Isolation and partial characterization of concanavalin A receptors on cloned cytotoxic T lymphocytes

Plant Lectins Activate the NLRP3 Inflammasome To Promote Inflammatory Disorders

Plant-derived dietary lectins have been reported to be involved in the pathogenesis of several inflammatory diseases, including inflammatory bowel disease, diabetes, rheumatoid arthritis, and celiac disease, but little is known about the molecular mechanisms underlying lectin-induced inflammation. In this study, we showed that plant lectins can induce caspase-1 activation and IL-1β secretion via the NLRP3 inflammasome. Lectins were internalized and subsequently escaped from the lysosome and then translocated to the endoplasmic reticulum. Endoplasmic reticulum-loaded plant lectins then triggered Ca²⁺ release and mitochondrial damage, and inhibition of Ca²⁺ release and mitochondrial reactive oxygen species by chemical inhibitors significantly suppressed NLRP3 inflammasome activation. In vivo, plant lectin-induced inflammation and tissue damage also depended on the NLRP3 inflammasome. Our findings indicate that plant lectins can act as an exogenous "danger signal" that can activate the NLRP3 inflammasome and suggest that dietary lectins might promote inflammatory diseases via the NLRP3 inflammasome.

Roles of reactive nitrogen intermediates and transforming growth factor-β produced by immunosuppressive macrophages in the expression of suppressor activity against T cell proliferation induced by TCR stimulation

The suppressor activity of splenic macrophages induced by Mycobacterium intracellulare infection (MI-M phi s) against T cell concanavalin A (Con A) mitogenesis is mediated by MI-M phi's mediators, such as reactive nitrogen intermediates (RNIs), phosphatidylserine, free fatty acids, prostaglandin E(2) and to a minor extent TGF-beta. Here, we have compared the roles of RNIs and TGF-beta in the expression of MI-M phi's suppressor activity against Con A mitogenesis and anti-CD3 monoclonal antibody (mAb)- and anti-CD28 mAb-induced mitogenesis (TCR signal-induced mitogenesis) of the target T cells, and have found the following. First, N(G)-monomethyl-L-arginine (NMMA) inhibited MI-M phi's suppressor activity against TCR signal-induced mitogenesis as well as Con A mitogenesis. Second, anti-TGF-beta mAb weakly restored the MI-M phi-mediated suppression only in the case of Con A mitogenesis, under limited conditions, such as very low cell densities of MI-M phi s. Third, the blocking effects of NMMA plus anti-TGF-beta mAb were somewhat more prominent in the case of Con A mitogenesis than in the case of TCR signal-induced mitogenesis. Fourth, Con A- or TCR signal-stimulated MI-M phi s secreted significant amounts of the latent TGF-beta but not the active one. These findings indicate that RNIs, but not TGF-beta, play important roles in the MI-M phi-mediated suppression of TCR signal-induced mitogenesis, as well as Con A mitogenesis, of the target T cells.

Receptor tyrosine phosphatase, CD45 binds galectin-1 but does not mediate its apoptotic signal in T cell lines

Galectin-1 (Gal-1) is an endogenous mammalian S-type lectin with highly pleiotropic effect on different tissues. The viability of the lymphoid cells is reduced by gal-1 by triggering apoptosis, however, the mechanism of the gal-1 induced apoptosis is still under investigation. The receptor tyrosine phosphatase, CD45, a heavily glycosylated cell surface molecule binds to gal-1 with high affinity, however, its contribution to the gal-1 induced apoptosis is still controversial. In this study we show that galectin-1 binds to cells deficient for CD45, although CD45 is one of the galectin-1-binding cell surface proteins on T cells. Moreover, the CD45 deficient Jurkat variant, J45.01 responds readily with tyrosine phosphorylation and subsequent apoptosis to galectin-1 treatment in a similar degree as its wild type counterpart, Jurkat does. These results strongly indicate that CD45 is not the receptor via gal-1 mediates the apoptotic signal into the cells as it was suggested in previous studies.

Growth of human cytomegalovirus in primary macrophages

Human cytomegalovirus (HCMV) is a major human pathogen that causes considerable disease among immunocompromised individuals. A primary infection results in life-long persistence of the virus in a latent form. HCMV is known to be transferred by blood products, bone marrow, and solid organs, but the cell type that carries the latent infection has been difficult to identify. We have recently demonstrated reactivation of latent HCMV in allogeneically stimulated monocyte-derived macrophages (Allo-MDM). Reactivation occurred only in macrophages produced by allogeneic but not mitogenic stimulation. The presence of dendritic cell markers on some Allo-MDM cells suggested that these macrophages were related to dendritic cells. However, dendritic cells obtained by stimulation of monocytes with interleukin-4 (IL-4) and granulocyte-macrophage colony stimulating factor (GM-CSF) were not permissive for HCMV infection. The cellular and cytokine components which are essential for HCMV replication and reactivation of virus were also examined in Allo-MDM. The importance of both CD4- or CD8-positive T cells in the generation of HCMV permissive Allo-MDM was demonstrated by negative selection or blocking experiments using antibodies directed against both HLA class I and HLA class II molecules. Examination of the cytokines essential for the generation of HCMV permissive Allo-MDM identified gamma-interferon (IFN-gamma, but not IL-1, IL-2, tumor necrosis factor alpha, or GM-CSF as critical components in the generation of these macrophages. However, addition of IFN-gamma to unstimulated macrophage cultures was insufficient to reactivate virus. These results indicate the importance of a specific moncyte stimulus in the generation of a unique HCMV permissive macrophage phenotype as well as why virus is commonly reactivated in transplant patients.

