Co-stimulation in T cell responses

CTLA-4 blockade augments human T lymphocyte-mediated suppression of lung tumor xenografts in SCID mice

Previous studies by others using transplantable murine tumor models have demonstrated that the administration of antibodies that block CTLA-4 interaction with B7 can provoke the elimination of established tumors, and that the tumor suppression is mediated by T-cells and/or cells expressing NK1.1. Studies from our lab have established in a human/severe combined immunodeficient (SCID) mouse chimeric model that autologous peripheral blood leukocytes (PBL) can suppress the growth of tumor xenografts in a PBL dose-dependent fashion, and that this suppression is dependent upon the patient's T and NK cells. Using this human/mouse chimeric model, we sought to determine whether an antibody blockade of CTLA-4 would enhance the anti-tumor response of a patient's PBL. It was first important to determine whether the tumor suppression observed in the SCID model was dependent upon CD28/B7 co-stimulation. Blockade of B7 with a human CTLA-4-Ig fusion protein completely abrogated the lymphocyte-mediated tumor suppression, confirming in this model that tumor suppression is dependent upon a CD28/B7 co-stimulation. Using two different CTLA-4 specific monoclonal antibodies, we observed that CTLA-4 blockade significantly enhanced the human lymphocyte-mediated tumor suppression in mice co-engrafted with PBL and tumor cells. This enhancement was observed in both an allogeneic setting (in which the PBL were allogeneic with respect to the tumor) and an autologous setting (in which the PBL and tumor were from the same patient). These results sustain the notion that human anti-tumor immune response can be augmented (in vivo) by blocking the interaction between CTLA-4 and B7.

A novel immunostimulator, N-[alpha-O-benzyl-N-(acetylmuramyl)-L-alanyl-D-isoglutaminyl]-N6-trans-(m-nitrocinnamoyl)-L-lysine, and its adjuvancy on the hepatitis B surface antigen

N(2)-[alpha-O-benzyl-N-(acetylmuramyl)-L-alanyl-D-isoglutaminyl]-N(6)-trans-(m-nitrocinnamoyl)-L-lysine (muramyl dipeptide C, or MDP-C) has been synthesized as a novel, nonspecific immunomodulator. The present study shows that MDP-C induces strong cytolytic activity by macrophages on P388 leukemia cells and cytotoxic activity by cytotoxic T lymphocytes (CTLs) on P815 mastocytoma cells. Our results also indicate that MDP-C is an effective stimulator for production of interleukin-2 and interleukin-12 by murine bone marrow derived dendritic cells (BMDCs) and production of interferon-gamma by CTLs. Additionally, MDP-C increases the expression levels of several surface molecules, including CD11c, MHC class I, and intercellular adhesion molecule-1 in BMDCs. Moreover, MDP-C remarkably enhances the immune system's responsiveness to hepatitis B surface antigen (HBsAg) in hepatitis B virus transgenic mice for both antibody production and specific HBsAg T-cell responses ex vivo. Our results indicate that MDP-C is an apyrogenic, nonallergenic, and low-toxicity immunostimulator with great potential for diagnostic, immunotherapeutic, and prophylactic applications in diseases such as hepatitis B and cancers.

Increased expression of plasma and cell surface co-stimulatory molecules CTLA-4, CD28 and CD86 in adult patients with allergic asthma

The co-stimulatory interactions of the B7 family molecules CD80 and CD86 on antigen-presenting cells, together with their T cell counter receptors CD28 and cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), modulate T lymphocyte-mediated immune responses in a reciprocal manner. To investigate whether there is altered expression and the clinical significance of soluble co-stimulatory molecules in asthmatic patients, plasma concentrations of sCTLA-4, sCD28, sCD80 and sCD86 in 51 adult allergic asthmatic adults with or without steroid treatment, and 35 sex- and age-matched control subjects were measured by enzyme-linked immunosorbent assay (ELISA). Cell surface expression of CTLA-4 and CD28 on peripheral blood mononuclear cells (PBMC) were analysed by flow cytometry. Results showed that the plasma sCTLA-4 concentration was significantly higher in all asthmatic patients while sCD28 and sCD86 concentrations were significantly higher in steroid and non-steroid treated asthmatic patients, respectively, compared with control subjects (all P < 0.01). Significantly increased cell surface expression of CD28 but not CTLA-4 on PBMC was found in asthmatic patients compared with controls (P < 0.05). The plasma concentration and cell surface expression of CTLA-4 were found to exhibit positive and significant correlations with those of CD28 (both P < 0.05). Serum total IgE concentration correlated positively and significantly with sCTLA-4 and sCD28 concentrations in allergic asthmatic patients (both P < 0.05). The increased expression of these soluble co-stimulatory molecules may reflect the dysregulation of T cell activation, thereby contributing to the immunopathogenesis of allergic asthma.

Constitutive and inducible expression of b7 family of ligands by human airway epithelial cells

Activated T cells have been implicated in chronic rhinosinusitis (CRS) and asthma and physically interact with epithelial cells in the airways. We now report that human airway epithelial cells display significant constitutive cell-surface expression of costimulatory ligands, B7-H1, B7-H2, B7-H3, and B7-DC. Expression of B7-H1 and B7-DC was selectively induced by stimulation of either BEAS2B or primary nasal epithelial cells (PNEC) with interferon (IFN)-gamma (100 ng/ml). The combination of IFN-gamma and tumor necrosis factor-alpha (100 ng/ml) selectively induced expression better than IFN-gamma alone. Fluticasone treatment (10(-7) M) reduced the baseline expression and inhibited the induction of B7-H1 and B7-DC in BEAS2B cells. In vitro exposure of PNEC to IFN-gamma also resulted in selective induction of B7-H1 and B7-DC. Monoclonal antibody blockade of B7-H1 or B7-DC enhanced IFN-gamma expression by purified T cells in co-culture experiments, suggesting that these two B7 homologs inhibit T cell responses at the mucosal surface. Immunohistochemical staining of human sinonasal surgical tissue confirmed the presence of B7-H1, B7-H2, and B7-H3 in the epithelial cell layer, especially in samples from patients diagnosed with Samter's Triad, a severe form of CRS. Real-time PCR analysis of sinonasal tissue revealed elevated levels of B7-H1 and B7-DC in CRS compared with controls. These results demonstrate that epithelial cells express functional B7 costimulatory molecules and that expression of selected B7 family members is inducible in vitro and in vivo. Epithelial B7 homologs could play a role in regulation of lymphocytic activity at mucosal surfaces.

Fine-tuning of helper T cell activation and apoptosis by antigen-presenting cells

The role of antigen-presenting cells (APC) in regulating helper T cell responses and activation-induced cell death (AICD) was investigated in vitro. T cell activation was monitored by measuring the early rise of intracellular free calcium [Ca+]ic, mRNA and cell surface expression of activation and apoptotic molecules, the production of cytokines and the activation of transcription factors. Our results demonstrate that the unique characteristics of a given APC can modify the threshold, kinetics and magnitude of the T cell response. The rapid and sustained rise of intracellular free calcium correlated well with the extent of cytokine production and the expression of activation molecules. Fas-dependent AICD could be induced by the most potent antigen-presenting cell (2PK3) only. Our results demonstrate that the response and fate of effector/memory CD4+ helper T lymphocytes is highly dependent on the individual properties of the APC they encounter.

Aberrant production of soluble costimulatory molecules CTLA-4, CD28, CD80 and CD86 in patients with systemic lupus erythematosus

OBJECTIVE

The costimulatory interactions of the B7 family molecules CD80 and CD86 on antigen-presenting cells with their T-cell counter-receptors CD28 and CTLA-4 modulate T lymphocyte-mediated immune responses in a reciprocal manner. We investigated the possible aberrant production of soluble (s) forms of the T-cell costimulatory molecules CD80, CD86, CD28 and CTLA-4 in plasma of patients with systemic lupus erythematosus (SLE), an autoimmune disease arising from T-lymphocyte dysregulation.

