Contingent genetic regulatory events in T lymphocyte activation

Molecular cloning and overexpression of the human FK506-binding protein FKBP

The potent immunosuppressive agent FK506 is highly effective in preventing organ transplant rejection in humans. Like cyclosporin A, FK506 inhibits the transcription of early T-cell activation genes, apparently by modulating the activity of transcriptional regulators such as nuclear factor of activated T cells. A remarkable finding is that the predominant binding proteins (immunophilins) for cyclosporin A and FK506, cyclophilin and FKBP respectively, are peptidyl-prolyl-cis-trans-isomerases that are potently and selectively inhibited by their respective ligands. Here we report the complementary DNA and derived amino-acid sequences of human FKBP from Jurkat cells and also the efficient overexpression in Escherichia coli of fully active, recombinant human FKBP. The human FKBP cDNA sequence shows significant similarity to an open reading frame in the Neisseria meningitidis genome.

Regulatory anatomy of the murine interleukin-2 gene

We have cloned the mouse IL2 gene and sequenced 2800 bp of 5' flanking DNA. Comparison to the previously reported human sequence revealed extensive identity (approximately 86%) between the two genes from +1 to -580 with additional small islands of homology further upstream. Proximal sites which have been shown to be important in regulation of the human IL2 gene are well conserved in sequence and location. Transfection experiments using hybrid gene constructs containing varying lengths of the mouse 5' flanking DNA linked to a CAT reporter gene have demonstrated the presence of several novel positive and negative regulatory elements. One negative regulatory region lying between -750 and -1000 consists primarily of alternating purines and pyrimidines and is absent from the human gene. The conserved region from -321 and -578, an upstream segment from -1219 to -1332, and another region of approximately 450 bp from -1449 to -1890, which contained a well-conserved sequence of 60 bp, were each associated with enhanced levels of expression. We found no evidence for intragenic or downstream enhancer elements in this gene. All the elements identified affect only the magnitude of the inducible response, for no region when deleted had the effect of altering either the need for induction, the kinetics of stimulation, or the cell-type specificity of expression. Deletion studies suggest a strong requirement for NFAT binding even in the presence of extensive 5' flanking sequence. Therefore we conclude that IL2 gene expression is controlled primarily through a central TH1-specific signaling pathway, which acts through proximal elements, while distal cis-elements exert a secondary modulating effect.

Activated T cells express a novel gene on chromosome 8 that is closely related to the murine ecotropic retroviral receptor

A novel cDNA clone (20.5) which is differentially expressed between two closely related T-lymphoma cell clones was isolated by subtraction-enriched differential screening. SL12.4 cells, from which the cDNA was isolated, have characteristics of thymocytes at an intermediate stage in development. A sister cell clone derived from the same tumor, SL12.3, does not express this mRNA, has a distinct phenotype, and expresses fewer genes required for mature T-cell function. The cDNA sequence predicts a highly hydrophobic protein (approximately 49.5 kilodaltons) which contains seven putative membrane spanning domains. The gene was expressed on concanavalin A-activated T lymphocytes and was designated Tea (T-cell early activation gene). The Tea gene mapped to chromosome 8 and appeared to be conserved among mammalian and avian species. The Tea gene is distinct from, but bears extensive amino acid and DNA sequence similarity with, the murine ecotropic retroviral receptor which is encoded by the Rec-1 gene. Neither gene product displayed significant homology with other known transmembrane-spanning proteins. Thus, the Tea and Rec-1 genes establish a new family encoding multiple membrane-spanning proteins.

The human interleukin-2 receptor beta-chain gene: genomic organization, promoter analysis and chromosomal assignment

The chromosomal gene for the human interleukin-2 receptor beta-chain (IL-2R beta) was isolated and characterized. The entire IL-2R beta gene is composed of ten exons spanning about 24.3 kilobases, in which the protein is encoded by the exons 2-10. The cysteine rich extracellular region which displays a significant evolutionary resemblance to other cytokine receptors, as well as growth hormone and prolactin receptors, is encoded primarily by exons 3 and 4, whereas the membrane proximal, cysteine poor domain showing a homology with type III modules of fibronectin is encoded by exon 7. Sequence analysis of the 5'-flanking region revealed the presence of potential binding sites for transcription factors such as Octamer binding factors, AP-1, AP-2 as well as the 'GC-clusters'. At least five potential cap sites were identified by S1 mapping analysis. The 850 bp DNA sequence of the 5'-flanking region exhibited constitutive promoter activity when it was linked upstream of the HSV-tk reporter gene and then transfected into YT cells, a human leukemic cell line. By applying the RFLP linkage analysis, the IL-2R beta gene has been assigned to chromosome 22q12-13.

Activated T cells express a novel gene on chromosome 8 that is closely related to the murine ecotropic retroviral receptor

A novel cDNA clone (20.5) which is differentially expressed between two closely related T-lymphoma cell clones was isolated by subtraction-enriched differential screening. SL12.4 cells, from which the cDNA was isolated, have characteristics of thymocytes at an intermediate stage in development. A sister cell clone derived from the same tumor, SL12.3, does not express this mRNA, has a distinct phenotype, and expresses fewer genes required for mature T-cell function. The cDNA sequence predicts a highly hydrophobic protein (approximately 49.5 kilodaltons) which contains seven putative membrane spanning domains. The gene was expressed on concanavalin A-activated T lymphocytes and was designated Tea (T-cell early activation gene). The Tea gene mapped to chromosome 8 and appeared to be conserved among mammalian and avian species. The Tea gene is distinct from, but bears extensive amino acid and DNA sequence similarity with, the murine ecotropic retroviral receptor which is encoded by the Rec-1 gene. Neither gene product displayed significant homology with other known transmembrane-spanning proteins. Thus, the Tea and Rec-1 genes establish a new family encoding multiple membrane-spanning proteins.

