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

B7-independent inhibition of T cells by CTLA-4

CTLA-4 is an inhibitory molecule that regulates T cell expansion and differentiation. CTLA-4 binding to B7-1/B7-2 is believed to be crucial for its inhibitory signal both by competing for CD28 binding to the same ligands and aggregating CTLA-4 to deliver negative signals. In this study, we demonstrate that B7 binding is not essential for CTLA-4 activity. CTLA-4 knockout T cells are hyperresponsive compared with wild-type T cells in B7-free settings. Expression of a B7-nonbinding CTLA-4 mutant inhibited T cell proliferation, cytokine production, and TCR-mediated ERK activation in otherwise CTLA-4-deficient T cells. Finally, transgenic expression of the ligand-nonbinding CTLA-4 mutant delayed the lethal lymphoproliferation observed in CTLA-4-deficient mice. These results suggest that ligand binding is not essential for the CTLA-4 function and supports an essential role for CTLA-4 signaling during T cell activation.

The B7/CD28 costimulatory family in autoimmunity

Host defense is dependent on the appropriate induction of immune responses. A central concept in immunology is the ability of the immune system to differentiate foreign from self-antigens. The failure of the immune response to recognize foreign pathogens can result in infection and disease in the host. The inappropriate response of the immune system to self-antigens is equally problematic, leading to autoimmune disease. Central and peripheral tolerance mechanisms control self-reactive T-cell responses and protect peripheral tissues from autoimmune attack. This review examines the roles of B7/CD28 family members, which can augment or antagonize T-cell receptor signaling, in the regulation of central and peripheral T-cell tolerance. We also discuss how B7/CD28 pathways influence both T-cell-intrinsic and -extrinsic mechanisms of regulation.

Therapeutic peptidomimetic strategies for autoimmune diseases: costimulation blockade*

Cognate interactions between immune effector cells and antigen-presenting cells (APCs) govern immune responses. Specific signals occur between the T-cell receptor peptide and APCs and nonspecific signals between pairs of costimulatory molecules. Costimulation signals are required for full T-cell activation and are assumed to regulate T-cell responses as well as other aspects of the immune system. As new discoveries are made, it is becoming clear how important these costimulation interactions are for immune responses. Costimulation requirements for T-cell regulation have been extensively studied as a way to control many autoimmune diseases and downregulate inflammatory reactions. The CD28:B7 and the CD40:CD40L families of molecules are considered to be critical costimulatory molecules and have been studied extensively. Blocking the interaction between these molecules results in a state of immune unresponsiveness termed 'anergy'. Several different strategies for blockade of these interactions are explored including monoclonal antibodies (mAbs), Fab fragments, chimeric, and/or fusion proteins. We developed novel, immune-specific approaches that interfere with these interactions. Using experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis mediated by central nervous system (CNS)-specific T-cells, we developed a multi-targeted approach that utilizes peptides for blockade of costimulatory molecules. We designed blocking peptide mimics that retain the functional binding area of the parent protein while reducing the overall size and are thus capable of blocking signal transduction. In this paper, we review the role of costimulatory molecules in autoimmune diseases, two of the most well-studied costimulatory pathways (CD28/CTLA-4:B7 and CD40:CD40L), and the advantages of peptidomimetic approaches. We present data showing the ability of peptide mimics of costimulatory molecules to suppress autoimmune disease and propose a mechanism for disease suppression.

Rational development of LEA29Y (belatacept), a high-affinity variant of CTLA4-Ig with potent immunosuppressive properties

Current success in organ transplantation is dependent upon the use of calcineurin-inhibitor-based immunosuppressive regimens. Unfortunately, current immunotherapy targets molecules with ubiquitous expression resulting in devastating non-immune side effects. T-cell costimulation has been identified as a new potential immunosuppressive target. The best characterized pathway includes CD28, its homologue CTLA4 and their ligands CD80 and CD86. While an immunoglobulin fusion protein construct of CTLA4 suppressed rejection in rodents, it lacked efficacy in primate transplant models. In an attempt to increase the biologic potency of the parent molecule a novel, modified version of CTLA4-Ig, LEA29Y (belatacept), was constructed. Two amino acid substitutions (L104E and A29Y) gave rise to slower dissociation rates for both CD86 and CD80. The increased avidity resulted in a 10-fold increase in potency in vitro and significant prolongation of renal allograft survival in a pre-clinical primate model. The use of immunoselective biologics may provide effective maintenance immunosuppression while avoiding the collateral toxicities associated with conventional immunsuppressants.

Crystal structure of a soluble CD28-Fab complex

Naive T cell activation requires signaling by the T cell receptor and by nonclonotypic cell surface receptors. The most important costimulatory protein is the monovalent homodimer CD28, which interacts with CD80 and CD86 expressed on antigen-presenting cells. Here we present the crystal structure of a soluble form of CD28 in complex with the Fab fragment of a mitogenic antibody. Structural comparisons redefine the evolutionary relationships of CD28-related proteins, antigen receptors and adhesion molecules and account for the distinct ligand-binding and stoichiometric properties of CD28 and the related, inhibitory homodimer CTLA-4. Cryo-electron microscopy-based comparisons of complexes of CD28 with mitogenic and nonmitogenic antibodies place new constraints on models of antibody-induced receptor triggering. This work completes the initial structural characterization of the CD28-CTLA-4-CD80-CD86 signaling system.

Ovine dendritic cells transduced with an adenoviral CTLA4eEGFP fusion protein construct induce hyporesponsiveness to allostimulation

CTLA4 (CD152) is a transmembrane molecule expressed on activated T cells and functions as a negative regulator of T cell activation upon binding to the costimulatory molecules CD80/86. In this study, CTLA4eEGFP constructs were engineered by cloning the extracellular domains of ovine and human CTLA4 (CTLA4e) 'in frame' with the enhanced green fluorescent protein (EGFP). Recombinant adenoviral vectors were generated by incorporation of the CTLA4eEGFP sequence into the adenoviral genome using homologous recombination in Esherichia coli. The functional activity of the adenoviral vectors was shown by the secretion of the CTLA4eEGFP upon infection of ovine fibroblasts and the binding of the fusion protein to the target ovine and human dendritic cells expressing CD80/86 receptors by flow cytometry. The EGFP tag facilitated molecular size determinations and quantification of the secreted ovine CTLA4 fusion protein by immunoprecipitation and enzyme-linked immunosorbent assay (ELISA), respectively, using anti-GFP mAbs. Ovine dendritic cells obtained from pseudoafferent lymphatic cannulation of sheep were characterized based on high major histocompatibility complex (MHC) class II expression and cross-reactivity with monoclonal antibodies to the human dendritic cell markers, CD83 and CMRF-56. In addition, ovine dendritic cells (DC) were transfected with the adenoviral CTLA4eEGFP and when used as stimulators in a mixed lymphocyte reaction showed a reduced capacity to induce allogeneic lymphocyte proliferation. This study verifies that the ovine CTLA4eEGFP fusion protein functions similarly to its human homologue and that DC modified with adenoviral CTLA4-EGFP may provide an effective therapeutic approach in targeting alloreactive T cells to prolong allograft acceptance in a preclinical ovine model of renal transplantation.