METHODS

Plasma concentration and ex vivo production of soluble costimulatory molecules of 79 SLE patients with or without active disease and 40 sex- and age-matched healthy subjects were measured by enzyme-linked immunosorbent assay.

RESULTS

Plasma sCTLA-4, sCD28, sCD80 and sCD86 concentrations of all SLE patients were significantly higher than concentrations in control subjects (all P<0.01). These increases were observed even in patients with inactive disease [SLE Disease Activity Index (SLEDAI) <3]. Plasma sCTLA-4 concentration in all SLE patients correlated significantly with SLEDAI score (r = 0.228, P = 0.043). Upon mitogen treatment of peripheral blood mononuclear cells, the percentage increases in ex vivo production of sCD28 and sCD80 and the percentage decrease in sCTLA-4 release were all significantly smaller in SLE patients with active disease than in healthy subjects (P<0.01, P<0.05 and P<0.0001, respectively).

CONCLUSION

The aberrant production of soluble T-cell costimulatory molecules is important in the immunopathogenesis of SLE, which occurs by the dysregulation of T-lymphocyte costimulation. Plasma sCTLA concentration could potentially serve as a surrogate marker of SLE disease activity.

Colorectal cancer vaccines: what we know and what we don't yet know

Humans have a sophisticated immune system that functions to clear invading organisms and abnormal cells. However, cancers are able to arise despite this immune system. Vaccines have the potential of benefiting cancer patients by stimulating an immune response against tumor-associated antigens (TAA). Our enhanced understanding of how the immune system processes and presents antigens has allowed an array of vaccine modalities to be developed and tested. The TAA with the greatest number of vaccine platforms tested in colorectal cancer is carcinoembyronic antigen (CEA). Trials to date have demonstrated safety and evidence for the induction of an immune response against CEA. This article will review trials conducted with a variety of CEA vaccines. Most studies conducted are phase I or II in the metastatic disease setting, limiting our understanding of the role of the immune response in controlling colon cancers. Phase III trials conducted to date have conflicting data with respect to improvements in disease-free and overall survival. It is our challenge to determine if and which vaccines have sufficient benefit to warrant large-scale trials in the adjuvant and prevention settings.

Immunological ignorance of solid tumors

Many peripheral solid tumors such as sarcomas and carcinomas express tumor-specific antigens that can serve as targets for immune effector T cells. Nevertheless, the immune surveillance against clinically manifest carcinomas and sarcomas seems relatively inefficient. Naïve cytotoxic T cells are activated exclusively in secondary lymphoid organs including the spleen and lymph nodes. Tumor antigen might be either cross-presented to naïve cytotoxic T cells by professional antigen-presenting cells (pAPC), or presented directly by tumor cells that migrated to secondary lymphoid organs. Direct priming is quite inefficient during early tumor development because metastasis to lymphoid organs is usually limited to advanced stage diseases. Similarly, the process of cross-priming by pAPC seems to depend on relatively large antigen amounts and on maturation stimuli for dendritic cells, and both requirements may be limiting during initial tumorigenesis. Therefore, the immunosurveillance of solid tumors may fail because they are ignored for too long by the immune system. However, these situations may prove promising for the induction of tumor-specific T cell immunity by vaccination, as the T cell repertoire against these antigens has a naïve phenotype and is not yet affected by tolerance mechanisms.

Inhibition of allogeneic T-cell responses by dendritic cells expressing transduced indoleamine 2,3-dioxygenase

BACKGROUND

Indoleamine 2,3-dioxygenase (IDO) is an enzyme involved in the catabolism of tryptophan and has been shown to prevent rejection of the fetus during pregnancy by inhibiting alloreactive T cells.

METHODS

In this study we investigated dendritic cells (DCs) that are transfected with IDO cDNA in the inhibition of T-cell proliferation after antigen-specific interaction. XS106 DCs, derived from A/J mice (H-2k), were transduced with IDO with a gene-delivery system using a recombinant adenoviral vector.

RESULTS

Western blotting and immune staining revealed IDO expression in XS106 DCs transduced with IDO (XS106-IDO DCs), and its catabolic effect was confirmed by an increase in kynurenine concentration. Fluorescence-activated cell sorting revealed that XS106-IDO DCs were not changeable for Ia, CD80, and CD86 expression. After XS106-IDO DCs were co-cultured with C57BL/6 allogeneic splenic T cells, the proliferation of the T cell was significantly inhibited. The co-cultured T cells with XS106-IDO DCs exhibited cell-cycle arrest. Furthermore, injection of XS160-IDO DCs into the footpads of C57BL/6 (H-2b) mice demonstrated a reduced T-cell response against allo-antigen.

CONCLUSIONS

These results suggest that overexpression of IDO in the DCs effectively inhibited T-cell proliferation, and may expand a new immunomodulatory strategy for the prevention of allo-rejection of organ transplantation.

CD80 and CD86, but not CD154, augment DNA vaccine-induced protection in experimental bovine tuberculosis

DNA vaccination is known to elicit robust cellular and humoral responses to encoded antigen. The co-administration of costimulatory molecules CD80 (B7-1), CD86 (B7-2) and CD154 (CD40L) has been shown to enhance immune responses in several murine models. The role of specific costimulatory molecules in non-rodent species remains incompletely characterized. In these studies, we demonstrate that the co-administration of CD80 and CD86, but not CD154, to an existing candidate subunit DNA vaccine (ESAT-6) against bovine tuberculosis, enhances protection after aerosol challenge with virulent Mycobacterium bovis. Additionally, we have shown that vaccination with M. bovis BCG is protective against tuberculosis following aerosol challenge in cattle. Two independent trials were conducted in cattle to determine the adjuvant effect of encoded antigen + CD80/CD86 and directly compare the adjuvant activities of CD80/CD86 to those of CD154. Co-administration of either CD80/CD86 or CD154 enhanced ESAT-6-specific IFN-gamma responses as compared to animals vaccinated with ESAT-6 DNA alone. However, following aerosol challenge, only animals vaccinated with CD80/CD86 possessed decreased pathology of the lungs and associated lymph nodes, as measured by gross examination, radiographic lesion morphometry and bacterial recovery. Collectively, these results demonstrate that the co-administration of costimulatory molecules with a protective antigen target enhances bovine immune responses to DNA vaccination, and that CD80/CD86 is superior to CD154 in augmenting DNA vaccine-induced protection in experimental bovine tuberculosis.

Soluble CD86 protein in serum samples of patients with asthma

BACKGROUND

Previous studies have reported that soluble (s) CD86 is involved in the initiation of the immune response. A study was undertaken to investigate the concentrations of sCD86 in serum samples from patients with bronchial asthma and to determine the cell origin of sCD86.

METHODS

Serum sCD86 concentrations were measured in 52 asthmatic subjects and 25 non-atopic normal volunteers using an enzyme linked immunosorbent assay, and the relationship of serum sCD86 concentrations to asthma severity and to total and differential white cell counts was analysed. Each type of white blood cell was purified and cultured in vitro to determine the cell origin of serum sCD86.

RESULTS

Serum samples from patients with an acute asthma exacerbation had much higher levels of sCD86 (585.4 (20.5) IU/ml) than those from stable asthmatics (479.6 (15.7) IU/ml, p<0.001) and healthy individuals (435.1 (13.8) IU/ml, p<0.001), and there was no difference between the latter two groups (p = 0.079). In asthmatic subjects the serum sCD86 level was inversely correlated with airway responsiveness, forced expiratory volume in 1 second, and with arterial carbon dioxide tension. In addition, the serum sCD86 level was positively correlated with numbers of lymphocytes, eosinophils, monocytes, but not neutrophils. The in vitro experiments indicated that sCD86 was produced by monocytes.

