key: cord-0005520-ci7sejyy
authors: Miller, Jim; Baker, Christina; Cook, Kevin; Graf, Beth; Sanchez-Lockhart, Mariano; Sharp, Katherine; Wang, Xia; Yang, Barbara; Yoshida, Takeshi
title: Two pathways of costimulation through CD28
date: 2009-02-13
journal: Immunol Res
DOI: 10.1007/s12026-009-8097-6
sha: 6aead323d4d66b2a670748159a592f43a74c9ec3
doc_id: 5520
cord_uid: ci7sejyy

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.

adhesion to allow for the formation of stable T cell:APC conjugates. Second, they provide costimulatory signals that work in concert with TCR signaling to promote T cell activation and differentiation. Clearly all proteins that interact between the two cells will provide some contribution to intercellular adhesion. In contrast, only a subset of the accessory molecules have been documented to provide effective costimulatory functions. Although the basic concept of costimulation is well established, the specific molecular mechanisms whereby costimulatory molecules influence T cell activation and differentiation events are not fully understood.

The two signal model and the concept of costimulation are well engrained in our understanding of the regulation of T cell activation and tolerance. T cell encounter with peptide-MHC ligands in the absence of an ongoing innate immune response generally does not lead to effective T cell activation and rather favors the induction of tolerance. One of the key consequences of the innate immune response is the upregulation on dendritic cells of CD80 and CD86, the ligands for CD28. Because CD28 is the major costimulatory molecule expressed on naïve T cells, CD28 can be viewed as the T cell-associated receptor for detection of the presence of a pathogen. At the same time, dendritic cells in the T cell zones of the lymph nodes can express high levels of CD80/CD86. However, as we and others have recently shown, engagement of CD28 requires continued stimulation through the TCR [1, 2] . Thus, this synergistic cross talk between TCR and CD28 provides a mechanism for coincidence detection to regulate T cell activation and control the initiation of T cell immune responses.

CD28 has been shown to have important functional consequences on T cell activation [3] [4] [5] [6] . CD28 costimulation leads to a dramatic upregulation in IL-2 expression mediated by enhanced transcription and mRNA stabilization [7, 8] . CD28 costimulation also plays an important role in T cell survival, inducing expression of the anti-apoptotic protein Bcl-XL [9] , and can regulate the metabolic activity of T cells [10] . T cell activation in the absence of CD28 leads to T cell anergy rather than activation, and costimulation through CD28 can protect against anergy induction [11, 12] . CD28 plays a key role in the generation of Th2 responses [13] . Finally, CD28 is required for the thymic maturation of NKT cells [14] and T regulatory cells [15] . Because CD28 is expressed on naïve T cells, it plays a critical role in initial T cell priming. Once T cells are activated, additional costimulatory molecules are upregulated, including ICOS, OX40, and 41BB that can enhance T cell survival, expansion and/or effector function [16, 17] .

The impact of CD28 costimulation on T cell function can sometimes appear paradoxical. For example, CD28 costimulation is required to protect T cells from the induction of anergy, but is also required for tolerance induction [18] . CD28 is not required for thymic maturation of conventional CD4 and CD8 T cells, but is essential for the development of NKT and regulatory T cells [14, 15] . CD28 costimulation plays an important role in promoting Th2 differentiation, but is not required for Th2 effector cytokine secretion. The opposite is true for Th1 cells, which can differentiate in the absence of CD28, but require CD28 for subsequent IL-2 secretion. Thus, although the importance of CD28 costimulation is well recognized, the molecular mechanisms whereby CD28 can regulate T cell development, activation, expansion, and differentiation and how these molecular mechanisms are integrated with signals from the TCR are not understood.

