CD16 cross-linking induces both secretory and extracellular signal-regulated kinase (ERK)-dependent cytosolic phospholipase A2 (PLA2) activity in human natural killer cells: involvement of ERK, but not PLA2, in CD16-triggered granule exocytosis

The phospholipase A2 (PLA2) enzymes play a central role in diverse cellular processes including phospholipid digestion and metabolism, host defense, and cell signaling. We investigated the ability of CD16 clustering to trigger PLA2 and extracellular signal-regulated kinase (ERK) activation in human NK cells, as well as their possible involvement in CD16-stimulated degranulation. Both secretory (sPLA2) and cytosolic (cPLA2) PLA2 were rapidly activated upon CD16 cross-linking; sPLA2 was found in the supernatant and also in a cell-associated form. cPLA2 activation was controlled by the ERK pathway as indicated by the close correlation between their kinetics of activation and by the ability of the specific MEK inhibitor, PD 098059, to abolish cPLA2 activation. CD16 stimulation also resulted in the generation of platelet-activating factor (PAF) and leukotrienes; both phospholipases contributed to their biosynthesis. Using the pharmacologic inhibitors AACOCF3, p-bromophenacyl bromide (pBPB), and PD 098059, which specifically inhibit cPLA2, sPLA2, and MEK, respectively, we demonstrated that the ERK signaling pathway, but not cytosolic or secretory PLA2, is required for CD16-triggered granule release.

Ligation of membrane IgM stimulates a novel c-Jun amino-terminal domain kinase activity in Daudi human B cells

Stress-activated protein kinases (SAPK; also known as JNK for c-Jun N-terminal kinase) phosphorylate Ser63 and Ser73 in the amino-terminus of the c-Jun protein and potentiate its transcriptional activity. We have analysed phosphorylation of GST fusion proteins containing the c-Jun N-terminal domain by lysates of Daudi human B lymphoblastoid cells stimulated with medium or anti-IgM. Crosslinking membrane IgM (mIgM) results in an increase in phosphorylation of GST-c-Jun (5-89) in an antibody dose-dependent manner. The kinase activity specifically phosphorylates the c-Jun N-terminal domain since it does not phosphorylate GST or GST-JunB. The activity preferentially phosphorylates the substrate that contains the sites for in vivo phosphorylation by SAPK/JNK and requires the delta domain of c-Jun, which is also required for SAPK/JNK activity. However, the c-Jun N-terminal kinase activity induced by mIgM ligation is not precipitatable with anti-SAPK/JNK antibodies. In addition, unlike SAPK/JNKs, the mIgM-dependent c-Jun N-terminal kinase activity is not detectable in assays for renaturable kinase activity (in-gel assay) or in assays that test activities that bind to c-Jun (solid-phase assay). The increased phosphorylation of c-Jun N-terminal domain in response to mIgM ligation is unlikely to be due to mIgM-activated ERKs as it was not suppressed by a selective MEK inhibitor. Thus, the mIgM-induced activity is distinct from the known SAPK/JNKs and may represent a novel mechanism for c-Jun phosphorylation in response to mIgM engagement in human B cells.

Induction of aminopeptidase N/CD13 on human lymphocytes after adhesion to fibroblast-like synoviocytes, endothelial cells, epithelial cells, and monocytes/macrophages

Aminopeptidase N (APN/CD13) is a transmembrane ectoenzyme occurring on a wide variety of cells. In contrast to monocytes and granulocytes, lymphocytes of peripheral blood do not express CD13 Ag. However, tumor-infiltrating T cells in renal cell cancer as well as synovial fluid T cells from patients suffering from various forms of arthritis can be CD13 positive. To learn more about expression of CD13 in these tissues, we cocultured lymphocytes with different adherent cell lines. CD13 expression was induced in T and B lymphocytes upon adhesion to fibroblast-like synoviocytes, HUVEC, renal tubular epithelial cells, and monocytes/macrophages but not always upon interaction with different tumor cell lines. Induction of APN was rapid, occurring as early as 1 h after coincubation. Expression persisted for >3 days and partially resisted inhibition by cycloheximide. Fixation of adherent cells with paraformaldehyde could not prevent induction of CD13 in lymphocytes. Soluble APN from human kidneys or placenta could not induce CD13 expression on lymphocytes. Induction of CD13 Ag on lymphocytes required direct cell-to-cell contact as shown in experiments using dual chambers. Lymphocytes exhibited an induction not only in CD13 protein but also in Ala-pNA-cleaving enzyme activity and in CD13 mRNA. Lymphocytic expression of CD13 represents a potentially increased cellular ability to inactivate inflammatory mediators. Furthermore, CD13 could be involved in adhesion, in lymphocytic migration, or in the Ag processing of peptides bound in the groove of MHC class II molecules.

