Expression of the T-cell surface molecule CD2 and an epitope-loss CD2 mutant to define the role of lymphocyte function-associated antigen 3 (LFA-3) in T-cell activation

CD58 Immunobiology at a Glance

The glycoprotein CD58, also known as lymphocyte-function antigen 3 (LFA-3), is a costimulatory receptor distributed on a broad range of human tissue cells. Its natural ligand CD2 is primarily expressed on the surface of T/NK cells. The CD2-CD58 interaction is an important component of the immunological synapse (IS) that induces activation and proliferation of T/NK cells and triggers a series of intracellular signaling in T/NK cells and target cells, respectively, in addition to promoting cell adhesion and recognition. Furthermore, a soluble form of CD58 (sCD58) is also present in cellular supernatant in vitro and in local tissues in vivo. The sCD58 is involved in T/NK cell-mediated immune responses as an immunosuppressive factor by affecting CD2-CD58 interaction. Altered accumulation of sCD58 may lead to immunosuppression of T/NK cells in the tumor microenvironment, allowing sCD58 as a novel immunotherapeutic target. Recently, the crucial roles of costimulatory molecule CD58 in immunomodulation seem to be reattracting the interests of investigators. In particular, the CD2-CD58 interaction is involved in the regulation of antiviral responses, inflammatory responses in autoimmune diseases, immune rejection of transplantation, and immune evasion of tumor cells. In this review, we provide a comprehensive summary of CD58 immunobiology.

Common Molecular Alterations in Canine Oligodendroglioma and Human Malignant Gliomas and Potential Novel Therapeutic Targets

Spontaneous canine (Canis lupus) oligodendroglioma (ODG) holds tremendous potential as an immunocompetent large animal model of human malignant gliomas (MG). However, the feasibility of utilizing this model in pre-clinical studies depends on a thorough understanding of the similarities and differences of the molecular pathways associated with gliomas between the two species. We have previously shown that canine ODG has an immune landscape and expression pattern of commonly described oncogenes similar to that of human MG. In the current study, we performed a comprehensive analysis of canine ODG RNAseq data from 4 dogs with ODG and 2 normal controls to identify highly dysregulated genes in canine tumors. We then evaluated the expression of these genes in human MG using Xena Browser, a publicly available database. STRING-database inquiry was used in order to determine the suggested protein associations of these differentially expressed genes as well as the dysregulated pathways commonly enriched by the protein products of these genes in both canine ODG and human MG. Our results revealed that 3,712 (23%) of the 15,895 differentially expressed genes demonstrated significant up- or downregulation (log2-fold change > 2.0). Of the 3,712 altered genes, ~50% were upregulated (n = 1858) and ~50% were downregulated (n = 1854). Most of these genes were also found to have altered expression in human MG. Protein association and pathway analysis revealed common pathways enriched by members of the up- and downregulated gene categories in both species. In summary, we demonstrate that a similar pattern of gene dysregulation characterizes both human MG and canine ODG and provide additional support for the use of the canine model in order to therapeutically target these common genes. The results of such therapeutic targeting in the canine model can serve to more accurately predict the efficacy of anti-glioma therapies in human patients.

Kinetics of gene expression in murine cutaneous graft-versus-host disease

The kinetics of gene expression associated with the development of cutaneous graft-versus-host disease (GVHD) were examined in a mouse model of MHC-matched allogeneic hematopoietic stem cell transplantation. Ear skin was obtained from recipient mice with or without GVHD between 7 and 40 days after transplantation for histopathological analysis and gene expression profiling. Gene expression patterns were consistent with early infiltration and activation of CD8(+) T and mast cells, followed by CD4(+) T, natural killer, and myeloid cells. The sequential infiltration and activation of effector cells correlated with the histopathological development of cutaneous GVHD and was accompanied by up-regulated expression of many chemokines and their receptors (CXCL-1, -2, -9, and -10; CCL-2, -5, -6, -7, -8, -9, -11, and -19; CCR-1 and CCR-5), adhesion molecules (ICAM-1, CD18, Ly69, PSGL-1, VCAM-1), molecules involved in antigen processing and presentation (TAP1 and TAP2, MHC class I and II, CD80), regulators of apoptosis (granzyme B, caspase 7, Bak1, Bax, and BclII), interferon-inducible genes (STAT1, IRF-1, IIGP, GTPI, IGTP, Ifi202A), stimulators of fibroblast proliferation and matrix synthesis (interleukin-1beta, transforming growth factor-beta1), and markers of keratinocyte proliferation (keratins 5 and 6), and differentiation (small proline-rich proteins 2E and 1B). Many acute-phase proteins were up-regulated early in murine cutaneous GVHD including serum amyloid A2 (SAA2), SAA3, serpins a3g and a3n, secretory leukocyte protease inhibitor, and metallothioneins 1 and 2. The kinetics of gene expression were consistent with the evolution of cutaneous pathology as well as with current models of disease progression during cutaneous GVHD.

LFA-3 co-stimulates cytokine secretion by cytotoxic T lymphocytes by providing a TCR-independent activation signal

T cell activation is known to depend not only on efficient antigen recognition and subsequent signaling through TCR, but also on interactions involving multiple adhesion and accessory molecules such as CD28/B7, LFA-1/ICAM-1 and LFA-3/CD2. The present study dissects the role of LFA-3/CD2 interactions in the activation of melanoma-specific CD8+ T cell clones. To this end we analyzed the influence of LFA-3 density on melanoma cells on lysis and cytokine production (TNF, IL-2, IFN-gamma) by T cells following activation by various amounts of antigenic peptides. Our results indicate that increasing LFA-3 density on melanoma cells variably affects their lysis susceptibility, but systematically and considerably enhances cytokine production by melanoma-specific cytotoxic T lymphocyte (CTL) clones. At any stimulatory antigen density, LFA-3 increased the fraction of responding cells and/or cytokine amounts produced by individual cells, without affecting TCR down-regulation. These results show that CD2 engagement increases cytokine gene activation essentially by providing to T cells a TCR-independent co-activation signal. From a practical point of view, our data demonstrate that the level of LFA-3 expressed on tumors critically affects cytokine production by specific CTL and thus the efficiency of specific immune reactions mediated by these cells.

Gene Structure, Promoter Characterization, and Basis for Alternative mRNA Splicing of the Human CD58 Gene

The 60-kDa lymphocyte function-associated Ag-3 (LFA-3/CD58), a highly glycosylated adhesion molecule that serves as ligand for the T cell-restricted glycoprotein CD2, is encoded by a gene at the human chromosome locus 1p13. We have elucidated the exon-intron organization of the entire human CD58 gene, including ∼2.5 kilobases (kb) of 5′-flanking DNA. Four overlapping genomic clones, spanning ∼65 kb, contained the entire ∼1-kb coding sequence of CD58 and consisted of six separate exons, which varied from 72 to 294 bp in size. At least two different CD58 mRNA precursors can be generated from the human gene as a result of alternative choice of one of the two acceptor splice sites located within exon 5. DNA sequence analysis of about 2.5 kb of 5′-flanking sequence of the CD58 gene indicated the absence of a CAAT box. However, potential binding sites for the transcriptional activators AP-2, GATA, PU.1, and Sp-1 are present. Two consensus TATAA elements, located ∼2.4 kb upstream of the transcriptional start site, have been identified. The 2.5-kb CD58 promoter sequence displayed functional activity in transient transfection assays in the hepatocellular carcinoma cell line HepG2. Comparing the response of CD58 promoter-driven luciferase plasmids to several cytokines and other agents suggests that the CD58 promoter is regulated by up-regulatory, enhancer-like and down-regulatory, silencer-like elements. Further analysis of this region should allow researchers to gain insight into the molecular mechanisms by which this gene is regulated, e.g., during inflammatory responses.

