Transforming growth factor beta1 inhibits Fas ligand expression and subsequent activation-induced cell death in T cells via downregulation of c-Myc

The proto-oncogene c-myc in hematopoietic development and leukemogenesis

The proto-oncogene c-myc has been shown to play a pivotal role in cell cycle regulation, metabolism, apoptosis, differentiation, cell adhesion, and tumorigenesis, and participates in regulating hematopoietic homeostasis. It is a transcription regulator that is part of an extensive network of interacting factors. Most probably, different biological responses are elicited by different overlapping subsets of c-Myc target genes, both induced and suppressed. Results obtained from studies employing mouse models are consistent with the need for at least one, and possibly two, mutations in addition to deregulated c-myc for malignant tumor formation. Repression of c-myc is required for terminal differentiation of many cell types, including hematopoietic cells. It has been shown that deregulated expression of c-myc in both M1 myeloid leukemic cells and normal myeloid cells derived from murine bone marrow, not only blocked terminal differentiation and its associated growth arrest, but also induced apoptosis, which is dependent on the Fas/CD95 pathway. There is evidence to suggest that the CD95/Fas death receptor pathway is an integral part of the apoptotic response associated with the end of the normal terminal myeloid differentiation program, and that deregulated c-myc expression can activate this signaling pathway prematurely. The ability of egr-1 to promote terminal myeloid differentiation when co-expressed with c-myc, and of c-fos to partially abrogate the block imparted by deregulated c-myc on myeloid differentiation, make these two genes candidate tumor suppressors. Several different transcription factors have been implicated in the down-regulation of c-myc expression during differentiation, including C/EBPalpha, CTCF, BLIMP-1, and RFX1. Alterations in the expression and/or function of these transcription factors, or of the c-Myc and Max interacting proteins, such as MM-1 and Mxi1, can influence the neoplastic process. Understanding how c-Myc controls cellular phenotypes, including the leukemic phenotype, should provide novel tools for designing drugs to promote differentiation and/or apoptosis of leukemic cells.

IL-18-independent cytotoxic T lymphocyte activation and IFN-gamma production during experimental acute graft-versus-host disease

Acute graft-versus-host disease (GvHD) is a serious complication after allogeneic bone marrow transplantation. Donor-derived T cells infiltrate recipient target organs and cause severe tissue damage, often leading to death of the affected patient. Tissue destruction is a direct result of donor CD8+ T cell activation and cell-mediated cytotoxicity. IL-18 is a novel pro-inflammatory cytokine with potent T(h)1 immune response-promoting and cytotoxic T lymphocyte (CTL)-inducing activity. IL-18 is strongly induced in experimental mouse models and human patients with acute GvHD. However, the precise role of IL-18 in the development of acute GvHD is still unknown. In this study, we have used IL-18-binding protein, a soluble IL-18 decoy receptor, to specifically neutralize IL-18 in vivo and in vitro. Our results demonstrate that IL-18 is induced during GvHD. However, its effect in the induction of GvHD appears to be redundant, since neutralization of IL-18 does not alter any disease parameter analyzed. Our study further shows that IFN-gamma production and CTL induction upon activation by T cell mitogens or by alloantigen does not involve IL-18-mediated amplification, in contrast to lipopolysaccharide-induced IFN-gamma production. We conclude that IL-18 expression correlates with the course of GvHD; however, its effect is dispensable for IFN-gamma and CTL induction for the initiation phase of this disease, most likely due to direct, IL-18-independent, CTL activation.

Adapting a transforming growth factor beta-related tumor protection strategy to enhance antitumor immunity

Transforming growth factor beta (TGF-beta), a pleiotropic cytokine that regulates cell growth and differentiation, is secreted by many human tumors and markedly inhibits tumor-specific cellular immunity. Tumors can avoid the differentiating and apoptotic effects of TGF-beta by expressing a nonfunctional TGF-beta receptor. We have determined whether this immune evasion strategy can be manipulated to shield tumor-specific cytotoxic T lymphocytes (CTLs) from the inhibitory effects of tumor-derived TGF-beta. As our model we used Epstein-Barr virus (EBV)-specific CTLs that are infused as treatment for EBV-positive Hodgkin disease but that are vulnerable to the TGF-beta produced by this tumor. CTLs were transduced with a retrovirus vector expressing the dominant-negative TGF-beta type II receptor HATGF-betaRII-Deltacyt. HATGF-betaRII-Deltacyt- but not green fluorescence protein (eGFP)-transduced CTLs was resistant to the antiproliferative and anticytotoxic effects of exogenous TGF-beta. Additionally, receptor-transduced cells continued to secrete cytokines in response to antigenic stimulation. TGF-beta receptor ligation results in phosphorylation of Smad2, and this pathway was disrupted in HATGF-betaRII-Deltacyt-transduced CTLs, confirming blockade of the signal transduction pathway. Long-term expression of TGF-betaRII-Deltacyt did not affect CTL function, phenotype, or growth characteristics. Tumor-specific CTLs expressing HATGF-betaRII-Deltacyt should have a selective functional and survival advantage over unmodified CTLs in the presence of TGF-beta-secreting tumors and may be of value in treatment of these diseases.

Role of p38 mitogen-activated protein kinase phosphorylation and Fas-Fas ligand interaction in morphine-induced macrophage apoptosis

In this study, we evaluated the molecular mechanisms involved in morphine-induced macrophage apoptosis. Both morphine and TGF-beta promoted P38 mitogen-activated protein kinase (MAPK) phosphorylation, and this phosphorylation was inhibited by SB 202190 as well as by SB 203580. Anti-TGF-beta Ab as well as naltrexone (an opiate receptor antagonist) inhibited morphine-induced macrophage P38 MAPK phosphorylation. Anti-TGF-beta Ab also attenuated morphine-induced p53 as well as inducible NO synthase expression; in contrast, N(G)-nitro-L-arginine methyl ester, an inhibitor of NO synthase, inhibited morphine-induced P38 MAPK phosphorylation and Bax expression. Morphine also enhanced the expression of both Fas and Fas ligand (FasL), whereas anti-FasL Ab prevented morphine-induced macrophage apoptosis. Moreover, naltrexone inhibited morphine-induced FasL expression. In addition, macrophages either deficient in FasL or lacking p53 showed resistance to the effect of morphine. Inhibitors of both caspase-8 and caspase-9 partially prevented the apoptotic effect of morphine on macrophages. In addition, caspase-3 inhibitor prevented morphine-induced macrophage apoptosis. These findings suggest that morphine-induced macrophage apoptosis proceeds through opiate receptors via P38 MAPK phosphorylation. Both TGF-beta and inducible NO synthase play an important role in morphine-induced downstream signaling, which seems to activate proteins involved in both extrinsic (Fas and FasL) and intrinsic (p53 and Bax) cell death pathways.

