Phosphorylation of T-cell antigen receptor-associated proteins: correlation with activation for killing and/or for gamma-interferon production by a cytolytic T-cell clone

The modulation of CD40 ligand signaling by transmembrane CD28 splice variant in human T cells

The role of CD40 ligand (CD40L)/CD40 signaling in T cell–dependent B cell differentiation and maturation has been amply documented. The mechanism of CD40 signaling in B cells has been well established, whereas the signaling mechanism of CD40L in T cell costimulation remains unknown. In this study we show that CD28i, a transmembrane splice variant of CD28 costimulatory receptor, complexes with CD40L in human T cells. The cross-linking of CD40L resulted in the coendocytosis of CD28i with CD40L. The tyrosine phosphorylation of CD28i followed the cross-linking of CD40L, and the overexpression of CD28i augmented the c-Jun NH₂-terminal kinase, p21-activated kinase 2, and nuclear factor κB activation. These data indicate that CD28i, by functioning as a signaling adaptor, transduces CD40L signaling as well as CD28 signaling in human T cells.

T cell receptor-induced Fas ligand expression in cytotoxic T lymphocyte clones is blocked by protein tyrosine kinase inhibitors and cyclosporin A

Fas/APO-1 is a member of the tumor necrosis factor receptor family of proteins that induces apoptosis when cross-linked with monoclonal antibody (mAb) or with its physiological ligand. Recently, both a perforin-based and a Fas-based mechanism have been proposed to account for T cell-mediated cytotoxicity. In the present study we used a murine CD8+ cytotoxic T lymphocyte (CTL) clone (KB5 C20) specific for H-2Kb and a T cell receptor (TcR)-negative variant of the same clone (2005-D4) to test (i) whether the same cell can exert both cytotoxic effector mechanisms and (ii) the role of TcR engagement in the induction of Fas-based cytotoxicity. We demonstrate that both the TcR+ and TcR- clones were able to express the Fas ligand after stimulation with phorbol 12-myristate 13-acetate (PMA)/ionomycin, and that TcR engagement of the KB5.C20 clone by means of antigen-bearing cells or of its anticlonotypic mAb (Désiré-1), which leads to Ca(2+)-dependent, presumably perforin-based, cytotoxicity, was also able to induce Fas-based cytotoxicity. In addition, using inhibitors we investigated the signal transduction pathway(s) involved in the induction of Fas-based cytotoxicity and expression of the Fas ligand mRNA in the CTL clones. The involvement of src-like protein tyrosine kinases (PTK) in Fas ligand induction through TcR engagement, was strongly suggested by inhibition with the src-like PTK inhibitor herbimycin A. Inhibition of Fas ligand induction by genistein, a more general TPK inhibitor, even upon stimulation by PMA plus ionomycin, suggested the possible involvement of PTK activities downstream of protein kinase C (PKC) in Fas ligand induction in CTL. Finally, the implication of the Ca2+/calmodulin-dependent protein phosphatase calcineurin in Fas ligand induction was demonstrated by the partial inhibition of Fas ligand induction with cyclosporin A. Thus, in CTL clones, Fas ligand expression is inducible by TcR engagement through a pathway similar to that involved in expression of some lymphokine genes.

Comparative analysis of phosphotyrosyl polypeptides in normal and leukemic human T lymphocytes activated via CD3 or CD2