The receptor function of CD2 in human CD2 transgenic mice is based on highly conserved associations with signal transduction molecules

The activation of human T cells via CD2 in response to mitogenic monoclonal antibodies (mAbs) typically requires that one mAb is specific for an epitope within the N-terminal Ig domain of CD2 and the other for a partially hidden epitope. We have examined the proliferative response of human T cells and human CD2 (huCD2) transgenic murine T cells to two novel CD2 monoclonal antibodies, AICD2.M1 and AICD2.M2, and have partially mapped the epitopes of these and other mitogenic CD2-specific monoclonal antibodies by way of recognition of CD2:CD58 chimeric proteins possessing either the N-terminal or the membrane proximal immunoglobulin domains of CD2. To understand the molecular basis of proliferation in huCD2 transgenic murine T cells, the interactions of huCD2 with signaling proteins in murine T cells were analyzed. The transgenic huCD2 molecule was found to interact with the murine tyrosine kinases p56lck and p59fyn and the CD3-epsilon and zeta chains of the TCR/CD3 signaling complex and to coimmunoprecipitate tyrosine phosphatase activity. These molecular associations resemble the situation in human T cells and suggest that human CD2 couples to the same signal transduction pathways in humans and transgenic mice.

A role for phosphatidylinositol 3-kinase in the regulation of beta 1 integrin activity by the CD2 antigen

The rapid and reversible upregulation of the functional activity of integrin receptors on T lymphocytes is a vital step in the adhesive interactions that occur during successful T cell recognition of foreign antigen and transendothelial migration. Although the ligation of several different cell surface receptors, including the antigen-specific CD3/T cell receptor complex, the CD2, CD7, and CD28 antigens, as well as several chemokine receptors, has been shown to rapidly upregulate integrin function, the intracellular signaling events that initiate this increase in adhesion remain poorly defined. In this study, we have used DNA-mediated gene transfer to explore the role of phosphatidylinositol 3-kinase (PI 3-K) in the upregulation of beta 1 integrin functional activity mediated by the CD2 antigen. CD2 was expressed in the myelomonocytic cell line HL60, which expresses beta 1 integrins that mediate adhesion to fibronectin and VCAM-1 in an activation-dependent manner. Antibody stimulation of CD2 expressed on HL60 transfectants resulted within minutes in increased beta 1-mediated adhesion to fibronectin and VCAM-1 at levels comparable to that obtained upon stimulation with the phorbol ester PMA. A role for PI 3-K in CD2-mediated increases in beta 1 integrin function is suggested by: (a) the ability of the PI 3-K inhibitor wortmannin to completely inhibit CD2-induced increases in beta 1 integrin activity; (b) the association of PI 3-K with CD2; and (c) induced PI 3-K activity upon CD2 stimulation. The mode of association of PI 3-K with CD2 is not mediated by tyrosine phosphorylation-dependent binding of PI 3-K via SH2 domains, since: (a) PI 3-K is associated with CD2 in unstimulated cells; (b) CD2 stimulation fails to increase the amount of associated PI 3-K; and (c) the CD2 cytoplasmic domain lacks tyrosine residues. A role for both protein kinase C and cytoskeletal rearrangements in CD2 regulation of integrin activity is also suggested, since a PKC inhibitor partially inhibits CD2-induced increases in beta 1 integrin function, and CD2 stimulation increases F-actin content in a wortmannin-sensitive manner. Analysis of human peripheral T cells indicated that CD2 stimulation also results in PI 3-K-dependent upregulation of beta 1 integrin activity. Thus, these results demonstrate that CD2 can function as an adhesion regulator in the absence of expression of the CD3/T cell receptor complex; and directly implicate PI 3-K as a critical intracellular mediator involved in the regulation of beta 1 integrin functional activity by the CD2 antigen.

Intracellular mediators regulate CD2 lateral diffusion and cytoplasmic Ca2+ mobilization upon CD2-mediated T cell activation

CD2 is a T cell surface glycoprotein that participates in T cell adhesion and activation. These processes are dynamically interrelated, in that T cell activation regulates the strength of CD2-mediated T cell adhesion. The lateral redistribution of CD2 and its ligand CD58 (LFA-3) in T cell and target membranes, respectively, has also been shown to affect cellular adhesion strength. We have used the fluorescence photobleaching recovery technique to measure the lateral mobility of CD2 in plasma membranes of resting and activated Jurkat T leukemia cells. CD2-mediated T cell activation caused lateral immobilization of 90% of cell surface CD2 molecules. Depleting cells of cytoplasmic Ca2+, loading cells with dibutyric cAMP, and disrupting cellular microfilaments each partially reversed the effect of CD2-mediated activation on the lateral mobility of CD2. These intracellular mediators apparently influence the same signal transduction pathways, because the effects of the mediators on CD2 lateral mobility were not additive. In separate experiments, activation-associated cytoplasmic Ca2+ mobilization was found to require microfilament integrity and to be negatively regulated by cAMP. By directly or indirectly controlling CD2 lateral diffusion and cell surface distribution, cytoplasmic Ca2+ mobilization may have an important regulatory role in CD2 mediated T cell adhesion.

Crystal structure of the extracellular region of the human cell adhesion molecule CD2 at 2.5 A resolution

BACKGROUND

The T-lymphocyte antigen CD2 is an adhesion molecule implicated in immune responses in vivo. The extracellular regions of the human and rat homologues of CD2 share only 45% sequence identity and bind different protein ligands. Comparison of the human and rat soluble CD2 (sCD2) structures should provide insights into the structural basis of cell surface recognition.

RESULTS

We therefore determined the crystal structure of a form of human sCD2 with single N-acetylglucosamine residues at each glycosylation site to 2.5 A resolution with an R-factor of 19.3%. It is composed of two immunoglobulin superfamily domains similar to those of rat sCD2, but the relative orientation of the domains in the two homologues differs by up to 20 degrees. An interaction involving the flat, highly charged, ligand binding GFCC'C" faces of crystallographically related human sCD2 molecules duplicates, in a different lattice, that observed in the rat sCD2 crystals.

CONCLUSIONS

Intramolecular flexibility appears to be a conserved feature of CD2. The head-to-head interaction between molecules represents a general model for interactions between adhesion molecules of this structural class. Ligand specificity may be influenced by the distribution of charged residues on the binding face.

Helper effector function of human T cells stimulated by anti-CD3 mAb can be enhanced by co-stimulatory signals and is partially dependent on CD40-CD40 ligand interaction

In this study we have investigated whether anti-CD3-induced human T cell help for immunoglobulin production could be enhanced by co-stimulation of the T cells via other T cell surface molecules, and the contribution of CD40-CD40 ligand interaction to the execution of T helper effector function induced by these different stimulatory signals. In a system in which irradiated tonsillar T cells were stimulated with immobilized anti-CD3 monoclonal antibody (mAb), it was found that ligation of CD2 with a mitogenic pair of mAb considerably enhanced anti-CD3-induced T cell help for immunoglobulin production. Likewise, ligation of CD28 with mAb enhanced T helper activity, although to a lesser extent. Upon addition of anti-CD28 and anti-CD2 mAb together, an even higher immunoglobulin production was observed. This combination resulted in a four- to fivefold increase in immunoglobulin production as compared to cultures in which T cells were stimulated with anti-CD3 mAb alone. The effect of ligation with B7, the natural ligand of CD28, was studied in a system which utilizes the presentation of anti-CD3 mAb on human Fc gamma RII-expressing mouse fibroblasts which were co-transfected with human B7. It appeared that B7 could stimulate help for immunoglobulin production much more efficiently than ligation of CD28 with mAb did. Physical separation of B cells from T cells led to complete abrogation of immunoglobulin production. Blocking of CD40 with specific mAb, which have no intrinsic B cell stimulatory properties, or the CD40 ligand with a soluble CD40-human IgM fusion protein, resulted in dose-dependent, but only partial, inhibition of T cell-dependent immunoglobulin production with all modes of T cell activation tested. A clear correlation was found between the induction of CD40 ligand expression on the T cells by the different modes of co-stimulation and subsequent immunoglobulin production by the B cells. It is concluded that ligation of CD28 and/or CTLA-4, and of CD2 can generate co-stimulatory signals for T cell help for immunoglobulin production, which was found to be only partially dependent on the CD40-CD40 ligand interaction.