Differential regulation of interleukin 4 and interleukin 5 gene expression: a comparison of T-cell gene induction by anti-CD3 antibody or by exogenous lymphokines

Murine T helper type 2 clones were stimulated with immobilized anti-CD3 antibody or with recombinant lymphokines to compare the expression of T-cell activation genes induced by these stimuli. Immobilized anti-CD3 antibody, recombinant interleukin 2 (IL-2), and recombinant interleukin 4 (IL-4) all induced proliferation of the T helper type 2 clones 10-5-17 and D10. Proliferation of these clones induced by anti-CD3 antibody was completely inhibited by cyclosporine A, whereas cyclosporine A had little effect on proliferation induced by recombinant IL-2 or recombinant IL-4. Both immobilized anti-CD3 antibody, and recombinant IL-2 induced the expression of the protooncogenes c-myc and c-myb. Immobilized anti-CD3 antibody also induced expression of the lymphokine genes IL-4, interleukin 5 (IL-5), and granulocyte-macrophage colony-stimulating factor. In contrast, recombinant IL-2 induced IL-5 mRNA expression but did not induce detectable expression of IL-4 or granulocyte-macrophage colony-stimulating factor mRNA. Likewise, recombinant IL-4 induced expression of IL-5 but not IL-4 mRNA. Thus, the IL-4 and IL-5 genes appear to be differentially regulated after stimulation with recombinant lymphokines. Effects of cyclosporine A and the protein synthesis inhibitors cycloheximide and anisomycin on IL-4 and IL-5 gene expression suggest that these genes are activated by different pathways after anti-CD3 stimulation. Cyclosporine A completely inhibited anti-CD3-induced expression of IL-4 mRNA but not of IL-5 mRNA, and protein-synthesis inhibitors completely inhibited induction of IL-5 mRNA but not of IL-4 mRNA. Together, our data show that T-cell receptor-mediated and lymphokine receptor-mediated signals induce different patterns of lymphokine gene expression and provide strong evidence that the IL-4 and IL-5 genes are differently regulated.

A role for protein kinase C activity in interleukin-1 (IL-1) induction of IL-2 gene expression but not in IL-1 signal transduction

Interleukin-1 (IL-1) is known to synergize with phorbol esters in the induction of interleukin-2 (IL-2) expression in T-lymphoid leukemia cells and proliferation of mouse thymocytes. We used a plasmid construct containing the bacterial gene for chloramphenicol acetyltransferase under the control of the human IL-2 promoter to study the nature of this synergism in the murine thymoma cell line EL4. Although IL-1 induction of the IL-2 promoter in these cells required costimulus with phorbol myristate acetate, the signal induced by IL-1 was qualitatively different. We provide evidence to support the hypothesis that the phorbol ester signal is mediated by protein kinase C, and we show that the IL-1 signal is not. That IL-1 and phorbol myristate acetate represent different stimuli was shown by their response to protein kinase C inhibitors, capacity to synergize with increased intracellular free calcium, and requirement for protein synthesis. In addition we show that pretreatment with IL-1 can prime EL4 cells to subsequent activation by concentrations of phorbol esters not normally sufficient to induce IL-2 expression. Pretreated cells remained primed for at least 40 h after removal of the IL-1. Neither phorbol myristate acetate nor a calcium ionophore was capable of preactivating EL4 cells.

Interferon-regulatory factor 1 is an immediate-early gene under transcriptional regulation by prolactin in Nb2 T cells

The pituitary peptide hormone prolactin (Prl) is a potent inducer of Nb2 T lymphoma cell proliferation. To analyze the early genetic response to the mitogenic signals of Prl, a cDNA library was constructed from Nb2 T cells stimulated for 4 h with Prl and the protein synthesis inhibitor cycloheximide. Of 26 distinct clones isolated by differential screening, one clone, designated c25, exhibited extremely rapid but transient kinetics of induction by Prl and superinduction by Prl plus cycloheximide. Run-on transcription analysis indicated that c25 gene transcription was induced greater than 20-fold within 30 to 60 min of Prl stimulation. Surprisingly, DNA sequence analysis of c25 cDNA revealed that this Prl-inducible early-response gene is the rat homolog of the mouse transcription factor interferon-regulatory factor 1 (IRF-1), sharing 91% coding sequence similarity with mouse IRF-1. At the protein level, rat IRF-1 shares 97% and 92% homology with mouse IRF-1 and human IRF-1, respectively, suggesting that this molecule has been functionally conserved throughout evolution. Our studies show that the gene for IRF-1 is an immediate-early gene in Prl-stimulated T cells, which suggests that IRF-1 is a multifunctional molecule. In addition to its role in regulating growth-inhibitory interferon genes, IRF-1 may, therefore, also play a stimulatory role in cell proliferation. The gene for IRF-1 is one of the earliest genes known to be transcriptionally regulated by Prl.

