ICOS co-stimulatory receptor is essential for T-cell activation and function

A role for inducible costimulator protein in the CD28- independent mechanism of resistance to Toxoplasma gondii

Long-term resistance to Toxoplasma gondii is dependent on the development of parasite-specific T cells that produce IFN-gamma. CD28 is a costimulatory molecule important for optimal activation of T cells, but CD28(-/-) mice are resistant to T. gondii, demonstrating that CD28-independent mechanisms regulate T cell responses during toxoplasmosis. The identification of the B7-related protein 1/inducible costimulator protein (ICOS) pathway and its ability to regulate the production of IFN-gamma suggested that this pathway may be involved in the CD28-independent activation of T cells required for resistance to T. gondii. In support of this hypothesis, infection of wild-type or CD28(-/-) mice with T. gondii resulted in the increased expression of ICOS by activated CD4(+) and CD8(+) T cells. In addition, both costimulatory pathways contributed to the in vitro production of IFN-gamma by parasite-specific T cells and when both pathways were blocked, there was an additive effect that resulted in almost complete inhibition of IFN-gamma production. Although in vivo blockade of the ICOS costimulatory pathway did not result in the early mortality of wild-type mice infected with T. gondii, it did lead to increased susceptibility of CD28(-/-) mice to T. gondi associated with reduced serum levels of IFN-gamma, increased parasite burden, and increased mortality compared with the control group. Together, these results identify a critical role for ICOS in the protective Th1-type response required for resistance to T. gondii and suggest that ICOS and CD28 are parallel costimulatory pathways, either of which is sufficient to mediate resistance to this intracellular pathogen.

Constitutive expression of the B7h ligand for inducible costimulator on naive B cells is extinguished after activation by distinct B cell receptor and interleukin 4 receptor-mediated pathways and can be rescued by CD40 signaling

The recently described ligand-receptor pair, B7h-inducible costimulator (ICOS), is critical for germinal center formation and antibody responses. In contrast to the induced expression of the related costimulatory ligands B7.1 and B7.2, B7h is constitutively expressed on naive B cells and is surprisingly extinguished after antigen engagement and interleukin (IL)-4 cytokine signaling. Although signaling through both B cell receptor (BCR) and IL-4 receptor (R) converge on the extinction of B7h mRNA levels, BCR down-regulation occurs through Ca2+ mobilization, whereas IL-4R down-regulation occurs through a distinct Stat6-dependent pathway. During antigen-specific B cell activation, costimulation through CD40 signaling can reverse both BCR- and IL-4R-mediated B7h down-regulation. These data suggest that the CD40-CD40 ligand signaling pathway regulates B7h expression on activated B cells and may control whether antigen-activated B cells can express B7h and costimulate cognate antigen-activated T cells through ICOS.

Characterization of mouse and human B7-H3 genes

T cell activation and immune function are regulated by costimulatory molecules of the B7 superfamily. Human B7-H3 is a recent addition to this family and has been shown to mediate T cell proliferation and IFN-gamma production. In this work we describe the identification of the mouse B7-H3 homolog, which is ubiquitously expressed in a variety of tissues. Activated CD4 and CD8 T cells express a putative receptor that can be recognized by soluble mouse B7-H3-Ig molecules. While the mouse B7-H3 gene was found to contain a single copy, we discovered a novel isoform of human B7-H3 (named as B7-H3b hereafter) with four Ig-like domains that results from gene duplication and differential splicing. B7-H3b is the major isoform expressed in several tissues. This structural information suggests a genetic variation of the B7-H3 gene in mammalian species.

The B7 family of ligands and its receptors: new pathways for costimulation and inhibition of immune responses

T cell activation is dependent upon signals delivered through the antigen-specific T cell receptor and accessory receptors on the T cell. A primary costimulatory signal is delivered through the CD28 receptor after engagement of its ligands, B7-1 (CD80) or B7-2 (CD86). Engagement of CTLA-4 (CD152) by the same B7-1 or B7-2 ligands results in attenuation of T cells responses. Recently, molecular homologs of CD28 and CTLA-4 receptors and their B7-like ligands have been identified. ICOS is a CD28-like costimulatory receptor with a unique B7-like ligand. PD-1 is an inhibitory receptor, with two B7-like ligands. Additional members of B7 and CD28 gene families have been proposed. Integration of signals through this family of costimulatory and inhibitory receptors and their ligands is critical for activation of immune responses and tolerance. Understanding these pathways will allow development of new strategies for therapeutic intervention in immune-mediated diseases.

