Ursula Malipiero

Modulation of the Immune Response by Transforming Growth Factor Beta

For the past several years immunologists have been fascinated by a series of experiments showing that transforming growth factor beta (TGF beta) suppresses T- and B-lymphocyte growth as well as IgM and IgG production by B cells. Moreover, while exerting chemotactic activity on monocytes and inducing expression of interleukin-1 and interleukin-6 by these cells, TGF beta interferes with bacterially induced tumor necrosis factor alpha production, oxygen radical formation and the adhesiveness of granulocytes to endothelial cells. These mechanisms may provide the basis for the effect of TGF beta to prevent the microvascular changes associated with brain edema formation in bacterial meningitis. Given the potential of lymphocytes as well as macrophages to produce TGF beta 1, this cytokine may exert negative feedback signals on the immune response, provided the cytokine is processed from its latent form to the bioactive homodimer. Potent effects of TGF beta have been observed in experimental animals including the inhibition of the generation of virus-specific cytotoxic T cells and antiviral antibodies as well as the diminution of cellular infiltrates with decreased major histocompatibility complex class-II expression and CD8+ T cells in the tissue of virally infected animals. TGF beta may also be of importance in tumor immunology. By the production of bioactive TGF beta as detected in glioblastoma and acute T-cell leukemia, tumor cells may induce an immunodeficiency state and escape immune surveillance. In inflammation, monitoring of TGF beta in the tissue will bring light on the immune regulation in acute and chronic inflammatory diseases.

TGFβ directs gene expression of activated microglia to an anti-inflammatory phenotype strongly focusing on chemokine genes and cell migratory genes

In experimental autoimmune encephalomyelitis, the acute phase of the disease is produced by T‐helper lymphocyte type 1 (TH1), which produces mainly TNFα and IFNγ. Recovery from the disease is mediated by T‐helper lymphocyte types 2 and 3 (TH2/TH3), which, among other cytokines, produce transforming growth factor β (TGFβ). To address the influence of TGFβ on TH1‐induced gene expression, microarray technology was used on murine primary microglial cells stimulated with IFNγ and TNFα in the absence or presence of TGFβ. The resulting data from an investigation of up to 5,500 genes provided the notion that TGFβ prevents the induction of a proinflammatory gene program within microglia exposed to a TH1 milieu. TH1 cytokines upregulated 175 genes comprising cytokine, chemokine, and genes involved in host response to infection and the TNFα/IFNγ intracellular signaling pathway. It is observed that TGFβ inhibits expression of 25% of the TNFα/IFNγ‐induced genes and a further 66 TNFα/IFNγ‐independent genes. The focus of TGFβ inhibition is observed to be directed in genes involved in chemotaxis (IL‐15, CXCL1, CXCL2, CCL3, CCL4, CCL5, CCL9), chemokine receptors (CCR5, CCR9), LIF receptor, and FPR2, and on genes mediating cell migration (MMP9, MMP13, MacMARCKS, endothelin receptor B, Ena/VASP, Gas7), apoptosis (FAS, TNF, TNF receptor, caspase‐1 and ‐11), and host response to infection (toll‐like receptor 6, Mx‐1, and MARCO). Taken collectively, the data strongly suggest that one of the main effects of TGFβ is to impair cell entry into the CNS and to hinder migration of microglia in the CNS parenchyma.

Dendritic cells and differential usage of the MHC class II transactivator promoters in the central nervous system in experimental autoimmune encephalitis

In the normal central nervous system (CNS) expression of MHC class II is minimal, but has been found to be highly up‐regulated on microglia cells in experimental autoimmune encephalitis (EAE). Here we used the EAE model to examine the regulation of expression of the class II transactivator (CIITA), which is required for activation of MHC class II genes. EAE was induced in C57BL / 6 mice by immunization with myelin oligodendrocyte glycoprotein peptide 35 – 55. CIITA mRNA form I (specific for dendritic cells) and form IV (IFN‐γ inducible) but not form III (B cell specific) were detected in brain and spinal cord of mice with acute EAE. In unimmunized or mock‐immunized mice, none of the three CIITA forms was found to be induced. Dendritic cells (DC) were identified by immunostainings for CD11c in perivascular and meningeal cell infiltrates in EAE spinal cord and brain. Time‐course analysis showed (1) the appearance of DC in the CNS shortly before onset of disease, (2) the recruitment of CD11b+ cells occuring much earlier and (3) the absence of CIITA and MHC class II expression in these CD11b+ cells at preclinical stages.

