Differential use of Fas ligand and perforin cytotoxic pathways by donor T cells in graft-versus-host disease and graft-versus-leukemia effect

In allogeneic bone marrow transplantation (BMT) donor T cells are primarily responsible for antihost activity, resulting in graft-versus-host disease (GVHD), and for antileukemia activity, resulting in the graft-versus-leukemia (GVL) effect. The relative contributions of the Fas ligand (FasL) and perforin cytotoxic pathways in GVHD and GVL activity were studied by using FasL-defective or perforin-deficient donor T cells in murine parent --> F1 models for allogeneic bone marrow transplantation. It was found that FasL-defective B6.gld donor T cells display diminished GVHD activity but have intact GVL activity. In contrast, perforin-deficient B6.pfp(-/-) donor T cells have intact GVHD activity but display diminished GVL activity. Splenic T cells from recipients of B6.gld or B6.pfp(-/-) T cells had identical proliferative and cytokine responses to host antigens; however, splenic T cells from recipients of B6.pfp(-/-) T cells had no cytolytic activity against leukemia cells in a cytotoxicity assay. In experiments with selected CD4(+) or CD8(+) donor T cells, the FasL pathway was important for GVHD activity by both CD4(+) and CD8(+) T cells, whereas the perforin pathway was required for CD8-mediated GVL activity. These data demonstrate in a murine model for allogeneic bone marrow transplantation that donor T cells mediate GVHD activity primarily through the FasL effector pathway and GVL activity through the perforin pathway. This suggests that donor T cells make differential use of cytolytic pathways and that the specific blockade of one cytotoxic pathway may be used to prevent GVHD without interfering with GVL activity.

Absence of mitochondrial superoxide dismutase results in a murine hemolytic anemia responsive to therapy with a catalytic antioxidant

Manganese superoxide dismutase 2 (SOD2) is a critical component of the mitochondrial pathway for detoxification of O2(-), and targeted disruption of this locus leads to embryonic or neonatal lethality in mice. To follow the effects of SOD2 deficiency in cells over a longer time course, we created hematopoietic chimeras in which all blood cells are derived from fetal liver stem cells of Sod2 knockout, heterozygous, or wild-type littermates. Stem cells of each genotype efficiently rescued hematopoiesis and allowed long-term survival of lethally irradiated host animals. Peripheral blood analysis of leukocyte populations revealed no differences in reconstitution kinetics of T cells, B cells, or myeloid cells when comparing Sod2(+/+), Sod2(-/-), and Sod2(+/-) fetal liver recipients. However, animals receiving Sod2(-/-) cells were persistently anemic, with findings suggestive of a hemolytic process. Loss of SOD2 in erythroid progenitor cells results in enhanced protein oxidative damage, altered membrane deformation, and reduced survival of red cells. Treatment of anemic animals with Euk-8, a catalytic antioxidant with both SOD and catalase activities, significantly corrected this oxidative stress-induced condition. Such therapy may prove useful in treatment of human disorders such as sideroblastic anemia, which SOD2 deficiency most closely resembles.

Targeting Src homology 2 domain-containing tyrosine phosphatase (SHP-1) into lipid rafts inhibits CD3-induced T cell activation

To study the mechanism by which protein tyrosine phosphatases (PTPs) regulate CD3-induced tyrosine phosphorylation, we investigated the distribution of PTPs in subdomains of plasma membrane. We report here that the bulk PTP activity associated with T cell membrane is present outside the lipid rafts, as determined by sucrose density gradient sedimentation. In Jurkat T cells, approximately 5--10% of Src homology 2 domain-containing tyrosine phosphatase (SHP-1) is constitutively associated with plasma membrane, and nearly 50% of SHP-2 is translocated to plasma membrane after vanadate treatment. Similar to transmembrane PTP, CD45, the membrane-associated populations of SHP-1 and SHP-2 are essentially excluded from lipid rafts, where other signaling molecules such as Lck, linker for activation of T cells, and CD3 zeta are enriched. We further demonstrated that CD3-induced tyrosine phosphorylation of these substrates is largely restricted to lipid rafts, unless PTPs are inhibited. It suggests that a restricted partition of PTPs among membrane subdomains may regulate protein tyrosine phosphorylation in T cell membrane. To test this hypothesis, we targeted SHP-1 into lipid rafts by using the N-terminal region of Lck (residues 1--14). The results indicate that the expression of Lck/SHP-1 chimera inside lipid rafts profoundly inhibits CD3-induced tyrosine phosphorylation of CD3 zeta/epsilon, IL-2 generation, and nuclear mobilization of NF-AT. Collectively, these results suggest that the exclusion of PTPs from lipid rafts may be a mechanism that potentiates TCR/CD3 activation.

Uncoupling p70(s6) kinase activation and proliferation: rapamycin-resistant proliferation of human CD8(+) T lymphocytes

Rapamycin is a fungal macrolide that inhibits the proliferation of T cells. Studies in both animals and humans have found that rapamycin significantly reduces graft rejection. However, though CD8(+) T cells are involved in graft infiltration and rejection, little is known regarding the effects of rapamycin on CD8(+) human T cell responses. In this study, we examined the mechanism of rapamycin-induced inhibition of Ag-driven activation of CD8(+) T cells. Surprisingly, a heterogeneous proliferative response in the presence of rapamycin was observed among different Ag-specific CD8(+) T cell clones; this was also observed in CD8(+) peripheral blood T cells activated with TCR cross-linking ex vivo. Inhibition of T cell proliferation by rapamycin was controlled by both the strength of signal delivered through the Ag receptor as well as the specific costimulatory signals received by the T cell. Rapamycin-resistant proliferation occurred despite inhibition of p70(s6) kinase activity. Moreover, rapamycin-resistant proliferation of the CD8(+) T cell clones was blocked by anti-IL-2 Abs, suggesting that while some of the parallel pathways triggered by IL-2R signaling are sensitive to the effects of rapamycin, others account for the Ag-driven rapamycin resistance. These data provide a new framework for examining the specific mechanism of action of rapamycin in human disease.

Regulation of Cbl molecular interactions by the co-receptor molecule CD43 in human T cells

CD43, one of the most abundant glycoproteins on the T cell surface, has been implicated in selection and maturation of thymocytes and migration, adhesion, and activation of mature T cells. The adapter molecule Cbl has been shown to be a negative regulator of Ras. Furthermore, it may also regulate intracellular signaling through the formation of several multi-molecular complexes. Here we investigated the role of Cbl in the CD43-mediated signaling pathway in human T cells. Unlike T cell receptor signaling, the interaction of the adapter protein Cbl with Vav and phosphatidylinositol 3-kinase, resulting from CD43-specific signals, is independent of Cbl tyrosine phosphorylation, suggesting an alternative mechanism of interaction. CD43 signals induced a Cbl serine phosphorylation-dependent interaction with the tau-isoform of 14-3-3. protein. Protein kinase C-mediated Cbl serine phosphorylation was required for this interaction, because the PKC inhibitor RO-31-8220 prevented it, as well as 14-3-3 dimerization. Moreover, mutation of Cbl serine residues 619, 623, 639, and 642 abolished the interaction between Cbl and 14-3-3. Overexpression of Cbl in Jurkat cells inhibited the CD43-dependent activation of the mitogen-activated protein kinase (MAPK) pathway and AP-1 transcriptional activity, confirming nevertheless a negative role for Cbl in T cell signaling. However, under normal conditions, PKC activation resulting from CD43 engagement was required to activate the MAPK pathway, suggesting that phosphorylation of Cbl on serine residues by PKC and its association with 14-3-3 molecules may play a role in preventing the Cbl inhibitory effect on the Ras-MAPK pathway. These data suggest that by inducing its phosphorylation on serine residues, CD43-mediated signals may regulate the molecular associations and functions of the Cbl adapter protein.