B7-1 and B7-2 selectively recruit CTLA-4 and CD28 to the immunological synapse

The reported affinity differences between CD28 and CTLA-4 binding to B7-1 and B7-2 may serve to selectively regulate CD28 and CTLA-4 function by differentially recruiting and/or stabilizing these molecules at the immunological synapse. Here we show that ligand binding is important for the accumulation of both CD28 and CTLA-4 at the synapse. While CD28 is recruited to the synapse in the absence of B7-1 and B7-2 binding, it is not effectively stabilized there, as its localization can be disrupted by CTLA-4. In the case of CTLA-4, ligand binding is critical for its concentration at the synapse. We also demonstrate that the affinity and avidity differences in ligand binding translate into selective recruitment of CD28 or CTLA-4 to the immunological synapse--B7-1 is the major ligand mediating CTLA-4 localization, while B7-2 is the main ligand for CD28 concentration at the synapse.

Significant enhancement by anti-ICOS antibody of suboptimal tacrolimus immunosuppression in rat liver transplantation

A member of the costimulatory molecule family, inducible costimulator (ICOS), is expressed on activated T cells and plays a critical role in their primary activation and cytokine production. ICOS is involved in different immune phenomena, such as Th1-mediated autoimmune disease and graft rejection. Although blockade of ICOS costimulation theoretically may protect grafts from rejection, a single dose of anti-ICOS antibody did not result in the prolongation of rat liver allograft survival. However, in this article, we report that anti-rat ICOS antibody markedly enhanced the immunosuppressive activity of a suboptimal dose of tacrolimus (FK506). After fully allogenic DA to LEW liver transplantation, recipients received a single injection of tacrolimus (1 mg/kg, intramuscularly) with or without anti-ICOS antibody (1 mg/kg, intravenously). Recipient survival was significantly prolonged in rats treated with both the antibody and suboptimal tacrolimus (median survival time 44 days vs. 28 days with tacrolimus alone, P <.01). The extent of cell infiltration into the graft was closely associated with prolongation of recipient survival. Our findings thus demonstrate that anti-ICOS antibody immunotherapy combined with suboptimal tacrolimus has a synergistic effect in preventing hepatic allograft rejection and that it may induce long-term graft acceptance intimately associated with a marked reduction of intragraft T lymphocyte infiltration.

Identification of Protein-Protein Interfaces Implicated in CD80-CD28 Costimulatory Signaling

The B7 ligands CD80 and CD86 on APCs deliver either costimulatory or inhibitory signals to the T cell when interacting with their counter-receptors CD28 and CD152 (CTLA-4) on the T cell surface. Although crucial for lymphocyte regulation, the structural basis of these interactions is still not completely understood. Using multivalent presentation and conditions mimicking clustering, believed to be essential for signaling through these receptors, and by applying a combined differential mass spectrometry and structural mapping approach to these conditions, we were able to identify a putative contact area involving hydrophilic regions on both CD28 and CD80 as well as a putative CD28 oligomerization interface induced by B7 ligation. Analysis of the CD80-CD28 interaction site reveals a well-defined interface structurally distinct from that of CD80 and CD152 and thus provides valuable information for therapeutic intervention targeted at this pathway, suggesting a general approach for other receptors.

Role of costimulatory pathways in the pathogenesis of multiple sclerosis and experimental autoimmune encephalomyelitis

Multiple sclerosis is an immune-mediated disorder of the central nervous system. T lymphocytes are thought to play a central role in the initiation and potentially in the propagation of this disease. Two signals are required for T-cell activation. The first signal consists of the interaction of the T-cell receptor with antigen presented by the MHC molecule on antigen-presenting cells. The second signal requires engagement of costimulatory receptors on T cells with their ligands on antigen-presenting cells. Several costimulatory pathways have been shown to play an important role in T-lymphocyte activation. Here we will review the current literature on the contribution of the B7-1/2-CD28/CTLA-4, inducible costimulatory molecule-B7h, programmed death pathway 1-programmed death pathway ligand 1/ligand 2, CD40-CD154, OX40-OX40 ligand, and CD137-CD137 ligand pathways to the pathogenesis of multiple sclerosis and their potential roles as therapeutic targets.

Treatment of rheumatoid arthritis by selective inhibition of T-cell activation with fusion protein CTLA4Ig

BACKGROUND

Effective new therapies are needed for rheumatoid arthritis. Current therapies target the products of activated macrophages; however, T cells also have an important role in rheumatoid arthritis. A fusion protein--cytotoxic T-lymphocyte-associated antigen 4-IgG1 (CTLA4Ig)--is the first in a new class of drugs known as costimulation blockers being evaluated for the treatment of rheumatoid arthritis. CTLA4Ig binds to CD80 and CD86 on antigen-presenting cells, blocking the engagement of CD28 on T cells and preventing T-cell activation. A preliminary study showed that CTLA4Ig may be effective for the treatment of rheumatoid arthritis.

METHODS

We randomly assigned patients with active rheumatoid arthritis despite methotrexate therapy to receive 2 mg of CTLA4Ig per kilogram of body weight (105 patients), 10 mg of CTLA4Ig per kilogram (115 patients), or placebo (119 patients) for six months. All patients also received methotrexate therapy during the study. The clinical response was assessed at six months with use of the criteria of the American College of Rheumatology (ACR), which define the response according to its extent: 20 percent (ACR 20), 50 percent (ACR 50), or 70 percent (ACR 70). Additional end points included measures of the health-related quality of life.

RESULTS

Patients treated with 10 mg of CTLA4Ig per kilogram were more likely to have an ACR 20 than were patients who received placebo (60 percent vs. 35 percent, P<0.001). Significantly higher rates of ACR 50 and ACR 70 responses were seen in both CTLA4Ig groups than in the placebo group. The group given 10 mg of CTLA4Ig per kilogram had clinically meaningful and statistically significant improvements in all eight subscales of the Medical Outcomes 36-Item Short-Form General Health Survey. CTLA4Ig was well tolerated, with an overall safety profile similar to that of placebo.

CONCLUSIONS

In patients with active rheumatoid arthritis who were receiving methotrexate, treatment with CTLA4Ig significantly improved the signs and symptoms of rheumatoid arthritis and the health-related quality of life. CTLA4Ig is a promising new therapy for rheumatoid arthritis.

Unifying concepts in CD28, ICOS and CTLA4 co-receptor signalling

Many studies have shown the central importance of the co-receptors CD28, inducible costimulatory molecule (ICOS) and cytotoxic T lymphocyte antigen 4 (CTLA4) in the regulation of many aspects of T-cell function. CD28 and ICOS have both overlapping and distinct functions in the positive regulation of T-cell responses, whereas CTLA4 negatively regulates the response. The signalling pathways that underlie the function of each of the co-receptors indicate their shared and unique properties and provide compelling hints of functions that are as yet uncovered. Here, we outline the shared and distinct signalling events that are associated with each of the co-receptors and provide unifying concepts that are related to signalling functions of these co-receptors.