The most striking changes in the immune response in the elderly are in the reduction of immune response to newly encountered antigens. This creates a major health problem in this population because of the frequency of exposure to new variants of highly mutable viruses, such as influenza and RSV, emerging pathogens, such as SARS, opportunistic bacterial infections, and possibly cancer. The reduced immune response during aging is a multifactorial process, including reduced hematopoietic stem cells and lymphoid precursors, thymic involution, reduced T and B cell proliferation, germinal center formation, and effector cell maturation, decreased cytokine signaling, increased ratio of memory to naïve T cells, and overproduction of regulatory T cells [19] [20] [21] [22] [23] [24] [25] [26] [27] . In spite of the potential complexity of understanding the relative importance and potential interplay among all of the factors associated with immune senescence, a major component of the reduced immune response in the elderly may be associated with a failure to provide CD28 costimulation. In mice, adoptive transfer experiments have identified an intrinsic defect in activation of naïve CD4 T cells [28] [29] [30] [31] [32] . Interestingly, activation of the innate immune response and, presumably, associated CD28 costimulation can restore antigen responses in T cell from old mice that have been adoptively transferred into young animal [31] . In addition, the defect in T cell proliferation and effector T cell generation in vitro can be rescued by the addition of exogenous IL-2 [33] , an analogous phenotype to T cell activation in the absence of CD28 costimulation [34] . In humans, aging is associated with a downmodulation of CD28 expression [35] . CD28-null T cells accumulate with age and up to 70% of CD4 and 95% of CD8 T cells do not express CD28 [35] . Even in young adults 20-30% of CD8 T cells can be CD28-null. CD28-null cells are thought to be generated by repeated antigen stimulation and so represent a form of memory T cell. These T cells are replicatively senescent with highly eroded telomeres, but are also resistant to apoptosis, which allows them to accumulate to high numbers. Functionally, CD28-null T cells are hyporesponsive, have defects in effector function, and may be immunosuppressive [35, 36] . Taken together these results from both animal models and human samples indicate that diminished CD28 costimulation may contribute to attenuation of immune responses in the elderly.

In spite of the current understanding of T cell signaling events and long-term interest in CD28 costimulation, the biochemical events associated with CD28 costimulation are still not well understood. CD28 signaling is associated with several different protein interaction motifs in the cytosolic tail that can mediate recruitment and activation of downstream signaling proteins (Fig. 1) . The most studied signaling pathway downstream of CD28 is activation of PI3K through SH2 domain interactions of p85 with phospho-Y170 within the YxxM motif [37, 38] . PI3K activation is predicted to impact on T cell activation through the enhanced recruitment of PH domain proteins to the membrane, including Itk and Akt. Itk phosphorylates and activates PLCc, increasing the calcium and PKC responses [39, 40] . Sustained calcium signaling leads to calcineurin-dependent nuclear localization of NFAT. PKCh plays an important role in the upregulation of NF-jB and AP1 [41] . Akt is a central regulator of cell activation, proliferation, and survival [42] . In T cells, Akt is thought to mediate the CD28-mediated, PI3K-dependent inhibition of the cell cycle regulator, p27 KIP , that promotes cell cycle entry and progression [43] , as well as the pro-survival factor, Bcl-XL [44] . Akt also plays a role in CD28-mediated glucose uptake, which may play a key role in CD28 regulation of T cell metabolism [45, 46] . Although the precise linkage is not yet established, Akt can cooperate with PKCh in CD28-mediated regulation of a NF-jB reporter construct in Jurkat T cells [47] . Akt can also inhibit GSK-3, the kinase that opposes calcineurin and drives NFAT back to the cytosol. Thus, Akt leads to sustained NFAT activation. All of these transcription factors, NF-jB, AP1, and NFAT, play an important role in the upregulation of IL-2 transcription. Although it is clear that CD28 can promote activation of PI3K, the functional role of PI3K activation in CD28 costimulation remains controversial. In murine tumor cell lines, disruption of the PI3K interaction site in CD28 inhibited IL-2 production [37, 38] , but this was not the case in transfected Jurkat cells [48, 49] . This discrepancy was thought to be resolved when Jurkat cells were found to lack both PTEN and SHIP-1, phosphatases that inactivate the products of PI3K [50, 51] . However, transgenic or retroviral reconstitution of normal murine T cells from CD28deficient mice with CD28 mutations in the YxxM motif had a limited phenotype [52] [53] [54] [55] [56] . A defect in Bcl-XL induction and T cell homing was noted [52, 54, 57] , but there was little overall impact on the immune response and T cell activation resulted in normal levels of IL-2 secretion. This discrepancy was resolved when we showed that the YxxM motif was required for CD28-mediated recruitment of PKCh, activation of NF-jB and upregulation of IL-2 transcription, but that this did not impact on IL-2 secretion because the primary pathway for CD28 induction of IL-2 secretion was mediated through mRNA stability [56, 58] (Wang X and Miller J, unpublished data). Although the YxxM motif appeared to be functionally relevant for CD28 costimulation, recent data indicate that this may not be mediated through activation of PI3K as originally proposed [59] .

