4 deletional mutant (-/-) bone marrow chimeras were used as tumor hosts. In both systems, there was a significant increase in the rate of rejection of C1498/B7-1 tumors. Resistance to C1498/B7-1 in CTLA-4(minus sign/minus sign) hosts was mediated by CD8+ T cells. Blocking or deletion of CTLA-4 did not affect the growth of parental C1498, indicating that B7-1 was important for the induction of CD8+ T-cell immunity in the absence of CTLA-4.

The lymphoid protein tyrosine phosphatase Lyp interacts with the adaptor molecule Grb2 and functions as a negative regulator of T-cell activation

OBJECTIVE

Following activation of T cells, phosphorylation of tyrosine residues occurs through a complex signaling process involving protein tyrosine kinases, phosphatases, and a variety of adapter molecules including Grb2. We have attempted to identify new signaling molecules that are important for the activation response.

METHODS

Using a protein interaction screening protocol based on phage display, T-cell signaling components that associate with the adapter molecule, Grb2, the lymphoid-specific tyrosine phosphatase Lyp was identified. Using transcriptional reporter assays, the role of Lyp in T-cell activation was studied by overexpression of wild-type or catalytically inactive mutants of Lyp.

RESULTS

A GST fusion containing the C-terminal SH3 domain of Grb2 bound to the nucleotide exchange factor Sos or Grb2-associated binder 2 (Gab2). In contrast, the N-terminal SH3-containing fusion bound to the protein tyrosine phosphatase Lyp. Grb2 was co-immunoprecipitated with Lyp in 293T cells overexpressing both proteins. Using Northern blot analysis, Lyp was found to be expressed predominantly in hematopoietic tissue, including spleen, lymph node, thymus, peripheral blood leukocytes, bone marrow, and fetal liver. Two human T-cell lines, Jurkat and HuT78, expressed both Lyp mRNA and protein. Overexpression of wild-type Lyp or a catalytically inactive, substrate-trapping mutant (D195A) in Jurkat cells inhibited transcriptional activity initiated by anti-CD3 and anti-CD28 antibodies. In contrast, two other catalytically inactive mutants (R233M or C227S) had no effect.

CONCLUSION

These data demonstrate a novel interaction between the phosphatase Lyp and the adaptor Grb2 and are consistent with a negative regulatory role for Lyp in T-cell signaling.

CD46/CD3 costimulation induces morphological changes of human T cells and activation of Vav, Rac, and extracellular signal-regulated kinase mitogen-activated protein kinase

Efficient T cell activation requires at least two signals, one mediated by the engagement of the TCR-CD3 complex and another one mediated by a costimulatory molecule. We recently showed that CD46, a complement regulatory receptor for C3b as well as a receptor for several pathogens, could act as a potent costimulatory molecule for human T cells, highly promoting T cell proliferation. Indeed, we show in this study that CD46/CD3 costimulation induces a synergistic activation of extracellular signal-related kinase mitogen-activated protein kinase. Furthermore, whereas T lymphocytes primarily circulate within the bloodstream, activation may induce their migration toward secondary lymphoid organs or other tissues to encounter APCs or target cells. In this study, we show that CD46/CD3 costimulation also induces drastic morphological changes of primary human T cells, as well as actin relocalization. Moreover, we show that the GTP/GDP exchange factor Vav is phosphorylated upon CD46 stimulation alone, and that CD46/CD3 costimulation induces a synergistic increase of Vav phosphorylation. These results prompted us to investigate whether CD46/CD3 costimulation induced the activation of GTPases from the Rho family. Indeed, we report that the small GTPase Rac is also activated upon CD46/CD3 costimulation, whereas no change of Rho and Cdc42 activity could be detected. Therefore, CD46 costimulation profoundly affects T cell behavior, and these results provide important data concerning the biology of primary human T cells.