Targeting T cell costimulation in autoimmune disease

Many factors contribute to the pathogenesis of autoimmune diseases. Targets for treating such debilitating diseases will become more apparent by understanding the nature of immune activation. This review examines the possibility of targeting costimulation and discusses the molecules found on the T cell and the antigen-presenting cell (APC) that participate in T cell activation. Although new molecules continue to be discovered, the functions of B7-1 (CD80), B7-2 (CD86), CD28, cytotoxic T lymphocyte antigen 4 (CTLA-4), inducible costimulator (ICOS), programmed death 1 (PD-1), OX 40 (CD134) and CD40 ligand (CD40L, CD154) are now sufficiently understood that immunologists are targeting them to manipulate T cells to slow the progression of autoimmune diseases or treat tumours through the increase in T cell activation. CD28, ICOS, OX 40 and CD40L are considered the costimulatory molecules that increase T cell activation. However, ICOS and OX 40 appear to act on memory cells while CD28 is predominantly a naive T cell activator. Most therapies in the treatment of autoimmunity that target these molecules work through blockade of their function with receptor specific immunoglobulin (Ig). CTLA-4 and PD-1 are considered to be the inhibitory T cell costimulators. While stimulating CTLA-4 has not been a widely used therapy, using soluble CTLA-4Ig to block B7 and disrupt the B7/CD28 pathway is fairly common. The majority of therapeutic use for PD-1 stems from targeting PD-1 with its natural ligand. It is hoped that therapies targeting costimulation may provide a means of conserving the patient's normal T cell repertoire and immune function whilst eliminating or suppressing autoreactive T cells and thus provide a more efficient means to treat autoimmune disease.

A co-stimulatory molecule on activated T cells, H4/ICOS, delivers specific signals in T(h) cells and regulates their responses

We examined the co-stimulatory activity of H4/ICOS on murine activated CD4(+) T cells and found that the cross-linking of H4/ICOS enhanced their proliferation, in addition to raising IFN-gamma, IL-4 and IL-10 production to levels comparable to those induced by CD28. However, IL-2 production was only marginally co-stimulated by H4/ICOS. This distinct pattern of lymphokine production appears to be induced by a specific intracellular signaling event. Compared with CD28, H4/ICOS dominantly elicited the Akt pathway via phosphatidylinositol 3-kinase. In addition, mitogen-activated protein kinase family kinases were activated in different ways by CD28 and H4/ICOS. The strong phosphorylation of p46 c-Jun N-terminal kinase was observed upon CD28 co-stimulation, but was less potently induced by H4/ICOS. The strain diversity in the induction of H4/ICOS was recognized. The expression of H4/ICOS on BALB/c activated CD4(+) T cells was >6-fold higher compared with C57BL/6 activated CD4(+) T cells. Furthermore, BALB/c activated CD4(+) T cells exhibited more T(h)2-deviated lymphokine production as compared with C57BL/6 activated CD4(+) T cells and signaling through H4/ICOS during the primary stimulation of naive CD4(+) T cells promoted the generation of T(h)2 cells. Thus, the difference in H4/ICOS expression on activated CD4(+) T cells, which is regulated among the mouse strains, may also regulate the polarization of T(h) cells.

The right place at the right time: novel B7 family members regulate effector T cell responses

Recent identification of novel members of the B7-family of costimulatory ligands has illustrated their importance for costimulation, not only for initiation of adaptive immune responses, but also for regulation of activated effector lymphocytes. Two key features that distinguish these novel molecules from classical B7.1 and B7.2 costimulatory ligands are their broader expression in non-lymphoid tissues and their binding to receptors induced on activated T cells. Whereas B7.1/B7.2-CD28 interactions are important for priming naïve T cells, novel costimulatory interactions appear critical in regulating effector lymphocytes at sites of infection in the periphery.

Negative co-receptors on lymphocytes

The past year has seen significant advances in our understanding of critical roles of negative immunoregulatory signals delivered through the B7-CD28 superfamily in regulating T cell activation and tolerance. Structural data on CTLA-4 have provided novel insights into the inhibitory functions of CTLA-4. Initial characterization of the PD-1-PD-1-ligand pathway has revealed that this pathway can downregulate TCR- and CD28-mediated signals. Recent studies indicate that ICOS exerts distinct effects at different phases of an immune response: ICOS can inhibit as well as stimulate T cell responses.