CD28 can also recruit the adaptor proteins Grb2 through SH2 and SH3 domains and GADS through SH3 domains [60] [61] [62] [63] . Both adaptor proteins can recruit SOS and so activate Ras. However, CD28-mediated activation of Ras appears to be limited to antibody cross-linking and is not induced by B7 [64] . Interestingly, GADS also interacts with SLP76 and LAT, which can bind the guanine nucleotide exchange factor Vav, and the Grb2 SH3 domain can interact with Vav directly [65] . Vav has been implicated downstream of CD28 costimulation, and PKCh has been shown to interact with Vav in Jurkat T cells [66] [67] [68] [69] . Mutation of the Grb2/GADS binding site on CD28 leads to a decrease in Vav Fig. 1 Signaling motifs in the cytosolic tail of CD28. The sequence of the cytosolic tail of CD28 and three potential docking sites for cell signaling molecules are shown below. The amino acid number of the start of the motif relative to the mature CD28 protein is shown above. Note that in some publications the amino acid positions in CD28 have been numbered to include the 19 amino acid leader sequence. Phosphorylation of Y170 creates a docking site for SH2 domains which has been shown to bind to PI3K (YxxM) and Grb2 (YxN). The YxxM motif mediates recruitment of PKCh to the cSMAC, upregulation of IL-2 transcription, and induction of Bcl-XL expression. The polyproline motif starting at 175 has been shown to interact with the SH3 domain of Itk and GADS. The polyproline motif starting at 187 can bind the SH3 domains of Lck and Grb2. The Lck SH2 domain can also bind to phosphorylated Y188. CD28-filamin interaction is also dependent on this polyproline motif. Y188 is important for CD28 localization to the cSMAC. The polyproline motif starting at P187 mediates IL-2 mRNA stability and contributes to cytokine expression, humoral responses, and the reorganization of lipid rafts. See text for references and further discussion phosphorylation and can inhibit IL-2 production [60] . CD28 costimulation can also enhance TCR signaling downstream, most notably through activation of JNK [70] and ERK, possibly through inhibition of the Ras antagonist, Rap1 [71, 72] . CD28 has been shown to bind to the SH3 domain of Itk [40, 73] ; however, the functional significance of this interaction and of the polyproline motif starting at 175 is not clear. Recently, filamin A has been shown to be recruited to the immunological synapse and may play a role in lipid raft organization or PKCh recruitment [74, 75] . Interestingly, filamin A recruitment to the immunological synapse is dependent on the polyproline motif starting at 187 [74] . Filamins are large, actin binding scaffolds that have been shown to interact with over 20 different proteins associated with cell migration and signaling [76, 77] . Finally, CD28 can interact with both the SH2 and SH3 domains of Lck [78, 79] . The original model was that recruitment of Lck to CD28 resulted in CD28 activation, because Lck was shown to mediate phosphorylation of Y170, creating the binding site for the SH2 domains of PI3K and Grb2 [79, 80] . However, more recently, Lck has been considered as an effector of CD28 costimulation. Double mutation of the prolines at 187/190 results in a defect in the thymic maturation and peripheral differentiation of T regulatory T cells, whereas mutation of Y170 has no effect [15, 81, 82] . Likewise, we have shown that the ability of CD28 to enhance IL-2 mRNA stability requires the polyproline motif, but not the YMNM motif [56, 58] (Wang X and Miller J, unpublished data). In vivo this translates to a more dramatic effect of the P187/P190 mutation compared to the Y170 mutation [52, 83] . Although the functional relevance of this polyproline motif is clear, whether any or all of these effects are mediated though Lck recruitment is not known.

One of the major complications in dissecting CD28 costimulation is that most of the specific pathways that are activated by CD28 costimulation are shared with TCR signaling. Both protein profiling of signaling intermediates [84] and genetic profiling of changes in gene expression [85, 86] have suggested that CD28 costimulation functions primarily to modify those signaling pathways that can be regulated by the TCR itself and it has been difficult to identify a unique contribution of CD28. One potential site where CD28 could impact on TCR signaling is within the cSMAC of the immunological synapse [87] [88] [89] [90] . The proteins that are recruited to the immunological synapse between a T cell and APC are not randomly distributed. They segregate into at least three subdomains, called supramolecular activation clusters [91] . The central region, cSMAC, is enriched for cell surface proteins such as the TCR, CD4, CD28, and a minor fraction of CD45, along with associated signaling proteins such as PKCh and Lck; the peripheral region, pSMAC, contains the integrin, LFA-1, and associated cytoskeletal components, such as talin; and an outer region that contains the majority of CD45 [92] [93] [94] [95] [96] [97] [98] [99] [100] . Additionally, some proteins such as CD43 are excluded from the interface altogether [101] . In addition to the spatial arrangement of proteins within the immunological synapse, there is a dramatic temporal organization as well. Initially, TCR and CD4 form small clusters that coalesce into the cSMAC, while LFA-1 moves out into the pSMAC [94, 95] . CD28 is also recruited to these TCR clusters in the immature immunological synapse [102] . This remodeling provides a brief colocalization of CD4 and the phosphatase, CD45, possibly accounting for the initial activation of the CD4-associated Src family kinase, Lck [96, 99] . Sustained TCR signaling is thought to take place within microclusters that form at the periphery of the synapse and transduce signals while being transported through the pSMAC enroute to the cSMAC [103] [104] [105] .