CD28 as a molecular amplifier extending TCR ligation and signaling capabilities

Evidence has gathered that CD28 costimulation facilitates T cell activation by potentiating TCR intrinsic-signaling. However, the underlying molecular mechanism is largely unknown. Here we show that, by enhancing T cell/APC close contacts, CD28 facilitates TCR signal transduction. Moreover, the signal supplied by CD28 does not lead to increased Zap-70 and Lat phosphorylation, but amplifies PLCgamma1 activation and Ca(2+) response. We provide evidence that the PTK Itk controls the latter function. Our data suggest that CD28 binding to B7 contributes to setting the level of TCR-induced phosphorylated Lat for recruiting signaling complexes, whereas the CD28 signal boosts multiple pathways by facilitating PLCgamma1 activation. These results should provide a conceptual framework for understanding quantitative and qualitative aspects of CD28-mediated costimulation.

The GTPase Rac-1 controls cell fate in the thymus by diverting thymocytes from positive to negative selection

The positive selection of CD4 or CD8 single-positive mature peripheral T lymphocytes and the deletion of self-reactive cells are crucial for central tolerance in the peripheral immune system. Previously, the guanine nucleotide binding protein Rac-1 has been shown to control pre-T cell development. The present report now describes the actions of Rac-1 in thymocyte selection. The study reveals that this molecule has the striking and unique ability to efficiently divert cells from positive selection into a pathway of negative selection and deletion. The ability of Rac-1 to switch thymocytes from a destiny of positive to negative selection identifies this molecule as a critical regulator of the developmental processes in T cells that are essential for immune homeostasis.

Alterations in transcription factor binding at the IL-2 promoter region in anergized human CD4+ T lymphocytes

BACKGROUND

The mechanisms responsible for the induction of clonal anergy are not well understood. We have utilized an in vitro model of human T cell anergy to explore the perturbations in cell signaling at the level of interleukin (IL)-2 gene transcription and to define the contribution of other cytokines to this effect.

METHODS

An in vitro model of clonal anergy was established by using CD4+ T lymphocytes from healthy human donors. Cells were anergized by prestimulation with an anti-CD3 monoclonal antibody (mAb) followed by restimulation 72 hr later with anti-CD3 mAb with or without anti-CD28.

RESULTS

CD4+ T cells, anergized with anti-CD3 monoclonal antibody (OKT3) prestimulation, displayed a marked reduction in proliferation (P=0.0036) and IL-2 production (P<0.0001). Co-incubation with IL-10 reduced cellular proliferation in OKT3/CD28 pretreated cells by 19% (P=NS) and reduced IL-2 production by 40% (P=0.0024). Anergized T cells demonstrated a reduced binding activity of the AP-1 complex to the IL-2 promoter. Supershift experiments and Western blots confirmed that the binding of c-Fos, JunB, and JunD, but not of FosB, was reduced in anergized cells. At the sis-inducible element (SIE)-binding region of the c-Fos promoter, Stat3 binding was reduced.

CONCLUSIONS

T cell anergy, induced by prestimulation with OKT3, is characterized by reduced proliferation and a profound decrease in IL-2 production. Anergy can be prevented by co-incubation with anti-CD28 and partially re-established by IL-10. Anergy is accompanied by a reduction in AP-1 binding to the IL-2 promoter, with selective reduction in binding of c-Fos, JunB, and JunD. Defective binding for Stat3 at the c-Fos promoter suggests an involvement of the Jak-Stat pathway.

Deficiency of small GTPase Rac2 affects T cell activation

Rac2 is a hematopoietic-specific GTPase acting as a molecular switch to mediate both transcriptional activation and cell morphological changes. We have examined the effect of Rac2 deficiency during T cell activation. In Rac2(-/-) T cells, proliferation was reduced upon stimulation with either plate-bound anti-CD3 or T cell receptor-specific antigen. This defect is accompanied with decreased activation of mitogen activated protein kinase extracellular signal-regulated kinase (ERK)1/2 and p38, and reduced Ca(2)+ mobilization. TCR stimulation-induced actin polymerization is also reduced. In addition, anti-CD3 cross-linking-induced T cell capping is reduced compared with wild-type T cells. These results indicate that Rac2 is important in mediating both transcriptional and cytoskeletal changes during T cell activation. The phenotypic similarity of Rac2(-/-) to Vav(-/-) cells implicates Rac2 as a downstream mediator of Vav signaling.