Biologic therapy of inflammatory bowel disease

Advancing knowledge regarding the biology of chronic inflammation has led to the development of specific biologic therapies that mechanistically target individual inflammatory pathways. Many biologic therapies are being evaluated for the treatment of the chronic inflammatory bowel diseases, Crohn's disease and ulcerative colitis. Biologic compounds proven to be effective for Crohn's disease include monoclonal antibodies to tumor necrosis factor (infliximab and CDP571) and to the leukocyte adhesion molecule alpha4 integrin (natalizumab). Other biologic compounds for which there is insufficient evidence to judge efficacy for inflammatory bowel disease include: p55 tumor necrosis factor binding protein (onercept); interferon alpha; interferon beta-1a; anti-interferon gamma antibody; anti-interleukin 12 antibody; p65 anti-sense oligonucleotide (blocks NF-kappaB); granulocyte colony stimulating factor, and granulocyte macrophage colony stimulating factor; anti-interleukin 2 receptor antibody; epidermal growth factor; keratinocyte growth factor 2 (repifermin); human growth hormone; anti-CD4 antibody; and anti-alpha4beta7 antibody. Biologic therapies that have been proven ineffective for inflammatory bowel disease include: interleukin 10; interleukin 11; anti-sense intercellular adhesion molecule-1; and the tumor necrosis factor receptor fusion protein etanercept. Based on the early successes of infliximab, CDP571 and natalizumab, it seems certain that biologic therapy will play an important role in the future treatment of inflammatory bowel disease.

Critical role of CD81 in cognate T-B cell interactions leading to Th2 responses

We previously demonstrated that CD81-/- mice fail to develop Th2-biased immune responses and allergen-induced airway hyper-reactivity. Because CD81 is expressed on both activated T and on B cells, we examined the role of CD81 expression by each cell type. We established an in vitro system by backcrossing the CD81 deletion to TCR transgenic (Tg) mice and to BCR Tg mice. Here we demonstrate that CD81 expression by T cells is critical for their induction of IL-4 synthesis by B cells. CD81-/- TCR Tg T cells were impaired in IL-4 production compared to CD81+/+ TCR Tg T cells, whereas CD81-/- and CD81+/+ BCR Tg B cells induced equivalent amounts of IL-4 in CD81+/+ TCR Tg T cells. CD81-/- TCR Tg T cells expressed reduced levels of ICOS, GATA-3, STAT6 and phosphorylated STAT6 when activated by antigen-presenting B cells. Taken together, these results indicate that CD81 expression by T cells greatly enhances cognate T-B cell interactions and greatly augments intracellular activation pathways leading to Th2 polarization.

ICOS-ligand, expressed on human endothelial cells, costimulates Th1 and Th2 cytokine secretion by memory CD4+ T cells

Endothelial cells (EC) play a central role in inflammatory immune responses and efficiently induce effector functions in T cells, despite lacking the classical costimulatory ligands CD80 and CD86. By using the mAb HIL-131 we now demonstrate that human inducible costimulator-ligand (ICOS-L), a molecule related to CD80/CD86, is constitutively expressed on human EC in vivo. In vitro, ICOS-L expression was strongly enhanced on human umbilical vein EC and microvascular EC by the inflammatory cytokines tumor necrosis factor alpha and IL-1beta, and to a lower extent by stimulation of EC by CD40 or lipopolysaccharide. Coculture of MHC class II(+) EC with resting memory CD4(+) T cells in the presence of superantigen led to a marked up-regulation of ICOS on T cells and to the production of Th1 (IFN-gamma, IL-2) and Th2 cytokines (IL-4, IL-10, IL-13). When these cocultures were performed in the presence of the inhibitory mAb HIL-131, secretion of all cytokines was reduced by about 50-80%, indicating that ICOS-L is a major costimulator in EC-mediated T cell activation. Taken together, our data suggest an important physiological role of ICOS-L in the reactivation of effector/memory T cells on the endothelium controlling the entry of immune cells into inflamed tissue.

AILIM/ICOS: its expression and functional analysis with monoclonal antibodies

Activation-inducible lymphocyte immuno-mediatory molecule (AILIM/ICOS) is the third member of the co-stimulatory molecule CD28/CTLA-4 (CD152) family, and an inducible cell surface glycoprotein expressed on lymphocytes following activation. To determine the expression profile of the molecule, we generated monoclonal antibodies (MAbs) against human, rat, and mouse AILIM/ICOS. None of the MAbs bound to AILIM/ICOS of other species. The numbers of AILIM/ICOS-positive cells among human peripheral blood mononuclear cells (PBMC), and rat and mouse splenocytes were very low (0.5, 0.4, and 1.2%, respectively), and the cells included many CD4-positive T cells except in the case of rat. Rat AILIM/ICOS-positive cells among splenocytes included many CD45RA-positive B cells, although the expression on lymph node cells was similar to that on human PBMC and mouse splenocytes. Among rat thymocytes, the AILIM/ICOS expression was mainly localized on CD4- and CD8-double positive T cells. The binding of AILIM/ICOS to B7h-Ig, which is the ligand-Fc chimeric protein, was inhibited by all AILIM/ICOS-specific MAbs except for SG430. The potency of the co-stimulatory activity of CD3 and AILIM/ICOS as to T-cell proliferation was found to be substantial in human. Interestingly, the levels of stimulation with the two types of MAbs were equal to that with CD3 and CD28 despite the different functions of the two MAbs in the AILIM/ICOS-B7h interaction. On the other hand, the potencies in rat and mouse, although two independent MAbs were tested, were relatively lower than that of CD28-mediated co-stimulation.

