Single administration of recombinant IL-6 restores the gene expression of lipogenic enzymes in liver of fasting IL-6-deficient mice.

BACKGROUND AND PURPOSE: Lipogenesis is intimately controlled by hormones and cytokines as well as nutritional conditions. IL-6 participates in the regulation of fatty acid metabolism in the liver. We investigated the role of IL-6 in mediating fasting/re-feeding changes in the expression of hepatic lipogenic enzymes. EXPERIMENTAL APPROACH: Gene and protein expression of lipogenic enzymes were examined in livers of wild-type (WT) and IL-6-deficient (IL-6(-/-) ) mice during fasting and re-feeding conditions. Effects of exogenous IL-6 administration on gene expression of these enzymes were evaluated in vivo. The involvement of STAT3 in mediating these IL-6 responses was investigated by using siRNA in human HepG2 cells. KEY RESULTS: During feeding, the up-regulation in the hepatic expression of lipogenic genes presented similar time kinetics in WT and IL-6(-/-) mice. During fasting, expression of lipogenic genes decreased gradually over time in both strains, although the initial drop was more marked in IL-6(-/-) mice. Protein levels of hepatic lipogenic enzymes were lower in IL-6(-/-) than in WT mice at the end of the fasting period. In WT, circulating IL-6 levels paralleled gene expression of hepatic lipogenic enzymes. IL-6 administration in vivo and in vitro showed that IL-6-mediated signalling was associated with the up-regulation of hepatic lipogenic enzyme genes. Moreover, silencing STAT3 in HepG2 cells attenuated IL-6 mediated up-regulation of lipogenic gene transcription levels. CONCLUSIONS AND IMPLICATIONS: IL-6 sustains levels of hepatic lipogenic enzymes during fasting through activation of STAT3. Our findings indicate that clinical use of STAT3-associated signalling cytokines, particularly against steatosis, should be undertaken with caution.

Diagnostic difficulties in glucokinase hyperinsulinism.

Glucokinase hyperinsulinism is a rare variant of congenital hyperinsulinism caused by activating mutations in the glucokinase gene and has been reported so far to be a result of overactivity of glucokinase within the pancreatic beta-cell. Here we report on a new patient with difficulties to diagnose persistent hyperinsulinism and discuss diagnostic procedures of this as well as the other reported individuals. After neonatal hypoglycemia, the patient was reevaluated at the age of 3 years for developmental delay. Morning glucose after overnight fast was 2.5-3.6 mmol/l. Fasting tests revealed supressed insulin secretion at the end of fasting (1.4-14.5 pmol/l). In addition, diagnostic data of the patients reported so far were reviewed. A novel heterozygous missense mutation in exon 10 c.1354G>C (p.Val452Leu) was found and functional studies confirmed the activating mutation. There was no single consistent diagnostic criterion found for our patient and glucokinase hyperinsulinism individuals in general. Often at the time of hypoglycemia low insulin levels were found. Therefore insulin concentrations at hypoglycemia, or during fasting test as well as reactive hypoglycemia after an oral glucose tolerance test were not conclusive for all patients. A glucose lowering effect in extra-pancreatic tissues independent from hyperinsulinism that results in diagnostic difficulties may contribute to underestimation of glucokinase hyperinsulinism. Mutational analysis of the GCK-gene should be performed in all individuals with unclear episodes of hypoglycemia even without documented hyperinsulinism during hypoglycemia. Delay of diagnosis might result in mental handicap of the affected individuals.

Presence of functional cannabinoid receptors in human endocrine pancreas.

