Crosslinking of surface antigens causes mobilization of intracellular ionized calcium in T lymphocytes

The lymphocyte glycoprotein CD6 contains a repeated domain structure characteristic of a new family of cell surface and secreted proteins

The isolation, characterization, and expression of a full-length cDNA encoding the human T cell glycoprotein CD6 is described. COS cells transfected with the CD6 clone express a 90-kD protein that reacts with all available anti-CD6 monoclonal antibodies. RNA blot hybridization analysis indicates that CD6 transcripts are predominantly restricted to cells in the T lineage. The predicted CD6 sequence is 468 amino acids long, with the typical features of a type I integral membrane protein. The cytoplasmic domain of CD6 contains two serine residues, one or both of which are substrates for phosphorylation during T cell activation. The extracellular domain of CD6 is significantly related to the extracellular domain of the human and mouse T cell antigen CD5, the cysteine-rich domain of the bovine and mouse type I macrophage scavenger receptor, the extracellular domain of the sea urchin spermatozoa protein that crosslinks the egg peptide speract, the mammalian complement factor 1, and the human lung tumor antigen L3. These molecules, therefore, constitute a new gene superfamily that is well conserved across species boundaries.

Homotypic aggregation of human cell lines by HLA class II-, class Ia- and HLA-G-specific monoclonal antibodies

Major histocompatibility complex (MHC) class II molecules have been implicated in cell adhesion in two ways. In addition to the well-established role of class II antigens in low-affinity adhesion provided by interactions between class II and CD4, recent data indicated that class II may also induce adhesion between T and B cells by activating the CD18/CD11a (LFA-1) adhesion pathway. Here we report that monoclonal antibodies (mAb) against HLA-DR (L243, p4.1, HB10a, VI15) and certain broad class II reacting mAb (TU35, TU39), but not anti-DQ (TU22, Leu-10) mAb, induced homotypic aggregation of human class II-positive monocytic (I937) and T leukemic (HUT78) tumor cell lines and Epstein-Barr virus (EBV) transformed B-lymphoid cell lines (EBV-LCL). Class II-negative cell lines (U-937 and the EBV-LCL mutant line 616) were not induced to aggregate. An HLA-G-transfected EBV-LCL, 221-AGN, but not the class I-negative parental line, 221, showed homotypic aggregation in response to an HLA-G specific mAb (87G) and a broad reacting class I-specific mAb (IOT2). Both cell lines responded with aggregation to anti-class II mAb (TU35). The anti-class I mAb, W6/32, had no effect on all cell lines tested and two anti-beta 2-microglobulin mAb had variable, weak effects. The aggregation response was an active, temperature-sensitive process which was almost totally abrogated by azide and by cytochalasins B and E, but unaffected by colchicine, EDTA, aphidicolin, actinomycin D and protein tyrosine kinase inhibitors (genistein, herbimycin A). Serine/threonine protein kinase inhibitors (staurosporin, H7) partly inhibited the aggregation responses. There was no strict correlation between induction of aggregation and epitope density. FcR were not involved in the aggregation response, since F(ab')2 fragments of anti-DR mAb, L243, were as effective as the whole antibody. The aggregation was not influenced by mAb against accessory molecules previously shown to be involved directly or indirectly in homotypic aggregation [CD11a (LFA-1)/CD18/CD54 (ICAM-1), CD58 (LFA-3)/CD2, BB1/CD28, CD43, and CD44]. In conclusion, these data provide further evidence that HLA molecules are implicated in a novel, cellular aggregation phenomenon involving the cytoskeleton.

CTLA-4 is a second receptor for the B cell activation antigen B7

Functional interactions between T and B lymphocytes are necessary for optimal activation of an immune response. Recently, the T lymphocyte receptor CD28 was shown to bind the B7 counter-receptor on activated B lymphocytes, and subsequently to costimulate interleukin 2 production and T cell proliferation. CTLA-4 is a predicted membrane receptor from cytotoxic T cells that is homologous to CD28 and whose gene maps to the same chromosomal band as the gene for CD28. It is not known, however, if CD28 and CTLA-4 also share functional properties. To investigate functional properties of CTLA-4, we have produced a soluble genetic fusion between the extracellular domain of CTLA-4 and an immunoglobulin C gamma chain. Here, we show that the fusion protein encoded by this construct, CTLA4Ig, bound specifically to B7-transfected Chinese hamster ovary cells and to lymphoblastoid cells. CTLA4Ig also immunoprecipitated B7 from cell surface 125I-labeled extracts of these cells. The avidity of 125I-labeled B7Ig fusion protein for immobilized CTLA4Ig was estimated (Kd approximately 12 nM). Finally, we show that CTLA4Ig was a potent inhibitor of in vitro immune responses dependent upon cellular interactions between T and B lymphocytes. These findings provide direct evidence that, like its structural homologue CD28, CTLA-4 is able to bind the B7 counter-receptor on activated B cells. Lymphocyte interactions involving the B7 counter-receptor are functionally important for alloantigen responses in vitro.

Mutations in CD8 that affect interactions with HLA class I and monoclonal anti-CD8 antibodies

The T cell co-receptor, CD8, binds to the alpha 3 domain of HLA class I (Salter, R.D., R.J. Benjamin, P.K. Wesley, S.E. Buxton, T.P.J. Garrett, C. Clayberger, A.M. Krensky, A.M. Norman, D.R. Littman, and P. Parham. 1990. Nature [Lond.]. 345:41; Connolly, J.M., T.A. Potter, E.M. Wormstall, and T.H. Hansen. 1988. J. Exp. Med. 168:325; and Potter, T.A., T.V. Rajan, R.F. Dick II, and J.A. Bluestone. 1989. Nature [Lond.]. 337:73). To identify regions of CD8 that are important for binding to HLA class I, we performed a mutational analysis of the CD8 molecule in the immunoglobulin (Ig)-like variable domain. Our mutational analysis was based on our finding that using a cell-cell adhesion assay murine CD8 (Lyt-2) did not bind to human class I. Since the interaction of human CD8 with HLA class I is species specific, we substituted nonconservative amino acids from mouse CD8 and analyzed the ability of the mutated CD8 molecules expressed in COS 7 cells to bind HLA class I-bearing B lymphoblastoid cells, UC. Mutants with the greatest effect on binding were located in a portion of the molecule homologous to the first and second hypervariable regions of an antibody combining site. In addition, a panel of 12 anti-CD8 monoclonal antibodies were used to stain the 10 CD8 mutants, and amino acids that affected antibody binding were localized on the crystal structure of the Bence-Jones homodimer, REI. Support for an Ig-like structure of CD8 can be found in the pattern of substitutions affecting antibody binding. This work supports the similar tertiary structure of the CD8 alpha-terminal domain and an Ig variable domain.