Interferon-gamma and tumor necrosis factor-alpha specifically induce formation of cytomegalovirus-permissive monocyte-derived macrophages that are refractory to the antiviral activity of these cytokines

Monocytes/macrophages are key cells in the pathogenesis of human cytomegalovirus (HCMV). Although HCMV infection in monocytes is restricted to early events of gene expression, productive infection has been demonstrated in differentiated macrophages in vitro. We examined the cellular and cytokine components that are essential for HCMV replication in Concanavalin A-stimulated monocyte-derived macrophages (MDM). By negative selection, depletion of CD8+ T lymphocytes, but not CD4+ T lymphocytes, CD19+ B cells, or CD56+ NK cells, resulted in a 60-70% reduction in the number of HCMV-infected MDM, and a 4 log decrease in virus production. Neutralization of IFN-gamma and TNF-alpha, but not IL-1, IL-2, or TGF-beta, decreased production of virus by 4 logs and 2 logs, respectively. Subsequently, addition of recombinant IFN-gamma or TNF-alpha to purified monocyte cultures was sufficient to produce HCMV-permissive MDM. While IFN-gamma and TNF-alpha possess antiviral properties, addition of these cytokines to permissive MDM cultures did not affect production of HCMV. Thus, rather than inhibiting replication of HCMV, IFN-gamma and TNF-alpha specifically induce differentiation of monocytes into HCMV-permissive MDM, which are resistant to the antiviral effects of these cytokines.

Inhibition of T cell mitogenesis by a novel anti-CD45R monoclonal antibody

CD45 consists of a major family of membrane glycoproteins which have protein tyrosine phosphatase activity and regulate early activation events, progression and maturation signals in leucocytes. Various isoforms of CD45 (Mr 180-240 kDa) regulate sets of intermolecular associations between different surface receptors, and appear to be differentially expressed on B and T cells (namely CD45RA, B or CD45RO). We describe a novel IgG2a mAb directed against restricted and unique CD45R modified epitopes expressed preferentially on peripheral blood T cells. This anti-CD45R antibody (I(2)4c) at concentrations of 50 and 200 ng/mL inhibited mitogenic T cell lectin and anti-CD3-stimulated lymphocyte proliferation and blocked associated IL-2 secretion in vitro. Phorbol ester-stimulated mitogenesis was unaltered suggesting that the inhibition occurs independent of protein kinase C-mediated pathways. Western blotting and immunoprecipitation of purified cell lysates reveals that I(2)4c preferentially binds the higher Mr bands of CD45 expressed on T cells. Following T cell activation in vitro, the 190 kDa band became more predominant and an additional 130 kDa protein, possibly a proteolytic fragment was recognized. I(2)4c may inhibit T cell mitogenesis by direct effects on CD45R alone or by preventing interaction with other membrane-associated proteins and hence adhesive interactions with monocytes. Such interactions may however inhibit the initiation of signal transduction and, as a consequence, alter cellular activation by mitogenic lectins and anti-CD3 in vitro.

Lectins inhibit the Aujeszky's disease virus-induced interferon-alpha production of porcine peripheral blood mononuclear cells

The interaction between virus and peripheral blood mononuclear cells (PBMC) required to elicit the production of interferon-alpha (IFN-alpha) by the so-called natural interferon-producing cell is unknown. However, results from inhibition experiments suggest that viral glycoproteins are essential in this IFN induction process. We demonstrate here that cellular glycoproteins also appear to be involved in the initiation of IFN-alpha production. Lectins, that is, sugar binding glycoproteins, inhibited the Aujeszky's disease virus-induced IFN-alpha production of porcine PBMC by up to 99%. The level of inhibition varied with lectin used (concanavalin A, Galanthus nivalis lectin, Helix pomatia lectin, and lentil lectin). Preincubation experiments with porcine cells and concanavalin A (ConA) revealed that the lectin exerted its major effect directly on the PBMC. Although the IFN-alpha production in some cases was reduced by more than 90%, the PBMC were still able to proliferate in response to mitogenic lectins. The ConA-mediated inhibition of the IFN-alpha production was reduced if the lectin was added later than 6-8 h after the start of induction and was not mediated by soluble factors. Both orthovanadate and staurosporine inhibited the IFN-alpha production and did not relieve the ConA-mediated inhibition. Thus, ConA seems to interfere with the early events during IFN-alpha induction, but the mechanisms behind this interference could not be clarified.

Glycosylation of CD45: carbohydrate processing through Golgi apparatus is required for cell surface expression and protein stability

The importance of glycosylation on cell surface expression, protein stability and function of the CD45 phosphotyrosine phosphatase leukocyte common antigen, has been studied in the K562 erythroleukemic cell line. Cell treatment with the N-glycosylation inhibitor tunicamycin generated unglycosylated CD45 polypeptides of 130 and 140 kDa. Immunofluorescence flow cytometry and Scatchard techniques revealed that CD45 cell surface expression was decreased in a time- and dose-dependent manner upon tunicamycin incubation. Moreover, a remarkable decrease in CD45 phosphatase activity was detected in tunicamycin-treated cells, which correlated with the diminished CD45 cell surface expression. Pulse-chase biosynthetic assays demonstrated that the fate of the unglycosylated CD45 proteins underwent a late non-lysosomal degradation, since it was not prevented either by monensin, an inhibitor of Golgi transport, or by the lysosomal function inhibitor chloroquine. These results demonstrate that intact glycosylation of CD45 molecules through Golgi compartment is required for stability and proper transport towards the plasma membrane of these phosphotyrosine phosphatase proteins.