Phosphotyrosyl polypeptides induced following CD3- or CD2- specific antibody stimulation were analysed in different human T cell lines by immunoblotting or by immunoprecipitation of 32P-labelled cell lysates using a phosphotyrosine-specific monoclonal antibody. In Jurkat cells, resting peripheral T lymphocytes, T lymphoblasts, CD8+ T lymphoblasts and a CD4+ T cell clone, CD3 stimulation induced a strong but transient tyrosine phosphorylation of at least 15 polypeptides. However, in peripheral T cells and T blasts, the kinetics of phosphorylation were considerably slower than in Jurkat cells. The pattern of phosphotyrosyl polypeptides induced by CD3 stimulation was similar, although some differences were noted between normal T cells and Jurkat, especially at the level of the extent of phosphorylation. As had been previously reported for Jurkat T cells, a qualitatively similar tyrosine phosphorylation response was induced upon CD2 or CD3 stimulation in each of the analysed T cell populations, suggesting that CD3 and CD2 share a common pathway of protein tyrosine kinase (PTK) activation. In HPB. ALL leukemia T cells (which express very low levels of CD45), both CD3 and CD2 stimulation induced only very weak protein tyrosyl phosphorylation. However, a 50 kDa polypeptide, which was part of an inducible doublet in Jurkat or normal T lymphocytes, was constitutively tyrosyl-phosphorylated in the HPB. ALL line. These results suggest that there is a common pathway of early PTK activation following CD3- or CD2-mediated stimulation in mature T cells, whether they express surface CD4 or CD8, and also that the PTK may be differently regulated in different T cell populations leading to different kinetics or intensity of tyrosyl phosphorylation.

Biochemical and functional association between CD8 and H-2 at the surface of a T cell clone

In an attempt to define structures interacting with CD8 molecules during activation of CD8+ cells, immunoprecipitates of CD8 and Tcr-CD3 molecules from lysates of a surface-labeled CTL clone were analyzed. No proteins other than the known Tcr alpha/beta and associated CD3 components were detected in either anti-Tcr or anti-CD3 immunoprecipitates, whether or not the CTL clone had been activated. However, anti-CD8 antibodies co-precipitated class I MHC heavy chain and associated beta 2-microglobulin in all conditions. The latter co-precipitation was shown to result from "cis-type" interactions between CD8 and class I MHC proteins on the same cell and to involve a degree of selectivity, as class I MHC molecules were absent from immunoprecipitates of highly expressed cell surface molecules such as LFA-1. A further analysis of cell surface molecular distribution during antigen-dependent CTL-target cell interaction by double fluorescence-microscopy in non-activating conditions indicated that an increased density of CTL class I molecules was found in the CTL-target cell contact zone of most conjugates with redistributed CD8 molecules. A possible role for "cis-type" class I MHC-CD8 interactions in the dynamics of CTL-target cell contacts is proposed.

T cell receptor/CD3 complex internalization following activation of a cytolytic T cell clone: evidence for a protein kinase C-independent staurosporine-sensitive step

The fate of the T cell receptor (TcR)/CD3 complex was examined on a cytotoxic T lymphocyte (CTL) clone (KB5.C20) activated either via binding of an anti-TcR monoclonal antibody (mAb) or by a Ca2+ ionophore and phorbol 12-myristate 13-acetate (PMA). After binding of the anti-TcR mAb, electron microscopy revealed internalization through coated vesicles followed by slow degradation of the antibody as shown by use of radiolabeled mAb. The influence of activation on TcR/CD3 internalization was analyzed. The Ca2+ ionophore alone had no effect on internalization, whereas PMA induced an accelerated internalization of anti-TcR mAb. PMA-induced internalization was dependent on protein kinase C (PKC) as shown by its absence in PKC-depleted cells or in the presence of the PKC inhibitor staurosporine. Anti-TcR mAb-induced internalization was maintained in PKC-depleted cells, but unexpectedly remained sensitive to inhibition by staurosporine. The monovalent anti-TcR mAb Fab fragment is non-stimulatory for the CTL. It was poorly internalized but its internalization was induced by PMA. Surprisingly, on PKC-depleted cells, the Fab was internalized more readily than in untreated cells and this internalization was sensitive to inhibition by staurosporine. Inhibition of PMA-induced phosphorylation of gamma and epsilon subunits of CD3 was demonstrated after depletion of PKC or in the presence of staurosporine, confirming that PKC function was inhibited in those conditions. Cross-linking of the TcR via plastic-coated anti-TcR mAb led to phosphorylation of CD3 gamma and epsilon and also of zeta, known to be phosphorylated on tyrosines. All of these phosphorylation events were inhibited by treatment with staurosporine. Our results indicate that staurosporine inhibits the receptor internalization induced by anti-TcR mAb by means other than inhibition of PKC, suggesting that other kinases may control a step of this internalization process.