The B-cell surface protein CD72/Lyb-2 is the ligand for CD5

The glycoprotein CD5 is expressed on the surface membrane of all mature T cells and a small proportion of B lymphocytes. Its exact role in immune interactions is still unknown. Studies indicate that CD5 functions both in mice and humans as a receptor, delivering co-stimulatory signals to T cells in a manner similar to CD2 (ref. 11) and CD28 (ref. 12). Anti-CD5 antibodies stimulate both T-cell proliferation mediated by CD3 in association with the T-cell receptor and secretion of interleukin-2 and expression of its receptor, as well as inducing an increase in intracellular Ca2+ concentration (refs 5-10). To identify the ligand for CD5 we purified the human CD5 protein, labelled it with biotin and used it as a probe. Here we report that CD5 specifically interacts with the cell-surface protein CD72 exclusive to B cells. This interaction is blocked by anti-CD72 antibodies, but not by any other anti-B-cell antibodies. Moreover, non-B cells (mouse L-cell fibroblasts and human Jurkat T cells) expressing a transfected human CD72 complementary DNA could bind to the CD5-biotin conjugate. The results demonstrate that the B-cell surface protein CD72 (Lyb-2 in mice) is the ligand for CD5.

Cholera toxin-mediated inhibition of signalling in Jurkat cells is followed by, but not due to a loss of T cell receptor complex

Cholera toxin treatment of the human T cell lymphoma Jurkat resulted in inhibition of signalling via the T cell antigen receptor complex (TcR/CD3-complex). Cholera toxin specifically ADP-ribosylated the alpha-subunit of the stimulatory G-protein of the adenylate cyclase (Gs alpha), no other proteins were modified in the intact cells. ADP-ribosylation of Gs alpha and its subsequent activation led to an increase of the cyclic AMP level and in addition, to a drastic reduction of the cell-surface density of the TcR/CD3-complex. Recently, we demonstrated that the effect of cholera toxin at the receptor level is not due to an increased cAMP level (4). As inhibition of signalling is also not cAMP-mediated (8), we examined whether the modulation of the TcR/CD3-complex could be the reason for the interruption of the signalling cascade. Analyzing the time courses of the multiple cholera toxin effects in Jurkat cells at 37 degrees C, the following sequence was found: ADP-ribosylation of Gs alpha--increase of cyclic AMP level--inhibition of signalling via the TcR/CD3-complex--decrease of cell-surface density of the TcR/CD3-complex. Treatment of Jurkat cells at 20 degrees C with cholera toxin resulted in an increase of cyclic AMP and inhibition of signal transduction, while no decrease of TcR/CD3-complex density could be observed. These data imply that receptor loss from the cell-surface is not causative for the inhibition of signalling. More likely, activation of Gs uncouples signal transduction in Jurkat cells via the TcR, which by a so far unknown mechanism is followed by a loss of the receptor from the cell surface.

Direct involvement of CD7 (gp40) in activation of TcR gamma/delta+ T cells

In this study we reported that on T cell receptor (TcR) gamma/delta+ cells from three cell lines Peer, MOLT-13 and ICRF-1, the T cell antigen CD7 (gp40) can be directly involved in the activation process. This is shown by a rapid increase in cytoplasmic free calcium after stimulation of these cells with an anti-CD7 monoclonal antibody (mAb). Activation through CD7 was further confirmed by measuring the production of interleukin 2 in ICRF-1 cells stimulated with anti-CD7 mAb. In addition induction of mRNA for tumor necrosis factor (TNF)-alpha and TNF-beta in Peer and for granulocyte-macrophage-colony-stimulating factor in MOLT-13 was observed in these anti-CD7-stimulated cells. The same anti-CD7 antibody was unable to activate TcR alpha/beta+ Jurkat cells or normal resting peripheral blood T lymphocytes. We further showed that normal resting TcR gamma/delta+ cells were likewise activated via the CD7 molecule. TcR gamma/delta+ cells obtained from a patient with acute lymphoblastic leukemia 3 months after autologous bone marrow transplantation were induced to proliferate, as measured by [3H]thymidine incorporation after stimulation with anti-CD7 mAb but not with anti-CD3 mAb. Interestingly TcR alpha/beta+ cells from the same donor tested in parallel were not stimulated by anti-CD7 but by anti-CD3 mAb. In essence these findings contribute to the idea that on TcR gamma/delta+ cell, the CD7 antigen could play an important role during T cell differentiation.

Calcium signals in single T cells on activation by lectin

Succinylation of concanavalin A (Con A) reduces its oligomer size while retaining its mitogenicity, and provides a probe of T cell activation. We have observed responses of cytosolic ionized calcium to succinyl Con A in suspensions of Jurkat and rat lymph node (LN) cells, using a fluorimeter, and in single cells settled on glass, using a dual wavelength video imaging system. In the fluorimeter a mitogenic level of succinyl Con A (30 micrograms/ml) produced only a 15-30 nM rise in average cell calcium in the suspended Jurkat or rat cells whereas the use of quantitative video imaging produced asynchronous 250-1000 nM pulses of free calcium in 35% of Jurkat cells and 300-850 nM pulses in 45% of rat LN cells. In Jurkat cells these pulses were sometimes repetitive, giving rise to apparent oscillations. In the fluorimeter 30 micrograms/ml of native Con A (a supra-mitogenic concentration) produced a 300 nM rise in average cell calcium in suspended Jurkat cells, and a 100 nM rise in rat LN cells; when major histocompatibility complex class II-bearing cells were removed the response rose. Mitogenic Con A (3 micrograms/ml) produced a much lower rise in calcium. With video imaging the response seen was greater. Levels greater than 30 micrograms/ml Con A caused 700-5000 nM pulses synchronously in 94% of Jurkat cells and 250-1000 nM pulses in 73% of rat LN cells. At 3 micrograms/ml Con A produced asynchronous 300-1100 nM pulses in 36% of rat LN cells. We conclude that the absence of a calcium signal in the fluorimeter can conceal asynchronous calcium responses in individual cells and that brief asynchronous cytosolic calcium pulses are sufficient for lectin to activate rat T cells.

Guanine ribonucleotide depletion inhibits T cell activation. Mechanism of action of the immunosuppressive drug mizoribine

The immunosuppressive drug, mizoribine, has been used to prevent rejection of organ allografts in humans and in animal models. Based on studies in cell lines, mizoribine has been postulated to be an inhibitor of inosine monophosphate (IMP) dehydrogenase (EC1.2.1.14), a pivotal enzyme in the formation of guanine ribonucleotides from IMP. To further characterize the mechanism of action of this drug, we studied the effect of mizoribine on human peripheral blood T cells stimulated with alloantigen, anti-CD3 MAb, or pharmacologic mitogens. Mizoribine (1-50 micrograms/ml) was able to inhibit T cell proliferation by 10-100% in a dose-dependent fashion to all stimuli tested. Measurements of purine ribonucleotide pools by HPLC showed that mizoribine led to a decrease in intracellular GTP levels, and that repletion of GTP reversed its antiproliferative effects. We also examined sequential events occurring after T cell stimulation. Early events in T cell activation, as assessed by steady-state mRNA levels of c-myc, IL-2, c-myb, histone, and cdc2 kinase, as well as surface IL-2 receptor expression, were unaffected. However, cell cycle analysis revealed decreased numbers of cells in S, G2, and M phases, and showed that the G1/S block was reversed with GTP repletion. These data indicate that mizoribine has an effect on T cell proliferation by a mechanism distinct from that of cyclosporine or corticosteroids, and therefore may be useful in combination immunosuppressive regimens.