Overexpression of c-fos inhibits down-regulation of a cyclin-dependent kinase-2 inhibitor p27Kip1 in splenic B cells activated by surface Ig cross-linking

Splenic B cells activated by surface Ig (sIg) cross-linking transiently express the c-fos gene within 0.5 h and then enter into S phase of the cell cycle within 48 h. To investigate a role of c-fos in cell cycle progression, we used splenic B cells from IFN-alphabeta-inducible c-fos transgenic mice (Mx-c-fos). In the absence of IFN, the cell cycle progression of Mx-c-fos B cells stimulated with anti-IgM Ab was similar to that in control B cells. The cell cycle was arrested in G1 phase when we added IFN to the culture within 12 h after anti-IgM Ab stimulation, suggesting that overexpression of c-fos until mid-G1 phase perturbs activation of the cell cycle regulatory machinery. In control B cells, cyclin E and cdk2 were induced within 24 to 48 h after stimulation, and this induction was accompanied by down-regulation of a cdk2 inhibitor p27Kip1. As a consequence of these activation processes, cdk2 kinase activity was induced in B cells in the late G1 phase. However, kinase activity was not detected in Mx-c-fos B cells, presumably because the down-regulation of p27 was perturbed. These data suggest that c-Fos can negatively control cell cycle regulatory machinery in sIg-stimulated B cells.

Surface expression of hemopoietic cell phosphatase fails to complement CD45 deficiency and inhibits TCR-mediated signal transduction in a Jurkat T cell clone

Previous studies have shown that the protein tyrosine phosphatase CD45 is required for initiation of signal transduction through several lymphoid receptors. In contrast, there is increasing evidence that another protein tyrosine phosphatase, hemopoietic cell phosphatase (known as HCP, SHP, PTP1C, SHPTP-1, or PTPN6), is a negative regulator of signaling in hemopoietic cells. To determine the effect of HCP on signal transduction through the TCR, HCP was expressed as a chimeric molecule with extracellular and transmembrane domains of the HLA-A2 molecule (A2/HCP) on wild-type Jurkat T cells and the CD45-deficient variant, J45.01. In this report, we show that expression of A2/HCP, unlike A2 chimeras containing the enzymatic regions of CD45, fails to rescue TCR-mediated signal transduction in J45.01. Furthermore, expression of A2/HCP on wild-type Jurkat T cells results in diminished inositol phosphate production following TCR ligation as well as markedly diminished nuclear factor of activated T cells promoter activity. Surprisingly, however, TCR-mediated tyrosine phosphorylation of phospholipase C gamma 1 remains intact in the Jurkat cells expressing the A2/HCP chimera. These experiments provide further evidence that HCP can serve a negative regulatory role in receptor-mediated signaling in immune cells. Additionally, our studies suggest that surface expression of HCP in T cells may provide a means to identify phosphotyrosine-containing proteins that are required for coupling signaling pathways initiated by ligation of the T cell Ag receptor.

Activation of human monocytes through CD40 induces matrix metalloproteinases

The activation of monocytes/macrophages to secrete pro-inflammatory cytokines and matrix metalloproteinases (MMPs) is critically important in the development of chronic inflammatory diseases. However, the consequence of interactions between activated T cells and monocytes in these inflammatory processes is not well understood. In this study we have investigated the induction of MMPs in human monocytes by activated T cells. We show that fixed cells and the cell membranes from a T cell line, BMS-2, that expresses high levels of the CD40 ligand gp39 (also called TRAP, TBAM, or CD40L) stimulate both the expression of mRNA and the production of MMPs by human monocytic cells. Activation of monocytes by the human T cells could be significantly inhibited by a F(ab')2 fragment of a neutralizing Ab specific for human gp39, but not by an Ab that recognizes murine gp39. Furthermore, recombinant soluble gp39 (sgp39) alone induced marked increases in the levels of a 92-kDa metalloproteinase (gelatinase) in both the human monocytic cell line, THP-1, and peripheral human monocytes, and induction was blocked by the anti-human gp39 Ab. Pretreatment with IFN-gamma significantly enhanced gp39 induction of MMPs in THP-1 cells but not in peripheral monocytes. Up-regulation of mRNA for the 92-kDa MMP by gp39 could be detected within 6 h of stimulation and was maximal 24 h after treatment. MMP enzymatic activity was detectable in the culture medium 12 to 18 h following stimulation of the cells and remained high through 48 h. These results suggest the interaction of T cells with monocytes/macrophages via the gp39-CD40 counter receptors may be significant in development or maintenance of chronic inflammatory lesions.