Early increase in lymphocyte cyclic nucleotide phosphodiesterase activity upon mitogenic activation of human peripheral blood mononuclear cells

Cyclic nucleotide phosphodiesterase activity of human peripheral blood mononuclear cells was significantly increased following a short (30 min) incubation with the mitogenic lectin Concanavalin A. Con A stimulated phosphodiesterase activity to the same extent whatever the subcellular compartment (homogenate, cytosol or pellet). Further separation of the Con A-activated mononuclear cells into lymphocyte-enriched and monocyte-enriched populations showed that the Con A-induced increase of phosphodiesterase activity exclusively affected the lymphocyte-enriched population. In lymphocytes, cyclic GMP phosphodiesterase activity was more importantly enhanced by Con A (+275%) than cyclic AMP phosphodiesterase activity (+75%). The increase of both activities occurred as early as from 10 min of Con A incubation and proved to be maximal with Con A doses of 2.5 and 5 micrograms per 10(6) cells, lower and higher doses being less effective. Inhibition experiments with reference inhibitors suggested that, among the high affinity phosphodiesterase isoforms, the cyclic GMP-inhibited enzyme might be more selectively enhanced by Con A than the cyclic AMP-specific, Rolipram-sensitive one. The non-mitogenic lectin Helix pomatia hemagglutinin, was not able to enhance cyclic nucleotide phosphodiesterase activity of human mononuclear cells whereas anti-CD3 monoclonal antibody, although being less effective than Con A, exhibited a significant stimulatory effect. Putting together these results suggest that the early increase in phosphodiesterase activity might be a normal step involved in the mitogenic activation of human lymphocyte.

Respective contribution of intracellular calcium release and extracellular calcium influx for interleukin-2 synthesis in activated T-cell hybrids

Triggering of the T-cell receptor (TcR)alpha beta/CD3 receptor complex with anti-allotypic antibodies or concanavalin A (Con A) induced a rapid release of intracellular calcium in a murine T-cell hybridoma model system. Internal calcium release preceded the influx of extracellular calcium, as judged by comparative analysis of time-dependent changes in Quin 2 fluorescence following T-cell activation in the presence and absence of extracellular calcium. The magnitude of intracellular calcium release and extracellular calcium influx depended on the degree of receptor-occupancy and cross-linking. Correlations between the concentration of stimulating ligand, cytosolic calcium increase and IL-2 synthesis indicated a positive but non-linear relationship. Our data suggest that TcR cross-linking may provide a third T-cell activation signal which, in conjunction with protein kinase C activation and cytosolic calcium elevation, together form a signal triad responsible for interleukin-2 (IL-2) synthesis.

Anti-leucocyte common (CD45) antibodies inhibit dendritic cell stimulation of CD4 and CD8 T-lymphocyte proliferation

Dendritic cells (DC) act as primary antigen-presenting cells and probably initiate T-cell responses via the formation of DC-T-cell clusters. Both DC and T cells express members of the leucocyte common (CD45) family of molecules as major cell membrane proteins. The possibility that the leucocyte common antigens (LCA) play a functional role in the cellular events of a primary T-cell response was tested, by examining the effect of CD45 antibodies on the T-cell proliferative response induced by stimulating with allogeneic DC in the mixed leucocyte reaction (MLR). The presence of individual CD45 monoclonal antibodies during the MLR inhibited T-cell proliferation only weakly, but a mixture of monoclonal antibodies against different epitopes of the LCA had a more profound inhibitory effect. The effect of the CD45 antibodies was maximal during the first 24 hr and preincubation experiments indicated that the major blocking effect occurred at the T-cell level, although some inhibition after preincubation with DC was noted. Purified allogeneic CD8 T-cell responses were more susceptible to inhibition than purified CD4 cell responses, but no difference in the inhibition of the CD45RA (CMRF-11) or CD45RO (UCHL1) LCA-positive T-cell subsets was observed. The presence of CD45 antibodies did not inhibit DC-T-cell clustering but reduced cluster stability, suggesting that certain epitopes of the LCA may be involved in the process of cell adhesion.

Signal delivery by physical interaction and soluble factors from accessory cells in the induction of receptor-mediated T-cell proliferation. Synergistic effect of BSF-2/IL-6 and IL-1

Our recent study revealed that soluble factors derived from accessory cells (AC; monocytes) and physical interaction with T cells of the accessory cells are both required for the induction of the proliferation of human peripheral blood T cells by anti-CD3 antibody coupled on latex beads. The accessory cell-derived soluble factor could be replaced by IL-1 and IL-6, and the role of live macrophages for physical interaction with T cells was found to be replaceable with paraformaldehyde(PFA)-fixed macrophages, provided the macrophages were pretreated with interferon-gamma (IFN-gamma) before fixation. Quantitative analysis in the present study revealed that IL-1 and IL-6 act synergistically to induce T-cell proliferation in the above system but either one of the factors alone reveals only a marginal or weak activity. Furthermore, it was shown that the potentiating activity of the culture supernatants of monocytes was substantially inhibited by anti-IL-6 antibody. Taken together with our previous results that anti-IL-1 serum strongly inhibited the potentiating activity of the culture supernatant, these results indicate that the main responsible molecules in the culture supernatant are IL-1 and IL-6, although a presence of other effective factors is not excluded. The anti-CD3-induced thymidine uptake by T cells in the presence of IL-1 and IL-6 was significantly inhibited by anti-Tac antibody, suggesting that the proliferation of T cells in this system is mostly mediated by a IL-2-dependent pathway. Our study further showed that accessory cells seem to acquire cell surface properties necessary for the effective interaction with T cells during 6-24 hr of culture with IFN-gamma. Presumably, a certain molecule(s) required for the interaction is induced on the cell surface of the AC by IFN-gamma.