Immobilized anti-CD5 together with prolonged activation of protein kinase C induce interleukin 2-dependent T cell growth: evidence for signal transduction through CD5

Monoclonal antibodies (mAb) identifying the CD5 antigen were used to stimulate human peripheral blood T lymphocytes. Three out of three anti-CD5 mAb, 10.2, OKT1 and anti-Leu-1 induced vigorous proliferation of purified T cells in the presence of 1.6 nM phorbol 12-myristate 13-acetate (PMA). Immobilization of anti-CD5 mAb on a solid support was necessary for the induction of a proliferative response. Neither 1.6 nM PMA, nor immobilized anti-CD5 mAb were mitogenic as a sole stimulus. mAb identifying CD4, CD7, CD11a, CD18, and major histocompatibility complex class I molecules were not comitogenic with PMA. Anti-CD5/PMA-induced cell proliferation proceeded by an interleukin 2 (IL 2)-dependent mechanism, as was demonstrated by the cell surface expression of the p55 chain of the IL 2 receptor (IL 2R), the production of IL 2 and the inhibition of the proliferative response by anti-IL 2R mAb anti-Tac. There was no strict requirement for detectable numbers of monocytes, although cell proliferation could be enhanced by the monocyte-derived cytokines IL 1 and IL 6. Phorbol 12,13-dibutyrate and mezerein could substitute for PMA in this activation pathway, but synthetic diacylglycerols and phorbol esters that do not activate protein kinase C (PKC) could not, indicating a need for prolonged activation of PKC. T cells activated by anti-CD5/PMA are sensitive to inhibition by cyclosporin A (CsA) and by prostaglandin E2 (PGE2). This contrasts with anti-CD28/PMA-induced T cell proliferation, which is resistant to CsA and PGE2. Cell surface expression of CD5 was strongly up-regulated by PMA, whereas CD3 expression was down-regulated. We conclude that T cell activation can be triggered by engagement of CD5 by immobilized anti-CD5 mAb, combined with prolonged activation of PKC. These data support a role for CD5 as an independent signal transducing molecule.

Isolation and characterization of the genomic human CD7 gene: structural similarity with the murine Thy-1 gene

The human CD7 molecule is a 40-kDa member of the immunoglobulin gene superfamily that is expressed on T-lymphoid and myeloid precursors in fetal liver and bone marrow. CD7 is also expressed on T lymphocytes in multiple stages of T-cell development, including a major subset of mature peripheral T cells. In this paper we report the isolation and characterization of the human CD7 gene and 5' flanking region. Sequence analysis revealed that the CD7 gene comprises four exons that span 3.5 kilobases. The 5' flanking region (506 base pairs) has a high G + C content and no "TATA" or "CCAAT" elements. DNase I sensitivity analysis of chromatin from the CD7+ progenitor cell leukemia line, DU528, and the CD7-, CD4+, CD8+, TCR alpha beta + T-cell line, DU980 (where TCR is the T-cell receptor), revealed two distinct hypersensitive sites 5' of the CD7 gene. Hypersensitive site 1, present in the DU980 T-cell line, was located 4.5 kilobases upstream of the presumed CD7 transcription initiation site. Only DNase I hypersensitive site 2, which mapped to the promoter region, was found in the DU528 line. Comparison of the organization of the CD7 gene with that of other members of the immunoglobulin gene superfamily revealed that the human CD7 gene most closely resembles the murine Thy-1 gene. Both CD7 and Thy-1 are encoded by small genes with four exons, contain TATA-less promoters, and have a similar functional organization. These structural similarities suggest that human CD7 and murine Thy-1 may be functional homologues.

Inverse relationship of CA2+ mobilization and cell proliferation in CD8+ memory and virgin T cells

Memory T cells can now be defined with various monoclonal antibodies but little is known about the functional properties of these cells as compared to virgin T cells. We have studied Ca2+ mobilization and proliferation of memory and virgin cells in both the CD4 and CD8 subsets in response to phytohemagglutin-in. Using two-color fluorescence (yellow, red) for cell surface staining combined with two-color fluorescence for the Ca2+ chelator indo-1 (violet, blue) we can demonstrate that CD8+UCHL1+ memory cells exhibit a 1,5-fold higher influx than the CD8+UCHL1- virgin cells. The same pattern was obtained when CD8 memory and virgin cells were defined by a reciprocal marker (Leu-18) indicating that the higher response of memory CD8 cells is independent of the monoclonal antibody used to identify the cells. For the respective CD4 subsets no such clear-cut pattern of Ca2+ influx was evident. Analysis of proliferation in both subsets indicates that CD8+UCHL1- cells strongly proliferate in response to PHA, while CD8+UCHL1+ memory cells show only a minimal response with the average cpm values being 6-fold lower. In some instances no proliferation at all was detectable in the CD8+UCHL1+ cells. Identification of the subsets by reciprocal markers confirmed the lower proliferative response in the CD8 memory cells. Hence, within the CD8 compartment the memory cells, after phytohemagglutinin stimulation, exhibit a high Ca2+ mobilization but a low to absent proliferation, while the converse is true for the naive cells. These data show that (a) Ca2+ mobilization and proliferation are inversely correlated and (b) an initially vigorous response of memory cells may be subject to negative control mechanisms.

The G protein coupling T cell antigen receptor/CD3-complex and phospholipase C in the human T cell lymphoma Jurkat is not a target for cholera toxin

Intact Jurkat cells could be stimulated by monoclonal antibodies against the Tcell antigen receptor complex (OKT3 directed against the CD3 complex, BMA031 directed against constant framework epitopes in the alpha/beta heterodimer). The accumulation of inositol phosphates was inhibited by prior incubation of the cells with cholera holotoxin. The inhibitory effect of cholera toxin (CT) was not cAMP mediated because forskolin (a direct activator of adenylate cyclase) did not mimic the inhibitory effect. When measuring phospholipase C (PLC) in a cell-free assay system by using [3H]inositol-labeled membranes, the enzyme could be stimulated by the poorly hydrolyzable GTP analogue guanosine 5'-O-(thiotriphosphate (GTP gamma S). Both anti-receptor antibodies augmented the GTP gamma S stimulatory effect, while the antibodies alone had no stimulatory capacity. In membranes from CT-pretreated cells, whereas the antibodies lost their stimulatory effect on PLC as in untreated cells, whereas the antibodies lost their stimulatory capacity in the presence of GTP gamma S. These data imply that CT exerts its inhibitory effect on signaling by acting at the receptor level while the PLC regulating G protein is not a target for CT-mediated alterations. This assumption is supported by the finding that in intact Jurkat cells CT, which ADP ribosylated only the alpha-subunit of the stimulatory G protein of the adenylate cyclase, led to a loss of the T cell antigen receptor complex from the cell surface as demonstrated by a decrease of receptor density using flow cytometry analysis. Receptor loss could not be achieved by forskolin treatment or incubation of the cells with the binding subunit of the toxin alone.