Uncoupling IL-2 production from apoptosis and TNF production by changing the signal through the TCR

T cells may discriminate between stimuli in a variety of ways, including the presence of cytokines or other costimulatory signals, the type of Ag (peptide, superantigen, or allorecognition), or the magnitude of the signal through the TCR. We have used anti-CD3 stimulation of T hybridomas to examine signals generated through the TCR in the absence of exogenous APCs. Soluble whole anti-CD3, but not F(ab')2 anti-CD3, was able to stimulate the T hybridomas to produce IL-2. Plastic-bound anti-CD3, in contrast, stimulated TNF production, G1 arrest, and apoptosis by the T hybridoma. Engagement of the CD4 coreceptor on these cells had no effect on the overall pattern of signaling observed. Although TNF production was correlated with apoptosis, anti-TNF treatment did not prevent cell death or G1 arrest. The response of the T hybridoma to both forms of anti-CD3 included significant IL-2 production even at the lowest dose tested. However, soluble anti-CD3 at the highest dose tested elicited only minor apoptosis, while plastic-bound anti-CD3 elicited significant apoptosis even at the lowest dose. The difference in response was not evident at the level of phosphotyrosine proteins two min after cross-linking of the TCR.

LPS-dependent cyclooxygenase-2 induction in human monocytes is down-regulated by IL-13, but not by IFN-gamma

We investigated the effects of Th2 cell-associated cytokines, IL-4, IL-10, and IL-13, on prostaglandin (PG) production by human peripheral blood monocytes (HPBM) in terms of four parameters: PGE2 synthesis; cyclooxygenase activity; protein; and mRNA of two cyclooxygenase isozymes (cyclooxygenase-1 and cyclooxygenase-2). LPS-stimulated PGE2 synthesis and cyclooxygenase activity were suppressed by IL-4, IL-10, or IL-13. Furthermore, the LPS-dependent increase of cyclooxygenase activity in HPBM was attributable to cyclooxygenase-2 because it was inhibited by NS-398 (a cyclooxygenase-2-specific inhibitor). Western and Northern blot analyses revealed that the LPS-induced increases in cyclooxygenase-2 protein and mRNA were attenuated by the addition of IL-4, IL-10, or IL-13. In contrast, cyclooxygenase-1 protein and mRNA were hardly detected in monocytes that were incubated with or without LPS in the presence or absence of IL-4, IL-10, and IL-13. These results suggest that the reduction of LPS-induced PGE2 synthesis and cyclooxygenase activity by IL-4, IL-10, and IL-13 in HPBM are mainly due to the down-regulation of cyclooxygenase-2 selectively induced by LPS. Conversely, IFN-gamma, a Th1 cell-associated cytokine, did not affect PGE2 production and cyclooxygenase activity. These data suggest a mechanism for modulation of inflammation by the anti-inflammatory Th2 cell-associated cytokines but not a Th1 cell-associated cytokine.

Glycosylphosphatidylinositol toxin of Plasmodium induces nitric oxide synthase expression in macrophages and vascular endothelial cells by a protein tyrosine kinase-dependent and protein kinase C-dependent signaling pathway

In this study, we demonstrate that glycosylphosphatidylinositol (GPI) is a major toxin of Plasmodium falciparum origin responsible for nitric oxide (NO) production in host cells. Purified malarial GPI is sufficient to induce NO release in a time- and dose-dependent manner in macrophages and vascular endothelial cells, and regulates inducible NO synthase expression in macrophages. GPI-induced NO production was blocked by the NO synthase-specific inhibitor L-N-monomethylarginine. GPI also synergizes with IFN-gamma in regulating NO production. The structurally related molecules dipalmitoylphosphatidylinositol and iM4 glycoinositolphospholipid from Leishmania mexicana had no such activity, and the latter antagonized IFN-gamma-induced NO output. GPI activates macrophages by initiating an early onset tyrosine kinase-mediated signaling process, similar to that induced by total parasite extracts. The tyrosine kinase antagonists tyrphostin and genistein inhibited the release of NO by parasite extracts and by GPI, alone or in combination with IFN-gamma, demonstrating the involvement of one or more tyrosine kinases in the signaling cascade. GPI-induced NO release was also blocked by the protein kinase C inhibitor calphostin C, demonstrating a role for protein kinase C in GPI-mediated cell signaling, and by pyrrolidine dithiocarbamate, indicating the involvement of the NF-kappa B/c-rel family of transcription factors in cell activation. A neutralizing mAb to malarial GPI inhibited NO production induced by GPI and total malarial parasite extracts in human vascular endothelial cells and murine macrophages, indicating that GPI is a necessary agent of parasite origin in parasite-induced NO output. Thus, in contrast to dipalmitoylphosphatidylinositol and glycoinositolphospholipids of Leishmania, malarial GPI initiates a protein tyrosine kinase- and protein kinase C-mediated signal transduction pathway, regulating inducible NO synthase expression with the participation of NF-kappa B/c-rel, which leads to macrophage and vascular endothelial cell activation and downstream production of NO. These events may play a role in the etiology of severe malaria.