A protein kinase C pseudosubstrate peptide inhibits phosphorylation of the CD3 antigen in streptolysin-O-permeabilized human T lymphocytes

Activation of human T lymphocytes leads to the phosphorylation of the CD3-antigen gamma polypeptide. We have investigated a possible role for protein kinase C (PKC) in mediating this phosphorylation event by using T cells permeabilized with streptolysin-O in the presence of 120 mM-K+ buffers containing Ca2+-EGTA. The gamma-chain was phosphorylated by [gamma-32P]ATP in permeabilized T lymphoblasts in the presence of phorbol 12,13-dibutyrate (Pdbu) or phytohaemagglutinin (PHA). Ca2+ alone in the range 0.5-1.0 microM also induced gamma-chain phosphorylation in some T-lymphoblast preparations; that in Jurkat-6 cells occurred at lower concentrations (50-500 nM). Two experimental approaches were used to investigate the possible involvement of PKC. Firstly, when permeabilization was carried out in buffer lacking free Ca2+, PKC was lost from the cells, and gamma-chain phosphorylation could then no longer be induced on subsequent addition of Pdbu or PHA in 400 nM-Ca2+, or 800 nM-Ca2+ alone, to permeabilized cells. However, when permeabilization was carried out in the presence of these three agents, PKC was translocated to intracellular membranes, and subsequent addition of [gamma-32P]ATP to these cells then resulted in gamma-chain phosphorylation. In the second approach, induction of gamma-chain phosphorylation by Pdbu, 1-oleoyl-2-acetylglycerol, 1,2-diolein, PHA or Ca2+ alone was effectively blocked by permeabilizing T cells in the presence of a PKC pseudosubstrate peptide (50 microM). Pseudosubstrate concentrations in the range 7-20 microM inhibited gamma-chain phosphorylation by 50%. In contrast, addition of four other 'irrelevant' basic peptides (50 microM) did not result in detectable inhibition, and 50 microM-pseudosubstrate did not inhibit the phosphorylation of 17 other polypeptides isolated from permeabilized T cells. These data suggest that Pdbu-, 1,2-diacylglycerol-, PHA- and Ca2+-induced phosphorylation of the CD3-antigen gamma chain in permeabilized T cells is mediated by PKC.

Interaction of concanavalin A and a divalent derivative with lymphocytes and reconstituted lymphocyte membrane glycoproteins

Both concanavalin A (con A) and its divalent derivative, succinyl-concanavalin A (S-con A) are mitogenic for porcine lymph node lymphocytes. We have compared the binding of these two lectins to intact porcine lymphocytes and phospholipid vesicles containing reconstituted lymphocyte membrane glycoproteins. Both con A and S-con A showed high- and low-affinity binding to intact cells, as indicated by LIGAND analysis of Scatchard plots of binding data. Despite the apparently identical saccharide specificities of the two lectins, high-affinity binding sites for S-con A were only one-third as numerous as high-affinity sites for the parent lectin. Large numbers of low-affinity binding sites existed for con A, while many fewer were present for S-con A. It is suggested that these sites result from hydrophobic association. Con A bound to lymphocytes in a positively cooperative fashion, while S-con A showed noncooperative behavior. Lectin binding to large unilamellar phospholipid vesicles containing reconstituted lymphocyte membrane glycoproteins was measured using a rapid filtration assay, and was linear with the glycoprotein content of the vesicles. Almost all of the outward-facing glycoprotein was functional in terms of lectin binding. Reconstituted glycoproteins showed only a single class of high-affinity binding sites for both con A and S-con A, with association constants similar to those measured for intact cells. Con A, but not S-con A, showed positively cooperative binding to reconstituted vesicles. Cooperativity was observed in both gel phase and liquid crystalline phase lipid, and was thus not dependent on long-range lateral rearrangement of glycoprotein receptors. Results suggested that con A induces a microredistribution of receptors on the lymphocyte membrane surface, leading to the exposure of glycoproteins that were previously inaccessible to the lectin. S-Con A does not cause glycoprotein redistribution, and a large fraction of the receptors remain cryptic.

Two distinct signals regulate induction of IL-2 responsiveness in CD8+ murine T cells

In the model system used here, cross-linking of T-cell receptor structures (TCR) by antigen-presenting cells (APCs) is substituted by the use of anti-F23.1 anti-T-cell receptor monoclonal antibody immobilized on Sepharose beads. We show that CR cross-linking of resting murine CD8+ T cells seeded at low cell densities is insufficient to induce responsiveness to the growth-promoting effect of interleukin-2 (IL-2), i.e. fails to induce expression of functional IL-2 receptors. The macrophage cell-line product, IL-2 receptor-inducing factor (RIF), but not IL-1, IL-3, IL-4 and interferon-gamma (IFN-gamma) functions efficiently as a co-stimulator. Once activated, growth of CD8+ T cells is driven entirely by IL-2. We conclude that two restriction points control the activation of resting CD8+ T cells. While cross-linking of TCR is essential as the first step, RIF is required as the competence factor to induce IL-2 responsiveness. We consider the possibility that the ability of APCs to produce RIF determines the immunogenicity of APCs towards antigen-reactive resting CD8+ T cells.

