ICOS costimulates invariant NKT cell activation

It has been reported that costimulatory molecules, CD80/86-CD28 and CD154-CD40, critically contribute to activation of CD1d-restricted invariant NKT (iNKT) cells. Here we have demonstrated that ICOS, a new member of the CD28 family, plays a substantial role in iNKT cell activation. iNKT cells constitutively expressed ICOS as well as CD28 independently, and ICOS expression was further up-regulated 2-3 days after alpha-galactosylceramide (alpha-GalCer) treatment. Blockade of ICOS-mediated costimulation by administration of anti-ICOS ligand (B7RP-1) mAb or by ICOS gene knockout substantially inhibited alpha-GalCer-induced IFN-gamma and IL-4 production, cytotoxic activity, and anti-metastatic effect. Moreover, blockade of both B7RP-1-ICOS and CD80/86-CD28 interactions mostly abolished the alpha-GalCer-induced immune responses. These findings indicate that iNKT cell activation is regulated by CD28 and IOCS independently.     

4-1BB engagement costimulates NKT cell activation and exacerbates NKT cell ligand-induced airway hyperresponsiveness and inflammation

Multiple studies have demonstrated that 4-1BB (CD137), a member of the TNF receptor superfamily, is expressed on several immune cells including activated T cells. However, the expression and the role of 4-1BB on natural killer T (NKT) cells have not been fully characterized. In this study, it was shown that 4-1BB was not expressed on naive NKT cells but was rapidly induced on activated NKT cells by TCR engagement with alpha-galactosylceramide (alpha-GalCer). Also, 4-1BB signaling provided by 3H3, an agonistic anti-4-1BB mAb, promoted NKT cell activation resulting in enhanced cytokine production of NKT cells driven by alpha-GalCer. When NKT cell-driven airway immune responses were evaluated by intranasal administration of alpha-GalCer, airway hyperresponsiveness (AHR) and lung inflammation were significantly more aggravated in mice treated with 3H3 and alpha-GalCer than in mice treated with alpha-GalCer alone. These aggravations were accompanied by up-regulation of IL-4, IL-13, and IFN-gamma production. Interestingly, AHR was not developed in IL-4Ralpha-deficient mice treated with alpha-GalCer with or without 3H3 but was exacerbated in IFN-gamma-deficient mice. Our study suggests that 4-1BB on NKT cells functions as a costimulatory molecule and exacerbates the induction of NKT cell-mediated AHR, which is dependent on the IL-4Ralpha-mediated pathway.     

Cutting edge: ligation of the glucocorticoid-induced TNF receptor enhances autoreactive CD4+ T cell activation and experimental autoimmune encephalomyelitis

The glucocorticoid-induced TNFR (GITR) is expressed at high levels on resting CD4(+)CD25(+) T regulatory (T(R)) cells and regulates their suppressive phenotype. Accordingly, we show that anti-GITR mAb treatment of SJL mice with proteolipid protein 139-151-induced experimental autoimmune encephalomyelitis significantly exacerbated clinical disease severity and CNS inflammation, and induced elevated levels of Ag-specific T cell proliferation and cytokine production. Interestingly, prior depletion of T(R) cells failed to result in exacerbated experimental autoimmune encephalomyelitis suggesting alternative targets for the anti-GITR mAb treatment. Importantly, naive CD4(+)CD25(-) T cells up-regulated GITR expression in an activation-dependent manner and anti-GITR mAb treatment enhanced the level of CD4(+) T cell activation, proliferation, and cytokine production in the absence of T(R) cells both in vivo and in vitro. Taken together, these findings suggest a dual functional role for GITR as GITR cross-linking both inactivates T(R) cells and increases CD4(+)CD25(-) T cell effector function, thus enhancing T cell immunity.     

