Induction of alloantigen-specific hyporesponsiveness in human T lymphocytes by blocking interaction of CD28 with its natural ligand B7/BB1

Novel NFAT sites that mediate activation of the interleukin-2 promoter in response to T-cell receptor stimulation

The transcription factors NFAT and AP-1 have been shown to be essential for inducible interleukin-2 (IL-2) expression in activated T cells. NFAT has been previously reported to bind to two sites in the IL-2 promoter: in association with AP-1 at the distal antigen response element at -280 and at -135. On the basis of DNase I footprinting with recombinant NFAT and AP-1 proteins, gel shift assays, and transfection experiments, we have identified three additional NFAT sites in the IL-2 promoter. Strikingly, all five NFAT sites are essential for the full induction of promoter activity in response to T-cell receptor stimulation. Four of the five NFAT sites are part of composite elements able to bind AP-1 in association with NFAT. These sites display a diverse range of cooperativity and interdependency on NFAT and AP-1 proteins for binding. One of the NFAT sites directly overlaps the CD28-responsive element. We present evidence that CD28 inducibility is conferred by the AP-1 component in NFAT-AP-1 composite elements. These findings provide further insight into the mechanisms involved in the regulation of the IL-2 promoter.

Ligation of either CD2 or CD28 rescues CD4+ T cells from HIV-gp120-induced apoptosis

Temporal or quantitative imbalance in signals delivered to T cells via T cell antigen receptor (TCR), the CD4 co-receptor, and accessory molecules can lead to anergy, apoptosis, or both. This has been observed following ligation of CD4 by HIV gp120 prior to TCR occupancy. The ability of molecules such as CD2 and CD28, interacting with their ligands LFA-3 and B7, to provide signals that protect T cells from the induction of anergy, has been reported. Here, we demonstrate that ligation of CD2 and CD28 in conjunction with TCR occupancy rescue T cells that have been programmed for apoptotic death by prior CD4 ligation to gp120. This appears to be the result of augmented interleukin-2 and interleukin-4 release by the T cells following these molecular interactions. In conclusion, our results suggest that an impairment of antigen-presenting accessory cell functions could favor gp120-mediated apoptosis in HIV-uninfected cells.

Signal transduction by B7/BB1 expressed on activated T lymphocytes: cross-linking of B7/BB1 induces protein tyrosine phosphorylation and synergizes with signalling through T-cell receptor/CD3

We report here that B7/BB1 molecules expressed on activated T lymphocytes are involved in signal transduction. Anti-B7/BB1 monoclonal antibody (mAb) enhanced allogeneic proliferative responses against three different B lymphoma lines in a dose-dependent manner, while the same mAb inhibited T-cell response against allogeneic T cells expressing B7/BB1. Induction of B7/BB1 expression on T cells with allogeneic stimulation was confirmed by flow cytometric analysis. With the purified preactivated T cells as responder cells, anti-B7/BB1 mAb costimulated these primed T cells with coimmobilized anti-CD3 mAb. Moreover, cross-linking of B7/BB1 molecules induced protein tyrosine phosphorylation in preactivated T cells with a phosphorylation pattern distinct from those induced by signalling through other T-cell molecules. These results suggest that B7/BB1 molecules function not only as costimulatory ligands expressed on antigen-presenting cells but as receptors on T cells to transduce the costimulatory signals into the cells and may play a role for T-cell-T-cell interactions leading to clonal expansion of activated T lymphocytes. However, the physiological relevance of our finding remains to be explored.

Heat-stable antigen is expressed by murine keratinocytes and delivers costimulatory signals in T-cell activation

Heat-stable antigen (HSA), expressed by various antigen-presenting cells (APC), has been described as a costimulatory molecule for CD4+ T cells. Recently, we observed that HSA also serves as an important costimulatory molecule on epidermal Langerhans cells (LC). During these studies, low levels of HSA staining were also detected on normal murine keratinocytes (KC). To investigate whether HSA also is involved in T-cell activation by KC, normal murine KC or the spontaneously transformed KC cell-line PAM 212 were treated with PDB or PMA to induce HSA-expression. FACS analyses showed induction of HSA expression on normal murine KC, as well as PAM 212 cells. In functional assays PDB or PMA-treated normal or transformed KC were far more potent inducers of primary allogeneic T-cell responses than untreated KC. Addition of anti-HSA-specific mAb 20C9 specifically inhibited the costimulatory activity of KC, an effect that was even more pronounced when CTLA-4Ig was added to the cultures. Cleavage of HSA on KC surfaces by a phosphoinositol-specific phospholipase C (PI-PLC) also significantly inhibited the costimulatory capacity of KC for naive CD4+ T cells. In aggregate, our data indicate that expression of HSA on activated KC contributes to the capacity of these cells to induce proliferation of allogeneic T cells.

Both extracellular immunoglobin-like domains of CD80 contain residues critical for binding T cell surface receptors CTLA-4 and CD28

The B7-related molecules CD80 and CD86 are expressed on antigen-presenting cells, bind the homologous T cell receptors CD28 and CTLA-4, and trigger costimulatory signals important for optimal T cell activation. All four molecules are immunoglobulin superfamily members, each comprising an extracellular Ig variable-like (IgV) domain, with CD80 and CD86 containing an additional Ig constant-like (IgC) domain. Despite limited sequence identity, CD80 and CD86 share similar overall receptor binding properties and effector functions. We have identified, by site-directed mutagenesis of soluble forms of CD80 and CD86, residues in both the IgV and IgC domains that are important for CTLA4Ig and CD28Ig binding. Mutagenesis in the IgV domain of CD80 identified 11 amino acids that support receptor binding. Many of these residues are conserved in the B7 family, are hydrophobic, and approximately map to the GFCC'C" beta-sheet face of an IgV fold. Mutagenesis of corresponding residues in CD86 established that some, but not all, of these residues also played a role in CD86 receptor binding. In general, mutations had a similar effect on CTLA4Ig and CD28Ig binding, thereby indicating that both receptors bind to overlapping sites on CD80 and CD86. Further, mutagenesis of several conserved residues in the ABED beta-sheet face of the IgC domain of CD80 completely ablated receptor binding. Point mutagenesis had a more pronounced effect than complete truncation of the IgC domain. Thus, full CTLA4Ig and CD28Ig binding to B7 molecules is dependent upon residues in the GFC'C" face of the IgV domain and the ABED face of the IgC domain.

Intercellular adhesion molecule-3 is the predominant co-stimulatory ligand for leukocyte function antigen-1 on human blood dendritic cells

Dendritic cells (DC) are potent stimulators of primary T lymphocyte responses to foreign antigen. The initial DC-T lymphocyte interaction involves the binding of the adhesion molecule leukocyte function antigen-1 (LFA-1; CD11a/CD18) on the T lymphocyte to an intercellular adhesion molecule (ICAM) on the DC. Although blood and tonsil DC express ICAM-1 (CD54) and ICAM-2 (CD102) on their surface, anti-ICAM-1 and anti-ICAM-2 monoclonal antibodies (mAb) have little inhibitory activity on the DC-stimulated mixed leukocyte reaction (MLR). We therefore examined the expression of the more recently identified LFA-1 ligand, ICAM-3 (CD50), in comparison to ICAM-1 and ICAM-2 on blood DC and sought a functional role for ICAM-3 in DC-mediated T lymphocyte responses. Resting blood DC expressed significantly more ICAM-3 than ICAM-1 or ICAM-2 as assessed by flow cytometry. Treatment of resting DC with interferon-gamma led to increased expression of ICAM-1; however, ICAM-2 and ICAM-3 levels remained relatively constant. Solid-phase recombinant chimeric molecules ICAM-1-, ICAM-2- and ICAM-3-Fc were able to co-stimulate CD4+ T lymphocyte proliferation in conjunction with suboptimal solid-phase CD3 mAb 64.1. However, the anti-ICAM-3 mAb CAL 3.10 inhibited a DC-stimulated MLR to a greater extent than anti-ICAM-1 or anti-ICAM-2 reagents and appeared to act by blocking the DC ICAM-3- T lymphocyte LFA-1 interaction. As ICAM-3 is the predominant LFA-1 ligand on resting blood DC, we postulate that DC may utilize ICAM-3 for initial DC- T lymphocyte interactions, and that ICAM-1, which is up-regulated upon DC activation, and/or ICAM-2, may contribute to DC migration or later phases of the T lymphocyte activation process.