Nitric oxide-mediated suppression of T cell responses during Trypanosoma brucei infection: soluble trypanosome products and interferon-gamma are synergistic inducers of nitric oxide synthase

African trypanosome infections result in lymphocyte unresponsiveness and anemia in the mammalian host. In murine infections, these effects are mediated by suppressor macrophages releasing nitric oxide (NO). We investigated the mechanism of activation of macrophages to produce NO during trypanosomiasis in vitro. A soluble component of trypanosome lysates induced NO synthesis in peritoneal macrophage cultures only when the macrophages were co-stimulated with interferon-gamma (IFN-gamma). The macrophage-activating factor was also released in a soluble form by live bloodstream-form trypanosomes, but not procyclic trypanosomes. When splenocyte cultures were exposed to IFN-gamma and trypanosomes, an NO-dependent suppression of T cell proliferation occurred. This is similar to the suppression observed in the spleens of trypanosome-infected mice, suggesting that a combination of trypanosome-released macrophage-activating factors and IFN-gamma are a trigger of immune dysfunction in trypanosomiasis.

An inducible lymphocyte nuclear Ca2+/Mg(2+)-dependent endonuclease associated with apoptosis

Apoptotic cell death is typically accompanied by internucleosomal chromatin fragmentation. Although a number of candidate enzymes have been proposed, there is as yet no direct evidence for the involvement of any particular endonuclease in this process. Here we demonstrate the existence of an endonuclease(s) that is up-regulated during apoptotic T cell death. The endonuclease(s) is located in the nucleus, and its activity is increased up to eightfold by a variety of stimuli or conditions that induce apoptosis in T cell hybridomas and thymocytes. Treatments that prevent TCR-mediated apoptosis, such as cyclosporin A or concomitant administration of glucocorticoids, also prevent the induction of enzyme activity. The endonuclease activity is associated with three molecular forms, designated A, B, and C, with apparent M(r) of 49K, 47K, and 45K, respectively, and constitutes the major endonuclease activity in T hybridoma cells. From A exists in resting cells, and its activity is increased threefold after the induction of apoptosis. Forms B and C are absent in resting cells and are induced up to 20-fold after stimuli that lead to apoptosis. All three forms are Ca2+/Mg2+ dependent and are inhibited by Zn2+. This enzyme(s) introduces double strand breaks and single strand nicks into supercoiled plasmid DNA, demonstrating the mode of DNA fragmentation characteristic of products of apoptotic chromatin degradation. The enzyme(s) produces DNA fragments with 5'-P and 3'-OH terminals, also consistent with apoptotic chromatin degradation. Finally, enzyme solubilized from cells activated to die cleaves chromatin in nuclei isolated from unstimulated T hybridoma cells, yielding the classic DNA ladder. Because of its biologic properties, we named this enzyme(s) inducible lymphocyte Ca2+/Mg(2+)-dependent endonuclease, or ILCME. Because inducible lymphocyte Ca2+/Mg(2+)-dependent endonuclease possesses the key features predicted for an apoptosis-specific enzyme, it is a new candidate for an enzyme(s) that participates in DNA cleavage in apoptotic T cells.

Interleukin-6 induces tyrosine phosphorylation of the Ras activating protein Shc, and its complex formation with Grb2 in the human multiple myeloma cell line LP-1

Like many other cytokines and growth factors, interleukin-6 (IL-6) activates p21ras. However, the precise biochemical mechanisms inducing this activation are unknown. Therefore, we investigated the effects of IL-6 on some recently identified signaling intermediates, Shc (Src homology and collagen) and Grb2 (growth factor receptor bound protein 2), known to activate p21ras. In the multiple myeloma cell line LP-1, IL-6 stimulated the tyrosine phosphorylation of Shc in a time- and concentration-dependent manner. This led to the complex formulation of Shc with Grb2, an adaptor protein known to relocate a p21ras-GDP exchange factor. Sos1 (Son-of-sevenless), to the cell membrane. Taken together, these findings suggest that IL-6 might activate the Ras signaling pathway via tyrosine phosphorylation of Shc and subsequent recruitment of Grb2. Further studies will elucidate which of the IL-6 receptor associated non-receptor tyrosine kinases of the Src kinase or Janus kinase family, mediate these effects.