Conversion of alloantigen-specific CD8+ T cell anergy to CD8+ T cell priming through in vivo ligation of glucocorticoid-induced TNF receptor

In this study, we investigated the effect of an agonistic mAb (DTA-1) against glucocorticoid-induced TNF receptor (GITR) in a murine model of systemic lupus erythematosus-like chronic graft-vs-host disease (cGVHD). A single dose of DTA-1 inhibited the production of anti-DNA IgG1 autoantibody and the development of glomerulonephritis, typical symptoms of cGVHD. DTA-1-treated mice showed clinical and pathological signs of acute GVHD (aGVHD), such as lymphopenia, loss of body weight, increase of donor cell engraftment, and intestinal damage, indicating that DTA-1 shifted cGVHD toward aGVHD. The conversion of cGVHD to aGVHD occurred because DTA-1 prevented donor CD8+ T cell anergy. Functionally active donor CD8+ T cells produced high levels of IFN-gamma and had an elevated CTL activity against host Ags. In in vitro MLR, anergic responder CD8+ T cells were generated, and DTA-1 stimulated the activation of these anergic CD8+ T cells. We further confirmed in vivo that donor CD8+ T cells, but not donor CD4+ T cells, were responsible for the DTA-1-mediated conversion of cGVHD to aGVHD. These results indicate that donor CD8+ T cell anergy is a restriction factor in the development of aGVHD and that in vivo ligation of GITR prevents CD8+ T cell anergy by activating donor CD8+ T cells that otherwise become anergic. In sum, our data suggest GITR as an important costimulatory molecule regulating cGVHD vs aGVHD and as a target for therapeutic intervention in a variety of related diseases.     

Principles of antibody-mediated TNF receptor activation

From the beginning of research on receptors of the tumor necrosis factor (TNF) receptor superfamily (TNFRSF), agonistic antibodies have been used to stimulate TNFRSF receptors in vitro and in vivo. Indeed, CD95, one of the first cloned TNFRSF receptors, was solely identified as the target of cell death-inducing antibodies. Early on, it became evident from in vitro studies that valency and Fcγ receptor (FcγR) binding of antibodies targeting TNFRSF receptors can be of crucial relevance for agonistic activity. TNFRSF receptor-specific antibodies of the IgM subclass and secondary cross-linked or aggregation prone dimeric antibodies typically display superior agonistic activity compared with dimeric antibodies. Likewise, anchoring of antibodies to cell surface-expressed FcγRs potentiate their ability to trigger TNFRSF receptor signaling. However, only recently has the relevance of oligomerization and FcγR binding for the in vivo activity of antibody-induced TNFRSF receptor activation been straightforwardly demonstrated in vivo. This review discusses the crucial role of oligomerization and/or FcγR binding for antibody-mediated TNFRSF receptor stimulation in light of current models of TNFRSF receptor activation and especially the overwhelming relevance of these issues for the rational development of therapeutic TNFRSF receptor-targeting antibodies.     

Distinct differences between TNF receptor 1- and TNF receptor 2-mediated activation of NFkappaB

Tumor necrosis factor (TNF) signaling is mediated via two distinct receptors, TNFR2 and TNFR1, which shows partially overlapping signaling mechanisms and biological roles. In the present study, TNFR2 and TNFR1 signal transduction mechanisms involved in activation of NFkappaB and CMV promoter-enhancer were compared with respect to their susceptibility towards inhibitors of intracellular signaling. For this, we used SW480 cells, where we have shown that TNF-signaling can occur independently through each of the two receptors. The TNFR1 response was inhibited by D609, bromophenacyl bromide (BPB), nordihydroguararetic acid (NDGA), and by sodium salicylate, while TNFR2-mediated activation of NFkappaB and CMV promoter-enhancer was resistant to these compounds. The signaling mechanisms known to be affected by these inhibitors include phospholipases as well as redox- and pH-sensitive intracellular components. Our results imply that TNFR2 signaling involved in NFkappaB activation proceeds independently of these inhibitor-sensitive signaling components, indicating distinct signaling pathways not shared with TNFR1.     