T cell lines recognizing the 70-kD protein of U1 small nuclear ribonucleoprotein (U1snRNP)

In sera of patients with mixed connective tissue disease (MCTD) high titres of IgG autoantibodies to U1snRNP-specific proteins (70 kD, A, C) are found, suggesting an antigen-driven and T-cell-dependent process. In order to establish U1snRNP-specific T cell lines we cultured under various culture conditions mononuclear cells from MCTD patients and healthy donors with a highly purified UsnRNP preparation from HeLa cells. Nine T cell lines were established by limiting dilution cloning from two MCTD patients and five T cell lines from a healthy individual. All T cell lines expressed the TCR alpha beta/CD3 complex. Surprisingly, most of the T cells lines exhibited the CD8 phenotype. Irrespective of this phenotype, all T cell lines showed a proliferative response to an N-terminal part (aa 51-195) of recombinant U1-specific 70-kD protein. One CD8+ T cell clone exhibited cytotoxic activity against an autologous B cell line pulsed with snRNP or recombinant fragments (aa 51-95 and aa 51-88). Interestingly, two T cell lines proliferated in response to four recombinant polypeptides representing different parts of the U1snRNP 70-kD protein. Since regions of sequence homology are distributed over the 70-kD molecule, it is suggested that conserved motifs may be recognized by the T cell lines.

Role of B7 signaling in the differentiation of naive CD4+ T cells to effector interleukin-4-producing T helper cells

Signaling through the T cell receptor must be accompanied by costimulatory signals for the differentiation of naive T cells to cytokine-producing effector T helper cells. The costimulatory signal through CD28 is required for T cell activation resulting in increased interleukin (IL)-2 production in vitro, but its role in the production of IL-4 and in the in vivo response is still unclear. We have examined the effects of blocking CTLA-4 (the CD28 homologue) ligand interactions on the in vivo development of IL-4-producing T helper effector cells during a primary mucosal immune response to the nematode parasite Heligmosomoides polygyrus and during a primary systemic immune response to immunogenic anti-IgD antibodies. Our results demonstrate that CD28 and/or CTLA-4 signaling is required for T cell priming leading to IL-4 cytokine production, B cell activation, and IgE secretion during both immune responses, suggesting that other signaling molecules do not substitute for these molecules in either of these two different immune responses. Furthermore, the CD28 ligands, B7-1 and B7-2, can substitute for each other in providing the required T cell costimulatory ligand interactions during the primary immune response to H. polygyrus. In contrast, memory T cells during the challenge immune response do not require CD28/CTLA-4 ligand interactions for IL-4 production and T helper effector function.

Transplantation-related complications predicted by cytokine gene expression in the mixed lymphocyte culture in allogeneic bone marrow transplants

In this study, we have investigated cytokine (IL-1 beta, IL-2, IL-5, IL-6, IFN-gamma, TNF-alpha) and T cell surface molecule (IL-2 receptor, CD28, CTLA-4) gene expression in two way mixed lymphocyte cultures (MLC) enhanced by concanavalin A (ConA) to assess whether this is a useful predictive method for severe graft-versus-host disease (GVHD) and graft failure in allogeneic bone marrow transplantation (allo BMT) patients. Our present study revealed increased mRNA expression of IL-2, IL-5 and IFN-gamma using this assay in patients with delayed engraftment followed by graft failure and patients who developed grade III acute GVHD. Elevated IL-2 and IFN-gamma levels in MLC medium were also observed in these patients. Concerning T cell surface molecule gene expression in our modified MLC, IL-2 receptor gene expression was not altered so much in allo BMT patients, however, CD28 and CTLA-4 gene expression were elevated in patients with graft failure and severe acute GVHD. The elevated expression of cytokines (IL-2, IL-5 and IFN-gamma) and T cell surface molecules (CD28 and CTLA-4) mRNA in our modified MLC, in patients who developed severe lethal transplantation-related complications may suggest an important role for these molecules in inducing a strong alloresponse. Therefore, the detection of increased gene expression of those molecules, in our modified MLC system, appeared to be useful for predicting transplantation-related complications in allo BMT patients. In addition, this modified MLC assay may also be useful for the selection of the most compatible related and unrelated donors.

Construction, purification, and functional incorporation on tumor cells of glycolipid-anchored human B7-1 (CD80)

To generate a potent cell-mediated immune response, at least two signals are required by T cells. One is engagement of the T-cell receptor with peptide-bearing major histocompatibility complex molecules. The other signal can be delivered by various molecules on the antigen-presenting cell, such as B7-1 (CD80). Many tumor cells escape immune recognition by failing to express these costimulatory molecules. Transfection of the B7 gene into some murine tumor cells allows for immune recognition and subsequent rejection of the parental tumor. We have studied an alternative approach for the introduction of B7-1 onto the surface of tumor cells. This method involves purified glycosyl-phosphatidylinositol (GPI)-anchored proteins which can spontaneously incorporate their lipid tail into cell membranes. We have created and purified a GPI-anchored B7-1 molecule (called GPI-B7) which is able to bind its cognate ligand, CD28, and incorporate itself into tumor cell membranes after a short incubation. Tumor cells that have been reconstituted with GPI-B7 can provide the costimulatory signal needed to stimulate T cells. These findings suggest an approach for the introduction of new proteins onto cell membranes to create an effective tumor vaccine for potential use in human immunotherapy.

Induction of T-cell hyporesponsiveness by intrahepatic modulation of donor antigen-presenting cells

In this study, we examined the ability of varying populations of donor cells from B6 mice to induce hyporesponsiveness in T lymphocytes from C3H mice in vitro and in vivo. Small, resting B lymphocytes were inefficient stimulators of T-lymphocyte proliferation compared to splenic mononuclear cells (SMNC) and lipopolysaccharide (LPS)-induced B-cell blasts in vitro (P < 0.05). Pretreatment of SMNC with anti-B7-1 or anti-intracellular adhesion molecule-1 (ICAM-1) monoclonal antibodies (mAb) similarly resulted in inefficient stimulation of T-cell proliferation in vitro (P < 0.05). However, in vivo, only intrahepatic, but not intravenous, injection of donor cells into C3H mice resulted in decreased T-lymphocyte proliferation in response to restimulation by alloantigen. This effect was most pronounced following intrahepatic injection of resting B lymphocytes or SMNC pretreated with anti-ICAM-1 mAb compared to uninjected or intravenously injected mice (P < 0.05). The hyporesponsiveness was associated with an increased production of interleukin-4 (IL-4) by the responder T lymphocytes and correlated with enhanced skin allograft survival. These data demonstrate that intrahepatic injection of donor-derived cells induces T-lymphocyte hyporesponsiveness. The mechanism appears to be modulated by an ICAM-1-mediated signal resulting in expansion of an IL-4-producing T-lymphocyte population.

