Requirement for Tec kinases Rlk and Itk in T cell receptor signaling and immunity.
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T cell receptor (TCR) signaling requires activation of Zap-70 and Src family tyrosine kinases, but requirements for other tyrosine kinases are less clear. Combined deletion in mice of two Tec kinases, Rlk and Itk, caused marked defects in TCR responses including proliferation, cytokine production, and apoptosis in vitro and adaptive immune responses to Toxoplasma gondii in vivo. Molecular events immediately downstream from the TCR were intact in rlk-/-itk-/- cells, but intermediate events including inositol trisphosphate production, calcium mobilization, and mitogen-activated protein kinase activation were impaired, establishing Tec kinases as critical regulators of TCR signaling required for phospholipase C-gamma activation. (+info)
Explaining high alloreactivity as a quantitative consequence of affinity-driven thymocyte selection.
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Interactions between alphabeta T cell receptors and peptides bound to molecules encoded by the MHC genes underly T cell activation. More than 1% of T cells are activated by foreign (allogenic) MHC molecules, a phenomenon called alloreactivity. Reconciling the high frequency of alloreactivity with the fact that only 1 T cell in 10(4)-10(6) responds to a given foreign antigen presented on self MHC has been a long-standing puzzle. We show, by using a quantitative model, that this difference follows from the affinity model of T cell selection. Further, we demonstrate that highly alloreactive pre- and post-selection repertoires can be obtained without assuming germline bias of T cell receptors toward recognition of allele-specific MHC residues. It has been proposed that alloreactivity occurs because self and foreign MHCs bind different subsets of self peptides or alter their conformation differently. We find that such effects decrease rather than increase alloreactivity. Overall, our results show that the affinity model of T cell selection can quantitatively explain both self MHC restriction and high alloreactivity. (+info)
Changes in the strength of co-stimulation through the B7/CD28 pathway alter functional T cell responses to altered peptide ligands.
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T cells require a TCR and a co-stimulatory signal for activation. We have examined the effect of the strength of TCR and co-stimulatory signals on proliferation and production of cytokines by differentiated T cell clones. The TCR signal was varied using antigen dose and altered peptide ligands. The co-stimulatory signal was varied by using as antigen-presenting cells, Chinese hamster ovary cell transfectants that express different levels of the B7-1 molecule with similar levels of MHC class II. Our results show that the level of co-stimulation has a profound effect on the response to an antigen, and that a strong co-stimulatory signal can convert a weak agonist into a full agonist and an agonist into a superagonist. Antigenicity is not absolute but a function of the strengths of the TCR and co-stimulatory signals. Increasing the strength of co-stimulation can lower antigen concentration required for maximal proliferative responses by T cell clones by 5 log. These results show that the level of expression of co-stimulatory molecules will profoundly regulate T cell clonal expansion and effector functions. (+info)
Rapid tyrosine phosphorylation of Lck following ligation of the tumor-associated cell surface molecule A6H.
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We have recently described the A6H antigen as a novel 120-140 kDa molecule which is co-expressed on human peripheral blood T cells and renal cell carcinoma cells. Engagement of the A6H antigen results in co-stimulation of CD4+ T cells but it remained unknown how cross-talk between the A6H antigen and the TCR-CD3 complex takes place and which signaling pathway might be involved. Here we show that ligation of the A6H antigen with mAb induces tyrosine phosphorylation of the Lck protein tyrosine kinase (PTK). Co-ligation of the A6H antigen with CD3 resulted in augmented Lck phosphorylation and mitogenesis. In addition, A6H ligation induced an up-regulation of CD3-mediated phosphorylation of the 23 kDa high mol. wt form of TCR zeta and the zeta-associated protein, ZAP-70. Co-precipitation of Lck and ZAP-70 was only seen in T cells activated by combined A6H and anti-CD3 stimulation. In contrast, another Src family PTK, Fyn, was not affected by A6H ligation. In conclusion, we now demonstrate, for the first time, that A6H ligation triggers Lck phosphorylation, and that cross-talk between A6H and the TCR-CD3 complex involves Lck, ZAP-70 and the slow migrating isoform of TCR zeta. These results further suggests that A6H ligation is sufficient for triggering some of the early events in T cell activation, whereas full activation of the T cell, characterized by proliferation and cytokine production, requires co-ligation of the TCR-CD3 complex. (+info)
Possible mechanisms of immunotherapy for maintaining pregnancy in recurrent spontaneous aborters: analysis of anti-idiotypic antibodies directed against autologous T-cell receptors.
