Cellular FLIP long form-transgenic mice manifest a Th2 cytokine bias and enhanced allergic airway inflammation

Bioinformatics analysis of ceRNA network of autophagy-related genes in pediatric asthma

The molecular underpinnings of pediatric asthma present avenues for targeted therapies. A deeper exploration into the significance of differentially expressed autophagy-related genes (DE-ARGs) and their interactions with the long noncoding RNA (lncRNA)-microRNA (miRNA)-mRNA network may offer insights into the pathogenesis of pediatric asthma. DE-ARGs were retrieved from the Gene Expression Omnibus and the Human Autophagy Database. These DE-ARGs were subjected to comprehensive analyses, including Gene Ontology, Kyoto Encyclopedia of Genes and Genomes pathway, Gene Set Enrichment Analysis, and protein-protein interaction networks. The identified DE-ARGs were further verified for core gene expression. The miRDB and ENCORI databases were used for inverse miRNA predictions. Furthermore, miRNA-lncRNA interactions were predicted using LncBase and ENCORI platforms. Following the exclusion of lncRNAs exclusively localized in the nucleus and extracellular space, a competitive endogenous RNA (ceRNA) network was established and subsequently subjected to detailed analysis. The mRNA expression patterns in the ceRNA network were validated using quantitative real-time PCR. In total, 31 DE-ARGs were obtained, of which 29 were up-regulated and 2 were down-regulated. Notably, the autophagy, regulation of apoptotic signaling pathways, interferon-α/β signaling, interferon γ signaling, autophagy-animal, and apoptosis pathways were predominantly enriched in pediatric asthma. Five hub genes (VEGFA, CFLAR, RELA, FAS, and ATF6) were further analyzed using the Gene Expression Omnibus dataset to verify their expression patterns and diagnostic efficacy. Four hub genes (VEGFA, CFLAR, RELA, and FAS) were obtained. Finally, a ceRNA network of 4 mRNAs (VEGFA, CFLAR, RELA, and FAS), 3 miRNAs (hsa-miR-320b, hsa-miR-22-3p, and hsa-miR-625-5p), and 35 lncRNAs was constructed by integrating data from literature review and analyzing the predicted miRNAs and lncRNAs. Moreover, the quantitative real-time PCR data revealed a pronounced upregulation of Fas cell surface death receptor. The identification of 4 DE-ARGs, especially Fas cell surface death receptor, has shed light on their potential pivotal role in the pathogenesis of pediatric asthma. The established ceRNA network provides novel insights into the autophagy mechanism in asthma and suggests promising avenues for the development of potential therapeutic strategies.

Regulation of autoimmune disease by the E3 ubiquitin ligase Itch

Itch is a HECT type E3 ubiquitin ligase that is required to prevent the development of autoimmune disease in both mice and humans. Itch is expressed in most mammalian cell types, and, based on published data, it regulates many cellular pathways ranging from T cell differentiation to liver tumorigenesis. Since 1998, when Itch was first discovered, hundreds of publications have described mechanisms through which Itch controls various biologic activities in both immune and non-immune cells. Other studies have provided insight into how Itch catalytic activity is regulated. However, while autoimmunity is the primary clinical feature that occurs in both mice and humans lacking Itch, and Itch control of immune cell function has been well-studied, it remains unclear how Itch prevents the emergence of autoimmune disease. In this review, we explore recent discoveries that advance our understanding of how Itch regulates immune cell biology, and the extent to which these clarify how Itch prevents autoimmune disease. Additionally, we discuss how molecular regulators of Itch impact its ability to control these processes, as this may provide clues on how to therapeutically target Itch to treat patients with autoimmune disease.

FLIP the Switch: Regulation of Apoptosis and Necroptosis by cFLIP

cFLIP (cellular FLICE-like inhibitory protein) is structurally related to caspase-8 but lacks proteolytic activity due to multiple amino acid substitutions of catalytically important residues. cFLIP protein is evolutionarily conserved and expressed as three functionally different isoforms in humans (cFLIP_L, cFLIP_S, and cFLIP_R). cFLIP controls not only the classical death receptor-mediated extrinsic apoptosis pathway, but also the non-conventional pattern recognition receptor-dependent apoptotic pathway. In addition, cFLIP regulates the formation of the death receptor-independent apoptotic platform named the ripoptosome. Moreover, recent studies have revealed that cFLIP is also involved in a non-apoptotic cell death pathway known as programmed necrosis or necroptosis. These functions of cFLIP are strictly controlled in an isoform-, concentration- and tissue-specific manner, and the ubiquitin-proteasome system plays an important role in regulating the stability of cFLIP. In this review, we summarize the current scientific findings from biochemical analyses, cell biological studies, mathematical modeling, and gene-manipulated mice models to illustrate the critical role of cFLIP as a switch to determine the destiny of cells among survival, apoptosis, and necroptosis.