RelA is a potent transcriptional activator of the CD28 response element within the interleukin 2 promoter

T-cell activation requires two different signals. The T-cell receptor's recognition of a specific antigen on antigen-presenting cells provides one, and the second signal comes from costimulatory molecules such as CD28. In contrast, T cells that are stimulated with antigen in the absence of the CD28 costimulatory signal can become anergic (nonresponsive). The CD28 response element (CD28RE) has been identified as the DNA element mediating interleukin 2 (IL-2) gene activation by CD28 costimulation. Our previous work demonstrates that the Rel/NF-kappa B family proteins c-Rel, RelA (p65), and NFKB1 (p50) are involved in the complex that binds to the CD28RE. We also showed that c-Rel, but not NFKB1 (p50), can bind to the CD28RE and activate CD28RE-driven transcription in cotransfection assays. However, the role of RelA (p65) in CD28 signaling has not yet been addressed. We provide evidence that RelA (p65) itself bound directly to the CD28RE of the IL-2 promoter and other lymphokine promoters. In addition, RelA (p65) was a potent transcriptional activator of the CD28RE in vivo. We show that a RelA (p65)-c-Rel heterodimer bound to the CD28RE and synergistically activated the CD28RE enhancer activity. We also demonstrate that activated Raf-1 kinase synergized with RelA (p65) in activating the CD28RE enhancer activity. Interestingly, a soluble anti-CD28 monoclonal antibody alone, in the absence of other stimuli, also synergized with RelA (p65) in activating the CD28RE. Furthermore, we show that RelA (p65) activated expression of the wild-type IL-2 promoter but not the CD28RE-mutated IL-2 promoter. A combination of RelA (p65) and NFKB1 (p50) also activated the IL-2 promoter through the CD28RE site. These results demonstrate the functional regulation of the CD28RE, within the IL-2 promoter, by Rel/NF-kappa B transcription factors.

Characteristics of antigen-independent and antigen-dependent interaction of dendritic cells with CD4+ T cells

Dendritic cells (DC) are the main antigen-presenting cells for the initiation of primary T cell-mediated immune responses. In the first stage of activation, T cells bind to DC in an antigen-independent manner. We studied the adhesion characteristics of human CD4+ T cells to DC generated from CD34+ hematopoietic progenitors following 12 to 13 days of culture in the presence of granulocyte/macrophage colony-stimulating factor and tumor necrosis factor-alpha. A majority of these cells had the morphology, phenotype and functions of DC. CD4+ T/DC adhesion was measured by means of fluorescence microscopy and flow cytometry. Four independent receptor/ligand pathways, LFA-1/ICAM, ICAM/LFA-1, CD2/LFA-3 and CD28/CD80, were involved in the transient adhesion of DC to CD4+ T cells in antigen-independent and specific alloantigen-dependent situations, as shown by blocking experiments using monoclonal antibodies. The antibodies also blocked a primary mixed lymphocyte reaction (MLR) in which DC were used as stimulatory cells. Adhesion of alloreactive CD4+ T cells to antigen-presenting DC was stronger than that of resting CD4+ T cells, while peak adhesion occurred after 5 and 20 min, respectively. The LFA-1 ligands involved in adhesion of resting CD4 T cells to DC and alloreactive CD4+ T cells to specific DC differed in part, since ICAM-3 on resting T cells and ICAM-1 on alloreactive T lymphocytes preferentially bound LFA-1. Studies of interactions between DC and phorbol ester-activated T cells expressing the CD40 ligand revealed a fifth independent adhesion pathway, CD40/CD40 ligand. CD4-mediated regulation of CD4+ T/DC adhesion was suggested by the observation that preincubation of CD4+ T cells and DC individually with anti-CD4 antibodies inhibited adhesion. In addition, antibodies specific for HLA class II molecules inhibited adhesion when used to pretreat DC but not alloactivated CD4+ T cells.

Direct and indirect control of T-cell activation by keratinocytes

Keratinocytes can function as antigen-presenting cells/accessory cells and regulate T cells with three distinct outcomes, depending on the nature of the stimulus. In the presence of alloantigen, it appears that a "null" event takes place between T cells and keratinocytes, with neither activation nor induction of tolerance. Using nominal antigen, keratinocytes induce antigen-specific tolerance. In contrast, with bacterial-derived superantigens, phytohemagglutinin, or immobilized CD3 monoclonal antibody, keratinocytes can significantly activate resting autologous T-cell proliferation and cytokine release. To understand these highly divergent responses, we focused on the two-signal model of T-cell activation, with particular emphasis on costimulatory molecules expressed by keratinocytes. Such second signals, as highlighted by the B7 and CD28 receptor families, provide useful insights into the complex interactions involving keratinocytes and T cells. In this review, we summarize recent evidence indicating that keratinocytes regulate T-cell activation in a direct and indirect manner by their mutual expression and responsiveness involving adhesion molecules, cytokines, and costimulatory signals. As investigative momentum continues to grow in the fields of immunology and keratinocyte biology, it is likely that manipulation of CD28:B7 interactions will not only provide a useful model to understand further the complexities of skin immune reactions, but will also serve as the basis for new therapeutic opportunities for numerous T-cell-mediated diseases that involve aberrant reactions with keratinocytes.

Differential activation through the TCR-CD3 complex affects the requirement for costimulation of human T cells

Although it is well established that T cells require at least two activation signals, the coordination of primary signaling through the TCR-CD3 complex with costimulatory signals through accessory molecules is incompletely understood. To mimic the signal provided by natural ligand for TCR, we used eight anti-TCR V-region-specific mAbs as well as two anti-TCR-CD3 mAbs, OKT3 and T10B9, to stimulate human peripheral blood T cells in the presence or absence of accessory cells. With accessory cells, only OKT3 in soluble form stimulated T cells, but when mABs were immobilized on plastic, all except the V alpha mAb, F1, induced proliferation. This result suggests that the signaling qualities of TCR V-region-specific mAb may differ from OKT3, which activates through CD3 epsilon. To address this issue, the costimulatory requirements of two V beta-specific mAbs, 1C1 and OT145, were also compared with OKT3 and T10B9 using T cells depleted of accessory cells. The V beta-specific mAb and T10B9 could only be complemented by costimulation through the CD28 molecules, whereas OKT3 was able to synergize with mAb directed not only at CD28, but also CD2 and CD11a. Furthermore, mAb specific for CD80 was able to block activation of T cells in the presence of accessory cells when V beta-specific mAbs were used to activate, whereas anti-CD80 had no effect on activation of T cells by immobilized OKT3. Thus the nature of the signal received through the TCR-CD3 complex, whether TCR alpha beta or CD3 epsilon, may determine the qualitative requirements for costimulation.

Long-term inhibition of murine experimental autoimmune encephalomyelitis using CTLA-4-Fc supports a key role for CD28 costimulation

T cell activation involves not only recognition of antigen presented by the MHC, but also nonspecific interactions termed "costimulation." The costimulatory molecules B7-1 and B7-2 are ligands on antigen-presenting cells for the CD28 and CTLA-4 receptors on T cells. Previously, a fusion protein consisting of human CTLA-4 linked to human Fc was shown to bind B7-1 and B7-2 with high avidity and to prevent specific T cell activation. Here we investigated the effects of a recombinant fusion protein consisting of the extracellular domain of human CTLA-4 bound to mouse IgG2a Fc (CTLA-4-Fc) upon experimental autoimmune encephalomyelitis, a T cell-mediated disease that serves as a model for multiple sclerosis. CTLA-4-Fc prevented experimental autoimmune encephalomyelitis in 26 of 28 CTLA-4-Fc-treated mice (median maximum score 0), whereas 28 of 30 mice treated with control mouse IgG2a developed disease (median maximum score 2.75). Less inflammation and virtually no demyelination or axonal loss occurred in CTLA-4-Fc-treated compared with control-treated mice. Activated splenocytes from CTLA-4-Fc-treated mice were able to transfer disease adoptively to naive recipients. These results indicate a key role for the B7/CD28 system in the development of actively induced murine experimental autoimmune encephalomyelitis, suggesting an area of investigation with therapeutic potential for multiple sclerosis.

