Post-transcriptional regulation in lymphocytes: the case of CD154

IRF5 mediates adaptive immunity via altered glutamine metabolism, mTORC1 signaling and post-transcriptional regulation following T cell receptor activation

Although dynamic alterations in transcriptional, translational, and metabolic programs have been described in T cells, the factors and pathways guiding these molecular shifts are poorly understood, with recent studies revealing a disassociation between transcriptional responses and protein expression following T cell receptor (TCR) stimulation. Previous studies identified interferon regulatory factor 5 (IRF5) in the transcriptional regulation of cytokines, chemotactic molecules and T effector transcription factors following TCR signaling. In this study, we identified T cell intrinsic IRF5 regulation of mTORC1 activity as a key modulator of CD40L protein expression. We further demonstrated a global shift in T cell metabolism, with alterations in glutamine metabolism accompanied by shifts in T cell populations at the single cell level due to loss of Irf5. T cell conditional Irf5 knockout mice in a murine model of experimental autoimmune encephalomyelitis (EAE) demonstrated protection from clinical disease with conserved defects in mTORC1 activity and glutamine regulation. Together, these findings expand our mechanistic understanding of IRF5 as an intrinsic regulator of T effector function(s) and support the therapeutic targeting of IRF5 in multiple sclerosis.

Upregulated LncRNA-LINC00659 expression by H. pylori infection promoted the progression of gastritis to cancer by regulating PTBP1 expression

CONTEXT

Helicobacter pylori ( H. pylori ), a spiral-shaped bacterium, is closely associated with chronic, progressive gastric mucosal damage, gastric atrophy, and even gastric cancer (GC). An increasing number of studies have addressed the correlation between long noncoding RNAs (lncRNAs) and H. pylori pathogenicity in GC.

OBJECTIVE

In this study, we found that the expression level of LINC00659 gradually increased in the progression from atrophic gastritis, intestinal metaplasia, and dysplasia to GC in H. pylori -infected patients. Thus, we aimed to further explore the function of LINC00659 in the progression of gastritis to cancer under H. pylori infection.

MATERIALS AND METHODS

StarBase predictions, ribonucleic acid (RNA)-binding protein immunoprecipitation assays, and gene ontology functional annotation (GO)/Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis were performed to identify the RNA-binding proteins of LINC00659; moreover, qRT‒PCR, western blotting, RNA interference, and immunofluorescence assays were used to investigate the function of LINC00659.

RESULTS

LINC00659 bound directly to the RNA-binding protein polypyrimidine tract-binding protein (PTBP1). Importantly, qRT‒PCR and western blot assays demonstrated that PTBP1 expression increased in the progression from inflammation to cancer in the stomach of H. pylori -infected patients and H. pylori -infected GES-1 cells. However, LINC00659 knockdown downregulated PTBP1 expression and inhibited PTBP1 binding under H. pylori infection. Finally, LINC00659 knockdown significantly reduced H. pylori -induced human gastric epithelial cell senescence and suppressed interleukin (IL)-6 and IL-8 secretion by reducing the phosphorylation level of NF-κB p65.

CONCLUSIONS

This study indicated that LINC00659 may have the potential to be a novel promising prognostic and therapeutic marker for H. pylori -associated gastric diseases.

A Posttranscriptional Pathway of CD40 Ligand mRNA Stability Is Required for the Development of an Optimal Humoral Immune Response

CD40 ligand (CD40L) mRNA stability is dependent on an activation-induced pathway that is mediated by the binding complexes containing the multifunctional RNA-binding protein, polypyrimidine tract-binding protein 1 (PTBP1) to a 3' untranslated region of the transcript. To understand the relationship between regulated CD40L and the requirement for variegated expression during a T-dependent response, we engineered a mouse lacking the CD40L stability element (CD40LΔ5) and asked how this mutation altered multiple aspects of the humoral immunity. We found that CD40LΔ5 mice expressed CD40L at 60% wildtype levels, and lowered expression corresponded to significantly decreased levels of T-dependent Abs, loss of germinal center (GC) B cells and a disorganized GC structure. Gene expression analysis of B cells from CD40LΔ5 mice revealed that genes associated with cell cycle and DNA replication were significantly downregulated and genes linked to apoptosis upregulated. Importantly, somatic hypermutation was relatively unaffected although the number of cells expressing high-affinity Abs was greatly reduced. Additionally, a significant loss of plasmablasts and early memory B cell precursors as a percentage of total GL7⁺ B cells was observed, indicating that differentiation cues leading to the development of post-GC subsets was highly dependent on a threshold level of CD40L. Thus, regulated mRNA stability plays an integral role in the optimization of humoral immunity by allowing for a dynamic level of CD40L expression on CD4 T cells that results in the proliferation and differentiation of pre-GC and GC B cells into functional subsets.