Single-cell analysis of signal transduction in CD4 T cells stimulated by antigen in vivo

Flow cytometry was used to study signaling events in individual CD4 T cells after antigen recognition in the body. Phosphorylation of c-jun and p38 mitogen-activated protein kinase was detected within minutes in all antigen-specific CD4 T cells in secondary lymphoid tissues after injection of peptide antigen into the bloodstream. The remarkable rapidity of this response correlated with the finding that most naive T cells are in constant contact with dendritic antigen-presenting cells. Contrary to predictions from in vitro experiments, antigen-induced c-jun and p38 mitogen-activated protein kinase phosphorylation did not depend on CD28 signals and was insensitive to inhibition by cyclosporin A. Our results highlight the efficiency of the in vivo immune response and underscore the need to verify which signaling pathways identified in vitro actually operate under physiological conditions.

ZAP-70 and SLP-76 regulate protein kinase C-theta and NF-kappa B activation in response to engagement of CD3 and CD28

The transcription factor NF-kappaB is a critical regulator of T cell function that becomes strongly activated in response to coengagement of TCR and CD28. Although events immediately proximal to NF-kappaB activation are well understood, uncertainty remains over which upstream signaling pathways engaged by TCR and CD28 lead to NF-kappaB activation. By using Jurkat T cell lines that are deficient or replete for either the protein tyrosine kinase ZAP-70 or the cytosolic adapter molecule SLP-76, the role of these proteins in modulating NF-kappaB activation was examined. NF-kappaB was not activated in response to coengagement of TCR and CD28 in either the ZAP-70- or SLP-76-negative cells, whereas stimuli that bypass these receptors (PMA plus A23187, or TNF-alpha) activated NF-kappaB normally. Protein kinase C (PKC) theta activation, which is required for NF-kappaB activation, also was defective in these cells. Reexpression of ZAP-70 restored PKCtheta and NF-kappaB activation in response to TCR and CD28 coengagement. p95(vav) (Vav)-1 tyrosine phosphorylation was largely unperturbed in the ZAP-70-negative cells; however, receptor-stimulated SLP-76/Vav-1 coassociation was greatly reduced. Wild-type SLP-76 fully restored PKCtheta and NF-kappaB activation in the SLP-76-negative cells, whereas 3YF-SLP-76, which lacks the sites of tyrosine phosphorylation required for Vav-1 binding, only partially rescued signaling. These data illustrate the importance of the ZAP-70/SLP-76 signaling pathway in CD3/CD28-stimulated activation of PKC theta and NF-kappaB, and suggest that Vav-1 association with SLP-76 may be important in this pathway.

Antigen-independent Th2 cell differentiation by stimulation of CD28: regulation via IL-4 gene expression and mitogen-activated protein kinase activation

To delineate the molecular mechanisms regulating Th2 cell differentiation, CD28-mediated generation of Th2 effectors was analyzed. In the absence of TCR ligation CD28 stimulation induced Th2 differentiation of memory but not of naive CD4(+) T cells, whereas costimulation via CD28 and the TCR enhanced Th2 differentiation from naive T cells but suppressed it from memory T cells. Stimulation of T cells via the CD28 pathway, therefore, provided critical signals facilitating Th2 cell differentiation. By comparing the responses to CD28 stimulation in memory and naive T cells and by using specific inhibitors, signaling pathways were defined that contributed to Th2 differentiation. CD28-induced Th2 differentiation required IL-4 stimulation and the activation of the mitogen-activated protein kinases p38 and extracellular signal-regulated kinases 1/2. CD28 engagement directly initiated IL-4 gene transcription in memory T cells and induced activation of phosphatidylinositol 3-kinase, p38, and c-Jun NH(2)-terminal kinase/stress-activated protein kinase pathways. Extracellular signal-regulated kinase phosphorylation that was necessary for Th2 differentiation, however, required stimulation by IL-2. These results indicate that optimal TCR-independent generation of Th2 effectors requires coordinate signaling via the CD28 and IL-2 pathways. TCR-independent generation of Th2 effectors might provide a mechanism to control Th1-dominated cellular inflammation.

