otherwise fatal overstimulation of the lymphoid system 6-9 . Here we report the identification of a third member of this family of molecules, inducible co-stimulator (ICOS), which is a homodimeric protein of relative molecular mass 55,000-60,000 (M r 55K-60K). Matching CD28 in potency, ICOS enhances all basic Tcell responses to a foreign antigen, namely proliferation, secretion of lymphokines, upregulation of molecules that mediate cell-cell interaction, and effective help for antibody secretion by B cells. Unlike the constitutively expressed CD28, ICOS has to be de novo induced on the T-cell surface, does not upregulate the production of interleukin-2, but superinduces the synthesis of interleukin-10, a B-cell-differentiation factor. In vivo, ICOS is highly expressed on tonsillar T cells, which are closely associated with B cells in the apical light zone of germinal centres, the site of terminal B-cell maturation. Our results indicate that ICOS is another major regulator of the adaptive immune system.We identified ICOS by generating monoclonal antibodies against activated human T cells. The ICOS-specific monoclonal antibody F44 did not react with resting human peripheral-blood T cells, but stained CD4 + and CD8 + T lymphocytes that had been activated by stimulation of the T-cell antigen receptor (TCR) complex (Fig. 1a). No signal was detected on resting or appropriately activated B cells, monocytes, natural killer cells, granulocytes, dendritic cells or platelets (data not shown). Immunoprecipitations using monoclonal Figure 1 Identification, purification and cloning of ICOS. a, Expression of ICOS on human T cells after 36 h of stimulation.Peripheral-blood CD4 + or CD8 + T cells were left untreated or were stimulated with the solid-phase-bound anti-CD3 monoclonal antibody OKT 3 (1:1,000 dilution of ascites), and were analysed by flow cytometry using the fluorescein isothiocyanate (FITC)-labelled monoclonal antibody F44. At 14 h after stimulation, ICOS could not yet be detected on CD8 + T cells, whereas CD4 + T cells expressed levels of ICOS that were similar to those expressed after 36 h (data not shown). The background signal obtained with the isotype-control monoclonal antibody MOPC-21 (Sigma) is shown in black. Stimulation by phorbol-12-myristate-13-acetate (PMA) and ionomycin led to the expression of ICOS on most CD4 + and CD8 + T cells (data not shown). b, Structure of the homodimeric ICOS protein. ICOS protein was immunoprecipitated from surface-iodinated MOLT-4V cells with monoclonal antibody F44 and separated by two-dimensional (nonreducing/reducing) SDS-PAGE. Numbers at the top and right side of the gel are M r values. The 55K-60K protein species on the diagonal corresponds to residual dimeric ICOS, which was not reduced by the in-gel reducing procedure required for the twodimensional SDS-PAGE (a full reduction of this species was routinely observed in onedimensional SDS-PAGE; data not shown). Identical data were obtained with iodinated activated primary T cells (data not shown). c, ICOS mRNA expression...

The T-cell-specific cell-surface receptors CD28 and CTLA-4 are important regulators of the immune system. CD28 potently enhances those T-cell functions that are essential for an effective antigen-specific immune response, and the homologous CTLA-4 counterbalances the CD28-mediated signals and thus prevents an otherwise fatal overstimulation of the lymphoid system. Here we report the identification of a third member of this family of molecules, inducible co-stimulator (ICOS), which is a homodimeric protein of relative molecular mass 55,000-60,000 (M(r) 55K-60K). Matching CD28 in potency, ICOS enhances all basic T-cell responses to a foreign antigen, namely proliferation, secretion of lymphokines, upregulation of molecules that mediate cell-cell interaction, and effective help for antibody secretion by B cells. Unlike the constitutively expressed CD28, ICOS has to be de novo induced on the T-cell surface, does not upregulate the production of interleukin-2, but superinduces the synthesis of interleukin-10, a B-cell-differentiation factor. In vivo, ICOS is highly expressed on tonsillar T cells, which are closely associated with B cells in the apical light zone of germinal centres, the site of terminal B-cell maturation. Our results indicate that ICOS is another major regulator of the adaptive immune system.

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

Numerous epidemiological studies have shown an inverse correlation between helminth infections and the manifestation of atopic diseases, yet the immunological mechanisms governing this phenomenon are indistinct. We therefore investigated the effects of infection with the filarial parasite Litomosoides sigmodontis on allergen-induced immune reactions and airway disease in a murine model of asthma. Infection with L. sigmodontis suppressed all aspects of the asthmatic phenotype: Ag-specific Ig production, airway reactivity to inhaled methacholine, and pulmonary eosinophilia. Similarly, Ag-specific recall proliferation and overall Th2 cytokine (IL-4, IL-5, and IL-3) production were significantly reduced after L. sigmodontis infection. Analysis of splenic mononuclear cells and mediastinal lymph nodes revealed a significant increase in the numbers of T cells with a regulatory phenotype in infected and sensitized mice compared with sensitized controls. Additionally, surface and intracellular staining for TGF-β on splenic CD4+ T cells as well as Ag-specific TGF-β secretion by splenic mononuclear cells was increased in infected and sensitized animals. Administration of Abs blocking TGF-β or depleting regulatory T cells in infected animals before allergen sensitization and challenges reversed the suppressive effect with regard to airway hyperreactivity, but did not affect airway inflammation. Despite the dissociate results of the blocking experiments, these data point toward an induction of regulatory T cells and enhanced secretion of the immunomodulatory cytokine TGF-β as one principle mechanism. In conclusion, our data support the epidemiological evidence and enhance the immunological understanding concerning the impact of helminth infections on atopic diseases thus providing new insights for the development of future studies.

Evasion of apoptosis can be caused by epigenetic silencing of caspase-8, a key component of the extrinsic apoptosis pathway. Loss of caspase-8 correlates with poor prognosis in medulloblastoma, which highlights the relevance of strategies to upregulate caspase-8 to break apoptosis resistance. Here, we develop a new combinatorial approach, that is treatment using histone deacetylase inhibitors (HDACI) together with interferon (IFN)-c, to restore caspase-8 expression and to overcome resistance to the death-receptor ligand TNF-related apoptosis-inducing ligand (TRAIL) in medulloblastoma in vitro and in vivo. HDACI, for example, valproic acid (VA), suberoylanilide hydroxamic acid (SAHA) and MS-275, cooperate with IFN-c to upregulate caspase-8 in cancer cells lacking caspase-8, thereby restoring sensitivity to TRAILinduced apoptosis. Molecular studies show that VA promotes histone acetylation and acts in concert with IFN-c to stimulate caspase-8 promoter activity. The resulting increase in caspase-8 mRNA and protein expression leads to enhanced TRAILinduced activation of caspase-8 at the death-inducing signaling complex, mitochondrial outer-membrane permeabilization and caspase-dependent cell death. Intriguingly, pharmacological or genetic inhibition of caspase-8 also abolishes the VA/IFN-cmediated sensitization for TRAIL-induced apoptosis. It is important to note that VA and IFN-c restore caspase-8 expression and sensitivity to TRAIL in primary medulloblastoma samples and significantly potentiate TRAIL-mediated suppression of medulloblastoma growth in vivo. These findings provide the rationale for further (pre)clinical evaluation of VA and IFN-c to restore caspase-8 expression and apoptosis sensitivity in cancers with caspase-8 silencing and open new perspectives to overcome TRAIL resistance.