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We examined whether immunotherapy for recurrent spontaneous abortion (RSA) using paternal lymphocytes induces anti-T-cell receptor (TCR) idiotypic antibodies in RSA patients. The sera of these patients were assessed for inhibitory activity against mixed lymphocyte reactions (MLR) between maternal responder cells and paternal stimulator cells. Sera of four of the five women who maintained pregnancy successfully after immunotherapy showed significant MLR inhibition, whereas none of the five women who had unsuccessful pregnancies showed significant MLR inhibition. These sera inhibited the MLR of autologous responder T-cells, when stimulated with lymphocytes having the same HLA-DR antigens as the patient's husband, but not when stimulated with lymphocytes having unrelated HLA-DR antigens. This MLR inhibitory activity was absorbed by autologous maternal T-lymphoblasts induced by stimulation with lymphocytes having the paternal HLA-DR type but not by those induced by stimulation with lymphocytes having other HLA-DR types. The maternal serum inhibited the proliferation of autologous T-cells, but not of non-autologous T-cells, stimulated with paternal lymphocytes. These results indicate that anti-TCR idiotypic antibodies were induced in RSA patients by immunotherapy. These antibodies may contribute to maintaining pregnancy by negatively regulating maternal T-cells directed against HLA-DR antigens of the fetus. (+info)
Grf40, A novel Grb2 family member, is involved in T cell signaling through interaction with SLP-76 and LAT.
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We molecularly cloned a new Grb2 family member, named Grf40, containing the common SH3-SH2-SH3 motif. Expression of Grf40 is predominant in hematopoietic cells, particularly T cells. Grf40 binds to the SH2 domain-containing leukocyte protein of 76 kD (SLP-76) via its SH3 domain more tightly than Grb2. Incidentally, Grf40 binds to linker for activation of T cells (LAT) possibly via its SH2 domain. Overexpression of wild-type Grf40 in Jurkat cells induced a significant increase of SLP-76-dependent interleukin (IL)-2 promoter and nuclear factor of activated T cell (NF-AT) activation upon T cell receptor (TCR) stimulation, whereas the COOH-terminal SH3-deleted Grf40 mutant lacked any recognizable increase in IL-2 promoter activity. Furthermore, the SH2-deleted Grf40 mutant led to a marked inhibition of these regulatory activities, the effect of which is apparently stronger than that of the SH2-deleted Grb2 mutant. Our data suggest that Grf40 is an adaptor molecule involved in TCR-mediated signaling through a more efficient interaction than Grb2 with SLP-76 and LAT. (+info)
Induction of Fas ligand expression by HIV involves the interaction of Nef with the T cell receptor zeta chain.
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During HIV/SIV infection, there is widespread programmed cell death in infected and, perhaps more importantly, uninfected cells. Much of this apoptosis is mediated by Fas-Fas ligand (FasL) interactions. Previously we demonstrated in macaques that induction of FasL expression and apoptotic cell death of both CD4(+) and CD8(+) T cells by SIV is dependent on a functional nef gene. However, the molecular mechanism whereby HIV-1 induces the expression of FasL remained poorly understood. Here we report a direct association of HIV-1 Nef with the zeta chain of the T cell receptor (TCR) complex and the requirement of both proteins for HIV-mediated upregulation of FasL. Expression of FasL through Nef depended upon the integrity of the immunoreceptor tyrosine-based activation motifs (ITAMs) of the TCR zeta chain. Conformation for the importance of zeta for Nef-mediated signaling in T cells came from an independent finding. A single ITAM motif of zeta but not CD3epsilon was both required and sufficient to promote activation and binding of the Nef-associated kinase (NAK/p62). Our data imply that Nef can form a signaling complex with the TCR, which bypasses the requirement of antigen to initiate T cell activation and subsequently upregulation of FasL expression. Thus, our study may provide critical insights into the molecular mechanism whereby the HIV-1 accessory protein Nef contributes to the pathogenesis of HIV. (+info)
Anchorage dependence of mitogen-induced G1 to S transition in primary T lymphocytes.
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Anchorage dependence defines the cellular requirement for integrin-mediated adhesion to substrate to initiate DNA replication in response to growth factors. In this study we investigated whether normal T cells, which spend extended periods in a nonadherent state, show similar requirements for cell cycle progression in response to TCR stimulation. Resting primary T lymphocytes were induced to enter the cell cycle by TCR triggering, and leukocyte integrins were either engaged using purified ICAM-1 or inhibited with function-blocking mAbs. Our data indicate that leukocyte integrins complement TCR-driven mitogenic signals not as a result of their direct clustering but, rather, via integrin-dependent organization of the actin cytoskeleton. Leukocyte integrin-dependent reorganization of the actin cytoskeleton cooperates with the TCR to effect mitogen-activated protein kinase activation, but also represents a required late (4-8 h poststimulation) component in the mitogenic response of normal T cells. Prolonged leukocyte integrin-dependent spreading, in the context of intercellular contact, is a requisite for the production of the mitogenic cytokine IL-2, which, in turn, is involved in the induction of D3 cyclin and is primarily responsible for the decrease in the cyclin-dependent kinase inhibitor p27kip, resulting in retinoblastoma protein inactivation and S phase entry. Thus, T lymphocytes represent a peculiar case of anchorage dependence, in which signals conveyed by integrins act sequentially with the activating stimulus to effect a sustained production of the essential mitogenic cytokine. (+info)