The CD28-related molecule ICOS is required for effective T cell-dependent immune responses

T cell activation

This year marks the 25th anniversary of the first Annual Review of Immunology article to describe features of the T cell antigen receptor (TCR). In celebration of this anniversary, we begin with a brief introduction outlining the chronology of the earliest studies that established the basic paradigm for how the engaged TCR transduces its signals. This review continues with a description of the current state of our understanding of TCR signaling, as well as a summary of recent findings examining other key aspects of T cell activation, including cross talk between the TCR and integrins, the role of costimulatory molecules, and how signals may negatively regulate T cell function.Acronyms and DefinitionsAdapter protein: cellular protein that functions to bridge molecular interactions via characteristic domains able to mediate protein/protein or protein/lipid interactions Costimulation: signals delivered to T cells by cell surface receptors other than the TCR itself that potentiate T cell activation cSMAC: central supramolecular activation cluster Immunoreceptor tyrosine-based activation motif (ITAM): a short peptide sequence in the cytoplasmic tails of key surface receptors on hematopoietic cells that is characterized by tyrosine residues that are phosphorylated by Src family PTKs, enabling the ITAM to recruit activated Syk family kinases Inside-out signaling: signals initiated by engagement of immunoreceptors that lead to conformational changes and clustering of integrins, thereby increasing the affinity and avidity of the integrins for their ligands NFAT: nuclear factor of activated T cells PI3K: phosphoinositide 3-kinase PKC: protein kinase C PLC: phospholipase C pMHC: peptide major histocompatibility complex (MHC) complex pSMAC: peripheral supramolecular activation cluster PTK: protein tyrosine kinase Signal transduction: biochemical events linking surface receptor engagement to cellular responses TCR: T cell antigen receptor

ICOS-gene variants are not associated with atopic disease susceptibility in European children

The inducible co-stimulatory molecule, ICOS, is an important regulator of T cell differentiation and effector function. Previously, it was reported that two variants in the ICOS promotor region, g.1-1413G>A and g.1-693G>A, were associated with sensitization to airborne allergens, elevated serum IgE levels and Th2 cytokine production in a Hutterite population. The aim of this study was to evaluate these two and four other selected ICOS variants for association with atopic phenotypes in two large European prospective pediatric cohorts. We investigated subjects from the German Multicenter Allergy Study (MAS), which followed over 800 children with atopic family history from birth until 13 yr of age, and from the Early Treatment of the Allergic Child Study (ETAC), which collected DNA and clinical data of over 330 children with atopic dermatitis during their first 2 yr of life. We genotyped DNA from these children by melting curve analysis using fluorescence resonance energy transfer (FRET) probes. We could not confirm the previously reported association of g.1-1413G>A and g.1-693G>A with atopic phenotypes in our pediatric cohorts. Also four other ICOS variants at putative binding sites for transcription factors showed no association with atopic dermatitis, asthma, allergic sensitization and allergic rhinitis. Our data suggest that these ICOS variants do not play a major role in the development of atopy in European children.

Increased expression of soluble inducible costimulator ligand (ICOSL) in patients with systemic lupus erythematosus

To investigate the level of costimulating molecules in systemic lupus erythematosus (SLE), we assessed the plasma concentrations of soluble forms of costimulatory molecules such as programmed death-1 (PD-1), B7-H1 (also called PD-L1 or CD274) and inducible costimulator ligand (ICOSL) in patients with SLE. Plasma concentrations of soluble PD-1, B7-H1 and ICOSL were measured by ELISA using plasma samples from 57 SLE patients with or without the active disease, 21 rheumatoid arthritis (RA) patients and 35 healthy subjects. We also evaluated surface ICOSL expression on B cells using flow cytometry to gain a better understanding of ICOSL expression. To compare the level of ICOSL mRNA expression, reverse transcriptase-polymerase chain reaction (RT-PCR) was performed using total RNA from peripheral blood mononuclear cells (PBMCs) isolated from eight healthy subjects and 11 patients with SLE. The concentration of plasma ICOSL was significantly higher in patients with SLE compared with healthy subjects ( P = 0.005). Plasma ICOSL concentrations in patients with active SLE were also significantly higher than those of either patients with inactive SLE or patients with RA ( P = 0.001, P = 0.015, respectively). Plasma ICOSL concentrations in patients with SLE correlated modestly with the SLE disease activity index score ( r = 0.298, P = 0.024). We also found a significant inverse correlation between the soluble ICOSL expression and the surface ICOSL expression on B cells ( r = −0.690, P = 0.001). However, ICOSL mRNA levels of patients with SLE were comparable with those of the control subjects. There was also no significant difference in plasma B7-H1 concentrations between groups, and plasma PD-1 was not detectable in any of the groups. The plasma concentration of soluble ICOSL might be correlated to the disease severity of lupus. The increased levels of ICOSL in active lupus suggest that this pathway is involved in the pathogenesis of SLE. The mechanism and physiological role of soluble ICOSL in the pathogenesis of SLE, however, remains to be investigated.

B7RP-1 blockade ameliorates autoimmunity through regulation of follicular helper T cells

Autoimmune diseases are marked by the presence of class-switched, high-affinity autoantibodies with pathogenic potential. Costimulation plays an important role in the activation of T cells and the development of T cell-dependent B cell responses. ICOS plays an indispensable role in the development of follicular helper T cells (T(FH) cells), which provide cognate help to germinal center (GC) B cells. We show that the levels of T(FH) cells and GC B cells in two different models of autoimmunity, the New Zealand Black/New Zealand White (NZB/NZW) F(1) mouse model of systemic lupus erythematosus and the collagen-induced arthritis model of rheumatoid arthritis, are dependent on the maintenance of the ICOS/B7RP-1 pathway. Treatment with an anti-B7RP-1 Ab ameliorates disease manifestations and leads to a decrease in T(FH) cells and GC B cells as well as an overall decrease in the frequency of ICOS(+) T cells. Coculture experiments of Ag-primed B cells with CXCR5(+) or CXCR5(-) T cells show that blocking B7RP-1 does not directly impact the production of IgG by B cells. These findings further support the role of ICOS in autoimmunity and suggest that the expansion of the T(FH) cell pool is an important mechanism by which ICOS regulates Ab production.

Defining dose-response relationships in the therapeutic blockade of B7RP-1-dependent immune responses

The ICOS (Inducible T cell Co-Stimulator)/B7RP-1 (B7-related protein 1) interaction is critical for the proper activation of a T lymphocyte. In this manuscript we describe a systematic in vivo approach to determine the level of blockade required to impair the generation of a T cell-dependent antibody response. We have developed an overall strategy for correlating drug exposure, target saturation, and efficacy in a biological response that can be generalized for most protein therapeutics. Using this strategy, we determined that low levels of B7RP-1 blockade are still sufficient to inhibit the immune response. These data suggest that contact between the T cell and the antigen-presenting cell during antigen presentation is much more sensitive to inhibition than previously believed and that ICOS/B7RP-1 blockade may be efficacious in the treatment of autoimmune diseases.