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.

B7 family molecules: novel immunomodulators at the maternal-fetal interface

The placenta utilizes both active and passive mechanisms to evade rejection by the maternal immune system. Recently, the mRNA for two newly cloned members of the B7 family of immunomodulatory cell-associated proteins have been identified in the human term placenta. In this article, we review the current knowledge of the B7 family member B7-H1, and discuss how it may participate in modulation of the maternal immune system at the maternal-fetal interface. B7-H1 has been found to possess immunostimulatory or immunoinhibitory properties, and immunohistological examination of first trimester and term placenta has revealed that this protein is abundant in the placenta. B7-H1 is highly expressed by both the syncytiotrophoblast and extravillous cytotrophoblast, both of which lie in direct contact with maternal blood and tissue. Further, treatment of the choriocarcinoma cell line, JEG-3, with recombinant human interferon (IFN)-gamma resulted in a dose-dependent increase in the abundance of the message for B7-H1, suggesting that IFN-gamma could regulate expression of B7-H1 by the trophoblast. These studies document that the positioning of B7-H1 at the maternal-fetal interface is such that it could participate in suppression of activated maternal leukocytes.

Lipid A analogue, ONO-4007, inhibits IgE response and antigen-induced eosinophilic recruitment into airways in BALB/c mice

BACKGROUND

Since antigen-specific IgE and eosinophils are major inducing factors of allergic inflammation of the airways, both factors are therapeutic targets of asthma. We investigated the effects of ONO-4007, a nontoxic lipid A analogue, on antigen-specific antibody response and the recruitment of eosinophils into airways in murine systems.

METHODS

BALB/c mice were injected ONO-4007 intraperitoneally during sensitization with ovalbumin (OVA) and aluminium hydroxide to determine its effects on the antigen-specific antibody response. ONO-4007 was also injected intravenously during either systemic sensitization and inhalation with OVA, or sensitization or inhalation alone to determine its effects on antigen-induced airway inflammation. In vitro effects of ONO-4007 on the functional differentiation of naive CD4+ T cells were investigated by culturing naive CD4+ T cells derived from DO11.10 mice and OVA-pulsed dendritic cells (CDCs) with ONO-4007.

RESULTS

ONO-4007 inhibited antigen-specific IgE and IgG1, but not IgG2a responses. ONO-4007 decreased the recruitment of eosinophils and the levels of IL-5 in bronchoalveolar lavage fluid, not only when it was injected during systemic sensitization and inhalation with OVA, but also during inhalation alone. ONO-4007 inhibited the differentiation of IL-4- and IL-13-producing CD4+ T cells in vitro, which was partly mediated by DCs.

CONCLUSIONS

ONO-4007 inhibited antigen-specific IgE and IgG1 responses and antigen-induced eosinophil recruitment into the airways in BALB/c mice. These effects were mediated, at least partly, by the modulation of DCs, although there may also be other mechanisms.

Predictive testing for autoimmunity

Many chemicals, in particular drugs, cause systemic allergy or autoimmune-like disorders. Due to complex pathogenesis and strong dependence on genetic make-up, these immunotoxicological effects are usually missed in standard toxicity testing. Besides, animal studies that demonstrate chemically induced systemic allergy or autoimmune-like disorders are scarce. Here, animal models are presented that would fit into a predictive two-tiered strategy, designed to allow screening for immunostimulatory potential in the first tier, and more elaborate testing for allergenic or autoimmunogenic potential of selected chemicals in the second tier. The popliteal lymph node assay (PLNA), with or without reporter antigens, would fit in the first tier, and relevant route of exposure protocols with selected strains of mice or rats may be further developed to compose the second tier. To date, the relevant route of exposure models mentioned here (with 'normal' inbred mice and/or Brown Norway rats) has been tested with only a few chemicals, and the PLNA, although tested with over 100 chemicals, is not validated as yet. Conceivably, a major challenge in immunotoxicology is to incorporate the present knowledge on chemical-induced systemic allergy and autoimmunity in further development and validation of predictive models and strategies.