Targeting the RNA-Binding Protein HuR Alleviates Neuroinflammation in Experimental Autoimmune Encephalomyelitis: Potential Therapy for Multiple Sclerosis

Multiple sclerosis (MS) is a chronic autoimmune inflammatory and neurodegenerative disease of the central nervous system characterized by demyelination, axonal loss, and motor dysfunction. Activated microglia are associated with the destruction of myelin in the CNS. Activated microglia produce cytokines and proinflammatory factors, favoring neuroinflammation, myelin damage, and neuronal loss, and it is thought to be involved in the disease pathogenesis. The present study investigated the role of post-transcriptional regulation of gene expression on the neuroinflammation related to experimental autoimmune encephalomyelitis (EAE) in mice, by focusing on HuR, an RNA-binding protein involved in inflammatory and immune phenomena. Spinal cord sections of EAE mice showed an increased HuR immunostaining that was abundantly detected in the cytoplasm of activated microglia, a pattern associated with its increased activity. Intrathecal administration of an anti-HuR antisense oligonucleotide (ASO) decreased the proinflammatory activated microglia, inflammatory infiltrates, and the expression of the proinflammatory cytokines IL-1β, TNF-α, and IL-17, and inhibited the activation of the NF-κB pathway. The beneficial effect of anti-HuR ASO in EAE mice corresponded also to a decreased permeability of the blood-brain barrier. EAE mice showed a reduced spinal CD206 immunostaining that was restored by anti-HuR ASO, indicating that HuR silencing promotes a shift to the anti-inflammatory and regenerative microglia phenotype. Mice that received anti-HuR ASO exhibited improved EAE-related motor dysfunction, pain hypersensitivity, and body weight loss. Targeting HuR might represent an innovative and promising perspective to control neurological disturbances in MS patients.

Molecular basis and therapeutic implications of CD40/CD40L immune checkpoint

The CD40 receptor and its ligand CD40L is one of the most critical molecular pairs of the stimulatory immune checkpoints. Both CD40 and CD40L have a membrane form and a soluble form generated by proteolytic cleavage or alternative splicing. CD40 and CD40L are widely expressed in various types of cells, among which B cells and myeloid cells constitutively express high levels of CD40, and T cells and platelets express high levels of CD40L upon activation. CD40L self-assembles into functional trimers which induce CD40 trimerization and downstream signaling. The canonical CD40/CD40L signaling is mediated by recruitment of TRAFs and NF-κB activation, which is supplemented by signal pathways such as PI3K/AKT, MAPKs and JAK3/STATs. CD40/CD40L immune checkpoint leads to activation of both innate and adaptive immune cells via two-way signaling. CD40/CD40L interaction also participates in regulating thrombosis, tissue inflammation, hematopoiesis and tumor cell fate. Because of its essential role in immune activation, CD40/CD40L interaction has been regarded as an attractive immunotherapy target. In recent years, significant advance has been made in CD40/CD40L-targeted therapy. Various types of agents, including agonistic/antagonistic monoclonal antibodies, cellular vaccines, adenoviral vectors and protein antagonist, have been developed and evaluated in early-stage clinical trials for treating malignancies, autoimmune diseases and allograft rejection. In general, these agents have demonstrated favorable safety and some of them show promising clinical efficacy. The mechanisms of benefits include immune cell activation and tumor cell lysis/apoptosis in malignancies, or immune cell inactivation in autoimmune diseases and allograft rejection. This review provides a comprehensive overview of the structure, processing, cellular expression pattern, signaling and effector function of CD40/CD40L checkpoint molecules. In addition, we summarize the progress, targeted diseases and outcomes of current ongoing and completed clinical trials of CD40/CD40L-targeted therapy.