Cellular FLICE-Inhibitory Protein Regulates Tissue Homeostasis

Cellular FLICE-inhibitory protein (cFLIP) is structurally related to caspase-8, but lacks its protease activity. Cflip gene encodes several splicing variants including short form (cFLIPs) and long form (cFLIP_L). cFLIP_L is composed of two death effector domains at the N terminus and a C-terminal caspase-like domain, and cFLIPs lacks the caspase-like domain. Our studies reveal that cFLIP plays a central role in NF-κB-dependent survival signals that control apoptosis and programmed necrosis. Germline deletion of Cflip results in embryonic lethality due to enhanced apoptosis and programmed necrosis; however, the combined deletion of the death-signaling regulators, Fadd and Ripk3, prevents embryonic lethality in Cflip-deficient mice. Moreover, tissue-specific deletion of Cflip reveals cFLIP as a crucial regulator that maintains tissue homeostasis of immune cells, hepatocytes, intestinal epithelial cells, and epidermal cells by preventing apoptosis and programmed necrosis.

Regulated cell death: signaling and mechanisms

Cell turnover is a fundamental feature in metazoans. Cells can die passively, as a consequence of severe damage to their structural integrity, or actively, owing to a more confined biological disruption such as DNA damage. Passive cell death is uncontrolled and often harmful to the organism. In contrast, active cell death is tightly regulated and serves to support the organism's life. Apoptosis-the primary form of regulated cell death-is relatively well defined. Necroptosis-an alternative, distinct kind of regulated cell death discovered more recently-is less well understood. Apoptosis and necroptosis can be triggered either from within the cell or by extracellular stimuli. Certain signaling components, including several death ligands and receptors, can regulate both processes. Whereas apoptosis is triggered and executed via intracellular proteases called caspases, necroptosis is suppressed by caspase activity. Here we highlight current understanding of the key signaling mechanisms that control regulated cell death.

The regulation and role of c-FLIP in human Th cell differentiation

The early differentiation of T helper (Th) cells is a tightly controlled and finely balanced process, which involves several factors including cytokines, transcription factors and co-stimulatory molecules. Recent studies have shown that in addition to the regulation of apoptosis, caspase activity is also needed for Th cell proliferation and activation and it might play a role in Th cell differentiation. The isoforms of the cellular FLICE inhibitory protein (c-FLIP) are regulators of CASPASE-8 activity and the short isoform, c-FLIP_S, has been shown to be up-regulated by IL-4, the Th2 driving cytokine. In this work, we have studied the expression and functional role of three c-FLIP isoforms during the early Th cell differentiation. Only two of the isoforms, c-FLIP_S and c-FLIP_L, were detected at the protein level although c-FLIP_R was expressed at the mRNA level. The knockdown of c-FLIP_L led to enhanced Th1 differentiation and elevated IL-4 production by Th2 cells, whereas the knockdown of c-FLIP_S diminished GATA3 expression and IL-4 production by Th2 cells. In summary, our results provide new insight into the role of c-FLIP proteins in the early differentiation of human Th cells.

cFLIP expression is altered in severe corticosteroid-resistant asthma

Dysregulation of alternative splicing of mRNA precursors is known to contribute to numerous human diseases. In this study we carried out the first systematic search for asthma-associated changes in alternative splicing events, using a model of Aspergillus fumigatus (A. fumigatus)-sensitized mice and an exon junction microarray to detect potential changes in alternative splicing. One of the sensitization-associated changes identified in the search was a shift in alternative splicing of the mRNA encoding cFLIP, a modulator of the caspase-mediated extrinsic apoptosis pathway. Expanding these studies to human asthma patients, we discovered a significant decrease in the expression of both cFLIP isoforms in severe corticosteroid-resistant asthmatics. Although it is unclear whether these changes were due solely to differences in alternative splicing, these findings provide evidence that dysregulation of the extrinsic apoptosis pathway is part of the underlying immunopathogenesis of severe refractory asthma.

Inhibition of caspase-8 activity promotes protective Th1- and Th2-mediated immunity to Leishmania major infection

We investigated how apoptosis pathways mediated by death receptors and caspase-8 affect cytokine responses and immunity to Leishmania major parasites. Splenic CD4 T cells undergo activation-induced apoptosis, and blockade of FasL-Fas interaction increased IFN-γ and IL-4 cytokine responses to L. major antigens. To block death receptor-induced death, we used mice expressing a T cell-restricted transgene for vFLIP. Inhibition of caspase-8 activation in vFLIP mice enhanced Th1 and Th2 cytokine responses to L. major infection, even in the Th1-prone B6 background. We also observed increased NO production by splenocytes from vFLIP mice upon T cell activation. Despite an exacerbated Th2 response, vFLIP mice controlled better L. major infection, with reduced lesions and lower parasite loads compared with WT mice. Moreover, injection of anti-IL-4 mAb in infected vFLIP mice disrupted control of parasite infection. Therefore, blockade of caspase-8 activity in T cells improves immunity to L. major infection by promoting increased Th1 and Th2 responses.