Cutting edge: inducible costimulator protein regulates both Th1 and Th2 responses to cutaneous leishmaniasis

The CD28 family member inducible costimulator protein (ICOS) has an important role in T cell differentiation and Ig class switching. To investigate the role of ICOS in vivo, ICOS-/- mice were infected s.c. with Leishmania mexicana. While wild-type mice developed large, cutaneous lesions, the growth of lesions and tissue histopathology was significantly delayed in ICOS-/- mice. ICOS-/- mice exhibited marked decreases in both Th1 and Th2 cytokine production and profound defects in L. mexicana-specific Ig isotype class switching to IgG1 and IgG2a and reduced total IgE levels. Our findings indicate that ICOS is a key regulator of both Th1 and Th2 responses and has a role in controlling cutaneous L. mexicana infection.

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.

Blocking the CD80 and CD86 costimulation molecules: lessons to be learned from animal models

The induction of tolerance for allografts, obviating the need for immunosuppression, is the ultimate goal in transplantation. Immunoregulatory antibodies preventing graft rejection are promising candidates for the induction of tolerance. Costimulation blockade could be a useful approach to inducing donor-specific nonresponsiveness in organ transplantation. Rodent studies and in vitro studies using human or nonhuman primate peripheral blood mononuclear cells indicated that this approach might indeed lead to specific T-cell anergy. Nonhuman primate studies, in which the B7 costimulation pathway was blocked, have so far not led to permanent drug-free graft acceptance. The results are promising, however, because during the treatment period with B7 costimulation blockade alone or combined with anti-CD40 or cyclosporine, no graft loss was observed and donor-specific antibody formation was prevented. Based on these findings, new approaches to inducing drug-free graft acceptance should be investigated.

Immunomodulation with CTLA4-Ig in islet transplantation

The need for permanent, nonspecific, and potentially harmful immunosuppression remains a major obstacle for islet transplantation. The response of a type 1 diabetic recipient to an islet graft includes a specific allogenic immune response and the recurrence of autoimmunity. Free or encapsulated in an immunoisolation device, islet cells are exposed to immune aggression, initiated by donor antigen-presenting cells or by indirect, host antigen-presenting cell-mediated antigen presentation. CTLA4-Ig is a genetically engineered fusion protein of human CTLA4 and the IgG 1 Fc region. It prevents T-cell activation by binding to human B7, which costimulates T cells through CD28. Interesting data were reported in experimental islet transplantation, suggesting that CTLA4-Ig may be slightly but significantly beneficial to islet allograft survival, although studies in autoimmune diabetes are scarce. The main limitations include transient and low levels of expression when CTLA4-Ig is delivered locally, a predominant effect on the direct recognition pathway, and the lack of effect on memory cells. Clinical trials in islet transplantation could be discussed in nonuremic patients, with steroid-free and anticalcineurin-free regimens, in combination with another costimulation blocker, rapamycin, and an anti-interleukin 2 receptor antibody, and with a strategy directed against the recurrence of autoimmunity.

CD28 function: a balance of costimulatory and regulatory signals

Both the recognition of MHC/antigen complex by the T-cell receptor and engagement of costimulatory molecules are necessary for efficient T-cell activation. CD28 has been widely recognized as the major costimulation pathway for naive T-cell activation, and the CD28/B7 pathway plays a central role in immune responses against pathogens, autoimmune diseases, and graft rejection. In this review, we will summarize evidence that CD28 is also prominent in the regulation of immune responses and the maintenance of peripheral tolerance. Indeed, CD28 engagement increases the expression of the down-modulatory molecule CTLA-4, induces the differentiation of Th2 cells that have a protective function in autoimmunity, and has an obligatory role in the homeostasis of regulatory T cells. Therefore, CD28/B7 interactions induce a balance of costimulatory and regulatory signals that have opposite outcomes on immune responses. This new perspective on CD28 function suggests that caution should be taken in the development of immunotherapies targeting costimulatory pathways.

Dendritic cells: immune regulators in health and disease

Dendritic cells (DCs) are bone marrow-derived cells of both lymphoid and myeloid stem cell origin that populate all lymphoid organs including the thymus, spleen, and lymph nodes, as well as nearly all nonlymphoid tissues and organs. Although DCs are a moderately diverse set of cells, they all have potent antigen-presenting capacity for stimulating naive, memory, and effector T cells. DCs are members of the innate immune system in that they can respond to dangers in the host environment by immediately generating protective cytokines. Most important, immature DCs respond to danger signals in the microenvironment by maturing, i.e., differentiating, and acquiring the capacity to direct the development of primary immune responses appropriate to the type of danger perceived. The powerful adjuvant activity that DCs possess in stimulating specific CD4 and CD8 T cell responses has made them targets in vaccine development strategies for the prevention and treatment of infections, allograft reactions, allergic and autoimmune diseases, and cancer. This review addresses the origins and migration of DCs to their sites of activity, their basic biology as antigen-presenting cells, their roles in important human diseases and, finally, selected strategies being pursued to harness their potent antigen-stimulating activity.