AIMS/HYPOTHESIS: We examined the presence of functional cannabinoid receptors 1 and 2 (CB1, CB2) in isolated human islets, phenotyped the cells producing cannabinoid receptors and analysed the actions of selective cannabinoid receptor agonists on insulin, glucagon and somatostatin secretion in vitro. We also described the localisation on islet cells of: (1) the endocannabinoid-producing enzymes N-acyl-phosphatidyl ethanolamine-hydrolysing phospholipase D and diacylglycerol lipase; and (2) the endocannabinoid-degrading enzymes fatty acid amidohydrolase and monoacyl glycerol lipase. METHODS: Real-time PCR, western blotting and immunocytochemistry were used to analyse the presence of endocannabinoid-related proteins and genes. Static secretion experiments were used to examine the effects of activating CB1 or CB2 on insulin, glucagon and somatostatin secretion and to measure changes in 2-arachidonoylglycerol (2-AG) levels within islets. Analyses were performed in isolated human islets and in paraffin-embedded sections of human pancreas. RESULTS: Human islets of Langerhans expressed CB1 and CB2 (also known as CNR1 and CNR2) mRNA and CB1 and CB2 proteins, and also the machinery involved in synthesis and degradation of 2-AG (the most abundant endocannabinoid, levels of which were modulated by glucose). Immunofluorescence revealed that CB1 was densely located in glucagon-secreting alpha cells and less so in insulin-secreting beta cells. CB2 was densely present in somatostatin-secreting delta cells, but absent in alpha and beta cells. In vitro experiments revealed that CB1 stimulation enhanced insulin and glucagon secretion, while CB2 agonism lowered glucose-dependent insulin secretion, showing these cannabinoid receptors to be functional. CONCLUSIONS/INTERPRETATION: Together, these results suggest a role for endogenous endocannabinoid signalling in regulation of endocrine secretion in the human pancreas.

Liver expression of proteins controlling interferon-mediated signalling as predictive factors in the response to therapy in patients with hepatitis C virus infection.

Combination therapy with interferon-alpha (IFNalpha) and ribavirin is the current treatment of choice for hepatitis C virus (HCV) infection. However, an important number of patients fail to respond to this therapeutic strategy. Factors determining IFN responsiveness are not well understood, and assessment of biomarkers that predict the response to IFN therapy in HCV patients is necessary. Several studies show that particular HCV proteins are able to block IFN function through interaction with important IFN-signal mediators, such as signal transducers and activators of transcription (STATs). We performed immunostaining analysis of STATs in liver tissue from IFN-responder vs. non-responder HCV patients in order to compare the expression profile of these proteins between both groups. Tissue arrays of liver biopsies were used to study the expression of STAT1, STAT2, STAT5 and PIAS1 (protein inhibitor of activated STAT1). Robust and higher expression levels of STAT1, STAT2 and STAT5 in liver tissue from HCV patients were found when compared with samples from healthy donors. However, no significant differences were observed between IFN-responder and -non-responder groups, but rather increasing levels of STAT1, STAT2 and STAT5 paralleled the degree of liver injury. Importantly, PIAS1 expression in the nucleus of most hepatocytes in HCV tissue biopsy sections, particularly of non-responder HCV patients, strongly indicated a regulatory effect on STAT1-DNA binding, likely affecting the IFN late signalling. In conclusion, our evidence indicates that intense PIAS1 nuclear staining, widely distributed in hepatocytes of infected livers, could be a good predictive factor of a defective response to IFN treatment, and a biomarker that is easily detectable by immunostaining during standard histopathological liver biopsy analysis.

The proliferative and antiapoptotic actions of growth hormone and insulin-like growth factor-1 are mediated through distinct signaling pathways in the Pro-B Ba/F3 cell line.

Biological actions of GH can be direct or mediated through insulin-like growth factor I (IGF-I). In the interleukin-3 (IL-3)-dependent Ba/F3 cell line, IGF-I induces cell cycle entry and proliferation. Ba/F3 cells expressing the rat GH receptor (Ba/F3 GHR cells) have been shown to escape from apoptosis and to proliferate under GH stimulation. Using the Ba/F3 GHR cell model, we sought to dissect the signals elicited specifically by IGF-I or GH. In contrast to IGF-I or IL-3, GH is able to maintain cell cycle entry of Ba/F3 GHR cells cultured for 7 days in the absence of serum. The presence of IGF-I messenger RNA was not detected by RT-PCR, and by RIA, IGF-I was not found in culture medium of Ba/F3 GHR cells, unstimulated or stimulated by GH. Moreover, the addition of an anti-IGF-I antibody that blocks IGF-I effects suggests that the actions of GH are not mediated by IGF-I, but appear to be direct. GH or IGF-I stimulation increased expression of cyclins A and D(1) with comparable kinetics, whereas expression of p21(waf1/cip1) seemed delayed in IGF-I-stimulated cells compared with that in GH-stimulated cells. Contrary to GH or IL-3, IGF-I did not induce nuclear factor-kappaB DNA-binding activity in Ba/F3 cells. Inhibition of nuclear factor-kappaB through expression of the mutant IkappaBalpha (A32/36) abrogated the GH-mediated survival signal, but did not result in alterations of the cell cycle in Ba/F3 GHR cells treated with IGF-I. Phosphatidylinositol 3-kinase was required for both survival and proliferative responses to IGF-I. Transfection of a dominant negative form of AKT (AH-AKT) resulted in suppression of IGF-I-mediated cell survival, but not of the antiapoptotic effect of GH in Ba/F3 GHR cells. Thus, GH and IGF-I are able to promote cell survival and proliferation through independent and different pathways in Ba/F3 cells.