Cutting edge: gab2 mediates an inhibitory phosphatidylinositol 3'-kinase pathway in T cell antigen receptor signaling

Phosphatidylinositol 3'-kinase (PI3K) is a key component of multiple signaling pathways, where it typically promotes survival, proliferation, and/or adhesion. Here, we show that in TCR signaling, the scaffolding adapter Gab2 delivers an inhibitory signal via PI3K. Overexpression of Gab2 in T cell lines inhibits TCR-evoked activation of the IL-2 promoter, blocking NF-AT- and NF-kappaB-directed transcription. Inhibition is abrogated by mutating the Gab2 p85-binding sites, by treatment with PI3K inhibitors or by cotransfection of phosphatase homolog of tensin. Our findings provide the first evidence of a negative function for a scaffolding adapter in T cells and identify Gab2/PI3K-containing complexes as novel regulators of TCR signaling.

CD43-mediated signals induce DNA binding activity of AP-1, NF-AT, and NFkappa B transcription factors in human T lymphocytes

Although numerous reports document a role for CD43 in T cell signaling, the direct participation of this molecule in cell activation has been questioned. In this study we show that CD43 ligation on human normal peripheral T cells was sufficient to induce interleukin-2, CD69, and CD40-L gene expression, without requiring signals provided by additional receptor molecules. This response was partially inhibited by cyclosporin A and staurosporine, suggesting the participation of both the Ca(2+) and the protein kinase C pathways in CD43 signaling. Consistent with the transient CD43-dependent intracellular Ca(2+) peaks reported by others, signals generated through the CD43 molecule resulted in the induction of NF-AT DNA binding activity. CD43-dependent signals resulted also in AP-1 and NFkappaB activation, probably as a result of protein kinase C involvement. AP-1 complexes bound to the AP-1 sequence contained c-Jun, and those bound to the NF-AT-AP-1 composite site contained c-Jun and Fos. NFkappaB complexes containing p65 could be found as early as 1 h after CD43 cross-linking, suggesting that CD43 participates in early events of T cell activation. The induction of the interleukin-2, CD69, and CD-40L genes and the participation of AP-1, NF-AT, and NFkappaB in the CD43-mediated signaling cascade implicate an important role for this molecule in the regulation of gene expression and cell function.

A peptide library approach identifies a specific inhibitor for the ZAP-70 protein tyrosine kinase

We utilized a novel peptide library approach to identify specific inhibitors of ZAP-70, a protein Tyr kinase involved in T cell activation. By screening more than 6 billion peptides oriented by a common Tyr residue for their ability to bind to ZAP-70, we determined a consensus optimal peptide. A Phe-for-Tyr substituted version of the peptide inhibited ZAP-70 protein Tyr kinase activity by competing with protein substrates (K(I) of 2 microM). The related protein Tyr kinases, Lck and Syk, were not significantly inhibited by the peptide. When introduced into intact T cells, the peptide blocked signaling downstream of ZAP-70, including ZAP-70-dependent gene induction, without affecting upstream Tyr phosphorylation. Thus, screening Tyr-oriented peptide libraries can identify selective peptide inhibitors of protein Tyr kinases.

The role of calcineurin in lymphocyte activation

Lymphokine gene transcription involves numerous signal transduction molecules and second messengers. The serine/threonine phosphatase calcineurin has been demonstrated to play a central role in the immediate, early activation of numerous lymphokines (such as interleukin [IL]-2) and in the regulation of cell surface receptors such as CD40L, CD95, and recently CD25 alpha (the alpha chain of the IL-2 receptor). In addition to lymphocyte activation, calcineurin functions include control of neuronal signaling, muscle contraction, muscle hypertrophy and cellular death. Therefore, calcineurin not only plays a vital role in the regulation of T lymphocyte function, but also functions in cellular environments outside the immune system.

Fas ligand-deficient gld mice are more susceptible to graft-versus-host-disease

BACKGROUND

The Fas/Fas ligand (FasL) pathway plays an important role in a number of apoptotic processes that could be important for the development of graft-versus-host disease (GVHD) after bone marrow transplantation (BMT), such as cytolysis of target cells by cytotoxic T cells, regulation of inflammatory responses, peripheral deletion of autoimmune cells, costimulation of T cells, and activation-induced cell death.

METHODS

To study the role of the Fas/FasL pathway in the complex pathophysiology of graft versus host disease (GVHD), we used FasL-deficient B6.gld mice as recipients in a Major Histocompatibility Antigen Complex-matched minor Histocompatibility Antigen-mismatched murine model for GVHD after allogeneic BMT (C3H.SW-->B6).

RESULTS

We found a significantly higher morbidity and mortality from GVHD compared to control B6 recipients. Histopathological analysis of the GVHD target organs demonstrated that B6.gld recipients developed significantly more thymic and intestinal GVHD. B6gld recipients with GVHD demonstrated an increased expansion of donor T cells and monocytes/ macrophages compared to control B6 recipients, whereas serum TNF-alpha levels were equivalent in B6.gld recipients and control B6 recipients.

CONCLUSION

This study demonstrates that the expression of FasL in the BMT recipient is important for the host's ability to control GVHD.

Calcineurin regulation of the mammalian G0/G1 checkpoint element, cyclin dependent kinase 4

Cyclin dependent kinase 4 (cdk4) activity is controlled by the binding of regulatory subunits and inhibitory factors, as well as tyrosine and serine/threonine phosphorylation. More recently the influence of calcium levels have been demonstrated. Using transient transfections in Jurkat cells, we observed specific binding between cdk4 and the calcium and calmodulin activated serine/threonine phosphatase, calcineurin. Furthermore, we demonstrated that the inhibition of the phosphatase activity of calcineurin with FK506 and cyclosporin A resulted in an overall increase in cdk4 kinase activity, suggesting that the phosphatase activity of calcineurin was inhibitory to the kinase activity of cdk4. In contrast, we were not able to observe a similar effect on the kinase activity of either cdk6 or cdk2, indicating that the phosphatase activity of calcineurin was specific for cdk4. In addition, using an in vitro phosphatase assay for calcineurin, we observed that the exogenous addition of calcineurin resulted in the dephosphorylation of cdk4, an event that downregulated the kinase activity of cdk4. Calcineurin could, therefore, play an opposing role to the action of the cyclin activating kinase complex, an enzyme that upregulates the kinase activity of cdk4, an important G0/G1 checkpoint element in mammalian cells. Oncogene (2000) 19, 2820 - 2827

Graft-versus-host-disease-associated thymic damage results in the appearance of T cell clones with anti-host reactivity

BACKGROUND

We studied whether T-cell clones, which appear in the periphery as a result of the failure of thymic negative selection during graft-versus-host disease (GVHD), have any in vivo anti-host reactivity and can cause GVHD in an adoptive transfer model.

METHODS

We performed our studies in a murine model (B10.BR into CBA/J) for allogeneic bone marrow transplantation with major histocompatibility complex-matched and minor histocompatibility antigen-mismatched unrelated donors and unique Vbeta T-cell deletion patterns in donors and recipients.

RESULTS

GVHD resulted in the appearance of Vbeta6+ T cells as a result of a loss of negative selection. We found that Vbeta6+ T cells from normal donors proliferated in vitro and in vivo. Depletion of Vbeta6+ T cells from the donor T-cell inoculum resulted in less GVHD morbidity and a decrease in the loss of thymic cellularity. To test the anti-host reactivity of de novo generated Vbeta6+ T cells in animals with GVHD, we developed an adoptive transfer model of splenic T cells from CBA/J host animals with GVHD into sublethally irradiated CBA/J recipients Depletion of Vbeta6+ T cells from the splenic T cells before adoptive transfer could significantly decrease the transient GVHD morbidity in the sublethally irradiated hosts.