Anti-CTLA-4 therapy results in higher CD4+ICOShi T cell frequency and IFN-gamma levels in both nonmalignant and malignant prostate tissues

Cytotoxic lymphocyte antigen-4 (CTLA-4) blockade is an active immunotherapeutic strategy that is currently in clinical trials in cancer. There are several ongoing trials of anti-CTLA-4 in the metastatic setting of prostate cancer patients with reported clinical responses consisting of decreases in the prostate specific antigen (PSA) tumor marker for some patients. Immunologic markers that correlate with these clinical responses are necessary to guide further development of anti-CTLA-4 therapy in the treatment of cancer patients. We recently reported that CD4(+) inducible co-stimulator (ICOS)(hi) T cells that produce interferon-gamma (IFN-gamma) are increased in the peripheral blood and tumor tissues of bladder cancer patients treated with anti-CTLA-4 antibody. Here we present data from the same clinical trial in bladder cancer patients demonstrating a higher frequency of CD4(+)ICOS(hi) T cells and IFN-gamma mRNA levels in nonmalignant prostate tissues and incidental prostate tumor tissues removed at the time of radical cystoprostatectomy. Our data suggest immunologic markers that can be used to monitor prostate cancer patients who receive anti-CTLA-4 therapy and indicate that the immunologic impact of anti-CTLA-4 antibody can occur in both tumor and nonmalignant tissues. These data should be taken into consideration for evaluation of efficacy as well as immune-related adverse events associated with anti-CTLA-4 therapy.

Down-regulation of inducible co-stimulator (ICOS) by intravitreal injection of small interfering RNA (siRNA) plasmid suppresses ongoing experimental autoimmune uveoretinitis in rats

BACKGROUND

RNA interference (RNAi) is now being exploited as a powerful tool for gene knockdown. Recently, we had shown that inducible co-stimulator (ICOS) was up-regulated in experimental autoimmune uveoretinitis (EAU). The aim of this study was to investigate whether intravitreal injection of small interfering RNA (siRNA) plasmid, targeting ICOS, suppresses the ongoing experimental autoimmune uveoretinitis (EAU) in rats.

METHODS

Oligonucleotide targeting ICOS was cloned into linearized pRNAT-U6.1/Neo eukaryotic expression vector to construct the recombinant plasmid (pRNAT-U6.1/Neo-ICOS). After transfecting activated rat T cells with the recombinant plasmid, ICOS mRNA and protein expression levels were determined by real-time RT-PCR and Western blot analysis respectively. Rats were immunized with IRBP R16 peptide emulsified in complete Freund's adjuvant (CFA) and given an intravitreal injection of pRNAT-U6.1/Neo-ICOS on day 6 after immunization. After 13days of immunization, the ICOS protein expression and CD4(+) ICOS (+) T cells were identified in retinae through Western blot analysis and flow cytometry respectively. Intraocular inflammation was assessed by the scores of the clinical and histological appearances. Delayed-type hypersensitivity (DTH) and lymphocyte proliferation were detected to evaluate the systemic effect of intravitreal injection of pRNAT-U6.1/Neo-ICOS.

RESULT

The recombinant plasmid (pRNAT-U6.1/Neo-ICOS) for the ICOS siRNA was successfully constructed. In vitro studies using the recombinant plasmid has showed the down-regulation of ICOS gene expression both at the mRNA and protein levels. Clinical and pathological scores showed that ocular inflammation of pRNAT-U6.1/Neo-ICOS-treated eyes was markedly less than that of vehicle-treated eyes. The expression of ICOS protein and the amount of CD4(+) ICOS(+) T cells in retinae significantly decreased by intravitreal injection of the recombinant plasmid, whereas delayed-type hypersensitivity response and lymphocyte proliferation were not impaired in rats treated with the recombinant plasmid.

CONCLUSION

Intravitreal injection of siRNA plasmid targeting ICOS effectively down-regulated the expression of ICOS, and was highly effective in suppressing the ongoing process of EAU without any side-effects on systemic cellular immunity.

Effector functions of donor-reactive CD8 memory T cells are dependent on ICOS induced during division in cardiac grafts

Alloreactive T-cell memory is present in every transplant recipient and endangers graft survival. Even in the absence of known sensitizing exposures, heterologous immunity and homeostatic T-cell proliferation generate 'endogenous' memory T cells with donor-reactivity. We have recently shown that endogenous donor-reactive CD8 memory T cells infiltrate murine cardiac allografts within hours of reperfusion and amplify early posttransplant inflammation by producing IFN-gamma. Here, we have tested the role of ICOS co-stimulation in eliciting effector function from these memory T cells. ICOS is not expressed on the cell surface of circulating CD8 memory T cells but is rapidly upregulated during cell division within the allograft parenchyma. Donor-reactive CD8 memory T-cell infiltration, proliferation and ICOS expression are regulated by donor class I MHC molecule expression. ICOS blockade significantly reduced IFN-gamma production and other proinflammatory functions of the activated CD8 memory T cells. Our data demonstrate that this induction of ICOS expression within peripheral tissues is an important feature of CD8 memory T-cell activation and identify ICOS as a specific target for neutralizing proinflammatory functions of endogenous CD8 memory T cells.

Inducible costimulator blockade prolongs airway luminal patency in a mouse model of obliterative bronchiolitis

BACKGROUND

In human lung transplantation, chronic rejection is accompanied by obliterative bronchiolitis (OB), a fibrosing inflammatory condition that leads to occlusion of the bronchial lumen and graft failure. The pathogenesis of this disorder is poorly understood, but likely involves antigen presentation by dendritic cells (DC). We studied the presence and activation status of DCs in transplanted tracheas in a mouse model of OB and studied the effect on graft luminal patency of blocking the costimulatory B7RP-1/inducible costimulator (ICOS) pathway.

METHODS

Tracheas from Balb/C or from C57Bl/6 mice were transplanted heterotopically under the dorsal skin of C57Bl/6 mice. Histologic, fluorescence-activated cell sorter, and quantitative-polymerase chain reaction analyses were performed after 1, 2, or 4 weeks. In some groups, treatment with blocking rat anti-mICOS antibodies or irrelevant rat immunoglobulin G was administered during the entire observation period.

RESULTS

After heterotopic transplantation, both CD103+CD11b- and CD103- CD11b+ MHC II+ DCs accumulated in the airway epithelium as early as 1 week after allogeneic (mismatched) but not syngeneic (matched) transplantation. Four weeks after Tx, infiltration with CD11c+ MHCII+ DCs and CD8+ lymphocytes, luminal fibrosis and epithelial damage were more pronounced in the allogeneic than in the syngeneic setting. There was a 10-fold up-regulation of ICOS mRNA and of chemokines involved in T-cell influx in the mismatched setting compared with the matched setting. Strikingly, anti-ICOS treatment without other immunosuppression prevented luminal fibrosis in mismatched transplants.