Anemia of uremia is associated with reduced in vitro cytokine secretion: immunopotentiating activity of red blood cells

Many in vitro studies demonstrate various stimulatory effects of red blood cells (RBC) on T cell reactivity. Only a few suggest a role for RBC in vivo, such as decreased B and T cell function in iron deficiency anemia. Immune deficiency of uremia is only partially corrected by dialysis treatment. We postulated therefore that this anemia may contribute in part to the immune deficiency of uremia. The aim of our study was to evaluate this postulate and to investigate the role RBC may have in the immune system in vivo. The in vitro secretion of interleukin-2 (IL-2), gamma-interferon (gamma-IFN), tumor necrosis factor (TNF) and colony stimulating factor (CSF) by human peripheral blood mononuclear cells isolated from patients and controls was used as a measure of immune function. The following protocols were carried out: IL-2 secretion was measured in patients with end-stage renal disease (ESRD) and in controls. RBCs were transfused to patients with ESRD and secretion of cytokines was measured before, and 4 hours, 4, 7 and 14 days afterwards; patients with ESRD received recombinant human erythropoietin (rHuEpo) and secretion of cytokines was measured before treatment and two and four months later. Finally, the effect of phlebotomy and transfusion of the autologous blood on cytokine secretion in healthy subjects was measured. Secretion of IL-2 by patients with ESRD was substantially lower than that of healthy subjects. In each group, IL-2 secretion correlated positively with hemoglobin level, r = 0.73, P < 0.01 and r = 0.71, P < 0.01.(ABSTRACT TRUNCATED AT 250 WORDS)

Separable portions of the CD2 cytoplasmic domain involved in signaling and ligand avidity regulation

Effective T cell immune responses require the molecular interplay between adhesive and signaling events mediated by the T cell receptor for antigen (TCR) and other cell surface coreceptor molecules. In this report, we have distinguished between the role of regulated adhesion and transmembrane signaling in coreceptor function using the T cell glycoprotein CD2. By binding its ligands on antigen-presenting cell (APC), CD2 serves both to initiate signal transduction events and to promote cellular adhesion. Furthermore, the avidity of CD2 for one ligand, CD58 (LFA-3), is regulated by TCR signaling. We have expressed wild type CD2 and a series of mutated CD2 molecules in an antigen-specific murine T cell hybridoma. Structure-function studies using these stably transfected cell lines identify two structurally and functionally distinct regions of the 116 amino acid (aa) cytoplasmic domain. One region is required for CD2-mediated signal transduction, and a separate COOH-terminal 21 aa portion is required for CD2 activity regulation. Cell lines expressing CD2 molecules lacking the cytoplasmic segment required for CD2-initiated IL-2 production retain the ability to upregulate CD2 avidity. Conversely, cell lines expressing CD2 mutants lacking the cytoplasmic segment required for avidity regulation retain the ability to initiate CD2-specific signaling. In antigen-specific T cell responses, basal binding of CD2 to its ligands enhances antigen responsiveness only minimally, whereas regulated avidity and transmembrane signaling are both required for optimal coreceptor function. Taken together, these studies demonstrate the independent contributions of regulated adhesion and intracellular signaling in CD2 coreceptor function.

1H resonance assignments and secondary structure of the 13.6 kDa glycosylated adhesion domain of human CD2

Human CD2, a glycosylated transmembrane receptor found on all T-lymphocytes, plays a key role in facilitating cellular adhesion between T-cells and target cells or antigen-presenting cells by binding to its counter receptor CD58 (LFA-3) present on the surface of those cells. All CD2 adhesion functions are localized within the amino-terminal 105-residue domain, which contains a single high mannose N-glycan required for maintaining both the conformational stability and CD58 binding properties of the glycoprotein. In order to better understand the structural basis for CD2-CD58-mediated adhesion and the critical role of the carbohydrate moiety in maintaining the functional stability of the molecule, we have determined the secondary structure of the N-glycosylated adhesion domain of human CD2 (hu-sCD2(105)) using NMR spectroscopy. Most of the 1H resonance assignments have been obtained from 1H-1H homonuclear 2D NMR spectra, which were further extended by applying 1H-15N heteronuclear 2D experiments on a hu-sCD2(105) sample selectively labeled with [15N]lysine. Thus, 98% of all backbone 1H resonances and over 80% of all side chain 1H resonances have been assigned. An overall topology characteristic of an immunoglobulin variable domain is observed, which consists of two beta-sheets comprised of three (residues 16-20, 67-71, and 60-63) and five (residues 94-103, 80-86, 32-37, 45-47, and 53-55) antiparallel beta-strands, respectively, with a hydrophobic core sandwiched between them. A ninth beta-strand (residues 7-12) makes parallel contacts to the carboxy-terminal beta-strand. NOEs between the N-linked glycan and the protein have tentatively been identified.

Mutational analysis of the CD2/CD58 interaction: the binding site for CD58 lies on one face of the first domain of human CD2

The adhesion interaction between the immunoglobulin superfamily molecules CD2 and CD58 (lymphocyte function-associated antigen 3) plays an important role in T cell and natural killer cell interaction with various antigen-presenting and target cells. Determination of the solution structure of rat CD2 domain 1 has allowed a model of human CD2 domain 1 to be generated, and a series of mutants based on this model have been made. Residues of domain 1 of human CD2 predicted to be solvent exposed were substituted with the equivalent residues present in the rat CD2 molecule. The ability of these mutants to mediate rosetting with human and sheep erythrocytes was studied. Results show that the binding site of CD2 for both human and sheep CD58 maps to the beta sheet containing beta strands CC'C"F and G. Residues K34 and E36 in beta strand C, R48 and K49 in beta strand C', and K91 and N92 in the loop connecting beta strands F and G are shown to be critical in the interaction. The data support the proposition that the interaction between CD2 and CD58 involves the major beta sheet face of CD2.

The mitogenic activity of human T-cell leukemia virus type I is T-cell associated and requires the CD2/LFA-3 activation pathway

The presence of a high number of activated T cells in the bloodstream and spontaneous proliferation of peripheral blood mononuclear cells in vitro are striking characteristics of human T-cell leukemia virus type I (HTLV-I) infection. The HTLV-I regulatory protein Tax and the envelope protein gp46 have been implicated in mediating the activation process. In this study, HTLV-I-producing cell lines and purified virus from the cell lines were examined for the ability to activate peripheral blood lymphocytes (PBLs) and Jurkat cells. Antisera and monoclonal antibodies against several cellular adhesion proteins involved in T-cell activation and against viral proteins were used to identify which molecules may be participating in the activation process. First, neither virus from a T-cell line, MT2, nor virus produced from the human osteosarcoma cell line HOS/PL was able to induce PBLs to proliferate. In contrast, both fixed and irradiated HTLV-I-producing T-cell lines induced proliferation of PBLs; HOS/PL cells did not activate PBLs. Second, HTLV-I-positive T-cell lines were capable of activating interleukin-2 mRNA expression in Jurkat cells. Induction of interleukin-2 expression was inhibited by anti-CD2 and anti-lymphocyte function-associated antigen 3 (LFA-3) monoclonal antibodies but not anti-human leukocyte antigen-DR, anti-CD4, anti-LFA-1, or anti-intercellular adhesion molecule 1. Similar results were obtained with PBLs as the responder cells. Furthermore, monoclonal antibodies and antisera against various regions of the HTLV-I envelope proteins gp46 and gp21 as well as p40tax did not block activation. These data indicate that HTLV-I viral particles are not intrinsically mitogenic and that infection of target T cells is not necessary for activation. Instead, the mitogenic activity is restricted to virus-producing T cells, requires cell-to-cell contact, and may be mediated through the LFA-3/CD2 activation pathway.

