Extending the B7 (CD80) gene family.

B7-1 and B7-2 are members of the immunoglobulin superfamily (IgSF) and important regulators of T cell-mediated immune responses. Despite sharing only limited sequence identity, B7-1 and B7-2 bind common receptors, CD28 and CTLA-4, on T cells and have similar functional properties. We have found that the extracellular V (ariable)-like domains of B7-1 and B7-2 share significant sequence similarities with 3 major histocompatibility complex (MHC)-encoded members of the IgSF: butyrophilin, myelin/oligodendrocyte glycoprotein, and the chicken MHC molecule, B-G.

Comparative analysis of B7-1 and B7-2 costimulatory ligands: expression and function.

Antigen-specific T cell activation requires the engagement of the T cell receptor (TCR) with antigen as well as the engagement of appropriate costimulatory molecules. The most extensively characterized pathway of costimulation has been that involving the interaction of CD28 and CTLA4 on the T cell with B7 (now termed B7-1) on antigen presenting cells. Recently, B7-2 a second costimulatory ligand for CTLA4, was described, demonstrating the potential complexity of costimulatory interactions.

Expression of functional B7 and CTLA4 on rheumatoid synovial T cells.

To assess the role of B7, CTLA4, and CD28 in the pathogenesis of chronic synovitis we analyzed the expression and function of these cell surface molecules in patients with rheumatoid arthritis, osteoarthritis, and psoriatic arthritis, and in normal controls. Immunoperoxidase staining of rheumatoid synovial membranes showed reactivity of 30% of T cells with anti-B7 mAb, in contrast to osteoarthritic and normal synovial membranes, in which no such staining was seen. In addition, rheumatoid synovial fluid T cells were positive by flow cytometric analysis for B7 (mean 20%, range 0 to 96%), as measured by staining with anti-B7 mAb or the CTLA4 Ig fusion protein, whereas no B7 expression was detected on peripheral blood T cells (mean 1%).

The effect of combination cyclosporine and CTLA4-Ig therapy on cardiac allograft survival.

Transplant rejection requires not only T cell receptor/CD3 complex activation by foreign MHC, but also additional costimulatory signals, as T cell receptor activation alone is insufficient for induction of the immune response. The CD28 receptor on helper T cells, interacting with its ligand B7 on activated B cells or macrophages, provides this costimulus to support T cell activity. CTLA4Ig (a soluble CD28 receptor analog), binds B7 and inhibits CD28 activation.

Polyclonal in vitro T cell proliferation and T cell-dependent B cell differentiation supported by activated autologous B cells.

Although anti-CD3-induced T cell proliferation and T cell-dependent B cell differentiation are not supported well by resting (nonactivated) B cells as AC, human peripheral blood B cells activated in vitro with SAC, rIL2, or anti-IgM effectively subserve AC function in a manner qualitatively similar to that of blood monocytes and support polyclonal anti-CD3-driven T cell-dependent B cell differentiation. Physical contact among T cells, responder B cells, and (irradiated) activated B cells is required, and the ability of activated B cells to support anti-CD3-driven generation of IgSC parallels surface B7 expression by the activated B cells. The fusion protein CTLA4Ig, which binds to B7 and B7-like molecules with high avidity and disrupts the interaction of T cell surface CD28 with B7, inhibits B cell differentiation in a dose-dependent fashion, whereas a control fusion protein has no such effect.

Soluble CTLA-4 can suppress autoantibody production and elicit long term unresponsiveness in a novel transgenic model.

Activation of Ag-specific T cells often requires costimulatory signals in addition to the primary signal mediated through the TCR. The CD28-B7 interaction provides one important costimulatory signal. Previous studies have shown that a soluble CD28 homologue, CTLA4lg, binds B7 with high affinity and can inhibit CD28-B7-mediated costimulation in vitro and in vivo.

Human T cell-dependent B cell differentiation induced by staphylococcal superantigens.

Microbial superantigens (SAgs), by virtue of their binding to TCR V beta elements on T cells and to class II MHC molecules on accessory cells (AC), trigger T cell activation. Although anti-CD3 mAb (which also trigger T cell activation via surface CD3/TCR) can readily induce T cell-dependent B cell differentiation in unmanipulated PBMC cultures, induction of Ig production in SAg-stimulated cultures has usually required special manipulation of the T cells, such as irradiating them or treating them with mitomycin C. We now demonstrate that eight different staphylococcal SAgs, typically at concentrations 10- to 100-fold lower than those required for proliferation, can each trigger unmanipulated peripheral blood and tonsil T cells to drive polyclonal B cell differentiation.

Transplantation tolerance induced by CTLA4-Ig.

The rejection of the transplanted allograft is dependent on T cell activation, which requires T cell receptor engagement by antigen and costimulatory signals delivered by T cell surface molecules such as CD28. CTLA4-Ig is a fusion protein that has previously been shown to block the CD28-mediated costimulatory signal and inhibit immune responses in vitro and in vivo. In this report we show that treatment of the C3H/He recipient of a BALB/c vascularized cardiac allograft with a 12-day course of CTLA4-Ig produced indefinite graft survival (> 100 days) in the majority of recipients.