Growth hormone exerts antiapoptotic and proliferative effects through two different pathways involving nuclear factor-kappaB and phosphatidylinositol 3-kinase.

Dependence of murine pro-B Ba/F3 cells on interleukin-3 can be substituted by GH when cells are stably transfected with the GH receptor (GHR) complementary DNA. Recently, we demonstrated that Ba/F3 cells produce GH, which is responsible for the survival of cells expressing the GHR. This GH effect involves the activation of nuclear factor-kappaB (NF-kappaB). Here, we examined the signaling pathways mediating proliferation of growth factor-deprived Ba/F3 GHR cells. Exogenous GH stimulation of Ba/F3 GHR cells induced cyclins E and A and the cyclin-dependent kinase inhibitor p21(waf1/cip1) and repressed cyclin-dependent kinase inhibitor p27(kip1). The presence of the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor Ly 294002 abolished proliferation induced by GH, arresting Ba/F3 GHR cells at the G(1)/S boundary, but did not promote apoptosis. Thus, the proliferative effect of GH is closely related to PI 3-kinase activation, whereas PI 3-kinase is not essential for GH-induced cell survival. Addition of Ly 294002 resulted in a moderate decrease in NF-kappaB activation by GH, suggesting a possible link between PI 3-kinase and NF-kappaB signaling by GH. Expression of c-myc was also induced by GH in Ba/F3 GHR cells, and inactivation of either PI 3-kinase or NF-kappaB reduced this induction. Overexpression of the dominant negative repressor mutant c-Myc-RX resulted in an inhibition of the GH proliferative effect, suggesting the involvement of c-myc in GH-induced proliferation. Taken together, these results suggest that the effects of GH on cell survival and proliferation are mediated through two different signaling pathways, NF-kappaB and PI 3-kinase, respectively; although cross-talk between them has not been excluded. NF-kappaB, which has been shown to be responsible for the antiapoptotic effect of GH, could also participate in GH-induced proliferation, as c-myc expression is promoted by PI 3-kinase, in an NF-kappaB-dependent and -independent manner.

Growth hormone prevents apoptosis through activation of nuclear factor-kappaB in interleukin-3-dependent Ba/F3 cell line.

The pro-B Ba/F3 cell line requires interleukin-3 and serum for growth, and their removal results in cell apoptosis. Ba/F3 cells transfected with the GH receptor (GHR) cDNA become able to proliferate in response to GH. To investigate the role of GH in the control of apoptosis, Ba/F3 cells expressing either the wild-type rat GHR (Ba/F3 GHR) or a mutated rat GHR (Ba/F3 ILV/T) were used. We show that Ba/F3 GHR cells, but not parental Ba/F3 or Ba/F3 ILV/T cells, were able to survive in the absence of growth factor. Furthermore, an autocrine/paracrine mode of GH action was suggested by the demonstration that Ba/F3 cells produce GH, and that addition of GH antagonists (B2036 and G120K) promotes apoptosis of Ba/F3 GHR cells. Consistent with survival, the levels of both antiapoptotic proteins Bcl-2 and Bag-1 were maintained in Ba/F3 GHR cells, but not in parental Ba/F3 cells upon growth factor deprivation. Constitutive activation of the transcription factor nuclear factor-kappaB (NF-kappaB), which has been shown to promote cell survival, was sustained in Ba/F3 GHR cells, whereas no NF-kappaB activation was detected in parental Ba/F3 cells in the absence of growth factor. Furthermore, addition of GH induced NF-kappaB DNA binding activity in Ba/F3 GHR cells. Overexpression of the mutated IkappaB alpha (A32/36) protein, known to inhibit NF-kappaB activity, resulted in death of growth factor-deprived Ba/F3 GHR cells, and addition of GH was no longer able to rescue these cells from apoptosis. Together, our results provide evidence for a new GH-mediated pathway that initiates a survival signal through activation of the transcription factor NF-kappaB and sustained levels of the antiapoptotic proteins Bcl-2 and Bag-1.