CD28-B7 blockade after alloantigenic challenge in vivo inhibits Th1 cytokines but spares Th2

Blocking the CD28-B7 T cell costimulatory pathway with the fusion protein CTLA4Ig inhibits alloimmune responses in vitro and in vivo and induces tolerance to cardiac allografts in mice and rats, but the mechanisms mediating the tolerant state in vivo are unknown. Here, we report the effects and potential mechanisms of CTLA4Ig in the rat renal allograft model. LEW rats were nephrectomized and received renal allografts from major histocompatibility complex-incompatible WF rats. While all untreated and control immunoglobulin (Ig)-treated animals acutely rejected their allografts and died, 86% of rats that received a single injection of CTLA4Ig on day 2 after transplantation had prolonged survival (> 60-100 days) with preserved renal function. By contrast, only 29% of animals that received CTLA4Ig on the day of engraftment had prolonged survival. Long-term survivors (> 100 days) exhibited donor-specific tolerance, accepting donor-matched WF but acutely rejecting third-party BN cardiac allografts. Immunohistological analysis of grafts sampled at 1 week after transplantation showed that both control and CTLA4Ig-treated animals had mononuclear cell infiltrates, with a higher percentage of CD4+ cells in the CTLA4Ig-treated group. However, while this was associated with vasculitis and tubulitis in control grafts, there was no evidence of tissue injury in CTLA4Ig-treated animals. The immune response leading to graft rejection in control animals was characterized by expression of the T helper (Th) type 1 cytokines interleukin (IL)-2 and interferon-gamma. In contrast, the persistent CD4+ infiltrate without graft rejection in CTLA4Ig-treated animals was associated with increased staining for the Th2-related cytokines IL-4 and IL-10. Furthermore, grafts from CTLA4Ig-treated animals had marked upregulation of intragraft staining for IgG1, but not IgG2a or IgG2b. Administration of rIL-2 to CTLA4Ig-treated animals restored allograft rejection in 50% of animals tested. These results confirm that blockade of the CD28-B7 pathway after alloantigenic challenge induces donor-specific acceptance of vascularized organ allografts, and indicates in this model that CTLA4Ig inhibits Th1 but spares Th2 cytokines in vivo.

Co-expression of B7-1 and ICAM-1 on tumors is required for rejection and the establishment of a memory response

Although the transfection of B7-1 cDNA into a few mouse tumor cell lines can induce anti-tumor T cell immunity, its expression alone is ineffective in many other tumor cell lines tested. We were interested to study what factors limit B7-1 co-stimulatory activity, and decided to investigate whether B7-1 requires the cooperation of ICAM-1 to provide the minimal co-stimulatory signal for establishing an efficient anti-tumor immunity. We show that the transfection of B7-1 cDNA into three ICAM-1+ (plasmocytoma J558L, T lymphomas EL-4 and RMA), but not into two ICAM-1- tumors cell lines (adenocarcinoma TS/A and melanoma B16.F1), is sufficient to induce their complete rejection in syngeneic mice. The expression of ICAM-1 is necessary for the rejection of the B7 expressing tumors, since the primary response elicited by B7-1+ EL-4 and RMA clones expressing reduced levels of ICAM-1 is severely reduced. Furthermore, super-transfection of ICAM-1 cDNA into B7-1+ adenocarcinoma and melanoma clones optimizes their primary rejection. Histologic examination of transfected tumors reveals that B7-1 and ICAM-1 exert a potent pro-inflammatory activity. The intra-tumor infiltration is composed of both eosinophils and lymphomonocytes, and is already massive 5 days after the tumor challenge. The primary rejection of the B7-1+ ICAM-1+ tumors depends critically on CD8+ T cells, natural killer cells and granulocytes, but is independent of CD4+ T cells. Remarkably, in addition to its effects on the early phases of the immune response, the co-expression of ICAM-1 and B7-1 on tumors is also necessary for the efficient induction of a memory response. In fact, only the primary challenge with B7-1+, ICAM-1+ tumor cells protects the majority of the mice from a second injection of parental tumor cells. Collectively, our findings indicate that B7-1 and ICAM-1 are fundamental components for triggering the primary rejection of tumors and establishing a protective memory response. These findings may help to define new strategies for the rational application of co-stimulation in tumor immunotherapy.

Expression of heat-stable antigen on tumor cells provides co-stimulation for tumor-specific T cell proliferation and cytotoxicity in mice

Heat-stable antigen (HSA/J11d/possibly homologous to CD24), a cell adhesion molecule capable of providing a co-stimulatory signal for T cell proliferation, is expressed on B cells, activated T cells, monocytes, granulocytes, Langerhans cells and thymocytes. Recent studies have demonstrated that co-stimulatory signals provided by cell adhesion molecules such as B7-1 play an essential role in generation of an anti-tumor immune response. To examine whether the co-stimulatory signal provided by HSA can induce an anti-tumor immune response, we have transfected HSA cDNA into the murine melanoma cell line K1735M2, and examined the ability of this transfected cell line to induce tumor-specific T cell responses. The results demonstrate that spleen cells from mice immunized with HSA-transfected K1735M2 cells showed enhanced T cell proliferation in a mixed lymphocyte tumor reaction (MLTR) assay and also demonstrated a significant anti-tumor cytotoxicity to the parent tumor cell (K1735M2). This anti-tumor cytolytic activity could be abrogated by pretreatment of effector cells with anti-mouse CD8 monoclonal antibody and complement. Under similar conditions, spleen cells from C3H mice immunized with vector-transfected K1735M2 cells neither actively proliferate in an MLTR assay, nor did they exert significant cytolytic activity against the respective tumor cells. In summary, our study demonstrated that HSA can provide a co-stimulatory signal for the T cell immune response against tumor cells in a murine model.

Mutational analysis and an alternatively spliced product of B7 defines its CD28/CTLA4-binding site on immunoglobulin C-like domain

Costimulatory molecules B7 and B7-2 interact with T cell surface receptors CD28/CTLA4 and deliver a costimulatory signal essential for T cell growth. However, the structure basis of this interaction is not known. B7 and B7-2 are members of immunoglobulin (Ig) superfamily and their extracellular portion consists of an IgV- and IgC-like domain. Here we report that a naturally occurring, alternatively spliced form of B7 reveals that exon 3-encoded IgC domain is essential for CD28/CTLA4 binding. Mutational analysis of B7 demonstrates a critical role of several amino acids around loops between strands B and C and D and E, for binding CTLA4/CD28. These amino acids are clustered to form a single binding site centered at 201Y. A comparison of the effects of mutations on the binding of CD28 and CTLA4 reveals that CD28 and CTLA4 binds to the same site on B7. These results have important implications on the role of CTLA4 and CD28 in T cell costimulation. The structure of the CD28/CTLA4-binding site also provides valuable information for immune intervention targeted at the B7/B7-2-CD28/CTLA4 interactions.