Stimulation of the glucocorticoid-induced TNF receptor family-related receptor on CD8 T cells induces protective and high-avidity T cell responses to tumor-specific antigens

Treatment of tumor-bearing mice with a stimulatory Ab to glucocorticoid-induced TNFR family-related receptor (GITR) has previously been shown to elicit protective T cell responses against poorly immunogenic tumors. However, the role of GITR stimulation on CD8 T cells and the nature of tumor rejection Ags have yet to be determined. In this study, we show that a stimulatory mAb to GITR (clone DTA-1) acts directly on CD8 T cells, but not on CD4(+)CD25(+) regulatory T (T(reg)) cells, in B16 tumor-bearing mice to induce concomitant immunity against secondary B16 tumors, as well as protective memory following surgical excision of the primary tumor. Melanoma growth itself induced GITR expression on tumor-specific CD8 T cells, providing a mechanism whereby these cells may respond to stimulatory anti-GITR. Unexpectedly, in contrast to T(reg) cell depletion therapy with anti-CD4, GITR stimulation induced very weak CD8 T cell responses to melanocyte differentiation Ags expressed by the tumor, and did not induce autoimmune vitiligo. Accordingly, GITR-stimulated hosts that were primed with B16 melanoma rejected B16, but not the unrelated JBRH melanoma, indicating that tumor rejection Ags are tumor-specific rather than shared. In support of this, we show that GITR stimulation induces CD8 T cell responses to a tumor-specific Ag, and that these responses are of higher functional avidity compared with those induced by T(reg) cell depletion. We conclude that stimulation of GITR on effector CD8 T cells results in high-avidity T cell responses to tumor-specific Ags, thereby inducing potent antitumor immunity in the absence of autoimmunity.     

IL-21 is produced by NKT cells and modulates NKT cell activation and cytokine production

The common gamma-chain cytokine, IL-21, is produced by CD4(+) T cells and mediates potent effects on a variety of immune cells including NK, T, and B cells. NKT cells express the receptor for IL-21; however, the effect of this cytokine on NKT cell function has not been studied. We show that IL-21 on its own enhances survival of NKT cells in vitro, and IL-21 increases the proliferation of NKT cells in combination with IL-2 or IL-15, and particularly with the CD1d-restricted glycosphingolipid Ag alpha-galactosylceramide. Similar to its effects on NK cells, IL-21 enhances NKT cell granular morphology, including granzyme B expression, and some inhibitory NK receptors, including Ly49C/I and CD94. IL-21 also enhanced NKT cell cytokine production in response to anti-CD3/CD28 in vitro. Furthermore, NKT cells may be subject to autocrine IL-21-mediated stimulation because they are potent producers of this cytokine following in vitro stimulation via CD3 and CD28, particularly in conjunction with IL-12 or following in vivo stimulation with alpha-galactosylceramide. Indeed, NKT cells produced much higher levels of IL-21 than conventional CD4 T cells in this assay. This study demonstrates that NKT cells are potentially a major source of IL-21, and that IL-21 may be an important factor in NKT cell-mediated immune regulation, both in its effects on NK, T, and B cells, as well as direct effects on NKT cells themselves. The influence of IL-21 in NKT cell-dependent models of tumor rejection, microbial clearance, autoimmunity, and allergy should be the subject of future investigations.     

Glucocorticoid-induced TNF receptor functions as a costimulatory receptor that promotes survival in early phases of T cell activation

Glucocorticoid-induced TNFR (GITR) is a member of the TNFR family that can inhibit the suppressive function of regulatory T cells and promote the survival and activation of T cells. However, little is known about the molecular mechanisms regulating T cell survival and activation downstream of GITR. To gain further insight into the cellular events and signaling pathways triggered by GITR, survival, proliferation, and cytokine production as well as activation of MAPKs and NF-kappaB were monitored after cross-linking of the receptor on naive and activated T cells. GITR cross-linking provided costimulation of naive and activated T cells and resulted in activation of MAPKs and NF-kappaB. Although GITR-induced signaling pathways augmented the survival of naive T cells, they were not sufficient to inhibit activation-induced cell death triggered by CD3 cross-linking of activated T cells. Differences in the contributions of GITR to cell survival between naive and activated T cells suggest that the receptor triggers specific pathways depending on the activation state of the T cell.     