CONCLUSIONS

Our data indicate that GVHD-associated thymic damage results in a loss of thymic negative selection, which leads to the appearance of T-cell clones with anti-host reactivity in vitro and in vivo.

Cytosolic tyrosine dephosphorylation of STAT5. Potential role of SHP-2 in STAT5 regulation

STAT5, a member of the signal transducers and activators of transcription (STATs), is important in modulating T cell functions through interleukin-2 (IL-2) receptors. Like other STAT proteins, STAT5 undergoes a rapid activation and inactivation cycle upon cytokine stimulation. Tyrosine phosphorylation and dephosphorylation are critical in regulating STAT5 activity. A number of protein tyrosine kinases have been shown to phosphorylate STAT5; however, the phosphatases responsible for STAT5 dephosphorylation remain unidentified. Using CTLL-20 as a model system, we provide evidence that tyrosine dephosphorylation of STAT5 subsequent to IL-2-induced phosphorylation occurs in the absence of STAT5 nuclear translocation and new protein synthesis. Nevertheless, down-regulation of the upstream Janus kinase activity during the deactivation cycle of IL-2-induced signaling does involve new protein synthesis. These findings point to the constitutive presence of STAT5 tyrosine phosphatase activity in the cytosolic compartment. We further demonstrate that SHP-2, but not SHP-1, directly dephosphorylates STAT5 in an in vitro tyrosine phosphatase assay with purified proteins. Furthermore, tyrosine-phosphorylated STAT5 associates with the substrate-trapping mutant (Cys --> Ser) of SHP-2 but not SHP-1. These results suggest a potential role for cytoplasmic protein-tyrosine phosphatases in directly dephosphorylating STAT proteins and in maintaining a basal steady state level of STAT activity.

Fas-deficient lpr mice are more susceptible to graft-versus-host disease

The Fas/Fas ligand (FasL) pathway is involved in a variety of regulatory mechanisms that could be important for the development of graft-vs-host disease (GVHD) after bone marrow transplantation (BMT), such as cytolysis of target cells by cytotoxic T cells, regulation of inflammatory responses, peripheral deletion of autoimmune cells, costimulation of T cells, and activation-induced cell death. To further evaluate the role of Fas/FasL in the complex pathophysiology of GVHD, we used Fas-deficient B6.lpr mice as recipients in a MHC-matched minor histocompatibility Ag-mismatched murine model for GVHD after allogeneic BMT (C3H.SW-->B6). We found a significantly higher morbidity and mortality from GVHD compared with control B6 recipients. In contrast, B6.lpr recipients had very little hepatic GVHD, although all other specific GVHD target organs (skin, intestines, and thymus) were more severely affected than in the control B6 recipients. B6.lpr recipients with GVHD demonstrated intact donor lymphoid engraftment and an increase in expansion of donor T cells and monocytes/macrophages compared with control B6 recipients. Serum levels of IFN-gamma and TNF-alpha were higher in B6.lpr recipients than in control B6 recipients, and monocytes/macrophages in B6.lpr recipients appeared more sensitized. B6.lpr recipients had more residual peritoneal macrophages after BMT, and peritoneal macrophages from B6.lpr mice could induce a greater proliferative response from C3H.SW splenocytes. This study demonstrates that the expression of Fas in the recipient is required for GVHD of the liver, but shows unexpected consequences when host tissues lack the expression of Fas for the development of GVHD in other organs and systemic GVHD.

Antigen-induced unresponsiveness results in altered T cell signaling

Pretransplant exposure to allogeneic lymphocytes can result in donor-specific unresponsiveness and prolonged allograft survival. Intracellular signaling events have been described in anergic T cell clones, but the biochemical events underlying in vivo induced unresponsiveness have not been studied in detail. We employed a TCR transgenic mouse, bearing the 2C TCR, providing adequate numbers of homogenous peripheral T cells to study biochemical aspects of T cell unresponsiveness in vivo. 2C mice exposed to semiallogeneic lymphocytes (H-2b x H-2d) experienced prolonged H-2d cardiac allograft survival, and cells from these mice did not proliferate or make IL-2 in response to alloantigen (H-2d). Importantly, there were marked differences in TCR-associated tyrosine phosphorylation activation patterns. The targets for the unresponsive state appear to be diminished Lck activation and absent ZAP-70 and LAT (linker for activation of T cells) phosphorylation. Our study demonstrates that Ag-induced tolerance in vivo is accompanied by altered early TCR-mediated signaling events.

Human 70-kDa SHP-1L differs from 68-kDa SHP-1 in its C-terminal structure and catalytic activity

The tyrosine phosphatase SHP-1 functions as a negative regulator in hematopoietic cell development, proliferation, and receptor-mediated cellular activation. In Jurkat T cells, a major 68-kDa band and a minor 70-kDa band were immunoprecipitated by a monoclonal antibody against the SHP-1 protein-tyrosine phosphatase domain, while an antibody against the SHP-1 C-terminal 19 amino acids recognized only the 68-kDa SHP-1. The SDS-gel-purified 70-kDa protein was subjected to tryptic mapping and microsequencing, which was followed by molecular cloning. It revealed that the 70-kDa protein, termed SHP-1L, is a C-terminal alternatively spliced form of SHP-1. SHP-1L is 29 amino acids longer than SHP-1, and its 66 C-terminal amino acids are different from SHP-1. The C terminus of SHP-1L contains a proline-rich motif PVPGPPVLSP, a potential Src homology 3 domain-binding site. In contrast to SHP-1, tyrosine phosphorylation of SHP-1L is not detected upon stimulation in Jurkat T cells. This is apparently due to the lack of a single in vivo tyrosine phosphorylation site, which only exists in the C terminus of SHP-1 (Y564). COS cell-expressed glutathione S-transferase-SHP-1L can dephosphorylate tyrosine-phosphorylated ZAP70. At pH 7.4, SHP-1L was shown to be more active than SHP-1 in the dephosphorylation of ZAP70. At pH 5.4, SHP-1L and SHP-1 exhibited similar catalytic activity. It is likely that these two isoforms play different roles in the regulation of hematopoietic cell signal transduction.

Requirement for Shc in TCR-mediated activation of a T cell hybridoma

Engagement of the TCR determines the fate of T cells to activate their functional programs, proliferate, or undergo apoptosis. The intracellular signal transduction pathways that dictate the specific outcome of receptor engagement have only been partially elucidated. The adapter protein, Shc, is involved in cytokine production, mitogenesis, transformation, and apoptosis in different cell systems. We found that Shc becomes phosphorylated on tyrosine residues upon stimulation of the TCR in DO11.10 hybridoma T cells; therefore, we investigated the role of Shc in activation-induced cell death in these cells by creating a series of stably transfected cell lines. Expression of Shc-SH2 (the SH2 domain of Shc) or Shc-Y239/240F (full-length Shc in which tyrosines 239 and 240 have been mutated to phenylalanine) resulted in the inhibition of activation-induced cell death and Fas ligand up-regulation after TCR cross-linking. Expression of wild-type Shc or Shc-Y317F had no significant effect. In addition, we found that Shc-SH2 and Shc-Y239/240F, but not Shc-Y317F, inhibited phosphorylation of extracellular signal-regulated protein kinase and production of IL-2 after TCR cross-linking. These results indicate an important role for Shc in the early signaling events that lead to activation-induced cell death and IL-2 production after TCR activation.