A cell culture model for T lymphocyte clonal anergy

T lymphocytes respond to foreign antigens both by producing protein effector molecules known as lymphokines and by multiplying. Complete activation requires two signaling events, one through the antigen-specific receptor and one through the receptor for a costimulatory molecule. In the absence of the latter signal, the T cell makes only a partial response and, more importantly, enters an unresponsive state known as clonal anergy in which the T cell is incapable of producing its own growth hormone, interleukin-2, on restimulation. Our current understanding at the molecular level of this modulatory process and its relevance to T cell tolerance are reviewed.

Two inflammatory mediator cytokine genes are closely linked and variably amplified on chromosome 17q

Mitogenic stimulation of resting T cells results in the de novo transcription of a large number of genes including those encoding regulatory molecules such as lymphokines. The genomic organization of two newly described induced lymphokine genes, 464.1 and 744.1, has been determined. 464.1 and 744.1 appear to be the human homologues of the recently cloned murine macrophage inflammatory proteins, MIP-1 alpha and MIP-1 beta, respectively. The 464.1 and 744.1 genes share 55% amino acid homology and demonstrate parallel regulation of induced expression in T cells. It was therefore of interest to observe that these genes are closely linked in the human genome, separated by 14 kb, and are organized in a head to head fashion. Each of the genes is present in an additional nonallelic copy (referred to as 464.2 and 744.2) as part of an apparent amplification unit in the genome of many individuals. The 464.2 gene is expressed and potentially encodes a protein highly related to 464.1, varying in 5 of 92 amino acids. As expected, 464.2 and 744.2 are also closely linked to each other as determined by population linkage disequilibrium studies. Individuals bearing a chromosome with a third amplification event, involving a 464-related gene but not a 744-related gene, are also infrequently observed. These genes are all located on chromosome 17 in bands q11-q21, the region implicated in von Recklinghausen neurofibromatosis (NF1) and in acute promyelocytic leukemia (AML-M3).

A method for measuring protein kinase C activity in permeabilized T lymphocytes by using peptide substrates. Evidence for multiple pathways of kinase activation

Activation of protein kinase C (PKC) in human T lymphocytes is an immediate consequence of mitogenic signalling via the antigen-receptor complex and CD2 antigen. In order to investigate further the signal-transduction pathways which result in PKC activation, we have established a novel PKC assay system using streptolysin-O-permeabilized T cells. Known peptide substrates of PKC were introduced into permeabilized cells in the presence of [gamma-32P]ATP, 3 mM-Mg2+ and 150 nM free Ca2+. The peptide found to have the lowest background phosphorylation had the sequence Pro-Leu-Ser-Arg-Thr-Leu-Ser-Val-Ala-Ala-Lys-Lys (peptide GS), and the phosphorylation of the peptide was increased up to 6-fold by direct activation of PKC with phorbol 12,13-dibutyrate. Induction of PKC activation with the UCHT1 antibody against the CD3 antigen, or with phytohaemagglutinin (PHA) or guanosine 5'-[gamma-thio]triphosphate (GTP[S]), increased peptide-GS phosphorylation by 2-3 fold. The specificity of PKC action on peptide GS was demonstrated by blocking increases in phosphorylation with a pseudosubstrate peptide PKC inhibitor. PKC activation by this technique could be detected within 1 min of adding external ligand. Dose-response curves revealed that PHA-induced production of inositol phosphates correlated closely with PKC activities, whereas only a partial correlation between these parameters was observed with GTP[S]. Our data are consistent with the presence of more than one G-protein-mediated pathway of PKC regulation in T cells. The quantitative PKC assay system described is both simple and reproducible, and its potential application to a wide range of cell types should prove useful in further investigations of PKC activation mechanisms.

A role for protein kinase C activity in interleukin-1 (IL-1) induction of IL-2 gene expression but not in IL-1 signal transduction

Interleukin-1 (IL-1) is known to synergize with phorbol esters in the induction of interleukin-2 (IL-2) expression in T-lymphoid leukemia cells and proliferation of mouse thymocytes. We used a plasmid construct containing the bacterial gene for chloramphenicol acetyltransferase under the control of the human IL-2 promoter to study the nature of this synergism in the murine thymoma cell line EL4. Although IL-1 induction of the IL-2 promoter in these cells required costimulus with phorbol myristate acetate, the signal induced by IL-1 was qualitatively different. We provide evidence to support the hypothesis that the phorbol ester signal is mediated by protein kinase C, and we show that the IL-1 signal is not. That IL-1 and phorbol myristate acetate represent different stimuli was shown by their response to protein kinase C inhibitors, capacity to synergize with increased intracellular free calcium, and requirement for protein synthesis. In addition we show that pretreatment with IL-1 can prime EL4 cells to subsequent activation by concentrations of phorbol esters not normally sufficient to induce IL-2 expression. Pretreated cells remained primed for at least 40 h after removal of the IL-1. Neither phorbol myristate acetate nor a calcium ionophore was capable of preactivating EL4 cells.