Treatment of Experimental Glioma by Administration of Adenoviral Vectors Expressing Fas Ligand

Fas ligand (FasL) is a cytokine, produced by activated T cells and NK cells, that triggers apoptosis of Fas-positive target cells including human glioma cells. As shown here, in vitro infection of rat F98 and human LN18 glioma cell lines with recombinant adenovirus (rAd) expressing FasL cDNA under control of the cytomegalovirus promoter (rAd-CMV-FasL) induced striking cytotoxicity in Fas-positive glioma cell lines but not in the Fas-negative F98 glioma subline F98/ZH. The extent of FasL-mediated cytotoxic effects outranged the expectations based on expression of beta-galactosidase (beta-Gal) by F98 cells infected with a control virus expressing the lacZ gene (rAd-CMV-lacZ). The detection of FasL bioactivity in supernatants of infected cells provides evidence of a bystander mechanism involving the cytotoxic action of FasL on uninfected cells. In F98 tumor-bearing rats, infection with rAd-CMV-FasL increased the mean survival time by 50% compared with infection with rAd-CMV-lacZ or untreated controls. These data suggest that viral vector transduction of the FasL gene could be part of a successful glioma gene therapy.

Production of macrophage colony-stimulating factor by astrocytes and brain macrophages

Morphological hallmarks of inflammatory and degenerative diseases of the brain are hypertrophy of astrocytes and accumulation of macrophages recruited from circulating blood monocytes and/or from resident macrophages, the so-called microglial cells. Recently, production of granulocyte-macrophage colony-stimulating factor (GM-CSF) by astrocytes has been suggested to contribute to the macrophage response. Here we report that in addition to GM-CSF, murine astrocytes also produce macrophage (M)-CSF upon stimulation with tumor necrosis factor alpha, interleukin-1 and lipopolysaccharides. The bioactivity detected in supernatant of astrocytes was characterized using the M-CSF-dependent cell line M-NFS-60 and neutralizing anti-M-CSF antibodies. RNase protection analysis showed M-CSF mRNA already in unstimulated astrocytes without striking up-regulation by the stimuli. Thus, in astrocytes the expression of the M-CSF gene is predominantly regulated at the posttranscriptional level.

TGF-beta-treated microglia induce oligodendrocyte precursor cell chemotaxis through the HGF-c-Met pathway

In acute experimental autoimmune encephalomyelitis (EAE), demyelination is induced by myelin‐specific CD4+ T lymphocytes and myelin‐specific antibodies. Recovery from the disease is initiated by cytokines which suppress T cell expansion and the production of myelin‐toxic molecules by macrophages. Th2/3 cell‐derived signals may also be involved in central nervous system (CNS) repair. Remyelination is thought to be initiated by the recruitment and differentiation of oligodendrocyte precursor cells (OPC) in demyelinated CNS lesions. Here, we report that unlike Th1 cytokines (TNF‐α, IFN‐γ), the Th2/3 cytokine TGF‐β induces primary microglia from C57BL/6 mice to secrete a chemotactic factor for primary OPC. We identified this factor to be the hepatocyte growth factor (HGF). Our studies show that TGF‐β‐1‐2‐3 as well as IFN‐β induce HGF secretion by microglia and that antibodies to the HGF receptor c‐Met abrogate OPC chemotaxis induced by TGF‐β2‐treated microglia. In addition we show spinal cord lesions in EAE induced in SJL/J mice to contain both OPC and HGF producing macrophages in the recovery phase, but not in the acute stage of disease. Taken these findings, TGF‐β may play a pivotal role in remyelination by inducing microglia to release HGF which is both a chemotactic and differentiation factor for OPC.

TGF‐β induces the expression of the FLICE‐inhibitory protein and inhibits Fas‐mediated apoptosis of microglia

During inflammatory reactions in the central nervous system (CNS), resident macrophages, the microglia, are exposed to Th1 cell‐derived cytokines and pro‐apoptotic Fas ligand (FasL). Despite the presence of TNF‐α and IFN‐γ, both being capable of sensitzing microglia to FasL, apoptosis of microglia is not a hallmark of inflammatory diseases of the CNS. In the present study, TGF‐β is found to counteract the effect of TNF‐α and IFN‐γ to sensitize microglia to FasL‐mediated apoptosis. Resistance to Fas‐mediated apoptosis by TGF‐β does not correlate with a down‐regulation of Fas expression. As a key inhibitor of Fas‐mediated apoptosis, we found expression of the cellular FLICE‐inhibitory protein (c‐FLIP) to be induced by TGF‐β in resting as well as in activated microglia. Induction of FLIP was found to depend on a mitogen‐activated protein kinase kinase (MKK)‐dependent pathway as shown by the use of the specific MKK‐inhibitor PD98059. The presence of FLIP strongly interfered with FasL‐induced activation of caspase‐8 and caspase‐3 preventing subsequent cell death. The presented data provide the first evidence for a TGF‐β‐mediated FLIP in macrophage‐like cells and suggest a mode of action for the anti‐apoptotic role of TGF‐β in the CNS.