Site-Specific Gene Editing of Human Hematopoietic Stem Cells for X-Linked Hyper-IgM Syndrome

X-linked hyper-immunoglobulin M (hyper-IgM) syndrome (XHIM) is a primary immunodeficiency due to mutations in CD40 ligand that affect immunoglobulin class-switch recombination and somatic hypermutation. The disease is amenable to gene therapy using retroviral vectors, but dysregulated gene expression results in abnormal lymphoproliferation in mouse models, highlighting the need for alternative strategies. Here, we demonstrate the ability of both the transcription activator-like effector nuclease (TALEN) and clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR/Cas9) platforms to efficiently drive integration of a normal copy of the CD40L cDNA delivered by Adeno-Associated Virus. Site-specific insertion of the donor sequence downstream of the endogenous CD40L promoter maintained physiologic expression of CD40L while overriding all reported downstream mutations. High levels of gene modification were achieved in primary human hematopoietic stem cells (HSCs), as well as in cell lines and XHIM-patient-derived T cells. Notably, gene-corrected HSCs engrafted in immunodeficient mice at clinically relevant frequencies. These studies provide the foundation for a permanent curative therapy in XHIM.

Hydroxychloroquine inhibits CD154 expression in CD4+ T lymphocytes of systemic lupus erythematosus through NFAT, but not STAT5, signaling

Background

Overexpression of membranous CD154 in T lymphocytes has been found previously in systemic lupus erythematosus (SLE). Because hydroxychloroquine (HCQ) has been used frequently in the treatment of lupus, we sought to identify the effects of HCQ on CD154 and a possibly regulatory mechanism.

Methods

CD4⁺ T cells were isolated from the blood of lupus patients. After stimulation with ionomycin or IL-15 and various concentrations of HCQ, expression of membranous CD154 and NFAT and STAT5 signaling were assessed.

Results

HCQ treatment had significant dose-dependent suppressive effects on membranous CD154 expression in ionomycin-activated T cells from lupus patients. Furthermore, HCQ inhibited intracellular sustained calcium storage release, and attenuated the nuclear translocation of NFATc2 and the expression of NFATc1. However, CD154 expressed through IL-15-mediated STAT5 signaling was not inhibited by HCQ treatment.

Conclusions

HCQ inhibited NFAT signaling in activated T cells and blocked the expression of membranous CD154, but not STAT5 signaling. These findings provide a mechanistic insight into SLE in HCQ treatment.

Silencing of the RNA-binding protein HuR attenuates hyperalgesia and motor disability in experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system associated with progressive neuronal loss and axonal degeneration. Neuronal lesions and dysfunction lead often to neuropathic pain, the most prevalent and difficult to treat pain syndrome observed in MS patients. Despite its widespread occurrence, the underlying neural mechanisms for MS pain are not fully understood. For a better clarification of the pathophysiology of MS-associated pain, we investigated the role of HuR, an RNA-binding protein that positively regulates the stability of many target mRNAs, including several cytokines. The influence of HuR in the generation of the hypernociceptive response in a mouse model of relapsing-remitting experimental autoimmune encephalomyelitis (RR-EAE), an experimental model of MS, was investigated. HuR silencing, obtained through the repeated intrathecal administration of an antisense oligonucleotide (aODN) anti-HuR, completely attenuated hindpaw mechanical allodynia and thermal hyperalgesia developed by RR-EAE mice. Anti-HuR aODN also reduced severity of motor deficits as reflected by a reduction of clinical EAE score and improvement of rotarod performance. RR-EAE mice showed demyelination in spinal cord sections that was significantly reduced by HuR silencing. Double-staining immunofluorescence studies showed a neuronal localization of HuR within dorsal horn spinal cord, consistent with a neuronal mechanism of action. Our findings suggest the involvement of HuR in the hypernociceptive behaviour of RR-EAE mice providing the first pharmacological assessment of an antiallodynic and antihyperalgesic effect of HuR silencing. These data may provide support for HuR modulation as a therapeutic perspective for the management of MS-related neuropathic pain.