Geniposide inhibits airway inflammation and hyperresponsiveness in a mouse model of asthma

Our group recently reported the strong anti-inflammatory effects of geniposide (Gen), a bioactive iridoid glucoside derived from gardenia jasminoides, in a mouse acute lung injury model. Herein, we hypothesized that Gen might also have potential therapeutic benefits in treatment of asthma, which was tested in a mouse model of ovalbumin (Ova)-induced allergic airway inflammation. Ova-sensitized and -challenged BALB/c mice, as compared with control animals, displayed airway hyperresponsiveness (AHR), bronchoalveolar lavage eosinophilia, mucus hypersecretion, and increased T help 2 (Th2)-associated cytokine and chemokine amounts, as well as serum Ova-specific immunoglobulin E (IgE) level. Being compared with the Ova-induced hallmarks of asthma, intraperitoneal Gen treatment prevented eosinophilic pulmonary infiltration, attenuated the increases in interleukin (IL)-4, IL-5, and IL-13, and reduced eotaxin and vascular cell adhesion molecule 1 (VCAM-1) expression. Also, Gen significantly ameliorated the Ova-driven airway hyperresponsiveness, mucus hypersecretion, and allergen-specific IgE level, which are the cardinal pathophysiological symptoms in allergic airway diseases. In addition, the efficacy of Gen was comparable to that of dexamethasone (Dex), a currently available anti-asthmatic drug. Collectively, our findings reveal that the development of immunoregulatory strategies based on Gen may be considered as an effective adjuvant therapy for allergic asthma.

Proliferative versus apoptotic functions of caspase-8 Hetero or homo: the caspase-8 dimer controls cell fate

Caspase-8, the initiator of extrinsically-triggered apoptosis, also has important functions in cellular activation and differentiation downstream of a variety of cell surface receptors. It has become increasingly clear that the heterodimer of caspase-8 with the long isoform of cellular FLIP (FLIP(L)) fulfills these pro-survival functions of caspase-8. FLIP(L), a catalytically defective caspase-8 paralog, can interact with caspase-8 to activate its catalytic function. The caspase-8/FLIP(L) heterodimer has a restricted substrate repertoire and does not induce apoptosis. In essence, caspase-8 heterodimerized with FLIP(L) prevents the receptor interacting kinases RIPK1 and -3 from executing the form of cell death known as necroptosis. This review discusses the latest insights in caspase-8 homo- versus heterodimerization and the implication this has for cellular death or survival. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

TNFRI is a positive T-cell costimulatory molecule important for the timing of cytokine responses

Tumor necrosis factor (TNF)- and TNF receptor I (TNFRI)-deficient mice are resistant to initiation and show delayed resolution of disease in paradigms of autoimmune disease, but the contribution of TNF/TNFRI signaling to T-cell activation and effector responses has not been determined. In this study, we investigated the role of TNFRI in T-cell receptor (TCR)-mediated T-cell activation in vitro and in vivo using CD3(+)-enriched primary T cells and mice deficient in TNFRI. Following TCR engagement, TNFRI knockout (KO) T cells showed significantly delayed proliferation, cell division, upregulation of interleukin 2 (IL-2) and IL-2 receptor alpha chain (CD25) mRNA and cell-surface expression of CD25 compared with wild-type (WT) cells. Thus, WT and TNFRI KO cells showed equivalent proliferation peaks at 48 and 72 h, respectively. TNFRI KO mice also developed a defective primary T-cell response to ovalbumin and an acute contact hypersensitivity response to oxazolone (4-ethoxymethylene-2-phenyl-2-oxazolin-5-one). However, TNFRI KO splenocytes that were stimulated by TCR engagement in vitro for 96 h produced significantly higher intracellular levels of interferon-gamma (IFN-gamma), IL-2 and TNF-alpha, but not IL-17, compared with WT cells, in correlation with their relatively higher proliferation rate at this time point. Further, TCR-stimulated CD3(+)-enriched TNFRI KO T cells showed similarly higher production and secretion of IFN-gamma and IL-2 compared with WT, suggesting that TNFRI-mediated cytokine regulation might involve a T-cell autonomous effect. Our results show a novel role for TNFRI as a positive T-cell costimulatory molecule that is important for timely T-cell activation and effector cytokine production and the development of primary immune responses in mice.

Altered thymic selection by overexpressing cellular FLICE inhibitory protein in T cells causes lupus-like syndrome in a BALB/c but not C57BL/6 strain

Cellular FLICE inhibitory protein (c-FLIP) is an endogenous inhibitor of the caspase-8 pro-apoptotic signaling pathway downstream of death receptors. Recent evidence indicates that the long form of c-FLIP (c-FLIP_L) is required for proliferation and effector T cell development. However, the role of c-FLIP_L in triggering autoimmunity has not been carefully investigated. We now report that c-FLIP_L transgenic (Tg) mice develop splenomegaly, lymphadenopathy, multi-organ infiltration, high titers of autoantibodies, and proliferative glomerulonephritis with immune complex deposition in a strain-dependent fashion. The development of autoimmunity requires CD4⁺ T cells and may result from impaired thymic selection. At the molecular level, c-FLIP_L over-expression inhibits the ZAP-70 activation, thus impairing the signaling pathway derived from ZAP-70 required for thymic selection. Therefore, we have identified c-FLIP_L as a susceptibility factor under the influence of epistatic modifiers for the development of autoimmunity.