SLAP, a dimeric adapter protein, plays a functional role in T cell receptor signaling

Engagement of the T cell antigen receptor (TCR) leads to rapid activation of protein tyrosine kinases, which in turn phosphorylate downstream enzymes and adapter proteins. Some adapter proteins, such as SLP-76, Vav, and LAT, positively regulate TCR-mediated signal transduction, whereas others, such as Cbl, play an inhibitory role. SLAP (Src-like adapter protein), an adapter protein containing a Src homology 3 and a Src homology 2 domain, was isolated from a yeast interacting screen by using N-terminal Cbl as bait. N-terminal Cbl interacts with SLAP in vivo and in vitro in a tyrosine phosphorylation-independent manner. We observed that SLAP is expressed in T cells, and upon TCR activation, SLAP interacts with ZAP-70, Syk, LAT, and TCRzeta chain in Jurkat T cells. In transiently transfected COS-7 cells, SLAP forms separate complexes with ZAP-70, Syk, and LAT through its Src homology 2 domain. Overexpression of a C-terminal-truncated SLAP mutant down-regulates nuclear factor of activated T cells-AP1 activity. We have evidence that SLAP forms homodimers through its C-terminal region. Serial truncations and mutations in the C terminus of SLAP demonstrate that there is a correlation between the loss of dimerization and the inhibition of nuclear factor of activated T cells-AP1 activity. The in vivo association of SLAP with key signaling molecules and its inhibition of T cell activation suggests that SLAP plays an important role in TCR-mediated signal transduction.

The extracellular signal-regulated kinase pathway is required for activation-induced cell death of T cells

T cells can undergo activation-induced cell death (AICD) upon stimulation of the T cell receptor-CD3 complex. We found that the extracellular signal-regulated kinase (ERK) pathway is activated during AICD. Transient transfection of a dominant interfering mutant of mitogen-activated/extracellular signal-regulated receptor protein kinase kinase (MEK1) demonstrated that down-regulation of the ERK pathway inhibited FasL expression during AICD, whereas activation of the ERK pathway with a constitutively active MEK1 resulted in increased expression of FasL. We also found that pretreatment with the specific MEK1 inhibitor PD98059 prevented the induction of FasL expression during AICD and inhibited AICD. However, PD98059 had no effect on other apoptotic stimuli. We found only very weak ERK activity during Fas-mediated apoptosis (induced by Fas cross-linking). Furthermore, preincubation with the MEK1 inhibitor did not inhibit Fas-mediated apoptosis. Finally, we also demonstrated that pretreatment with the MEK1 inhibitor could delay and decrease the expression of the orphan nuclear steroid receptor Nur77, which has been shown to be essential for AICD. In conclusion, this study demonstrates that the ERK pathway is required for AICD of T cells and appears to regulate the induction of Nur77 and FasL expression during AICD.

Structure of the amino-terminal domain of Cbl complexed to its binding site on ZAP-70 kinase

Cbl is an adaptor protein that functions as a negative regulator of many signalling pathways that start from receptors at the cell surface. The evolutionarily conserved amino-terminal region of Cbl (Cbl-N) binds to phosphorylated tyrosine residues and has cell-transforming activity. Point mutations in Cbl that disrupt its recognition of phosphotyrosine also interfere with its negative regulatory function and, in the case of v-cbl, with its oncogenic potential. In T cells, Cbl-N binds to the tyrosine-phosphorylated inhibitory site of the protein tyrosine kinase ZAP-70. Here we describe the crystal structure of Cbl-N, both alone and in complex with a phosphopeptide that represents its binding site in ZAP-70. The structures show that Cbl-N is composed of three interacting domains: a four-helix bundle (4H), an EF-hand calcium-binding domain, and a divergent SH2 domain that was not recognizable from the amino-acid sequence of the protein. The calcium-bound EF hand wedges between the 4H and SH2 domains and roughly determines their relative orientation. In the ligand-occupied structure, the 4H domain packs against the SH2 domain and completes its phosphotyrosine-recognition pocket. Disruption of this binding to ZAP-70 as a result of structure-based mutations in the 4H, EF-hand and SH2 domains confirms that the three domains together form an integrated phosphoprotein-recognition module.

Cloning of p97/Gab2, the major SHP2-binding protein in hematopoietic cells, reveals a novel pathway for cytokine-induced gene activation

Several components in cytokine signaling remain unidentified. We report the cloning and initial characterization of one such component, p97, a widely expressed scaffolding protein distantly related to Drosophila DOS and mammalian Gab1. Upon cytokine, growth factor, or antigen receptor stimulation, p97 becomes tyrosyl phosphorylated and associates with several SH2 domain-containing proteins, including SHP2. Expression of p97 mutants unable to bind SHP2 blocks cytokine-induced c-fos promoter activation, inhibiting Elk1-mediated and STAT5-mediated transactivation. Surprisingly, such mutants do not inhibit MAPK activation. Our results identify p97 as an important regulator of receptor signaling that controls a novel pathway to immediate-early gene activation and suggest multiple functions for SHP2 in cytokine receptor signaling.

The small GTP-binding protein Rho potentiates AP-1 transcription in T cells

The Rho family of small GTP-binding proteins is involved in the regulation of cytoskeletal structure, gene transcription, specific cell fate development, and transformation. We demonstrate in this report that overexpression of an activated form of Rho enhances AP-1 activity in Jurkat T cells in the presence of phorbol myristate acetate (PMA), but activated Rho (V14Rho) has little or no effect on NFAT, Oct-1, and NF-kappaB enhancer element activities under similar conditions. Overexpression of a V14Rho construct incapable of membrane localization (CAAX deleted) abolishes PMA-induced AP-1 transcriptional activation. The effect of Rho on AP-1 is independent of the mitogen-activated protein kinase pathway, as a dominant-negative MEK and a MEK inhibitor (PD98059) did not affect Rho-induced AP-1 activity. V14Rho binds strongly to protein kinase Calpha (PKCalpha) in vivo; however, deletion of the CAAX site on V14Rho severely diminished this association. Evidence for a role for PKCalpha as an effector of Rho was obtained by the observation that coexpression of the N-terminal domain of PKCalpha blocked the effects of activated Rho plus PMA on AP-1 transcriptional activity. These data suggest that Rho potentiates AP-1 transcription during T-cell activation.

Regulation of tyrosine phosphorylation in isolated T cell membrane by inhibition of protein tyrosine phosphatases

Jurkat T cells activated by the phosphotyrosine phosphatase inhibitors H2O2 or vanadate were found to have a similar pattern of tyrosine phosphorylation when compared with T cells stimulated by anti-CD3 Ab cross-linking, suggesting that protein tyrosine phosphatase (PTP) inhibitors affect the early steps of TCR signaling. To study the role of PTPs in the most proximal membrane events of tyrosine phosphorylation, subcellular fractions of T cells were treated with the PTP inhibitors in the presence of ATP. In the membrane fraction, tyrosine phosphorylation of Lck, Fyn, and CD3 zeta can be induced by PTP inhibitors, but not by anti-CD3. Detailed characterization of this cell-free system showed that the pattern and the order of induced tyrosine phosphorylation is similar to that induced in intact cells. Upon removal of the PTP inhibitor, the tyrosine-phosphorylated proteins, including Lck, Fyn, Syk, Zap70, and CD35 zeta are rapidly dephosphorylated. Preliminary characterizations indicate that a PTP distinct from CD45, SHP1, and SHP2 is present in T cell membranes and the inhibition of this yet unidentified PTP is most likely responsible for the Lck-dependent tyrosine phosphorylation triggered by PTP inhibitors.

The cytoplasmic domain of CD8 beta regulates Lck kinase activation and CD8 T cell development

Previous studies have shown that CD8 beta plays a role in both enhancing CD8 alpha-associated Lck kinase activity and promoting the development of CD8-lineage T cells. To examine the role of this enhancement in the maturation of CD8-lineage cells, we assessed CD8 alpha-associated Lck kinase activity in both T cell hybridomas and thymocytes of mice expressing CD8 beta mutations known to impair CD8 T cell development. Lack of CD8 beta expression or expression of a cytoplasmic domain-deleted CD8 beta resulted in a severalfold reduction in CD8 alpha-associated Lck kinase activity compared with that observed with cells expressing wild-type CD8 beta chain. This analysis indicated a critical role for the cytoplasmic domain of CD8 beta in the regulation of CD8 alpha-associated Lck activity. Decreased CD8 alpha-associated Lck activity observed with the various CD8 beta mutations also correlated with diminished in vivo cellular tyrosine phosphorylation. In addition, analysis of CD8 beta mutant mice (CD8 beta-/- or cytoplasmic domain-deleted CD8 beta transgenic) indicated that the degree of reduction in CD8 alpha-associated Lck activity associated with each mutation correlated with the severity of developmental impairment. These results support the importance of CD8 beta-mediated enhancement of CD8 alpha-associated Lck kinase activity in the differentiation of CD8 single-positive thymocytes.