Deregulated c-Myc prematurely recruits both Type I and II CD95/Fas apoptotic pathways associated with terminal myeloid differentiation

Previously we have reported that deregulated expression of c-myc in normal and leukemic myeloid cells blocked differentiation and, concomitantly, induced p53-independent apoptosis. Here, we show that this morbidity was due to premature recruitment of the Fas/CD95 cell death pathway which normally operates to induce apoptosis at the end of the terminal myeloid differentiation program. Analysis of the regulated components of this pathway revealed that IL6-mediated induction of differentiation resulted in rapid cell surface expression of CD95 receptor. Deregulated c-myc prevented the downregulation of CD95 ligand by maintaining its transcription, but caused premature downregulation of c-FLIP. First, the Type II (mitochondria-dependent, bcl-2-sensitive) and, then, the Type I (mitochondria-independent, bcl-2-insensitive) pathway were activated. Stable exogenous c-FLIP expression completely rescued the apoptotic phenotype. Furthermore, when the deregulated c-myc transgene was stably transduced into bone marrow cells from Fas(lpr/lpr) (CD95 receptor mutant) and FasL(gld/gld) (CD95 ligand mutant) mice, cell death was significantly suppressed relative to c-myc-transduced wild type bone marrow cells upon induction of differentiation. These data indicate that c-myc-mediated apoptosis associated with blocks in myeloid differentiation is dependent on the Fas/CD95 pathway. Our findings offer important new insights into understanding how deregulated c-myc alters normal blood cell homeostasis, and how additional mutations might promote leukemogenesis.

Negative regulation of CD95 ligand gene expression by vitamin D3 in T lymphocytes

Fas (APO-1/CD95) and its ligand (FasL/CD95L) are cell surface proteins whose interaction activates apoptosis of Fas-expressing targets. In T lymphocytes, the Fas/FasL system regulates activation-induced cell death, a fundamental mechanism for negative selection of immature T cells in the thymus and for maintenance of peripheral tolerance. Aberrant expression of Fas and FasL has also been implicated in diseases in which the lymphocyte homeostasis is compromised, and several studies have described the pathogenic functions of Fas and FasL in vivo, particularly in the induction/regulation of organ-specific autoimmune diseases. The 1,25(OH)(2)D(3) is a secosteroid hormone that activates the nuclear receptor vitamin D(3) receptor (VDR), whose immunosuppressive activities have been well studied in different models of autoimmune disease and in experimental organ transplantation. We and others have recently described the molecular mechanisms responsible for the negative regulation of the IFN-gamma and IL-12 genes by 1,25(OH)(2)D(3) in activated T lymphocytes and macrophages/dendritic cells. In this study, we describe the effect of 1,25(OH)(2)D(3) on the activation of the fasL gene in T lymphocytes. We show that 1,25(OH)(2)D(3) inhibits activation-induced cell death, fasL mRNA expression, and that 1,25(OH)(2)D(3)-activated VDR represses fasL promoter activity by a mechanism dependent on the presence of a functional VDR DNA-binding domain and ligand-dependent transcriptional activation domain (AF-2). Moreover, we identified a minimal region of the promoter containing the transcription start site and a noncanonical c-Myc-binding element, which mediates this repression. These results place FasL as a novel target for the immunoregulatory activities of 1,25(OH)(2)D(3), and confirm the interest for a possible pharmacological use of this molecule and its derivatives.

Viral IFN-regulatory factors inhibit activation-induced cell death via two positive regulatory IFN-regulatory factor 1-dependent domains in the CD95 ligand promoter

The CD95 (also called APO-1/Fas) system plays a major role in the induction of apoptosis in lymphoid and nonlymphoid tissues. The CD95 ligand (CD95L) is induced in response to a variety of signals, including IFN-gamma and TCR/CD3 stimulation. Here we report the identification of two positive regulatory IFN-regulatory factor-dependent domains (PRIDDs) in the CD95L promoter and its 5' untranslated region, respectively. EMSAs demonstrate specific binding of IFN-gamma-induced IFN-regulatory factor 1 (IRF-1) to the PRIDD sequences. Ectopic IRF-1 expression induces CD95L promoter activity. Furthermore, we demonstrate that PRIDDs play an important role in TCR/CD3-mediated CD95L induction. Most interestingly, viral IRFs of human herpes virus 8 (HHV8) totally abolish IRF-1-mediated and strongly reduce TCR/CD3-mediated CD95L induction. We demonstrate here for the first time that viral IRFs inhibit activation-induced cell death. Thus, these results demonstrate an important mechanism of HHV8 to modulate the immune response by down-regulation of CD95L expression. Inhibition of CD95-dependent T cell function might contribute to the immune escape of HHV8.

Transforming growth factor-beta in T-cell biology

Strict control of T-cell homeostasis is required to permit normal immune responses and prevent undesirable self-targeted responses. Transforming growth factor-beta (TGF-beta) has been shown to have an essential role in that regulation. Owing to its broad expression, and inhibitory effects on multiple cell types of the immune system, TGF-beta regulation is complex. Through advances in cell-specific targeting of TGF-beta signalling in vivo, the role of TGF-beta in T-cell regulation has become clearer. Recent in vitro studies provide a better understanding of how TGF-beta regulates T-cell homeostasis, through multiple mechanisms involving numerous cell types.

The potential of human regulatory T cells generated ex vivo as a treatment for lupus and other chronic inflammatory diseases

Regulatory T cells prevent autoimmunity by suppressing the reactivity of potentially aggressive self-reactive T cells. Contact-dependent CD4+ CD25+ 'professional' suppressor cells and other cytokine-producing CD4+ and CD8+ T-cell subsets mediate this protective function. Evidence will be reviewed that T cells primed with transforming growth factor (TGF)-beta expand rapidly following restimulation. Certain CD4+ T cells become contact-dependent suppressor cells and other CD4+ and CD8+ cells become cytokine-producing regulatory cells. This effect is dependent upon a sufficient amount of IL-2 in the microenvironment to overcome the suppressive effects of TGF-beta. The adoptive transfer of these suppressor cells generated ex vivo can protect mice from developing chronic graft-versus-host disease with a lupus-like syndrome and alter the course of established disease. These data suggest that autologous T cells primed and expanded with TGF-beta have the potential to be used as a therapy for patients with systemic lupus erythematosus and other chronic inflammatory diseases. This novel adoptive immunotherapy also has the potential to prevent the rejection of allogeneic transplants.

Transforming growth factor-beta has contrasting effects in the presence or absence of exogenous interleukin-12 on the in vitro activation of cells that transfer experimental autoimmune thyroiditis

Mouse thyroglobulin (MuTg)-sensitized spleen cells activated in vitro with MuTg induce experimental autoimmune thyroiditis (EAT) in recipient mice with a thyroid infiltrate consisting primarily of lymphocytes. A more severe and histologically distinct granulomatous form of EAT (G-EAT) is induced when donor cells are activated with MuTg together with anti-interferon-gamma (IFN-gamma), anti-interleukin-2 receptor (IL-2R) monoclonal antibody (mAb), and IL-12. Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine that can both suppress and exacerbate autoimmune diseases and often has inhibitory effects on lymphocytes. To determine if TGF-beta could modulate the in vitro activation of effector cells for G-EAT, TGF-beta was added to cultures of MuTg-sensitized donor spleen cells together with MuTg. Cells activated in the presence of 2 ng/ml TGF-beta induced moderately severe G-EAT in recipient mice. G-EAT induced by cells activated in the presence of TGF-beta was histologically similar but less severe than the G-EAT induced by cells activated in the presence of IL-12. IL-12 and TGF-beta modulate the activation of G-EAT effector cells by distinct mechanisms, as cells activated by TGF-beta could induce G-EAT in the presence of anti-IL-12, and TGF-beta inhibited the effects of IL-12 on EAT effector cells. TGF-beta exerted its activity during the first 24 h of the 72-h culture, whereas IL-12 functioned primarily during the final 24 h of culture. These results indicate that thyroid lesions with granulomatous histopathology can be induced by both IL-12-dependent and IL-12-independent mechanisms, and TGF-beta can exert both positive and negative effects on the effector cells for G-EAT.