Critical requirement for the membrane-proximal cytosolic tyrosine residue for CD28-mediated costimulation in vivo

The YMNM motif that exists in the CD28 cytoplasmic domain is known as a binding site for phosphatidylinositol 3-kinase and Grb-2 and is considered to be important for CD28-mediated costimulation. To address the role of the YMNM motif in CD28 cosignaling in primary T cells, we generated transgenic mice on a CD28 null background that express a CD28 mutant lacking binding ability to phosphatidylinositol 3-kinase and Grb-2. After anti-CD3 and anti-CD28 Ab stimulation in vitro, the initial proliferative response and IL-2 secretion in CD28 Y189F transgenic T cells were severely compromised, while later responses were intact. In contrast to anti-CD3 and anti-CD28 Ab stimulation, PMA and anti-CD28 Ab stimulation failed to induce IL-2 production from CD28 Y189F transgenic T cells at any time point. Using the graft-vs-host reaction system, we assessed the role of the YMNM motif for CD28-mediated costimulation in vivo and found that CD28 Y189F transgenic spleen cells failed to engraft and could not induce acute graft-vs-host reaction. Together, these results suggest that the membrane-proximal tyrosine of CD28 is required for costimulation in vivo. Furthermore, these results indicate that the results from in vitro assays of CD28-mediated costimulation may not always correlate with T cell activation in vivo.

Activation of antigen-specific T cells by artificial cell constructs having immobilized multimeric peptide-class I complexes and recombinant B7-Fc proteins

T cell activation results from the engagement of multiple receptors on T cells by their respective ligands on antigen presenting cells. Studies using artificial cell surface constructs have demonstrated that effective T cell response requires that antigen be presented on a solid surface with dimensions that approximate those of an intact cell. In this report, we describe the cloning and expression of recombinant B7-1-Fc and B7-2-Fc proteins and their incorporation onto 5-microm latex microspheres along with renatured peptide-MHC. These microspheres provide a simple and effective method for the in vitro or in vivo stimulation of antigen-specific T cells under precisely controlled antigen and costimulation conditions.

CD28 co-stimulation restores T cell responsiveness in NOD mice by overcoming deficiencies in Rac-1/p38 mitogen-activated protein kinase signaling and IL-2 and IL-4 gene transcription

Previously, we reported that T cell hyporesponsiveness induced by TCR ligation is causal to autoimmune diabetes in NOD mice. Neonatal CD28 co-stimulation reverses T cell hyporesponsiveness and protects NOD mice from diabetes by an IL-4-mediated mechanism, indicating that a deficiency in TCR signaling may be overcome by CD28/B7-2 co-stimulation in NOD T cells. To investigate which co-stimulation-induced signaling events mediate this protection, we analyzed the activity of Ras, Rac-1, mitogen-activated protein kinases (MAPK) and several transcription factors in TCR-activated NOD T cells in the presence or absence of CD28 co-stimulation. We show that CD28 co-stimulation restores normal TCR-induced activation of Rac-1 and p38 MAPK in NOD T cells. Deficiencies in TCR-induced nuclear expression of activating protein (AP)-1 binding proteins as well as activation of AP-1 and NF-AT in the IL-2 and IL-4 P1 promoters are also corrected by CD28 co-stimulation. Thus, CD28 co-stimulation reverses NOD T cell hyporesponsiveness by restoring TCR signaling leading to the activation of AP-1 and NF-AT during IL-2 and IL-4 gene transcription. Our findings provide additional evidence that CD28 co-stimulation amplifies signals delivered by the TCR and further explain the mechanism by which CD28 co-stimulation may protect against autoimmune diabetes.