T cell activation through the CD43 molecule leads to Vav tyrosine phosphorylation and mitogen-activated protein kinase pathway activation

CD43, the most abundant membrane protein of T lymphocytes, is able to initiate signals that lead to Ca2+ mobilization and interleukin-2 production, yet the molecular events involved in signal transduction pathway of the CD43 molecule are only beginning to be understood. We have shown recently that cross-linking CD43 on the cell surface of human T lymphocytes with the anti-CD43 monoclonal antibody L10 leads to CD43-Fyn kinase interactions and to Fyn phosphorylation on tyrosine residues. This interaction seems to be mediated by the SH3 domain of Fyn and a proline-rich sequence located in the cytoplasmic domain of CD43. Here we show that CD43-specific activation of human T lymphocytes induced tyrosine phosphorylation of the adaptor protein Shc and of the guanine exchange factor Vav, as well as the formation of a macromolecular complex that comprises Shc, GRB2, and Vav. CD43 ligation resulted in enhanced formation of Vav.SLP-76 complexes and in the activation and nuclear translocation of ERK2. Cross-linking of the CD43 molecule in 3T3-CD43(+) cells induced luciferase activity from a construct under the control of the Fos serum responsive element. Altogether, these data suggest that the mitogen-activated protein kinase pathway is involved in CD43-dependent interleukin-2 gene expression.

Monoallelic expression of the interleukin-2 locus

The lymphokine interleukin-2 (IL-2) is responsible for autocrine cell cycle progression and regulation of immune responses. Uncontrolled secretion of IL-2 results in adverse reactions ranging from anergy, to aberrant T cell activation, to autoimmunity. With the use of fluorescent in situ hybridization and single-cell polymerase chain reaction in cells with different IL-2 alleles, IL-2 expression in mature thymocytes and T cells was found to be tightly controlled by monoallelic expression. Because IL-2 is encoded at a nonimprinted autosomal locus, this result represents an unusual regulatory mode for controlling the precise expression of a single gene.

Constitutive activation of the Janus kinase-STAT pathway in T lymphoma overexpressing the Lck protein tyrosine kinase

The Lck protein, a Src family tyrosine kinase, plays a critical role in T cell maturation and activation. Dysregulation of Lck expression or Lck kinase activity has been implicated in T cell leukemias from mice to humans, although the mechanism underlying Lck-mediated oncogenesis is still largely unclear. We report here that both DNA binding activities and tyrosine phosphorylation of STAT3 and STAT5, but not STAT1, are constitutively enhanced in the mouse T cell lymphoma LSTRA, which is a well-characterized cell line that overexpresses Lck protein and exhibits high levels of Lck kinase activity. Furthermore, Janus kinase 1 (jak1) and Jak2 protein tyrosine kinases are constantly activated in these cells, as determined by their autophosphorylation in an in vitro kinase assay and increased levels of tyrosine phosphorylation on immunoblots. Therefore, like Src-transformed cells, Lck-overexpressing LSTRA cells also exhibit constitutive activation of distinct Jak and STAT proteins.

Constitutive activation of the Janus kinase-STAT pathway in T lymphoma overexpressing the Lck protein tyrosine kinase

The Lck protein, a Src family tyrosine kinase, plays a critical role in T cell maturation and activation. Dysregulation of Lck expression or Lck kinase activity has been implicated in T cell leukemias from mice to humans, although the mechanism underlying Lck-mediated oncogenesis is still largely unclear. We report here that both DNA binding activities and tyrosine phosphorylation of STAT3 and STAT5, but not STAT1, are constitutively enhanced in the mouse T cell lymphoma LSTRA, which is a well-characterized cell line that overexpresses Lck protein and exhibits high levels of Lck kinase activity. Furthermore, Janus kinase 1 (jak1) and Jak2 protein tyrosine kinases are constantly activated in these cells, as determined by their autophosphorylation in an in vitro kinase assay and increased levels of tyrosine phosphorylation on immunoblots. Therefore, like Src-transformed cells, Lck-overexpressing LSTRA cells also exhibit constitutive activation of distinct Jak and STAT proteins.

Evidence for a requirement for both phospholipid and phosphotyrosine binding via the Shc phosphotyrosine-binding domain in vivo

The adapter protein Shc is a critical component of mitogenic signaling pathways initiated by a number of receptors. Shc can directly bind to several tyrosine-phosphorylated receptors through its phosphotyrosine-binding (PTB) domain, and a role for the PTB domain in phosphotyrosine-mediated signaling has been well documented. The structure of the Shc PTB domain demonstrated a striking homology to the structures of pleckstrin homology domains, which suggested acidic phospholipids as a second ligand for the Shc PTB domain. Here we demonstrate that Shc binding via its PTB domain to acidic phospholipids is as critical as binding to phosphotyrosine for leading to Shc phosphorylation. Through structure-based, targeted mutagenesis of the Shc PTB domain, we first identified the residues within the PTB domain critical for phospholipid binding in vitro. In vivo, the PTB domain was essential for localization of Shc to the membrane, as mutant Shc proteins that failed to interact with phospholipids in vitro also failed to localize to the membrane. We also observed that PTB domain-dependent targeting to the membrane preceded the PTB domain's interaction with the tyrosine-phosphorylated receptor and that both events were essential for tyrosine phosphorylation of Shc following receptor activation. Thus, Shc, through its interaction with two different ligands, is able to accomplish both membrane localization and binding to the activated receptor via a single PTB domain.

A comparative analysis of the phosphoinositide binding specificity of pleckstrin homology domains

Pleckstrin homology (PH) and phosphotyrosine binding (PTB) domains are structurally related regulatory modules that are present in a variety of proteins involved in signal transduction, such as kinases, phospholipases, GTP exchange proteins, and adapter proteins. Initially these domains were shown to mediate protein-protein interactions, but more recently they were also found to bind phosphoinositides. Most studies to date have focused on binding of PH domains to phosphatidylinositol (PtdIns)-4-P and PtdIns-4,5-P2 and have not considered the lipid products of phosphoinositide 3-kinase: PtdIns-3-P, PtdIns-3,4-P2, and PtdIns-3,4,5-P3. Here we have compared the phosphoinositide specificity of six different PH domains and the Shc PTB domain using all five phosphoinositides. We show that the Bruton's tyrosine kinase PH domain binds to PtdIns-3,4, 5-P3 with higher affinity than to PtdIns-4,5-P2, PtdIns-3,4-P2 or inositol 1,3,4,5-tetrakisphosphate (Ins-1,3,4,5-P4). This selectivity is decreased by the xid mutation (R28C). Selective binding of PtdIns-3,4,5-P3 over PtdIns-4,5-P2 or PtdIns-3,4-P2 was also observed for the amino-terminal PH domain of T lymphoma invasion and metastasis protein (Tiam-1), the PH domains of Son-of-sevenless (Sos) and, to a lesser extent, the PH domain of the beta-adrenergic receptor kinase. The oxysterol binding protein and beta-spectrin PH domains bound PtdIns-3,4,5-P3 and PtdIns-4,5-P2 with similar affinities. PtdIns-3,4,5-P3 and PtdIns-4,5-P2 also bound to the PTB domain of Shc with similar affinities and lipid binding was competed with phosphotyrosine (Tyr(P)-containing peptides. These results indicate that distinct PH domains select for different phosphoinositides.