It was recently shown that CD28 can also be recruited to these microclusters and these may provide an additional site for TCR/CD28 signal integration [106] .

The functional relevance of the localization of proteins to specific domains within the immunological synapse is not clear. The best example is for CD45, the cell surface phosphatase that plays an important role in Lck activation (by removal of the inhibitory phosphate at Y505), but can also inhibit T cell activation by removal of the activating phosphate at Y354 of Lck and possibly other signaling molecules. CD45 is polarized toward the APC, but largely excluded from the immunological synapse. Thus, CD45 is at high concentration at the site of TCR microcluster formation, possibly enhancing association of TCR/CD4 with activated Lck. Microclusters then move through the pSMAC, which is a region of very low CD45 [96, 99, 105] . We have shown that the exclusion of CD45 from the immunological synapse is dependent on LFA-1 expression and the failure to exclude CD45 correlates with reduced calcium signaling [107] . Thus, the pSMAC may regulate the magnitude of sustained TCR signaling by segregating CD45 from activated TCR complexes. Whether TCR signaling is restricted to microclusters or persists after localization in the cSMAC is still controversial. Initially it was proposed that TCR signaling occurred within the cSMAC region; however, recent data indicate that the cSMAC is a site for TCR downregulation [108] . More recently it was shown that under conditions of suboptimal TCR engagement, signaling may persist in the cSMAC and so the cSMAC may be a site for regulating the threshold of TCR signaling for optimal T cell activation [109, 110] . Finally, CD28 signaling has been associated with localization of CD28 and PKCh to the cSMAC. Although T cells express a number of PKC isoforms, PKCh is selectively activated and recruited to the immunological synapse, where it is colocalized with TCR and CD28 in the cSMAC [93, 97] . PKCh plays an essential role in transducing TCR-mediated activation of NF-jB [41, 111, 112] . Expression of CD28 is required for the targeting of PKCh to the cSMAC and in the absence of CD28 PKCh is recruited to the immunological synapse, but it is diffusely distributed across the synapse and is not focused into the cSMAC [56, 113] . This disruption in PKCh localization in the absence of CD28 correlates with a loss in PKCh-dependent induction of NF-jB and IL-2 transcription. Interestingly, all of these functions of CD28 (recruitment of PKCh to the cSMAC, activation of NF-jB, and upregulation of IL-2 transcription) are lost by a single amino acid mutation of the PI3K interaction site in the cytosolic tail of CD28 [56] . In addition, we have mapped the cSMAC localization signal in the cytosolic tail of CD28 [1] . Mutation of a single amino acid (Y188F) reduces the efficiency of CD28 recruitment to the immunological synapse and disrupts localization of CD28 to the cSMAC. Interestingly, localization of PKCh mirrors CD28 localization, indicating that CD28, and not other signals within the cSMAC, is the primary signal for PKCh localization within the synapse. Finally, the Y188F mutation also results in reduced activation of NF-jB, suggesting that mislocalization of CD28 and correspondingly PKCh may reduce the magnitude of CD28 costimulation. Taken together these findings suggest that the magnitude of TCR signaling and the integration between TCR and CD28 signaling may occur within and through the spatial organization of proteins in the immunological synapse.