Transforming growth factor beta enhances epithelial cell survival via Akt-dependent regulation of FKHRL1

The Forkhead family of transcription factors participates in the induction of death-related genes. In NMuMG and 4T1 mammary epithelial cells, transforming growth factor beta (TGF beta) induced phosphorylation and cytoplasmic retention of the Forkhead factor FKHRL1, while reducing FHKRL1-dependent transcriptional activity. TGF beta-induced FKHRL1 phosphorylation and nuclear exclusion were inhibited by LY294002, an inhibitor of phosphatidylinositol-3 kinase. A triple mutant of FKHRL1, in which all three Akt phosphorylation sites have been mutated (TM-FKHRL1), did not translocate to the cytoplasm in response to TGF beta. In HaCaT keratinocytes, expression of dominant-negative Akt prevented TGF beta-induced 1) reduction of Forkhead-dependent transcription, 2) FKHRL1 phosphorylation, and 3) nuclear exclusion of FKRHL1. Forced expression of either wild-type (WT) or TM-FKHRL1, but not a FKHRL1 mutant with deletion of the transactivation domain, resulted in NMuMG mammary cell apoptosis. Evidence of nuclear fragmentation colocalized to cells with expression of WT- or TM-FKHRL1. The apoptotic effect of WT-FKHRL1 but not TM-FKHRL1 was prevented by exogenous TGF beta. Serum starvation-induced apoptosis was also inhibited by TGF beta in NMuMG and HaCaT cells. Finally, dominant-negative Akt abrogated the antiapoptotic effect of TGF beta. Taken together, these data suggest that TGF beta may play a role in epithelial cell survival via Akt-dependent regulation of FKHRL1.

Expression of Fas antigen (CD95) in peripheral blood lymphocytes and in liver-infiltrating, cytotoxic lymphocytes in patients with hepatocellular carcinoma

BACKGROUND

Fas-expressing cytotoxic T lymphocytes (CTLs) are important antitumor immune effector cells in patients with hepatocellular carcinoma (HCC). The role of transforming growth factor beta 1 (TGF-beta1) in modulating the expression of Fas by CTLs is not known in HCC. The objectives of this study were to characterize the expression of Fas by CTLs and natural killer (NK) cells among peripheral blood lymphocytes (PBLs) and tumor-infiltrating lymphocytes (TILs) in patients with HCC and to correlate the association, if any, with serum TGF-beta1 levels.

METHODS

PBLs from 18 patients with HCC and TILs from 5 HCC liver specimens were isolated, and Fas expression was analyzed by three-color flow cytometry. The results were compared with results from normal control volunteers (n = 19 individuals). Serum TGF-beta1 levels in patients with HCC were measured by enzyme-linked immunosorbent assay.

RESULTS

The median percentage of Fas expression by CD3 positive T cells was significantly higher in patients with HCC compared with normal controls (54.37% vs. 32.03%, respectively; P = 0.0036), and this was attributable solely to Fas expression by CD4 positive PBLs (54.46% vs. 34.90%, respectively; P = 0.0234). In contrast, Fas expression was significantly higher in all the subtypes of TILs (CD3 positive, CD4 positive, CD8 positive, NK cells, and natural T cells) compared with controls (all P values were < 0.001). Tumor size was inversely proportional to the TGF-beta1 levels (correlation coefficient [r] = -0.725; P < 0.0001), which were correlated inversely with Fas expression by CD4 positive PBLs (r = -0.516; P = 0.01).

CONCLUSIONS

In patients with HCC, TILs exhibit significantly increased expression of Fas compared with PBLs that may enhance their susceptibility to apoptotic mechanisms. Larger tumors were associated with lower serum TGFbeta1 levels, and this was correlated with greater Fas expression by CD4 positive PBLs.

p38-mediated regulation of an Fas-associated death domain protein-independent pathway leading to caspase-8 activation during TGFbeta-induced apoptosis in human Burkitt lymphoma B cells BL41

On binding to its receptor, transforming growth factor beta (TGFbeta) induces apoptosis in a variety of cells, including human B lymphocytes. We have previously reported that TGFbeta-mediated apoptosis is caspase-dependent and associated with activation of caspase-3. We show here that caspase-8 inhibitors strongly decrease TGFbeta-mediated apoptosis in BL41 Burkitt's lymphoma cells. These inhibitors act upstream of the mitochondria because they inhibited the loss of mitochondrial membrane potential observed in TGFbeta-treated cells. TGFbeta induced caspase-8 activation in these cells as shown by the cleavage of specific substrates, including Bid, and the appearance of cleaved fragments of caspase-8. Our data show that TGFbeta induces an apoptotic pathway involving sequential caspase-8 activation, loss of mitochondrial membrane potential, and caspase-9 and -3 activation. Caspase-8 activation was Fas-associated death domain protein (FADD)-independent because cells expressing a dominant negative mutant of FADD were still sensitive to TGFbeta-induced caspase-8 activation and apoptosis. This FADD-independent pathway of caspase-8 activation is regulated by p38. Indeed, TGFbeta-induced activation of p38 and two different inhibitors specific for this mitogen-activated protein kinase pathway (SB203580 and PD169316) prevented TGFbeta-mediated caspase-8 activation as well as the loss of mitochondrial membrane potential and apoptosis. Overall, our data show that p38 activation by TGFbeta induced an apoptotic pathway via FADD-independent activation of caspase-8.

Synergistic induction of apoptosis in primary CD4(+) T cells by macrophage-tropic HIV-1 and TGF-beta1

Depletion of CD4(+) T lymphocytes is a central immunological characteristic of HIV-1 infection. Although the mechanism of such CD4(+) cell loss following macrophage-tropic (R5) HIV-1 infection remains unclear, interactions between viral and host cell factors are thought to play an important role in the pathogenesis of HIV-1 disease. Based on the observation that TGF-beta1 enhanced expression of HIV chemokine coreceptors, the role of this host factor in virus effects was investigated using PBLs cultured in a nonmitogen-added system in the absence or presence of TGF-beta1. Most CD4 cells in such cultures had the phenotype CD25(-)CD69(-)DR(-)Ki67(-) and were CD45RO(bright)CD45RA(dim). Cultured cells had increased expression of CCR5 and CXCR4 and supported both HIV-1 entry and completion of viral reverse transcription. Virus production by cells cultured in the presence of IL-2 was inhibited by TGF-beta1, and this inhibition was accompanied by a loss of T cells from the culture and an increase in CD4(+) T cell apoptosis. Whereas R5X4 and X4 HIV-1 infection was sufficient to induce T cell apoptosis, R5 HIV-1 failed to induce apoptosis of PBLs in the absence of TGF-beta1 despite the fact that R5 HIV-1 depletes CD4(+) T cells in vivo. Increased apoptosis with HIV and TGF-beta1 was associated with reduced levels of Bcl-2 and increased expression of apoptosis-inducing factor, caspase-3, and cleavage of BID, c-IAP-1, and X-linked inhibitor of apoptosis. These results show that TGF-beta1 promotes depletion of CD4(+) T cells after R5 HIV-1 infection by inducing apoptosis and suggest that TGF-beta1 might contribute to the pathogenesis of HIV-1 infection in vivo.