CONCLUSIONS

Our results suggest that early infiltration by DC occurs in posttransplant OB. Blocking critical costimulatory molecules expressed on DCs, as in the B7RP1-ICOS pathway, prevents epithelial damage and luminal fibrosis.

The costimulatory molecule ICOS regulates the expression of c-Maf and IL-21 in the development of follicular T helper cells and TH-17 cells

The inducible costimulatory molecule ICOS has been suggested to be important in the development of interleukin 17 (IL-17)-producing helper T cells (T(H)-17 cells) and of follicular helper T cells (T(FH) cells). Here we show that ICOS-deficient mice had no defect in T(H)-17 differentiation but had fewer T(H)-17 cells after IL-23 stimulation and fewer T(FH) cells. We also show that T(FH) cells produced IL-17 and that T(FH) cells in ICOS-deficient mice were defective in IL-17 production. Both T(H)-17 and T(FH) cells had higher expression of the transcription factor c-Maf. Genetic loss of c-Maf resulted in a defect in IL-21 production and fewer T(H)-17 and T(FH) cells. Thus our data suggest that ICOS-induced c-Maf regulates IL-21 production that in turn regulates the expansion of T(H)-17 and T(FH) cells.

Antigen-independent adhesion and cell spreading by inducible costimulator engagement inhibits T cell migration in a PI-3K-dependent manner

Engagement of the costimulatory protein ICOS activates effector/memory T cells in tissue by enhancing TCR-mediated proliferation and cytokine production. We now report that in an antigen-independent manner, ICOS also induces adhesion and spreading in human effector/memory T cells, consequently inhibiting cell migration. T cell spreading and elongation after ICOS ligation are accompanied by the formation of two types of actin-rich membrane protrusions: thin, finger-like structures similar to filopodia and short, discrete microspikes. Although filopodia/microspike formation occurs independently of the PI-3K signaling cascade, ICOS-mediated T cell elongation depends on PI-3K activity, which inhibits the accumulation of GTP-bound RhoA. Further inhibition of RhoA activation exacerbates the ICOS-mediated, elongated phenotype. We propose that in inflamed tissue, ICOS engagement by ICOS ligand on a professional or nonprofessional APC prevents the forward motility of the T cell by inhibiting RhoA-dependent uropod retraction. The resulting ICOS-induced T cell spreading and filopodia/microspike formation may promote antigen recognition by enhancing a T cell's scanning potential of an adherent APC surface.

Lack of association between sCTLA-4 levels in human plasma and common CTLA-4 polymorphisms

BACKGROUND

Cytotoxic T lymphocyte antigen-4 (CTLA-4) is an important downregulatory molecule expressed on both T and B lymphocytes. Numerous population genetics studies have documented significant associations between autoimmune diseases and single nucleotide polymorphisms (SNPs) within and around the CTLA-4 region of chromosome 2 in man. Furthermore, circulating levels of a soluble form of CTLA-4 (sCTLA-4) have been reported in a variety of autoimmune mediated diseases. Despite these findings, the relationship between levels of sCTLA-4 protein, mRNA transcript levels, and SNPs within the CTLA-4 region have not been clearly defined. In order to further clarify this relationship, we have tested four different SNPs within the CTLA-4 region among subjects whom are negative (n = 53) versus positive (n = 28) for sCTLA-4.

RESULTS

Our data do not support a clear association between sCTLA-4 levels and any of the four SNPs tested.

CONCLUSION

The variation in the SNPs tested does not appear to effect sCTLA-4 protein levels, despite reports that they affect sCTLA-4 mRNA.

Suppression of T cell costimulator ICOS by Delta9-tetrahydrocannabinol

Inducible costimulator (ICOS), a prototypic T cell costimulator, is induced on activated T cells. ICOS regulates T cell activation and Th cell differentiation and is principally involved in humoral immune responses. Previous work showed that T cell accessory function is modulated by the plant-derived cannabinoid, delta-9-tetrahydrocannabinol (Delta(9)-THC). In light of an emerging role by ICOS in T cell-mediated immunity, the objective of this study was to investigate the effect of Delta(9)-THC on ICOS in activated mouse T cells. Induction of ICOS mRNA levels by phorbol ester (PMA) plus ionomycin (Io) activation in mouse splenocytes was attenuated by Delta(9)-THC in a concentration-related manner. Similar results were obtained in the mouse T cell line, EL4.IL-2. Anti-CD3/CD28 induced ICOS expression on CD4(+) splenic T cells, which was suppressed by Delta(9)-THC in a time- and concentration-related manner. The PMA/Io-induced icos promoter luciferase reporter activity was also down-regulated by Delta(9)-THC, suggesting that the suppression of ICOS expression by Delta(9)-THC occurs at the transcriptional level. Moreover, transcriptional activation of the NFAT was also down-regulated by Delta(9)-THC as shown by a NFAT luciferase reporter assay, which is consistent with a putative role of NFAT in regulating ICOS expression. Collectively, Delta(9)-THC suppresses ICOS expression in activated T cells, and this suppression may be related, in part, to its modulation of NFAT signaling. The emerging role of ICOS in a wide range of immune-related diseases also suggests that it may represent a potential therapeutic target, which could be modulated by cannabinoid compounds.

T-cell co-stimulatory pathways in autoimmunity

T-cell activation and differentiation depend on the signal strength received by the T-cell receptor and on signals provided by co-stimulatory molecules. The most prominent co-stimulatory molecule is CD28, which controls the activation of naïve and memory T cells by antigen presented on professional antigen-presenting cells. Blocking of the CD28-CD80/86 pathway has been an appealing strategy for inducing tolerance in autoimmune diseases where the disease-inducing autoantigens are not known. Although CD28 has maintained its unique position, the past decade has witnessed the recognition that a large number of regulatory molecules on T cells must be stimulated to generate a fully protective immune response. These regulatory receptors differ in their preferential expression on T-cell subsets, in the ligands that they recognize, and in the signaling pathways that they trigger. They have in common the fact that they provide information on the cellular environment in which the T-cell response occurs. By intercepting these signals, we may be able to influence disease-relevant T-cell responses in autoimmune diseases while potentially minimizing broad immunosuppression.