Anti-parasitic effector mechanisms in human brain tumor cells: role of interferon-gamma and tumor necrosis factor-alpha

Toxoplasma gondii, an obligate intracellular parasite, is able to replicate in human brain cells. We recently showed that interferon (IFN)-gamma-activated cells from glioblastoma line 86HG39 were able to restrict Toxoplasma growth. The effector mechanism responsible for this toxoplasmostatic effect was shown by us to be the IFN-gamma-mediated activation of indolamine 2,3-dioxygenase (IDO), resulting in the degradation of the essential amino acid tryptophan. In contrast, glioblastoma 87HG31 was unable to restrict Toxoplasma growth after IFN-gamma activation, and IFN-gamma-mediated IDO activation was weak. We observed that tumor necrosis factor (TNF)-alpha alone is unable to activate IDO or to induce toxoplasmostasis in any glioblastoma cell line tested. Interestingly, we found that TNF-alpha and IFN-gamma were synergistic in the activation of IDO in glioblastoma cells 87HG31, 86HG39 and U373MG and in native astrocytes. This was shown by the measurement of enzyme activity as well as by the detection of IDO mRNA in TNF-alpha + IFN-gamma activated cells. This IDO activity results in a strong toxoplasmostatic effect mediated by glioblastoma cells activated simultaneously by both cytokines. Antibodies directed against TNF-alpha or IFN-gamma were able to inhibit IDO activity as well as the induction of toxoplasmostasis in glioblastoma cells stimulated with both cytokines. Furthermore, it was found that the addition of L-tryptophan to the culture medium completely blocks the antiparasitic effect. We therefore conclude that both TNF-alpha and IFN-gamma may be involved in the defense against cerebral toxoplasmosis by inducing IDO activity as an antiparasitic effector mechanism in brain cells.

CD38 signal transduction in human B cell precursors. Rapid induction of tyrosine phosphorylation, activation of syk tyrosine kinase, and phosphorylation of phospholipase C-gamma and phosphatidylinositol 3-kinase

Ligation of CD38 inhibits proliferation and induces apoptosis of human immature B cells, but the molecular mechanisms underlying this function are unknown. We found that CD38 dimerization with the specific mAbs T16 and IB4 induces rapid and transient tyrosine phosphorylation of several intracellular proteins in the immature B cell lines RS4;11, REH, 380, Nalm6, and OP-1. This effect could be markedly reduced by incubating cells with the tyrosine kinase inhibitors genistein, staurosporine, and herbimycin A. CD38 dimerization induced tyrosine phosphorylation of the protein kinase syk and increased syk kinase activity. CD38 dimerization also induced tyrosine phosphorylation of phospholipase C-gamma and of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-K). The latter was accompanied by a distinct increase in PI 3-kinase activity in the immunoprecipitates obtained with an anti-phosphotyrosine Ab. In contrast to the signaling triggered by surface Ig engagement in B lymphocytes, CD38 ligation did not appear to induce tyrosine phosphorylation of the src-like protein tyrosine kinases lyn, fyn, and btk, or of vav- and ras-GTPase-activating protein, nor did it induce detectable changes in cytosolic CA2+ concentrations. CD38 signaling also differed from cytokine-induced signaling in that it did not cause tyrosine phosphorylation of Jak1 and Jak2. Finally, CD38 ligation did not inhibit IL-3-induced tyrosine phosphorylation of Jak2. These results identify CD38 as a cell surface receptor with signal transduction properties activated by dimerization. Induction of signal transduction by CD38 ligation implies the existence of a yet unidentified natural ligand of CD38.

Ceramide inhibits IgE-mediated activation of phospholipase D, but not of phospholipase C, in rat basophilic leukemia (RBL-2H3) cells

Ceramide, a product of sphingomyelin hydrolysis by sphingomyelinase, elicits various cellular functions and has recently been regarded as a second messenger. To investigate the role of ceramide in rat basophilic leukemia (RBL-2H3) cells, the effects of a cell-permeable analogue, N-acetylsphingosine (C2-ceramide), on Ag-mediated cellular responses were examined. C2-Ceramide inhibited Ag- or PMA-induced activation of phospholipase D (PLD), whereas Ca2+ ionophore A23187-induced PLD activation was not affected. C2-Ceramide failed to inhibit PLD activity in two different in vitro assay systems. Since PLD activity is known to be regulated by several factors, the effects of C2-ceramide on these factors were examined. We have previously reported the possible involvement of protein tyrosine kinase in Ag-mediated PLD activation. However, C2-ceramide had no effect on Ag-induced protein tyrosine phosphorylation, including mitogen-activated protein (MAP) kinases. In fura-2-loaded RBL-2H3 cells, C2-ceramide suppressed Ag-induced Ca2+ influx, leaving initial Ca2+ increase and inositol phosphate production unaffected. Western blot analysis revealed that Ag caused translocation of protein kinase C (PKC) alpha, beta 1, beta 2, delta and epsilon isozymes from cytosol to membrane fraction. Translocation of alpha, beta 1, and beta 2 isozymes was specifically prevented by C2-ceramide. Moreover, C2-ceramide suppressed Ag-induced serotonin release. In the absence of extracellular Ca2+, Ag-induced PLD activation and release reaction were greatly reduced. The inhibitory profile was nearly the same as that obtained in C2-ceramide-treated cells. These results suggest that C2-ceramide inhibits Ag-induced PLD activation and serotonin release, probably through the blockage of Ca2+ influx and translocation of Ca(2+)-dependent PKC isozymes in RBL-2H3 cells.