Accumulation and activation-induced release of preformed Fas (CD95) ligand during the pathogenesis of experimental graft-versus-host disease

Fas (CD95/APO-1) ligand (FasL)-mediated cytotoxicity has been implicated in tissue destruction in a variety of diseases, including acute graft-vs-host disease (GVHD). In this study, we have analyzed FasL expression and regulation during the course of experimental murine acute GVHD. Although activation-induced FasL-mediated cytotoxicity in control T cells was sensitive to the immunosuppressant cyclosporin A, we observed that functional FasL expression of GVHD T cells became increasingly cyclosporin A unresponsive. This was found to be the result of a massive in vivo accumulation and intracellular storage of FasL protein and its release in a transcription- and protein synthesis-independent manner. Immunohistochemistry analysis of FasL expression in situ revealed accumulation of FasL-expressing cells in the spleen, the liver, and small intestine, with a typical cytoplasmic and granular expression pattern. Thus, we conclude that the release of preformed FasL by infiltrating donor T cells may contribute to recipient tissue damage during the pathogenesis of acute GVHD.

Requirement for transforming growth factor beta1 in controlling T cell apoptosis

Transforming growth factor (TGF)-beta1, a potent immunoregulatory molecule, was found to control the life and death decisions of T lymphocytes. Both thymic and peripheral T cell apoptosis was increased in mice lacking TGF-beta1 (TGF-beta1(-/-)) compared with wild-type littermates. Engagement of the T cell receptor enhanced this aberrant T cell apoptosis, as did signaling through either the death receptor Fas or the tumor necrosis factor alpha receptor in peripheral T cells. Strikingly, TGF-beta was localized within the mitochondria of normal T cells, and the absence of TGF-beta1 resulted in disruption of mitochondrial membrane potential (Deltapsi(m)), which marks the point of no return in a cell condemned to die. This TGF-beta-dependent regulation of viability appears dissociable from the TGF-beta1 membrane receptor-Smad3 signaling pathway, but associated with a mitochondrial antiapoptotic protein Bcl-XL. Thus, TGF-beta1 may protect T cells at multiple sites in the death pathway, particularly by maintaining the essential integrity of mitochondria. These findings may have broad implications not only for T cell selection and death in immune responses and in the generation of tolerance, but also for defining the mechanisms of programmed cell death in general.

Attrition of bystander CD8 T cells during virus-induced T-cell and interferon responses

Experiments designed to distinguish virus-specific from non-virus-specific T cells showed that bystander T cells underwent apoptosis and substantial attrition in the wake of a strong T-cell response. Memory CD8 T cells (CD8(+) CD44(hi)) were most affected. During acute viral infection, transgenic T cells that were clearly defined as non-virus specific decreased in number and showed an increase in apoptosis. Also, use of lymphocytic choriomeningitis virus (LCMV) carrier mice, which lack LCMV-specific T cells, showed a significant decline in non-virus-specific memory CD8 T cells that correlated to an increase in apoptosis in response to the proliferation of adoptively transferred virus-specific T cells. Attrition of T cells early during infection correlated with the alpha/beta interferon (IFN-alpha/beta) peak, and the IFN inducer poly(I:C) caused apoptosis and attrition of CD8(+) CD44(hi) T cells in normal mice but not in IFN-alpha/beta receptor-deficient mice. Apoptotic attrition of bystander T cells may make room for the antigen-specific expansion of T cells during infection and may, in part, account for the loss of T-cell memory that occurs when the host undergoes subsequent infections.

Regulation of cell death and survival in intestinal intraepithelial lymphocytes

Intraepithelial lymphocytes (IEL) of the small murine bowel represent a unique population of mostly CD8(+) T lymphocytes that reside within the epithelial cell layer of the intestinal mucosa. The close interaction with epithelial cells appears to be crucial for IEL survival since isolation and ex vivo culture induces massive apoptosis in this lymphocyte population. Here, we provide evidence that this form of IEL cell death may be mediated at least in part by endogenously produced glucocorticoids since adrenalectomy or treatment of mice with a glucocorticoid receptor antagonist significantly enhanced ex vivo survival of IEL. We further demonstrate that ex vivo activation of IEL induces upregulation of anti-apoptotic gene products, compensates for the lack of survival cytokines and rescues from apoptotic cell death. Thus, similar to thymocytes and T cell hybridomas, IEL survival may be regulated by the antagonistic action of TCR activation and glucocorticoids.

Transforming growth factor-beta promotes 'death by neglect' in post-activated human T cells

Transforming growth factor-beta (TGF-beta) is central to the wound repair processes that follow local trauma and inflammation. In order to mimic the early events of wound-healing, we studied the effects of TGF-beta on mitogen-stimulated peripheral blood cells. TGF-beta added at the initiation of mitogenesis did not significantly alter T-cell activation, proliferation, CD45 isoform switching, or activation-induced cell death. By contrast, TGF-beta added 72 hr post-activation (or later) enhanced the cumulative increase in apoptotic T cells. TGF-beta had no effect on mitogen-induced up-regulation of Fas (CD95) or Fas ligand and did not enhance killing of the Fas-sensitive Jurkat cell line by activated T cells. Furthermore, TGF-beta had no direct effect on levels of mRNA for members of the bcl family (bcl-X, bfl-1, bik, bak, bax, bcl-2 and mcl-1). These findings suggest that TGF-beta does not directly induce apoptosis via the Fas system or by direct effects on bcl proteins. However, interleukin-2, which can 'rescue' lymphocytes from spontaneous apoptosis due to cytokine deprivation, abolished the pro-apoptotic effects of TGF-beta on post-activated T cells, thus demonstrating that TGF-beta increases the cytokine-dependence of T cells for survival. We propose a novel role for TGF-beta in the suppression of inflammation by promoting the elimination of post-activated T cells once the initiating stimulus has been resolved.

Vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide inhibit expression of Fas ligand in activated T lymphocytes by regulating c-Myc, NF-kappa B, NF-AT, and early growth factors 2/3

Activation-induced cell death in T cells, a major mechanism for limiting an ongoing immune response, is initiated by Ag reengagement and mediated through Fas/Fas ligand interactions. Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP), two multifunctional neuropeptides, modulate innate and adaptive immunity. We reported previously that VIP/PACAP protect T cells from activation-induced cell death through down-regulation of Fas ligand (FasL). In this study, we investigate the molecular mechanisms involved in the protective effect of VIP and PACAP. VIP/PACAP reduce in a dose-dependent manner anti-CD3-induced apoptosis in 2B4.11 T cell hybridomas. The protective effect is mediated through the specific type 2 VIP receptor, and the cAMP/protein kinase A pathway. A functional study demonstrates that VIP/PACAP inhibit activation-induced FasL expression. VIP/PACAP inhibit the expression and/or DNA-binding activity of several transcriptional factors involved in FasL expression, i.e., c-myc, NF-kappaB, NF-ATp, and early growth factors (Egr) 2/3. The inhibition of NF-kappaB binding is due to the stabilization of I-kappaB (inhibitory protein that dissociates from NF-kappaB), through the inhibition of I-kappaB kinase alpha activity. Subsequently, p65 nuclear translocation is significantly reduced. The inhibition in NF-ATp binding results from a calcineurin-independent reduction in NF-ATp nuclear translocation. VIP/PACAP inhibit the expression of Egr2 and 3, but not of Egr1. The effects on the transcriptional factors are mediated through type 2 VIP receptor with cAMP as secondary messenger.