B and T lymphocyte attenuator regulates T cell survival in the lung

The initiation, intensity, and duration of T cell-directed inflammatory responses are dependent upon the coordination of both activating and inhibitory signals mediated by specific receptors on the T lymphocyte. The recently described receptor, B and T lymphocyte attenuator (BTLA), has been demonstrated to have an important role in limiting the duration of inflammation in a murine model of allergic asthma. In this study, we have examined the role of BTLA on the proliferation, recruitment, and survival of T cells in response to inhaled allergen. We find that there is decreased cell death in T cells from BTLA-deficient mice, whereas proliferation and recruitment to the lungs are unchanged. Thus, the regulation of cell death through BTLA signaling is a key determinant of the inflammatory response in the lung.

Further study of anti-ICOS immunotherapy for rat cardiac allograft rejection

PURPOSE

To study the effect of B7-CD28 costimulatory signal blockade by adenovirus-mediated cytotoxic T-lymphocyte-associated antigen 4 immunoglobulin (AdCTLA-4Ig) on cardiac allograft survival in DA (RT1(a)) to LEW (Lewis RT1(l)) rat combinations.

METHODS

We evaluated the effect of combined AdCTLA-4Ig and anti-inducible costimulator (ICOS) antibody immunotherapy on rat cardiac allograft acceptance.

RESULTS

Unlike AdCTLA-4Ig alone, anti-ICOS immunotherapy combined with AdCTLA-4Ig induced stable tolerance without causing chronic rejection. The combined immunotherapy also prevented the accelerated cardiac rejection caused by donor-type test skin grafting. Immunohistochemical analyses revealed remarkable inflammatory mononuclear cell infiltration with typical vasculopathy, especially ICOS-positive cells in the grafts, in recipients treated with AdCTLA-4Ig alone. In contrast, anti-ICOS therapy combined with AdCTLA-4Ig reduced the ICOS-positive inflammatory cell infiltration of the graft significantly. The most important finding is that possible cardiac arrest caused by secondary donor-type skin graft was prevented by combined immunotherapy of AdCTLA-4Ig and anti-ICOS antibody, despite skin graft rejection.

CONCLUSIONS

Our results identified a major role played by the ICOS-ICOSL pathway in chronic and accelerated cardiac allograft rejection, providing a novel approach to preventing the chronic rejection of vascularized organ allografts.

Semaphorin 6D regulates the late phase of CD4+ T cell primary immune responses

The semaphorin and plexin family of ligand and receptor proteins provides important axon guidance cues required for development. Recent studies have expanded the role of semaphorins and plexins in the regulation of cardiac, circulatory and immune system function. Within the immune system, semaphorins and plexins regulate cell-cell interactions through a complex network of receptor and ligand pairs. Immune cells at different stages of development often express multiple semaphorins and plexins, leading to multivariate interactions, involving more than one ligand and receptor within each functional group. Because of this complexity, the significance of semaphorin and plexin regulation on individual immune cell types has yet to be fully appreciated. In this work, we examined the regulation of T cells by semaphorin 6D. Both in vitro and in vivo T cell stimulation enhanced semaphorin 6D expression. However, semaphorin 6D was only expressed by a majority of T cells during the late phases of activation. Consequently, the targeted disruption of semaphorin 6D receptor-ligand interactions inhibited T cell proliferation at late but not early phases of activation. This proliferation defect was associated with reduced linker of activated T cells protein phosphorylation, which may reflect semaphorin 6D regulation of c-Abl kinase activity. Semaphorin 6D disruption also inhibited expression of CD127, which is required during the multiphase antigen-presenting cell and T cell interactions leading to selection of long-lived lymphocytes. This work reveals a role for semaphorin 6D as a regulator of the late phase of primary immune responses.

JTA-009, a fully human antibody against human AILIM/ICOS, ameliorates graft-vs-host reaction in SCID mice grafted with human PBMCs

OBJECTIVE

Activation-inducible lymphocyte immunomediatory molecule (AILIM; also referred to as inducible costimulator [ICOS]) is the third molecule identified in the CD28 family participating in T-cell activation. AILIM/ICOS has been implicated in both effector and pathogenic T-cell functions, as evidenced by the beneficial effects of AILIM/ICOS blockade in several murine disease models. In the present study, the role of human AILIM/ICOS in T-cell function was investigated using a fully human monoclonal antibody specific to human AILIM/ICOS (JTA-009).

MATERIALS AND METHODS

The effect of JTA-009 on allogenic T-cell proliferation was examined using human mixed lymphocyte reaction (MLR). To investigate the efficacy of AILIM/ICOS blockade in vivo, a graft-vs-host disease (GVHD) model, in which severe combined immunodeficient (SCID) mice were grafted with human peripheral blood mononuclear cells (PBMCs), was used.

RESULTS

In MLR, suppressive effect of JTA-009 on allogenic T-cell proliferation was detected with comparable potency to CD28 blockade by cytotoxic T-lymphocyte antigen 4 (CTLA4)-Ig at an intermediate culture phase. JTA-009 acts as a blocking antibody in vivo and inhibited binding of human AILIM/ICOS to mouse AILIM/ICOS ligand (B7h). Treatment with JTA-009 significantly prolonged survival of mice, with reductions of human interferon-gamma levels in blood and number of human cells in spleens.

CONCLUSION

These results demonstrate that human AILIM/ICOS plays a role in the GVHD pathogenesis mediated by human T cells, and its blockade is attractive for abrogating undesired T-cell responses as is well-documented in mice.

ICOS ligation recruits the p50alpha PI3K regulatory subunit to the immunological synapse

ICOS ligation in concert with TCR stimulation results in strong PI3K activation in T lymphocytes. The ICOS cytoplasmic tail contains an YMFM motif that binds the p85alpha subunit of class IA PI3K, similar to the YMNM motif of CD28, suggesting a redundant function of the two receptors in PI3K signaling. However, ICOS costimulation shows greater PI3K activity than CD28 in T cells. We show in this report that ICOS expression in activated T cells triggers the participation of p50alpha, one of the regulatory subunits of class IA PI3Ks. Using different T-APC cell conjugate systems, we report that p50alpha accumulates at the immunological synapse in activated but not in resting T cells. Our results demonstrate that ICOS membrane expression is involved in this process and that p50alpha plasma membrane accumulation requires a functional YMFM Src homology 2 domain-binding motif in ICOS. We also show that ICOS triggering with its ligand, ICOSL, induces the recruitment of p50alpha at the synapse of T cell/APC conjugates. In association with the p110 catalytic subunit, p50alpha is known to carry a stronger lipid kinase activity compared with p85alpha. Accordingly, we observed that ICOS engagement results in a stronger activation of PI3K. Together, these findings provide evidence that p50alpha is likely a determining factor in ICOS-mediated PI3K activity in T cells. These results also suggest that a differential recruitment and activity of class IA PI3K subunits represents a novel mechanism in the control of PI3K signaling by costimulatory molecules.