Interferon-regulatory factor 1 is an immediate-early gene under transcriptional regulation by prolactin in Nb2 T cells

The pituitary peptide hormone prolactin (Prl) is a potent inducer of Nb2 T lymphoma cell proliferation. To analyze the early genetic response to the mitogenic signals of Prl, a cDNA library was constructed from Nb2 T cells stimulated for 4 h with Prl and the protein synthesis inhibitor cycloheximide. Of 26 distinct clones isolated by differential screening, one clone, designated c25, exhibited extremely rapid but transient kinetics of induction by Prl and superinduction by Prl plus cycloheximide. Run-on transcription analysis indicated that c25 gene transcription was induced greater than 20-fold within 30 to 60 min of Prl stimulation. Surprisingly, DNA sequence analysis of c25 cDNA revealed that this Prl-inducible early-response gene is the rat homolog of the mouse transcription factor interferon-regulatory factor 1 (IRF-1), sharing 91% coding sequence similarity with mouse IRF-1. At the protein level, rat IRF-1 shares 97% and 92% homology with mouse IRF-1 and human IRF-1, respectively, suggesting that this molecule has been functionally conserved throughout evolution. Our studies show that the gene for IRF-1 is an immediate-early gene in Prl-stimulated T cells, which suggests that IRF-1 is a multifunctional molecule. In addition to its role in regulating growth-inhibitory interferon genes, IRF-1 may, therefore, also play a stimulatory role in cell proliferation. The gene for IRF-1 is one of the earliest genes known to be transcriptionally regulated by Prl.

Expression of a zinc finger gene in HTLV-I- and HTLV-II-transformed cells

Gene products encoded by the human T cell leukemia virus (HTLV) types I and II mediate transformation by the transactivation of cellular genes necessary for proliferation, probably including transcriptional regulatory factors. By searching for factors that may control proliferation, a zinc finger gene (225) was identified that was constitutively expressed in all HTLV-I- or HTLV-II-transformed cell lines examined, whereas in normal T cells it was only transiently expressed after mitogenic stimulation. The 225 gene was also constitutively expressed in two HTLV-I-transformed helper T cell clones, but not in the parental cell lines. Thus this putative cellular transcriptional factor, which was abnormally expressed in retrovirus-infected cells, may have a role in transformation.

Anti-beta 2-microglobulin monoclonal antibodies mediate a delay in HIV1 cytopathic effect on MT4 cells

We attempted to select monoclonal antibodies (mAb) which reacted with T-cell surface molecules and were able to interfere with the human immunodeficiency virus type 1 (HIV1) replicative cycle in the MT4 T-leukaemic cell line. In comparison with OKT4A, an anti-CD4 mAb, only one mAb, HC11.151.1, was found to significantly delay HIV-induced cytopathic effect on MT4 cells among the 15 mAb tested which reacted with MT4 cell surface antigens. Biochemical and immunological characterization of HC11.151.1 demonstrated its specificity for beta 2-microglobulin (beta 2m), the light chain of human leukocyte antigen (HLA) class I molecules. Other beta 2m-specific mAb were tested in order to assess whether this effect represented an intrinsic capacity of HC11.151.1 or whether it was a common feature shared by all anti-beta 2m mAb. Three (B1.1G6, B2.62.2 and BBM1) of the four anti-beta 2m mAb demonstrated the same protective effect, whereas C21.48A, which was devoid of a functional effect, was directed towards a beta 2m epitope involved in binding to the HLA class I heavy chain molecule. The physiological relevance of this observation is discussed.

Reciprocal expression of human ETS1 and ETS2 genes during T-cell activation: regulatory role for the protooncogene ETS1

The expression of the protooncogenes ETS1 and ETS2 has been studied in purified human T cells activated either by cross-linking of the T-cell receptor-CD3 complex on their cell surface or by direct stimulation with phorbol esters and ionomycin. Our results show that resting T cells express high levels of ETS1 mRNA and protein, while expression of ETS2 is undetectable. Upon T-cell activation, ETS2 mRNA and proteins are induced, while ETS1 gene expression decreases to very low levels. Late after stimulation, ETS1 mRNA is reinduced and maintained at a high level, while ETS2 gene expression decreases to undetectable levels. Therefore, it appears that in human T cells, ETS2 gene products are associated with cellular activation and proliferation, while ETS1 gene products are preferentially expressed in a quiescent state.

Phytohemagglutinin-induced activity of cyclic AMP (cAMP) response elements from cytomegalovirus is reduced by cyclosporine and synergistically enhanced by cAMP

The 19-base-pair enhancer repeat of the human cytomegalovirus immediate-early 1 gene mediates cyclic AMP- and phytohemagglutinin-induced expression in Jurkat T cells. Synergistic activity was observed in the presence of both drugs, suggesting a convergence of the protein kinase A and C pathways on this transcription element. In addition, the immunosuppressive drug cyclosporine strongly reduced the ability of the 19-base-pair repeat to activate gene expression in phytohemagglutinin-stimulated T cells.

Absence of interleukin 2 production in a severe combined immunodeficiency disease syndrome with T cells

We have characterized a child with a severe combined immunodeficiency disease syndrome with increased numbers, but a normal distribution, of CD3+ T cells. This patient's immunological defect appears to be attributable to a selective deficiency in T cell production of IL-2, which may reflect a subtle abnormality in the IL-2 gene locus or a defect in a regulatory factor necessary for IL-2 transcription. The increased numbers of phenotypically normal T cells in this patient suggest that alternative pathways of T cell development exist in man or that IL-2 production intra- and extrathymically is controlled via distinct regulatory mechanisms.