CONCLUSIONS

The serum sCD86 protein level was significantly increased in asthmatic subjects during an exacerbation and correlated with the severity of asthma. sCD86 is most probably derived from monocytes in the peripheral blood.

CTLA-4 recombinant protein genetically fused to canine Fcepsilon receptor Ialpha enhances allergen specific lymphocyte responses in experimentally sensitized dogs

Vaccination with a recombinant antigen fused to a targeting molecule is a potential strategy for inducing efficient immune responses. For the therapeutic purpose of allergic diseases in dogs, a DNA construct which expresses recombinant fusion protein with two functional domains, cytotoxic T lymphocyte antigen (CTLA-4) and Fcepsilon receptor Ialpha, was developed to bridge antigen-presenting cells and IgE-allergen complex. The recombinant fusion protein expressed by the DNA construct was demonstrated to retain the ability to bind monocytes in PBMC and dog IgE, respectively. Additionally, the recombinant protein induced enhancement of allergen-induced lymphoproliferation in experimentally sensitized dogs under conditions of suboptimal allergen stimulation. These results indicated that the DNA construct could enhance allergen-induced immune responses in vivo, implying its usefulness for perspective application in immunotherapy in dogs.

Role of 4-1BB:4-1BB ligand in cancer immunotherapy

The activation of T cells plays a central role in antitumor immunity. In order to activate naïve T cells, two key signals are required. Signal one is provided through the T-cell receptor (TCR) while signal two is that of costimulation. The CD28:B7 molecules are one of the best-studied costimulatory pathways, thought to be the main mechanism through which primary T-cell stimulation occurs. However, a number of molecules have been identified which serve to amplify and diversify the T-cell response, following initial T-cell activation. These include the more recently described 4-1BB:4-1BB ligand (4-1BBL) molecules. 4-1BB:4-1BBL are a member of the TNFR:TNF ligand family, which are expressed on T cells and antigen-presenting cells (APCs), respectively. Therapies utilizing the 4-1BB:4-1BBL signaling pathway have been shown to have antitumor effects in a number of model systems. In this paper, we focus on the 4-1BB:4-1BBL costimulatory molecules. In particular, we will describe the structure and function of the 4-1BB molecule, its receptor and how 4-1BB:4-1BBL costimulation has and may be used for the immunotherapy of cancer.

Amplification of the lytic potential of effector/memory CD8+ cells by vector-based enhancement of ICAM-1 (CD54) in target cells: implications for intratumoral vaccine therapy

We demonstrated that enhanced expression of the costimulatory molecules CD80, CD54 and CD48 (designated rF-TRICOM) on target cells, as delivered via a recombinant fowlpox vector, results in an increased state of stimulation of CD8+ T cells, and consequent increased lysis of target cells. CTL studies in conjunction with antibody-blocking studies demonstrated that the enhanced effector activity of these CD8+ T cells is mediated mainly through CD54. Intracellular staining of CD8+ cells that interact with target cells infected with rF-TRICOM showed that they contain higher amounts of perforin and have a higher level of perforin message. Enhanced expression of costimulatory molecules (specifically CD54) on target cells using rF-TRICOM vectors also leads to the formation of stable conjugates/synapses between targets and T cells. The interaction of T cells with target cells that overexpress costimulatory molecules upon infection with rF-TRICOM leads to enhanced signaling through Lck, ZAP70, and STAT-1 in CD8+ T cells and heightened lytic activity of CD8+ cells through the formation of a greater number of immunological synapses. This, in turn, leads to enhanced signaling in T cells. Finally, studies were conducted in mice in which CEA is a self-antigen in an attempt to understand the potential clinical relevancy of intratumoral vaccine therapy. Mice were transplanted subcutaneously with CEA expressing tumors. Intratumoral (i.t.) vaccination was administered 8 days post tumor transplant. Mice vaccinated i.t. with rF-TRICOM demonstrated significantly reduced tumor growth and 40% of the mice had complete tumor regression. The antitumor effects were further improved by the addition of tumor antigen (CEA) in the vaccination by utilizing rF-CEA/TRICOM, with 80% of the mice experiencing complete tumor regression. These studies thus support the concept of intratumoral vaccination employing vectors expressing costimulatory molecules.

The ALX Src homology 2 domain is both necessary and sufficient to inhibit T cell receptor/CD28-mediated up-regulation of RE/AP

Activation of naive T cells occurs when two signals are received. The first signal is received through the T cell antigen receptor (TCR), and a second costimulatory signal is primarily provided by CD28. We have recently identified a novel adaptor molecule, ALX, which is expressed exclusively in hematopoietic cells. ALX contains several sites for potential protein-protein interaction, including an Src homology 2 (SH2) domain, four PXXP polyproline sequences, and two likely sites of tyrosine phosphorylation. Overexpression of ALX inhibits the transcriptional activation of the interleukin 2 promoter during T cell activation, specifically affecting CD28-mediated activation of the RE/AP element of the interleukin 2 promoter. To understand how ALX functions downstream of CD28, we generated a panel of site-directed mutants as well as truncations in which potential protein-binding sites were mutated or absent. We found that the ALX SH2 domain is both necessary and sufficient to mediate inhibition of RE/AP activation. Mutation of the SH2 domain did not affect ALX expression, relative localization in the cytoplasm and nucleus, phosphorylation, or a mobility shift in response to TCR signaling alone. However, an activation-induced mobility shift triggered by CD28 was reduced in the ALX SH2 domain mutant. In addition, the isolated ALX SH2 domain was found to associate with a phosphoprotein from Jurkat T cells on TCR/CD28 stimulation. Therefore, the ALX SH2 domain plays a critical role in ALX function downstream of CD28.

Cytotoxic T Lymphocyte Antigen-4 Gene in Breast Cancer

The exon 1 polymorphism (49A/G) of ctla-4 gene corresponds to an amino acid exchange (threonine to alanine) in the leader peptide of the expressed protein. There are reports concerning the higher level of G allele in subjects with various autoimmune diseases, which has resulted in the hypothesis that CTLA-4 may play a role in regulating self-tolerance by the immune system and in the pathogenesis of autoimmune disorders. This study was undertaken to investigate the correlation of exon 1 (49A/G) polymorphism in the ctla-4 gene and breast cancer. The ctla-4 49A/G polymorphism was studied in 197 women with primary breast cancer and 151 age/sex matched normal individuals. The results indicated a significant difference between frequency of ctla-4 genotypes in patients and controls. The frequency of GG genotype was significantly decreased in breast cancer patients compared to controls (4.6% v.s. 12.6%, P = 0.012). There was also a significant positive correlation between tumor size and the existence of AA genotype in patients (P = 0.016). In addition, a positive correlation between AA genotype and lymph node involvement was observed (P = 0.042). The observed decrease in the frequency of GG genotype in the breast cancer patients is contrary to the frequently reported increase of GG genotype in autoimmune diseases. In addition, the data implies that polymorphism of ctla-4 exon 1 contributes in tumor progression.