Mechanistic, functional and immunopharmacological implications of biochemical studies of antigen receptor-triggered cytolytic T-lymphocyte activation

Biochemical events that follow the engagement of cytotoxic T lymphocytes (CTL) with an Ag-bearing target cell (TC) or triggering by the crosslinking of the Ag-receptor (TcR) by immobilized anti-TcR mAb were studied using cloned CTL and a novel CTL activation assay. The approach described here was undertaken to shed light on the molecular mechanisms of "ON", "STOP" and "OFF" signalling that allow CTL to be activated, kill TC and disengage from the target cell after delivery of the "lethal hit" and then to proceed with the destruction of the next Ag-bearing target encountered. Biochemical studies of TcR-regulated and TcR-triggered constitutive exocytosis in CTL provided a detailed description of the molecular requirements for this important phenomenon in T lymphocytes and provided an alternative CTL activation assay; this assay measures the TcR-dependent response in the absence of a TC. These studies also helped to envision CTLs screening activities as a cycle of engagements-disengagements with the TC, where every surrounding cell is treated by the CTL as a potential Ag-bearing TC. Both constitutive and regulated exocytosis in CTL are triggered through a transmembrane signalling pathway which involves protein kinase C and extracellular Ca2+ that, most likely, is translocated through Ca2+ channels. This is followed by the involvement of calmodulin (CaM)-binding proteins, e.g., calcineurin, a CaM-dependent phosphatase, which was shown to be a major CaM-binding protein in murine lymphocytes. Unexpectedly, these biochemical studies demonstrated that the granule exocytosis model of CTL-mediated cytotoxicity cannot account for the mechanism of target cell lysis by CTL, at least in in vitro conditions in the absence of extracellular Ca2+. These results indicate the existence of an extracellular Ca2+-independent, TcR-regulated CTL response and raise the possibility that second messenger(s) other than Ca2+ and/or products of phosphoinositide turnover are involved in T-cell lysis. Predominance of "non-lethal" engagements between some CTL and TC, revealed during time-lapse cinematographic studies, together with comparative studies of TcR-regulated exocytosis of granules and of constitutive exocytosis of gamma-interferon, suggested that TC destruction by CTL may not be their only or even their most important function in vivo. It is possible that CTL, triggered by Ag recognition to exocytose storage granules and to synthesize and constitutively exocytose macrophage-activating factors, in turn promote tumor destruction by macrophages.(ABSTRACT TRUNCATED AT 400 WORDS)

Primary activation of murine CD8 T cells via cross-linking of T3 cell surface structures: two signals regulate induction of interleukin 2 responsiveness

In the model system used here to study the minimal signal requirements for the activation of murine resting CD8 T cells, cross-linking of T cell receptor structures by antigen-presenting cells is substituted for by the use of anti-CD3 monoclonal antibodies immobilized in Sepharose beads. We show that cross-linking of CD3 structures, even in combination with CD8 structures, is necessary but insufficient to induce responsiveness to the growth-promoting effect of interleukin 2 (IL2), i.e. fails to induce expression of functional IL2 receptors. A macrophage cell line product termed IL2 receptor-inducing factor (RIF), but not IL1, IL3, IL4 or tumor necrosis factor, efficiently functions as costimulator. Once activated, growth of CD8 T cells is entirely driven by IL2. We conclude that two restriction points control the activation of resting CD8 T cells. While cross-linking of CD3 structures is essential as a first step, RIF is required as competence factor to induce IL2 responsiveness. We consider the possibility that the ability of antigen-presenting cells to produce RIF determines the immunogenicity of presented antigen towards antigen-reactive resting CD8 T cells.

Analysis of differentiation and transformation of cells by lectins

During differentiation cells are known to change their biological behavior according to their genotype. This is thought to be accompanied by a modulation of cell surface determinants expressed on the outer cell membrane. Vice versa, cell surface molecules are suggested to mediate extracellular signals to the genome. Most of these molecules integrated in the cell membrane have been proven to be glycoconjugates. The carbohydrate moieties of these molecules can be detected by means of lectins that are characterized by their ability to react specifically with distinct terminal sugar sequences. Thus, lectins have been used as appropriate tools for studying the modulation of functionally important membrane-associated molecules during the differentiation of cells, in particular of B- and T-lymphocytes. Moreover, lectins have been proven to distinguish between differentiated cells and malignant cell clones, according to the hypothesis that transformed cells possess a glycoconjugate profile that corresponds to the stage of differentiation at which they are arrested. Since lectins, like monoclonal antibodies, make it possible to study functionally important molecules that are associated with differentiation and malignancy, they might be of value for diagnostic purposes and, moreover, for analyzing malignant transformation.

Evidence that Thy-1 and Ly-5 (T-200) antigens interact with sulphated carbohydrates

Recent studies have demonstrated that lymphocytes express an array of cell surface receptors for sulphated polysaccharides (SP). Experiments were undertaken to determine the binding characteristics of these receptors and establish whether any known lymphocyte cell surface antigens interact with sulphated carbohydrates. It was found that murine thymocytes lack receptors for chondroitin-4-sulphate but express saturable, high affinity binding sites for heparin, fucoidan and dextran sulphate, with an apparent affinity constant range of 0.03-2.6 x 10(-9) mol/l. Binding inhibition experiments revealed one class of binding sites on murine thymocytes that is shared by heparin, fucoidan and dextran sulphate and another class of sites that is dextran sulphate-specific. The cell surface receptors for the SP were affinity-purified by applying detergent lysates of 125I-labelled thymocyte membranes to SP-coupled solid supports. It was found that the Thy-1 and Ly-5 (T-200 or leucocyte common antigen) molecules of murine thymocytes bind to sulphated carbohydrates, although the two molecules differed substantially in their reactivity with the four different SP tested. Furthermore, only subpopulations of the Thy-1 and Ly-5 molecules interacted with sulphated sugars. Four additional sulphated carbohydrate-binding molecules were also detected. It is suggested that the SP-binding molecules are involved in the interaction of lymphocytes with glycosaminoglycans on other cells and in the interstitial space.