Polyionic compounds selectively alter availability of CD4 receptors for HIV coat protein rgp120

We studied the ability of several polyionic compounds, previously shown to have activity in vitro against human immunodeficiency virus (anti-HIV) to block binding of anti-CD4 and recombinant HIV gp120 to the CD4 receptor on human lymphocytes. We found that Evans blue and aurin tricarboxylic acid could completely inhibit binding of anti-CD4 (Leu3a) and rgp120 and have selectivity for the CD4 receptor. A number of other compounds, including dextran sulfate and heparin had no effect on binding of rgp120 and were shown to be nonspecific for inhibition of binding of monoclonal antibodies to different T-cell receptors. Studies using a number of membrane-active drugs showed that changes in membrane potential or ion fluxes were not involved in the inhibition of binding of rgp120 by Evans blue or aurin tricarboxylic acid.

Glycolipids and transmembrane signaling: antibodies to galactocerebroside cause an influx of calcium in oligodendrocytes

This is the first study to provide evidence that one function for the surface glycolipid galactocerebroside (GalC) is participation in the opening of Ca2+ channels in oligodendroglia in culture. This glycolipid is a unique differentiation marker for myelin-producing cells; antibodies to GalC have been shown to markedly alter oligodendroglial morphology via disruption of microtubules (Dyer, C. A., and J. A. Benjamins. 1988. J. Neurosci. 8:4307-4318). This study demonstrates that extracellular EGTA blocks anti-GalC-induced disassembly of microtubules in oligodendroglial membrane sheets, demonstrating that an influx of extracellular Ca2+ mediates the cytoskeletal changes. The Ca2+ influx was examined directly by loading oligodendroglia with the fluorescent dye Indo-1 in defined medium, and measuring changes in Ca2+ in individual cells with a laser cytometer. Upon addition of anti-GalC IgG, a marked sustained increase in intracellular Ca2+ occurred in 80% of the oligodendroglia observed. EGTA blocked the increase, indicating the increase is due to an influx of extracellular Ca2+, and not due to release from intracellular stores. The effect is specific, since Ca2+ levels remain normal in oligodendroglia treated with nonimmune IgG; astrocytes do not respond to the anti-GalC. The Ca2+ response in oligodendrocytes is dependent on concentration of antibody and GalC on the oligodendroglial membrane surface. The Ca2+ influx is not mediated by voltage-sensitive Ca2+ channels: it is not blocked by cadmium, and depolarization with K+ does not mimic the response. The kinetics of the response suggest that second messenger-mediated opening of Ca2+ channels is involved.

CD4-T-cell antigen receptor complexes on human leukemia T cells

CD4 and T-cell antigen receptor (TCR) comodulate from the surface of human and murine T cells following exposure to monoclonal anti-CD4 or anti-TCR. This comodulation may occur because expression of CD4 and TCR is regulated by similar transmembrane signals or because CD4 and TCR are physically associated. To study multimolecular assemblies on the plasma membrane, we developed a flow cytometric method for detecting singlet-singlet energy transfer between fluorescein isothiocyanate (FITC)- and tetramethylrhodamine isothiocyanate (TRITC)-conjugated monoclonal antibodies as sensitized TRITC emission on intact, single cells. Using this procedure, we detected CD4-TCR complexes on the surface of the transformed human leukemia T cells, HPB-ALL, in the absence of stimulation. More than one CD4 were found in association with one TCR. CD4-TCR complexes were not in rapid equilibrium with free CD4 and free TCR, and they were not induced by the dye-labeled anti-CD4 or anti-TCR.

T-cell activation. III. Attempts to activate MHC class I-negative and class I-transfected EL4 T-lymphoma cells by immobilized anti-CD3 antibody

The aim of this study was to examine whether the unresponsiveness of MHC class I-negative subclones of the EL4 thymoma to CD3 cross-linking can be restored by transfection of class I genes into the H-2-negative cells. Cell activation experiments with selected MHC class I-negative subclones and H-2b- and H-2Ld-positive transfectants showed that these cells are equally capable of secreting interleukin 2 (IL-2) after exposure to the phorbol ester phorbol 12-myristate 13-acetate (PMA) and ionomycin. In contrast, only the parental H-2-positive EL4 cells are capable of responding to treatment with immobilized anti-CD3 antibody with IL-2 secretion and IL-2 receptor expression. Measurements of intracellular free Ca2+ (Ca2+i) following anti-CD3 antibody-induced cross-linking of parental EL4 cells and H-2-negative and H-2b gene-transfected subclones showed that the parental cells and two of the class I transfectants, one H-2-positive and one H-2-negative, responded with a slow rise in Ca2+i, whereas one H-2-positive transfected cell clone was completely refractory to CD3 cross-linking. Modulation experiments using parental EL4 cells, H-2-negative subclones and H-2-positive transfectants demonstrated that the CD3 and class I molecules of these different cells are modulated to the same extent after exposure to specific antibodies. The present findings thus indicate that the unresponsiveness of H-2-negative EL4 subclone cells to CD3 cross-linking is not functionally associated with a lack of class I surface expression.

Cross-linking of the surface immunoglobulin on lymphocytes from the bursa of Fabricius results in second messenger generation

The bursa of Fabricius represents the major site of B lymphocyte development in avian species. Although the vast majority of bursal lymphocytes express cell surface immunoglobulin (sIg), it is generally considered that the bursa does not represent a significant site of antigen-induced B cell maturation to Ig secretion. However, the question as to whether antigen, either exogenous or self, can induce positive or negative selection of bursal lymphocytes in such a way as to modify the peripheral B cell repertoire remains open. Clearly, such intrabursal selection would require that bursal lymphocyte sIg have the molecular machinery to transduce signals into the cell as a consequence of its interaction with antigen. In this report we demonstrate that exposure of bursal lymphocytes to antibodies directed against sIg induced a rapid increase in cytosolic free calcium ion concentrations [Ca2+]i. Furthermore, such antibodies also induced a rapid increase in intracellular phosphatidic acid concentrations followed by a rise in intracellular phosphatidylinositol. Increased [Ca2+]i, phosphatidic acid and phosphatidylinositol levels required the cross-linking of sIg and were not induced by antibodies to other bursal cell surface antigens. Thus, cross-linking of the sIg on bursal lymphocytes results in second messenger generation, demonstrating that bursal sIg is a functional signal transduction element.

Interrelationship between signals transduced by phytohemagglutinin and interleukin 1

In the murine cell line LBRM-331A5, phytohemagglutinin (PHA) induces secretion of the T cell growth factor interleukin 2 (IL2). IL1 augments PHA-induced IL2 production. In this cell line, PHA stimulates a number of biochemical changes including phospholipid hydrolysis, increases in cytosolic free calcium [( Ca2+]i), membrane hyperpolarization, cytosolic alkalinization, and tyrosine phosphorylation of specific substrates. Using LBRM cells, we have studied the interrelationship between these events and the secretion of IL2. Increases in [Ca2+]i triggered by PHA or following addition of ionomycin result in membrane hyperpolarization but are not required for PHA-induced cytosolic alkalinization or tyrosine phosphorylation. Addition of IL1 to PHA-stimulated cells did not affect any of the biochemical parameters, although it significantly augmented PHA-induced IL2 secretion. Increasing [Ca2+]i with ionomycin did not trigger IL2 secretion, increases in cytosolic pH, or tyrosine phosphorylation in the presence or absence of IL1. Preventing increases in cytosolic pH did not alter PHA-induced changes in [Ca2+]i or membrane potential. These data are compatible with PHA including activation of phospholipase C and production of inositol phosphates resulting in both release of Ca2+ from internal stores and transmembrane uptake of Ca2+ as well as activation of protein kinase C. However, unlike other growth factor or mitogen-stimulated systems, the changes stimulated by PHA and IL1 in LBRM cells including IL2 secretion are not regulated by a pertussis toxin-sensitive G protein.