ICOS mediates the development of insulin-dependent diabetes mellitus in nonobese diabetic mice

Initiation of diabetes in NOD mice can be mediated by the costimulatory signals received by T cells. The ICOS is found on Ag-experienced T cells where it acts as a potent regulator of T cell responses. To determine the function of ICOS in diabetes, we followed the course of autoimmune disease and examined T cells in ICOS-deficient NOD mice. The presence of ICOS was indispensable for the development of insulitis and hyperglycemia in NOD mice. In T cells, the deletion of ICOS resulted in a decreased production of the Th1 cytokine IFN-gamma, whereas the numbers of regulatory T cells remained unchanged. We conclude that ICOS is critically important for the induction of the autoimmune process that leads to diabetes.

A fundamental role for interleukin-21 in the generation of T follicular helper cells

T cell help to B cells is a fundamental property of adaptive immunity, yet only recently have many of the cellular and molecular mechanisms of T cell help emerged. T follicular helper (Tfh) cells are the CD4(+) T helper cells that provide cognate help to B cells for high-affinity antibody production in germinal centers (GC). Tfh cells produce interleukin-21 (IL-21), and we show that IL-21 was necessary for GC formation. However, the central role of IL-21 in GC formation reflected its effects on Tfh cell generation rather than on B cells. Expression of the inducible costimulator (ICOS) was necessary for optimal production of IL-21, indicative of interplay between these two Tfh cell-expressed molecules. Finally, we demonstrate that IL-21's costimulatory capacity for T helper cell differentiation operated at the level of the T cell receptor signalosome through Vav1, a signaling molecule that controls T cell helper function. This study reveals a previously unappreciated role for Tfh cells in the formation of the GC and isotype switching through a CD4(+) T cell-intrinsic requirement for IL-21.

T cell Ig and mucin domain-1-mediated T cell activation requires recruitment and activation of phosphoinositide 3-kinase

Ligation of the transmembrane protein T cell Ig and mucin domain (Tim)-1 can costimulate T cell activation. Agonistic Abs to Tim-1 are also capable of inducing T cell activation without additional stimuli. However, little is known about the biochemical mechanisms underlying T cell stimulation or costimulation through Tim-1. We show that a tyrosine in Tim-1 becomes phosphorylated in a lck-dependent manner, whereupon it can directly recruit p85 adaptor subunits of PI3K. This results in PI3K activation, which is required for Tim-1 function. We also provide genetic evidence that p85 expression is required for optimal Tim-1 function. Thus, we describe a pathway from Tim-1 tyrosine phosphorylation to the PI3K signaling pathway, which appears to be a major effector of Tim-1-mediated T cell activation.

Critical role of activation-inducible lymphocyte immunomediatory molecule/inducible costimulator in the effector function of human T cells: a comparative in vitro study of effects of its blockade and CD28 blockade in human beings and monkeys

Activation-inducible lymphocyte immunomediatory molecule (AILIM; also referred to as inducible costimulator, ICOS) is the third homolog of the "professional" costimulatory molecule, CD28. To date, the characteristics and role of AILIM/ICOS, especially in effector function of T cells, have been determined through numerous studies in vitro and in vivo using mice. Considering potential differences among species, whether the AILIM/ICOS blockade acts as an efficacious immunomodulator for human diseases remains to be elucidated. In the present study, ability of AILIM/ICOS blockade to modulate immune responses of human and monkey cells was investigated using a fully human antibody (JTA-009), comparing the effect of CD28 blockade. JTA-009 blocked the response of human and monkey T cells co-stimulated with anti-CD3 and AILIM/ICOS ligand, B7h. AILIM/ICOS and CD28 blockade both inhibited human mixed lymphocyte reaction in different fashions, as well as cytokine production in T helper (Th) 1-/Th2-type recall responses. In monkeys however, CD28 blockade by CTLA4-Ig effectively prevented mixed lymphocyte reaction to a greater extent than AILIM/ICOS blockade. These results suggest that AILIM/ICOS blockade is valuable for suppressing both primary allogenic response and recall responses of T cell in human beings, and that there are differences between human and monkey use preferences for costimulatory molecules.

T follicular helper (TFH) cells in normal and dysregulated immune responses

T cell help for antibody production is a fundamental aspect of immune responses. Only recently has a better understanding of the cellular and molecular mechanisms for T cell help emerged. A subset of T cells, termed T follicular helper cells (T(FH) cells), provides a helper function to B cells and represents one of the most numerous and important subsets of effector T cells in lymphoid tissues. T(FH) cells are distinguishable from Th1 and Th2 cells by several criteria, including chemokine receptor expression (CXCR5), location/migration (B cell follicles), and function (B cell help). Central to the function of CD4(+) T cells is IL-21, a "helper" cytokine produced by T(FH) cells that potently stimulates the differentiation of B cells into Ab-forming cells through IL-21R. Consequently, dysregulation of T(FH) cell function, and over- or under-expression of T(FH) cell-associated molecules such as ICOS or IL-21, most likely contributes to the pathogenesis of certain autoimmune diseases or immunodeficiencies.

Induction of B7-H1 and B7-DC expression on airway epithelial cells by the Toll-like receptor 3 agonist double-stranded RNA and human rhinovirus infection: In vivo and in vitro studies

BACKGROUND

T-cell infiltration of the epithelium is a key feature of chronic rhinosinusitis and asthma. Viral infections are an important cause of disease exacerbations. We have found virus-induced expression of T cell-interacting ligands, B7 homolog costimulatory molecules, on airway epithelium.

OBJECTIVE

We tested the ability of human rhinovirus (HRV) 16 and double-stranded RNA (dsRNA) to alter the expression of B7 homologs on human airway epithelial cells.

METHODS

BEAS2B and primary human airway epithelial cells were exposed in vitro to dsRNA (25 microg/mL) or HRV-16, and then expression of cell-surface protein and mRNA for B7 homologs was assessed by means of flow cytometry and real-time PCR, respectively. Additionally, human subjects were infected with HRV-16 in vivo, and mRNA for B7 homologs was assessed by means of real-time PCR in fresh nasal epithelial cell scrapings obtained before and daily up to 4 days after infection.

RESULTS

dsRNA exposure of BEAS2B and human primary bronchial epithelial cells resulted in increased levels of cell-surface and mRNA expression of B7-H1 and B7-DC but not B7-H2 or B7-H3. Exposure of primary cells to HRV-16 resulted in induction of cell-surface expression of B7-H1 and B7-DC. Pretreatment with fluticasone propionate failed to suppress the induction of B7-H1 and B7-DC. Nasal scrapings taken at the time of peak symptom scores (3 days) after infection of 6 human subjects with HRV-16 displayed selective induction of levels of mRNA for B7-H1 and B7-DC.

CONCLUSION

These data show that HRV-16 infection or exposure to dsRNA induces epithelial B7-H1 and B7-DC.