Eosinophils function as antigen-presenting cells

Eosinophils release lipid mediators, including leukotriene C4, platelet-activating factor, and liposins, and contain four distinct granule cationic proteins, major basic protein, eosinophil peroxidase, eosinophil cationic protein, and eosinophil-derived neurotoxin, which may cause dysfunction and destruction of other cells. Eosinophils are primarily thought of as terminal effectors of allergic responses and of parasite elimination. Eosinophils are characteristically present within the airway lumina of asthmatics, and these airway eosinophils have been induced in vivo to express major histocompatibility complex II (MHC-II) complexes and costimulatory molecules, which are required for T lymphocytes to be functionally activated. In in vitro experiments, eosinophils can process antigen and express the costimulatory molecules, and after cytokine-elicited induction of MHC-II, expression can function as antigen-presenting cells in stimulating T lymphocyte responses. Airway luminal eosinophils can migrate into draining paratracheal lymph nodes, localized to T cell-rich paracortical areas, and stimulate antigen-specific T cell proliferation in vivo within paratracheal lymph nodes, which was CD80- and CD86-dependent and limited to CD4+ T cells. Furthermore, eosinophils within the lumina of airways promote expansion of T helper cell type 2 (Th2) by presenting antigen, suggesting that eosinophils actively modulate immune responses by amplifying Th2 cell responses.

Detection of circulating soluble CD28 in patients with systemic lupus erythematosus, primary Sjögren's syndrome and systemic sclerosis

The aim of this study was to evaluate the presence and the role of the serum soluble costimulatory molecule CD28 in patients with systemic lupus erythematosus (SLE), primary Sjögren's syndrome (SS), and systemic sclerosis (SSc). Soluble CD28 concentration was determined by ELISA in 45 patients with SLE, 45 patients with primary SS, 30 patients with SSc, and 45 healthy subjects. We also evaluated CD28 mRNA expression by semiquantitative RT-PCR, and the biological activity of recombinant soluble CD28 on T lymphocyte activity. Concentrations of soluble CD28 were significantly higher in patients with SLE, primary SS and SSc than in healthy subjects. Soluble CD28 concentrations were higher in patients with systemic primary SS than in patients with glandular-limited primary SS. PCR analysis suggested that soluble CD28 resulted from the shedding of the membrane form. In vitro assay revealed that soluble CD28 inhibits the anti-CD3 mAb induced T cell proliferation. Soluble CD28, which modulates the proliferation of T lymphocytes, could be associated with disease severity in patients with autoimmune disease, especially primary SS. These results suggest that soluble CD28 could play an important role in the regulation of autoimmune diseases.

Revival of the regulatory T cell: new targets for drug development

Compelling new evidence supports the idea that regulatory T cells play a major role in our immune system. Several subsets of these regulators have been identified recently. Differences in the phenotypical and functional characteristics of these subsets have immunological implications. From our growing knowledge of the field of immunology, we could potentially generate a new class of therapeutic agents that target immune-related diseases.

Genetic analysis of CD28 signaling

T cell activation is central to initiating an immune response. Two signals are required: an antigen-specific signal through the T cell receptor (TCR) and an antigen-independent costimulatory signal, primarily through CD28 in naïve T cells. Although many of the molecules involved in TCR signal transduction have been identified, the signaling pathways downstream of CD28 involved in costimulation are not well-defined. Through mutagenesis, we have generated a panel of Jurkat T cell lines in which CD28 costimulation fails to upregulate the RE/AP composite element of the IL-2 promoter. Biochemical analysis and genetic rescue of the defects in these cell lines will lead to a better understanding of CD28 signal transduction.

Selective cytoplasmic translocation of HuR and site-specific binding to the interleukin-2 mRNA are not sufficient for CD28-mediated stabilization of the mRNA

The interleukin-2 mRNA is a labile transcript containing AU-rich elements that is transiently stabilized by CD28 receptor signaling. For a number of proto-oncogenes and cytokines, the HuR protein has been shown to avidly bind the AU-rich elements that confer instability upon their mRNAs. HuR was originally thought to participate in mRNA degradation but subsequent studies indicated that it actually functions to stabilize mRNA. Binding of HuR to the mouse interleukin-2 mRNA has not been studied. We tested if HuR binds the interleukin-2 mRNA and whether such binding is related to CD28-mediated stabilization of the mRNA. First, we confirm that T cell receptor signaling, which is sufficient to induce interleukin-2 transcription, also triggers translocation of HuR from the nucleus to the cytoplasm. Interestingly, T cell receptor-triggered translocation is selective as heterogeneous nuclear ribonucleoprotein A1 does not shuttle under the same conditions. Engagement of both the T cell and CD28 receptors, which enhance interleukin-2 transcription and induce stabilization of the mRNA, did not further increase the level of cytoplasmic HuR. Using an in vitro binding assay, we demonstrate that HuR binds the interleukin-2 mRNA and localize binding to a sequence downstream of the single nonameric AU-rich element that is present in its 3'-untranslated region. However, we conclude that HuR binding to the interleukin-2 mRNA, both in vitro and in vivo, is not associated with alterations in mRNA stability.

Immunology of B7-H1 and its roles in human diseases

B7-H1 was originally identified by homology analysis in comparison with B7-1 and B7-2, two molecules with important immunoregulatory functions. B7-H1, however, was broadly induced in the majority of peripheral tissues as well as hematopoietic cells. Upon binding to an as yet unidentified costimulatory receptor on primed T-cells, B7-H1 costimulates T-cell proliferation and preferentially induces interleukin 10 and interferon gamma. The costimulatory function of B7-H1 may be critical for enhancing maturation and differentiation of T-cells in lymphoid organs. Conversely, by binding to programmed death 1 receptors on activated T-cells and B-cells, B7-H1 may inhibit ongoing T-cell responses in peripheral tissues by inducing apoptosis and arresting cell-cycle progression. Although a positive regulatory role of B7-H1 has been demonstrated in vitro and in various animal models, a negative regulatory role of B7-H1 has also been documented in human diseases, including cancer, rheumatoid arthritis, and human immunodeficiency virus infection. Delineation of the complex interactions between B7-H1 and its receptors as well as its interplay with other ligands is critical for understanding this new immunoregulatory system. Precise manipulation of B7-H1 and its receptors may provide unique opportunities for designing new disease treatments.

Expression of costimulatory molecules B7/CD28 in systemic lupus erythematosus

The expression of the costimulatory molecules B7/CD28 in peripheral blood mononuclear cells (PBMC) of the patients with systemic lupus erythematosus (SLE) and its relation to the pathogenesis of SLE were studied. The expression of the costimulatory molecules in PBMC in 30 patients with active SLE and 20 cases of healthy controls was detected by using the techniques of immunofluorescence and flow cytometer. The result showed that the expression percentage of CD28+, CD4+ CD28+ in T cells of PBMC from the patients with SLE decreased significantly as compared with that in healthy control group, while the expression percentage of CD80+, CD19+ CD80+ in B cells was significantly increased than that in healthy control group (P<0.01). It suggested that the abnormal expression of costimulatory molecules B7/CD28 played a role in the pathogenesis of SLE.

CTL Are Inactivated by Herpes Simplex Virus-Infected Cells Expressing a Viral Protein Kinase

Numerous cell-to-cell signals tightly regulate CTL function. Human fibroblasts infected with HSV type 1 or 2 can generate such a signal and inactivate human CTL. Inactivated CTL lose their ability to release cytotoxic granules and synthesize cytokines when triggered through the TCR. Inactivation requires cell-to-cell contact between CTL and HSV-infected cells. However, inactivated CTL are not infected with HSV. The inactivation of CTL is sustainable, as CTL function remains impaired when the CTL are removed from the HSV-infected cells. IL-2 treatment does not alter inactivation, and the inactivated phenotype is not transferable between CTL, distinguishing this phenotype from traditional anergy and T regulatory cell models. CTL inactivated by HSV-infected cells are not apoptotic, and the inactivated state can be overcome by phorbol ester stimulation, suggesting that inactivated CTL are viable and that the signaling block is specific to the TCR. HSV-infected cells require the expression of U(S)3, a viral protein kinase, to transmit the inactivating signal. Elucidation of the molecular nature of this signaling pathway may allow targeted manipulation of CTL function.