Immune function of the blood-brain barrier: incomplete presentation of protein (auto-)antigens by rat brain microvascular endothelium in vitro

The endothelial blood-brain barrier (BBB) has a critical role in controlling lymphocyte traffic into the central nervous system (CNS), both in physiological immunosurveillance, and in its pathological aberrations. The intercellular signals that possibly could induce lymphocytes to cross the BBB include immunogenic presentation of protein (auto-)antigens by BBB endothelia to circulating T lymphocytes. This concept has raised much, though controversial, attention. We approached this problem by analyzing in vitro immunospecific interactions between clonal rat T lymphocyte lines with syngeneic, stringently purified endothelial monolayer cultures from adult brain micro-vessels. The rat brain endothelia (RBE) were established from rat brain capillaries using double collagenase digestion, density gradient fractionation and selective cytolysis of contaminating pericytes by anti-Thy 1.1 antibodies and complement. Incubation with interferon-gamma in most of the brain-derived endothelial cells induced Ia-antigens in the cytoplasm and on the cell surface in some of the cells. Before the treatment, the cells were completely Ia-negative. Pericytes were unresponsive to IFN-gamma treatment. When confronted with syngeneic T cell lines specific for protein (auto-)antigens (e.g., ovalbumin and myelin basic protein, MBP), RBE were completely unable to induce antigen-specific proliferation of syngeneic T lymphocytes irrespective of pretreatment with IFN-gamma and of cell density. RBE were inert towards the T cells, and did not suppress T cell activation induced by other "professional" antigen presenting cells (APC) such as thymus-derived dendritic cells or macrophages. IFN-gamma-treated RBE were, however, susceptible to immunospecific T cell killing. They were lysed by MBP-specific T cells in the presence of the specific antigen or Con A. Antigen dependent lysis was restricted by the appropriate (MHC) class II product. We conclude that the interaction of brain endothelial cells with encephalitogenic T lymphocytes may involve recognition of antigen in the molecular context of relevant MHC products, but that this interaction per se is insufficient to initiate the full T cell activation program.

Protein kinase C and T cell activation

Understanding the intracellular mechanisms by which binding of ligands, such as hormones and growth factors, to their specific receptors elicits the appropriate cellular response has long been a topic of great interest. Considerable excitement was generated when it was recognised that several receptor-ligand interactions operate via the hydrolysis of inositol phospholipids. This yields, at least, two 'second messengers', namely, inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], which causes the release of Ca2+ from intracellular stores, and 1,2-diacylglycerol (ac2Gro), which activates the serine/threonine-specific enzyme, protein kinase C(PKC), reviewed in [1] and [2]. The pertinent question that follows is, how do PKC activation and elevation of the intracellular Ca2+ concentration evoke cell responses? In this review, attention has been focused on PKC, and the consequences of its activation in resting human T cells. Evidence that PKC activity is, at least partially, responsible for activation of resting human T cells will be examined, and some of the more recent research investigating how PKC activation elicits this cell response will be described.

Increased intracellular Ca2+ is necessary for maximal expression of the proto-oncogene c-jun in the Jurkat T-cell line

The time course and signal-transduction requirements for proto-oncogene c-jun expression in T-cells were investigated. Expression of c-jun mRNA was evident at 30 min after stimulation. Both the activation of Ca2+/phospholipid-dependent kinase as well as an increased intracellular free Ca2+ concentration were necessary for the maximal induction of c-jun mRNA and synthesis of Jun protein 1 h after stimulation.

An early postinfection signal mediated by monoclonal anti-beta 2 microglobulin antibody is responsible for delayed production of human immunodeficiency virus type 1 in peripheral blood mononuclear cells

We recently found (C. Devaux, J. Boucraut, G. Poirier, P. Corbeau, F. Rey, M. Benkirane, B. Perarneau, F. Kourilsky, and J.C. Chermann, submitted for publication) a latency in the human immunodeficiency virus (HIV) type 1 cytopathic effect in the human T-cell lymphotropic virus type I immortalized T-cell line MT4 that was mediated by anti-beta 2 microglobulin (beta 2m) monoclonal antibodies (MAb). Here we describe a delay in viral particle production in peripheral blood mononuclear cells (PBMC) that was mediated by three (B1-1G6, B2-62-2, and HC11-151-1) of four anti-beta 2m MAb tested, the nonefficient MAb (C21-48A) being specific for an epitope on beta 2m that was masked by association with the human leukocyte antigen class I heavy chain. Experiments were designed to determine the mechanism of interference. PBMC incubated with anti-beta 2m MAb before viral exposure were not protected from HIV infection. In addition, anti-beta 2m MAb were not efficient in preventing syncytium formation between HIV-infected PBMC and CD4-positive MT4 cells. In contrast, anti-beta 2m MAb treatment of freshly infected PBMC significantly delayed HIV production in these cells. The window of cell sensitivity to anti-beta 2m MAb treatment took place during a very early post-HIV-binding stage. The possible mechanism of anti-beta 2m MAb action is discussed.

Inhibition of T cell development in thymic organ culture: implications for the mechanism of action of cyclosporin A

We have examined the effects of the immunosuppressive drug cyclosporin A (CsA) on the phenotypic maturation of T cells in thymic organ cultures begun at day 16 of gestation. CsA specifically inhibited the generation of cells expressing high levels of alpha/beta TcR/CD3 complexes and a mature phenotype defined by CD4 and CD8 surface markers. Adding interleukin (IL) 1 beta, IL 2 or IL 4 failed to reverse the effects of CsA, and major histocompatibility complex class II expression in the thymic medulla was preserved. Possible mechanisms of CsA-mediated inhibition of T cell development are discussed.