Analysis of C-MYC function in normal cells via conditional gene-targeted mutation

Germline inactivation of c-myc in mice causes embryonic lethality. Therefore, we developed a LoxP/Cre-based conditional mutation approach to test the role of c-myc in mouse embryonic fibroblasts (MEFs) and mature B lymphocytes. Cre expression resulted in reduced proliferation of wild-type MEFs, but c-Myc-deficient MEFs showed a further reduction. In contrast to fibroblasts, Cre expression had no apparent affect on wild-type B cell proliferation. Deletion of both c-Myc genes in B cells led to severely impaired proliferation in response to anti-CD40 plus IL-4. However, treated cells did upregulate several early activation markers but not CD95 or CD95 ligand. We discuss these findings with respect to potential c-Myc functions in proliferation and apoptosis and also discuss potential limitations in the Cre-mediated gene inactivation approach.

TGF-beta 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-alpha and IFN-gamma, both being capable of sensitizing microglia to FasL, apoptosis of microglia is not a hallmark of inflammatory diseases of the CNS. In the present study, TGF-beta is found to counteract the effect of TNF-alpha and IFN-gamma to sensitize microglia to FasL-mediated apoptosis. Resistance to Fas-mediated apoptosis by TGF-beta 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-beta 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-beta-mediated FLIP in macrophage-like cells and suggest a mode of action for the anti-apoptotic role of TGF-beta in the CNS.

Death the Fas way: regulation and pathophysiology of CD95 and its ligand

Apoptotic cell death mediated by the members of the tumor necrosis factor receptor family is an essential process involved in the regulation of cellular homeostasis during development, differentiation, and pathophysiological conditions. Among the cell death receptors comprising the tumor necrosis factor receptor superfamily, CD95/APO-1 (Fas) is the best characterized. The specific interaction of Fas with its cognate ligand, Fas ligand (FasL), elicits the activation of a death-inducing caspase (cysteine aspartic acid proteases) cascade, occurring in a transcription-independent manner. Caspase activation executes the apoptosis process by cleaving various intracellular substrates, leading to genomic DNA fragmentation, cell membrane blebbing, and the exposure of phagocytosis signaling molecules on the cell surface. Recent studies have shown that the Fas/FasL pathway plays an important role in regulating the life and death of the immune system through activation-induced cell death. In addition, these molecules have been implicated in aging, human immunodeficiency virus infection, drug abuse, stress, and cancer development. In this review, we will focus on the mechanisms that regulate Fas and FasL expression, and how their deregulation leads to diseases.

CD30 signals integrate expression of cytotoxic effector molecules, lymphocyte trafficking signals, and signals for proliferation and apoptosis

Although CD30 has long been recognized as an important marker on many lymphomas of diverse origin and as activation molecule on B cells and T cells, its primary function has remained obscure. We now report that CD30 signals may serve to inhibit effector cell activity by integrating gene expression changes of several pathways important for cytotoxic NK and T cell effector function. In the large granular lymphoma line YT, CD30 signals down-regulate the expression of cytotoxic effector molecules, Fas ligand, perforin, granzyme B, and abrogate cytotoxicity. c-myc, a regulator of proliferation and an upstream regulator of Fas ligand expression, is completely suppressed by CD30. Furthermore, CD30 signals strongly induce CCR7, suggesting a role for CD30 signals in the homing of lymphocytes to lymph nodes. The up-regulation of Fas, death receptor 3, and TNF-related apoptosis-inducing ligand by CD30 indicates an increase in susceptibility to apoptotic signals whereas up-regulation of TNFR-associated factor 1 and cellular inhibitor of apoptosis 2 protect cells from certain types of apoptosis. Using gene microarrays, 750 gene products were induced and 90 gene products were suppressed >2-fold by CD30 signals. Signals emanating from CD30 use both TNFR-associated factor 2-dependent and -independent pathways. The integration of CD30 signals in a lymphoma line suggests that CD30 can down-modulate lymphocyte effector function and proliferation while directing the cells to lymph nodes and increasing their susceptibility to certain apoptotic signals. These studies may provide a molecular mechanism for the recently observed CD30-mediated suppression of CTL activity in vivo in a diabetes model.

Keloid-derived fibroblasts are refractory to Fas-mediated apoptosis and neutralization of autocrine transforming growth factor-beta1 can abrogate this resistance

The pathogenesis of keloid remains poorly understood. As no effective therapy for keloid is as yet available, an insight into its pathogenesis may lead to novel approaches. Apoptosis has been found to mediate the decrease in cellularity during the transition between granulation tissue and scar. Here, we report that in contrast to hypertrophic scar-derived and normal skin-derived fibroblasts, keloid-derived fibroblasts are significantly resistant to both Fas-mediated and staurosporine-induced apoptosis. The caspases-3, -8, and -9 were not activated indicating that the block in the apoptotic pathway in keloid is upstream of the caspases. There were no significant differences in the level of expression of Fas, Bcl-2, and Bax between the three groups but addition of transforming growth factor (TGF)-beta1 significantly inhibited Fas-mediated apoptosis in hypertrophic scar-derived and normal skin-derived fibroblasts and neutralization of autocrine TGF-beta1 with anti-TGF-beta1 antibody abrogated the resistance of keloid-derived fibroblasts. Anti-apoptotic activity was not observed with TGF-beta2. This is the first study linking refractory Fas-mediated apoptosis to cellular phenotype in keloids and indicating a pivotal role for the anti-apoptotic effect of TGF-beta1 in this resistance. Hence, it becomes important to treat keloids as a separate entity different from hypertrophic scars and enhancement of Fas-sensitivity could be a promising therapeutic target.

A 'non-canonical' DNA-binding element mediates the response of the Fas-ligand promoter to c-Myc

Cell number is regulated by maintaining a balance between cell proliferation and cell death through apoptosis. Key regulators of this balance include the oncogene product c-Myc, which promotes either entry into the cell cycle or apoptosis [1]. Although the mechanism of c-Myc-induced apoptosis remains unclear, it is susceptible to regulation by survival factors [2,3] and can proceed through the interaction of Fas ligand (FasL) with its receptor, Fas [4]. Activated T lymphocytes are eliminated by an apoptotic process known as activation-induced cell death (AICD), which requires the transcriptional induction of FasL expression [5-7] and sustained levels of c-Myc [8]. The FasL promoter can be driven by c-Myc overexpression, and functional inhibitors of Myc and its binding partner, Max, inhibit the transcriptional activity of the FasL promoter [9,10]. We identified a non-canonical binding site (ATTCTCT) for c-Myc-Max heterodimers in the FasL promoter, which, when mutated, abolished activity in response to c-Myc. Exchange of the canonical c-Myc responsive elements (CACGTG) in the ornithine decarboxylase (ODC) promoter [11] with the non-canonical sequence in the FasL promoter generated an ODC-FasL promoter that was significantly more responsive to c-Myc than the wild-type ODC promoter. Our findings identify a precise physiological role for c-Myc in the induction of apoptosis as a transcriptional regulator of the FasL gene.