Topological requirements and signaling properties of T cell-activating, anti-CD28 antibody superagonists

Full activation of naive T cells requires both engagement of the T cell antigen receptor (TCR; signal 1) and costimulatory signaling by CD28 (signal 2). We previously identified two types of rat CD28-specific monoclonal antibodies (mAbs): "conventional," TCR signaling-dependent costimulatory mAbs and "superagonistic" mAbs capable of inducing the full activation of primary resting T cells in the absence of TCR ligation both in vitro and in vivo. Using chimeric rat/mouse CD28 molecules, we show that the superagonists bind exclusively to the laterally exposed C"D loop of the immunoglobulin-like domain of CD28 whereas conventional, costimulatory mAbs recognize an epitope close to the binding site for the natural CD80/CD86 ligands. Unexpectedly, the C"D loop reactivity of a panel of new antibodies raised against human CD28 could be predicted solely on the basis of their superagonistic properties. Moreover, mouse CD28 molecules engineered to express the rat or human C"D loop sequences activated T cell hybridomas without TCR ligation when cross-linked by superagonistic mAbs. Finally, biochemical analysis revealed that superagonistic CD28 signaling activates the nuclear factor kappaB pathway without inducing phosphorylation of either TCRzeta or ZAP70. Our findings indicate that the topologically constrained interactions of anti-CD28 superagonists bypass the requirement for signal 1 in T cell activation. Antibodies with this property may prove useful for the development of T cell stimulatory drugs.

ICOS costimulation in inflammatory bowel disease

For years, medical researchers have striven to develop selective immunotherapies that could specifically ameliorate pathogenic immune responses without immunocompromising the patient. Blockade of many known receptors on T cells can inhibit the initiation of immune responses. However, this approach is problematic in that it is not possible to predict the onset of disease in patients. Current immunotherapies are unsatisfactory for the sporadic exacerbating type of diseases such as multiple sclerosis and inflammatory bowel disease (IBD), because they require either long-term treatment or acute treatment with high-dose immunosuppressants. With regard to this issue, the inducible and inflammatory site-specific molecule, inducible costimulator (ICOS), may be particularly useful as an ideal targeting molecule for the strategy of treatment of human IBD patients.

Suppression of experimental autoimmune encephalomyelitis using peptide mimics of CD28

The B7:CD28/CTLA-4 costimulatory pathway plays a critical role in regulating the immune response and thus provides an ideal target for therapeutic manipulation of autoimmune disease. Previous studies have shown that blockade of CD28 signaling by mAbs can both prevent and exacerbate experimental autoimmune encephalomyelitis (EAE). In this study, we have designed two CD28 peptide mimics that selectively block B7:CD28 interactions. By surface plasmon resonance, both the end group-blocked CD28 peptide (EL-CD28) and its retro-inverso isomer (RI-CD28) compete effectively with the extracellular domain of CD28 for binding to B7-1. Both the CD28 peptide mimics inhibited expansion of encephalitogenic T cells in vitro. A single administration of EL-CD28 or RI-CD28 peptide significantly reduced disease severity in EAE. Importantly, we show that either CD28 peptide mimic administered during acute disease dramatically improved clinical signs of EAE, suppressing ongoing disease. The ratio of CD80:CD86 expression was significantly lower on CD4(+) and F4/80(+) spleen cells in CD28 peptide-treated mice. Peripheral deletion of Ag-specific CD4(+) T cells occurs following in vivo blockade of CD28 with synthetic CD28 peptides.

Alefacept, an immunomodulatory recombinant LFA-3/IgG1 fusion protein, induces CD16 signaling and CD2/CD16-dependent apoptosis of CD2(+) cells

Alefacept, an immunomodulatory recombinant fusion protein composed of the first extracellular domain of LFA-3 fused to the human IgG1 hinge, C(H)2, and C(H)3 domains, has recently been shown in phase II and III clinical trials to safely reduce disease expression in patients with chronic plaque psoriasis. Alefacept modulates the function of and selectively induces apoptosis of CD2(+) human memory-effector T cells in vivo. We have sought to gain further understanding of the mechanisms of action that influence the biological activity of alefacept and may contribute to its efficacy and patient responsiveness. Specifically evaluated is the ability of alefacept to activate intracellular signals mediated via CD2 and/or Fc gamma RIII (CD16). Experimentation using isoforms of alefacept engineered to have amino acid substitutions in the IgG1 C(H)2 domain that impact Fc gamma R binding indicate that alefacept mediates cognate interactions between cells expressing human CD2 and CD16 to activate cells, e.g., increase extracellular signal-regulated kinase phosphorylation, up-regulate cell surface expression of the activation marker CD25, and induce release of granzyme B. In the systems used, this signaling is shown to require binding to CD2 and CD16 and be mediated through CD16, but not CD2. Experimentation using human CD2-transgenic mice and isoforms of alefacept confirmed the requirement for Fc gamma R binding for detection of the pharmacological effects of alefacept in vivo. Thus alefacept acts as an effector molecule, mediating cognate interactions to activate Fc gamma R(+) cells (e.g., NK cells) to induce apoptosis of sensitive CD2(+) target cells.

Structural mechanisms of costimulation

Recent studies have highlighted the structural requirements for T cell costimulation and have revealed unusual modes of dimerization for the cytolytic T lymphocyte-associated antigen 4 (CTLA-4) costimulatory receptor and its B7 ligands. These distinctive quaternary structures potentially endow both receptor and ligand with bivalent binding properties, which suggests a number of mechanistic features relevant to signaling. These include the potential to form a highly ordered, alternating network of CTLA-4 and B7 homodimers that may represent the organization of these molecules and their associated signaling partners within the immunological synapse. Primary sequence and structural considerations suggest that some aspects of the organizational and mechanistic features associated with the CTLA-4-B7 complexes may extend to other members of the costimulatory receptor-ligand family. An examination of the signaling mechanisms within the costimulatory receptor-ligand family provides an excellent framework to consider the general principles that are relevant to cell surface receptor-mediated signaling events.

Ligand binding sites of inducible costimulator and high avidity mutants with improved function

Interaction between inducible costimulator (ICOS) and its ligand is implicated in the induction of cell-mediated and humoral immune responses. However, the molecular details of this interaction are unknown. We report here a mutagenesis analysis of residues in ICOS that are critical for ligand binding. A three-dimensional model of the extracellular immunoglobulin-like domain of ICOS was used to map the residues conserved within the CD28 family. This analysis identified a surface patch containing the characteristic "PPP" sequence and is conserved in human and mouse ICOS. Mutations in this region of human ICOS reduce or abolish ligand binding. Our results suggest that the ligand binding site in ICOS maps to a region overlapping yet distinct from the CD80/CD86 binding sites in CD28 and cytotoxic T lymphocyte antigen (CTLA)-4. Thus, the analysis suggests that differences in ligand binding specificity between these related costimulatory molecules have evolved by utilization of overlapping regions with different patterns of conserved and nonconserved residues. Two site-specific mutants generated in the course of our studies bound ICOS ligand with higher avidity than wild-type ICOS. An S76E mutant protein of ICOS blocked T cell costimulatory function of ICOS ligand and inhibited T cell response to allogeneic antigens superior to wild-type ICOS. Our studies thus identified critical residues involving in ICOS receptor-ligand interaction and provide new modulators for immune responses.