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.

Assembly and annotation of human chromosome 2q33 sequence containing the CD28, CTLA4, and ICOS gene cluster: analysis by computational, comparative, and microarray approaches

Human chromosome 2q33 is an immunologically important region based on the linkage of numerous autoimmune diseases to the CTLA4 locus. Here, we sequenced and assembled 2q33 bacterial artificial chromosome (BAC) clones, resulting in 381,403 bp of contiguous sequence containing genes encoding a NADH: ubiquinone oxidoreductase, the costimulatory receptors CD28, CTLA4, and ICOS, and a HERV-H type endogenous retrovirus located 366 bp downstream of ICOS in the reverse orientation. Genomic microarray expression analysis using differentially activated T-cell RNA against a subcloned CTLA4/ICOS BAC library revealed upregulation of CTLA4 and ICOS sequences, plus antisense ICOS transcripts generated by the HERV-H, suggesting a potential mechanism for ICOS regulation. We identified four nonlinked, polymorphic, simple repetitive sequence elements in this region, which may be used to delineate genetic effects of ICOS and CTLA4 in disease populations. Comparative genomic analysis of mouse genomic Icos sequences revealed 60% sequence identity in the 5' UTR and regions between exon 2 and the 3' UTR, suggesting the importance of ICOS gene function.

Terminal defects of B lymphocyte differentiation

In humans several abnormalities can occur during terminal B cell differentiation, leading to primary humoral immunodeficiencies. A recent study provided evidence of a qualitative defect of the affinity antibody maturation in some patients affected with common variable immunodeficiency syndrome, the molecular basis of which remains unknown. Several genetic defects in class switch recombination leading to a hyper-IgM syndrome have recently been delineated. Besides the well-known role of CD40-CD40 ligand interaction, they definitively demonstrate the requirement of CD40-mediated nuclear factor kappa B activation and the essential role of a newly described molecule, the activation-induced cytidine deaminase, in B cell terminal differentiation.

Opposing effects of anti-activation-inducible lymphocyte-immunomodulatory molecule/inducible costimulator antibody on the development of acute versus chronic graft-versus-host disease

The functional role of inducible costimulator (ICOS)-mediated costimulation was examined in an in vivo model of alloantigen-driven Th1 or Th2 cytokine responses, the parent-into-F(1) model of acute or chronic graft-vs-host disease (GVHD), respectively. When the Ab specific for mouse ICOS was injected into chronic GVHD-induced mice, activation of B cells, production of autoantibody, and development of glomerulonephritis were strongly suppressed. In contrast, the same treatment enhanced donor T cell chimerism and host B cell depletion in acute GVHD induced host mice. Blocking of B7-CD28 interaction by injection of anti-B7-1 and anti-B7-2 Abs inhibited both acute and chronic GVHD. These observations clearly indicate that the costimulatory signal mediated by CD28 caused the initial allorecognition resulting in the clonal expansion of alloreactive T cells, whereas the costimulatory signal mediated by ICOS played a critical role in the functional differentiation and manifestation of alloreactive T cells. Furthermore, treatment with anti-ICOS Ab selectively suppresses Th2-dominant autoimmune disease.

B7H costimulates clonal expansion of, and cognate destruction of tumor cells by, CD8(+) T lymphocytes in vivo

B7H/B7RP (hereby called B7H) is a new member of the B7 family of costimulatory molecules and interacts with inducible costimulatory molecule (ICOS). Its function for CD8 T cells has not been reported. We report here that expression of B7H on the tumor cells reduced tumorigenicity and induced immunity to subsequent challenge with parental tumor cells. The immune protection correlates with an enhanced cytotoxic T lymphocyte (CTL) response against P1A, the major tumor antigen expressed in the J558 tumor. To understand the mechanism of immune protection, we adoptively transferred transgenic T cells specific for tumor antigen P1A into mice that bore P1A-expressing tumors. We found that while the transgenic T cells divided faster in mice bearing the B7H(+) tumors, optimal B7H-induced clonal expansion of P1CTL required costimulation by B7-1 and B7-2 on the endogenous host antigen-presenting cells (APCs). Interestingly, when B7H(+) and B7H(-) tumors were coinjected, P1CTL selectively eliminated the B7H(+) tumor cells. Moreover, B7H expressed on the tumor cells made them highly susceptible to destruction by CTL in vivo, even if the CTL was administrated into mice with large tumor burdens. Tumors that recurred in the P1CTL-treated mice lost transfected B7H and/or H-2L(d), the class I molecule that presents the P1A peptide. Taken together, our results reveal that B7H costimulates clonal expansion of, and cognate destruction by CD8(+) T lymphocytes in vivo.