Transforming growth factor-β: pleiotropic role in the regulation of hematopoiesis

Hematopoiesis is a remarkable cell-renewal process that leads to the continuous generation of large numbers of multiple mature cell types, starting from a relatively small stem cell compartment. A highly complex but efficient regulatory network is necessary to tightly control this production and to maintain the hematopoietic tissue in homeostasis. During the last 3 decades, constantly growing numbers of molecules involved in this regulation have been identified. They include soluble cytokines and growth factors, cell–cell interaction molecules, and extracellular matrix components, which provide a multifunctional scaffolding specific for each tissue. The cloning of numerous growth factors and their mass production have led to their possible use for both fundamental research and clinical application.

Transforming growth factor-β: pleiotropic role in the regulation of hematopoiesis

Hematopoiesis is a remarkable cell-renewal process that leads to the continuous generation of large numbers of multiple mature cell types, starting from a relatively small stem cell compartment. A highly complex but efficient regulatory network is necessary to tightly control this production and to maintain the hematopoietic tissue in homeostasis. During the last 3 decades, constantly growing numbers of molecules involved in this regulation have been identified. They include soluble cytokines and growth factors, cell–cell interaction molecules, and extracellular matrix components, which provide a multifunctional scaffolding specific for each tissue. The cloning of numerous growth factors and their mass production have led to their possible use for both fundamental research and clinical application.

Apoptosis induced by TGF-beta 1 in Burkitt's lymphoma cells is caspase 8 dependent but is death receptor independent

TGF-beta is a potent inducer of apoptosis in many Burkitt's lymphoma (BL) cell lines. In this study, we characterize this apoptotic process in the EBV-negative BL41 cell line. Induction of apoptosis was detected as early as 8 h after TGF-beta treatment, as assayed by TUNEL and poly(ADP-ribose) polymerase cleavage. FACS analysis demonstrates that this proceeds predominately from the G1, but also from the G2/M phases of the cell cycle. We observed no early detectable changes in the steady-state levels of Bcl-2 and several of its family members after TGF-beta treatment. We detected cleavage of caspases 2, 3, 7, 8, and 9 into their active subunits. Consistent with the involvement of these enzymes in TGF-beta-mediated apoptosis, the broad spectrum caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(Ome)-flouromethylketone (ZVAD-fmk) blocked TGF-beta-induced apoptosis and revealed a G1 arrest in treated cells. Use of specific caspase inhibitors revealed that the induction of apoptosis is caspase 8 dependent, but caspase 3 independent. Activation of caspase 8 has been shown to be a critical event in death receptor-mediated apoptosis. However, TGF-beta treatment of BL41 cells was found not to affect the cell surface expression of Fas, TNF-R1, DR3, DR4, or DR5, or the steady-state expression levels of Fas ligand, TNF-R1, DR3, DR4, and DR5. Furthermore, blocking experiments indicated that TGF-beta-mediated apoptosis is not dependent on Fas ligand, TNF-alpha, tumor necrosis-like apoptosis-inducing ligand, or TNF-like weak inducer of apoptosis signaling. Therefore, it appears that TGF-beta induces apoptosis in BL cell lines via caspase 8 in a death receptor-independent fashion.

The role of the immune system in conjunctival wound healing after glaucoma surgery

The immune system has a fundamental role in the development and regulation of ocular healing, which plays an important role in the pathogenesis of most blinding diseases. This review discusses the mechanisms of normal wound healing, describing the animal and fetal wound healing models used to provide further insight into normal wound repair. In particular, conjunctival wound repair after glaucoma filtration surgery will be used to illustrate the contributions that the different components of the immune system make to the healing process. The potential role of macrophages, the possible regulatory effect of lymphocytes, and the important role of growth factors and cytokines in the wound healing reaction are discussed. The significance of the immune system in the pathogenesis of aggressive conjunctival scarring is addressed, particularly assessing the predisposing factors, including drugs, age, and ethnicity. The rationale behind the pharmacological agents currently used to modulate the wound healing response and the effects these drugs have on the function of the immune system are described. Finally, potential new therapeutic approaches to regulating the wound healing response are reported.

Induction of Fas ligand-mediated apoptosis by interferon-alpha

Interferon-alpha (IFN-alpha) was among the first cytokines studied and the earliest to be used in clinical medicine for the treatment of viral infections and malignancies. Although the capacity of IFN-alpha to augment NK cell cytotoxicity against virus-infected target cells or tumor cells is well established, the mechanism has not been fully elucidated. Here we report that IFN-alpha stimulation of PBMC from healthy donors induces Fas (CD95) ligand (FasL) transcription and leads to increased cell surface FasL expression exclusively on the NK cell fraction. Furthermore, IFN-alpha augments the FasL-mediated cytotoxicity of normal PBMC against Fas-sensitive lymphoid tumor cells. In the context of innate immunity, induction of FasL by IFN-alpha can be viewed as an efficient mechanism to potentiate NK cell cytotoxicity in the presence of harmful targets, such as virally infected or transformed cells.

Expression of Fas ligand in activated T cells is regulated by c-Myc

The transcription factor c-Myc is important for the control of cell cycle progression, neoplasia, and apoptotic cell death. c-Myc dimerizes with its partner Max to form an active transcription factor complex. Little is known, however, about the transcriptional targets of c-Myc and their roles in c-Myc-induced cell death. Here we demonstrate that T cell activation-induced expression of Fas ligand (FasL, CD95-L, APO-1-L), which can induce apoptotic cell death in many different cell types, is regulated by c-Myc. Down-modulation of c-Myc protein via antisense oligonucleotides blocked activation-induced FasL mRNA and protein expression and functional FasL expression in activated T cells and T cell lines. Further, FasL promoter activity in T cells is driven by overexpression of c-Myc and inhibited by expression of dominant-negative mutants of c-Myc and Max. Our findings indicate that c-Myc controls apoptotic cell death in T cells through regulation of FasL expression.

Engagement of the α2β1 integrin inhibits Fas ligand expression and activation-induced cell death in T cells in a focal adhesion kinase-dependent manner

T-cell receptor (TCR)-mediated apoptosis, also known as activation-induced cell death (AICD), plays an important role in the control of immune response and in the development of T-cell repertoire. Mechanistically, AICD has been largely attributed to the interaction of Fas ligand (Fas-L) with its cell surface receptor Fas in activated T cells. Signal transduction mediated by the integrin family of cell adhesion receptors has been previously shown to modulate apoptosis in a number of different cell types; in T cells, integrin signaling is known to be important in cellular response to antigenic challenge by providing a co-stimulatory signal for TCR. In this study we demonstrate that signaling via the collagen receptor 2β1 integrin specifically inhibits AICD by inhibiting Fas-L expression in activated Jurkat T cells. Engagement of the 2β1 integrin with monoclonal antibodies or with type I collagen, a cognate ligand for 2β1, reduced anti-CD3 and PMA/ionomycin-induced cell death by 30% and 40%, respectively, and the expression of Fas-L mRNA by 50%. Further studies indicated that the 2β1-mediated inhibition of AICD and Fas-L expression required the focal adhesion kinase FAK, a known component in the integrin signaling pathways. These results suggest a role for the 2β1 integrin in the control of homeostasis of immune response and T-cell development.