Cloning of the Human Puromycin‐Sensitive Aminopeptidase and Evidence for Expression in Neurons

Abstract: The puromycin‐sensitive aminopeptidase (PSA) is thought to contribute to the degradation of enkephalins. Besides being the most abundant aminopeptidase in the brain, PSA is expressed in other organs as well. From a human fetal brain cDNA library, we have isolated cDNA encoding the human PSA (huPSA) protein. The isolated cDNA gave rise to a protein with a molecular mass of 99 kDa. Compared with mouse PSA, homology at the amino acid and cDNA level was 98 and 93%, respectively. Translation of the huPSA was found to be initiated at the second of two possible start codons, as shown by studies with antibodies directed against peptide sequences of both potential N‐terminal regions. Northern blot analysis with RNA isolated from different human organs demonstrated that the huPSA transcript is strongest but not exclusively expressed in the brain. Vesicular stomatitis virus epitope‐tagged huPSA protein was expressed in HeLa cells and found to be localized in the cytoplasm, especially in the perinuclear region. By in situ hybridization, huPSA transcript could be identified in cortical and cerebellar neurons, whereas glial cells and blood vessels remained negative.

Chemosensitivity of human malignant glioma: modulation by p53 gene transfer

Loss of wild-type p53 activity is one of the most common molecular abnormalities in human cancers including malignant gliomas. The p53 status is also thought to modulate sensitivity to irradiation and chemotherapy. Here, we studied the effect of a p53 gene transfer on the chemosensitivity of three human glioma cell lines with different endogenous p53 status (LN-229, wild-type; LN-18, mutant; LN-308, deleted), using the murine temperature-sensitive p53 val135 mutant. Expression of mutant p53 enhanced proliferation of LN-308 cells but reduced proliferation in the other cell lines. Expression of wild-type p53 caused reversible growth arrest of all cell lines but failed to induce apoptosis. Growth arrest induced by wild-type p53 was associated with strong induction of p21 expression. Strong induction of BAX expression and loss of BCL-2 expression, which are associated with p53-dependent apoptosis rather than growth arrest, were not observed. Wild-type p53 failed to sensitize glioma cells to cytotoxic drugs including BCNU, cytarabine, doxorubicin, teniposide and vincristine. The combined effects of wild-type p53 gene transfer and drug treatment were less than additive rather than synergistic, suggesting that the intracellular cascades activated by p53 and chemotherapy are rebundant. Unexpectedly, forced expression of mutant p53 modulated drug sensitivity in that it enhanced the toxicity of some drugs but attenuated the effects of others. These effects may represent a dominant negative effect of mutant p53 in LN-229 cells which have wild-type p53 activity but must be considered a gain of function-type effect in the other two cell lines which have no wild-type p53 activity. Importantly, no clear-cut pattern emerged among the three cell lines studied. We conclude that somatic gene therapy based on the reintroduction of p53 will limit the proliferation of human malignant glioma cells but is unlikely to induce clinically relevant sensitization to chemotherapy in these tumors.

A new look at the role of p53 in leukemia cell sensitivity to chemotherapy

A gene encoding the p53 val135 mutant, which assumes mutant conformation at 38.5°C and wild-type conformation at 32.5°C, was introduced into p53-deficient K562 myeloid leukemia cells. Forced expression of wild-type, but not mutant, p53 resulted in growth arrest, accumulation of p21 and Bax proteins, and delayed cell death. Wild-type p53 enhanced the cytotoxic effects of some drugs and attenuated those of others. Compared with wild-type p53, mutant p53 induced much stronger sensitization to drug cytotoxicity. This occurred in the absence of effects on cell cycle progression or activation of several p53 target genes. Although both mutant and wild-type p53 induced changes of immunophenotype, no specific pattern of differentiation was associated with enhanced chemosensitivity. Thus, (1) induction of growth arrest and activation of p53 target genes such as p21 and bax are linked to the wild-type conformation of p53; (2) p53 induces immunophenotypic changes of myeloid leukemia cells suggestive of multidirectional differentiation in a conformation-dependent manner; and (3) (so-called) mutant p53 induces chemosensitization in the absence of effects on cell cycle progression, activation of bax, p21, gadd45 and mdm-2, or a specific pattern of differentiation; and (4) chemosensitization mediated by wild-type p53 may be masked by transcription-dependent induction of growth arrest.