ICOS controls the pool size of effector-memory and regulatory T cells

ICOS is an important regulator of T cell effector function. ICOS-deficient patients as well as knockout mice show severe defects in T cell-dependent B cell responses. Several in vitro and in vivo studies attributed this phenomenon to impaired up-regulation of cell surface communication molecules and cytokine synthesis by ICOS-deficient T cells. However, we now could show with Ag-specific T cells in a murine adoptive transfer system that signaling via ICOS does not significantly affect early T cell activation. Instead, ICOS substantially contributes to the survival and expansion of effector T cells upon local challenge with Ag and adjuvant. Importantly, the observed biological function of ICOS also extends to FoxP3+ regulatory T cells, as can be observed after systemic Ag delivery without adjuvant. In line with these findings, absence of ICOS under homeostatic conditions of nonimmunized mice leads to a reduced number of both effector-memory and FoxP3+ regulatory T cells. Based on these results, we propose a biological role for ICOS as a costimulatory, agonistic molecule for a variety of effector T cells with differing and partly opposing functional roles. This concept may reconcile a number of past in vivo studies with seemingly contradictory results on ICOS function.

CD4+ICOS+ T lymphocytes inhibit T cell activation 'in vitro' and attenuate autoimmune encephalitis 'in vivo'

The inducible co-stimulator (ICOS, CD278) is essential to the efficient development of normal and pathological immune reactions. Since ICOS-deficient mice have enhanced susceptibility to experimental allergic encephalomyelitis (EAE), we have functionally analyzed a CD4+ICOS+ population comprising 6-15% of all CD4+ T cells in secondary lymphoid organs of unmanipulated wild-type mice and checked for their ability to suppress EAE. In C57BL/6 mice, CD4+ICOS+ cells were a major source of cytokines including IFN-gamma, IL-2, IL-4, IL-10 or IL-17A. Upon activation, these cells showed preferentially enhanced production of IL-4 or IL-10 but inhibited IFN-gamma production. In contrast, CD4+ICOS- cells mainly produced IFN-gamma. Interestingly, CD4+ICOS+ cells partially suppressed the proliferation of CD4+ICOS- or CD4+CD25- lymphocytes 'in vitro' by an IL-10-dependent mechanism. Furthermore, CD4+ICOS+ activated and expanded under appropriate conditions yielded a population enriched in cells producing IL-10 and T(h)2 cytokines that also suppressed the proliferation of CD4+CD25- lymphocytes. CD4+ICOS+ cells, before or after expansion in vitro, reduced the severity of EAE when transferred to ICOS-deficient mice. In the same EAE model, lymph node cells from ICOS-deficient mice receiving ICOS+ cells showed reduced IL-17A production and enhanced IL-10 secretion upon antigen activation in vitro. Thus, naturally occurring CD4+ICOS+ cells, expanded or not in vitro, are functionally relevant cells able of protecting ICOS-deficient mice from severe EAE.

PIP3 pathway in regulatory T cells and autoimmunity

Regulatory T cells (Tregs) play an important role in preventing both autoimmune and inflammatory diseases. Many recent studies have focused on defining the signal transduction pathways essential for the development and the function of Tregs. Increasing evidence suggest that T-cell receptor (TCR), interleukin-2 (IL-2) receptor (IL-2R), and co-stimulatory receptor signaling are important in the early development, peripheral homeostasis, and function of Tregs. The phosphoinositide-3 kinase (PI3K)-regulated pathway (PIP3 pathway) is one of the major signaling pathways activated upon TCR, IL-2R, and CD28 stimulation, leading to T-cell activation, proliferation, and cell survival. Activation of the PIP3 pathway is also negatively regulated by two phosphatidylinositol phosphatases SHIP and PTEN. Several mouse models deficient for the molecules involved in PIP3 pathway suggest that impairment of PIP3 signaling leads to dysregulation of immune responses and, in some cases, autoimmunity. This review will summarize the current understanding of the importance of the PIP3 pathway in T-cell signaling and the possible roles this pathway performs in the development and the function of Tregs.

CD86 has sustained costimulatory effects on CD8 T cells

CD80 and CD86 both costimulate T cell activation. Their individual effects in vivo are difficult to study as they are coordinately up-regulated on APCs. We have studied mice expressing rat insulin promoter (RIP)-CD80 and RIP-CD86 on the NOD and NOD.scid genetic background to generate in vivo models, using diabetes as a readout for cytotoxic T cell activation. Accelerated spontaneous diabetes onset was observed in NOD-RIP-CD80 mice and the transfer of diabetes from 6-wk-old NOD mice to NOD.scid-RIP-CD80 mice was greater compared with NOD-RIP-CD86 and NOD.scid-RIP-CD86 mice, respectively. However, the secondary in vivo response was maintained if T cells were activated through CD86 costimulation compared with CD80. This was demonstrated by greater ability to cause recurrent diabetes in NOD-RIP-CD86 diabetic mice transplanted with 6-wk-old NOD islets and adoptively transferred diabetes from diabetic NOD-RIP-CD86 mice to NOD.scid mice. In vitro, CD80 costimulation enhanced cytotoxicity, proliferation, and cytokine secretion in activated CD8 T cells compared with CD86 costimulation. We demonstrated increased CTLA-4 and programmed death-1 inhibitory molecule expression following costimulation by both CD80 and CD86 (CD80 > CD86). Furthermore, T cells stimulated by CD80 were more susceptible to inhibition by CD4(+)CD25(+) T cells. Overall, while CD86 does not stimulate an initial response as strongly as CD80, there is greater sustained activity that is seen even in the absence of continued costimulation. These functions have implications for the engineered use of costimulatory molecules in altering immune responses in a therapeutic setting.

Immunological decision-making: how does the immune system decide to mount a helper T-cell response?

Aberrant T-cell responses underpin a range of diseases, including asthma and allergy and autoimmune diseases. Pivotal immune elements of these diseases are the development of antigen-specific effector T-helper type 2 (Th2) cells, Th1 cells, or the recently defined Th17 cells that are associated with the clinical features and disease progression. In order to identify crucial processes in the pathogenesis of these diseases it is critical to understand how the development of these T cells occurs. The phenotype of a polarized T-cell that differentiates from a naïve precursor is determined by the complex interaction of antigen-presenting cells with naïve T cells and involves a multitude of factors, including the dominant cytokine environment, costimulatory molecules, type and load of antigen presented and a plethora of signaling cascades. The decision to take the immune response in a certain direction is not made by one signal alone, instead many different elements act synergistically, antagonistically and through positive feedback loops to activate a Th1, Th2, or Th17 immune response. The elucidation of the mechanisms of selection of T-cell phenotype will facilitate the development of therapeutic strategies to intervene in the development of deleterious T-cell responses. This review will focus on the pathways and key factors responsible for the differentiation of the various subsets of effector CD4 T cells. We will primarily discuss what is known of the Th1 and Th2 differentiation pathways, while also reviewing the emerging research on Th17 differentiation.