Prolonged survival in rat liver transplantation with mouse monoclonal antibody against an inducible costimulator (ICOS)

BACKGROUND

An inducible costimulator (ICOS), a recently identified costimulatory receptor with a close structural homology to CD28 and CTLA4, is expressed on activated T cells. Interaction with its ligand on antigen-presenting cells stimulates T-cell proliferation to produce a different spectrum of cytokine. The inhibition of ICOS-mediated signal transduction by an anti-ICOS antibody is considered to be capable of protecting against graft rejection in organ transplantation.

METHODS

An anti-rat ICOS antibody was intravenously administered into recipients of dark Agouti-to-Lewis liver transplantations. The recipient lymphocytes from mesenteric lymph nodes were harvested on day 7 after transplantation for fluorescence-activated cell sorting analysis, and tissue specimens from the grafts were removed for histologic evaluation. Antigen-specific T-cell proliferation responses were assessed in vitro with anti-ICOS antibody.

RESULTS

Monotherapy with the antibody significantly prolonged the graft survival time by inhibiting T-cell activation and its proliferation response. The graft-infiltrating cells, both CD4 and CD8 T cells, were not completely reduced even when rats were administered the antibody, whereas the expression of ICOS almost completely disappeared in these cells.

CONCLUSIONS

T-cell activation through the ICOS costimulatory pathway plays an important role in graft rejection, and manipulating its pathway is an effective method for modulating transplantation immunity.

The B7-CD28 superfamily

The B7-1/B7-2-CD28/CTLA-4 pathway is crucial in regulating T-cell activation and tolerance. New B7 and CD28 molecules have recently been discovered and new pathways have been delineated that seem to be important for regulating the responses of previously activated T cells. Several B7 homologues are expressed on cells other than professional antigen-presenting cells, indicating new mechanisms for regulating T-cell responses in peripheral tissues. Some B7 homologues have unknown receptors, indicating that other immunoregulatory pathways remain to be described. Here, we summarize our current understanding of the new members of the B7 and CD28 families, and discuss their therapeutic potential.

Expression of CTLA-4 by human monocytes

Cytotoxic T lymphocyte-associated molecule-4 (CTLA-4) is a receptor present on T cells that plays a critical role in the downregulation of antigen-activated immune responses. CTLA-4 interacts with the ligands CD80 and CD86 on antigen-presenting cells (APC), and also directs the assembly of inhibitory signalling complexes that lead to quiescence or anergy. In this study, we show that human monocytes constitutively express CTLA-4. About 3% of monocytes expressed CTLA-4 on the cell surface, whereas the intracellular expression was higher and present in about 20% of the monocytes. The sequences of the cDNAs from human monocytes were identical to the sequences of CTLA-4 from T cells. Expression of CTLA-4 was also confirmed in the activated myelomonocytic cell lines U937 and THP-1. Monocytes, but not T cells, activated by interferon (IFN)-gamma also secreted soluble CTLA-4 in vitro. The CTLA-4 expression was upregulated upon treatment with phorbol 12-myristate 13-acetate (PMA) and IFN-gamma. This increased expression could be partially abolished by staurosporine, an inhibitor of protein kinase C (PKC). Ligation of CTLA-4 in the monocyte-like cell-line U937 with antibodies against CTLA-4 partially inhibited the proliferation of cells and the upregulation of cell-surface markers CD86, CD54, HLA-DR and HLA-DQ induced by IFN-gamma and Staphylococcus aureus, Cowan I strain (SAC). Ligation of CTLA-4 suppressed the PMA-stimulated activation of transcription activator protein 1 (AP-1) and nuclear factor (NF)-kappaB in the U937 cell line, indicating the involvement of an inhibitory signal transduction. These data provide the first evidence that CTLA-4 is constitutively expressed by monocytes and thus might be important for the regulation of immune mechanisms associated with monocytes.

Tumor necrosis factor-alpha regulates the expression of inducible costimulator receptor ligand on CD34(+) progenitor cells during differentiation into antigen presenting cells

The inducible costimulator receptor (ICOS) is a third member of the CD28 receptor family that regulates T cell activation and function. ICOS binds to a newly identified ligand on antigen presenting cells different from the CD152 ligands CD80 and CD86. We used soluble ICOSIg and a newly developed murine anti-human ICOS ligand (ICOSL) monoclonal antibody to further characterize the ICOSL during ontogeny of antigen presenting cells. In a previous study, we found that ICOSL is expressed on monocytes, dendritic cells, and B cells. To define when ICOSL is first expressed on myeloid antigen presenting cells, we examined ICOSL expression on CD34(+) cells in bone marrow. We found that CD34(bright) cells regardless of their myeloid commitment were ICOSL(-), whereas ICOSL was first expressed when CD34 expression diminished and the myeloid marker CD33 appeared. However, acute myeloid leukemia cells were ICOSL-negative, whereas among B-cell malignancies only some cases of the most mature tumors such as prolymphocytic leukemia and hairy cell leukemia were positive. Next, we investigated purified CD34(+) hematopoietic progenitor cells that did not constitutively express ICOSL but were induced to express ICOSL within 12 h after granulocyte/macrophage colony-stimulating factor/tumor necrosis factor alpha (TNF-alpha) stimulation. Interestingly, ICOSL was induced prior to CD80/CD86 induction on CD34(+) cells so that ICOSL was expressed in the absence of CD80/CD86. This suggests that ICOSL is an early differentiation marker along the monocytic/dendritic maturation pathway. Induction of ICOSL was dependent on TNF-alpha and was regulated via NF-kappa B as revealed by use of inhibitors specific for I kappa B alpha phosphorylation such as BAY 11-7082 and BAY 11-7085. The antigen presenting capacity of TNF-alpha stimulated CD34(+) cells was strongly inhibited by ICOSIg fusion proteins or by NF-kappa B inhibition. Thus, TNF-alpha-induced ICOSL expression seemed to be functionally important for the costimulatory capacity of CD34(+) hematopoietic progenitor cells.

Costimulation of memory T-cells by ICOS: a potential therapeutic target for autoimmunity?

Approaches that target costimulatory receptors are independent of T-cell receptor specificity and may be useful for T-cell-mediated diseases in which the antigens involved are not well defined. However, the proper costimulatory pathways need to be targeted. For example, therapies for human T-cell-mediated diseases need to be effective against previously activated memory cells. In this review, we use autoimmune demyelination as a paradigm for established immune-mediated pathogenesis. Studies with the human disease multiple sclerosis and the rodent model experimental autoimmune encephalomyelitis have suggested that the effectiveness of CD28 blockade, as a therapeutic strategy for established autoimmune demyelination, may be limited. ICOS, a receptor that appears to be involved in the costimulation of previously activated T-cells, may be an attractive alternative.