CD45 ligation in T cells regulates signal transduction through both the interleukin-2 receptor and the CD3/Ti T-cell receptor complex

The cytoplasmic domain of the CD45 leukocyte cell surface antigen has recently been shown to possess protein tyrosine phosphatase (PTPase) activity. The existence of a cell membrane-bound PTPase may represent a mechanism by which an activation signal, initiated by ligand binding to a surface receptor, is down-regulated following delivery of the signal. Both the interleukin-2 (IL2) growth factor receptor and the CD3/Ti T-cell antigen receptor contain a subunit which is phosphorylated on tyrosine by an activated protein kinase (PTK) during T-cell activation. We compared the effect of CD45 ligation on signal transduction mediated by the binding of IL2 or anti-CD3 to these two receptors. Immunoblotting with anti-phosphotyrosine antiserum was used to investigate the effect of CD45 ligation on anti-CD3- or IL2-induced protein tyrosine phosphorylation. When CD3 and CD45 were triggered together, changes in the pattern of tyrosine phosphorylation of specific substrates was observed in comparison to the stimulus triggered through CD3 alone. In contrast, CD45 ligation did not alter the pattern of tyrosine-phosphorylated proteins in "resting" T-cell blasts responding to IL2, except for a mobility shift of a 55 kDa protein and increased phosphorylation of a 112 kDa substrate. The proliferative response of T cells to both anti-CD3 or IL2 was inhibited by ligating CD45. The CD45 molecule down-regulated CD3-induced T-cell activation when the CD45 and CD3 molecules were ligated simultaneously with immobilized antibodies. In contrast, immobilized CD45 mAb alone inhibited IL2-induced proliferation, and the inhibition was not potentiated by simultaneously using a CD25 mAb which was non-competitive for IL2-binding.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

UVB radiation and human monocyte accessory function: differential effects on pre-mitotic events in T-cell activation

Purified T lymphocytes fail to proliferate in response to antigenic and mitogenic stimuli when cultured in the presence of accessory cells that have been exposed in vitro to sublethal doses of UVB radiation. Because proliferation represents a final stage in the T-cell activation process, the present study was conducted to determine whether T cells were able to progress through any of the pre-mitotic stages when UVB-irradiated monocytes were used as model accessory cells. In these experiments, monoclonal anti-CD3 antibodies were employed as the mitogenic stimulus. Culture of T cells with UVB-irradiated monocytes did allow the T cells to undergo an increase in intracellular free calcium, which is one of the first steps in the activation sequence. The T cells expressed interleukin-2 receptors, although at a reduced level. However, T cells failed to produce interleukin-2 above background levels when they were placed in culture with monocytes exposed to UVB doses as low as 50 J/m2. Incubation of T cells with UVB-irradiated monocytes did not affect the subsequent capacity of T cells to proliferate, since they developed a normal proliferative response in secondary culture when restimulated with anti-CD3 antibodies and unirradiated monocytes. These studies indicate that T lymphocytes become partially activated when cultured with UVB-irradiated monocytes and mitogenic anti-CD3 monoclonal antibodies. In addition, they suggest that interleukin-2 production is the T-cell activation step most sensitive to inhibition when UVB-irradiated monocytes are employed as accessory cells.

Definition of a population of CD4-8- T cells that express the alpha beta T-cell receptor and respond to interleukins 2, 3, and 4

Whereas most T cells express surface CD4 or CD8 molecules, a minority lacks both. CD4-8- cells usually express the gamma delta T-cell receptor, but here we describe a population of CD4-8- T cells from the peripheral blood that express the alpha beta heterodimer. These cells have different surface antigens than gamma delta+ T cells, expressing CD5 but lacking CD16, and differ in function from gamma delta+ T cells. CD4-8- alpha beta+ cells lack non-major histocompatibility complex-restricted cytolytic function but can be induced to lyse their target cells after activation of their T-cell receptors. A peculiar characteristic of these cells is their responsiveness to interleukin 3. Since these cells have not altered their phenotype or function over a 12-month period in culture, they appear to be mature T cells. The results indicate that normal human peripheral blood contains two subsets of CD4-8- T cells, expressing either gamma delta or alpha beta receptors, that differ in function, phenotype, and growth control.

T-cell receptor cross-linking transiently stimulates adhesiveness through LFA-1

Effective interaction between T cells and their targets requires that recognition of specific antigen be coordinated with increased cell-cell adhesion. We show that antigen-receptor cross-linking increases the strength of the adhesion mechanism between lymphocyte function-associated molecule-1 (LFA-1) and intercellular adhesion molecules (ICAMs), with intracellular signals transmitted from the T-cell antigen receptor to the LFA-1 adhesion molecule. The increase in avidity is rapid and transient, providing a dynamic mechanism for antigen-specific regulation of lymphocyte adhesion and de-adhesion.

Activation of T lymphocytes via monoclonal antibodies against rat cell surface antigens with particular reference to CD2 antigen

In the rat, monoclonal antibodies (mAbs) against the T-lymphocyte receptor (TCR), CD3 antigen and CD2 antigen are mitogenic for T lymphocytes. Antibodies against CD43 (leukosialin) and CD5 are not, but can enhance the effects of other stimuli. Activation of a T-lymphocyte cell line in terms of triggering an increase in free cytoplasmic Ca2+ [( Ca2+]i) was mediated by mAbs against Class 1 MHC and CD4 in addition to the mitogenic mAbs. In the presence of non-activating levels of anti-TCR mAb a strong synergistic signal was seen with anti-CD4, but incubation with anti-CD4 prior to activation with anti-CD3 inhibited the CD3 signal. Stimulation of DNA synthesis in resting T cells via anti-CD2 mAbs required a combination of two non-competitive antibodies and addition of a third anti-CD2 mAb inhibited the activation. The anti-CD2 mAbs were active on all types of mature T cells from peripheral lymphoid organs and the thymus, but cells in the thymus that lacked a TCR could not be activated via CD2. Rat CD2 was transfected into Jurkat cells and cross-linking with anti-rat CD2 mAbs gave an increase in [Ca2+]i similar to that seen with an anti-human CD3 mAb. All three types of anti-rat CD2 mAbs gave a signal including the mAb that inhibited mitogenesis. Signal transduction via CD2 was not seen in L cells and was only weakly observed in TCR-ve Jurkat cells that had been transfected with rat CD2. The cytoplasmic domain of transfected CD2 was shown to be important for the signal transduction since no increase in [Ca2+]i could be triggered in cells expressing mutant CD2 that lacked the cytoplasmic domain. Mutant forms with increasing lengths of cytoplasmic domain showed functions that progressively approached that of the native function.