Although TCR and CD28 signaling normally occur within the context of the immunological synapse, the ability of CD28 to upregulate IL-2 mRNA stability can be transduced in trans, i.e. from a separate site on the cell surface from TCR engagement [114] . This argues that TCR and CD28 signaling integration can take place downstream from plasma membrane-associated events. Furthermore, we have shown that the CD28 induced IL-2 transcription and mRNA stability are mediated through independent motifs within the CD28 cytosolic tail (Wang X and Miller J, unpublished data). The induction of mRNA stability is transduced through a polyproline-like motif; however, the specific signals that are activated by this motif are not clearly identified. The regulation of mRNA stability is largely controlled by AU-rich elements (ARE) within the 3 0 UTR of the mRNA [115] [116] [117] . ARE-mediated mRNA degradation plays an important role in the regulation of many genes [118, 119] , including cytokines [7] . RNA stability can also be regulated by micr-oRNA, although the relationship between microRNA and ARE-mediated mRNA decay are not well established [120] . The current model for regulated mRNA stability is that AUbinding proteins that induce mRNA instability, such as TTP, bind to the 3 0 UTR in unstimulated cells [121] . T cell activation leads to an increase in expression of TTP, and TTP can bind to the AU-rich region in the IL-2 3 0 UTR and drive IL-2 mRNA degradation [122, 123] . TTP recruits the multi-component exosome, allowing for deadenylation and 3 0 exonuclease digestion of the mRNA. In some cases, mRNA molecules can also be targeted by a 5 0 de-capping enzyme allowing for 5 0 exonuclease digestion as well [117, 124] . In the absence of ARE-mediated mRNA degradation, either by genetic disruption of TTP expression [125] or the deletion of the ARE from TNF [126] , overexpression of TNF results in the induction of autoimmune inflammatory diseases. The stability of AREcontaining mRNAs can be enhanced during cell activation events, although the mechanisms that mediate this stabilization are not well understood [115] [116] [117] . One model that has been proposed is that cell signaling induces the recruitment of different AU-binding proteins, such as HuR, that may compete with TTP for binding to the 3 0 UTR and, thus, interfere with TTP-dependent recruitment of the exosome. Although HuR can bind to the ARE in IL-2 mRNA, HuR does not appear to be involved in CD28-mediated mRNA stabilization [127] . Alternatively, MAP kinase activation has been implicated in the induction of mRNA stability. JNK can induce IL-2 and IL-3 mRNA stability [128] [129] [130] ; p38 has been implicated in the stabilization of IL-2, IL-6, IL-8, and TNFa mRNA [131] [132] [133] ; and ERK can stabilize COX-2 mRNA in smooth muscle cells. One potential target of MAP kinase activation is TTP itself. Phosphorylation of TTP appears to increase the stability of TTP, probably through the association with 14-3-3. However, whether phosphorylation itself directly impacts on TTP binding affinity for ARE or whether 14-3-3 association regulates the ability of TTP to recruit the exosome degradation machinery remain controversial [134] [135] [136] .

The initial focus on the regulation of mRNA stability in T cells was the JNK pathway. A JNK-response element was identified by mutagenesis in the 5 0 end of IL-2 mRNA and two proteins, YB-1 and nucleolin, were found to bind to this element [129] . YB-1 and nucleolin are ubiquitously expressed multifunctional nucleic acid binding proteins [137, 138] . However, these proteins alone are not sufficient to induce mRNA stability and require at least one protein that might bind to the 3 0 UTR. In addition, the ability of CD28 to enhance JNK activity [70, 139] and the importance of JNK in IL-2 expression [140] remain controversial. More recently the focus has been on NF90, which was originally identified as a transcription factor associated with NFAT [141] , and is now recognized as an RNA-binding protein. NF90 can compete with TTP for binding to the AU-rich region of the IL-2 mRNA [142] . NF90 is localized in the nucleus in resting cells, but T cell activation results in movement of the majority of NF90 to the cytosol and this nuclearcytosolic shuttle event is linked to IL-2 mRNA stabilization. Recent analysis of NF90 knock out T cells has confirmed an important role of NF90 in the regulation of IL-2 secretion, but it is not clear how much of this effect is mediated through NF90's effect on IL-2 transcription or mRNA stability [143] . The signals that regulate NF90 localization/ function, how CD28 might impact on regulation of NF90, and how NF90 might interact with other factors that impact on mRNA stability are not understood.