Scaffold protein JLP mediates TCR-initiated CD4+T cell activation and CD154 expression

CD4⁺ T-cell activation and its subsequent induction of CD154 (CD40 ligand, CD40L) expression are pivotal in shaping both the humoral and cellular immune responses. Scaffold protein JLP regulates signal transduction pathways and molecular trafficking inside cells, thus represents a critical component in maintaining cellular functions. Its role in regulating CD4⁺ T-cell activation and CD154 expression, however, is unclear. Here, we demonstrated expression of JLP in mouse tissues of lymph nodes, thymus, spleen, and also CD4⁺ T cells. Using CD4+ T cells from jlp-deficient and jlp-wild-type mice, we demonstrated that JLP-deficiency impaired T-cell proliferation, IL-2 production, and CD154 induction upon TCR stimulations, but had no impacts on the expression of other surface molecules such as CD25, CD69, and TCR. These observed impaired T-cell functions in the jlp-/- CD4⁺ T cells were associated with defective NF-AT activation and Ca²⁺ influx, but not the MAPK, NF-κB, as well as AP-1 signaling pathways. Our findings indicated that, for the first time, JLP plays a critical role in regulating CD4⁺ T cells response to TCR stimulation partly by mediating the activation of TCR-initiated Ca²⁺/NF-AT.

The RNA-Binding Protein, Polypyrimidine Tract-Binding Protein 1 (PTBP1) Is a Key Regulator of CD4 T Cell Activation

We have previously shown that the RNA binding protein, polypyrimidine tract-binding protein (PTBP1) plays a critical role in regulating the expression of CD40L in activated CD4 T cells. This is achieved mechanistically through message stabilization at late times of activation as well as by altered distribution of CD40L mRNA within distinct cellular compartments. PTBP1 has been implicated in many different processes, however whether PTBP1 plays a broader role in CD4 T cell activation is not known. To examine this question, experiments were designed to introduce shRNA into primary human CD4 T cells to achieve decreased, but not complete ablation of PTBP1 expression. Analyses of shPTB-expressing CD4 T cells revealed multiple processes including cell proliferation, activation-induced cell death and expression of activation markers and cytokines that were regulated in part by PTBP1 expression. Although there was an overall decrease in the steady-state level of several activation genes, only IL-2 and CD40L appeared to be regulated by PTBP1 at the level of RNA decay suggesting that PTBP1 is critical at different regulatory steps of expression that is gene-specific. Importantly, even though the IL-2 protein levels were reduced in cells with lowered PTBP1, the steady-state level of IL-2 mRNA was significantly higher in these cells suggesting a block at the translational level. Evaluation of T cell activation in shPTB-expressing T cells revealed that PTBP1 was linked primarily to the activation of the PLCγ1/ERK1/2 and the NF-κB pathways. Overall, our results reveal the importance of this critical RNA binding protein in multiple steps of T cell activation.

Post-transcriptional Regulation of Immunological Responses through Riboclustering

Immunological programing of immune cells varies in response to changing environmental signals. This process is facilitated by modifiers that regulate the translational fate of mRNAs encoding various immune mediators, including cytokines and chemokines, which in turn determine the rapid activation, tolerance, and plasticity of the immune system. RNA-binding proteins (RBPs) recruited by the specific sequence elements in mRNA transcripts are one such modifiers. These RBPs form RBP-RNA complexes known as "riboclusters." These riboclusters serve as RNA sorting machinery, where depending upon the composition of the ribocluster, translation, degradation, or storage of mRNA is controlled. Recent findings suggest that this regulation of mRNA homeostasis is critical for controlling the immune response. Here, we present the current knowledge of the ribocluster-mediated post-transcriptional regulation of immune mediators and highlight recent findings regarding their implications for the pathogenesis of acute or chronic inflammatory diseases.

Post-transcriptional coordination of immunological responses by RNA-binding proteins

Immunological reactions are propelled by ever-changing signals that alter the translational ability of the RNA in the cells involved. Such alterations are considered to be consequential modifications in the transcriptomic decoding of the genetic blueprint. The identification of RNA-binding protein (RBP) assemblies engaged in the coordinative regulation of state-specific RNAs indicates alternative and exclusive means for determining the activation, plasticity and tolerance of cells of the immune system. Here we review current knowledge about RBP-regulated post-transcriptional events involved in the reactivity of cells of the immune system and the importance of their alteration during chronic inflammatory pathology and autoimmunity.