Immunodeficiency after allogeneic bone marrow transplantation in man. Effect of phorbol ester (phorbol myristate acetate) and calcium ionophore (A23187) in vitro

This study was undertaken to clarify the mechanism behind the severely decreased lymphocyte proliferative response upon stimulation with mitogens and antigens seen after allogeneic bone marrow transplantation (BMT) in man. We investigated eight BMT patients and eight controls and found that the proliferative response of patient cells was reduced both when the cells were stimulated with phytohaemagglutinin (PHA) and when they were stimulated with a combination of phorbol myristate acetate (PMA), which is an activator of protein kinase C (PKC), and the calcium ionophore A23187, which irreversibly opens for calcium transport into the cell (median relative responses were 41 and 37%, respectively). However, the PHA-induced increase in the concentration of intracellular free calcium in post-BMT cells was not significantly different from the values found in control cells and the expression of interleukin 2 (IL-2) receptors (CD25) was only slightly decreased. However, the production of IL-2 was severely decreased in patient cells after stimulation with A23187/PMA (median 3541 units), although it was higher than in PHA-stimulated control cells (median 354 units). These results show that a direct activation of PKC by PMA combined with an increase in intracellular free calcium by A23187 cannot overcome the lymphocyte proliferation deficiency in cells from patients after allogeneic BMT. The data suggests that the defect is affecting the diacylglycerol pathway considerably more than the inositol triphosphate pathway.

Complex Ca2+ flux inhibition as primary mechanism of staurosporine-induced impairment of T cell activation

The inhibitory effect of the highly effective drug staurosporine on the early activation signal Ca2+ flux was investigated via multiparameter flow cytometry in human peripheral blood T lymphocytes. Staurosporine has been reported to be a specific inhibitor of protein kinase C. However, we show that it inhibits the Ca2+ influx in anti-CD3 and phytohemagglutinin-stimulated human CD4+ and CD8+ lymphocytes at concentrations between 1.0 and 10.0 ng/ml. Staurosporine decreases the number of Ca2+-positive CD4+ and CD8+ lymphocytes as well as the Ca2+ influx per cell; the drug also delays the time of the maximum response to polyclonal stimulation. In addition, we demonstrate that staurosporine affects the primary Ca2+ response via inhibition of the release of the membrane-bound Ca2+ from the endoplasmic reticulum in CD4+ and CD8+ lymphocytes. Binding studies of the anti-CD3 antibody to T lymphocytes indicate normal binding capacities in the presence of staurosporine. With respect to the classical scheme of T cell activation via phospholipase C, our data suggest that staurosporine may inhibit T cell activation primarily by its effect on the early Ca2+ flux signal.

Leukocyte function-associated antigen 1 is an activation molecule for human T cells

The leukocyte function-associated antigen 1 (LFA-1) molecule is well established as a surface protein involved in cellular adhesion and interaction, but there has been little information about whether engagement of this molecule can also directly modify cellular activation. These studies demonstrate that crosslinking the LFA-1 molecule on human T cell clones transmits a unique signal to the cell. Crosslinking LFA-1 alone did not increase intracellular calcium ([ CA2+]i), nor did crosslinking LFA-1 activate the cells as measured by IL-2 production or [3H]thymidine incorporation. However, when CD3 and LFA-1 were crosslinked, a more prolonged calcium signal was observed than when CD3 alone was crosslinked. Moreover, IL-2 production and DNA synthesis were greatly augmented. These responses could be demonstrated when LFA-1 was crosslinked via either the alpha or the beta chain, and required surface expression of the LFA-1 molecule as no enhancement was observed in T cell clones from a child with leukocyte adhesion deficiency. The enhancement of cellular activation by LFA-1 did not require that it be directly crosslinked to the CD3 complex. Thus, crosslinking LFA-1 alone with isotype-specific secondary antibodies on cells also pretreated with an anti-CD3 mAb of a different Ig isotype stimulated the cells as effectively as crosslinking both surface antigens with GaMIg. Similarly, a delayed, but sustained increase in [Ca2+]i was elicited. This increase in [Ca2+]i and the enhanced functional responses required engagement of CD3 with an intact bivalent anti-CD3 mAb, as crosslinking LFA-1 on cells also reacted with Fab fragments of an anti-CD3 mAb did not increase [Ca2+]i, nor activate the cells. These data indicate that LFA-1 can convey activation signals to T cells. Synergism in signaling can be observed upon crosslinking of LFA-1 and independently crosslinking CD3. In the physiologic interaction between T cells and accessory cells, the interaction of LFA-1 with its ligand, intercellular adhesion molecule 1, may therefore not only facilitate cellular adhesion, but also may amplify T cell activation by delivering costimulatory signals.

CD3/T-cell receptor coupling to a pertussis and cholera toxin-insensitive G-protein

We have analyzed the effect of CD3/T-cell receptor stimulation on GTP hydrolysis and GTP binding. We show that stimulation of Jurkat, T-cell, membranes with OKT3 results in a 50% increase in GTP hydrolysis which is specifically inhibited by GDP. Pretreatment of the membranes with neither pertussis toxin nor cholera toxin inhibited the GTP hydrolysis. We also show that stimulation with OKT3 increases the binding of GTP gamma S to Jurkat membranes. These data strongly implicate the involvement of a G-protein in CD3/T-cell receptor signalling.

Mitogen-induced genes are subject to multiple pathways of regulation in the initial stages of T-cell activation

The delivery of a mitogenic signal to T cells via any one of several cell surface molecules elicits a variety of intracellular responses, some or all of which regulate subsequent gene expression events. The expression of nine novel mitogen-induced genes in response to various T-cell-activating agents was examined to evaluate the diversity of pathways which regulate such genes. The relative contribution of distinct secondary signals, individually or together, to mitogen-stimulated gene induction and the capability of individual genes to respond to the sometimes divergent signals generated from different cell surface structures is addressed. The activation of T cells with mitogenic monoclonal antibodies directed against the CD2 or CD3 cell surface molecules, or with phytohemagglutinin, induced all nine genes. Thus, stimulation by fully mitogenic agents regardless of cell surface-binding specificity correlated with the expression of all of the genes studied. However, heterogeneous patterns of gene expression, encompassing five regulatory classes, were revealed by the use of phorbol 12-myristate 13-acetate, calcium ionophore, and anti-CD28 monoclonal antibody, agents which mediated only a subset of intracellular events and thus an incomplete mitogenic signal. Interleukin-2 and two novel lymphokines represented one regulatory class that appeared to require unique transcriptional activation signals relative to the other mitogen-induced genes. As demonstrated in the accompanying paper (P. F. Zipfel, S. G. Irving, K. Kelly, and U. Siebenlist, Mol. Cell. Biol. 9:1041-1048, 1989), the immediate transcriptional response of T cells to mitogenic stimulation is quite complex, involving numerous genes beyond those which have been previously described. Furthermore, the discrimination of several regulatory phenotypes among these nine genes suggests that a multiplicity of signaling pathways extends from the cell surface to the level of transcription.