The comparative efficacy of CTLA-4 and L-selectin targeted DNA vaccines in mice and sheep

The access of antigens to antigen presenting cells (APCs) appears to be a rate-limiting step in the generation of immune responses to DNA vaccines. The cytotoxic T lymphocyte antigen 4 (CTLA-4) and L-selectin represent attractive ligands for use in the targeting of antigen to APCs and lymph nodes. CTLA-4 binds with high affinity to the B7 membrane antigen on APCs, while L-selectin functions as a lymphocyte homing marker and binds to CD34 on the surface of high endothelial venule cells. DNA vaccines encoding human immunoglobulin (HIg), fused to either CTLA-4 or L-selectin, have been shown to generate up to 10,000-fold higher anti-HIg antibody responses than DNA vaccines encoding HIg alone. In this study, the ability of CTLA-4 or L-selectin mediated targeting to enhance the humoral immune response to an alternate vaccine antigen was investigated. DNA vaccines encoding CTLA-4-HIg and L-selectin-HIg fused to the host-protective 45W antigen from Taenia ovis were constructed. In BALB/c mice, the L-selectin targeted vaccine did not improve either the magnitude or speed of antibody responses of vaccinated mice. In contrast, the CTLA-4 targeted DNA vaccine generated 45W-specific antibody responses which were up to 30-fold higher than those achieved with non-targeted DNA vaccination. The kinetic of the antibody response generated following CTLA-4 targeted DNA vaccination was also significantly faster than that achieved with non-targeted DNA vaccination, or with adjuvanted protein vaccination. Vaccination of outbred sheep with DNA vaccines expressing either murine or ovine CTLA-4 targeted antigen failed to enhance immune responses. These findings indicate that CTLA-4 targeting may find application in the improvement of DNA vaccines, but requires further development for applications in large animal species.

Building novel binding ligands to B7.1 and B7.2 based on human antibody single variable light chain domains

Ligands specific for B7.1 (CD80) and B7.2 (CD86) have applications in disease indications that require inhibition of T-cell activity. As we observed significant sequence and structural similarity between the B7-binding ligand, cytotoxic T-lymphocyte associated protein-4 (CTLA-4), and antibody variable light chain domains (VLs), we have explored the possibilities of making novel B7 binding molecules based on single VL domains. We first describe the "rational" design and construction of a VL/CTLA-4 hybrid molecule in which we have grafted both the CDR1 and CDR3-like loops of CTLA-4 onto a single VL light chain, at sites determined by sequence and structure-based alignment. This molecule was secreted as a soluble product from Escherichia coli, but did not show any binding to B7.1 and B7.2. In a second approach we constructed a VL library in which human VL genes derived from B-cells were spiked with the CDR3-like loop of CTLA-4 and further diversified by DNA shuffling. This library was displayed on phage, and after selection gave B7.1 binding ligands which competed with CTLA-4. In order to evaluate the possible general utility of VL domains as binding ligands, we have constructed a non-biased VL library. From this DNA-shuffled human VL library we have selected single VL domains specific for B7.1, B7.2 or human IgG. Two B7.1-specific VL ligands and one B7.2-specific VL ligand showed competition with CTLA-4. One candidate VL domain-specific for B7.1 was affinity matured by simultaneous randomisation of all CDR loops using DNA shuffling with degenerate CDR-spiking oligonucleotides. From this library, a single VL domain with affinity of 191 nM for B7.1 was obtained, which also showed binding to B7.1 in situ. This VL had mutations in CDR1 and CDR3, indicating that antigen recognition for this single VL is most likely mediated by the same regions as in the VL domain of whole antibodies. The B7.1 and B7.2-specific VL domains described in this study may form the basis of a new family of immunomodulatory recombinant molecules. Furthermore, our studies suggest that it is feasible to create specific single VL domains to diverse targets as is the case for single VH domains.

A retro-inverso peptide mimic of CD28 encompassing the MYPPPY motif adopts a polyproline type II helix and inhibits encephalitogenic T cells in vitro

Complete activation of T cells requires two signals: an Ag-specific signal delivered via the TCR by the peptide-MHC complex and a second costimulatory signal largely provided by B7:CD28/CTLA-4 interactions. Previous studies have shown that B7 blockade can either ameliorate experimental autoimmune encephalomyelitis by interfering with CD28 signaling or exacerbate the disease by concomitant blockade of CTLA-4 interaction. Therefore, we developed a functional CD28 mimic to selectively block B7:CD28 interactions. The design, synthesis, and structural and functional properties of the CD28 free peptide, the end group-blocked CD28 peptide, and its retro-inverso isomer are shown. The synthetic T cell-costimulatory receptor peptides fold into a polyproline type II helical structure commonly seen in regions of globular proteins involved in transient protein-protein interactions. The binding determinants of CD28 can be transferred onto a short peptide mimic of its ligand-binding region. The CD28 peptide mimics effectively block the expansion of encephalitogenic T cells in vitro suggesting the potential usefulness of the peptides for the treatment of autoimmune disease conditions requiring down-regulation of T cell responses.

CD28-independent costimulation of T cells in alloimmune responses

T cell costimulation by B7 molecules plays an important role in the regulation of alloimmune responses. Although both B7-1 and B7-2 bind CD28 and CTLA-4 on T cells, the role of B7-1 and B7-2 signaling through CTLA-4 in regulating alloimmune responses is incompletely understood. To address this question, we transplanted CD28-deficient mice with fully allogeneic vascularized cardiac allografts and studied the effect of selective blockade of B7-1 or B7-2. These mice reject their grafts by a mechanism that involves both CD4(+) and CD8(+) T cells. Blockade of CTLA-4 or B7-1 significantly accelerated graft rejection. In contrast, B7-2 blockade significantly prolonged allograft survival and, unexpectedly, reversed the acceleration of graft rejection caused by CTLA-4 blockade. Furthermore, B7-2 blockade prolonged graft survival in recipients that were both CD28 and CTLA-4 deficient. Our data indicate that B7-1 is the dominant ligand for CTLA-4-mediated down-regulation of alloimmune responses in vivo and suggest that B7-2 has an additional receptor other than CD28 and CTLA-4 to provide a positive costimulatory signal for T cells.

The expanding world of co-stimulation: the two-signal model revisited

The crucial role for CD28, its homolog CTLA-4 and their binding partners B7-1 and B7-2 in the generation of effective T-cell responses has been well documented. Recently, two new pairs of the CD28/B7 families were identified. The ability of these molecules to regulate T-cell expansion and effector function and the dynamic integration of the co-stimulatory and T-cell receptor signals are just beginning to be explored. Understanding these processes will be crucial for designing clinically relevant approaches to manipulate the adaptive immune system.