Ethanol blocks leukocyte recruitment and endothelial cell activation in vivo and in vitro

Immune system impairment and increased susceptibility to infection among alcohol abusers is a significant but not well-understood problem. We hypothesized that acute ethanol administration would inhibit leukocyte recruitment and endothelial cell activation during inflammation and infection. Using LPS and carrageenan air pouch models in mice, we found that physiological concentrations of ethanol (1-5 g/kg) significantly blocked leukocyte recruitment (50-90%). Because endothelial cell activation and immune cell-endothelial cell interactions are critical regulators of leukocyte recruitment, we analyzed the effect of acute ethanol exposure on endothelial cell activation in vivo using the localized Shwartzman reaction model. In this model, ethanol markedly suppressed leukocyte accumulation and endothelial cell adhesion molecule expression in a dose-dependent manner. Finally, we examined the direct effects of ethanol on endothelial cell activation and leukocyte-endothelial cell interactions in vitro. Ethanol, at concentrations within the range found in human blood after acute exposure and below the levels that induce cytotoxicity (0.1-0.5%), did not induce endothelial cell activation, but significantly inhibited TNF-mediated endothelial cell activation, as measured by adhesion molecule (E-selectin, ICAM-1, VCAM-1) expression and chemokine (IL-8, MCP-1, RANTES) production and leukocyte adhesion in vitro. Studies exploring the potential mechanism by which ethanol suppresses endothelial cell activation revealed that ethanol blocked NF-kappaB nuclear entry in an IkappaBalpha-dependent manner. These findings support the hypothesis that acute ethanol overexposure may increase the risk of infection and inhibit the host inflammatory response, in part, by blocking endothelial cell activation and subsequent immune cell-endothelial cell interactions required for efficient immune cell recruitment.     

Macrophage activation in vivo and in vitro

Morphology, lysosomal enzyme activity and phagocytic ability were tested in peritoneal macrophage cultures after stimulation in vivo or in vitro with endotoxin, mineral oil or latex particles, and compared to the same parameters in normal peritoneal macrophages. Treatment with latex did not give changes in the parameters tested after in vivo or in vitro stimulation. In all other types of stimulation the cells displayed varying degrees of spreading and changes in granule content. Extensive ruffling of cell membrane was obvious in endotoxin-stimulated cells. The pattern of lysosomal enzyme activity was complex and depended on the means of stimulation. Acid phosphatase showed the greatest increase after both in vivo and in vitro stimulation, N-acetyl-glucosaminidase could not be increased in vitro. Internalization of opsonized red cells mediated by the Fc receptor increased after in vivo stimulation. No such change was observed after in vitro stimulation. Normal peritoneal macrophages do not internalize significantly via the C₃ receptor. In vivo stimulation triggered the capacity to internalize up to 45% of the attached red cells. A similar reaction was obtained in vitro when both endotoxin and FCS were added to the culture medium, but not when endotoxin or FCS were used alone. We conclude that the use of the term activation of macrophages should always be based on quantitative changes in well defined parameters. Changes in one parameter will not necessarily be accompanied by the whole range of biochemical and morphological perturbations. The capacity to ingest via the C₃ receptor may be the most useful parameter.     

IL-10 inhibits vascular smooth muscle cell activation in vitro and in vivo

The anti-inflammatory cytokine IL-10 inhibits intimal hyperplasia after stent implantation via a powerful inactivation of monocytes. We tested the hypothesis that IL-10 may also inhibit vascular smooth muscle cell (SMC) activation via the inhibition of the NF-kappaB/I-kappaB system. The IL-10 receptor was detected in rat SMCs in vitro and in vivo. In LPS-stimulated rat SMCs, 1 ng/ml recombinant murine IL-10 (mIL-10) reduced I-kappaBalpha and I-kappaBbeta degradation, NF-kappaB activation, as well as the expression of the NF-kappaB-dependent gene IL-6 by 32%, 31%, 75%, and 19%, respectively (P < 0.05 for all). Similar results were obtained in vivo 6 h and 4 days after balloon abrasion of the rat aorta, a model in which intimal hyperplasia results essentially from SMC activation. Moreover, mIL-10 reduced SMC proliferation and migration in vitro (by 60% for both, P < 0.0001), resulting in reduced SMC proliferation and intimal growth 14 days after balloon abrasion of the rat aorta (by 76% and 75%, respectively; P < 0.005). In conclusion, mIL-10 has a direct inhibitory effect on SMCs in vitro and in vivo. This effect is mediated in part by NF-kappaB inactivation and may participate in the overall protective effect of IL-10 on postangioplasty restenosis.     