Recent attempts to dissect the different roles of CD28 in normal T cells have suggested that distinct functions might be mediated by different signaling pathways [55, 56] . We have defined two independent pathways that impact on the ability of CD28 to upregulate IL-2 expression [1, 56, 114] (Wang X and Miller J, unpublished data). It is well established that CD28 costimulation can enhance cytokine secretion through an increase in transcription and through the induction of mRNA stability. But the relationship between these molecular events and their relative impact on the levels of cytokine secretion were not understood. We have recently shown that these two mechanisms involved in the upregulation of IL-2 secretion are mediated through independent signaling pathways. First, mutation of M173 in the cytosolic tail of CD28, a mutation that disrupts the ability of CD28 to recruit and activate PI3K, results in a failure to recruit PKCh to the cSMAC, drive nuclear localization of NF-jB, and enhance IL-2 transcription [56] . Disruption of this site also results in a loss in Bcl-XL upregulation, a defect in the generation of graft-versus-host responses and an alteration in T cell trafficking, but has little effect on overall cytokine expression [52] [53] [54] 57] . However, this mutation did not affect the ability of CD28 to promote IL-2 mRNA stability and had little effect on the level of IL-2 protein secretion. Second, mutation of a polyproline motif (PYAPARDF) disrupts the ability of CD28 to induce IL-2 mRNA stability (Wang X and Miller J, unpublished data). This element is associated with Lck and Grb2 recruitment and mutation of the proline residues interferes with T regulatory cell development and results in a general reduction in cytokine expression and humoral responses [15, 60, 61, 79, [81] [82] [83] 144] . Importantly, the two pathways are independent. Mutation of the YMNM motif blocked upregulation of IL-2 transcription without affecting mRNA stability, whereas mutation of the PYAPARDF motif blocked the induction of mRNA stability without affecting transcription. In addition, disruption of mRNA stability had a greater effect on the levels of IL-2 secretion than did disruption of CD28-enhanced IL-2 transcription. Understanding these pathways and the biochemical events associated with signal transduction will provide important insight into how CD28 costimulation can impact on so many aspects of T cell activation, differentiation, and tolerance.

Signals and sequences that control CD28 localization to the central region of the immunological synapse

T cell-dendritic cell immunological synapses contain TCR-dependent CD28-CD80 clusters that recruit protein kinase Ctheta

CD28/B7 system of T cell costimulation

Upstream-downstream: CD28 cosignaling pathways and T cell function

CD28-mediated co-stimulation; a quantitative support for TCR signaling

Unifying concepts in CD28, ICOS and CTLA4 co-receptor signalling

Regulation of lymphokine messenger RNA stability by a surface-mediated T cell activation pathway

Regulation of interleukin-2 gene enhancer activity by the T cell accessory molecule CD28

CD28 costimulation can promote T cell survival by enhancing the expression of Bcl-xL

The CD28 signalling pathway regulates glucose metabolism

Inhibition of antigen-specific proliferation of type 1 murine T cell clones after stimulation with immobilized anti-CD3 monoclonal antibody

CD28-mediated signalling co-stimulates murine T cells and prevents induction of anergy in T-cell clones

CD28 costimulation promotes the production of Th2 cytokines

Regulation of thymic NKT cell development by the B7-CD28 costimulatory pathway

CD28 costimulation of developing thymocytes induces Foxp3 expression and regulatory T cell differentiation independently of interleukin 2

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

TNF family ligands define niches for T cell memory

Role of B7 in T cell tolerance

Age-related changes in lymphocyte development and function

T cell aging: naive but not young

Cytokine receptor signalling and aging

Intrinsic versus environmental influences on T-cell responses in aging

The effect of age on the cognate function of CD4? T cells

Why aging T cells fail: implications for vaccination

CD4?CD25?Foxp3? T cells and CD4?CD25-Foxp3? T cells in aged mice

High accumulation of T regulatory cells prevents the activation of immune responses in aged animals

Signal transduction in the aging immune system

Relative contribution of T and B cells to hypermutation and selection of the antibody repertoire in germinal centers of aged mice

Antigen-specific CD4 T cell clonal expansion and differentiation in the aged lymphoid microenvironment. I. The primary T cell response is unaffected

Age-related defects in CD4 T cell cognate helper function lead to reductions in humoral responses

Inflammatory cytokines overcome age-related defects in CD4 T cell responses in vivo

Newly generated CD4 T cells in aged animals do not exhibit age-related defects in response to antigen

Interleukin 2, but not other common gamma chain-binding cytokines, can reverse the defect in generation of CD4 effector T cells from naive T cells of aged mice

CD28/B7 interactions deliver a unique signal to naive T cells that regulates cell survival but not early proliferation

CD28 extinction in human T cells: altered functions and the program of T-cell senescence

Lower GrB? CD62Lhigh CD8 TCM effector lymphocyte response to influenza virus in older adults is associated with increased CD28null CD8 T lymphocytes

Binding of phosphatidylinositol-3-OH kinase to CD28 is required for T-cell signalling

Selective CD28pYMNM mutations implicate phosphatidylinositol 3-kinase in CD86-CD28-mediated costimulation

Analysis of CD28 cytoplasmic tail tyrosine residues as regulators and substrates for the protein tyrosine kinases, EMT and LCK