CD48 is a counter-receptor for mouse CD2 and is involved in T cell activation

CD2 is an intercellular adhesion molecule that has been implicated in T cell activation and differentiation both in humans and mice. Although the ligand for human CD2 has been defined as LFA-3, that for murine CD2 has not been identified yet. To identify the ligand for mouse CD2, we generated a chimeric molecule consisting of the extracellular domain of mouse CD2 and human immunoglobulin (Ig)G1 Fc (mCD2Rg). A hamster monoclonal antibody (mAb), HM48-1, was established by screening mAbs that could block the binding of mCD2Rg to T cell lines at the ligand site. The putative mouse CD2 ligand recognized by this mAb was a glycosyl phosphatidylinositol-anchored glycoprotein with an apparent molecular mass of 45 kD, which were shared characteristics with human LFA-3. However, its expression was predominantly restricted to hematopoietic cells, unlike human LFA-3. Protein microsequencing analysis for the NH2-terminal 18 amino acid residues of the affinity-purified HM48-1 antigen revealed that it is almost identical with mouse CD48. This identity was further confirmed by the reactivity of HM48-1 with a soluble recombinant CD48 (sCD48) protein and the molecule recognized by a rat mAb raised against sCD48. A rat anti-CD48 mAb blocked the mCD2Rg binding as well as HM48-1. Moreover, sCD48 also inhibited the mCD2Rg binding to the cellular ligand. Finally, like anti-CD2 mAb, HM48-1 inhibited the phytohemagglutinin response and, when crosslinked, augmented the anti-CD3 response of splenic T cells. These results indicate that CD48 is a ligand for mouse CD2 and is involved in regulating T cell activation.

Correlation of calcineurin phosphatase activity and programmed cell death in murine T cell hybridomas

Ligation of T cell receptor/CD3 complexes induces programmed cell death, or apoptosis, in immature thymocytes and many T cell hybridomas. While it has been demonstrated that T cell receptor-mediated apoptosis requires an increase in intracellular calcium concentration, the specific calcium-dependent signalling events leading to cell death are poorly defined. We have previously shown that T cell receptor/CD3-mediated induction of apoptosis in a murine T cell hybridoma is inhibited by the immunosuppressive drugs cyclosporin A (CsA) and FK506. Recently, it has been determined that these agents inhibit the activity of calcineurin, a calcium- and calmodulin-dependent serine/threonine phosphatase. Using an assay which measures calcineurin activity in cell lysates, we find that calcineurin-dependent dephosphorylation of a phosphopeptide substrate is potently inhibited in hybridomas treated with CsA or FK506. Drug dose-response analyses indicate that the level of cellular calcineurin activity correlates closely with the ability of these cells to undergo apoptosis. Thus, calcineurin appears to be a critical mediator of T cell receptor/CD3 signalling leading to programmed cell death in T cell hybridomas.

A distinct cytoplasmic domain of CD2 regulates ligand avidity and T-cell responsiveness to antigen

The T-cell glycoprotein CD2 not only contributes to intercellular adhesion but also plays a direct role in T-cell activation. Here we demonstrate that the interaction of CD2 with its ligand lymphocyte function-associated antigen 3 (CD58) is regulated by T-cell receptor-CD3 signaling. T-cell receptor-CD3 crosslinking by specific antigen or monoclonal antibodies rapidly increases the avidity with which cell-surface CD2 binds immunoaffinity-purified CD58. Mutational analysis of the CD2 cytoplasmic domain demonstrates that the carboxyl-terminal asparagine is essential for T-cell receptor-induced changes in CD2 avidity but is not essential for CD2-mediated signaling, establishing that the cytoplasmic portion of CD2 consists of distinct functional domains. Furthermore, cell lines expressing CD2 molecules incapable of avidity regulation exhibit a marked deficiency in an antigen-specific response. Thus, the regulation of CD2 adhesiveness has a profound effect on the ability of CD2 to enhance antigen responsiveness. These observations demonstrate that adhesion strengthening resulting from increased CD2 avidity contributes directly to T-cell responsiveness independently of CD2-mediated signal transduction.

Phorbol ester stimulates a protein-tyrosine/threonine kinase that phosphorylates and activates the Erk-1 gene product

The regulation of the Erk (extracellular-signal-regulated kinase) gene-encoded protein kinase activity by reversible phosphorylation has been reported to involve either an activator of autophosphorylation or an upstream protein kinase. In this communication we describe assays utilizing the Erk-1 protein fused to glutathione S-transferase that permit the identification of protein kinase(s) that phosphorylate and activate the myelin basic protein kinase activity encoded by the Erk-1 gene. A phorbol ester-stimulated protein kinase activity was identified that phosphorylated a kinase-negative Erk-1 gene product on tyrosine and threonine. The protein kinase phosphorylated and activated wild-type protein expressed in bacteria from 20- to 50-fold. The activation of the Erk-1-encoded myelin basic protein kinase required ATP and correlated directly with the degree of phosphorylation on the same amino acid residues previously shown to be phosphorylated in vivo. Conversion of the tyrosine site of phosphorylation to phenylalanine yielded an Erk-1 gene product that could not be activated. Similar results were obtained when the threonine site was mutated to valine. It is likely that the phorbol ester-stimulated protein-tyrosine/threonine kinase(s) is an up-stream target for multiple extracellular signals.

Purification of a murine protein-tyrosine/threonine kinase that phosphorylates and activates the Erk-1 gene product: relationship to the fission yeast byr1 gene product

We report the purification to near homogeneity of a 45-kDa phorbol ester-stimulated protein kinase that phosphorylates and activates the Erk-1 gene product. This kinase, which we provisionally denote MEK for MAPK/Erk kinase, phosphorylated kinase-inactive Erk-1 protein primarily on a tyrosine residue and, to a lesser extent, on a threonine. We extend our previous results and show that two forms of purified MEK activated the myelin basic protein kinase encoded by Erk-1. MEK was inactivated by the serine/threonine phosphatase 2A but not by the protein-tyrosine phosphatase 1B. Sequence analysis of peptides generated by trypsin digestion of MEK revealed similarity to the proteins encoded by the Schizosaccharomyces pombe byr1 and Saccharomyces cerevisiae STE7 genes. These data are discussed with regard to a possible signal transduction mechanism.