In vivo blockade of CD28/CTLA4: B7/BB1 interaction with CTLA4-Ig reduces lethal murine graft-versus-host disease across the major histocompatibility complex barrier in mice.

We tested whether the in vivo infusion of recombinant, soluble CTLA4 fused with Ig heavy chains, as a surrogate ligand used to block CD28/CTLA4 T-cell costimulation, could prevent efficient T-cell activation and thereby reduce graft-versus-host disease (GVHD). Lethally irradiated B10.BR recipients of major histocompatibility complex disparate C57BL/6 donor grafts received intraperitoneal injections of human CTLA4-Ig (hCTLA4-Ig) or murine CTLA4-Ig (mCTLA4-Ig) in various doses and schedules beginning on day -1 or day 0 of bone marrow transplantation (BMT).

Effect of CTLA-4 chimeric protein on rat autoimmune anti-glomerular basement membrane glomerulonephritis.

The interaction of the T cell receptor with the antigen/major histocompatibility class II complex is insufficient to induce optimal T cell activation. Co-stimulatory signals, including those provided by CD28/CTLA-4 on T cells and B7 molecules (B7-1, -2, and -3) on antigen-presenting cells, are also required. CD28-B7 interactions can be blocked by a soluble human CTLA-4 chimeric protein (CTLA4Ig).

Unique antigen recognition by a herpesvirus-specific TCR-gamma delta cell.

TCR-gamma delta cells, a T cell subset present in the epithelial and lymphoid tissues, have been implicated in viral and bacterial infections. We have identified a TCR-gamma delta clone (TgI4.4) that, unlike TCR-alpha beta cells, recognizes a herpes simplex virus type 1 transmembrane glycoprotein, gI, in an MHC class I- and class II-independent fashion.

Regulation of immunostimulatory function and costimulatory molecule (B7-1 and B7-2) expression on murine dendritic cells.

Dendritic cells (DC) play a critical role in the initiation of T cell-mediated immune responses, and express costimulatory molecules that are required for optimal activation of unprimed T cells. Studies on the regulation of the costimulatory molecules on DC have produced evidence from several systems that GM-CSF can up-regulate expression of CTLA4 counter receptor (CTLA4-CR) (but not intercellular adhesion molecule 1 (ICAM-1) and heat stable Ag (HsAg)) on DC. This is demonstrated on splenic DC, Langerhans cells, kidney DC in culture, and in a skin-explant culture system, in which the increased expression of CTLA4-CR on Langerhans cells (LC) occurs concomitantly with their migration out of skin.

Costimulator B7-1 confers antigen-presenting-cell function to parenchymal tissue and in conjunction with tumor necrosis factor alpha leads to autoimmunity in transgenic mice.

Tolerance to peripheral antigens is thought to result from the inability of parenchymal tissue to stimulate T cells--an inability that is believed to relate to the lack of expression of the costimulatory signal(s) required for T-cell activation. To test this model, we generated transgenic mice expressing costimulatory molecule B7-1 on the B cells of the pancreas. We find that islets from these transgenic mice are immunogenic for naive T cells in vitro and in vivo.

T cells of staphylococcal enterotoxin B-tolerized autoimmune MRL-lpr/lpr mice require co-stimulation through the B7-CD28/CTLA-4 pathway for activation and can be reanergized in vivo by stimulation of the T cell receptor in the absence of this co-stimulatory signal.

The CD28/CTLA-4 receptors on T cells interact with the B7 molecule on antigen-presenting cells (APC) to produce a co-stimulatory signal that determines the outcome of activation. The role of this co-stimulatory signal in T cell activation and loss of tolerance in autoimmune MRL-lpr/lpr mice has not been investigated previously. The present study examines the contribution of the CD28/CTLA-4 co-stimulatory pathway to the loss of T cell tolerance in V beta 8 transgenic MRL-lpr/lpr and (-)+/+ mice in which neonatal tolerance has been induced by the superantigen staphylococcal enterotoxin B (SEB).

Identification of an alternatively spliced form of the murine homologue of B7.

B7 on antigen presenting cells is a costimulatory ligand necessary for full activation of T cell. Receptors for B7 have been known as CD28 or CTLA4. We here show that in addition to B7 mRNA, an alternatively spliced mRNA (designated as MB7-2 mRNA), that immunoglobulin (Ig)C-like domain coded by exon 3 has been spliced out, is found in activated murine splenic B cells by reverse transcriptase-polymerase chain reaction analysis.

Costimulation of T lymphocytes with integrin ligands intercellular adhesion molecule-1 or vascular cell adhesion molecule-1 induces functional expression of CTLA-4, a second receptor for B7.

Costimulation by the CD28 ligand B7/BB1 plays an important role during T cell proliferation primarily by augmenting synthesis of IL-2 and other cytokines. Resting CD4+ T cells express CD28 but not CTLA-4 on their surface. Costimulation of T cells with ICAM-1 or VCAM-1 induced CTLA-4 expression and up-regulated CD28 expression.