The SH3 domain of Itk/ Emt binds to proline-rich sequences in the cytoplasmic domain of the T cell costimulatory receptor CD28

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

Protein kinase B (Akt) regulation and function in T lymphocytes

CD28 costimulation mediates downregulation of p27 kip1 and cell cycle progression by activation of the PI3 K/PKB signaling pathway in primary human T cells

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

Glucose uptake is limiting in T cell activation and requires CD28-mediated Akt-dependent and independent pathways

Fuel feeds function: energy metabolism and the T-cell response

Akt provides the CD28 costimulatory signal for upregulation of IL-2 and IFN-c but not Th2 cytokines

CD28 delivers costimulatory signals independently of its association with phosphatidylinositol 3-kinase

CD28-mediated costimulation in the absence of phosphatidylinositol 3-kinase association and activation

Deficiency of PTEN in Jurkat T cells causes constitutive localization of Itk to the plasma membrane and hyperresponsiveness to CD3 stimulation

Evidence that SHIP-1 contributes to phosphatidylinositol 3,4,5-trisphosphate metabolism in T lymphocytes and can regulate novel phosphoinositide 3-kinase effectors

A point mutation in CD28 distinguishes proliferative signals from survival signals

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

Distinct motifs within CD28 regulate T cell proliferation and induction of Bcl-XL

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

CD28-mediated transcriptional and posttranscriptional regulation of IL-2 expression are controlled through different signaling pathways

Physiologic and aberrant regulation of memory T-cell trafficking by the costimulatory molecule CD28

CD28-mediated mRNA stabilty and IL-2 production is controlled by a polyprolinelike motif

CD28 provides T-cell costimulation and enhances PI3K activity at the immune synapse independently of its capacity to interact with the p85/p110 heterodimer

Growth factor receptor-bound protein 2 SH2/SH3 domain binding to CD28 and its role in co-signaling

Grb2 forms an inducible protein complex with CD28 through a src homology 3 domain-proline interaction

GRID: a novel Grb-2 related adapter protein that interacts with the activated T cell costimulatory receptor CD28

Grb2 and Gads exhibit different interactions with CD28 and play distinct roles in CD28-mediated costimulation

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

Novel recognition mode between Vav and Grb2 SH3 domains

Synergistic activation of NF-jB by functional cooperation between vav and PKCh in T lymphocytes

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

CD28 utilizes Vav-1 to enhance TCR-proximal signaling and NF-AT activation

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

JNK is involved in signal integration during costimulation of T lymphocytes

Costimulation through CD28 suppresses T cell receptor-dependent activation of the Ras-like small GTPase Rap1 in human T lymphocytes

CD28 and the tyrosine kinase Lck stimulate mitogen-activated protein kinase activity in T cells via inhibition of the small G protein Rap1

CD28 is associated with and induces the immediate tyrosine phosphorylation and activation of the TEC family kinase ITK/EMT in the human Jurkat leukemic T-cell line

CD28 interaction with filamin-A controls lipid raft accumulation at the T-cell immunological synapse

Filamin A is required for T cell activation mediated by protein kinase C-theta

The many faces of filamin: a versatile molecular scaffold for cell motility and signalling

Filamins: promiscuous organizers of the cytoskeleton

Multiple modes of interaction between Lck and CD28

Proline residues in CD28 and the Src homology (SH)3 domain of Lck are required for T cell costimulation

p56Lck and p59Fyn regulate CD28 binding to phosphatidylinositol 3-kinase, growth factor receptor-bound protein GRB-2, and T cellspecific protein-tyrosine kinase ITK: implications for T-cell costimulation

CD28 controls differentiation of regulatory T cells from naive CD4 T cells

Induction of autoimmune disease in CTLA-4-/-mice depends on a specific CD28 motif that is required for in vivo costimulation

A dose-dependent requirement for the proline motif of CD28 in cellular and humoral immunity revealed by a targeted knockin mutant

CD28 as a molecular amplifier extending TCR ligation and signaling capabilities

Genomic expression programs and the integration of the CD28 costimulatory signal in T cell activation

Modulation of TCR-induced transcriptional profiles by ligation of CD28, ICOS, and CTLA-4 receptors

The immunological synapse

Signaling takes shape in the immune system

Formation and function of the immunological synapse

T-cell-antigen recognition and the immunological synapse

Three-dimensional segregation of supramolecular activation clusters in T cells

Cells that present both specific ligand and costimulatory activity are the most efficient inducers of clonal expansion of normal CD4 T cells