Prolactin blocks glucocorticoid induced cell death by inhibiting the disruption of the mitochondrial membrane.

Prolactin (PRL) has been reported to inhibit dexamethasone (Dex) induced cell death. Nevertheless, the mechanism through which PRL exerts its protective effect is still not unravelled. Here, we analyse the effect of PRL at different stages of the glucocorticoid (GC) apoptotic pathway in PRL dependent cells (Nb2 cells). PRL blocks completely the GC induced loss of the mitochondrial transmembrane potential (delta psi(m)) and consequently phosphatidylserine (PS) exposure and loss of DNA content. Although PRL promotes an upregulation of the bcl-2 expression, simultaneous addition of PRL to GC fails to maintain even the normal levels of this anti-apoptotic protein. This finding excludes a critical role for bcl-2 in the PRL protective effect against GC. GC induced delta psi(m) disruption can be inhibited by the ICE-like inhibitor zVAD-fmk but not by ICE inhibitor tetrapeptide acetyl-Tyr-Val-Ala-Asp.chloromethylketone (YVAD-cmk) nor by caspase-3 inhibitor zDEVD. It can be speculated that PRL blocks delta psi(m) disruption by inhibiting an unknown caspase activated by GC.

Vaccinia virus-induced apoptosis in immature B lymphocytes: role of cellular Bcl-2.

Apoptosis is a form of physiological cell death which can be initiated in response to various stimuli including virus infections. We show that vaccinia virus (VV) infection induces apoptosis in an immature B lymphocyte line, WEHI-231. In these cells, several VV-specific proteins were synthesized during the infection, but neither virus production nor viral DNA synthesis were detected. The intracellular levels of the proto-oncogene Bcl-2, which effectively protects cells from programmed cell death, were found to be down-regulated by the VV infection, suggesting that this down-regulation might be involved in the viral induction of apoptosis in WEHI-231 cells. Stable transfectants overexpressing human Bcl-2 were shown to be resistant to the apoptosis produced by the infection, a finding consistent with the proposed role for the down-regulation of endogenous Bcl-2 in VV-induced apoptotic death.

B cell receptor cross-linking prevents Fas-induced cell death by inactivating the IL-1 beta-converting enzyme protease and regulating Bcl-2/Bcl-x expression.

In the A20 cell line, we examined the mechanisms that modulate the Fas-mediated apoptotic pathway through the B cell receptor. As in other systems, Fas signaling activates cysteine proteases, leading to specific proteolysis of poly(ADP-ribose) polymerase (PARP) and protein kinase C (PKC) delta. We describe that PKC-epsilon and PKC-zeta proteins are two new IL-1 beta-converting enzyme (ICE) substrates; we found that ICE activation and its proteolytic effects are inhibited by surface IgG (sIgG) cross-linking. Apoptosis induced by Fas ligation is consequently abrogated after sIgG engagement, and sIgG signaling therefore interferes with the apoptotic signal upstream of ICE protease activation. Since the PKC inhibitor bisindolylmaleimide I completely abolishes the protective effect of the sIgG signal, a member of the PKC family is probably responsible for the prevention of ICE cascade activation. Direct activation of PKC by PMA partially mimics the protective effect of sIgG cross-linking against Fas-mediated death in A20 cells. Nevertheless, PMA inhibits neither ICE activation nor the subsequent proteolysis of ICE substrates, suggesting that the PKC responsible for ICE inactivation is a non-PMA-sensitive PKC. In this system, Fas ligation also triggers Bcl-2/Bcl-x down-regulation, an effect inhibited by sIgG cross-linking, the cysteine protease inhibitor acetyl-Tyr-Val-Ala-Asp-chloromethyl ketone, and PMA treatment. In A20 cells, Fas signaling may thus trigger both ICE activation and Bcl-x and Bcl-2 down-regulation. These results indicate that sIgG signaling gives rise to two pathways after PKC activation, one presumably promoted by non-PMA-sensitive PKC, which inactivates the ICE cascade, and another produced by PMA-sensitive PKC, which maintains normal Bcl-2/Bcl-x levels.