Structural basis for co-stimulation by the human CTLA-4/B7-2 complex

Regulation of T-cell activity is dependent on antigen-independent co-stimulatory signals provided by the disulphide-linked homodimeric T-cell surface receptors, CD28 and CTLA-4 (ref. 1). Engagement of CD28 with B7-1 and B7-2 ligands on antigen-presenting cells (APCs) provides a stimulatory signal for T-cell activation, whereas subsequent engagement of CTLA-4 with these same ligands results in attenuation of the response. Given their central function in immune modulation, CTLA-4- and CD28-associated signalling pathways are primary therapeutic targets for preventing autoimmune disease, graft versus host disease, graft rejection and promoting tumour immunity. However, little is known about the cell-surface organization of these receptor/ligand complexes and the structural basis for signal transduction. Here we report the 3.2-A resolution structure of the complex between the disulphide-linked homodimer of human CTLA-4 and the receptor-binding domain of human B7-2. The unusual dimerization properties of both CTLA-4 and B7-2 place their respective ligand-binding sites distal to the dimer interface in each molecule and promote the formation of an alternating arrangement of bivalent CTLA-4 and B7-2 dimers that extends throughout the crystal. Direct observation of this CTLA-4/B7-2 network provides a model for the periodic organization of these molecules within the immunological synapse and suggests a distinct mechanism for signalling by dimeric cell-surface receptors.

CD28-independent induction of experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is a T cell-mediated disease initiated by antigen-specific CD4(+) T cells. Signaling through CD28 is a critical second signal for activation of T cells, and CD28 knockout (CD28KO) mice have been reported to be resistant to induction of EAE. We now report that CD28KO mice have no intrinsic defect in mediating disease, because they developed EAE after passive transfer of primed T cells. After immunization, peripheral T cells from CD28KO mice were primed and developed memory phenotype, but had decreased antigen-specific IFN-gamma production as compared with cells from wild-type (WT) animals. Reimmunization of CD28KO mice brought out clinical disease and increased IFN-gamma production in vitro. Pathologically, there were cellular infiltrates in the central nervous system, in contrast to single-immunized mice. We show furthermore that blocking B7-1 or CTLA4, but not B7-2, in CD28KO mice induces disease after a single immunization. Thus, EAE can be induced in animals lacking CD28-dependent costimulation, suggesting that alternative costimulatory pathways were used. Blocking the OX40-OX40L costimulatory pathway differentially affected disease induction in CD28KO mice as compared with WT controls. Our data show that EAE may develop in the absence of CD28 T-cell costimulation. These findings have implications for therapies aimed at blocking costimulatory signals in autoimmune diseases.

Ribosome display and affinity maturation: from antibodies to single V-domains and steps towards cancer therapeutics

Protein affinity maturation using molecular evolution techniques to produce high-affinity binding proteins is an important step in the generation of reagents for cancer diagnosis and treatment. Currently, the most commonly used molecular evolution processes involve mutation of a single gene into complex gene repertoires followed by selection from a display library. Fd-bacteriophage are the most popular display vectors, but are limited in their capacity for library presentation, speed of processing and mutation frequency. Recently, the potential of ribosome display for directed molecular evolution was recognised and developed into a rapid and simple affinity selection strategy using ribosome complexes to display antibody fragments (scFv). Ribosome display and selection has the potential to generate and display large libraries more representative of the theoretical optima for naïve repertoires (10(14)). Even more important is the application of ribosome display for the affinity maturation of individual proteins by rapid mutation and selection cycles. These display strategies can apply to other members of the immunoglobulin superfamily; for example single V-domains which have an important application in providing specific targeting to either novel or refractory cancer markers. We discuss the application of ribosome display and selection in conjunction with variable domain (CTLA-4) libraries as the first step towards this objective and review affinity maturation strategies for in vitro ribosome display systems.

Induction, binding specificity and function of human ICOS

Recently, we have identified the inducible co-stimulator (ICOS), an activation-dependent, T cell-specific cell surface molecule related to CD28 and CTLA-4. Detailed analysis of human ICOS presented here shows that it is a 55-60-kDa homodimer with differently N-glycosylated subunits of 27 and 29 kDa. ICOS requires both phorbol 12-myristate 13-acetate and ionomycin for full induction, and is sensitive to Cyclosporin A. ICOS is up-regulated early on all T cells, including the CD28- subset, and continues to be expressed into later phases of T cell activation. On stimulation of T cells by antigen-presenting cells, the CD28/B7, but not the CD40 ligand/CD40 pathway is critically involved in the induction of ICOS. ICOS does not bind to B7-1 or B7-2, and CD28 does not bind to ICOS ligand; thus the CD28 and ICOS pathways do not cross-interact on the cell surface. In vivo, ICOS is expressed in the medulla of the fetal and newborn thymus, in the T cell zones of tonsils and lymph nodes, and in the apical light zones of germinal centers (predominant expression). Functionally, ICOS co-induces a variety of cytokines including IL-4, IL-5, IL-6, IFN-gamma, TNF-alpha, GM-CSF, but not IL-2, and superinduces IL-10. Furthermore, ICOS co-stimulation prevents the apoptosis of pre-activated T cells. The human ICOS gene maps to chromosome 2q33 - 34.

Regulation of cell surface expression of CTLA-4 by secretion of CTLA-4-containing lysosomes upon activation of CD4+ T cells

CTLA-4 is expressed on the surface of activated T cells and negatively regulates T cell activation. Because a low-level expression of CTLA-4 on the cell surface is sufficient to induce negative signals in T cells, the surface expression of CTLA-4 is strictly regulated. We previously demonstrated that the association of CTLA-4 with the clathrin-associated adaptor complex AP-2 induces internalization of CTLA-4 and keeps the surface expression low. However, the mechanism to induce high expression on the cell surface upon stimulation has not yet been clarified. To address this, we investigated the intracellular dynamics of CTLA-4 by analyzing its localization and trafficking in wild-type and mutant CTLA-4-transfected Th1 clones. CTLA-4 is accumulated in intracellular granules, which we identified as lysosomes. CTLA-4 is degraded in lysosomes in a short period, and the degradation process may serve as one of the mechanisms to regulate CTLA-4 expression. Upon TCR stimulation, CTLA-4-containing lysosomes are secreted as proven by the secretion of cathepsin D and beta-hexosaminidase in parallel with the increase of surface expression of CTLA-4 and lysosomal glycoprotein 85, a lysosomal marker. These results suggest that the cell surface expression of CTLA-4 is up-regulated upon stimulation by utilizing a mechanism of secretory lysosomes in CD4(+)T cells.

Structure of murine CTLA-4 and its role in modulating T cell responsiveness

The effective regulation of T cell responses is dependent on opposing signals transmitted through two related cell-surface receptors, CD28 and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4). Dimerization of CTLA-4 is required for the formation of high-avidity complexes with B7 ligands and for transmission of signals that attenuate T cell activation. We determined the crystal structure of the extracellular portion of CTLA-4 to 2.0 angstrom resolution. CTLA-4 belongs to the immunoglobulin superfamily and displays a strand topology similar to Valpha domains, with an unusual mode of dimerization that places the B7 binding sites distal to the dimerization interface. This organization allows each CTLA-4 dimer to bind two bivalent B7 molecules and suggests that a periodic arrangement of these components within the immunological synapse may contribute to the regulation of T cell responsiveness.