Cloning and characterization of ALX, an adaptor downstream of CD28

T cell activation requires two signals: specific recognition of antigen through the T cell receptor (TCR) and a costimulatory signal provided primarily by CD28 in naïve T cells. We cloned a novel gene with considerable homology to RIBP/TSAd/Lad, an adaptor involved in T cell activation and interleukin-2 (IL-2) promoter activation. Expression of this gene is limited to the spleen and thymus. We have named this gene ALX, adaptor in lymphocytes of unknown function X. Because the related adaptor RIBP is involved in IL-2 regulation, we investigated whether ALX had a similar function. ALX overexpression in Jurkat T cells results in inhibition of IL-2 promoter activation after stimulation with superantigen. The IL-2 promoter contains several binding sites for transcription factors including the composite element RE/AP, which is the primary site of CD28 transcriptional activation. ALX overexpression had the greatest effect on the activation of a RE/AP reporter as opposed to an AP-1 reporter. Interestingly, ALX overexpression strongly inhibited RE/AP activation in response to anti-CD28/phorbol 12-myristate 13-acetate (PMA) stimulation but had minimal effect when anti-TCR/PMA was used. Therefore, it appears that ALX may function downstream of CD28 costimulation during T cell activation. In addition, the mobility of ALX shifts upon TCR/CD28 costimulation to a greater extent than what is observed with either stimulus alone demonstrating that ALX is a target of both TCR and CD28 costimulatory signaling pathways.

Effects of CD80 and CD86 on cytokine production in patients with wasp-venom allergy who receive venom immunotherapy

Several studies have provided evidence that activation of antigen-specific T cells requires interactions between CD28 on T cells and its ligands, CD80 and CD86, on antigen-presenting cells (APCs). However, the effects of CD80 and CD86 on cytokine production in patients with Hymenoptera venom allergy who receive venom immunotherapy remain unclear. We examined the effects of CD80 and CD86 on Th1- and Th2-cytokine production before and after venom immunotherapy in patients with wasp-venom allergy. Peripheral blood mononuclear cells (PBMCs) were isolated from patients with wasp-venom allergy before and after three months of venom immunotherapy. CD4+ T cells and monocytes were isolated as APCs from PBMCs and were cocultured with wasp venom in the presence of anti-CD80 or -CD86 blocking antibodies. Interleukin (IL)-4, IL-10, and interferon (IFN)-gamma were measured by enzyme-linked immunosorbent assay. The expression of CD80 and CD86 on CD14+ PBMCs was detected by fluorescence-activated cell-sorter analysis. The expression of CD86, but not that of CD80, on CD14+ PBMCs cocultured with venom increased after three months of venom immunotherapy, but not before venom immunotherapy. Blockade of CD86 reduced IL-10 production after three months of venom immunotherapy. IL-10 production promoted by CD86 costimulation may be involved in the mechanism of venom immunotherapy in patients with venom allergy.

The tolerogenic molecule CD95-L is expressed on the plasma membrane of human activated, but not resting, Langerhans' cells

Although dendritic cells (DC) are well known for their immunogenic capacities, they may even induce peripheral T-cell tolerance, and such a tolerogenic potential can be exerted in mouse through the expression on the DC plasma membrane of the CD95-ligand (CD95-L) molecule, which is able to trigger apoptosis of CD95-expressing antigen-specific T cells. We therefore asked whether epidermal DC, namely Langerhans' cells (LC), either resting (i.e. within the epidermis, 'in situ') or activated (i.e. suspended from the epidermis) or both, could express the CD95-L molecule on the plasma membrane. For such a purpose, two colloidal gold-immunoelectron microscopy (IEM) double-step procedures were carried out: an 'in situ' method, able to investigate resting LC, was performed on ultrathin frozen sections obtained by ultracryomicrotomy (UCMT) of normal skin biopsies; a pre-embedding (P-E) method, able to investigate suspended LC, was performed on epidermal cells (EC) suspended from normal skin specimens. In UCMT/IEM sections, resting LC showed gold particles within the cytoplasm but very rarely within organelles and never along the plasma membrane: resting LC are therefore capable of synthesizing CD95-L but not of expressing it in a functional location, thus autoreactive phenomena against CD95-expressing EC being avoided in normal epidermis. On the other hand, in P-E/IEM preparations, suspended LC showed several gold particles along the plasma membrane: activated LC are therefore capable of expressing CD95-L in a functional location, thus bearing the potential to exert tolerogenic capabilities against CD95-expressing T cells, e.g. to prevent inflammatory/autoimmune cutaneous disorders and/or favor the resolution thereof.

In vitro co-stimulation with anti-CD28 synergizes with IL-12 in the generation of T cell immune responses to leukaemic cells; a strategy for ex-vivo generation of CTL for immunotherapy

The existence of an immune based graft-versus-leukaemia (GvL) effect highlighted the prospect of managing relapsed leukaemias with T cell-based adoptive immunotherapy. Thus, various strategies have been explored for the in vitro expansion of acute myeloid leukaemia (AML)-specific T cells. In a popular approach, AML blasts have been genetically modified to express co-stimulatory molecules essential for effective T cell priming. One such tactic has been the modification of AML cells to express the B7/CD80 co-stimulatory molecule that binds to CD28 on T cells initiating events that culminate in enhanced cytokine production, proliferation and development of effector functions by T cells. The success of these strategies has been limited by difficulties in attaining sufficient transduction efficiencies and associated high levels of CD80 expression. We demonstrate that these problems can be circumvented by using anti-CD28 monoclonal antibody. Furthermore, we show that the synergistic relationship between CD80/CD28 pathway and interleukin 12 cytokine (IL-12), documented in the generation of cytotoxic T lymphocytes (CTL) for solid tumours, also applies to AML. CD28/IL-12 synergy facilitated the proliferation of allogeneic T cells in response to stimulation with primary AML blasts. The synergy also favoured generation of a Th1-type immune response, evidenced by gamma interferon (IFN-gamma) secretion and facilitated naive and memory T cell proliferation. Unlike some methods of in vitro T cell expansion, use of CD28/IL-12 synergy left T cells in the physiologically appropriate CD45RA-/CCR7- subsets known to be associated with immediate cytotoxic functions.

Impaired germinal center formation and recall T-cell-dependent immune responses in mice lacking the costimulatory ligand B7-H2

B7-H2, which is expressed constitutively on B cells and binds the inducible costimulator (ICOS) on antigen-activated T cells, is a member of the B7 family of costimulatory ligands. We have inactivated B7-H2 in the mouse. B7-H2-/- mice generate normal populations of B and T cells in their various lymphoid organs but have lower basal levels of heavy chain class-switched antibodies in their sera. These mice are able to mount normal immune responses to both type I and type II T-cell-independent antigens. However, their pattern of responses to a T-cell-dependent antigen is altered, with greatly reduced production of antigen-specific heavy chain class-switched antibodies, the levels of which could not be elevated even with repeated immunizations. This suggests a critical role for B7-H2 in the recall phases of the immune response. Germinal center formation is also impaired in the mutant mice. While B cells from the mutant mice could response normally to anti-IgM, anti-CD40, and lipopolysaccharide stimulation, the production of T-helper-type II cytokines such as interleukin-4 (IL-4) and IL-10 by primed CD4+ T cells from mutant mice were reduced. This indicated that the defects in humoral responses and germinal center formation in B7-H2-deficient mice are due to the lack of T-cell-mediated help to the B cells. Hence, B7-H2 on B cells is important for recruiting T-cell help via its interaction with ICOS and plays a critical role in costimulating humoral immune responses.