Knockout mice: a paradigm shift in modern immunology

In the past decade, advances in genetic engineering and mouse knockout technology have transformed our understanding of the immune system. In particular, new perspectives on T-cell development, co-stimulation and activation have emerged from the study of single and multiple gene-knockout animals, as well as from conditional knockout and 'knock-in' mutants. Analysis of these animals has clarified important intracellular signalling pathways and has shed light on the regulatory mechanisms that govern normal immune responses and autoimmunity.

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.

ICOS ligand costimulation is required for T-cell encephalitogenicity

The interaction of ICOS with its ligand on APC provides a costimulatory signal to previously activated T-cells. In these studies, we blocked the ICOS:ICOS ligand interaction with ICOS-Ig during the in vitro activation of MBP-reactive transgenic CD4(+) T-cells. The presence of ICOS-Ig in these cultures inhibited the ability of the transgenic T-cells to transfer EAE, although they entered the brains of the recipient mice. ICOS-Ig increased apoptosis in the transgenic T-cells, especially in the memory population. This enhanced apoptosis was accompanied by an increase in the BAX/BCL-2 mRNA ratio. ICOS-Ig did not prevent IL2 production, demonstrating that IL-2 production is ICOS ligand independent. IFN-gamma and IL-10 production by the transgenic T-cells, however, was suppressed. Finally, ICOS-Ig injection into mice after the first signs of EAE ameliorated clinical disease. Therefore, ICOSL provides a signal distinct from CD28 costimulation that is required for the activation and viability of encephalitogenic T-cells.

Separable effector T cell populations specialized for B cell help or tissue inflammation

We identified specialized B helper and tissue inflammatory CD4(+) T cell subsets that developed concurrently from common naïve precursors during the primary immune response. These separable populations were distinguishable by their expression of adhesion and chemoattractant receptors that directed their homing to the appropriate effector sites in vivo and also showed intrinsic differences in their ability to support B cell antibody production and produce effector cytokines in vitro. Thus, our data show a previously unappreciated functional specialization among CD4(+) effector T cells, further defining their diversity and role in adaptive immunity.

Inducible costimulator regulates Th2-mediated inflammation, but not Th2 differentiation, in a model of allergic airway disease

A novel costimulatory molecule expressed on activated T cells, inducible costimulator (ICOS), and its ligand, B7-related protein-1 (B7RP-1), were recently identified. ICOS costimulation leads to the induction of Th2 cytokines without augmentation of IL-2 production, suggesting a role for ICOS in Th2 cell differentiation and expansion. In the present study, a soluble form of murine ICOS, ICOS-Ig, was used to block ICOS/B7RP-1 interactions in a Th2 model of allergic airway disease. In this model, mice are sensitized with inactivated Schistosoma mansoni eggs and are subsequently challenged with soluble S. mansoni egg Ag directly in the airways. Treatment of C57BL/6 mice with ICOS-Ig during sensitization and challenge attenuated airway inflammation, as demonstrated by a decrease in cellular infiltration into the lung tissue and airways, as well as by a decrease in local IL-5 production. These inhibitory effects were not due to a lack of T cell priming nor to a defect in Th2 differentiation. In addition, blockade of ICOS/B7RP-1 interactions during ex vivo restimulation of lung Th2 effector cells prevented cytokine production. Thus, blockade of ICOS signaling can significantly reduce airway inflammation without affecting Th2 differentiation in this model of allergic airway disease.

The costimulatory molecule ICOS plays an important role in the immunopathogenesis of EAE

The inducible costimulatory molecule (ICOS) is expressed on activated T cells and participates in a variety of important immunoregulatory functions. After the induction of experimental allergic encephalomyelitis in SJL mice with proteolipid protein (PLP), brain ICOS mRNA and protein were up-regulated on infiltrating CD3+ T cells before disease onset. ICOS blockade during the efferent immune response (9-20 days after immunization) abrogated disease, but blockade during antigen priming (1-10 days after immunization) exacerbated disease. Upon culture with PLP and compared with immunized controls, splenocytes produced either decreased interferon-gamma (IFN-gamma, in efferent blockade) or excessive IFN-gamma (in priming blockade). PLP-specific immunoglobulin G1 was decreased in animals treated with anti-ICOS during antigen priming, but not in other groups.