Antigen processing: the gateway to the immune response

The T-lymphocyte response to an antigen is governed by the source of that antigen and the way in which it is processed. Before recognition by T lymphocytes, proteins must be degraded to peptides by antigen-presenting cells. The peptides are then presented on major histocompatibility complex (MHC) molecules for recognition by the T cells. Antigens arising outside the cell (e.g., bacteria) are phagocytosed and processed by the exogenous pathway for presentation on MHC class II molecules (e.g., DR) to CD4+ cells. Antigens derived from the cytoplasm (e.g., viral proteins) are processed by the endogenous pathway for presentation by MHC class I molecules (e.g., HLA-A, -B, -C) to CD8+ cells. The response to a hapten or drug is a function of the antigen processing pathway and is determined by its chemical properties. Antigen processing also governs the T-cell response to pathogens, vaccines, and autoimmune conditions.

Alloantigen priming induces a state of unresponsiveness in human umbilical cord blood T cells

Induction of alloreactivity in human adult and umbilical cord blood T cells was evaluated in mixed leukocyte culture by exposure to an allogeneic lymphoblastoid line that expresses known costimulatory molecules. Initial exposure to alloantigen-presenting cells (allo-APC) induced strong proliferative responses in both adult and cord blood T cells. However, in contrast to adult T cells, cord blood T cells exhibited little proliferation after restimulation with donor APC. Primed cord blood T cells could respond to interleukin 2 (IL-2), but unresponsiveness to alloantigen was not overcome by addition of exogenous IL-2. Unresponsiveness was long-lasting and appeared to be maintained by a combination of induction of anergy and activity of CD8+ suppressor cells. This information may contribute to use of human cord blood as an allogeneic source of transplantable stem cells.

Memory B cells from human tonsils colonize mucosal epithelium and directly present antigen to T cells by rapid up-regulation of B7-1 and B7-2

Human memory B cells that carry mutated IgV region genes were isolated from tonsils by negative selection of IgD+ naive B cells and CD38+ germinal center B cells and plasma cells. They were mainly found within the intraepithelial areas, but not in the B cell follicles of human tonsils. Memory B cells but not naive B cells have the capacity to present antigen directly to T cells, owing to the constitutive expression of the accessory molecules B7-1/CD80 and B7-2/CD86. Signals through antigen receptors and CD40 antigen result in these two molecules being further up-regulated more rapidly and strongly on memory B cells than on naive B cells. The unique anatomical localization of memory B cells beneath the surface of mucosa, together with their strong APC capacity, may explain the well-known prompt and robust secondary antibody responses.

Specific hyporesponsiveness of alloreactive peripheral T cells induced by CD4 antibodies

We investigated whether exposure of naive and in vitro pre-activated T cells to CD4 monoclonal antibodies (mAb) could induce specific hyporesponsiveness to a subsequent challenge in the absence of CD4 mAb. Unfractionated peripheral blood mononuclear cells were cultured with mitomycin-treated B cell lines as stimulator cells, in the presence or absence of CD4 mAb, then challenged with the same or unrelated stimulator cells. The kinetics of [3H] thymidine incorporation, blast transformation and CD25 expression were determined. Cells activated in primary or secondary culture in the presence of CD4 mAb demonstrated a markedly decreased response to subsequent challenge in the absence of antibody. This effect was reproduced with three different CD4 mAb of the IgG1 and IgG2a subclasses, which recognize two distinct epitopes of the CD4 molecule. Addition of recombinant interleukin-2 (rIL-2) during exposure to CD4 mAb failed to prevent the induction of specific hyporesponsiveness. Similarly, exogenous rIL-2, added together with stimulating cells, failed to restore the specific proliferative response, indicating that the mechanisms were different from those of classical anergy. The hyporesponsiveness was clonally restricted since CD4 mAb-pretreated cells developed a normal primary response to third-party stimulator cells. No increase in the percentage of apoptotic cells was observed in hyporesponsive cell populations, but selective clonal deletion cannot be excluded. The data demonstrate a delayed effect of CD4 ligation on T cell responses to a subsequent challenge.

Human alveolar macrophages present antigen ineffectively due to defective expression of B7 costimulatory cell surface molecules

Alveolar macrophages, resident phagocytic cells in the lung that derive from peripheral blood monocytes, are paradoxically ineffective in presenting antigen to T cells. We found that antigen presentation by alveolar macrophages could be restored by the addition of anti-CD28 mAb to cultures of T cells and macrophages, indicating that costimulation by alveolar macrophages via the CD28 pathway was defective. In addition, we found that alveolar macrophages activated with IFN-gamma failed to express B7-1 or B7-2 antigens, which normally ligate CD28 on T cells and provide a costimulatory signal required for the activation of T cells. These observations are the first to demonstrate the inability of a "professional" antigen-presenting cell type to effectively express the costimulatory molecules B7-1 and B7-2. Inasmuch as immune reactions within the lung are inevitably associated with inflammatory injury to pulmonary tissue, these observations suggest that reduced expression of B7-1 and B7-2 by alveolar macrophages may be advantageous, as a critical mechanism involved in the induction of peripheral tolerance to the abundance of antigens to which mucosal tissues are continuously exposed.

B7-1 expression of Langerhans cells is up-regulated by proinflammatory cytokines, and is down-regulated by interferon-gamma or by interleukin-10

Langerhans cells (LC) act as potent antigen-presenting cells (APC) for primary and secondary T cell-dependent immune responses. LC express several costimulatory and/or adhesion molecules such as B7/BB1, which has been implicated as one of the important determinants for professional APC. Recent studies have shown that B7/BB1 antigens comprise three distinct molecules termed B7-1, B7-2, and B7-3. We have examined the regulatory properties of B7-1 expression in LC using various cytokines including interleukin (IL)-1 alpha, IL-1 beta, IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, IL-10, interferon (IFN)-gamma, granulocyte/macrophage colony-stimulating factor (GM-CSF), and tumor necrosis factor (TNF)-alpha. We have demonstrated: 1) that the B7-1 expression of LC is reproducibly up-regulated by either GM-CSF, TNF-alpha, IL-1 alpha, IL-1 beta, or IL-4 in a dose- and time-dependent manner, 2) that GM-CSF exhibits the most active effect on B7-1 up-regulation in each experiment, 3) that IFN-gamma or IL-10 profoundly inhibits the B7-1 expression of LC in a dose- and time-dependent manner, and 4) that the down-regulatory ability of IFN-gamma or IL-10 neutralizes the activity of up-regulatory cytokines. The enhancing or inhibitory action of these cytokines on B7-1 expression occurs selectively because none of the cytokines consistently affects I-A expression of LC. These data suggest that the B7-1 expression of LC may be dynamically regulated by these up- and down-regulatory cytokines in normal and inflammatory epidermal microenvironment.