Conditional knockout of the RNA-binding protein HuR in CD4⁺ T cells reveals a gene dosage effect on cytokine production

The posttranscriptional mechanisms by which RNA binding proteins (RBPs) regulate T-cell differentiation and cytokine production in vivo remain unclear. The RBP HuR binds to labile mRNAs, usually leading to increases in mRNA stability and/or translation. Previous work demonstrated that HuR binds to the mRNAs encoding the Th2 transcription factor trans-acting T-cell-specific transcription factor (GATA-3) and Th2 cytokines interleukin (IL)-4 and IL-13, thereby regulating their expression. By using a novel conditional HuR knockout (KO) mouse in which HuR is deleted in activated T cells, we show that Th2-polarized cells from heterozygous HuR conditional (OX40-Cre HuR(fl/+)) KO mice had decreased steady-state levels of Gata3, Il4 and Il13 mRNAs with little changes at the protein level. Surprisingly, Th2-polarized cells from homozygous HuR conditional (OX40-Cre HuR(fl/fl)) KO mice showed increased Il2, Il4 and Il13 mRNA and protein via different mechanisms. Specifically, Il4 was transcriptionally upregulated in HuR KO T cells, whereas Il2 and Il13 mRNA stabilities increased. Additionally, when using the standard ovalbumin model of allergic airway inflammation, HuR conditional KO mice mounted a robust inflammatory response similar to mice with wild-type HuR levels. These results reveal a complex differential posttranscriptional regulation of cytokines by HuR in which gene dosage plays an important role. These findings may have significant implications in allergies and asthma, as well as autoimmune diseases and infection.

New insights into functional roles of the polypyrimidine tract-binding protein

Polypyrimidine Tract Binding Protein (PTB) is an intensely studied RNA binding protein involved in several post-transcriptional regulatory events of gene expression. Initially described as a pre-mRNA splicing regulator, PTB is now widely accepted as a multifunctional protein shuttling between nucleus and cytoplasm. Accordingly, PTB can interact with selected RNA targets, structural elements and proteins. There is increasing evidence that PTB and its paralog PTBP2 play a major role as repressors of alternatively spliced exons, whose transcription is tissue-regulated. In addition to alternative splicing, PTB is involved in almost all steps of mRNA metabolism, including polyadenylation, mRNA stability and initiation of protein translation. Furthermore, it is well established that PTB recruitment in internal ribosome entry site (IRES) activates the translation of picornaviral and cellular proteins. Detailed studies of the structural properties of PTB have contributed to our understanding of the mechanism of RNA binding by RNA Recognition Motif (RRM) domains. In the present review, we will describe the structural properties of PTB, its paralogs and co-factors, the role in post-transcriptional regulation and actions in cell differentiation and pathogenesis. Defining the multifunctional roles of PTB will contribute to the understanding of key regulatory events in gene expression.

Posttranscriptional gene regulation of IL-17 by the RNA-binding protein HuR is required for initiation of experimental autoimmune encephalomyelitis

IL-17 is a proinflammatory cytokine produced by activated Th17 cells and other immune cells. IL-17-producing Th17 cells are major contributors to chronic inflammatory and autoimmune diseases, such as multiple sclerosis, rheumatoid arthritis, and inflammatory bowel disease. Although the transcriptional regulation of Th17 cells is well understood, the posttranscriptional regulation of IL-17 gene expression remains unknown. The RNA-binding protein HuR positively regulates the stability of many target mRNAs via binding the AU-rich elements present in the 3' untranslated region of many inflammatory cytokines including IL-4, IL-13, and TNF-α. However, the regulation of IL-17 expression by HuR has not been established. CD4(+) Th17 cells from HuR knockout mice had decreased IL-17 steady-state mRNA and protein levels compared with wild-type Th17 cells, as well as decreases in frequency of IL-17(+) cells. Moreover, we demonstrated that HuR directly binds to the IL-17 mRNA 3' untranslated region by using RNA immunoprecipitation and biotin pulldown assays. In addition, the knockout of HuR decreased cellular proliferation of CD4(+) T cells. Mice with adoptively transferred HuR KO Th17 cells had delayed initiation and reduced disease severity in the onset of experimental autoimmune encephalomyelitis compared with wild-type Th17 cells. Our results reveal a HuR-induced posttranscriptional regulatory mechanism of Th17 differentiation that influences IL-17 expression. These findings may provide novel therapeutic targets for the treatment of Th17-mediated autoimmune neuroinflammation.