Importance of ICOS-B7RP-1 costimulation in acute and chronic allograft rejection

Primary T cell activation requires B7-CD28 and CD40-CD154 costimulation, but effector T cell functions are considered to be largely independent of these costimulatory pathways. Although blockade of costimulation with cytolytic T lymphocyte-associated antigen 4-immunoglobulin (CTLA-4-Ig) or monoclonal antibody (mAb) to CD154 prolongs allograft survival, chronic rejection follows, which suggests that additional key costimulatory pathways are active in vivo. We found that both antibody to inducible costimulator (anti-ICOS) and an ICOS-Ig fusion protein suppressed intragraft T cell activation and cytokine expression and prolonged allograft survival in a manner similar to that in ICOS-/- allograft recipients. The combination of anti-ICOS therapy and cyclosporin A led to permanent engraftment. In addition, ICOS-B7RP-1 costimulation was required for the development of chronic rejection after CD40-CD154 blockade. These data demonstrate a key role for the ICOS-B7RP-1 pathway in acute and chronic rejection and highlight the benefits of targeting this pathway in combination with the use of conventional immunosuppressive agent.

ICOS is critical for T helper cell-mediated lung mucosal inflammatory responses

We examined the requirement for and cooperation between CD28 and inducible costimulator (ICOS) in effective T helper (TH) cell responses in vivo. We found that both CD28 and ICOS were critical in determining the outcome of an immune response; cytolytic T lymphocyte-associated antigen 4-immunoglobulin (CTLA-4-Ig), ICOS-Ig and/or a neutralizing ICOS monoclonal antibody attenuated T cell expansion, TH2 cytokine production and eosinophilic inflammation. CD28-dependent signaling was essential during priming, whereas ICOS-B7RP-1 regulated TH effector responses, and the up-regulation of chemokine receptors that determine T cell migration. Our data suggests a scenario whereby both molecules regulate the outcome of the immune response but play separate key roles: CD28 primes T cells and ICOS regulates effector responses.

ICOS costimulation: It's not just for TH2 cells anymore

A current paradigm has ICOS participating in TH2 costimulation. New data indicates ICOS regulates not only TH2 cells, but also TH1s.

Resident and infiltrating central nervous system APCs regulate the emergence and resolution of experimental autoimmune encephalomyelitis

During experimental autoimmune encephalomyelitis (EAE), autoreactive Th1 T cells invade the CNS. Before performing their effector functions in the target organ, T cells must recognize Ag presented by CNS APCs. Here, we investigate the nature and activity of the cells that present Ag within the CNS during myelin oligodendrocyte glycoprotein-induced EAE, with the goal of understanding their role in regulating inflammation. Both infiltrating macrophages (Mac-1(+)CD45(high)) and resident microglia (Mac-1(+)CD45(int)) expressed MHC-II, B7-1, and B7-2. Macrophages and microglia presented exogenous and endogenous CNS Ags to T cell lines and CNS T cells, resulting in IFN-gamma production. In contrast, Mac-1(-) cells were inefficient APCs during EAE. Late in disease, after mice had partially recovered from clinical signs of disease, there was a reduction in Ag-presenting capability that correlated with decreased MHC-II and B7-1 expression. Interestingly, although CNS APCs induced T cell cytokine production, they did not induce proliferation of either T cell lines or CNS T cells. This was attributable to production by CNS cells (mainly by macrophages) of NO. T cell proliferation was restored with an NO inhibitor, or if the APCs were obtained from inducible NO synthase-deficient mice. Thus, CNS APCs, though essential for the initiation of disease, also play a down-regulatory role. The mechanisms by which CNS APCs limit the expansion of autoreactive T cells in the target organ include their production of NO, which inhibits T cell proliferation, and their decline in Ag presentation late in disease.

B7-DC, a new dendritic cell molecule with potent costimulatory properties for T cells

Dendritic cells (DCs), unique antigen-presenting cells (APCs) with potent T cell stimulatory capacity, direct the activation and differentiation of T cells by providing costimulatory signals. As such, they are critical regulators of both natural and vaccine-induced immune responses. A new B7 family member, B7-DC, whose expression is highly restricted to DCs, was identified among a library of genes differentially expressed between DCs and activated macrophages. B7-DC fails to bind the B7.1/2 receptors CD28 and cytotoxic T lymphocyte-associated antigen (CTLA)-4, but does bind PD-1, a receptor for B7-H1/PD-L1. B7-DC costimulates T cell proliferation more efficiently than B7.1 and induces a distinct pattern of lymphokine secretion. In particular, B7-DC strongly costimulates interferon gamma but not interleukin (IL)-4 or IL-10 production from isolated naive T cells. These properties of B7-DC may account for some of the unique activity of DCs, such as their ability to initiate potent T helper cell type 1 responses.

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.