Rapid turnover of endogenous endonuclease activity in thymocytes: effects of inhibitors of macromolecular synthesis

Previous work has shown that inhibitors of protein or mRNA synthesis block endonuclease activation in thymocytes undergoing programmed cell death. In the present study we used isolated nuclei to investigate the effects of cycloheximide and actinomycin D, inhibitors of protein and mRNA synthesis, respectively, on endogenous endonuclease activity in thymocytes. We observed a rapid loss of Ca2(+)-dependent endonuclease activity in nuclei isolated from thymocytes treated with these inhibitors. In contrast, pretreatment of cells with antipain and leupeptin, inhibitors of proteases, prevented the depletion of endonuclease activity in the nuclei, suggesting that proteolysis was involved. The effects of cycloheximide and actinomycin D were mimicked by incubating thymocytes with treatments known to exert their effects via activation of protein kinase C. Our results suggest that endonuclease activity in thymocyte nuclei undergoes rapid, spontaneous turnover. Agents interfering with macromolecular synthesis may therefore block DNA fragmentation in thymocytes by depleting nuclei of endogenous endonuclease activity.

Interleukin-2 inhibits the replication of human herpesvirus-6 in mature thymocytes

Human herpesvirus-6 (HHV-6) is a recently identified T lymphotropic virus. We have examined the ability of HHV-6 to replicate in mature and immature human thymocytes. Infection of both cell populations revealed that only mitogen-activated mature thymocytes could support efficient virus replication. Because interleukin-2 (IL-2) plays a central role in T cell activation we investigated its effect on HHV-6 replication. Unexpectedly, addition of recombinant IL-2 at concentrations-exceeding 10 U/ml strongly inhibited the virus-induced cytopathic effect. Electron microscopic examinations and immunofluorescence assays revealed a threefold reduction in the fraction of infected cells, and almost total absence of extracellular virions in the IL-2-treated cultures. It will therefore be of interest to determine whether the IL-2-mediated inhibitory effect plays some role in the establishment of HHV-6 latency in the human host.

The dose dependent effect of cyclic AMP on ribonucleotide reductase in mitogen stimulated mononuclear cells

Cyclic adenosine monophosphate arrests proliferating T lymphocytes in the G1 phase of the cell cycle. Here we demonstrate that exogenous and endogenous elevations in cyclic AMP concentration result in diminished mitogen stimulation, cell cycle arrest, and decreased ribonucleotide reductase messenger RNA concentrations in peripheral blood mononuclear cells. At lower concentrations (less than 1mM) of dibutyryl cyclic AMP that do not generate cell cycle arrest there is inhibition of ribonucleotide reductase activity without decreased messenger RNA concentration for the M2 subunit of ribonucleotide reductase. However, at higher concentrations of dibutyryl cyclic AMP there is G1 cell cycle arrest and reduced M2 specific messenger RNA concentration. Thus, cyclic AMP inhibition of lymphocyte activation may occur by different mechanisms that are dose dependent.

Effect of cyclosporin A on suppressor T cells

The effect of cyclosporin A (CsA) on suppressor T cells (Ts) was evaluated by in vitro incubation with afferent-phase Ts (Ts-aff) and efferent-phase Ts (Ts-eff). 2,4-Dinitro-1-fluorobenzene (DNFB) and hen egg-white lysozyme (HEL) were used as antigens. Both Ts-aff and Ts-eff were resistant to high- and low-dose CsA treatments. However, T cells associated with delayed type hypersensitivity (DTH) [TDTH] to DNFB or HEL were sensitive to these CsA treatments. These results indicate that the effect of CsA treatment on contact sensitivity and DTH influences TDTH and not Ts.

Activation of human T lymphocytes by phorbol-12,13-dibutyrate and ionomycin

The calcium ionophore ionomycin and the phorbol ester phorbol-12,13-dibutyrate (PDBu) are shown to have a synergistic effect upon interleukin 2 (IL-2) production, interleukin 2 receptor expression, and T-lymphocyte proliferation. The proliferative response was inhibited by addition of a monoclonal antibody directed against the IL-2R (Tac antigen) demonstrating that PDBu and ionomycin induce T-cell growth through an IL-2-dependent autocrine pathway. Sequential stimulation with PDBu and ionomycin failed to induce IL-2 production, IL-2R expression, and consequently proliferation of the T cells, indicating that T-cell activation requires simultaneous activation of protein kinase C (PKC) and elevation of cytosolic calcium. Exposure of T cells to both agents for different times resulted in IL-2 production, IL-2R expression, and proliferation in proportion to the duration of incubation with at least 4 h required for maximal T-cell activation. Further, in the presence of PDBu maximal T-cell activation was found to require stimulation with ionomycin for 4 h, indicating that a sustained increase in free cytoplasmic calcium of several hours' duration is essential for T-cell activation. In contrast T cells incubated with ionomycin were induced to produce IL-2 and express IL-2Rs upon brief exposure to PDBu with a 2-h incubation period being sufficient for maximal T-cell activation. Thus transient activation of PKC seems to be sufficient for activation of the IL-2 gene and IL-2R gene. However, maximal T-cell activation requires activation of PKC for at least 2 h.