Interaction between the CD45 antigen and phytohemagglutinin. Inhibitory effect on the lectin-induced T cell proliferation by anti-CD45 monoclonal antibody

Immunoprecipitation studies from 125I-labeled T cells, previously activated with the lectin phytohemagglutinin (PHA) from Phaseolus vulgaris, showed a preferential association between the CD45 glycoprotein family containing members of 220, 205, 190 and 180 kDa, and a protein of 33 kDa. This 33-kDa protein, with an identical molecular mass as PHA, was not associated with other highly expressed molecules such as class I histocompatibility antigens (HLA-A,B,C), transferrin receptor, CDw44 and CD11a. Further immunoprecipitation analysis from peripheral blood lymphocytes incubated with 125I-labeled PHA confirmed the identification of the CD45-associated 33-kDa protein as the lectin PHA. These results prompted us to analyze the possible involvement of the CD45 molecules in the T cell activation process induced by PHA. Thus, two monoclonal antibodies specific for CD45 antigens were able to inhibit the T cell proliferation induced by the lectin. This inhibitory ability was exerted by affecting both the interleukin 2 production and the interleukin 2 receptor expression.

A role of L3T4+ antigen in the Con A response of regenerating splenic L3T4+ T cells after Cy treatment

The role of L3T4 antigens in the concanavalin A (Con A) response of regenerating spleen cells (Cy-SCs) after cyclophosphamide (Cy) treatment was studied. Anti-L3T4 monoclonal antibodies (Mabs) markedly inhibited the Con A response of the regenerating Cy-SCs, which do not require Ia molecules expressed on accessory cells (ACs) for Con A activation. However, the Con A response of normal spleen cells (N-SCs), which do require Ia molecules on ACs, was not inhibited by the same Mabs, although the Con A response of N-SCs, as well as that of Cy-SCs, was demonstrated to be mediated by L3T4+ T cells. The optimal times for the inhibitory effect of anti-L3T4 Mab was 7 days after Cy treatment, when the number of spleen cells increased to a maximum following a regenerative phase. Its inhibitory effect was reduced by high concentrations of Con A, and was restricted to the early phase of the Con A response. A short time exposure of the Cy-SCs to the anti-L3T4 Mabs was sufficient to decrease the response to Con A. Our results cannot explain the hypothesis that the L3T4 molecule functions solely by interacting with non-polymorphic parts of Ia molecules on ACs. Taken together, these results and those of other groups of investigators suggest that Con A-induced T-cell activation may be mediated by at least two or more interaction mechanisms involving either Ia or L3T4 molecules. Firstly, normal L3T4+ T cells may mainly interact with Con A involving self Ia molecules on the ACs. The extent of this interaction is sufficient to induce T-cell activation, and then does not need another L3T4 molecule. Secondly, the regenerating L3T4+ T cells may usually interact with the cell surface antigens of other T cells, including L3T4+, by the binding of both cell surface molecules to Con A in the absence of ACs, and then transmit a signal for T-cell activation. Anti-L3T4 Mabs may exert inhibitory effects somewhere in this process.

Antigen-receptor interaction requirement for conjugate formation and lethal-hit triggering by cytotoxic T lymphocytes can be bypassed by protein kinase C activators and Ca2+ ionophores

We show that phorbol esters and Ca2+ ionophores can trigger the lysis of nonantigen-bearing target cells by cytotoxic T lymphocytes. This effect obviates the requirement for antigen-receptor-mediated recognition of the antigen; the intensity of lysis is dose and Ca2+ dependent and requires contact between cytotoxic T lymphocytes and target cells. Using a fluorescence-activated cell sorter to enumerate cytotoxic T lymphocyte-target cell conjugates, we show that phorbol esters at concentrations that triggered lysis of non-antigen-bearing target cells also increased the number of stable conjugates with these target cells. The results point to the importance of the antigen-nonspecific engagements of cytotoxic T lymphocytes in immunologic surveillance. The data also show that the linkage between the T-cell receptor and antigen is not mandatory for conjugate formation, for the strengthening of conjugates, and for lysis.

Pleiotropic loss of activation pathways in a T-cell receptor alpha-chain deletion variant of a cytolytic T-cell clone

Untransformed T-cell clones maintained in culture are dependent on signals transmitted through their antigen receptors (Ti; alpha and beta chains associated with the CD3 molecules) for growth and effector function. For cytolytic T cells (CTL), Ti stimulation also activates the killing machinery and induces synthesis of gamma interferon (IFN-gamma) messenger RNA and IFN-gamma secretion. The Thy-1 molecule, expressed on all murine cells of the T-cell lineage, has been suggested to function in transmembrane signalling, based on the ability of some anti-Thy-1 monoclonal antibodies (mAb) to activate T cells. Recently, it was suggested that Thy-1 could function as a signal-transduction molecule when expressed in B-cell lymphomas after transfection of the gene, leading to speculation that the molecule was part of an activation pathway independent of the Ti/CD3 structures. Here we report the immunoselection of a variant CTL clone which has lost expression of mRNA for the alpha-chain of the Ti. The Ti- variant was defective in lectin-mediated activation whether measured by increase in intracytoplasmic Ca2+, CTL effector function or IFN-gamma synthesis. The variant, which expressed normal levels of Thy-1, was also unresponsive to Thy-1 mAb activation as measured by IFN-gamma secretion, whereas it responded to calcium ionophore plus phorbol ester. These results indicate that in a non-transformed, functional mature T-cell, Thy-1 mediated signalling is not an alternative to, but might depend on elements associated with the Ti/CD3-mediated T-cell activation pathway.

Cell growth cycle block of T cell hybridomas upon activation with antigen

Stimulation of antigen-specific T cell hybridomas with the appropriate antigen/MHC combination, at concentrations that resulted in the secretion of the lymphokine interleukin 2, resulted in a dose-dependent decrease in both [3H]thymidine incorporation and cell growth. Flow cytometric studies demonstrated that stimulation with antigen resulted in a cell cycle block that was most evident at the G1/S border, and mixing studies revealed that bystander T cells of different antigen specificities were not affected. For at least the large majority of T cells, the G1/S cell cycle block appeared to be irreversible after 24 h of exposure to antigen. This cell cycle block may be useful as a rapid and quantitative measure of T cell hybridoma activation, as a means of selecting T cell hybridomas that have functional alterations in the reception of stimulatory signals, and may serve as a model of the induction of tolerance in immature T cells.