Restoration of the LFA-3 adhesion pathway in Burkitt's lymphoma cells using an LFA-3 recombinant vaccinia virus: consequences for T cell recognition

Conjugate formation between cytotoxic T lymphocytes (CTL) and target B cells, as observed in vitro, is mediated by interactions between adhesion molecules on the two cell surfaces rather than involving immune recognition through the T cell receptor. It is still not clear to what extent such adhesive contacts facilitate the process of immune recognition and target cell lysis. However, work on the Epstein-Barr virus (EBV)-associated malignancy Burkitt's lymphoma (BL) has suggested that down-regulation of one particular adhesion molecule, the lymphocyte function-associated antigen LFA-3, on the tumor cell surface is a key factor in allowing these target cells to escape EBV-specific T cell surveillance. To examine this directly, we used a cDNA for the full-length transmembrane form of LFA-3 to construct a recombinant vaccinia virus (Vacc-LFA 3), which is capable of restoring surface LFA-3 in adhesion molecule-negative BL cell lines to levels as high as seen in EBV-transformed lymphoblastoid cell lines (LCL); biochemical studies confirmed expression of the authentic N-glycosylated protein. The recombinant vaccinia-encoded LFA-3 was functional as an adhesion molecule since BL cells acutely infected with Vacc-LFA-3 then acquired the ability to form conjugates with activated T cells in vitro. However, there was no clear dependence upon LFA-3 when such BL cell lines were tested as targets for cytotoxic T lymphocytes (CTL). Firstly, LFA-3- BL cells could be killed by allospecific CTL recognizing HLA class I alloantigens, in some cases as efficiently as the corresponding LCL. In other cases where lysis was slightly below that of the LCL, Vacc-LFA-3 infection of the BL cells increased lysis up to, but never beyond, LCL values. Secondly, we studied the sensitivity of BL to EBV-specific HLA class I-restricted CTL using a BL target line which was LFA-3- but which expressed the same spectrum of EBV target proteins as an LCL. This line was not recognized by appropriately HLA-matched effectors, even after restoration of LFA-3 expression. We conclude that the LFA-3 status of BL cells influences their conjugate forming ability in in vitro assays but not necessarily their sensitivity to immune T cell-mediated cytolysis.

Overlapping but nonidentical binding sites on CD2 for CD58 and a second ligand CD59

The interaction of the T cell glycoprotein CD2 with one ligand, CD58, contributes to T cell function. We have identified CD59, a glycoprotein with complement-inhibitory function, as a second physiological ligand for CD2. Antibodies to CD59 inhibit CD2-dependent T cell activation in murine T cell hybridomas expressing human CD2. In an in vitro binding assay with purified CD58 and CD59, CD2+ cells bind not only immobilized CD58 but also CD59. With two complementary approaches, it was demonstrated that the binding sites on CD2 for CD58 and CD59 are overlapping but nonidentical. These observations suggest that direct interactions between CD2 and both CD58 and CD59 contribute to T cell activation and adhesion.

CD43, a molecule defective in Wiskott-Aldrich syndrome, binds ICAM-1

THE protein CD43 (also known as sialophorin, leukosialin, large sialoglycoprotein or gp115) is expressed on the surface of T lymphocytes, monocytes, neutrophils, platelets and some B lymphocytes. Expression of CD43 is deficient and/or defective in the X-chromosome-linked immunodeficiency disorder Wiscott-Aldrich syndrome, suggesting that CD43 might have a role in T-cell activation. We have shown that expression of human CD43 in an HLA-DR-specific murine T-cell hybridoma enhances the antigen-specific response to stimulation by the human lymphoblastoid cell line Daudi, and that Daudi cells bind specifically to purified immobilized CD43. These data indicate that the specific interaction of CD43 with a ligand on the surface of Daudi cells might contribute to T-cell activation. Here we report evidence that intercellular adhesion molecule-1 (ICAM-1, or CD54), is a ligand for CD43.

B-cell surface antigen B7 provides a costimulatory signal that induces T cells to proliferate and secrete interleukin 2

Occupancy of the T-cell receptor complex does not appear to be a sufficient stimulus to induce a T-cell-mediated immune response. Increasing evidence suggests that cognate cell-cell interaction between an activated T cell and an antigen-presenting cell may provide such a stimulus. A candidate T-cell surface molecule for this costimulatory signal is the T-cell-restricted CD28 antigen. Following crosslinking with anti-CD28 mAb, suboptimally stimulated CD28+ T cells show increased proliferation and markedly increased secretion of a subset of lymphokines. Recently, the B-cell surface activation antigen B7 was shown to be a natural ligand for the CD28 molecule, and both B7 and CD28 are members of the immunoglobulin superfamily. Here we report that B7-transfected CHO cells can induce suboptimally activated CD28+ T cells to proliferate and secrete high levels of interleukin 2. The response is identical whether T cells are submitogenically stimulated with either phorbol myristate acetate or anti-CD3 to activate the T cells. This response is specific and can be totally abrogated with anti-B7 monoclonal antibody. As has previously been observed for anti-CD28 monoclonal antibody, B7 ligation induced secretion of interleukin 2 but not interleukin 4. We have previously demonstrated that B7 expression is restricted to activated B lymphocytes and interferon gamma-activated monocytes. Since these two cellular populations are involved in antigen presentation as well as cognate interaction with T lymphocytes, B7 is likely to represent a central constimulatory signal that is capable of amplifying an immune response.

Adhesion of normal and neoplastic lymphoid cells to fibroblasts

Adhesion between lymphocytes and other cells is critical to many processes in the normal immune system. Alteration in the expression of cell adhesion molecules may be important in determining the behaviour of malignant lymphomas. In this study, the adhesion of normal lymphocytes to fibroblasts is compared with the adhesion of the T-cell lymphomas J6 and Hut 78 ICRF. J6 was significantly more adherent to fibroblasts than either Hut78 or PBL despite the fact that J6 expresses almost no LFA-1. Anti-LFA-1 had little effect on the basal adhesion of Hut78 ICRF or PBL. Addition of anti-CD2 caused enhanced adhesion of J6 and PBL but not Hut78 ICRF, which expresses little of this molecule. This enhancement was abrogated by anti-LFA-1. Anti-CD45 also caused enhanced adhesion of PBL. This was largely due to LFA-1-mediated homotypic cell adhesion. The tumour cell lines display no such homotypic adhesion and the small enhancement of fibroblast adhesion was much less affected by the anti-LFA-1 antibody. These results show the complex interactions which occur between adhesion molecules and that differences in patterns of expression between normal and neoplastic cells could be a major determinant of tumour cell behaviour.

The effect of the immunosuppressant FK-506 on alternate pathways of T cell activation

Structurally unrelated, FK-506 and cyclosporin (CsA) bind to and inhibit the action of distinct cytoplasmic receptors, FK-506-binding protein (FKBP) and cyclophilin (CyP), respectively. These receptors, termed immunophilins, share no sequence similarity, and yet both have been demonstrated to be capable of catalyzing the cis-trans isomerization of peptidyl-prolyl bonds (rotamase activity). Because FK-506 and CsA bind to different intracellular target structures, we investigated the spectrum of action of FK-506, in comparison to CsA, on T cell activation. We have shown that FK-506, like CsA, is able to inhibit T cell activation mediated not only by the T cell receptor-CD3 complex, but also via another surface molecule, CD2. T cell proliferation, stimulation of interleukin 2 production, and induction of apoptosis were all sensitive to inhibition by both FK-506 and CsA. With each parameter of activation, FK-506 is approximately 10-100-fold more effective than CsA. In contrast, FK-506 did not affect T cell proliferation induced by anti-CD28 monoclonal antibody in the presence of phorbol 12-myristate 13-acetate. This CD28 pathway, however, was inhibited by a structural homology of FK-506, rapamycin, demonstrating that the mechanism of action of FK-506 has specificity. These data suggest that immunophilins or the complex of drug coupled to immunophilin (i.e. FK-506/FKBP, CsA/CyP) are involved in and regulate selective pathways of T cell stimulation.