TGF-beta influences the life and death decisions of T lymphocytes

TGF-beta is a powerful mediator of immune cell phenotype and function. In TGF-beta1 homozygous null mice, aberrant regulation of the immune response culminates in lethal cardiopulmonary inflammation. In dissecting the underlying mechanisms leading to the attack of self, a role for TGF-beta1 in controlling apoptosis and T cell selection patterns was uncovered. Increased levels of apoptosis and TCR mediated cell death disrupted normal negative and positive T cell selection in the thymus. Moreover, in peripheral T cell populations, increased T lymphocyte death was associated with increased expression of apoptosis-inducing receptors. Persistent activation of T cells engendered unchecked apoptosis which, rather than reducing, further exacerbated, tissue inflammation due to the absence of TGF-beta1. TGF-beta, normally generated by macrophages during clearance of apoptotic cells contributes to dampening of inflammatory sequelae associated with phagocytosis. Collectively, these data demonstrate a pivotal role for TGF-beta in multiple stages of T cell apoptosis, selection, activation and clearance.

TGF-beta mediates CTLA-4 suppression of cellular immunity in murine kalaazar

Recent studies indicate important roles for CTLA-4 engagement in T cells, and for TGF-beta production in the immunopathogenesis of murine kalaazar or visceral leishmaniasis, but a functional link between these two pathways in helping intracellular parasite growth is unknown. Here we report that Ag or anti-CD3 activation of splenic CD4+ T cells from visceral leishmaniasis leads to intense CTLA-4-mediated TGF-beta1 production, as assessed either by CTLA-4 blockade or by direct CTLA-4 cross-linkage. Production of TGF-beta1 accounted for the reciprocal regulation of IFN-gamma production by CTLA-4 engagement. Following CD4+ T cell activation, intracellular growth of Leishmania chagasi in cocultured splenic macrophages required both CTLA-4 function and TGF-beta1 secretion. Cross-linkage of CTLA-4 markedly increased L. chagasi replication in cocultures of infected macrophages and activated CD4+ T cells, and parasite growth could be completely blocked with neutralizing anti-TGF-beta1 Ab. Exogenous addition of rTGF-beta1 restored parasite growth in cultures protected from parasitism by CTLA-4 blockade. These results indicate that the negative costimulatory receptor CTLA-4 is critically involved in TGF-beta production and in intracellular parasite replication seen in murine kalaazar.

Sensitivity to myc-induced apoptosis is retained in spontaneous and transplanted lymphomas of CD2-mycER mice

To study the effects of the Myc oncoprotein in a regulatable in vivo system, we generated lines of transgenic mice in which a tamoxifen inducible Myc fusion protein (c-mycER) is expressed under the control of the CD2 locus control region. Activation of the Myc oncoprotein resulted in both proliferation and apoptosis in vivo. Lines with a high transgene copy number developed spontaneous lymphomas at low frequency, but the tumour incidence was significantly increased with tamoxifen treatment. Surprisingly, we found that cellular sensitivity to Myc-induced apoptosis was retained in tumours from these mice and in most lymphoma cell lines, even when null for p53. Resistance to Myc-induced apoptosis could be conferred on these cells by co-expression of Bcl-2. However, acquired resistance is clearly not an obligatory progression event as sensitivity to apoptosis was retained in transplanted tumours in athymic mice. In conclusion, lymphomas arising in CD2-mycER mice retain the capacity to undergo apoptosis in response to Myc activation and show no phenotypic evidence of the presence of an active dominant inhibitor.

Vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide inhibit antigen-induced apoptosis of mature T lymphocytes by inhibiting Fas ligand expression

Apoptosis in T and B lymphocytes is a major element controlling the immune response. The Ag-induced cell death (AICD) in T cells is a main mechanism for maintaining peripheral tolerance and for limiting an ongoing immune response. AICD is initiated by Ag re-engagement of the TCR and is mediated through Fas/Fas ligand (FasL) interactions. Vasoactive intestinal peptide (VIP) and the structurally related pituitary adenylate cyclase-activating polypeptide (PACAP) are two multifunctional neuropeptides present in the lymphoid microenvironment that act primarily as anti-inflammatory agents. In the present study we investigated whether VIP and PACAP affect AICD in mature peripheral T cells and T cell hybridomas. VIP and PACAP reduce in a dose-dependent manner anti-CD3-induced apoptosis in Con A/IL-2-preactivated peripheral T cells and the murine T hybridomas 2B4.11 and A1.1. A functional study demonstrates that the inhibition of AICD is achieved through the inhibition of activation-induced FasL expression at protein and mRNA levels. VIP/PACAP-mediated inhibition of both AICD and FasL expression is mediated through the specific receptors VPAC1 and VPAC2. Of obvious biological significance is the fact that VIP and PACAP prevent Ag-induced clonal deletion of CD4+ T cells, but not that of CD8+ T cells. By affecting FasL expression, VIP and PACAP may play a physiological role in both the generation of memory T cells and the inhibition of FasL-mediated T cell cytotoxicity.

The inhibitory effects of transforming growth factor-beta-1 (TGF-beta1) in autoimmune diseases

The importance of transforming growth factor-beta-1 (TGF-beta1) in immunoregulation and tolerance has been increasingly recognized. It is now proposed that there are populations of regulatory T cells (T-reg), some designated T-helper type 3 (Th3), that exert their action primarily by secreting this cytokine. Here, we emphasize the following concepts: (1) TGF-beta1 has multiple suppressive actions on T cells, B cells, macrophages, and other cells, and increased TGF-beta1 production correlates with protection and/or recovery from autoimmune diseases; (2) TGF-beta1 and CTLA-4 are molecules that work together to terminate immune responses; (3) Th0, Th1 and Th2 clones can all secrete TGF-beta1 upon cross-linking of CTLA-4 (the functional significance of this in autoimmune diseases has not been reported, but TGF-beta1-producing regulatory T-cell clones can produce type 1 inflammatory cytokines); (4) TGF-beta1 may play a role in the passage from effector to memory T cells; (5) TGF-beta1 acts with some other inhibitory molecules to maintain a state of tolerance, which is most evident in immunologically privileged sites, but may also be important in other organs; (6) TGF-beta1 is produced by many cell types, is always present in the plasma (in its latent form) and permeates all organs, binding to matrix components and creating a reservoir of this immunosuppressive molecule; and (7) TGF-beta1 downregulates adhesion molecules and inhibits adhesion of leukocytes to endothelial cells. We propose that rather than being passive targets of autoimmunity, tissues and organs actively suppress autoreactive lymphocytes. We review the beneficial effects of administering TGF-beta1 in several autoimmune diseases, and show that it can be effectively administered by a somatic gene therapy approach, which results in depressed inflammatory cytokine production and increased endogenous regulatory cytokine production.