Selective modulation of protein kinase C-h during T cell activation

The immunological synapse: a molecular machine controlling T cell activation

Differential clustering of CD4 and CD3f during T cell recognition

A supramolecular basis for CD45 tyrosine phosphatase regulation in sustained T cell activation

The immunological synapse and CD28-CD80 interactions

T cell receptor signaling precedes immunological synapse formation

Staging and resetting T cell activation in SMACs

T cell receptor ligation induces the formation of dynamically regulated signaling assemblies

TCR signaling induces selective exclusion of CD43 from the T cell-antigen-presenting cell contact site

CD28 signals in the immature immunological synapse

Actin and agonist MHC-peptide complex-dependent T cell receptor microclusters as scaffolds for signaling

Newly generated T cell receptor microclusters initiate and sustain T cell activation by recruitment of Zap70 and SLP-76

T cell receptor-proximal signals are sustained in peripheral microclusters and terminated in the central supramolecular activation cluster

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

LFA-1-mediated T cell costimulation through increased localization of TCR/class II complexes to the central supramolecular activation cluster and exclusion of CD45 from the immunological synapse

The immunological synapse balances T cell receptor signaling and degradation

The balance between T cell receptor signaling and degradation at the center of the immunological synapse is determined by antigen quality

The stimulatory potency of T cell antigens is influenced by the formation of the immunological synapse

Protein kinase Ch: a new essential superstar on the T-cell stage

PDK1 nucleates T cell receptor-induced signaling complex for NF-kappaB activation

CD28 plays a critical role in the segregation of PKCh within the immunological synapse

Engagement of CD28 outside of the immunological synapse results in upregulation of IL-2 mRNA stability, but not IL-2 transcription

Post-transcriptional control of cytokine production

The highways and byways of mRNA decay

The control of mRNA stability in response to extracellular signals

Patterns of coordinate down-regulation of ARE-containing transcripts following immune cell activation

Genome-wide analysis of mRNA decay in resting and activated primary human T lymphocytes

Decoding ARE-mediated decay: is microRNA part of the equation?

Genome-wide analysis identifies interleukin-10 mRNA as target of tristetraprolin

HuA and tristetraprolin are induced following T cell activation and display distinct overlapping RNA binding specificities

Tristetraprolin downregulates IL-2 gene expression through AU-rich element-mediated mRNA decay

Transcriptional and post-transcriptional control of gene expression in inflammation

Feedback inhibition of macrophage tumor necrosis factor-a production by tristetraprolin

Impaired on/off regulation of TNF biosynthesis in mice lacking TNF AU-rich elements: implications for joint and gut-associated immunopathologies

Selective cytoplasmic translocation of HuR and site-specific binding to the interleukin-2 mRNA are not sufficient for CD28-mediated stabilization of the mRNA

Stabilization of IL-2 mRNA by the c-jun NH2-terminal kinase pathway

Nucleolin and YB-1 are required for JNK-mediated interleukin-2 mRNA stabilization during T-cell activation

c-jun N-terminal kinase is involved in AUUUA-mediated IL-3 mRNA turnover in mast cells

The p38 MAP kinase pathway signals for cytokine-induced mRNA stabilization via MAP kinase activated protein kinase-2 and an AU-rich region-targeted mechanism

Mitogen-activated protein kinase p38 controls the expression and posttranslational modification of tristetraprolin, a regulator of tumor necrosis factor alpha mRNA stability

Endothelial cell co-stimulation through OX40 augments and prolongs T cell cytokine synthesis by stabilization of cytokine mRNA

Mitogen-activated protein kinase-activated protein kinase 2 regulates tumor necrosis factor mRNA stability and translation mainly by altering tristetraprolin expression, stability, and binding to adenine/uridine-rich element

Structure/function analysis of tristetraprolin (TTP): p38 stress-activated protein kinase and lipopolysaccharide stimulation do not alter TTP function

MK2-induced tristetraprolin:14-3-3 complexes prevent stress granule association and ARE-mRNA decay

Structure and functions of nucleolin

The pleotropic functions of the Y-boxbinding protein, YB-1

CD28 is not required for c-Jun N-terminal kinase activation in T cells

JNK is required for effector T cell function but not for T cell activation

Purification by DNA affintiy chromatography of two polypeptides that contact the NFAT DNA binding site in the IL-2 promoter

Nuclear export of NF90 is required for interleukin-2 mRNA stabilization

NF90 regulates inducible IL-2 gene expression in T cells

Regulation of Lck activity by CD4 and CD28 in the immunological synapse