Infection with the intracellular protozoan parasite Theileria parva induces constitutively high levels of NF-kappa B in bovine T lymphocytes

The intracellular protozoan parasite Theileria parva causes a lymphoproliferative disease of T cells in cattle and uncontrolled lymphocyte proliferation in culture. We have identified and characterized in infected cells the transcriptional activator, NF-kappa B, whose recognition motifs have been identified in several gene enhancers important for lymphocyte-specific gene expression. NF-kappa B is normally constitutively activated in nuclear extracts derived from B cells and can be induced in T cells and nonlymphoid cells by phorbol esters. Theileria-infected lymphocytes contained constitutively high levels of activated NF-kappa B in nuclear fractions and inactive NF-kappa B in cytoplasmic fractions. The inactive cytoplasmic precursor could be activated by treatment of extracts with deoxycholate, which was shown previously to dissociate NF-kappa B from an inhibitor, I kappa B. Treatment of lymphocyte extracts with 3 mM GTP stimulated NF-kappa B binding to its recognition motif in vitro, thereby distinguishing it from a related nuclear factor, H2-TF1. Selective killing of the parasite, which left the host cells intact, resulted in a rapid loss of NF-kappa B from the nuclear fractions and a slower loss from the cytoplasmic fractions. In parasitized cells, NF-kappa B could not be further stimulated by treatment with 12-O-tetradecanoylphorbol-13-acetate whereas in cells treated to remove the parasite, this compound stimulated elevated levels of NF-kappa B. We propose that high levels of activated NF-kappa B are maintained by the presence of the parasite in infected T cells. Similarly, we propose that the high levels of inactive cytoplasmic precursor are a result of increased synthesis due to the presence of the parasite.

Lymphokine gene expression in vivo is inhibited by cyclosporin A

Murine T cells were stimulated in vivo by administering allogeneic cells or mitogens into the foot pads and then examining the draining popliteal lymph nodes. Allogeneic spleen cells induced the expression of IL2 and IFN-gamma mRNAs in a time- and dose-dependent manner. Induction of these transcripts also was detected after administration of Con A and anti-CD3 mAb. An increase in DNA-synthesizing cells was observed by 48 h, and these were shown to be T cells because of their sensitivity to anti-Thy-1 but not anti-B220 mAb and complement, and because of their localization to the T-dependent areas of the lymph node. The in vivo administration of cyclosporin A (CSA) reduced several of these T cell responses. The level of DNA synthesis and the frequency of cells synthesizing DNA were decreased by approximately 75%, while the induction of IL-2 responsiveness was not substantially diminished. IL-2 and IFN-gamma transcripts were inhibited at least 70-90%, as determined by Northern blot and in situ hybridization. Although the inhibition by CSA was not as complete in animals as observed previously in tissue culture, our findings indicate that in both systems, a major site of action of CSA is to inhibit T cell growth by inhibiting lymphokine production.

Human memory T cells express intercellular adhesion molecule-1 which can be increased by interleukin 2 and interferon-gamma

We have shown that low levels of intercellular adhesion molecule-1 (ICAM-1) expression can be detected on approximately 40% of the resting peripheral blood T cell population. The ICAM-1+ T cells have the phenotypic markers of memory cells which can be distinguished functionally from naive T cells by their ability to respond rapidly to previously experienced antigen. These cells appear to be in a state of marginal activation in that they also express low levels of the interleukin 2 receptor (Tac antigen) and have increased cell size as compared to the naive T cells. In addition we have shown that the cytokines interleukin 2 and interferon-gamma, both of which are products of activated T cells, are able to increase the expression of ICAM-1 on T cells. Finally, pretreating T cells with an anti-ICAM-1 monoclonal antibody inhibits their response to recall antigens, strongly suggesting a functional role for this protein on the memory T cell.

Oxidatively stressed lymphocytes remain in G0/G1a on mitogenic stimulation

Thiol modifiers and oxidants inhibit lymphocyte activation. To investigate which of the many cell functions sensitive to oxidation are critical in this inhibition, mouse splenic lymphocytes were treated with oxidants prior to exposure to mitogen, and progression into the cell cycle was assayed. Different treatments were used to chemically dissect different potential targets within the cell: copper phenanthroline (CuP), to oxidize surface sulfhydryls; N-ethyl maleimide (NEM), to alkylate extra- and intracellular thiols; and hydrogen peroxide, which generates the highly reactive hydroxyl radical within the cell. Progression into the cell cycle was assayed with acridine orange (AO) and assays of interleukin-2 (IL-2) production and IL-2 receptor (IL-2R) expression. The contribution of ADP-ribosylation to inhibition of mitogenesis was assessed using 3-aminobenzamide (3AB) to inhibit adenosine 5'-diphosphate (ADP)-ribose transferases. The results indicate that the CuP and NEM treatments both produce two independent inhibitory effects, that is, a failure in the production of and response to IL-2. Cells treated with these compounds were able to progress only through G1a upon mitogenic stimulation. H2O2 had more complex effects. Both ADP-ribosylation and modulations of cytosolic Ca2+ were involved in the inhibitory effects. With lower inhibitory doses of H2O2, lymphocytes were completely unresponsive to mitogen and failed to exit Go upon mitogenic stimulation. If intra- and extracellular Ca2+ were buffered before treatment with H2O2, higher concentrations were required, and under these conditions cells were able to enter G1a but could not progress into G1b. Under neither of these conditions could cells produce IL-2 or express IL-2R.