Phosphatidylinositol 3-kinase activity is not essential for CD28 costimulatory activity in Jurkat T cells: studies with a selective inhibitor, wortmannin

The interaction of CD28 with its counter-receptor, B7-1 (CD 80), on antigen-presenting cells induces a co-signal in T cells required to promote antigen-dependent interleukin-2 (IL-2) production and to prevent clonal anergy. CD28 stimulation causes both protein-tyrosine kinase and phosphatidylinositol3-kinase (PI3-K) activation, suggesting a possible role for these enzyme activities in CD28 co-signal transduction. Here, we investigate the effect of wortmannin, a selective and irreversible PI3-K inhibitor on CD28 co-signaling events in the Jurkat T cell line. Wortmannin added to cell cultures partially inhibits CD28-induced tyrosine phosphorylation of the putative p110 catalytic subunit of PI3-K, but does not block CD28-induced association of the p85 PI3-K regulatory subunit with the CD28 receptor. Wortmannin inhibits in a dose-dependent manner both total cellular PI3-K activity and CD28-induced receptor-associated PI3-K activity. Wortmannin (1 microM) inhibits cellular PI3-K activity by 90% with complete inhibition achieved at 10 microM. The inhibitory effect of wortmannin on cellular PI3-K activity is prolonged ( > 18 h), suggesting that the drug is not readily metabolized by Jurkat T cells. Wortmannin, at concentrations that blocked PI3-K activity, fails to inhibit the synergistic effect of CD28 on IL-2 secretion in the presence of phorbol 12-myristate 13-acetate and ionomycin. These data demonstrate that CD28-induced signaling events other than the activation of PI3-K catalytic activity contribute to the control of IL-2 secretion.

Prostaglandin E2 promotes the induction of anergy during T helper cell recognition of myelin basic protein

Remission of experimental autoimmune encephalomyelitis (EAE) in Lewis rats may involve mediators such as prostaglandins (PG) that are produced within demyelinating lesions and are known to potently inhibit T cell responses. In support, this study shows that PGE2 inhibited myelin basic protein (MBP)-specific responses of proliferation and IL-2 production by continuously propagated lines of T-helper cells. Simultaneous exposure to PGE2 and immunogenic MBP rendered T cells profoundly anergic. Even after several weeks of propagation in IL-2-containing medium, anergic T cells exhibited marked reductions in MBP-stimulated proliferation and IL-2 production responses when restimulated with optimal concentrations of MBP and irradiated splenocytes (SPL). PGE2 did not block other measures of MBP-dependent activation, including induction of postactivation refractoriness in IL-2 production pathways, activation-dependent decreases in MBP reactivity, and activation-dependent increases in PGE2 sensitivity. Proliferative responses by anergic T cells were reduced in magnitude but were not altered in their sensitivity to MBP. PGE2-mediated anergy was manifest as an intrinsic deficit rather than an acquired suppressive activity and was associated with reduced mitogenic responsiveness and a block in IL-2 production pathways. Anergic T cells were responsive to IL-2 and eventually regained full antigenic reactivity after extended propagation in IL-2-supplemented medium. In summary, a limited exposure to PG had long-lasting inhibitory effects on subsequent T cell responsiveness to the target autoantigen MBP. These findings support the hypothesis that PG may promote disease remission by inducing anergy in helper T cells.

Effect of CD28 signal transduction on c-Rel in human peripheral blood T cells

Optimal T-cell activation requires both an antigen-specific signal delivered through the T-cell receptor and a costimulatory signal which can be delivered through the CD28 molecule. CD28 costimulation induces the expression of multiple lymphokines, including interleukin 2 (IL-2). Because the c-Rel transcription factor bound to and activated the CD28 response element within the IL-2 promoter, we focused our study on the mechanism of CD28-mediated regulation of c-Rel in human peripheral blood T cells. We showed that CD28 costimulation accelerated the kinetics of nuclear translocation of c-Rel (and its phosphorylated form), p50 (NFKB1), and p65 (RelA). The enhanced nuclear translocation of c-Rel correlated with the stimulation of Il-2 production and T-cell proliferation by several distinct anti-CD28 monoclonal antibodies. This is explained at least in part by the long-term downregulation of I kappa B alpha following CD28 signalling as opposed to phorbol myristate acetate alone. Furthermore, we showed that the c-Rel-containing CD28-responsive complex is enhanced by, but not specific to, CD28 costimulation. Our results indicate that c-Rel is one of the transcription factors targeted by CD28 signalling.

Human B7-1 (CD80) and B7-2 (CD86) bind with similar avidities but distinct kinetics to CD28 and CTLA-4 receptors

B7-0 or B7-2 (CD86) is a T cell costimulatory molecule that binds the same receptors (CD28 and CTLA-4) as B7-1 (CD80), but shares with it only approximately 25% sequence identity and is expressed earlier during an immune response. Here we show that human CD86 maintains similar (within approximately 2- to 3-fold) overall receptor binding and T cell costimulatory properties as CD80. However, CD80 and CD86 did not bind equivalently to CTLA-4: CD80 bound Y100A, a form of CTLA4lg with a mutation in the CDR3-like region, > 200-fold better than did CD86; inhibition of CD80-mediated cellular responses required approximately 100-fold lower CTLA4lg concentrations; and CD80-CTLA4lg complexes dissociated 5- to 8-fold more slowly, Thus, CD80 and CD86 utilize different binding determinants and have different kinetics of binding to CD28 and CTLA-4.

Complementarity determining region 1 (CDR1)- and CDR3-analogous regions in CTLA-4 and CD28 determine the binding to B7-1

T cell surface receptors CD28 and CTLA-4 are homologous members of the immunoglobulin superfamily (IgSF), each comprising a single V-like extracellular domain. CD28 and CTLA-4 bind to the B7-1 and B7-2 counter-receptors on antigen presenting cells (APCs), thereby triggering a costimulatory pathway important for optimal T cell activation in vitro and in vivo. Soluble forms of CD28 and CTLA-4 in which the V-like extracellular domains were fused to Ig constant domains (CD28Ig and CTLA4Ig), have been used to study their interactions with B7-1 and B7-2, with CTLA4Ig binding B7-1 more strongly than CD28Ig (approximately 20-fold higher avidity). We have now, by site-specific and homologue mutagenesis, identified regions in CTLA4Ig important for strong binding to B7-1. A hexapeptide motif (MYPPPY) in the complementarity determining region 3 (CDR3)-like region is fully conserved in all CD28 and CTLA-4 family members. Alanine scanning mutagenesis through the motif in CTLA4Ig and at selected residues in CD28Ig reduced or abolished binding to B7-1. Chimeric molecules HS4, HS4-A, and HS4-B were constructed in which CDR3-like regions of CTLA-4, COOH-terminally extended to include nonconserved residues, were grafted onto CD28Ig. These homologue mutants showed stronger binding to B7-1 than did CD28Ig. Grafting of the CDR1-like region of CTLA-4, which is not conserved in CD28 and is predicted to be spatially adjacent to CDR3, into HS4 and HS4-A, resulted in chimeric molecules (HS7 and HS8) which bound B7-1 even better. Inclusion of the CDR2-like domain of CTLA-4 into HS7 and HS8 did not further increase binding. Thus, the MYPPPY motifs of CTLA4Ig and CD28Ig are important for their binding to B7-1, but the increased strength of this binding by CTLA4Ig is mediated by nonconserved residues in the CDR1- and CDR3-analogous regions.

Effect of CD28 signal transduction on c-Rel in human peripheral blood T cells

Optimal T-cell activation requires both an antigen-specific signal delivered through the T-cell receptor and a costimulatory signal which can be delivered through the CD28 molecule. CD28 costimulation induces the expression of multiple lymphokines, including interleukin 2 (IL-2). Because the c-Rel transcription factor bound to and activated the CD28 response element within the IL-2 promoter, we focused our study on the mechanism of CD28-mediated regulation of c-Rel in human peripheral blood T cells. We showed that CD28 costimulation accelerated the kinetics of nuclear translocation of c-Rel (and its phosphorylated form), p50 (NFKB1), and p65 (RelA). The enhanced nuclear translocation of c-Rel correlated with the stimulation of Il-2 production and T-cell proliferation by several distinct anti-CD28 monoclonal antibodies. This is explained at least in part by the long-term downregulation of I kappa B alpha following CD28 signalling as opposed to phorbol myristate acetate alone. Furthermore, we showed that the c-Rel-containing CD28-responsive complex is enhanced by, but not specific to, CD28 costimulation. Our results indicate that c-Rel is one of the transcription factors targeted by CD28 signalling.

