The outcome of T-cell responses after T-cell encounter with specific antigens is modulated by co-stimulatory signals, which are required for both lymphocyte activation and development of adaptive immunity. ICOS, an inducible co-stimulator with homology to CD28, is expressed on activated, but not resting T cells, and shows T-cell co-stimulatory function in vitro. ICOS binds specifically to its counter-receptor B7RP-1 (refs 5,6,7), but not to B7-1 or B7-2. Here we provide in vivo genetic evidence that ICOS delivers a co-stimulatory signal that is essential both for efficient interaction between T and B cells and for normal antibody responses to T-cell-dependent antigens. To determine the physiological function of ICOS, we generated and characterized gene-targeted ICOS-deficient mice. In vivo, a lack of ICOS results in severely deficient T-cell-dependent B-cell responses. Germinal centre formation is impaired and immunoglobulin class switching, including production of allergy-mediating IgE, is defective. ICOS-deficient T cells primed in in vivo and restimulated in vitro with specific antigen produce only low levels of interleukin-4, but remain fully competent to produce interferon-gamma.

Activation of 'initiator' (or 'apical') caspases-2, -8 or -9 (refs 1-3) is crucial for induction of apoptosis. These caspases function to activate executioner caspapses that, in turn, orchestrate apoptotic cell death. Here, we show that a cell-permeable, biotinylated pan-caspase inhibitor (bVAD-fmk) both inhibited and 'trapped' the apical caspase activated when apoptosis was triggered. As expected, only caspase-8 was trapped in response to ligation of death receptors, whereas only caspase-9 was trapped in response to a variety of other apoptosis-inducing agents. Caspase-2 was exclusively activated in heat shock-induced apoptosis. This activation of caspase-2 was also observed in cells protected from heat-shock-induced apoptosis by Bcl-2 or Bcl-xL. Reduced sensitivity to heat-shock-induced death was observed in caspase-2(-/-) cells. Furthermore, cells lacking the adapter molecule RAIDD failed to activate caspase-2 after heat shock treatment and showed resistance to apoptosis in this setting. This approach unambiguously identifies the apical caspase activated in response to apoptotic stimuli, and establishes caspase-2 as a proximal mediator of heat shock-induced apoptosis.

Costimulation through the inducible costimulator (ICOS) and its ligand (ICOSL) is essential for T cell-dependent B cell responses, but the cellular and temporal dynamics underlying its in vivo effects are poorly defined. Here we have shown that Icosl(-/-) and Icos(-/-) mice had similar phenotypes and that ICOS-ICOSL costimulation modulated the early but not late phases of IgG1 affinity maturation. Exploiting the adoptive transfer of T or B cells from primed Icosl(-/-) mice, we provided genetic evidence that costimulation through ICOSL was essential for primary but not secondary helper T cell responses and for the control of both T and B cell activities, resulting in T cell-dependent IgG1 production.

The p53 tumor suppressor promotes cell cycle arrest or apoptosis in response to diverse stress stimuli. p53-mediated cell death depends in large part on transcriptional up-regulation of target genes. One of these targets, P53-induced protein with a death domain (PIDD), was shown to function as a mediator of p53-dependent apoptosis. Here we show that PIDD is a cytoplasmic protein, and that PIDD-induced apoptosis and growth suppression in embryonic fibroblasts depend on the adaptor protein receptorinteracting protein (RIP)-associated ICH-1͞CED-3 homologous protein with a death domain (RAIDD). We provide evidence that PIDD-induced cell death is associated with the early activation of caspase-2 and later activation of caspase-3 and -7. Our results also show that caspase-2 ؊/؊ , in contrast to RAIDD ؊/؊ , mouse embryonic fibroblasts, are only partially resistant to PIDD. Our findings suggest that caspase-2 contributes to PIDD-mediated cell death, but that it is not the sole effector of this pathway.caspase-2 T he tumor suppressor p53 is a sequence-specific transcription factor that promotes cell cycle arrest or apoptosis in response to cellular stress (1). Transcriptional activation of the p21 WAF1 cyclin-dependent kinase inhibitor plays a key role in the induction of cell cycle arrest by p53 (2). p53-dependent apoptosis is regulated, at least in part, by transcriptional activation of its target genes (3), and this process highly depends on cytochrome c release and the Apaf-1͞caspase-9 activation pathway (4, 5). Although a number of candidate p53-effector molecules have been reported, it is yet unclear whether each contributes a part of the full response, or whether specific subsets of these genes are required for death in different cell types or in response to different signals (3).Among the identified apoptotic effectors of p53, P53-induced protein with a death domain (PIDD)͞leucine-rich DD (LRDD) is a 915-aa protein in mice (910 aa in humans) containing seven tandem LR repeats in the N terminus and a DD in the C terminus (6, 7). The dual domain structure of PIDD suggests that it may function as a key adapter protein that links additional components of the p53 apoptosis pathway. Using the method of differential display, PIDD was identified as a p53-up-regulated gene in a p53-null Friend-virus-transformed mouse erythroleukemia cell line (DP16.1͞p53ts) that stably expresses a temperature-sensitive (ts) Trp-53 mutant allele. DP16.1͞p53ts cells undergo apoptosis after expression of the wild-type p53 conformation at 32°C. PIDD mRNA is induced by ␥-irradiation in a p53-dependent manner, and the basal level of PIDD mRNA depends on p53 gene status. Overexpression of PIDD also inhibits cell growth in a p53-like manner by inducing apoptosis. Antisense inhibition of PIDD expression was shown to attenuate apoptosis in response to p53 activation and DNA damage, suggesting that PIDD expression is required for p53-dependent death (7).Recently, PIDD was found to be present in a large protein complex containing caspase-2 and the ad...