Involvement of proteasomes in regulating Jak-STAT pathways upon interleukin-2 stimulation

Interleukin-2 (IL-2) activates the receptor-associated Janus family tyrosine kinases, Jak1 and Jak3, which in turn phosphorylate and activate specific STAT proteins (signal transducers and activators of transcription), such as STAT5. Activation of Jak and STAT proteins by IL-2 is transient and the mechanism for the subsequent down-regulation of their activity is largely unknown. We report here that IL-2-induced DNA-binding activity and tyrosine phosphorylation of STAT5 are stabilized by a proteasome inhibitor MG132; however, no detectable ubiquitination of the STAT proteins is observed. This sustained STAT5 activation can be blocked by protein kinase inhibitors, which is consistent with the ability of the proteasome inhibitor to stabilize IL-2-induced tyrosine phosphorylation of Jak1 and Jak3. These results suggest that proteasome-mediated protein degradation modulates protein-tyrosine phosphatase activity that negatively regulates the Jak-STAT signaling pathways.

CD43-specific activation of T cells induces association of CD43 to Fyn kinase

CD43, the most abundant membrane protein of T lymphocytes, is able to initiate signal transduction pathways that lead to Ca2+ mobilization and interleukin-2 production, yet the molecular events involved in CD43's signal transduction pathway are poorly understood. In the present report we show that activation of both purified T lymphocytes and Jurkat cells, through CD43 cross-linking with the anti-CD43 L10 monoclonal antibody, induced CD43 association to Fyn kinase. This association is mediated by the Src homology 3 (SH3) domain of Fyn, since a glutathione S-transferase-Fyn SH3 fusion protein was able to precipitate CD43 from lysates of CD43-activated T cells. A synthetic peptide containing the SH3 binding sites of p85, located within the amino acid sequence 300ERQPAPALPPKPPKP314, was able to inhibit binding of CD43 to Fyn as well as to the glutathione S-transferase-Fyn SH3 fusion protein. We also provide evidence that upon CD43 cross-linking, Fyn is tyrosine-phosphorylated in a time-dependent manner. Our results suggest that CD43 cross-linking on the T cell surface induces the interaction between CD43 and Fyn, presumably through the Fyn SH3 domain and a putative SH3 binding site in CD43, leading to Fyn tyrosine phosphorylation and signal propagation.

Role of the Lck Src homology 2 and 3 domains in protein tyrosine phosphorylation

Many protein tyrosine phosphorylation events that occur as a result of T cell receptor (TCR) stimulation are enhanced when CD4 is co-cross-linked with the TCR, and this increased phosphorylation is thought to be a mechanism by which T cell functions are augmented by CD4. Such enhanced tyrosine phosphorylation was originally attributed to the kinase activity of the CD4-associated tyrosine kinase Lck. However, it has been shown that CD4-associated Lck lacking the catalytic domain can enhance T cell functions, suggesting that the noncatalytic domains of Lck are also important in CD4 signaling. Using T cells expressing various CD4-Lck chimeric molecules, we assessed the role of different Lck domains in early T cell signaling. Following TCR-CD4 co-cross-linking, cells expressing a CD4-Lck full-length chimera showed enhanced tyrosine phosphorylation of many cellular proteins in a CD4-dependent manner. Surprisingly, cells expressing a CD4-Lck chimera lacking the catalytic domain (termed CD4-N32) also showed enhanced phosphorylation. This enhancement of phosphorylation required both the Src homology 2 (SH2) and SH3 domains of Lck. Lck has been postulated to dimerize through the SH2 and SH3 domains. In this way CD4-N32 may interact with endogenous Lck, and although it lacks intrinsic kinase activity, it may be capable of enhancing phosphorylation through the associated full-length Lck. Consistent with this model, when CD4-Lck chimeric molecules were expressed in J. CaM1.6 cells lacking endogenous Lck, CD4-N32 failed to enhance tyrosine phosphorylation. Moreover, a Lck SH2 and SH3 domain fragment expressed as a glutathione S-transferase fusion protein associated with Lck when incubated with activated Jurkat T cell lysates, suggesting that the SH2 and SH3 domains of Lck can associate with endogenous full-length Lck upon activation. Thus, our data suggest that dimerization is an important mechanism of Lck function in T cell activation.

Lack of SHPTP1 results in src-family kinase hyperactivation and thymocyte hyperresponsiveness

Protein tyrosine phosphorylation and dephosphorylation are key regulatory events in T-cell receptor (TCR) signaling. We investigated the role of the tyrosine phosphatase SHPTP1 in TCR signaling by analysis of TCR signal transduction in motheaten (me/me) mice, which lack SHPTP1 expression. As revealed by flow cytometric analysis, thymocyte development was normal in me/me mice. However, me/me thymocytes hyperproliferated (3-to 5-fold) in response to TCR stimulation, whereas their response to interleukin 2 stimulation was unchanged compared with normal thymocytes. TCR-induced hyperproliferation of me/me thymocytes was reproduced in purified single-positive thymocytes. Moreover, me/me thymocytes produced increased amounts of interleukin 2 production upon TCR stimulation. Biochemical analysis revealed that, in response to TCR or TCR/CD4 stimulation, thymocytes lacking SHPTP1 showed increased tyrosyl phosphorylation of several cellular substrates, which correlated with increased activation of the src-family kinases Lck and Fyn. Taken together, our data suggest that SHPTP1 is an important negative regulator of TCR signaling, acting at least in part to inactivate Lck and Fyn.

Tyrosine-phosphorylated Cbl binds to Crk after T cell activation

Crk is a Src homology 2 (SH2)/Src homology 3 (SH3)-containing adapter protein that has been implicated in intracellular signaling in fibroblasts and PC12 pheochromocytoma cells. Crk has been shown to bind to a tyrosine-phosphorylated protein of 116 kDa after TCR-mediated T cell activation. Here we demonstrate that the Crk-associated p116 phosphoprotein is not the Crk-associated substrate (Cas) but, rather, is a protein product of the c-cbl proto-oncogene. Whereas Cas was not tyrosine-phosphorylated after T cell activation, Cbl became highly phosphorylated. Crk immunoprecipitates from activated T cell lysates contain tyrosine-phosphorylated Cbl. This association is mediated by the SH2 domain of Crk, as evidenced by the interaction between Cbl and the fusion protein product of a glutathione S-transferase (GST) expression construct encoding the Crk-SH2 domain in vitro. Furthermore, phosphopeptide-binding studies revealed that the GST-Crk SH2 domain binds to a tyrosine-phosphorylated peptide corresponding to amino acids 770-781 of Cbl with high affinity. Cbl is a protein tyrosine kinase (PTK) substrate that becomes phosphorylated after engagement of numerous cell surface receptors including the TCR. Data revealed by genetic studies in the nematode, Caenorhabditis elegans, implicates a Cbl-like molecule, Sli-1, as a negative regulator of the Let-23-signaling pathway. Because the signal from the Let-23 pathway affects the activation status of the Let-60 (Ras homologue in C. elegans) pathway, the activation-dependent association between Crk and Cbl may represent another TCR-generated signal leading to Ras-related pathways.