Two distinct signal transmission pathways in T lymphocytes are inhibited by complexes formed between an immunophilin and either FK506 or rapamycin

Proliferation and immunologic function of T lymphocytes are initiated by signals from the antigen receptor that are inhibited by the immunosuppressant FK506 but not by its structural analog, rapamycin. On the other hand, interleukin 2 (IL-2)-induced signals are blocked by rapamycin but not by FK506. Remarkably, these two drugs inhibit each other's actions, raising the possibility that both act by means of a common immunophilin (immunosuppressant binding protein). We find that the dissociation constant of rapamycin to the FK506 binding protein FKBP (Kd = 0.2 nM) is close to the dissociation constant of FK506 to FKBP (Kd = 0.4 nM) and to their effective biologic inhibitory concentrations. However, an excess of rapamycin is needed to revert FK506-mediated inhibition of IL-2 production, apoptosis, and transcriptional activation of NF-AT, a T-cell-specific transcription factor necessary for IL-2 gene activation. Similarly, an excess of FK506 is needed to revert rapamycin-mediated inhibition of IL-2-induced proliferation. The drug concentrations required for antagonism may be explained by the relative affinity of the drugs to, and by the abundance of, the immunophilin FKBP. FKBP has been shown to catalyze the interconversion of the cis- and trans-rotamers of the peptidyl-prolyl amide bond of peptide substrates; here we show that rapamycin, like FK506, is a potent inhibitor of the rotamase activity of FKBP (Ki = 0.2 nM). Neither FKBP binding nor inhibition of rotamase activity of FKBP alone is sufficient to explain the biologic actions of these drugs. Rather, these findings suggest that immunophilin bound to FK506 interferes with antigen receptor-induced signals, while rapamycin bound to the immunophilin interferes with IL-2-induced signals.

Probing immunosuppressant action with a nonnatural immunophilin ligand

The immunosuppressants FK506 and rapamycin bind to the same immunophilin, FK506 binding protein (FKBP), and inhibit distinct signal transduction pathways in T lymphocytes. A nonnatural immunophilin ligand, 506BD, which contains only the common structural elements of FK506 and rapamycin, was synthesized and found to be a high-affinity ligand of FKBP and a potent inhibitor of FKBP rotamase activity. Whereas 506BD does not interfere with T cell activation, it does block the immunosuppressive effects of both FK506 and rapamycin. Thus, the common immunophilin binding element of these immunosuppressants, which is responsible for rotamase inhibition, is fused to different effector elements, resulting in the inhibition of different signaling pathways. Inhibition of rotamase activity is an insufficient requirement for mediating these effects.

LFA-3, CD44, and CD45: physiologic triggers of human monocyte TNF and IL-1 release

The monocyte-derived cytokines, tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta), are central regulators of the immune response, but the physiologic stimuli for their release remain largely undefined. Engagement of three monocyte glycoproteins, LFA-3, CD44, and CD45, by specific monoclonal antibodies immobilized on plastic induced TNF-alpha and IL-1 beta release. In addition, TNF-alpha was released when monocyte LFA-3 bound immobilized, purified CD2, which is its physiologic receptor. Thus, a receptor-ligand interaction that mediates cell-cell adhesion can transmit the necessary signals for the release of monokines.

Differential effects of eight metal ions on lymphocyte differentiation antigens in vitro

In vitro studies were conducted to determine the effects of metal ions known to be released from metallic implants in vivo on the expression of lymphocyte surface antigens. Normal human peripheral blood lymphocytes were exposed to various concentrations of metal ions (Fe3+, Ni2+, Co2+, Mo6+, V5+, Cr6+, Cr3+, and Ti3+) for 30 min at 37 degrees C in a 5% CO2 atmosphere, and then analyzed for their ability to form rosettes with sheep red blood cells. Following this preliminary analysis, lymphocytes were exposed to the metal ions found to inhibit the E-rosette reaction (Fe3+, Ni2+, and Co2+) in order to determine which of the following surface antigens were affected: CD2, CD3, CD4, CD8, CD1, CD22, CD10, and HLA-DR. Our results showed that the in vitro treatment of lymphocytes with Fe3+ or Co2+ caused inhibition of CD2 only, whereas Ni2+ caused inhibition of both CD2 and CD3 antigens. These findings suggest that Fe3+, Co2+, and Ni2+ ions may interfere with T cell activation since both CD2 and CD3 are involved in that process.

Mapping the dextran sulfate binding site on CD2

This study has analysed the binding of a series of anti-CD2 monoclonal antibodies (MoAbs) to T cells in the presence of the sulfated polysaccharide dextran sulfate (2.3 sulfates/monosaccharide, 500 kDa) (DXS) to define the DXS binding site on CD2. The results show that DXS interacts primarily at the T11(2) epitope. Thus five anti-CD2 MoAbs which bound to the T11(2) epitope were inhibited in their binding by DXS. In contrast, seven anti-CD2 MoAbs that totally inhibited sheep red blood cells (SRBC) rosetting (identifying the T11(1) epitope) were unaffected in their binding to T cells in the presence of DXS. Three MoAbs which partially inhibited SRBC rosetting and thereby defining only part of the T11(1) epitope, were also inhibited in their binding by DXS. Consistent with the conclusion that the DXS binding site on CD2 is associated with the T11(2) epitope was the observation that interaction of DXS with CD2 resulted in augmented binding of the four MoAbs defining the T11(3) epitope, possibly reflecting an increased expression of the T11(3) (activation, CD2R) epitope of CD2. Collectively, the data presented support the notion that a natural ligand for the T11(2) epitope of CD2 will be identified as a sulphated carbohydrate structure.

A novel model for antigen-dependent activation of normal human T cells. Transmembrane signaling by crosslinkage of the CD3/T cell receptor-alpha/beta complex with the cluster determinant 2 antigen

Transmembrane signaling of normal human T cells was explored with mAbs directed at TCR, CD2, CD4, CD5, or CD8 antigens and highly purified CD4+ T cells and CD8+ T cells. Our experiments explicitly show that: (a) crosslinkage of TCR with the CD2 antigen, and not independent crosslinking of TCR and of CD2 antigen or crosslinking of either protein with the CD4 or CD8 antigen induces significant proliferation independent of co-stimulatory signals (e.g., accessory cells, recombinant lymphokines, or tumor promoter), (b) F(ab')2 fragments of mAb directed at the TCR and F(ab')2 anti-CD2, crosslinked with F(ab')2 fragments of rabbit anti-mouse IgG, promote the proliferation of highly purified T cells, (c) a prompt and sustained increase in intracellular free Ca2+ concentration results from crosslinkage of TCR with the CD2 antigen, (d) T cell proliferation induced by this novel approach is curtailed by EGTA and by direct or competitive inhibitors of PKC, (e) crosslinkage of TCR with the CD2 antigen results in the transcriptional activation and translation of the gene for IL-2 and in the expression of IL-2 receptor alpha (CD25), (f) anti-CD25 mAbs inhibit T cell proliferation initiated by crosslinkage of TCR with the CD2 antigen, and recombinant IL-2 restores the proliferative response. Our first demonstration that crosslinkage of TCR with the CD2 antigen induces proliferation of normal human CD4+ T cells and CD8+ T cells, in addition to revealing a novel activation mechanism utilizable by the two major subsets of T cells, suggest that the CD2 antigen might be targeted for the regulation of antigen-specific T cell immunity (e.g., organ transplantation).