Interleukin-1 beta induces the expression of hsp70, heme oxygenase and Mn-SOD in FACS-purified rat islet beta-cells, but not in alpha-cells

The cytokine IL-1 beta has previously been demonstrated to induce the expression of the stress genes iNOS, hsp70, heme oxygenase and Mn-SOD in rat pancreatic islets in vitro. The aim of this study was to determine whether the IL-1 beta-induced effects are specific for the insulin producing beta-cell, or whether other islet cells, such as the glucagon-producing alpha-cell, respond to IL-1 beta addition. Purified rat alpha- and beta-cell suspensions were obtained by fluorescence-activated cell sorting and incubated with or without IL-1 beta (25 U/ml) for 24 h. The alpha- and beta-cell contents of hsp70, heme oxygenase and Mn-SOD and medium nitrite levels were determined. It was found that IL-1 beta exposure induced the production of nitric oxide in beta-cells, but not in alpha-cells. Moreover, the expression of hsp70, heme oxygenase and Mn-SOD was also induced in beta-cells, but not in alpha-cells. There were no detectable levels of hsp70 in alpha-cells. It is concluded that the stress gene response following IL-1 beta exposure is markedly different in alpha- and beta-cells. This finding may be of importance for the understanding of the autoimmune destruction of beta-cells in insulin-dependent diabetes mellitus.

Lack of inducible nitric oxide synthase activity in T cell clones and T lymphocytes from naive and Leishmania major-infected mice

Nitric oxide (NO) generated by the inducible isoform of nitric oxide synthase (iNOS) is implicated in a number of immunological processes including killing of intracellular parasites, suppression of T cell proliferation, production of cytokines and destruction of tissue in autoimmune diseases. Considering that cytokine-activated mouse macrophages, fibroblasts and endothelial cells are potent producers of NO, we investigated whether T cells, as central participants in immune responses, can also be activated for the release of NO. Neither thymocytes nor type 1 or type 2 T helper cell clones generated significant amounts of nitrite (the stable end product of NO in culture supernatants) when stimulated by T cell mitogens, cytokines or antigen in the presence of irradiated antigen-presenting cells. Similarly, T cells freshly isolated from mice acutely infected with the intracellular pathogen Leishmania major did not produce NO upon restimulation in vitro. The lack of NO production was not due to the expression of enzymatically inactive iNOS, as we were unable to detect any iNOS protein in activated T helper clones or in freshly isolated T cells from infected mice by Western (protein) blot analysis. Finally, we tested whether iNOS expression in T cells might be restricted to a minor subpopulation and therefore only detectable on a single cell level. After immunofluorescence staining of lymph node or spleen cells from infected mice with antibodies against iNOS, F4/80- or Thy-1-antigen, macrophages, but no T cells, were found to express iNOS. Thus, we have no evidence that activated T helper cell clones or T cells from L. major-infected mice are high producers of NO.

Cells resistant to interferon-beta respond to interferon-gamma via the Stat1-IRF-1 pathway

The mechanism responsible for the induction of the 2-5A synthetase gene by Interferon-gamma (IFN-gamma) (type II) was studied in Friend leukemia cells. It was previously shown that activation of 2-5A synthetase gene expression by IFN-gamma in the 3Cl8 cell, a clone resistant to IFN-alpha,beta (type I), correlates with the formation of two major complexes, designated Fg and Fc, that bind to the interferon-stimulated responsive element of the gene. Conversely, in a clone resistant to both types of IFNs (3 gamma R8), no induction of DNA-protein complexes or of 2-5A synthetase gene expression was detected. In the present report the Fg complex has been characterized as including the interferon regulatory factor 1 (IRF-1), whereas the Fc factor, present also in control cells, has been characterized as composed of IRF-2. Incubation of cell extracts with antibodies to IRF-1 abolishes the formation of the Fg complex, and antibodies to IRF-2 abolish the formation of the Fc complex. Moreover, in the 3Cl8 cell, IFN-gamma is able to induce in few minutes the formation of a complex between a DNA element identified as the IFN-gamma activation site (GAS), present on the IRF-1 gene promoter, and the STAT1 protein. These findings suggest that in cells resistant to type I IFN, IFN-gamma is able, through the activation of the STAT1 protein, to induce the expression of the IRF-1 factor which in turn seems to be sufficient to transactivate the 2-5A synthetase gene.