Evidence that aggregation of mouse sperm receptors by ZP3 triggers the acrosome reaction

In the mouse, considerable evidence indicates that initial sperm binding to the zona pellucida (ZP) is mediated by ZP3. In addition, this same glycoprotein is also responsible for inducing the acrosome reaction (AR). Whereas the O-linked oligosaccharides of ZP3 appear to mediate sperm-ZP binding, the portion of ZP3 bearing AR activity has not been defined. To try to understand the bifunctional role of ZP3 (binding and AR inducing activities), we have examined the hypothesis that ZP3 aggregates sperm receptor molecules. By analogy with findings in a variety of other extracellular signal transducing systems, including receptors for growth factors and insulin, this aggregation event could initiate the cascade resulting in the AR. To test this hypothesis, we have generated monospecific polyclonal antibodies against ZP2 and against ZP3, and examined the effects of these probes on capacitated sperm incubated in the absence or presence of various ZP protein preparations. For some experiments, we have used proteolytic fragments of ZP3, a preparation known to retain specific binding, but not AR-inducing, activity. We show here that capacitated mouse sperm, incubated with ZP glycopeptides, displayed ARs when incubated subsequently with anti-ZP3 IgG; ARs did not occur when parallel sperm samples were incubated with anti-ZP2 IgG or with anti-ZP3 Fab fragments. When capacitated sperm were treated successively, with (a) ZP3 glycopeptides, (b) anti-ZP3 Fab fragments, and (c) goat anti-rabbit IgG, ARs occurred in the majority of sperm. An alternative approach to examine this hypothesis used ZP proteins obtained from tubal eggs treated previously with bioactive phorbol diester (12-O-tetradecanoyl phorbol-13-acetate [TPA]). This preparation arrests capacitated sperm in an intermediate state of the AR. We demonstrate here that these sperm can be induced to undergo a complete AR by subsequent treatment with anti-ZP3 IgG. Together, these findings are consistent with the hypothesis under examination, and suggest that the aggregation of sperm molecules recognized by ZP3 glycopeptides or by TPA-treated ZP is sufficient to trigger the events that occur during acrosomal exocytosis.

Expression and regulation of CD5 on in vitro activated human B cells

The T cell-associated antigen CD5 has been shown to play an important role in the regulation of T cell activation. Monoclonal antibodies directed against CD5 upregulate helper function, and induce interleukin 2 (IL2) production by mature T cells as well as thymocytes. CD5 is also expressed on subsets of B cells associated with autoantibody production, and CD5+ B cells are present in increased numbers in patients with rheumatoid arthritis and systemic lupus erythematosis. More recently CD5 has been found to be present on human B lymphocytes following in vitro activation with phorbol myristate acetate. To date a similar functional role for CD5 has not to date been demonstrated for B cells. In this study we have shown that structurally similar CD5 molecules are present on activated B cells and T cells. In addition, CD5 on both stimulated B cells and T cells is phosphorylated, which may be important in the function of CD5 following activation. CD5 protein or mRNA was not detected on unstimulated splenic B cells depleted of any CD5+ cells. To investigate the control of CD5 expression, we examined a series of cytokines either alone or in combination for their effect on the induction of CD5. CD5 expression was specifically inhibited by IL4 but not by the other cytokines tested. This inhibition was very specific as IL4 did not inhibit the expression of other B cell activation antigens including CD25, B5, T9 and CD23 as well as the pan-B cell antigen CD20. The addition of other cytokines did not increase or reverse the inhibition of CD5 expression by IL4. This inhibition was demonstrated by immunofluorescence and flow cytometric analysis. Immunoprecipitation studies of 125I-labeled activated B cells demonstrated that there was a decrease in cell surface CD5 protein, and not simply inhibition of expression of a particular epitope. Northern blot analysis demonstrated that the expression of CD5 mRNA was markedly inhibited in the presence of IL4, whereas the induction of the protooncogene c-myb was unaffected. This suggests that IL4 inhibits CD5 protein expression on activated B cells by reducing the amount of CD5 mRNA transcription or increasing the degradation of CD5 mRNA. The role of the T cell-derived lymphokine IL4 in regulating CD5 expression may be important in the disease states characterized by increased numbers of CD5+ B cells.

Mitogen-induced genes are subject to multiple pathways of regulation in the initial stages of T-cell activation

The delivery of a mitogenic signal to T cells via any one of several cell surface molecules elicits a variety of intracellular responses, some or all of which regulate subsequent gene expression events. The expression of nine novel mitogen-induced genes in response to various T-cell-activating agents was examined to evaluate the diversity of pathways which regulate such genes. The relative contribution of distinct secondary signals, individually or together, to mitogen-stimulated gene induction and the capability of individual genes to respond to the sometimes divergent signals generated from different cell surface structures is addressed. The activation of T cells with mitogenic monoclonal antibodies directed against the CD2 or CD3 cell surface molecules, or with phytohemagglutinin, induced all nine genes. Thus, stimulation by fully mitogenic agents regardless of cell surface-binding specificity correlated with the expression of all of the genes studied. However, heterogeneous patterns of gene expression, encompassing five regulatory classes, were revealed by the use of phorbol 12-myristate 13-acetate, calcium ionophore, and anti-CD28 monoclonal antibody, agents which mediated only a subset of intracellular events and thus an incomplete mitogenic signal. Interleukin-2 and two novel lymphokines represented one regulatory class that appeared to require unique transcriptional activation signals relative to the other mitogen-induced genes. As demonstrated in the accompanying paper (P. F. Zipfel, S. G. Irving, K. Kelly, and U. Siebenlist, Mol. Cell. Biol. 9:1041-1048, 1989), the immediate transcriptional response of T cells to mitogenic stimulation is quite complex, involving numerous genes beyond those which have been previously described. Furthermore, the discrimination of several regulatory phenotypes among these nine genes suggests that a multiplicity of signaling pathways extends from the cell surface to the level of transcription.

Human T cell activation. IV. T cell activation and proliferation via the early activation antigen EA 1

A new mAb G38 was generated against purified EA 1, an early activation antigen. In immunoprecipitation, it was reactive with the same complex precipitated by the initial anti-EA 1 mAb P8. mAb G38 augmented PMA-induced proliferation of PBMC. It was shown to be mitogenic for purified T cells in collaboration with PMA in a dose-dependent manner. This effect was independent of monocytes and other accessory cells. mAb G38 augmented PMA-induced IL-2-R expression. In conjunction with PMA, it induced IL-2 synthesis and secretion. Its effects on IL-2-R and IL-2 expression were documented at both protein and mRNA levels. Both anti-EA 1 mAbs did not induce Ca2+ influx by themselves in PMA-treated T cells. However, the addition of second anti-mouse Ig antibodies induced readily detectable increases in [Ca2+]i. Ca2+-mediated pathways may be utilized as the transduction signal mechanisms. mAb Leu-23 was shown to be reactive with EA 1. mAb Leu-23 was also mitogenic for T cells in the presence of PMA. These findings provide evidence for a functional role for EA 1 in T cell activation and proliferation.

Monoclonal antibodies specific for murine CD2 reveal its presence on B as well as T cells

Monoclonal antibodies specific for the murine CD2 antigen were identified by an efficient screening method utilizing murine CD2 cDNA transfectants. An unexpected expression of CD2 on murine B cells was revealed by immunofluorescence and immunoprecipitation studies with these monoclonal antibodies and by RNA blot analysis for the murine CD2 transcript.