The role of inducible co-stimulator (ICOS)/B7-related protein-1 (B7RP-1) interaction in the functional development of Peyer's patches

T-cell co-stimulatory molecule, inducible co-stimulator (ICOS)/B7-related protein-1 (B7RP-1) interactions play an essential role of T-cell-dependent B-cell activation in peripheral lymphoid organs such as spleen and lymph nodes. Here, we investigate the role of ICOS/B7RP-1 interactions in the development of Peyer's patches (PPs). In ICOS(-/-) mice, the number of PPs was not decreased, although PPs in ICOS(-/-) mice were significantly reduced in size. Phenotypic analysis showed no obvious differences between ICOS(-/-) and ICOS(+/-) mice in the distribution of T-cells, B-cells, macrophages and dendritic cells. However, PNA(+) cells characteristic of intestinal germinal centers were totally absent in ICOS(-/-) mice. Moreover, production of IgA and IgG, but not IgM was significantly reduced in PPs in ICOS(-/-) mice. These data suggest that ICOS/B7RP-1 interactions may not affect the organogenesis, but involve in the functional development of PPs.

A complicated relationship: fulfilling the interactive needs of the T lymphocyte and the dendritic cell

T cells recognize antigenic peptides displayed on the surface of MHC-bearing antigen-presenting cells (APCs), and with sufficient costimulation become activated. However, the ability of an APC (even bearing the correct peptide) to initiate and fulfill the requirements for T cell activation is not easily achieved. Naive T cells use multiple copies of a single receptor to survey the vast array of peptides presented on an APC, and require multiple receptor engagements to initiate T cell activation. Dendritic cells (DCs) are specialized cells with optimal capabilities for priming naive CD4+ T cells. Activation occurs, after initial antigen recognition by T cells, followed by a rapid dialogue between the T cells and the DCs. The resulting changes in both the cytoskeleton and the expression or regulation of cell-surface molecules on both cell types act to further strengthen engagement. In this report, we review the fundamentals of CD4+ T helper cell : DC interactions and discuss recent data concerning the molecular characteristics of this engagement.

Dynamics of Cell Surface Molecules During T Cell Recognition

Recognition of foreign antigens by T lymphocytes is a very important component of vertebrate immunity-vital to the clearance of pathogenic organisms and particular viruses and necessary, indirectly, for the production of high affinity antibodies. T cell recognition is mediated by the systematic scanning of cell surfaces by T cells, which collectively express many antigen receptors. When the appropriate antigenic peptide bound to a molecule of the major histocompatibility complex is found-even in minute quantities-a series of elaborate cell-surface molecule and internal rearrangements take place. The sequence of events and the development of techniques required to observe these events have significantly enhanced our understanding of T cell recognition and may find application in other systems of transient cell:cell interactions as well.

Responding naive T cells differ in their sensitivity to Fas engagement: early death of many T cells is compensated by costimulation of surviving T cells

Engagement of Fas (CD95) induces death of activated T cells but can also potentiate T-cell response to CD3 ligation. Yet, the effects of Fas-mediated signals on activation of naive T cells have remained controversial. We followed naive T cells responding under Fas ligation. Ligation of Fas simultaneously with activation by antigen-bearing dendritic cells promoted early death in half of the responding naive murine CD4 T cells. Surprisingly, it simultaneously accelerated cell division and interferon-gamma (IFN-gamma) production among surviving T cells. These cells developed quickly an activation-associated phenotype (CD44(hi), CD62L(lo)), responded vigorously to antigen rechallenge, were partially resistant to subsequent induction of cell death via Fas, and were long-lived in vivo. Compared with cells becoming apoptotic, the surviving cells expressed lower levels of Fas and higher levels of T-cell receptor (TCR), CD4, and interleukin-2 receptor (IL-2R). Their survival was associated with expression of antiapoptotic cellular FLICE-inhibitory protein (c-FLIP), Bcl-X(L), and Bcl-2. Thus, at the time of T-cell activation there is a subtle balance in the effects of Fas ligation that differs on a cell-to-cell basis. Factors that predict cell survival include expression levels of Fas, TCR, CD4, and IL-2R. Early death of some cells and a pronounced response of the surviving cells suggest that Fas ligation can both up- and down-regulate a primary T-cell response.

Stochastic model of T cell proliferation: a calculus revealing IL-2 regulation of precursor frequencies, cell cycle time, and survival

The outcome of Ag exposure is dictated by complex regulation of T cell proliferation. The rates of proliferation and survival are altered by numerous signals that the cell receives and integrates to achieve a net response. We have illustrated previously how small changes in kinetic parameters can lead to large differences, even under conditions of saturating IL-2. In this study, we examine the effect of varying IL-2 concentration on T cell response and develop a model incorporating additional parameters of proliferation and survival. Strikingly, the proportion of cells that enter the first division, but not the time at which they enter, is dramatically altered by IL-2. Furthermore, the survival and average division time of cells in later divisions are also altered by IL-2 concentration. Together, the small simultaneous effects on these parameters result in large differences in total cell number. These results reveal how in vitro systems may exaggerate the contribution of IL-2, and thus how costimuli or additional helper cells that alter IL-2 concentration, even by relatively small amounts, will generate large in vitro differences in cell number and therefore appear obligatory. Furthermore, they illustrate how a quantitative model of T cell activation can clarify how complex signal integration is handled by T cells in situ, and therefore more appropriately aid development of a theory of behavior.

Molecular modeling and functional mapping of B7-H1 and B7-DC uncouple costimulatory function from PD-1 interaction

B7-H1 and B7-DC are ligands for PD-1, a receptor implicated in negative regulation of T and B cell functions. These ligands, however, also costimulate T cell responses. It remains elusive whether or not costimulation is mediated through PD-1. By comparative molecular modeling and site-directed mutagenesis, we found that nonconserved residues between these ligands on the A'GFCC'C" face mediate interaction with PD-1. This indicates significant structural heterogeneity of the interactions between PD-1 and its ligands. Importantly, ligand mutants with abolished PD-1 binding capacity could still costimulate proliferation and cytokine production of T cells from normal and PD-1-deficient mice. Our results reveal unique binding characteristics of B7-H1 and B7-DC and provide direct evidence for an independent costimulatory receptor other than PD-1.

Expression of the costimulatory molecule B7-H2 (inducible costimulator ligand) by human airway epithelial cells

Tissue structural cells are known in some situations to play a role in the presentation of antigen and in immunoregulation. We assessed the expression of B7 homologs, known to be involved in antigen presentation and lymphocyte costimulation, in human airway epithelial cells. Flow cytometry performed on the airway epithelial cell line BEAS-2B, as well as primary bronchial epithelial cells (PBEC), showed that B7-H2 was constitutively expressed on both BEAS-2B and PBEC, whereas B7-1 and B7-2 were undetectable on either epithelial cell type. B7-H2 expression was confirmed by Western blot using a specific antibody. Stimulation with various cytokines, including tumor necrosis factor-alpha, interferon-gamma, and interleukin-4, slightly downregulated B7-H2 expression detected by flow cytometry, but did not significantly alter the apparent level of protein as assessed by Western blotting. Northern blotting detected mRNA for B7-H2 and B7-1, but not B7-2. B7-H2 was cloned from BEAS-2B cells and the sequence verified. Expression of B7-H2 mRNA was detected by real-time reverse transcriptase-polymerase chain reaction in PBEC from three independent donors. Immunohistochemical analysis of airway derived from autopsies revealed expression of B7-H2 in human airway epithelial cells. These results demonstrate that airway epithelial cells express the costimulatory molecule B7-H2, and suggest the possibility that B7-H2 may participate in antigen presentation by epithelial cells.