Differential regulation and function of Fas expression on glial cells

Fas/Apo-1 is a member of the TNF receptor superfamily that signals apoptotic cell death in susceptible target cells. Fas or Fas ligand (FasL)-deficient mice are relatively resistant to the induction of experimental allergic encephalomyelitis, implying the involvement of Fas/FasL in this disease process. We have examined the regulation and function of Fas expression in glial cells (astrocytes and microglia). Fas is constitutively expressed by primary murine microglia at a low level and significantly up-regulated by TNF-alpha or IFN-gamma stimulation. Primary astrocytes express high constitutive levels of Fas, which are not further affected by cytokine treatment. In microglia, Fas expression is regulated at the level of mRNA expression; TNF-alpha and IFN-gamma induced Fas mRNA by approximately 20-fold. STAT-1alpha and NF-kappaB activation are involved in IFN-gamma- or TNF-alpha-mediated Fas up-regulation in microglia, respectively. The cytokine TGF-beta inhibits basal expression of Fas as well as cytokine-mediated Fas expression by microglia. Upon incubation of microglial cells with FasL-expressing cells, approximately 20% of cells underwent Fas-mediated cell death, which increased to approximately 60% when cells were pretreated with either TNF-alpha or IFN-gamma. TGF-beta treatment inhibited Fas-mediated cell death of TNF-alpha- or IFN-gamma-stimulated microglial cells. In contrast, astrocytes are resistant to Fas-mediated cell death, however, ligation of Fas induces expression of the chemokines macrophage inflammatory protein-1beta (MIP-1beta), MIP-1alpha, and MIP-2. These data demonstrate that Fas transmits different signals in the two glial cell populations: a cytotoxic signal in microglia and an inflammatory signal in the astrocyte.

Cloak and dagger in the avoidance of immune surveillance

CD95 and CD95-ligand (CD95L) are physiological mediators of apoptosis required for the control of cell numbers in the human immune system. Discoveries in CD95-dependent mechanisms of immune evasion by tumours suggest regulation by oncogene expression. Clonal contraction of lymphocytes by a CD95/CD95L-independent mechanism has been reported and new evidence supports a role for CD95-dependent peripheral lymphocyte deletion by non-lymphoid tissue. Additionally, factors affecting the pro- and anti-inflammatory effects of CD95L point to a balance of cytokines and growth factors.

Modulation of hematopoietic stem/progenitor cell engraftment by transforming growth factor beta

OBJECTIVE

To investigate if cell cycle progression plays a role in modulating the engraftment potential of mouse hematopoietic stem and progenitor cells (HSPC).

MATERIALS AND METHODS

HSPC were isolated from adult mouse bone marrow, cultured in vitro under conditions promoting cell cycle arrest, and subsequently were evaluated for cell cycle status, clonogenic activity, and transplant potential. RESULTS In the presence of steel factor (STL) as a survival cytokine, transforming growth factor beta (TGF-beta) increased the G0/G1 fraction of cycling progenitor cells (Rh(high)) after a 20-hour culture. Clonogenic activity of quiescent long-term repopulating (Rh(low)) HSPC was unaffected by this culture, whereas clonogenic potential of Rh(high) cells decreased by about 30%. In competitive repopulation assays, Rh(low) cells cultured in STL + TGF-beta engrafted better than cells cultured in STL alone. However, culture in STL + TGF-beta did not overcome the failure of Rh(high) cells to engraft after transplant. We also utilized a two-stage culture system to first induce proliferation of Rh(low) HSPC by a 48-hour culture in STL + interleukin 6 + Flt-3 ligand, followed by shifting the culture to STL + TGF-beta for 24 hours to induce cycle arrest. A competitive repopulation assay demonstrated a relative decrease in repopulating potential in cultures that were cycle arrested compared to those that were not.

CONCLUSION

Cell cycle progression by itself cannot account for the decrease in repopulating potential that is observed after ex vivo expansion. Other determinants of engraftment must be identified to facilitate the transplantation of cultured HSPC.

TGF-beta1: immunosuppressant and viability factor for T lymphocytes

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine with multiple roles in the immune system. To date, it has been difficult to develop a comprehensive picture of the effect of TGF-beta on T lymphocytes, because TGF-beta not only acts directly on T lymphocytes, but also acts indirectly by regulating the function of antigen-presenting cells. In early studies, it was mostly the inhibitory function of TGF-beta that was demonstrated; recently, however TGF-beta was recognized as an antiapoptotic survival factor for T lymphocytes. The outcome of the TGF-beta effect on T lymphocytes was shown to strongly depend on their stage of differentiation and on the cytokine milieu. TGF-beta cannot be classified as a classical Th1 or Th2 cytokine. However, recently the existence of the TGF-beta-producing Th3 subset was described which might play an important regulatory role during an immune response. A better understanding of the molecular mechanism of how TGF-beta inhibits or stimulates T lymphocytes will help to predict the complex functions of this cytokine.

The Role of the Common Cytokine Receptor γ-Chain in Regulating IL-2-Dependent, Activation-Induced CD8+ T Cell Death

IL-2-dependent, activation-induced T cell death (AICD) plays an important role in peripheral tolerance. Using CD8+ TCR-transgenic lymphocytes (2C), we investigated the mechanisms by which IL-2 prepares CD8+ T cells for AICD. We found that both Fas and TNFR death pathways mediate the AICD of 2C cells. Neutralizing IL-2, IL-2Rα, or IL-2Rβ inhibited AICD. In contrast, blocking the common cytokine receptor γ-chain (γc) prevented Bcl-2 induction and augmented AICD. IL-2 up-regulated Fas ligand (FasL) and down-regulated γc expression on activated 2C cells in vitro and in vivo. Adult IL-2 gene-knockout mice displayed exaggerated γc expression on their CD8+, but not on their CD4+, T cells. IL-4, IL-7, and IL-15, which do not promote AICD, did not influence FasL or γc expression. These data provide evidence that IL-2 prepares CD8+ T lymphocytes for AICD by at least two mechanisms: 1) by up-regulating a pro-apoptotic molecule, FasL, and 2) by down-regulating a survival molecule, γc.

Crosstalk Between Keratinocytes and T Lymphocytes via Fas/Fas Ligand Interaction: Modulation by Cytokines

Apoptosis mediated by Fas/FasL interaction plays an important role during many inflammatory skin disorders. To estimate whether the expression of FasL, the ligand for Fas, might be regulated by cytokines we stimulated primary human keratinocytes with several pro- and anti-inflammatory cytokines. Keratinocytes cultured to subconfluence expressed FasL constitutively. Cells stimulated with the proinflammatory cytokines IL-1β, TNF-α, IFN-γ, and IL-15, respectively, increased significantly their intracellular as well as cell surface-bound FasL expression in a time- and dose-dependent manner. This cytokine-induced FasL expression was dependent on new protein synthesis. Despite enhanced expression of cell surface-bound FasL, no release of soluble FasL was measured in the cell supernatants determined by ELISA. Stimulation of the cells with IL-6, IL-10, IL-12, TGF-β1, and GM-CSF did not modulate the constitutive FasL expression, but IFN-γ-mediated FasL up-regulation was significantly diminished by IL-10 and TGF-β1, respectively. Up-regulation of FasL on IFN-γ-stimulated keratinocytes led to increased apoptosis within monolayers cultured for 48 h. Moreover, coculture experiments performed with Fas+ Jurkat T cells revealed that enhanced FasL expression on IFN-γ-stimulated keratinocytes induced apoptosis in cocultured T cells, demonstrating that up-regulated FasL was functionally active. In summary, our data suggest the important regulatory role of cytokine-controlled Fas/FasL interaction in the cross-talk between keratinocytes and skin-infiltrating T cells for maintenance of homeostasis in inflammatory skin processes.