Bacterial toxins affect early events of T lymphocyte activation

The effects of pertussis toxin and cholera toxin on early events of T lymphocyte activation were examined in the T lymphocyte cell line, Jurkat. Pertussis toxin treatment of these T cells increased inositol phosphates production and led to increases in intracellular free calcium concentration. These effects were produced by the isolated B (binding) subunit of pertussis toxin, alone. Inositol phosphates production resulting from perturbation of the T cell antigen receptor-CD3 complex by MAb was not affected by pertussis toxin treatment but was markedly inhibited by cholera toxin. This effect of cholera toxin paralleled elevations in cAMP content. However, forskolin, in concentrations equipotent for cAMP production, was a weaker inhibitor of inositol phosphates production. Cholera toxin inhibition of inositol phosphates production did not result from inhibition of baseline incorporation of inositol into phosphoinositide substrates of phospholipase C. These studies underline the complexity of toxin effects on cellular systems and suggest that other approaches will be required to implicate guanine nucleotide-binding regulatory proteins in control of the early events of T lymphocyte activation. However, the data presented here provide a molecular basis for the clinical observations of lymphocytosis and the in vitro observations of lymphocyte mitogenesis after pertussis toxin stimulation.

Evidences for protein kinase C. Activation in T lymphocytes by stimulation of either the CD2 or CD3 antigens

Phosphorylation of protein kinase C (PKC) substrates in T lymphocytes was analyzed after stimulation with specific pairs of anti-CD2 monoclonal antibodies (mAb) or an anti-CD3 mAb. The results show that the appropriate stimulation of both CD2 or CD3 antigens results in phosphorylation of a 80-kDa putative PKC substrate and that this phosphorylation event is sensitive to a PKC inhibitor, sphinganine. CD2- and CD3-dependent phosphorylation was found to be strongly dependent on an extensive cross-linking of surface antigens. The biological importance of cross-linking of CD2 and CD3 was also evident for other biological responses such as interleukin 2 production and induction of an autocrine growth response. Finally, we also present evidence for interaction between the CD2 and CD3 signal transducing pathways.

Alterations of the lymphocyte-specific protein tyrosine kinase (p56lck) during T-cell activation

The lymphocyte-specific tyrosine protein kinase p56lck is abundantly expressed in L3T4+ (CD4+) and Lyt-2+ (CD8+) T-lymphocytes, where it is predominantly phosphorylated in vivo on the carboxy-terminal tyrosine residue 505 (Y-505). Upon exposure to activating signals (mitogenic lectins, antibodies to the T-cell receptor), the p56lck expressed in normal cloned murine T-cells is modified into a product which migrates at approximately 59 kilodaltons on sodium dodecyl sulfate-polyacrylamide gels and which possesses several amino-terminal serine phosphorylations. The changes in both mobility and amino-terminal phosphorylation can be reproduced by known activators of protein kinase C (4 alpha-phorbol 12 beta-myristate, dioctanoylglycerol), suggesting that this signal transduction pathway (or related pathways) mediates at least part of these events. Interestingly, agents raising intracellular calcium (such as A23187) cause the appearance of several of these amino-terminal phosphorylation changes but do not cause the pronounced shift in electrophoretic mobility. These data suggest that at least two serine kinase systems are implicated in the alterations of p56lck associated with T-cell activation and that the lck gene product plays a critical role in normal T-cell physiology.

Cell-cell adhesion mediated by CD8 and MHC class I molecules

CD4 and CD8 are cell-surface glycoproteins expressed on mutually exclusive subsets of peripheral T cells. T cells that express CD4 have T-cell antigen receptors that are specific for antigens presented by major histocompatibility complex class II molecules, whereas T cells that express CD8 have receptors specific for antigens presented by MHC class I molecules (reviewed in ref. 1). Based on this correlation and on the observation that anti-CD4 and anti-CD8 antibodies inhibit T-cell function, it has been suggested that CD4 and CD8 increase the avidity of T cells for their targets by binding to MHC class II or MHC class I molecules respectively. Also, CD4 and CD8 may become physically associated with the T-cell antigen receptor, forming a higher-affinity complex for antigen and MHC molecules, and could be involved in signal transduction. Cell-cell adhesion dependent CD4 and MHC II molecules has recently been demonstrated. To determine whether CD8 can interact with MHC class I molecules in the absence of the T-cell antigen receptor, we have developed a cell-cell binding assay that measures adhesion of human B-cell lines expressing MHC class I molecules to transfected cells expressing high levels of human CD8. In this system, CD8 and class I molecules mediate cell-cell adhesion, showing that CD8 directly binds to MHC class I molecules.

CD45 regulates signal transduction and lymphocyte activation by specific association with receptor molecules on T or B cells

Evidence is presented that the leukocyte common antigen CD45 can regulate both signal transduction by lymphocyte receptor molecules and T- and B-cell proliferation in a manner dependent on specific interactions between these receptors on the cell surface. Formation of homoaggregates of CD3, CD2, or CD28 on the surface of T cells induced by crosslinking with monoclonal antibodies (mAbs) results in an increase in cytoplasmic free calcium concentration ([Ca2+]i). This increase in [Ca2+]i was abolished when these receptors were crosslinked to CD45 on the cell surface. In contrast, the increase in [Ca2+]i induced by formation of homoaggregates of CD4 was strongly amplified when CD4 was coupled to CD45. T-cell proliferation initiated by immobilized anti-CD3 was inhibited by anti-CD45 or anti-CD45R when immobilized on the same surface, but not when in solution. Similarly, proliferation after stimulation of the CD2 and CD28 receptors was inhibited when a CD45 mAb was crosslinked to either CD2 or CD28 mAbs, but not when a CD45-specific mAb was bound to the cell surface separately. In B cells, the increase in [Ca2+]i and resulting proliferation induced by crosslinking either the CD19 or Bgp95 receptors was inhibited by coupling these molecules to CD45. Thus, CD45 appears to modify other cellular receptors functionally when brought into close physical association with them. The homology of the CD45 conserved cytoplasmic domains with a major human placental protein tyrosine phosphatase suggests that the effects of CD45 described here result from alterations in the phosphorylation state of tyrosyl residues in membrane-associated proteins.

Non-productive infection of human newborn blood mononuclear cells with herpes simplex virus: effect on T cell activation, IL-2 production and proliferation

Proliferative responses by T cells from newborn cord blood stimulated with PHA or CD3 were reduced following infection with live (but not killed) herpes simplex virus in vitro although activation (measured by calcium flux) and IL-2 production were unaffected. The impairment of proliferation was not reversed by exogenous IL-2, phosphonoacetic acid, indomethacin or anti-alpha or anti-gamma interferon antibodies. HSV DNA was detected by hybridization in DNA extracted from unseparated MNC and from subsets sorted for CD3+ and for HLA DR+ expression. HSV DNA replication was not detected by thymidine uptake and only small amounts of virus were recovered in an infectious centre assay, suggesting that infection was non-permissive. Nevertheless, in-vitro synthesis of a limited range of HSV proteins including ICP4 was detected by metabolic labelling.