Signaling lymphocytic activation molecule-associated protein controls NKT cell functions

X-linked lymphoproliferative disease (XLP) is a fatal immunological disorder that typically manifests following EBV infection. XLP patients exhibit a number of immune defects including abnormal T, B, and NK lymphocyte function. These defects have been attributed to mutations of Src homology 2 domain-containing gene 1A (SH2D1A), the gene encoding signaling lymphocytic activation molecule-associated protein (SAP), an intracellular adaptor molecule expressed in lymphocytes. We have observed that SAP knockout (SAPKO) mice and humans with XLP have a complete lack of CD1d-restricted NKT cells. As expected, SAPKO mice injected with the NKT cell agonist, alpha-galactosylceramide failed to generate NKT cell IFN-gamma or IL-4. Furthermore, in contrast to wild-type littermates, SAPKO mice coinjected with OVA and alpha-galactosylceramide failed to mount OVA-specific CTL responses. These data suggest that an absence of NKT cells may underlie part of the immune dysregulation seen in SAPKO mice and in XLP patients.     

NKT cell activation mediates neutrophil IFN-gamma production and renal ischemia-reperfusion injury

Previous work has shown that ischemia-reperfusion (IR) injury (IRI) is dependent on CD4(+) T cells from naive mice acting within 24 h. We hypothesize that NKT cells are key participants in the early innate response in IRI. Kidneys from C57BL/6 mice were subjected to IRI (0.5, 1, 3, and 24 h of reperfusion). After 30 min of reperfusion, we observed a significant increase in CD4(+) cells (145% of control) from single-cell kidney suspensions as measured by flow cytometry. A significant fraction of CD4(+) T cells expressed the activation marker, CD69(+), and adhesion molecule, LFA-1(high). Three hours after reperfusion, kidney IFN-gamma-producing cells were comprised largely of GR-1(+)CD11b(+) neutrophils, but also contained CD1d-restricted NKT cells. Kidney IRI in mice administered Abs to block CD1d, or deplete NKT cells or in mice deficient of NKT cells (Jalpha18(-/-)), was markedly attenuated. These effects were associated with a significant decrease in renal infiltration and, in activation of NKT cells, and a decrease in IFN-gamma-producing neutrophils. The results support the essential role of NKT cells and neutrophils in the innate immune response of renal IRI by mediating neutrophil infiltration and production of IFN-gamma.     

Engagement of the OX-40 receptor in vivo enhances antitumor immunity

The OX-40 receptor (OX-40R), a member of the TNFR family, is primarily expressed on activated CD4+ T lymphocytes. Engagement of the OX-40R, with either OX-40 ligand (OX-40L) or an Ab agonist, delivers a strong costimulatory signal to effector T cells. OX-40R+ T cells isolated from inflammatory lesions in the CNS of animals with experimental autoimmune encephalomyelitis are the cells that respond to autoantigen (myelin basic protein) in vivo. We identified OX-40R+ T cells within primary tumors and tumor-invaded lymph nodes of patients with cancer and hypothesized that they are the tumor-Ag-specific T cells. Therefore, we investigated whether engagement of the OX-40R in vivo during tumor priming would enhance a tumor-specific T cell response. Injection of OX-40L:Ig or anti-OX-40R in vivo during tumor priming resulted in a significant improvement in the percentage of tumor-free survivors (20-55%) in four different murine tumors derived from four separate tissues. This anti-OX-40R effect was dose dependent and accentuated tumor-specific T cell memory. The data suggest that engagement of the OX-40R in vivo augments tumor-specific priming by stimulating/expanding the natural repertoire of the host's tumor-specific CD4+ T cells. The identification of OX-40R+ T cells clustered around human tumor cells in vivo suggests that engagement of the OX-40R may be a practical approach for expanding tumor-reactive T cells and thereby a method to improve tumor immunotherapy in patients with cancer.     