Prevention of T cell anergy by signaling through the gamma c chain of the IL-2 receptor

When stimulated through their antigen receptor without requisite costimulation, T cells enter a state of antigen-specific unresponsiveness termed anergy. In this study, signaling through the common gamma chain of the interleukin-2 (IL-2), IL-4, and IL-7 receptors in the presence of antigen was found to be sufficient to prevent the induction of anergy. After culture with IL-2, IL-4, or IL-7, Jak3 kinase was tyrosine-phosphorylated, which correlated with the prevention of anergy. Therefore, a signal through the common gamma chain may regulate the decision of T cells to either clonally expand or enter a state of anergy.

CD28 signal transduction: tyrosine phosphorylation and receptor association of phosphoinositide-3 kinase correlate with Ca(2+)-independent costimulatory activity

The interaction of CD28 with its counter-receptor, B7, induces a cosignal in T cells required to prevent clonal anergy and to promote antigen-dependent interleukin-2 production. The molecular basis of the CD28 cosignal is not well understood but involves the activation of protein tyrosine kinase(s) (PTK). In this report we demonstrate that CD28 cross-linking on Jurkat T leukemic cells causes the activation of at least two PTK pathways. A CD28-induced, p56lck kinase-independent pathway causes tyrosine-phosphorylation of a 110-kDa substrate while recruitment of p56lck kinase activity is apparently required for CD28-induced tyrosine-phosphorylation of 97- and 68-kDa substrates as well as CD28-induced increases in intracellular calcium. The tyrosine phosphorylation of p110, but not p97 or p68, correlated with CD28 calcium-independent costimulatory activity. The pp110 molecule was tentatively identified as the catalytic subunit of phosphoinositide (PI)-3 kinase based upon its coimmunoprecipitation with the p85 regulatory subunit of PI-3 kinase. PI-3 kinase protein and catalytic activity were found complexed with the CD28 receptor if the receptor was "activated" by cross-linking on the surface of intact cells prior to detergent solubilization. The kinetics of association of PI-3 kinase with the "activated" CD28 receptor was rapid, occurring within 30 s of receptor cross-linking and was stable for at least 30 min. Analysis of the CD28 cytoplasmic peptide sequence revealed a putative PI-3 kinase src homology 2 binding motif and CD28 tyrosine phosphorylation site, DYMNM. Tyrosine phosphorylation of CD28 was detected in pervanadate-treated Jurkat B2.7 cells, but not untreated cells. Pervanadate-induced tyrosine phosphorylation of CD28 correlated with receptor association of PI-3 kinase in the absence of CD28 cross-linking, suggesting that CD28 association with PI-3 kinase uses a tyrosine phosphorylation-dependent mechanism. These data provide a model for CD28 signal transduction and support a role for PI-3 kinase in mediating the CD28 calcium-independent, cyclosporin A-insensitive costimulatory signal.

CD2 is involved in maintenance and reversal of human alloantigen-specific clonal anergy

Induction and maintenance of a state of T cell unresponsiveness to specific alloantigen would have significant implications for human organ transplantation. Using human histocompatibility leukocyte antigen DR7-specific helper T cell clones, we demonstrate that blockade of the B7 family of costimulatory molecules is sufficient to induce alloantigen-specific T cell clonal anergy. Anergized cells do not respond to alloantigen and a variety of costimulatory molecules, including B7-1, B7-2, intercellular adhesion molecule-1 (ICAM-1), and lymphocyte function-associated molecule (LFA)-3. However, after culture in exogenous interleukin (IL)-2 for at least 7 d, anergized cells can respond to alloantigen in the presence of LFA-3. LFA-3 costimulation subsequently restores responsiveness to alloantigen in the presence of previously insufficient costimulatory signals. Expression of CD2R epitope is downregulated on anergic cells and is restored after 7 d of IL-2 culture. The loss of the CD2R is temporally associated with the inability of anergized cells to respond to LFA-3. These results suggest that in addition to blockade of B7 family members, inhibition of CD2 and, potentially, other costimulatory pathways that might reverse anergy will be necessary to maintain prolonged alloantigen-specific tolerance.

Decline in CD28+ T cells in centenarians and in long-term T cell cultures: a possible cause for both in vivo and in vitro immunosenescence

The dramatic decline in immune function with age, especially in T cell proliferative activity, has been documented extensively in experimental animal models and in clinical studies of the elderly. A similar proliferative decline is also seen in long-term T lymphocyte cultures used to study in vitro cellular senescence. We have compared the peripheral blood T lymphocytes of centenarians and younger controls for the cell surface expression of CD28, a costimulatory molecule that is required for optimal activation and proliferation following engagement of the T cell receptor. Our analysis shows a significant decrease (p < 0.001) in the percentage of T cells expressing CD28 in the elderly cohort, with values ranging from 44% to 90%, as compared to the mean control value of 91%. The decline in the percentage of CD28+ T cells correlates with a reduction in the CD4/CD8 ratio (r2 = 0.695, p < 0.0001). Concommitantly, experiments using an in vitro T cell culture system showed a progressive loss of CD28 expression with culture "age." The concordance of proliferative decline and loss of CD28 in the centenarians and in the in vitro cultures suggest that a Hayflick phenomenon may operate in vivo leading to immunosenescence.

Blockade of the CD28 co-stimulatory pathway: a means to induce tolerance

Presentation of antigen to the T-cell receptor (TCR) without co-stimulation results in a state of antigen-specific unresponsiveness on rechallenge, known as anergy in vitro and tolerance in vivo. Mounting evidence suggests that inhibition of the B7-CD28 co-stimulatory pathway is both necessary and sufficient to induce antigen-specific T-cell anergy. Anergy is not static because specific signals are required to maintain this state and prevent its reversal. Attention to these issues will be critical to translate these basic studies to the clinical arenas of transplantation, tumor immunity and autoimmunity.

Immunological functions of non-professional antigen-presenting cells: new insights from studies of T-cell interactions with keratinocytes

T-cell activation in the absence of co-stimulatory signals can lead to induction of anergy. Professional antigen-presenting cells (APCs) of bone marrow origin, such as macrophages and dendritic cells, can provide co-stimulation through molecules such as B7-1 and B7-2. In addition, cells of epithelial origin can function as 'non-professional' APCs when activated. In these circumstances, the functional consequences of the T cell-APC interaction may differ, perhaps due to the nature of the co-stimulatory pathways utilized and/or the cytokines encountered by the T cell. Here, Brian Nickoloff and Laurence Turka suggest that these differences may be important in regulating immune responses to local antigens and also in maintaining self-tolerance.

Induction of tolerance in freshly isolated alloreactive CD4+ T cells by activated T cell stimulators

Activated human T cells express major histocompatibility complex class II proteins, and their potential to present antigens to T cell clones has been documented extensively. The effect of such TT presentation on responder T cell clones has been shown to be the induction of tolerance, sometimes accompanied by activation. To investigate whether freshly isolated responder T cells are also susceptible to such induction of tolerance by activated T cells functioning as antigen-presenting cells (APC), we have used the capability of unprimed ex vivo T cells to respond in a proliferation assay in vitro to alloligands on professional APC. We show that purified human T cells ex vivo, when exposed to alloligand on activated T cells for primary allorecognition in vitro, fail to mount a proliferative response. Priming of responder CD4+ T cells with alloligand expressed on activated T cells results in the induction of nonresponsiveness to a subsequent challenge by competent allo-APC. This ability of activated, HLA-DR+ T cells to induce nonresponsiveness to subsequent challenge in bulk CD4+ T cell populations ex vivo has interesting implications for infections involving T cells such as human immunodeficiency virus.