The stimulation of T cells with anti-CD2 monoclonal antibodies facilitates the induction of polyclonal B-cell responses but does not enhance the activation of antigen-specific B cells

In this report we compare the effect of stimulation of peripheral mononuclear cells (PBMC) by using two monoclonal antibodies (MoAb) directed against the CD2 receptor on T cells or by using autologous erythrocytes (E) which express on their surface lymphocyte function-associated antigen 3 (LFA3), a natural ligand for CD2. The addition of autologous erythrocytes to pokeweed mitogen (PWM)-stimulated PBMC results in the enhancement of polyclonal immunoglobulin synthesis and of antigen-specific B-cell responses. Because B cells lack the CD2 molecule, it can be concluded that their enhanced activity is a consequence of the delivery of activating signals by activated T lymphocytes. When PBMC cultures were stimulated with a pair of anti-CD2 MoAb (Leu5b and VIT13) we were able to induce polyclonal immunoglobulin synthesis, particularly IgM, in cultures supplemented with interleukin 2(IL-2). Specific responses to tetanus toxoid (TT) and keyhole limpet haemocyanin (KLH) were also enhanced by the addition of autologous E to PWM-stimulated PBMC. Significant anti-TT responses were observed in cultures stimulated with E + TT + IL-2. In contrast, stimulation of PBMC with VIT13 + Leu5b + IL-2 + antigen was not effective in inducing anti-TT antibody and only weakly effective in inducing anti-KLH antibodies. Replacing Leu5b by anti-CD3 had no effect on the induction of specific antibody responses; in contrast, replacement of Leu5b by E enhanced anti-TT antibody production while the effect on polyclonal production of IgM was minimal. Therefore, it appears that the signal delivered by the association of CD2 with LFA3 is a better potentiating signal for specific B-cell responses than the signal delivered by pairs of MoAb to different epitopes of CD2 or to CD2 and CD3 epitopes.

The structural biology of CD2

The CD2 molecule is a 50-55KD transmembrane glycoprotein expressed on the vast majority of thymocytes and virtually all peripheral T lymphocytes. Its functions are two-fold: adhesion and activation. CD2 serves to facilitate conjugate formation between the T-lineage cell and its cognate partner via intermolecular interaction of CD2 and LFA-3 on the former and latter cells, respectively. Perturbation of the CD2 extracellular segment by certain combinations of anti-CD2 MAbs or LFA-3 and a single anti-CD2 MAb activate T-lineage function. These CD2-mediated activation events also synergize with signals mediated through the TCR to augment T-cell response. Based on microchemical analysis of immunoaffinity-purified human CD2 and cDNA and genomic cloning of mouse and human molecules, considerable structural information is now available. The mature surface human CD2 molecule consists of 327 amino acids: a 185 aa extracellular segment; a 25 aa hydrophobic transmembrane segment; and a 117 aa cytoplasmic domain rich in prolines and basic residues. The CD2 gene is comprised of five exons which span approximately 12 Kb on chromosome 1. A similar protein structure and gene exon organization is found for the mouse CD2 homologue. The CD2 adhesion domain is approximately 103 aa in length and is encoded by a single exon (exon 2). This domain is resistant to proteolysis, even though it lacks any intrachain disulfides and, like the entire extracellular segment protein expressed in a baculovirus system, binds to its cellular ligand, LFA-3. The latter occurs with a micromolar Kd. This relatively low affinity suggests that multivalent interactions among CD2 monomers on the T cells and individual LFA-3 structures on the cognate partner are important in enhancing the avidity of the T-cell interaction with its target or stimulator cell. The affinity of the CD2 extracellular segment for LFA-3 is not affected by truncations in the CD2 cytoplasmic domain, implying that ligand binding is not regulated by intracellular mechanisms. Given that CD2 mRNA expression and surface CD2 copy number are increased by more than one order of magnitude post-TCR stimulation, it is more likely that adhesion via CD2 is modulated by alteration in surface copy number. Analysis of early transduction events occurring via CD3-Ti (TCR) and CD2 including single channel Ca2+ patch-clamp recordings on living human T lymphocytes indicate a virtual identity of signals.(ABSTRACT TRUNCATED AT 400 WORDS)

CD2 expression and function in lepromatous leprosy

Leprosy is a spectral disease in which clinical presentation is thought to be related to the host immune response. Previous investigations have suggested that selective unresponsiveness to Mycobacterium leprae in patients with lepromatous leprosy is due to the presence of M. leprae-specific T-suppressor cells. However, it has recently been suggested that CD2 modulation was the mechanism for the observed impaired immune response in lepromatous patients. Therefore, we studied the expression of CD2 and CD3 on lymphocytes in lepromatous skin lesions and peripheral blood mononuclear cells (PBMC). Using immunohistochemical techniques, we found that virtually all of the CD3+ cells in leprosy skin lesions expressed CD2. In addition, indirect immunofluorescence flow cytometry demonstrated that most CD3+ cells in the peripheral blood possessed the CD2 marker, suggesting that CD2 expression of T-lymphocytes is normal. T-cell activation using paired anti-T11(2) and anti-T11(3) or anti-CD3 monoclonal antibodies demonstrated similar 3H-thymidine incorporation and gamma interferon production in the PBMC of lepromatous patients in comparison with the PBMC of their contacts and tuberculoid patients. However, lepromatous PBMC did not proliferate or produce gamma interferon in response to M. leprae. Our data suggest not only that CD2 expression is normal on T lymphocytes in lepromatous leprosy skin lesions but also that CD2 expression in peripheral blood lymphocytes is functional in T-cell activation. Defective CD2 modulation does not appear to be the mechanism for specific unresponsiveness in lepromatous leprosy.

A novel subset of CD2-, CD3/T cell receptor alpha/beta+ human peripheral blood T cells. Phenotypic and functional characterization of interleukin 2-dependent CD2-CD3+ T cell clones

It is generally believed that CD2 (T11, sheep erythrocyte receptor) is expressed on all human T cells. In the present study we have identified and characterized a minor subset of CD2- CD3/TCR alpha/beta+ T cells in the peripheral blood of healthy individuals. CD2-CD3+ T cells were enriched in PBMC depleted of plastic-adherent macrophages, E-rosetting (i.e., CD2+) T cells and surface Ig+ B cells. CD2-CD3+ T cells accounted for 0.1-0.8% of PBMC in six individuals. IL-2-dependent long-term clones of CD2-CD3+ T cells neither reacted with a panel of anti-CD2 mAbs nor expressed detectable levels of CD2 mRNA by Northern blot analysis. These clones, however, expressed a full-length TCR C beta mRNA and reacted with mAbs against TCR-alpha/beta, CD3, and CD4, and thus were mature T cells. CD2-CD3/TCR+ T cell clones could be triggered into proliferation, IL-2 production, and cytotoxic effector activity by anti-CD3 and anti-TCR mAbs. We conclude that (a) a minor subset of CD2-, CD3/TCR-alpha/beta+ T cells is present in normal peripheral blood; and (b) expression of CD2 at the level of protein and/or mRNA is not required for T cell signaling via the CD3/TCR molecular complex.

Expression and ontogeny of murine CD2

CD2 was first defined as the erythrocyte rosetting protein on the surface of human T cells. Recently, the rat and murine homologues have been identified by cDNA cloning. In this report we demonstrate that CD2 is expressed on the surface of most adult murine peripheral lymphocytes and thymocytes by indirect immunofluorescence using an anti-murine CD2 antiserum. The expression of CD2 on murine B cells was unexpected since in rat and human species it has been defined as a T cell-specific marker. Furthermore, CD2 appears very early on fetal thymocytes during development. The level of surface expression increases from day 13 of gestation to day 17, after which the surface density appears to reach a steady state. Thus, CD2 is expressed on day-13 thymocytes at the same stage that Thy-1, Pgp1 and the TcR gamma/delta/CD3 complex have been shown to be expressed.