Expression of CD80 on Kupffer cells is enhanced in cadaveric liver transplants

In experimental animals inhibition of T cell co-stimulation immediately after organ transplantation effectively prevents rejection. We investigated whether the expression of co-stimulatory molecules is enhanced in cadaveric liver transplants, whether their expression is influenced by the transplantation procedure, and whether variation in expression between liver transplants is related to the occurrence of acute rejection. Expression of CD80, CD86 and the macrophage marker CD68 were determined by immunohistochemistry in biopsies from 40 clinical liver transplants obtained at different time-points during the transplantation procedure, and in normal liver tissue obtained from 10 human livers. Expression of CD80 and CD86 on Kupffer cells was graded by comparison with CD68-staining. In a subgroup CD80 and CD86 mRNA was quantified by real-time detection polymerase chain reaction. CD86 was expressed in all liver transplants and normal livers on the majority of Kupffer cells. CD80 was absent or sporadically expressed in normal liver tissue, but in 18 of 40 liver transplants at least one-quarter of Kupffer cells expressed CD80. CD80- and CD86-mRNA and protein expression in liver transplants did not change during the warm ischaemic and reperfusion phases of the transplantation procedure. CD80-expression on Kupffer cells varied strongly between individual donor livers; this variation was, however, not significantly related to the occurrence of acute rejection after transplantation. In conclusion, in nearly half of cold-preserved cadaveric liver transplants an increased proportion of Kupffer cells express CD80 at the time of transplantation in comparison with normal liver tissue. The expression was not further induced by warm ischaemia and reperfusion. However, the observed variation in CD80-expression between liver transplants is not a accurate predictive measure for acute rejection.

Induction of cytotoxic T lymphocyte responses by cholera toxin-treated bone marrow-derived dendritic cells

Cholera toxin (CT), a powerful mucosal adjuvant, is a potent inducer of Th2-type responses via activation of co-stimulatory molecules for the induction of IgA antibody responses. Less appreciated is the ability of CT to induce and regulate cytotoxic T lymphocyte (CTL) responses. In order to help for clarifying mechanisms underlying the CTL-inducing ability of CT, we have examined the effects of CT on dendritic cells (DCs) that could lead to the induction of cytotoxic CD8(+) T cells. When bone marrow-derived DCs (BM-DCs) were cultured with CT in vitro, B7-1 but not B7-2 molecules were significantly enhanced and allogenic CTL responses were induced. Also, increased numbers of IFN-gamma-secreting CD8(+) T cells were elicited when CT-treated BM-DCs were co-cultured with allogenic CD8(+) CTLs. Antibody blockade of B7-1 on CT-treated BM-DCs suppressed allogenic CTL responses, further indicating the importance of CT-induced B7-1 molecules on DCs for the acquisition of cytolytic function by CTL precursors. CD40 signaling was proven not necessary for the CT-induced CTL response since CT-treated CD40(-/-) BM-DCs developed CTL responses equivalent to those detected in CT-treated BM-DCs derived from normal mice. Our results suggest that CT-treated DCs are effective inducers of CD8(+) CTL, and this induction is mediated through CT's ability to enhance B7-1 expression on DCs.

Prolongation of liver xenograft survival by adenovirus vector-mediated CTLA-4Ig gene transfer

Cytotoxic T lymphocyte-associated antigen-4/immunoglobulin fusion products (CTLA-4Ig), a structural homologue of CD28, has been shown to inhibit cellular and humoral immune responses. In this study, we investigated the efficacy of an adenovirus vector containing the CTLA-4Ig gene (AdCTLA-4Ig) on recipient survival after hamster-to-rat liver xenografting. AdCTLA-4Ig was administrated intravenously immediately after grafting. Gene expression was achieved a maximum of 7 days after vector injection and continued for more than 4 weeks. The proportion of CD25(+) T-cells in recipient lymph nodes was significantly reduced 7 days after administration of AdCTLA-4Ig, compared to a group given an adenoviral vector containing LacZ gene (AdLacZ) or to an untreated control group. AdCTLA-4Ig markedly reduced CD2(+) T-cell infiltration into the graft and significantly prolonged recipient survival time (9.2+/-4.12 days), compared to the untreated group (5.4+/-0.78 days) (P<0.001) and the AdLacZ-treated group (5.2+/-0.28 days) (P<0.001). These results indicate that a blockade of T-cell co-stimulation by AdCTLA-4Ig inhibited T-cell activation and attenuated CD2(+) T-cell infiltration into the xenograft, resulting in significant prolongation of recipient survival time. Thus, AdCTLA-4Ig therapy may provide a novel approach to immune regulation.

The EphB6 receptor inhibits JNK activation in T lymphocytes and modulates T cell receptor-mediated responses

EphB6 is the most recently identified member of the Eph receptor tyrosine kinase family. EphB6 is primarily expressed in thymocytes and a subpopulation of T cells, suggesting that it may be involved in regulation of T lymphocyte differentiation and functions. We show here that overexpression of EphB6 in Jurkat T cells and stimulation with the EphB6 ligand, ephrin-B1, results in the selective inhibition of TCR-mediated activation of JNK but not the MAPK pathway. EphB6 appears to suppress the JNK pathway by preventing T cell receptor (TCR)-induced activation of the small GTPase Rac1, a critical event in initiating the JNK cascade. Furthermore, EphB6 blocked anti-CD3-induced secretion of IL-2 and CD25 expression in a ligand-dependent manner. Dominant negative EphB6 suppressed the inhibitory activity of the endogenous receptor and enhanced anti-CD3-induced JNK activation, CD25 expression, and IL-2 secretion, confirming the requirement for EphB6-specific signaling. Activation of the JNK pathway and the establishment of an IL-2/IL-2R autocrine loop have been shown to play a role in the negative selection of CD4(+)CD8(+) self-reacting thymocytes. In agreement, stimulation of murine thymocytes with ephrin-B1 not only blocked anti-CD3-induced CD25 up-regulation and IL-2 production, but also inhibited TCR-mediated apoptosis. Thus, EphB6 may play an important role in regulating thymocyte differentiation and modulating responses of mature T cells.

Gallium arsenide exposure impairs splenic B cell accessory function

Gallium arsenide (GaAs) is utilized in industries for its semiconductor and optical properties. Chemical exposure of animals systemically suppresses several immune functions. The ability of splenic B cells to activate antigen-specific helper CD4(+) T cell hybridomas was assessed, and various aspects of antigen-presenting cell function were examined. GaAs-exposed murine B cells were impaired in processing intact soluble protein antigens, and the defect was antigen dependent. In contrast, B cells after exposure competently presented peptides to the T cells, which do not require processing. Cell surface expression of major histocompatibility complex (MHC) class II molecules and several costimulatory molecules on splenic B cells, which are critical for helper T cell activation, was not affected by chemical exposure. GaAs exposure also did not influence the stability of MHC class II heterodimers, suggesting that the defect may precede peptide exchange. GaAs-exposed B cells contained a normal level of aspartyl cathepsin activity; however, proteolytic activities of thiol cathepsins B and L were approximately half the control levels. Furthermore, two cleavage fragments of invariant chain, a molecular chaperone of MHC class II molecules, were increased in GaAs-exposed B cells, indicative of defective degradation. Thus, diminished thiol proteolytic activity in B cells may be responsible for their impaired antigen processing and invariant chain degradation, which may contribute to systemic immunosuppression caused by GaAs exposure.

Pooled human gammaglobulin modulates surface molecule expression and induces apoptosis in human B cells

We have previously shown that the pooled human gammaglobulin (IVIG) inhibited mixed lymphocyte reaction (MLR). In this study, we examined (1) if IVIG contains blocking antibodies reactive with cell surface molecules required for alloantigen recognition and (2) if IVIG modulates these surface molecule expressions using flow cytometry. IVIG does not contain significant amounts of blocking antibodies against CD3, CD4, CD8, CD20, CD14, CD40, MHC class I and class II. It reduces the number of intact B cells and monocytes, reduces or modulates CD19, CD20 and CD40 expression on B cells, and induces morphological changes in B cells. This B-cell modulation results primarily because of apoptosis. IVIG also induces apoptosis in T cells and monocytes, but to a lesser degree. Induction of apoptosis requires the intact IgG molecule. Reduction of intact B cell and monocyte cell numbers, modulation of surface molecule expression on B cells, and deletion of B and T cells by apoptosis could result in inhibition of optimal T-cell activation. This likely represents the primary mechanism responsible for IVIG suppression of the MLR, and may account for many of the observed beneficial effects of IVIG seen in the treatment of human autoimmune and alloimmune disorders.