CD40L--a costimulatory molecule involved in the maturation of antigen presenting cells in Atlantic salmon (Salmo salar)

The CD40L/CD40 signalling pathway is critically involved in the final stage of the maturation of DCs. This paper reports the identification and functional characterization of CD40L and CD40 from Atlantic salmon (Salmo salar). Salmon CD40L is a type II membrane-bound protein with a TNF homology domain in its extracellular C-terminal region, while CD40 is a type I membrane-bound receptor with a sequence pattern of four cysteine-rich domains in its extracellular N-terminal region. The salmon CD40L and CD40 were widely expressed, particularly in immune tissues, and while CD40L expression was induced by in vitro stimulation of HKLs with PHA and ConA, CpG increased CD40 expression. A CD40L construct was overexpressed in the CHSE-214 cell line and co-cultivation of the CD40L-CHSE transfectants with HKL induced a rapid and long-lasting upregulation of important costimulatory molecules like CD40, CD83, B7-H1 and the cytokines IL-12p40, IL-10, IL-1β and IFNs, which all are involved in T-helper cell responses. Furthermore, the CD40L transfected cells increased the percentage of HKLs expressing surface MHCIIβ but unlike other APC maturation stimuli, like CpG, they did not reduce the capacity to internalise antigen. Our results provide the first evidence for the existence of a functional CD40L mediated costimulatory pathway in Atlantic salmon.

Differential modulation by delta9-tetrahydrocannabinol (∆9)-THC) of CD40 ligand (CD40L) expression in activated mouse splenic CD4+ T cells

The anti-inflammatory activity of cannabinoids has been widely demonstrated in experimental animal models and in humans. CD40-CD40-ligand (L) interactions are among the most crucial initiators of inflammation. This study investigated the effects of ∆(9)-THC on CD40L expression in mouse splenic T cells after activation with various stimuli. Time course studies demonstrated that peak surface expression of CD40L by CD4(+) T cells after anti-CD3/CD28 or phorbol ester plus calcium ionophore (PMA/Io) occurred 8 h post activation. Peak CD40L mRNA levels were observed at 2 h post PMA/Io treatment and at 4 h post anti-CD3/CD28 treatment. Pretreatment with ∆(9)-THC significantly impaired the upregulation of CD40L induced by anti-CD3/CD28 at both the protein and mRNA level. By contrast, ∆(9)-THC did not affect PMA/Io-induced surface CD40L expression on CD4(+) T cells. Additionally, ∆(9)-THC also attenuated anti-CD3/CD28-induced CD40L expression on CD4(+) T cells derived from CB1(-/-)/CB2(-/-) mice. We investigated whether the mechanism by which ∆(9)-THC suppressed CD40L expression involved putative cannabinoid activation of the glucocorticoid receptor (GR). Although activation of GR resulted in suppression of CD40L induction by anti-CD3/CD28, no interaction between ∆(9)-THC and GR was observed by a glucocorticoid response element (GRE) luciferase reporter assay in HEK293T cells. Collectively, these results suggest that ∆(9)-THC targets proximal T cell receptor-associated signaling in a cannabinoid receptor- and glucocorticoid receptor-independent manner. These findings identify suppression of CD40L expression as a novel part of the mechanism by which ∆(9)-THC exerts anti-inflammatory activity.