Identification and characterization of rat AILIM/ICOS, a novel T-cell costimulatory molecule, related to the CD28/CTLA4 family

Activation-inducible lymphocyte immuno-mediatory molecule (AILIM) is an inducible cell surface glycoprotein expressed on thymocytes and activated lymphocytes. Specific monoclonal antibody to rat AILIM induced the cell aggregation of a rat thymoma cell line and ConA-activated splenocytes. In the present study, we identified the primary structure of two species of rat AILIM by expression cloning. We also cloned mouse and human AILIM homologues and the predicted amino acid sequences were identical to those of the inducible costimulator ICOS/CRP-1, which belongs to the CD28/CTLA4 family. Although the human and mouse AILIM/ICOS molecule is localized on T-cells, the major population of AILIM/ICOS-positive cells in rat splenocyte was CD45RA-positive B-cells. The expression level of AILIM/ICOS on T-cells was relatively low; however, its expression was drastically induced by the treatment with PMA plus Ca-ionophore or the engagement of CD3 and these costimulatory molecules. Almost all T-cells exhibited potency as to its expression. Functional analysis of AILIM/ICOS demonstrated that AILIM-mediated costimulation was relatively weak compared to that of human.

Porcine CTLA4-Ig lacks a MYPPPY motif, binds inefficiently to human B7 and specifically suppresses human CD4+ T cell responses costimulated by pig but not human B7

The CTLA4 receptor (CD152) on activated T lymphocytes binds B7 molecules (CD80 and CD86) on APC and delivers a signal that inhibits T cell proliferation. Several regions involved in binding to B7 are known, but the relative importance of these is not clear. We have cloned porcine CTLA4 (pCTLA4). Although highly homologous to human CTLA4 (hCTLA4), the predicted protein sequence contains a leucine for methionine substitution at position 97 in the MYPPPY sequence. A fusion protein constructed from the extracellular regions of pCTLA4 and the constant regions of human IgG1 (pCTLA4-Ig) bound porcine CD86 with equivalent affinity to that of hCTLA4-Ig. However, pCTLA4-Ig bound poorly to human CD80 and CD86 expressed on transfectants and EBV-transformed human B cells. In functional assays with MHC class II-expressing porcine endothelial cells and human B cells, pCTLA4-Ig blocked human CD4+ T cell responses to pig but not human cells, whereas control hCTLA4-Ig inhibited responses to both. Comparison between mouse, human, and porcine CTLA4-Ig suggests that the selective binding of pCTLA4-Ig to porcine CD86 molecules is due to the L for M substitution at position 97. Our results indicate that pCTLA4-Ig may be a useful reagent to define the precise nature of the interaction between B7 and CTLA4. By failing to inhibit the delivery of costimulatory signals provided by human B7, it may also prove to be a relatively specific inhibitor of the direct human T cell response to immunogenic pig tissue.

T lymphocyte costimulatory molecules in host defense and immunologic diseases

BACKGROUND

Costimulation is an essential component for the optimal induction of T cell-mediated immune responses. Manipulation of the costimulatory pathway with antibodies or genetically-engineered fusion proteins is an important strategy to treat immune-related diseases including allergy, asthma, transplantation and cancer. Recent advances have revealed several new costimulatory molecules, and the functional characteristics of each costimulatory pathway are now becoming clearer.

LEARNING OBJECTIVES

In this review, we summarize basic outlines of the costimulatory systems in terms of molecular structure, expression kinetics and immunological function. We further discuss involvement and therapeutic manipulation of costimulation in several clinical diseases.

DATA SOURCE

The MEDLINE database was used to review the literature related to costimulation.

CONCLUSION

Costimulatory pathways play an essential role in the activation and regulation of T cell immune responses and the induction of T cell tolerance. Therapeutic manipulation of the costimulatory system demonstrates beneficial effects to treat immunological diseases in murine models as well as some clinical situations.

CTLA4-Ig: a novel immunosuppressive agent

Activation of naive T-cells requires two signals: one is antigen-specific and based on T-cell receptor (TCR) recognition of a peptide-MHC complex and the second is antigen-nonspecific and delivered by specific T-cell receptors after ligation with their ligands (costimulatory molecules) expressed by antigen-presenting cells (APCs). Engagement of the B7 family of molecules on APCs with their T-cell associated ligands, CD28 and CTLA-4 (CD152), provides a pivotal costimulatoty signal in T-cell activation. The lack of costimulation after engagement of the T-cell receptor by antigen, results in a state of antigen-specific unresponsiveness, termed anergy. Manipulation of CD28/B7 pathway has therefore been envisioned as a potential strategy for achieving therapeutically useful immunosuppression or tolerance. CTLA4-Ig has been initially developed by Bristol-Myers Squibb as a competitive inhibitor of CD28/B7 pathway (BMS-188667). Thereafter, CTLA4-Ig was produced by Repligen and also in some individual laboratories. In various animal models, discussed in this paper, CTLA4-Ig has been shown to inhibit T-cell-dependent antibody responses, significantly prolong transplanted organ survival, induce long-term donor-specific tolerance in some models, slow progression of autoimmune disease and to have immunomodulatory function in several other immunological disease models. Recently, CTLA4-Ig has entered Phase I clinical trials for the treatment of psoriasis, a T-cell mediated skin disease and treatment of graft-versus-host disease in allogeneic bone marrow transplantation. Large clinical randomised trials on the use of CTLA4-Ig are missing, nevertheless, its immunosuppressive effects coupled with features such as specificity of interaction and low toxicity, make CTLA4-Ig a promising new therapeutic agent for induction of donor-specific immunological tolerance, the ultimate goal of clinical immunosuppression.

Molecular cloning and expression of feline CD28 and CTLA-4 cDNA

Feline CD28 and CTLA-4 (CD152) cDNA were cloned from Con-A stimulated feline peripheral blood mononuclear cells (PBMC) by rapid amplification of cDNA end-PCR (RACE-PCR). Both CD28 and CTLA-4 proteins belong to the immunoglobulin superfamily (Ig SF) and are composed of a signal sequence, an extracellular domain, a transmembrane domain and a cytoplasmic domain. The open reading frame (ORF) of CD28 cDNA encoded a predicted protein of 221 amino acids and that of CTLA-4 cDNA encoded a predicted protein of 223 amino acids. The B7 ligands binding motif MYPPPY hexamer was found on the extracellular Ig V-like domains of both receptors and phosphatidylinositol 3-kinase (PI 3-kinase) binding motifs pYMNM for CD28 and pYVKM for CTLA-4 were identified in the cytoplasmic domains. Comparisons of amino acid sequences of feline proteins with known sequences of other species indicated that rabbit CD28 and CTLA-4 were most closely related and mouse molecules were the least conserved with feline molecules. Comparison of each domain of both molecules with that of other animals showed that the cytoplasmic domain of CTLA-4 was 100% conserved and that of CD28 was the most conserved domain. The cloned CD28 and CTLA-4 cDNA could be expressed in transfected mammalian cells. Expression of feline CD28 and CTLA-4 mRNA in freshly isolated feline PBMC was demonstrated by RT-PCR. Stimulation of PBMC with Con-A similarly increased the expression of both CD28 and CTLA-4 mRNA.