Both CD28 ligands CD80 (B7-1) and CD86 (B7-2) activate phosphatidylinositol 3-kinase, and wortmannin reveals heterogeneity in the regulation of T cell IL-2 secretion

In this report, the co-stimulatory signals provided by CD80 (B7-1) or CD86 (B7-2) were compared to CD28 ligation by mAb. We demonstrate that while both anti-CD3 and anti-CD28 antibodies induced activation of phosphoinositide (PI) 3-kinase, the kinetics of activation differed. Anti-CD28 produced a sustained activation of PI 3-kinase while anti-CD3 induced activation was transient. Both B7-1 and B7-2 could induce prolonged activation of PI 3-kinase. The co-stimulatory effects of B7-1 and B7-2 were dependent on CD28 cross-linking, based on complete inhibition of PI 3-kinase activation by CD28 antibody Fab fragments. While Jurkat T cells co-stimulated with anti-CD3 and B7-1 or B7-2 secreted high levels of IL-2, there were distinct effects of anti-CD28 mAb and B7-1 or B7-2 on IL-2 secretion in conjunction with protein kinase C activation. To assess functional effects of CD28 ligation, pharmacologic inhibitors of PI 3-kinase were evaluated. In Jurkat cells, efficient inhibition of PI 3-kinase activation after B7-2 stimulation was achieved using wortmannin; however, we observed a surprising increase in IL-2 secretion after B7 or anti-CD28 stimulation. The effect of wortmannin was concentration dependent. Moreover, the effect was specific for receptor-mediated activation as wortmannin did not enhance phorbol ester plus ionomycin-induced IL-2 secretion. Another inhibitor of PI 3-kinase, LY294002, also resulted in augmentation of anti-CD28-induced IL-2 secretion by Jurkat cells. The effects of wortmannin on IL-2 secretion were also examined in primary T cells. In marked contrast, wortmannin resulted in a potent inhibition of anti-CD3 plus B7-1 or anti-CD28-induced IL-2 secretion while phorbol ester plus ionomycin-induced IL-2 secretion was wortmannin resistant. Together these observations demonstrate that signal transduction by both B7-1 and B7-2 involves PI 3-kinase, and that PI 3-kinase or other wortmannin-sensitive targets are important for IL-2 secretion. Finally, treatment of Jurkat cells with PI 3-kinase inhibitors alone was sufficient to induce low levels of IL-2 secretion. This is consistent with the notion that a wortmannin-sensitive target such as PI 3-kinase may down-regulate IL-2 secretion in Jurkat cells.

Alternative models for low dose-response analysis of biochemical and immunological endpoints for tetrachlorodibenzo-p-dioxin

As part of the current reassessment of dioxin, the empirical relationships between administered tetrachlorodibenzo-p-dioxin and selected immune and biochemical endpoints were investigated. A dose-response analysis from the published literature was performed using Linear, Sigmoid-Emax and Power Law functions. The results of this analysis indicate that the use of a wide dose range may bias the interpretation of low-dose phenomenon. The Power Law function was applied exclusively to low-dose subsets enabling estimation of dose response in the low-dose range. Subsequently, high-dose data were fit using Power Law subset analysis. This approach resulted in a change in slope value from low- to high-dose subsets for thymic atrophy, immune suppression, benzo(a)pyrene hydroxylase activity, and ethoxyresorufin-o-deethylase activity. This change suggests that there is a high probability that there is a tissue concentration along the dose-response continuum which results in biological activity from low to high dose. This analysis also demonstrates that the Power Law functional fit to the low-dose data differs quantitatively from the fit to the high-dose data for several noncancer endpoints. Therefore, by using the Power Law function a more accurate and biologically relevant assessment of risk can be produced for non-cancer endpoints.

IL-9 induces expression of granzymes and high-affinity IgE receptor in murine T helper clones

Interleukin 9 (IL-9) is a TH2 cytokine that has been shown to promote the antigen-independent growth of some mouse T helper clones. To characterize the specificity of IL-9-mediated T cell activation, we used a murine T cell clone that could grow with either IL-9 or IL-2. After differential hybridization of a cDNA library, we isolated three genes that were expressed preferentially in the presence of IL-9. Two of them correspond respectively to granzyme A and granzyme B, two proteases expressed by activated T cells. By Northern blot hybridization and functional assays, we found that IL-9 induced the expression of granzyme B in several T cell clones as well as in mast cell lines. In addition, other proteases such as the mouse mast cell proteases were also found to be expressed by IL-9-activated T cell clones. The third IL-9-induced cDNA corresponds to the alpha-chain of the high-affinity receptor for IgE. Several T cell clones expressed this IgE receptor mRNA and were able to bind IgE with high affinity. Taken together, our results indicate that IL-9 induces a mast cell-like phenotype in T cell clones.