Role of breast regression protein 39 (BRP-39)/chitinase 3-like-1 in Th2 and IL-13-induced tissue responses and apoptosis

Mouse breast regression protein 39 (BRP-39; Chi3l1) and its human homologue YKL-40 are chitinase-like proteins that lack chitinase activity. Although YKL-40 is expressed in exaggerated quantities and correlates with disease activity in asthma and many other disorders, the biological properties of BRP-39/YKL-40 have only been rudimentarily defined. We describe the generation and characterization of BRP-39(-/-) mice, YKL-40 transgenic mice, and mice that lack BRP-39 and produce YKL-40 only in their pulmonary epithelium. Studies of these mice demonstrated that BRP-39(-/-) animals have markedly diminished antigen-induced Th2 responses and that epithelial YKL-40 rescues the Th2 responses in these animals. The ability of interleukin13 to induce tissue inflammation and fibrosis was also markedly diminished in the absence of BRP-39. Mechanistic investigations demonstrated that BRP-39 and YKL-40 play an essential role in antigen sensitization and immunoglobulin E induction, stimulate dendritic cell accumulation and activation, and induce alternative macrophage activation. These proteins also inhibit inflammatory cell apoptosis/cell death while inhibiting Fas expression, activating protein kinase B/AKT, and inducing Faim 3. These studies establish novel regulatory roles for BRP-39/YKL-40 in the initiation and effector phases of Th2 inflammation and remodeling and suggest that these proteins are therapeutic targets in Th2- and macrophage-mediated disorders.

c-FLIP protects mature T lymphocytes from TCR-mediated killing

Although c-FLIP has been identified as an important player in the extrinsic (death receptor-induced) apoptosis pathway, its endogenous function in mature T lymphocytes remains undefined. c-FLIP may inhibit or promote T cell death as previous data demonstrate that the c-FLIP(L) isoform can promote or inhibit caspase 8 activation while the c-FLIP(S) isoform promotes or inhibits T cell death when overexpressed. Although the c-FLIP(R) isoform inhibits cell death in cell lines, its function in T cells remains unknown. To investigate the function of c-FLIP in mature T cells, we have generated several genetic mouse models with c-FLIP or its individual isoforms deleted in mature T cells. Surprisingly, we found that c-FLIP protects mature T cells not only from apoptosis induced by the death receptors Fas and TNFR but also from TCR-mediated and spontaneous apoptosis. Thus, c-FLIP plays an essential role in protecting mature T cells from a death signal induced through the TCR itself and is required for naive T cell survival. Our results demonstrate that c-FLIP functions beyond the extrinsic death pathway.

The long isoform of cellular FLIP is essential for T lymphocyte proliferation through an NF-kappaB-independent pathway

Although the long isoform of cellular FLIP (c-FLIP(L)) has been implicated in TCR-mediated signaling, its role in T cell proliferation remains controversial. Some studies have demonstrated that overexpression of c-FLIP(L) promotes T cell proliferation and NF-kappaB activation, whereas others have reported that c-FLIP(L) overexpression has no effect or even inhibits T cell proliferation. To establish the role of c-FLIP(L) in T lymphocyte proliferation, we have generated a conditional knockout mouse strain specifically lacking c-FLIP(L) in T lymphocytes. c-FLIP(L)(-/-) mice exhibit severely impaired effector T cell development after Listeria monocytogenes infection in vivo and c-FLIP(L)-deficient T cells display defective TCR-mediated proliferation in vitro. However, c-FLIP(L)(-/-) T cells exhibit normal NF-kappaB activity upon TCR stimulation. These results demonstrate that c-FLIP(L) is essential for T lymphocyte proliferation through an NF-kappaB-independent pathway.

Cellular FLIP long isoform transgenic mice overcome inherent Th2-biased immune responses to efficiently resolve Leishmania major infection

c-FLIP(L) expression in T cells is required for mounting effective T cell responses and can also be critical for effector T cell differentiation, as has recently been shown by a number of in vivo studies in conditional knockout and transgenic mouse systems. Available data supports therefore a novel immunomodulatory role of this anti-apoptotic protein besides its traditionally proposed function in homeostatic maintenance of T cell populations. In this study, the responses to infection with Leishmania major of mice over-expressing FLIP(L) specifically in the T cell compartment (TgFLIP(L)) are assessed. Although previous studies have shown that FLIP(L) drives T cells towards a T(h)2 differentiation programme in various autoimmune and allergic paradigms, in this study, we show that TgFLIP(L) are able to overcome this T(h)2 bias in a dermal L. major infection model to mount a robust T(h)1 response to pathogen and effectively clear infection. Our results suggest that vaccination protocols designed to enhance FLIP(L) expression in T cells may be useful for the treatment of autoimmune diseases like multiple sclerosis, without necessarily compromising immune responses towards infectious agents.