Inhibition of apoptosis by the African swine fever virus Bcl-2 homologue: role of the BH1 domain.

The function of the African swine fever virus (ASFV) bcl-2 homologue, gene A179L, in the regulation of apoptosis was investigated using as a model system the human myeloid leukemia cell line K562 induced to die by apoptosis with inhibitors of macromolecular synthesis, a process that is prevented by overexpression of human bcl-2. It is shown that transfection of K562 cells with the ASFV A179L gene protects these cells from apoptotic cell death induced by a combination of cycloheximide and actinomycin D or by treatment with cytosine arabinoside. To test the functional role of the highly conserved BH1 domain present in the A179L protein, the Gly residue at position 85 was mutated to Ala, since it has been shown that substitution of the corresponding Gly in human Bcl-2 abrogates its death-repressor activity. It was found that the Gly-to-Ala mutation in the BH1 domain of the viral protein abolished its capacity to protect the K562 cells from apoptosis, indicating that this Gly is essential for A179L action. This finding stresses the functional similarity of the BH1 domains of the viral protein and cellular Bcl-2.

CD2 ligation abrogates antigen-independent apoptosis in B cells.

We have described recently the prevention of apoptosis by CD2-soluble CD48 interaction on antigen B cell receptor occupancy. Here, we show that CD2 ligation is also able to interfere with B cell receptor-independent apoptosis pathways such as spontaneous death in spleen B cells or serum deprivation and hydrogen peroxide exposure in the BAL-17 cell line. In all cases, CD2 ligation induces a signal that prevents the downregulation of Bcl-2 expression. The specific CD2 signal pathway involved in this phenomenon is still unknown. As reported, CD2 did not appear to induce Ca2+ mobilization, phosphatidylinositol turnover, or PKC translocation in B cells. Nevertheless, we show that CD2 receptor ligation is coupled to the tyrosine phosphorylation pathway in B cells. These observations indicate that CD2 is functionally able to trigger at least an early signal that could play a role in apoptosis blockage B cells in addition to the adhesion one. The results suggest the participation of cellular membrane receptors other that CD40 in apoptosis rescue, not only in the antigen-dependent but also in the antigen-independent phases of B cell lymphopoiesis.

Decrease in cAMP levels promoted by CD48-CD2 interaction correlates with inhibition of apoptosis in B cells.

The authors recently reported that CD2 ligation rescues B cells from antigen-induced apoptosis by upregulation of intracellular Bcl-2 levels. However, the characterization of the early signals involved in apoptosis rescue by CD2 ligation has not been well established. In this context, CD2 does not promote either phosphatidylinositol turnover or CA2+ mobilization in B cells. In this paper the authors show that CD2 interaction with its ligand CD48 also reduces the apoptosis induced by forskolin and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine and, to a much lesser extent, the apoptosis induced by cholera toxin in murine B splenocytes. Using a cAMP detection system sensitive to the picomolar range, the authors demonstrate that CD2-CD48 interaction decreases the intracellular cAMP concentrations induced by forskolin but not by cholera toxin. In comparison with the CD2-CD48 interaction, CD40-CD40 ligand interaction completely inhibits the apoptosis induced by cAMP increases without affecting the intracellular cAMP levels promoted by forskolin or cholera toxin. These results indicate that CD2 can also control the apoptosis at the very early steps after receptor signalling, such as the adenylate cyclase activity. Given that heterotrimeric G-proteins can mediate the adenylate cyclase activity the authors suggest that CD2 signalling could act through these small proteins, which would explain the inability of CD2 signalling to rescue from the apoptosis induced by cholera toxin, a Gs-protein activator. Conversely, CD40 seems to control apoptosis further downstream of the cAMP-PKA pathway where the survival and apoptotic signals are confluent, which might therefore render it a more efficient system to block apoptosis.