Conditions of anti-Lyt-2-mediated inhibition of TcR/CD3-induced IFN-gamma secretion by a CTL clone

The relationship between the T cell receptor (TcR) for antigen (Ag) and the Lyt-2/3 molecule during T cell activation was studied using the T cell clone KB5.C20, which is dependent upon Lyt-2 for target cell killing. This cytolytic T cell clone can be activated to secrete IFN-gamma by stimulation with H-2Kb expressing cells or with monoclonal antibodies directed against a clonotypic structure of the TcR or against associated CD3 molecules. IFN-gamma production induced by H-2Kb can be inhibited by anti-Lyt-2mAb. In addition, TcR-mediated activation using the anticlonotypic mAb Désiré-1 in soluble form can be inhibited by anti-Lyt-2 mAb in soluble form either as a divalent IgG or as its monovalent Fab fragment. Anti-Lyt-2 mAb immobilized on plastic wells was also inhibitory. Stimulation induced by the anti-TcR mAb or by anti-CD3 mAb immobilized on plastic can be inhibited only with plastic immobilized and not with soluble anti-Lyt-2mAb, however. These results are discussed in terms of local interactions between TcR and Lyt-2 molecules.

Interactions between the Thy-1 and T-cell antigen receptor pathways in the activation of cytotoxic T cells: evidence from synergistic effects, loss variants, and anti-CD8 antibody-mediated inhibition

The relationship between activation of cytolytic T-lymphocyte (CTL) clones via the T-cell receptor (Ti) or the Thy-1 molecule was investigated. Anti-Ti and anti-Thy-1 monoclonal antibodies (mAb) can activate CTL clones to secrete interferon-gamma (IFN-gamma). Suboptimal doses of anti-Ti and anti-Thy-1 mAb, as well as suboptimal doses of two different anti-Thy-1 mAb, can synergize to activate T-cell clones. The addition of phorbol myristic acetate (PMA), which is not stimulatory by itself, can enhance the synergistic effect of mAb on IFN-gamma production. Although the Ti and Thy-1 molecules were not found associated at the cell surface, the results presented here indicate that these molecules are functionally associated. Use of Ti loss variants of a CTL clone confirms that Thy-1-mediated signaling is not an alternative to, but is dependent on the Ti-mediated activation pathway. Additionally, use of anti-Lyt-2/3 mAb, previously described as interfering with class I MHC-Ti binding and/or activation and, in some cases, with anti-Ti-mediated activation revealed that anti-Thy-1 mAb-mediated activation was also greatly reduced by the presence of Lyt-2/3-specific mAb. Thus the interaction between Thy-1 and Ti might also involve Lyt-2 (Lyt-3) molecules.

Sequential activation and lethal hit measured by [Ca2+]i in individual cytolytic T cells and targets

Changes in cytosolic free calcium ([Ca2+]i) have been continuously imaged during the interaction of the H-2Kb specific cytotoxic T cell lymphocyte (CTL) BM 3.3, with either the H-2Kb EL4.BU or the H-2Kk RDM4 cell lines. Activation of the CTLs by EL4.BU raises [Ca2+]i to several hundred nanomolar in the CTL. Frequently [Ca2+]i is preferentially elevated in the region of the CTL furthest from the site of target contact. These responses require external Ca2+ suggesting that they are generated by the plasma membrane and not internal stores. Inappropriate targets such as RDM4 evoke no changes in [Ca2+]i. Activation of the BM 3.3 CTL is followed by increases of [Ca2+]i to several micromolar or higher in the EL4.BU targets. This massive increase can be mimicked by direct application of cytolytic granules isolated from rat natural killer cells. The increase in plasma membrane permeability is ion-specific since external Mn2+ can also readily enter target cells that have been 'hit', as evidenced by the rapid selective quenching of fura-2 in those targets. The flood of Ca2+ into the target cell is followed by a leakage of the trapped fura-2. Since both processes continue after the CTL has disengaged, they provide a useful assay for the lethal hit. Furthermore, this technique can be used to follow complete cycles of CTL activation and lethal hit delivery, which under some circumstances can be as rapid as 6 min per cycle.