Contrasting effects of FLIPL overexpression in human T cells on activation-induced cell death and cytokine production

There have been disparate findings about the role of FLIP in the survival of mouse T cells and human tumor cell lines. The role of cellular FLIP in human T cell activation and function needs to be clarified further. To study this role, we have overexpressed long transcript FLIP (FLIPL) in primary T cells, including self-antigen-reactive, melanoma-specific T cells. We found that FLIPL overexpression protects human T cells from activation-induced cell death and enhances their proliferative capacity but suppresses the ability of these cells to produce the proinflammatory cytokines IL-2 and IFN-gamma in response to CD3 or antigen-specific stimulation. The multiple effects of FLIPL indicate that this protein may influence T cell responses to antigenic stimulation.

Promise and pitfalls in animal-based asthma research: building a better mousetrap

Asthma is one of the leading chronic diseases in the world today. An essential component of the asthma research endeavor is the animal-based experimental disease system, which provides knowledge that is not attainable through study of patients alone. Animal research is especially valuable for elucidating pathophysiology, drug testing, and as an adjunct for interpreting the results of human clinical trials. However, controversies surrounding animal systems data and at the interface between animal and human studies raise questions regarding the true utility of experimental asthma research. We consider here the considerable promise and important limitations of animal-based systems and their prospects for the future study asthma.

Lead and immune function

The heavy metal lead is a widely deposited environmental toxicant known to impact numerous physiological systems, including the reproductive, neurological, hepatic, renal, and immune systems. Studies illustrating the capacity of lead to impair immune function and/or host resistance to disease date back to at least the 1960s. However, it has only been in recent years that lead has been recognized among a new category of immunotoxicants-those that dramatically shift immune functional capacity while producing only modest changes to immune cell populations and lymphoid organs. These relatively noncytotoxic immunomodulating chemicals and drugs represent the immunotoxic hazards most difficult to identify and problematic for risk assessment using historic approaches. As a result, such environmental factors are also among the most likely to contribute to chronic immune-related disease at relevant exposure levels. This review considers the animal and human evidence that lead exposure can produce a stark shift in immune functional capacity with a skewing predicted to elevate the risk of atopic and certain autoimmune diseases. At the same time, host defenses against infectious agents and cancer may be reduced. Age-based exposure studies also suggest that levels of blood lead previously thought to be safe, that is, below 10 microg/dl, may be associated with later life immune alterations.

Reduced myocarditis following Coxsackievirus infection in cellular FLICE inhibitory protein--long form-transgenic mice

Cellular FLICE inhibitory protein--long form (c-FLIP(L)) is a caspase-defective homologue of caspase-8 that blocks apoptosis by death receptors. c-FLIP(L) expression in T cells can also augment activation of the mitogen-activated protein kinase, extracellular signal-related kinase, as well as nuclear factor-kappaB. This contributes to increased production of interleukin-2 and CD25, resulting in hyperproliferation of T cells from c-FLIP(L)-transgenic mice. c-FLIP also heterodimerizes with and activates caspase-8, resulting in increased death of T cells and a selection of a T helper 2 cytokine profile. The effects of c-FLIP on cytolytic function of CD8(+) T cells have not been examined previously. We studied the cytolytic capacity of T cells from c-FLIP(L)-transgenic mice using an antigen-specific system, as well as the consequences during a viral immune response to Coxsackievirus B3 (CVB3). The increased T-cell receptor (TCR) signalling due to c-FLIP did not alter the cytolytic machinery but did reduce cytotoxicity because of decreased surface expression of TCR and CD8. It also produced a Tc2 cytokine profile. These effects of c-FLIP collectively served to diminish the severity of CVB3-induced myocarditis.

Transgenic Expression of the Viral FLIP MC159 Causeslpr/gld-Like Lymphoproliferation and Autoimmunity