Development and application of cytotoxic T lymphocyte-associated antigen 4 as a protein scaffold for the generation of novel binding ligands

We have explored the possibilities of using human cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) as a single immunoglobulin fold-based scaffold for the generation of novel binding ligands. To obtain a suitable protein library selection system, the extracellular domain of CTLA-4 was first displayed on the surface of a filamentous phage as a fusion product of the phage coat protein p3. CTLA-4 was shown to be functionally intact by binding to its natural ligands B7-1 (CD80) and B7-2 (CD86) both in vitro and in situ. Secondly, the complementarity determining region 3 (CDR3) loop of the CTLA-4 extracellular domain was evaluated as a permissive site. We replaced the nine amino acid CDR3-like loop of CTLA-4 with the sequence XXX-RGD-XXX (where X represents any amino acid). Using phage display we selected several CTLA-4-based variants capable of binding to human alphavbeta3 integrin, one of which showed binding to integrins in situ. To explore the construction of bispecific molecules we also evaluated one other potential permissive site diametrically opposite the natural CDR-like loops, which was found to be tolerant of peptide insertion. Our data suggest that CTLA-4 is a suitable human scaffold for engineering single-domain molecules with one or possibly more binding specificities.

Characterization of human inducible costimulator ligand expression and function

The inducible costimulator (ICOS) is the newest member of the CD28/CD152 receptor family involved in regulating T cell activation. We constructed a soluble-Ig fusion protein of the extracellular domain of human ICOS and used it as a probe to characterize expression patterns of the ICOS ligand (ICOSL). ICOSIg did not bind to CD80- or CD86-transfected Chinese hamster ovary cell lines, demonstrating that ICOSL is distinct from those ligands identified for CD28/CD152. ICOSIg showed selective binding to monocytic and B cell lines, whereas binding was undetectable on unstimulated monocytes and peripheral blood T and B cells. Expression of ICOSL was induced on monocytes after integrin-dependent plastic adhesion. Pretreatment of monocytes with mAb to the beta2-integrin subunit CD18 decreased adhesion and abolished ICOSL up-regulation but had no effect on CD80/86 (CD152 ligand (CD152L)) expression. Both ICOSL and CD152L were up-regulated on monocytes by IFN-gamma but by distinct signaling pathways. Unlike CD152L expression, ICOSL expression did not change when monocytes were differentiated into dendritic cells (DCs) or after DCs were induced to mature by LPS, TNF-alpha, or CD40 ligation. Addition of ICOSIg to allogeneic MLRs between DCs and T cells reduced T cell proliferative responses but did so less efficiently than CTLA4Ig (CD152Ig) did. Similarly, ICOSIg also blocked Ag-specific T cell proliferation to tetanus toxoid. Thus, ICOSL, like CD80/86, is expressed on activated monocytes and dendritic cells but is regulated differently and delivers distinct signals to T cells that can be specifically inhibited by ICOSIg.

Molecular cloning and sequencing of a cDNA encoding the feline cytotoxic T-lymphocyte-associated protein 4 (CTLA4) homologue

The feline CTLA4 cDNA encodes a transmembrane protein which shares 87.9% sequence identity with the human CTLA4 molecule. The cytoplasmic region of the feline CTLA4 is identical to that of humans and mice, which suggests conserved function(s) such as the regulation of cell-surface expression of this molecule.

LICOS, a primordial costimulatory ligand?

In mammals, the classical B7 molecules expressed on antigen-presenting cells, B7-1 (CD80) and B7-2 (CD86), bind the structurally related glycoproteins CD28 and CTLA-4 (CD152), generating costimulatory signals that regulate the activation state of T cells. A recently identified human CD28-like protein, ICOS, also induces costimulatory signals in T cells when crosslinked with antibodies, but it is unclear whether ICOS is part of a B7-mediated regulatory pathway of previously unsuspected complexity, or whether it functions independently and in parallel. Here, we report that, rather than binding B7-1 or B7-2, ICOS binds a new B7-related molecule of previously unknown function that we call LICOS (for ligand of ICOS). At 37 degrees C, LICOS binds only to ICOS but, at lower, non-physiological temperatures, it also binds weakly to CD28 and CTLA-4. Sequence comparisons suggest that LICOS is the homologue of a molecule expressed by avian macrophages and of a murine protein whose expression is induced in non-lymphoid organs by tumour necrosis factor alpha (TNFalpha). Our results define the components of a distinct and novel costimulatory pathway and raise the possibility that LICOS, rather than B7-1 or B7-2, is the contemporary homologue of a primordial vertebrate costimulatory ligand.

Inhibition of M-tropic HIV-1 infection by the fd phage-gene 3 protein with MIP-1alpha-binding activity

CCR5 is a chemokine receptor with seven transmembrane-domains. It is expressed on T cells and macrophages and functions as the principal co-receptor for macrophage (M)-tropic strains of HIV-1. The anti-CCR5 monoclonal antibody (mAb) 2D7 inhibits the binding and chemotaxis of the three natural beta-chemokine ligands of CCR5, macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, and RANTES, to CCR5(+) cells. The mAb also efficiently blocks the infectivity of several M-tropic and dual-tropic HIV-1 strains in vitro. In this study, we attempted to determine the peptide motif recognized with the 2D7 mAb. We isolated phage clones by panning a phage display library using 2D7 and identified three peptide motifs. One of these phage clones (M23) showed a marked inhibitory activity on HIV-1 infection. The unique sequence of 15 amino acids with an internal disulfide bond was inserted in the g3p of the M23 phage clone (M23-g3p). The M23-g3p was purified by fast-performance liquid chromatography (FPLC). We show here that (1) M23-g3p was specifically recognized with anti-CCR5 mAb; (2) M23-g3p showed inhibitory activity on the infectivity of M-tropic but not T-tropic HIV-1 strains; (3) M23-g3p bound to MIP-1alpha, MIP-1beta, and RANTES but not MCP-1. These results suggested that the M23-g3p might mimic the CCR5-binding domain shared by beta-chemokines, MIP-1alpha, MIP-1beta, and RANTES as well as the HIV-1 infection.

A soluble form of CTLA-4 generated by alternative splicing is expressed by nonstimulated human T cells

CTLA-4, expressed by activated T cells, transduces an inhibitory signal. We show here that PCR amplification of the coding sequence of CTLA-4 in nonstimulated human T lymphocytes results in the amplification of two transcripts of 650 and 550 bp. Sequencing shows that the larger form codes for membrane CTLA-4 and the 550-bp transcript is a spliced variant in which exon 2 coding for the transmembrane region is deleted. This spliced cDNA has been named CTLA-4delTM. The splicing induces a frame shift which results in the addition of 22 extra amino acids before a translational termination. Activation of T cells with phorbol 12-myristate 13-acetate plus ionomycin or anti-CD3 plus anti-CD28 monoclonal antibodies induces a suppression of CTLA-4delTM mRNA expression associated with a preferential expression of the membrane CTLA-4 mRNA, showing that CTLA-4delTM mRNA expression is restricted to nonactivated T cells. A soluble immunoreactive form of CTLA-4 was detected in the serum of 14 / 64 healthy subjects. These results suggest that nonstimulated T cells may constitutively produce a soluble form of CTLA-4 which may have an important role in the regulation of immune homeostasis.