SummaryT cell receptor recognition of antigen can lead either to T lymphocyte differentiation and proliferation or to a state of unresponsiveness, which is dependent on whether appropriate costimulatory signals are provided to the mature T cell. We have investigated a novel intracellular signaling pathway provided by the costimulatory molecule CD28. CD28 engagement triggers the activation of an acidic sphingomyelinase (A-SMase), which results in the generation of ceramide, an important lipid messenger intc~i~ediate. A-SMase activation by CD28 occurred in resting as well as in activated primary T cells or leukemic Jurkat cells. In contrast, ligation of either CD3 or CD2 did not result in A-SMase activation. Overexpression of recombinant A-SMase in Jurkat T cells substituted for CD28 with regard to nuclear factor-gB activation. These data suggest that CD28 provides an important costimulatory signal by activation of an acidic sphingomyelinase pathway.

Rapid activation of immune responses is necessary for antibacterial defense, but excessive immune activation can result in life-threatening septic shock. Understanding how these processes are balanced may provide novel therapeutic potential in treating inflammatory disease. Fc receptors are crucial for innate immune activation. However, the role of the putative Fc receptor for IgM, known as Toso/Faim3, has to this point been unclear. In this study, we generated Toso-deficient mice and used them to uncover a critical regulatory function of Toso in innate immune activation. Development of innate immune cells was intact in the absence of Toso, but Toso-deficient neutrophils exhibited more reactive oxygen species production and reduced phagocytosis of pathogens compared with controls. Cytokine production was also decreased in Toso −/− mice compared with WT animals, rendering them resistant to septic shock induced by lipopolysaccharide. However, Toso −/− mice also displayed limited cytokine production after infection with the bacterium Listeria monocytogenes that was correlated with elevated presence of Listeria throughout the body. Accordingly, Toso −/− mice succumbed to infections of L. monocytogenes , whereas WT mice successfully eliminated the infection. Taken together, our data reveal Toso to be a unique regulator of innate immune responses during bacterial infection and septic shock.

Superior vena cava syndrome results from the obstruction of blood flow through the superior vena cava and is most often due to thoracic malignancy. However, benign etiologies are on the rise secondary to more frequent use of intravascular devices such as central venous catheters and pacemakers. Although rarely a medical emergency, the symptoms can be alarming, particularly to the patient. Traditionally, superior vena cava syndrome has been managed with radiotherapy and chemotherapy. But interventional endovascular techniques have made inroads that offer a safe, rapid, and durable response. In many cases, it may be the only reasonable treatment. Because of this, an approach to endovascular treatment of this condition must be in the armamentarium of the interventional radiologist. This review will provide the reader with an insight into the etiology, pathophysiology, and various management principles of superior vena cava syndrome. The focus will be on understanding the techniques used during various endovascular interventions, including angioplasty, stenting, and pharmacomechanical thrombolysis. Discussion will also be centred on possible complications and current evidence as well as controversies regarding these approaches.

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