Death receptor-induced programmed cell death (PCD) is crucial for the maintenance of immune homeostasis. However, interference of downstream death receptor signaling by genetic ablation or transgenic (Tg) expression of different apoptosis inhibitors often impairs lymphocyte activation. The viral FLICE (caspase-8)-like inhibitor proteins (v-FLIPs) are potent inhibitors of death receptor-induced apoptosis and programmed necrosis. We generated Tg mice expressing the v-FLIP MC159 from Molluscum contagiosum virus under the control of the H2Kb class I MHC promoter to examine the role of death receptor-induced PCD in the control of immune functions and homeostasis. We found that expression of MC159 led to lymphoproliferation and autoimmunity as exemplified by T and B lymphocyte expansion, accumulation of TCRalphabeta+ CD3+ B220+ CD4- CD8- lymphocytes in secondary lymphoid organs, elevated serum Ig levels, and increased anti-dsDNA Ab titers. These phenotypes were caused by defective death receptor-induced apoptosis, but not by defective passive cell death in the absence of mitogenic stimulation. Lymphocyte activation was normal, as demonstrated by normal thymidine incorporation and CSFE dilution of T cells stimulated with anti-CD3 and anti-CD28 Abs. In addition, effector CD8+ T cell responses to acute and memory lymphocytic choriomeningitis virus infections were unaffected in the Tg mice. These phenotypes are reminiscent of the lpr and gld mice, and show that the v-FLIP MC159 is a bona fide PCD inhibitor that does not interfere with other essential lymphocyte functions. Thus, the MC159-Tg mice provide a model to study the effects of PCD in immune responses without hampering other important lymphocyte functions.

A network-based analysis of allergen-challenged CD4+ T cells from patients with allergic rhinitis

We performed a network-based analysis of DNA microarray data from allergen-challenged CD4(+) T cells from patients with seasonal allergic rhinitis. Differentially expressed genes were organized into a functionally annotated network using the Ingenuity Knowledge Database, which is based on manual review of more than 200,000 publications. The main function of this network is the regulation of lymphocyte apoptosis, a role associated with several genes of the tuber necrosis factor superfamily. The expression of TNFRSF4, one of the genes in this family, was found to be 48 times higher in allergen-challenged cells than in diluent-challenged cells. TNFRSF4 is known to inhibit apoptosis and to enhance Th2 proliferation. Examination of a different material of allergen-stimulated peripheral blood mononuclear cells showed a higher number of interleukin-4(+) type 2 CD4(+) T (Th2) cells in patients than in controls (P<0.01), as well as a higher number of non-apoptotic Th2 cells in patients (P<0.01). The number of Th2 cells expressing TNFRSF4, TNFSF7 and TNFRSF1B was also significantly higher in patients. Treatment with anti-TNFSF4 resulted in a significantly decreased number of Th2 cells (P<0.05). A logical inference from all this is that the proliferation of allergen-challenged Th2 cells is associated with a decreased apoptosis of Th2 cells and an increase in TNFRSF4 signalling.

Caspases at the crossroads of immune-cell life and death

Caspases are responsible for crucial aspects of inflammation and immune-cell death that are disrupted in a number of genetic autoimmune and autoinflammatory diseases. The caspase family of proteases can be divided into pro-apoptotic and pro-inflammatory members based on their substrate specificity and participation in separate signalling cascades. However, as discussed here, evidence has emerged over the past few years that a number of the caspases thought to be involved solely in apoptosis also contribute to specific aspects of immune-cell development, activation and differentiation, and can even protect cells from some forms of cell death.

cFLIP regulation of lymphocyte activation and development

Cellular caspase-8 (FLICE)-like inhibitory protein (cFLIP) was originally identified as an inhibitor of death-receptor signalling through competition with caspase-8 for recruitment to FAS-associated via death domain (FADD). More recently, it has been determined that both cFLIP and caspase-8 are required for the survival and proliferation of T cells following T-cell-receptor stimulation. This paradoxical finding launched new investigations of how these molecules might connect with signalling pathways that link to cell survival and growth following antigen-receptor activation. As discussed in this Review, insight gained from these studies indicates that cFLIP and caspase-8 form a heterodimer that ultimately links T-cell-receptor signalling to activation of nuclear factor-kappaB through a complex that includes B-cell lymphoma 10 (BCL-10), mucosa-associated-lymphoid-tissue lymphoma-translocation gene 1 (MALT1) and receptor-interacting protein 1 (RIP1).

Proteolytic regulation of nuclear factor of activated T (NFAT) c2 cells and NFAT activity by caspase-3

The nuclear factor of activated T (NFAT) cell family of transcription factors is important in regulating the expression of a broad array of genes, including cytokines, T cell surface receptors, and other transcription factors. NFATc1 and NFATc2 are two principal NFAT members that are expressed in peripheral T cells. Levels of NFAT expression in T cells are partly transcriptionally regulated, but less is understood regarding their post-transcriptional control. We show here that NFATc1 and NFATc2 are rapidly degraded in apoptotic T cells. NFATc2 is highly sensitive to cleavage by caspase-3, whereas NFATc1 is only weakly sensitive to caspase-3 or caspase-8. Two potential caspase-3 cleavage sites were identified in the N-terminal transactivation domain. These sites were confirmed by in vitro caspase cleavage assays. Abolition of NFATc2 cleavage by mutation of these two cleavage sites resulted in augmented NFAT transcriptional activity. Furthermore, NFAT activity could be augmented in wild-type effector T cells by inhibition of caspase activity. Of particular interest was that non-apoptotic T cells from cellular FLIP long transgenic (c-FLIP(L)-Tg) mice that manifest elevated caspase activity have greatly reduced levels of NFATc2 protein and NFAT transcriptional activity. Our findings reveal a new post-transcriptional regulation of NFATc2 that operates, not only during apoptosis, but also in non-apoptotic effector T cells.