Antibody to the ligand of CD40, gp39, blocks the occurrence of the acute and chronic forms of graft-vs-host disease

Chronic and acute graft-versus-host disease (cGVHD and aGVHD) result from donor cells responding to host disparate MHC alleles. In cGVHD (H-2d-->H-2bd), heightened polyclonal immunoglobulin production is due to the interaction of donor allospecific helper T cells (Th) and the host B cells. In vivo administration of antibody to the ligand for CD40, gp39, blocked cGVHD-induced serum anti-DNA autoantibodies, IgE production, spontaneous immunoglobulin production in vitro, and associated splenomegaly. Antibody production remained inhibited for extended periods of time after termination of anti-gp39 administration. Antiallogeneic CTL responses induced in a GVHD were also prevented by the in vivo administration of anti-gp39 as was the associated splenomegaly. These data suggest that CD40-gp39 interactions are critical in GVHD and that CD40-gp39 may be a valuable ligand-receptor pair for targeting immunotherapeutic agents to control GVHD.

Selective targeting of the immune response in autoimmune demyelination

Recent developments in molecular and cellular immunology have led to the formulation of refined models that describe how tolerance to self-antigens is broken and autoimmunity develops. This knowledge can now be used to develop alternative approaches to conventional immunosuppression for the treatment of autoimmune demyelinating disorders. The ideal therapy would reverse established disease or prevent further progression by selectively eliminating the aggressive effector molecules or cells while leaving the immune system virtually intact. Indeed, several groups are engaged in preliminary or advanced clinical studies of promising specific immunotherapies for multiple sclerosis and other autoimmune conditions. Current data suggest that the immune response that results in organ-specific autoimmunity is highly complex and redundant in humans. This suggests that antigen-specific approaches may be less successful than broader immunotherapeutic strategies for treating multiple sclerosis and related diseases.

Absence of B7-dependent responses in CD28-deficient mice

Costimulation of T cell proliferation can occur through the CD28 signal transduction pathway. In addition, other cell surface receptors, including the CD28 homolog CTLA-4, have been proposed to be capable of providing costimulatory signals. We have examined the response of CD28-deficient T cells to activation by a variety of agonists. We demonstrate that proliferation of CD28-deficient T cells in the presence of antigen-presenting cells or B7-1 transfectants is markedly reduced. Although CTLA-4 can be expressed on CD28-deficient T cells, we observed no B7-dependent costimulation in the absence of CD28. This data demonstrates that CD28 is the major B7-binding costimulatory ligand on T cells. Furthermore, our data suggest that CD28 is the primary, and perhaps exclusive, costimulatory receptor used by traditional antigen-presenting cells to augment the proliferation of antigen-activated T cells.

Treatment of murine lupus with CTLA4Ig

The interaction of B7-related molecules on antigen-presenting cells with CD28 or CTLA-4 antigens on T cells provides a second signal for T cell activation. Selection inhibition of the B7-CD28 or B7-CTLA-4 interactions produces antigen-specific T cell unresponsiveness in vitro and suppresses immune function in vivo. To determine whether selective inhibition of the B7-CD28 or B7-CTLA-4 interactions could suppress spontaneous autoimmune disease, a B7-binding protein was generated by genetic fusion of the extracellular domain of murine CTLA-4 to the Fc portion of a mouse immunoglobulin G2a monoclonal antibody (muCTLA4Ig). In lupus-prone NZB/NZW filial generation (F1) mice, treatment with muCTLA4Ig blocked autoantibody production and prolonged life, even when treatment was delayed until the most advanced stage of clinical illness. These findings suggest a possible role for human CTLA4Ig in the treatment of autoimmune diseases in humans.

Exogenous superantigens acutely trigger distinct levels of peripheral T cell tolerance/immunosuppression: dose-response relationship

Ligand-specific immunosuppression requires an understanding of the parameters that control peripheral T cell tolerance. T cell receptor (TcR) transgenic mice offer a clear advantage for studying post-thymic tolerance mechanisms in vivo that are operational in a monoclonal T cell population with preselected antigen specificity. Yet it is unclear whether the rules defined in monoclonal T cells of genetically manipulated mice reflect those operative in clonally diverse peripheral T cells of normal mice. To analyze acute tolerance mechanisms in unselected peripheral T cells, we challenged normal mice with the superantigen staphylococcal enterotoxin B (SEB) and analyzed ligand-reactive V beta 8+ T cells for TcR-triggered tolerance mechanisms such as anergy, TcR down-regulation, or apoptosis. Upon challenge with graded doses of SEB (0.001-10 micrograms) V beta 8+ T cells become anergic within 6-16 h. Importantly, a dosage effect of SEB in regard to the level of anergy induced was observed. Anergy induced by low concentrations of SEB (0.001-0.1 microgram) is transient and is overcome by clonal growth, while higher concentrations of SEB (0.1-10 micrograms) cause long-lasting anergy resistant to cell cycle progression. At high SEB concentrations (1-10 mg) about 50% of the anergic V beta 8+ T cells additionally down-regulate their TcR-CD3 complex, followed by a loss of CD2, CD4, CD8 accessory molecules. In parallel, T cell phenotype-negative but genotypically V beta 8+ T cells are generated. The T cell phenotype-negative cells reacquire their V beta 8+ T cell phenotype upon culture in vitro. In vivo, a subset of V beta 8+ cells, defined by an intermediate stage of TcR down-regulation, i.e. V beta 8lowCD3+ cells, but not T cell phenotype-negative cells are selectively programmed for apoptosis, which occurs within 1 h. These data suggest that SEB triggers distinct tolerance pathways which operate in a hierarchical fashion in clonally diverse ligand-reactive T cells. Specifically, the results illustrate the power of exogenous superantigens to exploit these distinct tolerance pathways, thereby achieving distinct levels of immunosuppression.

CTLA-4 ligands are required to induce an in vivo interleukin 4 response to a gastrointestinal nematode parasite

The costimulatory signal provided to T cells through CTLA-4-ligand interactions is required for T cell activation resulting in increased interleukin 2 (IL-2) production in vitro, but its role in the production of IL-4 and other cytokines is unclear and few in vivo studies have been performed to confirm results of in vitro experiments. We have examined the in vivo effects of blocking CTLA-4 ligands on the T helper cell 2 (Th2)-associated mucosal immune response that follows oral infection of mice with the nematode parasite, Heligmosomoides polygyrus. CTLA-4Ig administration inhibited H. polygyrus-induced increases in mesenteric lymph node (MLN) B cell major histocompatibility complex class II expression and size and T cell-derived IL-4 gene expression. In addition, CTLA-4 immunoglobulin (Ig) partially blocked increased IL-3, IL-5, and IL-9 cytokine gene expression in Peyer's patch (PP) and MLN 8 d after primary inoculation of mice with the parasite. Increases in the number of IL-4- but not IL-5-secreting cells were also inhibited by CTLA-4Ig. H. polygyrus-induced elevations in serum IgE levels but not blood eosinophils, were markedly inhibited by CTLA-4Ig. These results suggest that stimulation of CD28 and/or CTLA-4 is required for T cell priming leading to IL-4 cytokine production, B cell activation, and IgE secretion during a Th2-like, mucosal immune response to a nematode parasite.