Pertussis toxin-sensitive GTP-binding proteins regulate activation-induced apoptotic cell death of human natural killer cells.

Apoptosis of natural killer (NK) cells can be induced by non-specific physical damage (UV irradiation, heat shock) or by simultaneous ligation of the CD16 and the interleukin-2 receptor (IL-2R) molecules, but not with either anti-CD16 or IL-2 alone. Whereas blockade of GTP-binding protein (G protein)-mediated signal transduction using ADP-ribosylating bacterial toxins or the GTPase-resistant GTP analog guanosine 5'-0-(3-thiotriphosphate (GTP gamma S) does not affect non-specific induction of NK cell apoptosis, such interventions do inhibit induction of apoptosis by anti-CD16/IL-2. The G proteins involved in the regulation of activation-induced NK apoptosis are sensitive to pertussis toxin (PTX) and to the non-specific GTP analog GTP gamma S but not to cholera toxin, Pseudomonas exotoxin A or diphtheria toxin. A pertussis toxin mutant that lacks ADP-ribosylating activity, but conserves the membrane translocating and T cell-mitogenic effects of the native molecule, fails to inhibit NK apoptosis. To exert their apoptosis-inhibitory effect, PTX and GTP gamma S must be employed before cells are activated. Later addition has no effect, suggesting the implication of G proteins in the transmission of apoptosis-inducing signals, but not in the effector stage of apoptosis. Pre-incubation with PTX or GTP gamma S does not affect the activation of NK cells by CD16 cross-linking, IL-2 stimulation- or both, as assessed by the induction of CD69 expression, protein tyrosine phosphorylation and calcium mobilization. Moreover, neither PTX nor GTP gamma S compromise the effector function of NK cells or the susceptibility of target cells to NK-mediated lysis. These data suggest apoptosis as a novel mechanism by which NK responses may be controlled in vivo, as well as an experimental and therapeutical strategy to counteract endogenous down-regulation of NK responses.

Pertussis toxin interferes with superantigen-induced deletion of peripheral T cells without affecting T cell activation in vivo. Inhibition of deletion and associated programmed cell death depends on ADP-ribosyltransferase activity.

Intravenous injection of a bacterial superantigen such as Staphylococcus aureus enterotoxin B (SEB) causes transient activation and expansion of SEB-reactive V beta 8+ T cells, as well as specific down-regulation of the immune response, through partial deletion of superantigen-reactive T cells. Here we demonstrate that co-administration of pertussis toxin (PTX) and SEB reduces the SEB-induced deletion of V beta 8+ T cells, although it does not affect T cell activation and proliferation. PTX abrogates the SEB-driven deletion of V beta 8+CD4+ (not V beta 8+CD8+) splenocytes that is observed early (12-24 h) after SEB injection. Moreover, it antagonizes the late (> or = 4 days) deletion of V beta 8+CD4+ and V beta 8+CD8+ peripheral T cells that follows transient expansion of such cells. This phenomenon is associated with significant reductions in apoptosis and endonucleolysis and is not caused by a compensatory increase in proliferation of SEB-reactive T cells, as we determined by using a combined fluorometric analysis of cell cycle and DNA alterations, which are associated with programmed cell death. These effects are also observed in thymectomized animals, thus excluding the possibility that PTX might act by enhancing the maturation and export of thymic T cells to the periphery. Moreover, the SEB-induced reduction of V beta 8+ splenocytes is antagonized by PTX in vitro. The capacity of PTX to reduce clonal deletion depends critically on its ADP-ribosyltransferase activity, inasmuch as a non-enzymatic PTX mutant fails to act in this biologic system. We conclude that PTX selectively antagonizes or impedes the delivery of negative signals to T cells, which are stimulated by superantigens, without interfering with the transmission of stimulatory signals.

Differential in vivo effects of a superantigen and an antibody targeted to the same T cell receptor. Activation-induced cell death vs passive macrophage-dependent deletion.