Outcome of Kupffer cell antigen presentation to a cloned murine Th1 lymphocyte depends on the inducibility of nitric oxide synthase by IFN-gamma

The interaction between lymphocytes and the resident hepatic macrophage, the Kupffer cell (KC), is relevant to the phenomenon of immune tolerance to Ags entering the liver. Tolerance to Ag administered via the portal vein can be prevented by the rare earth lanthanide metal, gadolinium (Gd). Therefore, we studied the ability of OVA-responsive, H-2d-restricted Th1 clones to proliferate in response to KCs from DBA/2J (H-2d) mice that had been injected with either saline (control) or a Gd solution. Whereas control KCs functioned as effective APCs, KCs from Gd-injected mice (GdKC) were incapable of sustaining the proliferative response of the Th1 clone to the 16 mer of OVA (323-339). This lack of proliferation was determined not to be caused by impaired Ag processing, but rather was the result of IFN-gamma-stimulated nitric oxide (NO) release by the APC: 1) In vitro addition of the nitric oxide synthase (NOS) inhibitor NG-methyl-L-arginine (NMMA) restored the ability of the Gd-treated KC to stimulate clone proliferation. 2) Additional of anti-IFN-gamma, but not anti-IL-2 or anti-IL-4, prevented the induction of NOS in the Gd-exposed KC and was associated with clone proliferation. 3) IFN-gamma levels from clone-GdKC-OVA cocultures closely paralleled the nitrite released by GdKCs. 4) Only the addition of rIFN-gamma, and not IL-2 or IL-4, to cultures of purified GdKCs resulted in the release of nitrite. The results of the study suggest an autocrine loop initiated by the interaction of the clone's TCR with the class II MHC molecule presenting processed OVA on the surface of KC. This interaction stimulates the Th1 lymphocyte to release IFN-gamma, which in turn induces NO release by KCs isolated from Gd-injected mice. This release of NO blocks Th1 proliferation. Such a feedback loop may have particular relevance to Ag-specific tolerance, which is not only induced by the administration of Ag into the portal vein, but is also prevented by Gd pretreatment of the recipient animal.

Nitric oxide-mediated apoptosis in murine peritoneal macrophages

Nitric oxide (NO) synthase, the enzyme responsible for the generation of the cytotoxic compound NO from L-arginine, is induced in macrophages during activation. Previous work demonstrated that the cytotoxicity of NO extends to the macrophages that produce it, because the activity of NO synthase in these cells correlates inversely with their life span in culture. Data presented here demonstrate that the NO-dependent death of murine peritoneal macrophages activated in vitro with IFN-gamma and LPS is mediated through apoptosis. Evidence in this direction was provided by microscopic examination of the cells, which revealed the presence of nuclear and cytoplasmic alterations characteristic of apoptosis, and by the specific pattern of internucleosomal DNA fragmentation detected by electrophoresis. That these alterations resulted from the production of NO was confirmed by the preventive effects of cell activation in L-arginine-restricted medium or in medium containing an inhibitor of NO synthase, NG-monomethy L-arginine, and more directly by the induction of apoptosis by exposure of the cells to authentic NO gas. Additional results demonstrated that glucose starvation, the inhibition of the tricarboxylic acid cycle with fluorocitrate or of glycolysis with iodoacetate, but not the suppression of the electron transport chain with potassium cyanide, also induced macrophage apoptosis. The potential role of metabolic inhibition as a mechanism for NO-mediated apoptosis, as well as the relationship of these findings with events occurring in wounds and other sites of macrophage infiltration are discussed.

Differences in cytokine response and induction of nitric oxide synthase in endotoxin-resistant and endotoxin-sensitive mice after intravenous gram-negative infection

Previous reports have suggested that the endotoxin-resistant C3H/HeJ strain of mouse is more susceptible to infection than is the endotoxin-sensitive parent strain, C3H/HeN, although they have never been compared in an i.v. model of sepsis. We therefore have used these mouse strains in an i.v. model of Gram-negative sepsis to compare their sensitivities to infection, their cytokine responses, and the levels of induction of the enzyme nitric oxide synthase assayed in their livers. By using i.v. infection with Escherichia coli we have found that both strains are approximately equally sensitive to this organism, despite the C3H/HeJ mice having a markedly attenuated TNF-alpha response. IFN-gamma levels after infection were identical in the two strains; the levels of nitric oxide synthase induced in their livers were about fourfold greater in the C3H/HeJ mice. This difference could not be explained by differences in bacterial load. These experiments suggest that factors other than TNF-alpha are important in determining outcome from Gram-negative sepsis and that TNF-alpha is not a major factor in the induction of hepatic nitric oxide synthase after infection in vivo.