Genetic defects in common variable immunodeficiency

Common variable immunodeficiency (CVID) is the most frequent clinically manifested primary immunodeficiency. According to clinical and laboratory findings, CVID is a heterogeneous group of diseases. Recently, the defects of molecules regulating activation and terminal differentiation of B lymphocytes have been described in some patients with CVID. In this study, we show the overview of deficiencies of inducible costimulator, transmembrane activator and calcium-modulator and cytophilin ligand interactor, CD19 molecules, their genetic basis, pathogenesis and clinical manifestations.

A costimulation-initiated signaling pathway regulates NFATc1 transcription in T lymphocytes

T cell activation and differentiation is accompanied and mediated by transcriptional reprogramming. The NFATc1 transcription factor is strongly induced upon T cell activation and controls numerous genes involved in the T cell effector function. However, its regulation by physiological stimuli in primary T cells has not been well understood. We previously found that ICOS synergizes with TCR and CD28 to greatly enhance NFATc1 expression in primary T cells. In this study, we have examined the signaling mechanisms whereby costimulation regulates NFATc1 expression. We found that CD28 and ICOS regulate sustained PI3K activity in primary T cells, which is required for NFATc1 up-regulation. CD28 and ICOS costimulation, possibly through Itk, a Tec kinase downstream of the PI3K, enhanced phosphorylation of phospholipase C gamma1 and increased and sustained Ca(2+) flux in T cells. Costimulation of T cells potentiated transcription of the Nfatc1 gene P1 promoter in a PI3K-dependent manner. This work demonstrates an important role for costimulatory receptors in sustaining T cell activation programs leading to Nfatc1 gene transcription and has implications in our understanding of the immune response and tolerance.

Inducible co-stimulator (ICOS) is upregulated in experimental autoimmune uveoretinitis

PURPOSE

To study the expression of inducible co-stimulator (ICOS) and its association with T cell effector function in experimental autoimmune uveoretinitis (EAU).

METHODS

Eighteen Lewis rats were immunized by retinal S-antigen (50 microg) emulsified in complete Freund's adjuvant (CFA). Twelve normal rats served as normal controls and 18 receiving injection of CFA and PBS as CFA controls for studying the influence of CFA on the expression of ICOS in CD4+CD25+ T cells. ICOS expression on cells from the spleens, inguinal nodes and retinae on day 0 (normal rats), 7, 13 and 21 was investigated using fluorescent quantitative real-time-PCR and Western blot. Expression of B7RP-1, an ICOS ligand, was also studied by Western blot. The phenotype of the cells from the aforementioned three tissues was identified with flow cytometry using antibodies to ICOS, CD4 and CD25. ICOS+ cells from the lymph nodes, and spleens on day 13 were magnetically sorted and cultured with S-antigen to study the cytokines production with enzyme-linked immunosorbent assay.

RESULT

An obvious uveitis was induced in all the immunized rats on day 13 after S-antigen immunization. The mRNA and protein of ICOS were scarcely detectable in normal rat spleens. In EAU rats, an up-regulation of ICOS could be observed on day 7 and was very pronounced on day 13, followed by a decrease on day 21 in the spleens, draining nodes and retinae. Similarly, B7RP-1 expression seemed to be up-regulated during EAU. Flow cytometry showed that ICOS+ cells were mostly CD4 positive. Kinetics of ICOS+CD4+CD25+ T cells was similar to that of ICOS+ cells. CFA alone was also able to induce increased expression of ICOS in CD4+CD25+ T cells. IFN-gamma was secreted predominantly by ICOS+ T cells.

CONCLUSION

ICOS expression is up-regulated in association with T cell effector capacity in EAU. It is presumed that the ICOS/B7RP-1 costimulatory pathway may play a role in the development of EAU.

The contribution of immunology to the rational design of novel antibacterial vaccines

In most cases, a successful vaccine must induce an immune response that is better than the response invoked by natural infection. Vaccines are still unavailable for several bacterial infections and vaccines to prevent such infections will be best developed on the basis of our increasing insights into the immune response. Knowledge of the signals that determine the best possible acquired immune response against a given pathogen - comprising a profound T- and B-cell memory response as well as long-lived plasma cells - will provide the scientific framework for the rational design of novel antibacterial vaccines.

ICOS and B7 costimulatory molecule expression identifies activated cellular subsets in rheumatoid arthritis

To better define important cell subsets expressing activation markers in rheumatoid arthritis (RA), we compared selective lymphocyte and monocyte B7H1, B7H2, B7RP.1, B7RP.2, and inducible costimulatory molecule (ICOS) expression from normal peripheral blood (NL PB), RA PB, and RA synovial fluid (SF) by multicolor flow cytometry and immunohistochemistry. RA SF memory lymphocytes expressed B7RP.1 and B7RP.2, suggesting that T-cells may function as antigen presenting cells (APCs) in RA joints. We found similar results for ICOS expression. RA SF CD14+ monocytes also expressed B7RP.1 (an ICOS ligand) and the homologous ligand B7RP.2, identifying monocytes as potential mediators of antigen processing and lymphocyte activation in RA. Furthermore, we found an increased population of RA SF CD14+ monocytes expressing B7H1 and B7H2. [The FACS analysis was supported by immunohistochemistry, showing intense lymphocyte and APC (macrophages with dendritic morphology) ICOS staining in RA synovial tissue (ST). Overall, these results define elevated populations of memoryT-lymphocytes expressing proinflammatory B7 molecules in RA SF that either stimulate T cells through ICOS (via ICOS ligands B7RP.1 and B7RP.2), or down-regulate RA ST T-lymphocytes through B7H1 and B7H2.] Therefore, in the same joint, there may exist positive and negative influences on the inflammatory response, and perhaps, the negative signals dominate as joint inflammation resolves.

IL-10 inhibits CD28 and ICOS costimulations of T cells via src homology 2 domain-containing protein tyrosine phosphatase 1

BACKGROUND

Specific T-cell activation requires T-cell receptor stimulation and the generation of costimulatory signals. Major costimulatory signals are delivered to T cells by the interaction of CD28 and inducible costimulator (ICOS).

OBJECTIVE

To investigate the molecular pathways involved in direct T-cell suppression by IL-10.

METHODS

T-cell proliferation analysis, immunoprecipitations, and Western blots were performed after T-cell receptor and CD28 and ICOS stimulations in the absence or presence of IL-10. Dominant-negative src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP-1) overexpression, small inhibitory RNA, and SHP-1-deficient and IL-10-deficient mice were used.