Costimulation via glucocorticoid-induced TNF receptor in both conventional and CD25+ regulatory CD4+ T cells

The glucocorticoid-induced TNF receptor (GITR), which is a member of the TNF receptor family, is expressed preferentially at high levels on CD25+CD4+ regulatory T cells and plays a key role in the peripheral tolerance that is mediated by these cells. GITR is also expressed on conventional CD4+ and CD8+ T cells, and its expression is enhanced rapidly after activation. In this report we show that the GITR provides a potent costimulatory signal to both CD25+ and CD25- CD4+ T cells. GITR-mediated stimulation induced by anti-GITR mAb DTA-1 or GITR ligand transfectants efficiently augmented the proliferation of both CD25-CD4+ and CD25+CD4+ T cells under the limited dose of anti-CD3 stimulation. The augmentation of T cell activation was further confirmed by the enhanced cell cycle progression; early induction of the activation Ags, CD69 and CD25; cytokine production, such as IL-2, IFN-gamma, IL-4, and IL-10; anti-CD3-induced redirected cytotoxicity; and intracellular signaling, assessed by translocation of NF-kappaB components. GITR costimulation showed a potent ability to produce high amounts of IL-10, which resulted in counter-regulation of the enhanced proliferative responses. Our results highlight evidence that GITR acts as a potent and unique costimulator for an early CD4+ T cell activation.     

In vitro and in vivo activation of the insulin receptor kinase in control and denervated skeletal muscle

Skeletal muscle rapidly develops severe insulin resistance following denervation, although insulin binding is unimpaired. Insulin-stimulated receptor tyrosyl kinase activity was studied in intact and 24-h denervated rat hind limb muscles using three preparations: (a) solubilized insulin receptors incubated +/- insulin with gamma-[32P]ATP and histone H2b; (b) soleus muscles prelabeled in vitro with [32P]phosphate with subsequent insulin-stimulated phosphorylation of the receptor in situ; (c) assessment of in vivo activation of muscle receptor tyrosyl kinase by insulin. The latter was achieved by solubilizing muscle insulin receptors in the presence of phosphoprotein phosphatase and kinase inhibitors and measuring receptor-catalyzed histone H2b phosphorylation in the presence of limiting (5 microM) gamma-[32P]ATP. Receptors isolated 5 and 30 min after intravenous insulin injection catalyzed 32P incorporation into histone H2b twice as fast as those from saline-treated controls; insulin stimulated histone H2b labeling exclusively on tyrosine. In vivo activation was demonstrated using solubilized and insulin-agarose-bound receptors. Autophosphorylation of the beta-subunit and receptor tyrosyl kinase activity toward histone H2b was stimulated by insulin in denervated muscles as in controls, although the biological response to insulin, in vitro and in vivo, was markedly impaired after denervation, suggesting a postreceptor defect. The method developed to assess insulin-stimulated receptor activation in vivo seems useful in characterizing mechanisms of insulin resistance.     

TNF type 2 receptor (p75) lowers the threshold of T cell activation.

T cell activation requires a threshold amount of TCR-mediated signals, an amount that is reduced by signals mediated through costimulatory molecules expressed on the T cell surface. Here the role of TNFR2 (p75) as a putative costimulatory receptor for T cell activation was examined. It was found that p75 deficiency in CD8(+) T cells increased the requirements for TCR agonist approximately 5-fold. Furthermore, p75(-/-) T cells display a marked reduction in the proliferative response to TCR agonist. This hypoproliferative response was associated with delayed kinetics of induction of the acute activation markers CD25 and CD69 as well as a marked decrease in the production of IL-2 and IFN-gamma. The net result is that very few cells are recruited into the dividing population. Interestingly, CD28 costimulation was only partially effective in rescuing the proliferative defect of p75(-/-)CD8(+) T cells. Thus, p75 provides an important costimulatory signal in addition to that provided by CD28 toward optimal T cell proliferation.