Tyrosine-phosphorylated Cbl binds to Crk after T cell activation

Crk is a Src homology 2 (SH2)/Src homology 3 (SH3)-containing adapter protein that has been implicated in intracellular signaling in fibroblasts and PC12 pheochromocytoma cells. Crk has been shown to bind to a tyrosine-phosphorylated protein of 116 kDa after TCR-mediated T cell activation. Here we demonstrate that the Crk-associated p116 phosphoprotein is not the Crk-associated substrate (Cas) but, rather, is a protein product of the c-cbl proto-oncogene. Whereas Cas was not tyrosine-phosphorylated after T cell activation, Cbl became highly phosphorylated. Crk immunoprecipitates from activated T cell lysates contain tyrosine-phosphorylated Cbl. This association is mediated by the SH2 domain of Crk, as evidenced by the interaction between Cbl and the fusion protein product of a glutathione S-transferase (GST) expression construct encoding the Crk-SH2 domain in vitro. Furthermore, phosphopeptide-binding studies revealed that the GST-Crk SH2 domain binds to a tyrosine-phosphorylated peptide corresponding to amino acids 770-781 of Cbl with high affinity. Cbl is a protein tyrosine kinase (PTK) substrate that becomes phosphorylated after engagement of numerous cell surface receptors including the TCR. Data revealed by genetic studies in the nematode, Caenorhabditis elegans, implicates a Cbl-like molecule, Sli-1, as a negative regulator of the Let-23-signaling pathway. Because the signal from the Let-23 pathway affects the activation status of the Let-60 (Ras homologue in C. elegans) pathway, the activation-dependent association between Crk and Cbl may represent another TCR-generated signal leading to Ras-related pathways.

Intrathymic signals in thymocytes are mediated by p38 mitogen-activated protein kinase

Thymocytes develop into mature functional T cells in the inductive environment of the thymus where thymocyte-stromal cell interactions and cytokines provide survival and differentiation signals as cues for thymocyte maturation. Disruption of the thymic microenvironment results in attenuation of T cell maturation, suggesting that intrathymic signals are essential for differentiation and repertoire selection. We have previously shown that several inducible nuclear factors such as AP-1, NF-AT, and NF-kappaB are activated in response to intrathymic signals. Here we demonstrate that in thymocytes p38 mitogen-activated protein (MAP) kinase, a member of the MAP kinase family of proteins that include the extracellular-signal regulated kinases and Jun aminoterminal kinases, is highly activated in response to intrathymic signals in vivo. These studies suggest a role for p38 MAP kinase in T cell survival and differentiation.

Intrathymic signals in thymocytes are mediated by p38 mitogen-activated protein kinase

Thymocytes develop into mature functional T cells in the inductive environment of the thymus where thymocyte-stromal cell interactions and cytokines provide survival and differentiation signals as cues for thymocyte maturation. Disruption of the thymic microenvironment results in attenuation of T cell maturation, suggesting that intrathymic signals are essential for differentiation and repertoire selection. We have previously shown that several inducible nuclear factors such as AP-1, NF-AT, and NF-kappaB are activated in response to intrathymic signals. Here we demonstrate that in thymocytes p38 mitogen-activated protein (MAP) kinase, a member of the MAP kinase family of proteins that include the extracellular-signal regulated kinases and Jun aminoterminal kinases, is highly activated in response to intrathymic signals in vivo. These studies suggest a role for p38 MAP kinase in T cell survival and differentiation.

Natural killer cell development is blocked in the context of aberrant T lymphocyte ontogeny

Over-expression of human or mouse CD3-epsilon transgenes profoundly disturbs T lymphocyte and natural killer (NK) cell development. One of these transgenic strains, termed tgepsilon26, displays a very early block in T lymphocyte and NK cell development. We showed previously that the absence of early thymocyte progenitors results in an abnormal thymic microenvironment. Due to this thymic defect, T cell development could not be restored by bone marrow (BM) transplantation in adult tgepsilon26 mice but could in fetal tgepsilon26 mice. Here we examine the effect of this abnormal thymic environment on NK cell development. We demonstrate that NK cell maturation in tgepsilon26 mice was reconstituted by BM derived from completely T cell-deficient mice, i.e. RAG-2(-/-) and TCRbeta x delta-/-, but not from wild-type mice. Moreover, tgepsilon26 mice transplanted with BM from partially T cell-deficient mice, i.e. TCRalpha-/-, TCRbeta-/- and TCRdelta-/- mice, did not reconstitute their NK cell compartment. We conclude from these studies that the thymic environment is not required for NK cell development, but that aberrantly educated alphabeta or gammadelta T lymphocytes can influence NK cell ontogeny. Furthermore, high serum levels of tumor necrosis factor (TNF) were detected in the vast majority of tgepsilon26 mice transplanted with BM cells derived from partially T cell-deficient mice, but never from tgepsilon26 mice transplanted with BM cells derived from completely T cell-deficient mice. The high levels of TNF may play an important role in the observed inhibition of NK cell development, since in vivo treatment with an anti-TNF antibody restored NK cell development.

Evidence for a role for the phosphotyrosine-binding domain of Shc in interleukin 2 signaling

Stimulation via the T-cell growth factor interleukin 2 (IL-2) leads to tyrosine phosphorylation of Shc, the interaction of Shc with Grb2, and the Ras GTP/GDP exchange factor, mSOS. Shc also coprecipitates with the IL-2 receptor (IL-2R), and therefore, may link IL-2R to Ras activation. We have further characterized the Shc-IL-2R interaction and have made the following observations. (i) Among the two phosphotyrosine-interaction domains present in Shc, the phosphotyrosine-binding (PTB) domain, rather than its SH2 domain, interacts with the tyrosine-phosphorylated IL-2R beta chain. Moreover, the Shc-PTB domain binds a phosphopeptide derived from the IL-2R beta chain (corresponding to residues surrounding Y338, SCFTNQGpYFF) with high affinity. (ii) In vivo, mutant IL-2R beta chains lacking the acidic region of IL-2Rbeta (which contains Y338) fail to phosphorylate Shc. Furthermore, when wild type or mutant Shc proteins that lack the PTB domain were expressed in the IL-2-dependent CTLL-20 cell line, an intact Shc-PTB domain was required for Shc phosphorylation by the IL-2R, which provides further support for a Shc-PTB-IL-2R interaction in vivo. (iii) PTB and SH2 domains of Shc associate with different proteins in IL-2- and T-cell-receptor-stimulated lysates, suggesting that Shc, through the concurrent use of its two different phosphotyrosine-binding domains, could assemble multiple protein complexes. Taken together, our in vivo and in vitro observations suggest that the PTB domain of Shc interacts with Y338 of the IL-2R and provide evidence for a functional role for the Shc-PTB domain in IL-2 signaling.

Evidence for a physical association between the Shc-PTB domain and the beta c chain of the granulocyte-macrophage colony-stimulating factor receptor

Granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates the growth and function of several myeloid cell types at different stages of maturation. The effects of GM-CSF are mediated through a high affinity receptor that is composed of two chains: a unique, ligand-specific alpha chain and a beta common chain (beta c) that is also a component of the receptors for interleukin 3 (IL-3) and IL-5. Beta c plays an essential role in the transduction of extra cellular signals to the nucleus through its recruitment of secondary messengers. Several downstream signaling events induced by GM-CSF stimulation have been described, including activation of tyrosine kinases and tyrosine phosphorylation of cellular proteins (including beta c) and activation of the Ras/mitogen-activated protein kinase and the JAK/STAT pathways. A region within the beta c cytoplasmic tail (amino acids 517-763) has been reported to be necessary for tyrosine phosphorylation of the adapter protein, Shc, and for the subsequent GM-CSF-induced activation of Ras. In this paper, we describe a physical association between the tyrosine phosphorylated GM-CSF receptor (GMR)-beta c chain and Shc in vivo. Using a series of cytoplasmic truncation mutants of beta c and various mutant Shc proteins, we demonstrate that the N-terminal phosphotyrosine-binding (PTB) domain of Shc binds to a short region of beta c (amino acids 549-656) that contains Tyr577. Addition of a specific phosphopeptide encoding amino acids surrounding this tyrosine inhibited the interaction between beta c and shc. Moreover, mutation of a key residue within the phosphotyrosine binding pocket of the Shc-PTB domain abrogated its association with beta c. These observations provide an explanation for the previously described requirement for Tyr577 of beta c for GM-CSF-induced tyrosine phosphorylation of Shc and have implications for Ras activation through the GM-CSF, IL-3, and IL-5 receptors.