RESULTS

IL-10 receptor-associated tyrosine kinase Tyk-2 acts as a constitutive reservoir for SHP-1 in resting T cells, and then tyrosine phosphorylates SHP-1 on IL-10 binding. SHP-1 rapidly binds to CD28 and ICOS costimulatory receptors and dephosphorylates them within minutes. In consequence, the binding of phosphatidylinositol 3-kinase to either costimulatory receptor no longer occurs, and downstream signaling is inhibited. Accordingly, spleen cells from SHP-1-deficient mice showed increased proliferation with CD28 and ICOS stimulation in comparison with wild-type mice, which was not suppressed by IL-10. Generation of dominant-negative SHP-1-overexpressing T cells or silencing of the SHP-1 gene by small inhibitory RNA both altered SHP-1 functions and abolished the T-cell suppressive effect of IL-10.

CONCLUSION

The rapid inhibition of the CD28 or ICOS costimulatory pathways by SHP-1 represents a novel mechanism for direct T-cell suppression by IL-10.

CLINICAL IMPLICATIONS

Molecular mechanisms of direct T-cell suppression by IL-10 may provide a novel target for therapy of allergy/asthma and autoimmune disease.

A critical role for ICOS co-stimulation in immune containment of pulmonary influenza virus infection

Lung pathology observed during influenza infection is due to direct damage resulting from viral replication and bystander damage caused by overly exuberant antiviral immune mechanisms. In the absence of universally effective vaccines and antiviral therapies, knowledge of the cellular components required for immune containment of influenza is essential. ICOS is a late co-stimulatory molecule expressed by T cells 12-24 h after activation. We show for the first time that inhibition of ICOS with a monoclonal antibody reduces pulmonary T cell inflammation and associated cytokine expression. Surprisingly however, this reduction in T cells was not accompanied by an alleviation of weight loss and illness. Furthermore, lung viral titres were elevated following anti-ICOS treatment, suggesting that the beneficial outcome of reducing T cell pathology was masked by enhanced virus-induced damage and innate inflammation. This study demonstrates the delicate balance that exists between pathogen burden and pulmonary T cell inflammation during influenza infection and highlights the critical role of ICOS in this response.

VSIG4, a B7 family-related protein, is a negative regulator of T cell activation

T cell activation by APCs is positively and negatively regulated by members of the B7 family. We have identified a previously unknown function for B7 family-related protein V-set and Ig domain-containing 4 (VSIG4). In vitro experiments using VSIG4-Ig fusion molecules showed that VSIG4 is a strong negative regulator of murine and human T cell proliferation and IL-2 production. Administration to mice of soluble VSIG4-Ig fusion molecules reduced the induction of T cell responses in vivo and inhibited the production of Th cell-dependent IgG responses. Unlike that of B7 family members, surface expression of VSIG4 was restricted to resting tissue macrophages and absent upon activation by LPS or in autoimmune inflammatory foci. The specific expression of VSIG4 on resting macrophages in tissue suggests that this inhibitory ligand may be important for the maintenance of T cell unresponsiveness in healthy tissues.

Role of nitric oxide in the regulation of T cell functions

There is a close relation between T helper (Th) 1 cells and nitric oxide in disease. Thus it is possible that a reciprocal regulatory mechanism exists between them. This paper briefly describes the experimental studies which have helped elucidate the mechanism by which nitric oxide selectively enhances Th 1 cell proliferation and the potential effect of nitric oxide on regulatory T (Treg) cells. On the basis of the results the authors propose that nitric oxide represents an additional signal for the induction of T cell subset response, contributing to the increasingly complex network of immune regulation essential for health and disease.

Tec kinase Itk forms membrane clusters specifically in the vicinity of recruiting receptors

The Tec family of tyrosine kinases transduces signals from antigen and other receptors in cells of the hematopoietic system. In particular, interleukin-2 inducible T cell kinase (Itk) plays an important role in modulating T cell development and activation. Itk is activated by receptors via a phosphatidylinositol 3-kinase-mediated pathway, which results in recruitment of Itk to the plasma membrane via its pleckstrin homology domain. We show here that membrane localization of Itk results in the formation of clusters of at least two molecules within 80 A of each other, which is dependent on the integrity of its pleckstrin homology domain. By contrast, the proline-rich region within the Tec homology domain, SH3 or SH2 domains, or kinase activity were not required for this event. More importantly, these clusters of Itk molecules form in distinct regions of the plasma membrane as only receptors that recruit phosphatidylinositol 3-kinase reside in the same membrane vicinity as the recruited Itk. Our results indicate that Itk forms dimers in the membrane and that receptors that recruit Itk do so to specific membrane regions.

Costimulation blockade and its possible future use in clinical transplantation

The nonimmune effects of currently used immunosuppressive drugs result in a high incidence of late graft loss due to nephrotoxicity and death of patients. As an immune-specific alternative to conventional immunosuppressants, new biotechnology tools can be used to block the costimulation signals of T-cell activation. Many experimental studies--particularly preclinical studies in nonhuman primates--have focused on blocking the 'classical' B7/CD28 and CD40/CD40L pathways, which are critical in primary T-cell activation. Here, we review the limitations, the recent advances and the first large-scale clinical application of the CTLA4-Ig fusion protein to block the B7/CD28 costimulation pathway. We also focus on new B7/CD28 and tumor necrosis factor (TNF)/TNF-R family costimulatory molecules that can deliver positive or negative costimulation signals regulating the alloimmune response. Strategies that use single agents to block costimulation have often proved to be insufficient. Given the diversity of the different costimulation molecules, future strategies for human transplantation may involve the simultaneous blockade of several selected pathways or the simultaneous use of conventional immunosuppressants.

ICOS cooperates with CD28, IL-2, and IFN-γ and modulates activation of human naïve CD4+ T cells

Several sets of data indicate that ICOS regulates cytokine production in activated T cells, but is less effective on naïve T cells. This work evaluates ICOS function in human naïve CD4+ T cells through an assessment of the effect of soluble forms of the ICOS and CD28 physiological ligands on activation driven by anti-CD3 mAb. ICOS strikingly potentiated secretion of IL-2, IFN-gamma, IL-10, and TNF-alpha, but not IL-4, promoted by optimal stimulation of CD3+CD28, and it was the key switching-factor of activation when cells received suboptimal stimulation of CD3+CD28 or stimulation of CD3 alone in the presence of exogenous IL-2. In these conditions, blockade of IL-2 and IFN-gamma showed that ICOS builds up a positive feedback loop with IFN-gamma, which required IL-2 and was inhibited by IL-4. By contrast, in the absence of CD28 triggering or exogenous IL-2, ICOS-induced costimulation mainly supported expression of TGF-beta1 and FoxP3 and differentiation of regulatory T cells capable to inhibit proliferation of naïve CD4+ T cells driven by allogeneic cells. These data suggest that ICOS favors differentiation of Th effector cells when cooperates with appropriate activation stimuli such as CD3+CD28 or CD3+IL-2, whereas it supports differentiation of regulatory T cells when costimulatory signals are insufficient.