Cell surface molecules and early events involved in human T lymphocyte activation

The physiologic activation of human T cells by antigen involves events that occur between ligands and receptors at the interface of the T cell and antigen-presenting cell (or target cell). These events have been examined by identifying the cell surface receptors involved in such interactions using mAb. Whereas the T3/T cell antigen receptor plays a central role in such interactions, other T cell receptors have been identified which may also contribute to T cell activation in providing primary activation signals or by functioning as accessory molecules. Although the ligands of these other receptors are currently unknown or ill defined, it is likely that this will provide a fruitful area of investigation. The use of mAb as probes to mimic these putative ligands has facilitated the study of the requirements for activation and the biochemical events initiated by the receptors involved. The T cell receptor, a multisubunit complex, has been most intensively studied. Ligands that bind to T3/Ti cannot initiate activation by themselves and require the participation of accessory molecules. Stimulation of T3/Ti results in the formation of at least two potent intracellular second messengers, IP3 and DG, through the hydrolysis of PIP2. These second messengers, in turn, induce an increase in [Ca2+]i and the activation of pkC. These two events appear to be essential in the transcriptional activation of certain targeted genes through ill-defined pathways leading to the manifestations of T cell activation.

NK-9, a distinct sialylated antigen of the T200 family

The nature and properties of antigens detected by a novel monoclonal antibody, NK-9, were studied. The NK-9 antigens had apparent molecular masses of 190, 200 and 220 kDa and were sensitive to neuraminidase and sodium metaperiodate treatments, which destroy the sialic acid residues of the cell surface glycoproteins. Trypsin treatment also removed the NK-9 reactivity, but the antigens were restored within a few hours thereafter. Tunicamycin, which inhibits the N-linked glycosylation after neuraminidase treatment, had no effect on the reappearance of the NK-9 positivity. Neither did endoglycosidase F, which removes the N-linked sugars, abolish the NK-9 antigenicity. Monensin, which blocks the cellular secretion, inhibited the restoration of the antigens, and monensin block also without preceding treatment with neuraminidase led to the disappearance of NK-9 reactivity, suggesting possible recycling molecules as carriers of the NK-9 detected epitopes. The NK-9 antigens appear to belong to the T200 antigen family, but are distinct from the antigens reactive with the available anti-T200 antibodies anti-LC, T29/33 and HLe-1, based on their different cell type distribution and absence of cross-reactivity in sequential immunoprecipitations.

Glycosylation of the T-cell antigen-specific receptor and its potential role in lectin-mediated cytotoxicity

Cytotoxic T lymphocytes (CTLs) normally destroy only those cells ("target cells") whose surface antigens they recognize. However, in the presence of lectins such as Con A, CTLs destroy virtually any cell, regardless of its antigens. The oligosaccharides of the T-cell antigen-specific receptor, a dimeric surface glycoprotein composed of disulfide-linked alpha and beta subunits, are of interest because of their potential involvement in this lectin-dependent cytotoxic activity. We report here that three or four asparagine-linked oligosaccharides could be enzymatically removed from each of the receptor subunits expressed by a cloned line of murine CTLs (clone 2C), consistent with the presence of glycosylation sites deduced from cDNA sequences of the alpha and beta genes expressed in this clone. All the N-linked glycans on the alpha subunit were of the complex type (i.e., resistant to endoglycosidase H), but the beta subunit carried two or three endoglycosidase H-sensitive (high-mannose) oligosaccharides. High-mannose glycans can bind tightly to Con A and, indeed, this lectin was found to bind specifically to solubilized 2C T-cell receptor. The Con A-dependent cytotoxic activity of clone 2C, but not of other CTL clones, was inhibited by a monoclonal antibody (1B2) that is specific for the T-cell receptor of clone 2C. Antibody 1B2 also inhibited clone 2C cytotoxicity mediated by phytohemagglutinin, lentil-lectin, and wheat-germ agglutinin. These results suggest that, although lectin-dependent lysis of target cells by CTLs is antigen nonspecific, the cytolytic activity can be triggered by binding of the lectin to the T-cell antigen-specific receptor.

Activation of resting T lymphocytes by a monoclonal antibody directed against an allotypic determinant on the T cell receptor

The murine monoclonal antibody F23.1 reacts with an allotypic determinant on the beta chain of the T cell receptor expressed by approximately 20% of T helper and cytotoxic T lymphocytes of most common mouse strains. This IgG2a antibody, either in soluble form or covalently coupled to Sepharose beads, can activate resting T cells from naive animals to proliferate. Interestingly, under all conditions of activation, the antibody can only induce proliferation if exogenous lymphokines in the form of Con A supernatant are provided. Thus, it is unlike most lectins and anti-T3 antibodies in this regard. Furthermore, under all conditions of culture, F23.1 activates preferentially the Lyt-2+ subset of T cells. This is the case even in the presence of accessory cells. Further evidence is provided that two soluble lymphokines, different from IL2, are required to initiate IL2-dependent growth and to allow the expression of lytic activity.

Monoclonal antibodies specific for H-2K and H-2D antigens on cytotoxic T cells can inhibit their function

Antibodies specific for murine major histocompatibility gene complex (MHC) class I H-2K and H-2D molecules present on cytotoxic T (Tc) cells have been shown to inhibit their function of target cell lysis. This could only be demonstrated by using a more sensitive assay for T-cell-mediated lysis, and many monoclonal antibodies of different Ig class, origin, and specificity can be shown to inhibit alloreactive as well as MHC-restricted Tc cells. These antibodies inhibit different activated T-cell populations to varying extents, and anti-H-2K but not anti-H-2D antibodies show a synergistic effect with anti-Lyt-2 antibodies. Data here suggest that MHC molecules may be located in or near the T-cell receptor complex on these cells.