Induction of alloantigen-specific tolerance by B cells from CD40-deficient mice

Interaction between CD40 on B cells and CD40 ligand molecules on T cells is pivotal for the generation of a thymus-dependent antibody response. Here we show that B cells deficient in CD40 expression are unable to elicit the proliferation of allogeneic T cells in vitro. More importantly, mice immunized with CD40-/- B cells become tolerant to allogeneic major histocompatibility complex (MHC) antigens as measured by a mixed lymphocyte reaction and cytotoxic T-cell assay. The failure of CD40-/- B cells to serve as antigen presenting cells in vitro was corrected by the addition of anti-CD28 mAb. Moreover, lipopolysaccharide stimulation, which upregulates B7 expression, reversed the inability of CD40-/- B cells to stimulate an alloresponse in vitro and abrogated the capacity of these B cells to induce tolerance in vivo. These results suggest that CD40 engagement by CD40 ligand expressed on antigen-activated T cells is critical for the upregulation of B7 molecules on antigen-presenting B cells that subsequently deliver the costimulatory signals necessary for T-cell proliferation and differentiation. Our experiments suggest a novel strategy for the induction of antigen-specific tolerance in vivo.

Irradiated B7-1 transduced primary acute myelogenous leukemia (AML) cells can be used as therapeutic vaccines in murine AML

Recent studies have shown that tumor cells genetically modified by transduction of B7-1, a natural ligand for the T-cell costimulatory molecules CD28 and CTLA-4, are rejected in syngeneic hosts. In these reports, transformed cell lines and drug-selected cells have been used for vaccinations. To determine the effectiveness of B7-1-transduced primary acute myelogenous leukemia (AML) cells on the induction of antitumor immunity, we have studied a murine AML model in which primary AML cells were retrovirally transduced with the murine B7-1 cDNA. A defective retroviral producer clone expressing B7-1 and secreting a high titer of virus was used for infection of AML cells. Unselected transduced AML cells, expressing a high level of B7-1, were used for in vivo vaccinations. Our results show that one intravenous (IV) injection of irradiated B7-1-positive (B7-1+) AML cells can provide long-lasting (5 to 6 months) systemic immunity against subsequent challenge with wild-type AML cells. Furthermore, one exposure to irradiated B7-1+ AML cells results in rejection of leukemia by leukemic mice when the vaccination occurs in the early stages of the disease. The antileukemia immunity is CD8+ T-cell-dependent and B7/CD28-mediated, since in vivo treatment of mice with anti-CD8 monoclonal antibody or CTLA-4 Ig leads to abrogation of the specific antileukemia immune response. These results emphasize that B7-1 vaccines may have therapeutic usefulness for patients with AML.

Measurement of Calcineurin Phosphatase Activity in Cell Extracts

The calmodulin-dependent phosphatase calcineurin is an important molecular target of the immunosuppressive drugs cyclosporin A (CsA) and FK506. Complexes of CsA and FK506 with their immunophilin ligands interact with calcineurin in vitro, resulting in the inhibition of its serine/threonine phosphatase activity toward polypeptide substrates. In order to demonstrate that CsA and FK506 inhibit calcineurin in vivo, we developed an assay to measure calcineurin phosphatase activity in crude cell extracts. We have previously reported that incubation of intact cells with nanomolar concentrations of either CsA or FK506 results in potent inhibition of calcineurin activity in a variety of cell lines. Here we discuss in detail the methodology and applications of the cell extract calcineurin assay.

Identification of a physical interaction between calcineurin and nuclear factor of activated T cells (NFATp)

In T lymphocytes, the calcium/calmodulin-dependent serine/threonine phosphatase, calcineurin, plays a pivotal role in transducing membrane-associated signals to the nucleus. One of the putative targets of calcineurin is the pre-existing, cytosolic component of the nuclear factor of activated T cells (NFATp; also referred to as NFAT1), which is one of several transcription factors required for the expression of interleukin 2. Inhibition of calcineurin by the immunosuppressive drugs cyclosporin A and FK506 prevents dephosphorylation of NFATp and its translocation to the nucleus. However, a physical interaction between calcineurin and NFATp has not been demonstrated. Here we demonstrate the binding of NFATp from lysates of T cells to immobilized calcineurin. Stimulation of T cells with calcium ionophore induced a shift in the molecular weight of NFATp that is due to its dephosphorylation. This dephosphorylation was inhibited by treatment of T cells with cyclosporin A or FK506 prior to stimulation. Of note, both the phosphorylated and the dephosphorylated form of NFATp bound to calcineurin. Furthermore, the binding of both forms of NFATp to calcineurin was inhibited by pretreatment of calcineurin with a complex of FK506 and its ligand FKBP12. Taken together these data strongly suggest a direct interaction of calcineurin with NFATp and that this interaction does not depend upon the phosphorylation sites of NFATp affected by activation.

CD4 and signal transduction

The CD4 molecule plays an important role in the development of CD4+T lymphocytes and it also acts as a coreceptor to enhance responses mediated via the TCR. It is now established that CD4 functions both as an adhesion molecule favoring the T cell: APC interaction and as a signaling molecule. The coreceptor function mediated via CD4 depends on its association with Lck, a src-family tyrosine kinase. Lck, while interacting via its unique NH2-terminal domain with CD4, also interacts via its SH2 and SH3 domains with other intracellular signaling proteins. Although the Lck association with CD4 is essential for CD4 coreceptor activity, the tyrosine kinase activity of CD4-associated Lck appears to be dispensable for CD4 function. Given the necessity of Lck kinase activity for T lymphocyte development and for mature T cell functions, perhaps Lck may function at different stages during T cell activation and at some stages the kinase activity of Lck may not be necessary. This raises an intriguing possibility that CD4-associated Lck may function more as an adapter protein than a kinase and may help to recruit other signaling proteins into the TCR/CD3 complex. However, determination of the precise role of Lck in CD4 coreceptor activity and the domains of Lck that are necessary for CD4-dependent and CD4-independent functions awaits further experiments.

Binding affinities of tyrosine-phosphorylated peptides to the COOH-terminal SH2 and NH2-terminal phosphotyrosine binding domains of Shc

The adaptor protein Shc has been implicated in Ras signaling via association with many tyrosine-phosphorylated receptors, including growth factor receptors, antigen receptors on T and B cells, and cytokine receptors. Shc could interact with the activated receptors through the carboxyl-terminal Src homology 2 (SH2) domain or the structurally unrelated amino-terminal phosphotyrosine binding (PTB) domain. Using NMR and surface plasmon resonance techniques, we have measured the binding affinities of the SH2 and the PTB domains of Shc to a series of phosphotyrosine-containing peptides derived from known Shc binding sites. Tyrosine-phosphorylated peptides derived from Trk (pY490), polyoma virus middle T-antigen (pY250), ErbB3 (pY1309), and epidermal growth factor receptor (pY1086, pY1148, and pY1114) that contain NPXpY sequences bind preferentially to the PTB domain of Shc with Kd values of 0.02-5.3 microM. The binding affinities of these peptides to the Shc SH2 domain were in the range of 220-1290 microM. In contrast, tyrosine-phosphorylated peptides from epidermal growth factor receptor (pY992, pY1173) and the zeta chain of the T-cell receptor bind preferentially to the SH2 domain (Kd = 50-130 microM) versus the PTB domain (Kd > 680 microM). From these studies, the relative contribution of the individual domains of Shc for binding to the phosphotyrosine-containing portions of these proteins was determined. In addition, our data indicate that the high affinity binding of the PTB domain to the NPXpY-containing peptides results from a very high association rate and a rapid dissociation rate, which is similar to previous results observed for the SH2-phosphopeptide complexes.