No apparent damage in the thyroid of transgenic mice expressing antiapoptotic FLIP

FLIP is an antiapoptotic protein that has been demonstrated to play an important role in inflammation, cancer, and autoimmune diseases. However, it is not known whether increased expression of FLIP (FLICE inhibitory protein) in thyrocytes would alter the development of the thyroid and/or pathogenesis of thyroiditis. To examine the effects of overexpression of this antiapoptotic molecule on the thyroid, we have developed transgenic mouse lines that specifically express FLIP in thyrocytes. A DNA construct designed with an in-frame coding sequence for the E8 protein, a viral FLIP, was put under the control of the thyroglobulin (Tg) promoter (the Tg-FLIP transgene). In 8 of 12 resultant transgenic mouse lines, FLIP expression in thyrocytes driven by the Tg promoter was documented, and confirmed at RNA and protein levels. These Tg-FLIP transgenic mice were monitored for 1 year. Throughout the entire observation period, the transgenic mice remained alive and healthy without evidence of thyroid dysfunction. Adult mice were able to breed. Histologic examination of thyroids obtained at various time points did not reveal significant differences between transgenic mice and their control littermates. Therefore, transgenic mice with thyrocyte-specific expression of FLIP have normal thyroid development with no significant changes in thyroid cell death or proliferation.

An essential role for c-FLIP in the efficient development of mature T lymphocytes

Apoptosis-related genes play important roles in thymocyte maturation. We show that cellular FLICE-like inhibitory protein (c-FLIP), a procaspase-8-like apoptotic regulator, plays an essential role in the efficient development of mature T lymphocytes. Mice conditionally lacking c-FLIP in T lymphocytes display severe defects in the development of mature T cells, as indicated by a dramatically reduced number of CD4+ and CD8+ T cells in the spleen and lymph nodes of mutant mice. The impaired T lymphocyte maturation in c-FLIP conditional knockout mice occurs at the single-positive thymocyte stage and may be caused by enhanced apoptosis in vivo. Moreover, although c-FLIP has been implicated in T cell receptor signaling through nuclear factor (NF)-kappaB and Erk pathways, activation of NF-kappaB and Erk in c-FLIP-deficient thymocytes appears largely intact. Collectively, our data suggest that the primary role of c-FLIP in thymocyte maturation is to protect cells from apoptosis.

Caspase-8 activity prevents type 2 cytokine responses and is required for protective T cell-mediated immunity against Trypanosoma cruzi infection

During Trypanosoma cruzi infection, T cells up-regulate caspase-8 activity. To assess the role of caspase-8 in T cell-mediated immunity, we investigated the effects of caspase-8 inhibition on T cells in viral FLIP (v-FLIP) transgenic mice. Compared with wild-type controls, increased parasitemia was observed in v-FLIP mice infected with T. cruzi. There was a profound decrease in expansion of both CD4 and CD8 T cell subsets in the spleens of infected v-FLIP mice. We did not find differences in activation ratios of T cells from transgenic or wild-type infected mice. However, the numbers of memory/activated CD4 and CD8 T cells were markedly reduced in v-FLIP mice, possibly due to defective survival. We also found decreased production of IL-2 and increased secretion of type 2 cytokines, IL-4 and IL-10, which could enhance susceptibility to infection. Similar, but less pronounced, alterations were observed in mice treated with the caspase-8 inhibitor, zIETD. Furthermore, blockade of caspase-8 by zIETD in vitro mimicked the effects observed on T. cruzi infection in vivo, affecting the generation of activated/memory T cells and T cell cytokine production. Caspase-8 is also required for NF-kappaB signaling upon T cell activation. Blockade of caspase-8 by either v-FLIP expression or treatment with zIETD peptide decreased NF-kappaB responses to TCR:CD3 engagement in T cell cultures. These results suggest a critical role for caspase-8 in the establishment of T cell memory, cell signaling, and regulation of cytokine responses during protozoan infection.

Cellular FLIP (long form) regulates CD8+ T cell activation through caspase-8-dependent NF-kappa B activation