Superantigens have multiple pleiotropic effects in vivo, causing the activation, proliferation, and deletion of specific T cells. In our study, we analyzed the effects of the bacterial superantigen Staphylococcus aureus enterotoxin B (SEB) on peripheral T cells in vivo. As an internal control we took advantage of a IgG2a mAb, F23.1 (anti-V beta 8), that recognizes products from the same V beta gene family as that recognized by SEB. Suprisingly, not only SEB, but also F23.1 primes peripheral T cells to undergo oligonucleosomal DNA fragmentation typical for programmed cell death (PCD). Nonetheless the deletion and induction of PCD imposed by both agents obey rather different principles. First, SEB, not F23.1-induced PCD, concerns T cells that have passed through the S phase of the cell cycle, as demonstrated by experiments in which the thymidine analogue 5-bromo-2'desoxyuridine was detected in mono- and oligonucleosomal fragments of T cells undergoing PCD. Second, deletion of V beta 8+ T cells induced by SEB, not F23.1, can be blocked in vivo by high doses of retinol and, during the early phase, by glucocorticoid receptor blockade with RU-38486. Inasmuch as retinol fails to antagonize the glucocorticoid-induced PCD, at least two pathways are involved in early SEB-driven deletion, one that depends on the presence of endogenous glucocorticoid, and another that can be inhibited by retinol. Third, depletion of phagocytes in vivo by means of liposome-encapsulated dichloromethylene diphosphonate does not impede the activation and deletion of V beta 8+ cells by SEB, although it partially prevents the elimination of T cells binding F23.1 in vivo. Thus, macrophages are not rate-limiting for the action of SEB. In a further series of experiments, we demonstrate that SEB causes the secretion of a variety of cytokines (IL-1, -2, -4, -10, granulocyte-macrophage-CSF, IFN-gamma, and TNF) that may cause lethal septic shock. In contrast, F23.1 that efficiently induces all these mediators in vitro, fails to do so in vivo. In synthesis, the elimination of T cells induced by two different agents specific for V beta 8 obeys different principles: activation-induced cell death in the case of SEB and passive macrophage-mediated elimination in the case of F23.1.

Characterization of the lymphocyte activation gene 3-encoded protein. A new ligand for human leukocyte antigen class II antigens.

The lymphocyte activation gene 3 (LAG-3), expressed in human activated T and natural killer (NK) cells, is closely related to CD4 at the gene and protein levels. We report here the initial characterization of the LAG-3-encoded protein. We have generated two monoclonal antibodies after immunization of mice with a 30-amino acid peptide that corresponds to an exposed extra loop region present in the LAG-3 immunoglobulin-like first domain. The reactivity of these reagents is directed against LAG-3 since they recognize both membrane-expressed and soluble recombinant LAG-3 molecules produced in a baculovirus expression system. The two antibodies are likely to react with the same or closely related epitope (termed LAG-3.1) exposed on the LAG-3 first domain extra loop, as assessed in competition experiments on LAG-3-expressing activated lymphocytes. Cellular distribution analysis indicated that the LAG-3.1 epitope is expressed on activated T (both CD4+ and CD8+ subsets) and NK cells, and not on activated B cells or monocytes. In immunoprecipitation experiments performed on activated T and NK cell lysates, a 70-kD protein was detected after SDS-PAGE analysis. 45-kD protein species were also immunoprecipitated. Both the 70- and 45-kD proteins were shown to be N-glycosylated. In Western blot analysis, only the former molecule was recognized by the anti-LAG-3 antibodies, demonstrating that it is LAG-3 encoded. These anti-LAG-3 antibodies were used to investigate whether the LAG-3 protein interacts with the CD4 ligands. By using a high-level expression cellular system based on COS-7 cell transfection with recombinant CDM8 vectors and a quantitative cellular adhesion assay, we demonstrate that rosette formation between LAG-3-transfected COS-7 cells and human leukocyte antigen (HLA) class II-bearing B lymphocytes is specifically dependent on LAG-3/HLA class II interaction. In contrast to CD4, LAG-3 does not bind the human immunodeficiency virus gp120. This initial characterization will guide further studies on the functions of this molecule, which may play an important role in immune responses mediated by T and NK lymphocytes.