A monoclonal antibody against altered LFA-1 induces proliferation and lymphokine release of cloned T cells

A murine monoclonal antibody (I-17, IgM) has the following functional effects on murine long-term T cell clones: inhibition of cell-mediated lysis, induction of proliferation, release of lymphokines and change of the cell morphology. The determinant detected by I-17 is expressed on long-term T lines but not on thymocytes, lymph node cells and spleen cells. I-17 precipitated proteins with apparent molecular mass of 220 kDa, 170 kDa, 150 kDa and 100 kDa. Biochemical studies indicate that the determinant recognized by I-17 is tunicamycin sensitive and that I-17 binds to the alpha chain of the lymphocyte function-associated antigen (LFA-1).

Activation of Lyt-2+ T cells by antibodies towards brain-associated antigens. II. Antibody-dependent induction of "nonspecific" cell-mediated cytotoxicity

Purified IgG from a rabbit anti-mouse brain antiserum (RaMB), previously shown to activate most Lyt-2+ T cells, was probed for lectin-like effects on cell-mediated cytolysis (CML) and found to induce "nonspecific" killing of syngeneic B cell targets by allospecific cytotoxic T lymphocytes (CTL) as effectively as concanavalin A, and at levels comparable to antigen-specific cytolysis of target cells. Interestingly, RaMB-mediated "nonspecific" CML of syngeneic targets, by polyclonally or specifically activated CTL, was restricted to B cell targets and cold target inhibition experiments indicated that syngeneic target cell blasts do not functionally interact with effector CTL in the presence of RaMB. The role of target cell Fc receptors was demonstrated by the competitive inhibition of RaMB-dependent CML by normal rabbit IgG. We conclude that RaMB both activates and bridges the effector CTL to target cells, RaMB-mediated "nonspecific" CML being a similar phenomenon to lectin-facilitated "nonspecific" cytolysis, and the mirror image of classical ADCC. These observations allowed us to interpret the effects of RaMB on allo-major histocompatibility complex-specific CML. Since RaMB stimulates specific CML of B cell targets while selectively blocking that of T cell targets, we conclude that RaMB antibodies interact with structures associated with the CTL antigen receptor, explaining the contradictory effects previously reported with monoclonal antibodies against the CTL receptor complex, which can both stimulate and inhibit T cell functions.

Functional modifications of cytotoxic T-lymphocyte T200 glycoprotein recognized by monoclonal antibodies

Plasma membrane glycoproteins of cytotoxic T lymphocytes (CTLs) are involved in the binding to and subsequent destruction of appropriate target cells. The electrophoretic profile of surface proteins of mature CTLs, particularly those of high relative molecular mass (Mr), is markedly different from that of naive peripheral T cells or non-cytolytic T cells, suggesting the possible involvement of these molecules in the activation of CTLs and/or in the lytic process itself. By generating monoclonal antibodies to cell-surface proteins of CTL clones, we have now detected CTL-specific modifications in one of these high-Mr membrane proteins, T200. Although forms of T200 are found on a wide variety of cell types, the neoantigenic determinants recognized by our antibodies are present exclusively on activated T cells and in high concentrations only on CTLs. Furthermore, the expression of the modifications recognized by our antibodies is influenced by soluble factors and also seems to have functional significance, as monoclonal antibodies specific for these novel epitopes block cytolytic activity.

Cell surface structures involved in T cell activation

We have discussed four specific models which provide different kinds of information about the requirements for T cell activation. The first utilized a CTL clone designated L3, which is reactive specifically with Ld alloantigen, to study the involvement of the associative recognition structure Lyt-2 in cytolysis. The apparent requirements for activation of this CTL clone differ depending on whether the target cells bear specific alloantigen or are hybridoma cells which express on their cell surface a clonotypic antibody which reacts specifically with the L3 T cell receptor for antigen. When the antigen receptor reacts with alloantigen on the allogeneic target cell, cytolysis is inhibited by anti-Lyt-2 antibody. However, when the clonotypic antibody of the target cell reacts with the antigen receptor of the T cell, cytolysis is much less inhibited by anti-Lyt-2 antibody. The antigen receptor seems to be responsible for the specificity of both these interactions but the avidity of the interaction between CTL and target cell seems to differ in the two situations. Evidence that participation of the L3T4 associative recognition structure on HTL is less important for cloned T cells which have higher affinity antigen receptors was provided by the second model system which used cloned HTL selected for optimal responses to different concentrations of nominal antigen. Proliferative responses of those clones which responded to lower antigen concentrations were less readily inhibited by anti-L3T4 mAb. Evidence provided by these two model systems is consistent with the concept that associative recognition structures are of lesser importance for T cell activation for those T cells which have higher affinity antigen receptors. In the third model system, we have identified several monoclonal antibodies which augment proliferative response of cloned T cells to sub-optimal amounts of IL-2, probably by reacting with the antigen receptor or with the associated Leu-4/T3 structure. The reactivity patterns of these antibodies indicate that several different epitopes are being recognized. Some appear to be clonotypic although they do not block functional activity of the clone with which they react. Others react with all T clones which we have tested. Several of these react with a cell surface antigen which is expressed at about the same level as the clonotypic structures: these antibodies may react with the murine equivalent of the human Leu-4/T3 molecular complex. One of the "pan-T cell" antibodies which augments IL-2-induced T cell proliferation appears to react with Thy-1; this antibody is similar to one described recently by Gunter et al. (1984).(ABSTRACT TRUNCATED AT 400 WORDS)