The outcome of T-cell costimulation through intercellular adhesion molecule-1 differs from costimulation through leucocyte function-associated antigen-1

Optimal T-cell activation requires both an antigen-specific and a costimulatory signal. The outcome of T-cell activation can be influenced by the nature of the costimulatory signal the T cell receives. We recently demonstrated the ability of stimulation through intercellular adhesion molecule-1 (ICAM-1), resident on the T-cell surface, to provide a second signal for T-cell activation, and have extended that work here to begin an examination of the functional outcome of this set of signals. Costimulation through ICAM-1 resulted in a greater percentage of cells having undergone more than three divisions when compared to costimulation through leucocyte function-associated antigen-1 (LFA-1). Costimulation through ICAM-1 also had an effect similar to costimulation through CD28 in its ability to down-regulate the cyclin dependent kinase inhibitor p27kip1. Costimulation through ICAM-1 provided greater protection from apoptosis than costimulation through LFA-1, especially in cells having divided more than three times. This was supported by the ability of costimulation through ICAM-1 to up-regulate the anti-apoptotic protein Bcl-2. Finally, costimulation through ICAM-1 or CD28 produced a greater number of T cells with a memory phenotype than costimulation through LFA-1.

Costimulating aberrant T cell responses by B7-H1 autoantibodies in rheumatoid arthritis

A pathogenic hallmark of rheumatoid arthritis (RA) is persistent activation of self-reactive CD4(+) T cells. The cause of this aberrant activity remains elusive. We report here detection of autoantibodies against B7-H1, a recently described member of the B7 family, in 29% of patients with RA versus 4% of healthy donors. High-level expression of cell surface B7-H1 are found on activated human CD4(+), CD8(+), and CD45RO(+) T cells. Immobilized autoantibodies to B7-H1 are capable of costimulating the proliferation of CD4(+) T cells in vitro, and the presence of these autoantibodies correlates with active disease status. Using immobilized B7-H1 mAb's and programmed death 1Ig, we demonstrate that engagement of B7-H1 on CD4(+) T cells costimulates proliferation and secretion of IL-10, and subsequently leads to programmed cell death, accompanied with upregulated expression of TNF-related apoptosis-inducing ligand and activation of caspase-3. Taken together with our previous findings, these data indicate a bidirectional signaling role of B7-H1 in T cell costimulation and apoptosis and implicate B7-H1 autoantibodies as contributing to the progression of RA by inducing aberrant T cell responses.

Inhibition of tumor rejection by gammadelta T cells and IL-10

Although many tumors express tumor-specific antigens, most fail to stimulate effective immune responses. Tumors generally lack co-stimulatory molecules, which can lead to tolerance of tumor-specific T cells and progressive tumor growth. Here, we demonstrate that the ovalbumin (OVA) transfected EL4 tumor, E.G7-OVA, grows progressively in syngeneic mice even though the tumor can be rejected if the mice are immunized with OVA in adjuvant. E.G7-OVA grew more rapidly in RAG-1 deficient than sufficient mice suggesting that normal mice make an abortive immune response to this tumor. Depletion of gammadelta T cells or IL-10 augmented the ability of B6 mice to reject E.G7-OVA. Spleen cells from normal, but not IL-10 knockout, mice reconstituted rapid tumor growth in gammadelta T cell-deficient mice. Thus, gammadelta T cells play an important role in preventing immune elimination of this tumor by a mechanism that directly or indirectly involves IL-10.

Blockade of CD86 in BALB/c mice infected with Leishmania major does not prevent the expansion of low avidity T cells

The interactions between CD28 and its ligand CD86 are critical for the regulation of T cell responses. However, it is not clear whether CD4+ T cells expressing low and high avidity TCR are equally dependent on CD28 costimulation for their activation and expansion. To address this issue, we have used multimers of I-Ad molecules linked to a peptide derived from the Leishmania major homolog for the receptor of activated C kinase (LACK) antigen to compare the fate of LACK-specific CD4+ T cells in Leishmania-infected BALB/c mice which have been treated or not with anti-CD86 mAb. Although the administration of anti-CD86 mAb did not completely prevent the expansion of LACK-specific T cells, their frequency and number were markedly reduced. In mice treated with anti-CD86 mAb as well as in control animals, L. major induced the clonal expansion of LACK-specific T cells which expressed a canonical low avidity Valpha8/Vbeta4 TCR. Taken together, our results suggest that the molecular interactions between CD28 on T cells and CD86 on APC serve to amplify and modulate T cell responses without promoting breadth in the TCR repertoire.

Principles of tumor immunosurveillance and implications for immunotherapy

Although antigen loss variants, major histocompatibility (MHC) class I down-regulation, or the expression of inhibitory molecules may explain the failure of immunosurveillance against some tumors, this seems not to apply for many other solid peripheral or lymphohematopoietic tumors. Why then is immunosurveillance so ineffective and can it be improved? This review focuses on one important aspect of tumor immunity, namely the relevance of antigen dose and localization. Immune responses in vivo are induced in organized lymphoid tissues, i.e., in lymph nodes and spleen. The antigen dose that reaches secondary lymphoid organs over time is a crucial parameter that drives antiviral and antitumoral immune responses. Tumors use various strategies to prevent efficient presentation of their antigens in lymphoid organs. A major obstacle to the induction of an endogenous tumor-specific cytotoxic T lymphocyte (CTL) response is the inefficient presentation of tumor antigen on MHC class I molecules of professional antigen-presenting cells. Peripheral solid tumors that develop outside lymphoid organs are, therefore, often ignored by the immune system. In other situations, tumors - especially of lymphohematopoietic origin - may tolerize specific CTLs. Understanding tumor immunosurveillance is key to the design of efficient antitumor vaccines. Attempts to improve immunity to tumors include vaccination strategies to (a) provide the tumor antigen to secondary lymphoid organs using recombinant viruses or dendritic cells as carriers, (b) express costimulatory signals on tumor cells, or (c) improve the efficiency of cross-priming.

Costimulation of multiple NK cell activation receptors by NKG2D

The activation of NK cells is mediated through specific interactions between activation receptors and their respective ligands. Little is known, however, about whether costimulation, which has been well characterized for T cell activation, occurs in NK cells. To study the function of NKG2D, a potential NK costimulatory receptor, we have generated two novel hamster mAbs that recognize mouse NKG2D. FACS analyses demonstrate that mouse NKG2D is expressed on all C57BL/6 IL-2-activated NK (lymphokine-activated killer (LAK)) cells, all splenic and liver NK cells, and approximately 50% of splenic NKT cells. Consistent with limited polymorphism of NKG2D, its sequence is highly conserved, and the anti-NKG2D mAbs react with NK cells from a large number of different mouse strains. In chromium release assays, we show that stimulation of NK cells with anti-NKG2D mAb can redirect lysis. Also, enhanced lysis of transfected tumor targets expressing NKG2D ligand could be inhibited by addition of anti-NKG2D mAb. Interestingly, stimulation of LAK cells via NKG2D alone does not lead to cytokine release. However, stimulation of LAK via both an NK activation receptor (e.g., CD16, NK1.1, or Ly-49D) and NKG2D leads to augmentation of cytokine release compared with stimulation through the activation receptor alone. These results demonstrate that NKG2D has the ability to costimulate multiple NK activation receptors.