Calcium-dependent DNA fragmentation in human synovial cells exposed to cold shock

Exposure of confluent human synovial McCoy's cells to near-freezing temperatures followed by rewarming at 37 degrees C resulted in endonuclease activation and cell death characteristic of a suicide process known as apoptosis. Both DNA fragmentation and cell killing were dependent on a sustained increase in the cytosolic Ca2+ concentration. Sensitivity to cold shock-induced endonuclease activation was critically dependent on the cell cycle (proliferative) status and limited to confluent cells, whereas cells in the logarithmic growth phase were completely resistant. However, DNA fragmentation was promoted in the proliferating McCoy's cells pretreated with H-7 or sphingosine, inhibitors of protein kinase C. In addition, phorbol ester, known to activate PKC, inhibited DNA fragmentation in the confluent cells. Our findings indicate that cold shock-induced DNA fragmentation in McCoy's cells is dependent on a sustained Ca2+ increase, and sensitivity to the process appears to be regulated by the status of protein kinase C.

Overexpression of src family gene for tyrosine-kinase p59fyn in CD4-CD8- T cells of mice with a lymphoproliferative disorder

Overexpression of a src family gene, lck, has been associated with differentiation of the murine thymic lymphoma line LSTRA. Recent findings by several groups strongly suggest a functional role for the gene product p56lck protein-tyrosine kinase (PTK) in the activation of normal T cells. A single recessive gene, lpr or gld, induces a lymphoproliferative disorder concomitant with autoimmune disease in mice. In this study, a 10-fold elevated activity of PTK encoded by fyn, another src family gene, was demonstrated in CD4-CD8- T cells in mutant mice. The increased PTK activity was consistent with overexpression of fyn mRNA. The elevated fyn mRNA expression appeared to be a characteristic of CD4-CD8- T cells, since it was not observed in normal T cells at any stage of differentiation. The fact that fyn mRNA expression was markedly induced in normal T cells by mitogenic stimulation with anti-T3 epsilon antiserum supports the possibility that p59fyn PTK is a signal-generating molecule in T cells. Thus, our findings provide insight into the physiological role for a src gene family kinase in T-cell development and contribute to a better understanding of the molecular mechanisms of disease-inducing recessive genes.

G1 events and regulation of cell proliferation

Cells prepare for S phase during the G1 phase of the cell cycle. Cell biological methods have provided knowledge of cycle kinetics and of substages of G1 that are determined by extracellular signals. Through the use of biochemical and molecular biological techniques to study effects of growth factors, oncogenes, and inhibitors, intracellular events during G1 that lead to DNA synthesis are rapidly being discovered. Many cells in vivo are in a quiescent state (G0), with unduplicated DNA. Cells can be activated to reenter the cycle during G1. Similarly, cells in culture can be shifted between G0 and G1. These switches in and out of G1 are the main determinants of post-embryonic cell proliferation rate and are defectively controlled in cancer cells.

Infection with the intracellular protozoan parasite Theileria parva induces constitutively high levels of NF-kappa B in bovine T lymphocytes

The intracellular protozoan parasite Theileria parva causes a lymphoproliferative disease of T cells in cattle and uncontrolled lymphocyte proliferation in culture. We have identified and characterized in infected cells the transcriptional activator, NF-kappa B, whose recognition motifs have been identified in several gene enhancers important for lymphocyte-specific gene expression. NF-kappa B is normally constitutively activated in nuclear extracts derived from B cells and can be induced in T cells and nonlymphoid cells by phorbol esters. Theileria-infected lymphocytes contained constitutively high levels of activated NF-kappa B in nuclear fractions and inactive NF-kappa B in cytoplasmic fractions. The inactive cytoplasmic precursor could be activated by treatment of extracts with deoxycholate, which was shown previously to dissociate NF-kappa B from an inhibitor, I kappa B. Treatment of lymphocyte extracts with 3 mM GTP stimulated NF-kappa B binding to its recognition motif in vitro, thereby distinguishing it from a related nuclear factor, H2-TF1. Selective killing of the parasite, which left the host cells intact, resulted in a rapid loss of NF-kappa B from the nuclear fractions and a slower loss from the cytoplasmic fractions. In parasitized cells, NF-kappa B could not be further stimulated by treatment with 12-O-tetradecanoylphorbol-13-acetate whereas in cells treated to remove the parasite, this compound stimulated elevated levels of NF-kappa B. We propose that high levels of activated NF-kappa B are maintained by the presence of the parasite in infected T cells. Similarly, we propose that the high levels of inactive cytoplasmic precursor are a result of increased synthesis due to the presence of the parasite.

Interleukin-1 costimulatory activity on the interleukin-2 promoter via AP-1

Interleukin-1 (IL-1) is a major regulator of inflammation and immunity. IL-1 induces T lymphocyte growth by acting as a second signal (together with antigen) in enhancing the production of interleukin-2 (IL-2). An IL-1-responsive element in the promoter region of the human IL-2 gene was similar to the binding site for the transcription factor AP-1. IL-1 enhanced expression of c-jun messenger RNA, whereas the antigenic signal enhanced messenger RNA expression of c-fos. Thus, the two components of the AP-1 factor are independently regulated and the AP-1 factor may serve as a nuclear mediator for the many actions of IL-1 on cells.