Cellular FLIP long form (c-FLIP(L)) was originally identified as an inhibitor of Fas (CD95/Apo-1). Subsequently, additional functions of c-FLIP(L) were identified through its association with receptor-interacting protein (RIP)1 and TNFR-associated factor 2 to activate NF-kappaB, as well as by its association with and activation of caspase-8. T cells from c-FLIP(L)-transgenic (Tg) mice manifest hyperproliferation upon activation, although it was not clear which of the various functions of c-FLIP(L) was involved. We have further explored the effect of c-FLIP(L) on CD8(+) effector T cell function and its mechanism of action. c-FLIP(L)-Tg CD8(+) T cells have increased proliferation and IL-2 responsiveness to cognate Ags as well as to low-affinity Ag variants, due to increased CD25 expression. They also have a T cytotoxic 2 cytokine phenotype. c-FLIP(L)-Tg CD8(+) T cells manifest greater caspase activity and NF-kappaB activity upon activation. Both augmented proliferation and CD25 expression are blocked by caspase inhibition. c-FLIP(L) itself is a substrate of the caspase activity in effector T cells, being cleaved to a p43(FLIP) form. p43(FLIP) more efficiently recruits RIP1 than full-length c-FLIP(L) to activate NF-kappaB. c-FLIP(L) and RIP1 also coimmunoprecipitate with active caspase-8 in effector CD8(+) T cells. Thus, one mechanism by which c-FLIP(L) influences effector T cell function is through its activation of caspase-8, which in turn cleaves c-FLIP(L) to allow RIP1 recruitment and NF-kappaB activation. This provides a partial explanation of why caspase activity is required to initiate proliferation of resting T cells.

A role for caspases in controlling IL-4 expression in T cells

Although caspase activation is critical for T cell proliferation following activation, the role of caspases in T cell differentiation is unclear. In this study, we have examined the effect of inhibition of caspases on the process of Th1/Th2 differentiation. Naive CD4+ T cells activated under neutral differentiation conditions in the presence of the pan caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp (Z-VAD) fluoromethylketone showed increased Th2 cell differentiation concomitant with an up-regulation of GATA-3. Z-VAD induced optimal Th2 differentiation when T cells were stimulated under strong primary activation conditions. Treatment of naive CD4+ T cells with Z-VAD under strong activation conditions led to a 6-fold increase in IL-4 mRNA compared with control-treated T cells. The Z-VAD-induced increase in IL-4 transcription occurred within 24 h of activation and was independent of Stat6. IFN-gamma mRNA expression was not affected by Z-VAD at the 24-h time point. Z-VAD did not augment IL-4 expression from a committed Th2 cell, suggesting that caspases regulate IL-4 expression specifically during primary T cell activation. Z-VAD did not augment IL-12-driven Th1 differentiation. Activation of T cells in the presence of Z-VAD led to a specific increase in the expression of the transcription factor c-fos. Lastly, retrovirus-mediated expression of the antiapoptotic protein Bcl-2 resulted in an enhancement of Th2 cytokine expression, suggesting that inhibition of caspase activation by Bcl-2 can also modulate IL-4 expression. These findings reveal a novel regulatory mechanism of cytokine expression by caspases, and may explain how signaling pathways that inhibit apoptosis tend to promote Th2 differentiation.

Cellular FLIP (long isoform) overexpression in T cells drives Th2 effector responses and promotes immunoregulation in experimental autoimmune encephalomyelitis

Cellular FLIP (c-FLIP) is an endogenous inhibitor of death receptor-induced apoptosis through the caspase 8 pathway. It is an NF-kappaB-inducible protein thought to promote the survival of T cells upon activation, and its down-regulation has been implicated in activation-induced cell death. We have generated transgenic mice overexpressing human c-FLIP long form (c-FLIP(L)) specifically in T cells using the CD2 promoter (TgFLIP(L)). TgFLIP(L) mice exhibit increased IgG1 production upon stimulation by a T cell-dependent Ag and a markedly enhanced contact hypersensitivity response to allergen. In addition to showing augmented Th2-type responses, TgFLIP(L) mice are resistant to the development of myelin oligodendrocyte glycoprotein 35-55 peptide-induced experimental autoimmune encephalomyelitis, a Th1-driven autoimmune disease. In vitro analyses revealed that T cells of TgFLIP(L) mice proliferate normally, but produce higher levels of IL-2 and show preferential maturation of Th2 cytokine-producing cells in response to antigenic stimulation. After adoptive transfer, these (Th2) cells protected wild-type recipient mice from experimental autoimmune encephalomyelitis induction. Our results show that the constitutive overexpression of c-FLIP(L) in T cells is sufficient to drive Th2 polarization of effector T cell responses and indicate that it might function as a key regulator of Th cell differentiation.

T-helper cell intrinsic defects in lupus that break peripheral tolerance to nuclear autoantigens

Special populations of T helper cells drive B cells to produce IgG class switched, pathogenic autoantibodies in lupus. The major source of antigenic determinants (epitopes) that trigger interactions between lupus T and B cells is nucleosomes of apoptotic cells. These epitopes can be used for antigen-specific therapy of lupus. Secondly, the autoimmune T cells of lupus are sustained because they resist anergy and activation-induced programmed cell death by markedly upregulating cyclooxygenase (COX) 2 along with the antiapoptotic molecule c-FLIP. Only certain COX-2 inhibitors block pathogenic anti-DNA autoantibody production in lupus by causing death of autoimmune T helper cells. Hence COX-2 inhibitors may work independently of their ability to block the enzymatic function of COX-2, and structural peculiarities of these select inhibitors may lead to better drug discovery and design.