Immunoglobulin signal transduction guides the specificity of B cell-T cell interactions and is blocked in tolerant self-reactive B cells.

The specificity of antibody (Ab) responses depends on focusing helper T (Th) lymphocyte signals to suitable B lymphocytes capable of binding foreign antigens (Ags), and away from nonspecific or self-reactive B cells. To investigate the molecular mechanisms that prevent the activation of self-reactive B lymphocytes, the activation requirements of B cells specific for the Ag hen egg lysozyme (HEL) obtained from immunoglobulin (Ig)-transgenic mice were compared with those of functionally tolerant B cells isolated from Ig-transgenic mice which also express soluble HEL. To eliminate the need for surface (s)Ig-mediated Ag uptake and presentation and allow the effects of sIg signaling to be studied in isolation, we assessed the ability of allogeneic T cells from bm12 strain mice to provide in vivo help to C57BL/6 strain-transgenic B cells.

CD28-mediated costimulation of interleukin 2 (IL-2) production plays a critical role in T cell priming for IL-4 and interferon gamma production.

Naive T cells require interleukin 4 (IL-4) to develop into IL-4-producing T cells and IL-4 blocks development of such cells into interferon gamma (IFN-gamma) producers. Prior studies in accessory cell-independent priming systems using antireceptor antibodies as agonists have demonstrated that IL-2 is also necessary for the development of IL-4-producing cells under these culture conditions. Here we have examined the role of IL-2 and the CD28 costimulation pathway in priming for IL-4 and IFN-gamma production using a more physiologic model.

The binding pattern of a neutralizing monoclonal antibody to mutant oncostatin M molecules is correlated with functional activity.

Oncostatin M (OM) is a member of the cytokine family that includes leukaemia inhibitory factor (LIF), granulocyte colony stimulating factor (G-CSF), and interleukin 6 (IL-6). We previously reported the characterization of a monoclonal antibody (MAb) to OM, termed OM2, which neutralizes its functional activity. To gain information about the epitope detected by this MAb, we utilized a sandwich enzyme-linked immunoassay (EIA) to examine OM2 binding on a series of mutant recombinant OM molecules generated by site-directed mutagenesis, encompassing amino acid insertions, deletions, or alterations throughout the molecule.

Single-chain mono- and bispecific antibody derivatives with novel biological properties and antitumour activity from a COS cell transient expression system.

Single-chain antibody molecules were expressed from modified eukaryotic expression vectors as individual protein domains encoded on interchangeable cDNA cassettes. Two different single-chain antibody derivatives were constructed by linking individual light- and heavy-chain variable domains. The first was specific for the L6 tumour-associated antigen and the second was specific for human CD3.

CD28-mediated costimulation is necessary for the activation of T cell receptor-gamma delta+ T lymphocytes.

The role of costimulation in the activation of TCR-gamma delta cells in normal mice and mice transgenic (tg) for a TCR-gamma delta receptor was investigated. Activation of TCR-gamma delta cells required two signals. One signal was mediated by TCR occupancy, whereas a second signal was provided by accessory cells.

Costimulation of T-cell activation and virus production by B7 antigen on activated CD4+ T cells from human immunodeficiency virus type 1-infected donors.

Infection with the human immunodeficiency virus type 1 (HIV-1) requires T-cell activation. Recent studies have shown that interactions of the T-lymphocyte receptors CD28 and CTLA-4 with their counter receptor, B7, on antigen-presenting cells are required for optimal T-cell activation. Here we show that HIV-1 infection is associated with decreased expression of CD28 and increased expression of B7 on CD4+ T-cell lines generated from seropositive donors by alloantigen stimulation.

Identification of an alternative CTLA-4 ligand costimulatory for T cell activation.

Stimulation of T cell proliferation generally requires two signals: The first signal is provided by the T cell receptor binding to antigen, and the second signal or costimulus is provided by a different receptor-ligand interaction. In mouse and human, the CD28-B7 interaction has been identified as a source of costimulatory signals. We have identified a cell surface molecule (GL1) that is distinct from B7 and abundantly expressed on activated B cells.

Long-term acceptance of major histocompatibility complex mismatched cardiac allografts induced by CTLA4Ig plus donor-specific transfusion.

Allograft rejection is a T cell-dependent process. Productive T cell activation by antigen requires antigen engagement of the T cell receptor as well as costimulatory signals delivered through other T cell surface molecules such as CD28. Engagement of CD28 by its natural ligand B7 can be blocked using a soluble recombinant fusion protein, CTLA4Ig.

Costimulation of T cells for tumor immunity.

Tumor specific antigens can be demonstrated on many neoplasms by immunization and challenge experiments; however, these antigens do not normally elicit a sufficiently strong immune response to prevent tumor growth in immunocompetent hosts. Recent studies have demonstrated that efficient activation of T cells requires costimulation of the CD28 receptor via the B7 molecule on antigen-presenting cells. Inadequate costimulation of tumor-reactive T cells may contribute to the fact that antigenic tumors are not normally rejected by the immune system, and weak anti-tumor immune responses may be amplified by upregulation of CD28 triggering.