A monoclonal antibody to LFA-3, the CD2 ligand, specifically immobilizes major histocompatibility complex proteins

T cells are activated when the antigen-specific T cell receptor recognizes antigen in association with major histocompatibility complex (MHC) proteins. The T cell surface protein CD2 (T11, LFA-2, the T erythrocyte receptor) and its target or stimulator cell ligand, lymphocyte function-associated antigen-3 (LFA-3), are also involved in T cell adhesion and activation. The molecular mechanisms by which the CD2/LFA-3 interaction affects T cell adhesion and activation are unclear. The CD2/LFA-3 interaction may be modeled by the interaction between LFA-3 and anti-LFA-3 monoclonal antibody (mAb). We used the fluorescence photobleaching recovery technique to investigate the effect of anti-LFA-3 mAb on the lateral mobility of MHC proteins in plasma membranes of JY, a human Epstein-Barr virus-transformed B cell line. Anti-LFA-3 mAb induced immobilization of class I MHC proteins labeled with bivalent but not monovalent fluorescein-conjugated W6/32 mAb. Anti-LFA-3 mAb also caused immobilization of class II MHC proteins labeled with bivalent fluoresceinated LB3.1 mAb. In contrast, anti-LFA-3 mAb did not affect the mobilities of either a B cell membrane protein labeled with bivalent fluoresceinated anti-CD45 (human leukocyte antigen) mAb or a membrane lipid analogue. Unlike anti-LFA-3 mAb, anti-LFA-1 mAb did not affect class I MHC protein mobility. These results suggest that CD2 binding to LFA-3 may trigger a physiological response in which target cell MHC proteins, cross-linked by receptors on the T cell surface, are immobilized at and thereby localized to the T cell-target cell interface.

Blast-1 possesses a glycosyl-phosphatidylinositol (GPI) membrane anchor, is related to LFA-3 and OX-45, and maps to chromosome 1q21-23

Blast-1 is a human activation-associated glycoprotein expressed on the surface of leukocytes. Analysis of a translated sequence from a Blast-1 cDNA reveals a single hydrophobic sequence which could traverse the plasma membrane, but is devoid of charged residues that might represent a cytoplasmic tail. Consistent with this characteristic, Blast-1 is demonstrated here to be anchored to the cell surface through a glycosyl-phosphatidylinositol (GPI)-containing lipid. Comparison of Blast-1 to other GPI-anchored membrane proteins revealed a striking primary and secondary structure similarity with MRC OX45 and the lymphocyte function antigen LFA-3. The degree of overall amino acid sequence homology reveals that OX45 is a rat homologue of Blast-1. The greatest homology to LFA-3 occurs between their NH2-terminal Ig-like domains. Evidence is presented that demonstrates that Blast-1 and LFA-3 possess a disulfide-bonded second domain. These common characteristics demonstrate a structural and evolutionary relationship between Blast-1, OX45, LFA-3, and CD2, which in turn suggests a functional role for Blast-1 in cell-cell interactions in the immune response. The gene for Blast-1 has been localized to chromosome 1 q21-q23, indistinguishable from the CD1 cluster of Ig superfamily genes, raising the possibility that they may be linked.

Proliferation of highly purified T cells in response to signaling via surface receptors requires cell-cell contact

Lymphocyte proliferation is associated with cell-cell aggregation. In order to assess the importance of cell-cell contact in T-cell proliferation we examined the effect of disruption of cellular aggregation by anti LFA-1(4) mAb on T-cell proliferation. Monocyte-dependent T-cell proliferation induced by anti-CD3 mAb, pairs of anti-CD2 mAbs, or PHA was inhibited by anti-LFA-1 mAb. Monocyte-independent proliferation of highly purified T cells to anti-CD3 mAb plus PMA or plus IL-2 and to PHA plus IL-2 was, surprisingly, also inhibited by anti-LFA-1 mAb. Anti-LFA-1 mAb caused the partial inhibition of both low-affinity and high-affinity IL-2 receptor and the complete inhibition of IL-2 synthesis. In contrast to the above, the proliferation of highly purified T cells to PMA plus ionomycin was not inhibited by anti-LFA-1 mAb. These results suggest that optimal activation of highly purified T cells via cell surface receptors requires LFA-1-dependent cell-to-cell contact between proliferating T cells as well as between T cells and accessory cells. Such contact appears to be crucial for initiating IL-2 production and for optimal action of IL-2 through its receptor.

Restricted expression of CD2 among subsets of sheep thymocytes and T lymphocytes

A monoclonal antibody (mAb) generated against sheep T-cell blasts, called I/35 A, blocks sheep autologous E rosetting and competes with purified T11 target structure (TS), the sheep form of LFA3, for binding sites on the sheep T-cell surface. Immunoprecipitation from lysates of surface iodinated sheep T cells identifies the cell surface molecule recognized by mAb I/35 A as a single chain polypeptide migrating as a diffuse band of MW 55,000. From its binding properties and the biochemical nature of the target antigen, we conclude that mAb I/35 A is directed at sheep CD2. This finding makes sheep the first animal model in which the CD2-LFA3 (T11TS) system is defined by mAbs to both receptor and ligand. When analysed by two-colour flow cytometry and by immunohistochemistry, the cellular expression of CD2 in sheep differs significantly to that reported in humans. In peripheral blood, CD2 is found exclusively on CD4+8- and CD4-8+ T cells, while the third, CD4-8- (predominantly SBU-T19+) subset of sheep T cells (around 20% in peripheral blood) is CD2-. In thymus, only low to moderate levels of CD2 expression occurs on 80% of cells. Among these, medullary 'single positive' thymocytes express the highest level of CD2, whereas the CD4-8- 'double negative' population (which in contrast to peripheral CD4-8- T cells contains only very few SBU-T19+ cells) consists of CD2- and weakly positive cells. In peripheral lymphoid organs, CD2+ lymphocytes occur in the T-cell regions of spleen, lymph nodes and jejunal Peyer's patches (JPP). Tissue macrophages found in B-cell follicles of lymph nodes and JPP are also CD2+. The implications of these findings are discussed in terms of the role CD2 plays in the proliferation of immature thymocytes and of the possible importance of CD2/LFA3 interactions in lymphocyte recirculation.

Correlation of CD2 binding and functional properties of multimeric and monomeric lymphocyte function-associated antigen 3

LFA-3 was purified with an intact (mLFA-3) or an enzymatically removed membrane-anchoring domain (sLFA-3). Gel filtration and sucrose gradient sedimentation showed sLFA-3 to be a single highly glycosylated polypeptide chain in solution, while mLFA-3 formed micelles of 8 LFA-3 monomers. 125I-mLFA-3 bound to Jurkat T leukemic cell surface CD2 with much higher avidity than sLFA-3. mLFA-3 binding had characteristics of a multivalent interaction with cell surface CD2 and had an avidity of 1.5 nM for Jurkat cells and 12 nM for resting T cells. Two CD2 mAbs tested did not block mLFA-3 binding: 9-1 and CD2.1. These mAbs were tested in combination with LFA-3 for their ability to activate T cells. The combination of mLFA-3 and CD2.1 mAbs induced a rapid increase in cytosolic free Ca2+ in Jurkat cells which was proportional to mLFA-3 occupation of CD2 sites. sLFA-3 showed no activity in the Ca2+ flux assay. The combination of mLFA-3 and the CD2.1 mAbs significantly stimulated proliferation of PBMC. The combination of mLFA-3 and 9-1 mAbs was weakly or not mitogenic for the same cells. The combination of CD2.1 and sLFA-3 at concentrations up to 480 nM was not consistently mitogenic. Therefore, monomeric LFA-3/CD2 interaction appears to have a relatively low affinity, while multimeric LFA-3 binds with high avidity. T cell activation by binding of LFA-3 to CD2 appears to require occupation of 10(4) to 10(5) CD2 sites, which is likely to occur during adhesion, but is rare in receptor systems with soluble ligands.