Posttranscriptional Gene Regulation: Novel Pathways for Glucocorticoids' Anti-inflammatory Action

Posttranscriptional gene regulation (PTR) is a fundamental biological process that integrates with the master transcriptional control of gene expression, in ways that only in the last decade have been increasingly understood [1, 2]. While epigenetic and transcriptional events shape cell response qualitatively, deciding the pattern of gene expression to 'switch on or off' in response to endogenous or environmental triggers, the key task of PTR is to act as a 'rheostat' and rapidly adapt the cellular response by providing the appropriate amplitude and timing to the protein expression patterns [3, 4]. The pivotal role of this mechanism comes to the forefront in inflammatory and immune response, where the changes in amplitude and duration in the expression of dangerous and protective genes are in delicate balance, and are critical in determining either the successful resolution of the immune response or its chronic overexpression [5]. This brief review introduces members of the main classes of molecules mediating the cytoplasmic arm of gene regulation, namely RNA-binding proteins and micro-RNA (miRNA), and summarizes experimental data that underscore the role of these molecules in the pathophysiology of chronic inflammation, as well as their promising value as mechanisms conveying the anti-inflammatory effect of synthetic glucocorticoids.

Preformed CD40L is stored in Th1, Th2, Th17, and T follicular helper cells as well as CD4+ 8- thymocytes and invariant NKT cells but not in Treg cells

CD40L is essential for the development of adaptive immune responses. It is generally thought that CD40L expression in CD4⁺ T cells is regulated transcriptionally and made from new mRNA following antigen recognition. However, imaging studies show that the majority of cognate interactions between effector CD4⁺ T cells and APCs in vivo are too short to allow de novo CD40L synthesis. We previously showed that Th1 effector and memory cells store preformed CD40L (pCD40L) in lysosomal compartments and mobilize it onto the plasma membrane immediately after antigenic stimulation, suggesting that primed CD4⁺ T cells may use pCD40L to activate APCs during brief encounters. Indeed, our recent study showed that pCD40L is sufficient to mediate selective activation of cognate B cells and trigger DC activation in vitro. In this study, we show that pCD40L is present in Th1 and follicular helper T cells developed during infection with lymphocytic choriomeningitis virus, Th2 cells in the airway of asthmatic mice, and Th17 cells from the CNS of animals with experimental autoimmune encephalitis (EAE). pCD40L is nearly absent in both natural and induced Treg cells, even in the presence of intense inflammation such as occurs in EAE. We also found pCD40L expression in CD4 single positive thymocytes and invariant NKT cells. Together, these results suggest that pCD40L may function in T cell development as well as an unexpectedly broad spectrum of innate and adaptive immune responses, while its expression in Treg cells is repressed to avoid compromising their suppressive activity.

Coordinate regulation of GATA-3 and Th2 cytokine gene expression by the RNA-binding protein HuR

The posttranscriptional mechanisms whereby RNA-binding proteins (RBPs) regulate T cell differentiation remain unclear. RBPs can coordinately regulate the expression of functionally related genes via binding to shared regulatory sequences, such as the adenylate-uridylate-rich elements (AREs) present in the 3' untranslated region (UTR) of mRNA. The RBP HuR posttranscriptionally regulates IL-4, IL-13, and other Th2 cell-restricted transcripts. We hypothesized that the ARE-bearing GATA-3 gene, a critical regulator of Th2 polarization, is under HuR control as part of its coordinate posttranscriptional regulation of the Th2 program. We report that in parallel with stimulus-induced increase in GATA-3 mRNA and protein levels, GATA-3 mRNA half-life is increased after restimulation in the human T cell line Jurkat, in human memory and Th2 cells, and in murine Th2-skewed cells. We demonstrate by immunoprecipitation of ribonucleoprotein complexes that HuR associates with the GATA-3 endogenous transcript in human T cells and found, using biotin pulldown assay, that HuR specifically interacts with its 3'UTR. Using both loss-of-function and gain-of-function approaches in vitro and in animal models, we show that HuR is a critical mediator of stimulus-induced increase in GATA-3 mRNA and protein expression and that it positively influences GATA-3 mRNA turnover, in parallel with selective promotion of Th2 cytokine overexpression. These results suggest that HuR-driven posttranscriptional control plays a significant role in T cell development and effector function in both murine and human systems. A better understanding of HuR-mediated control of Th2 polarization may have utility in altering allergic airway inflammation in human asthmatic patients.

Polypyrimidine tract-binding protein is critical for the turnover and subcellular distribution of CD40 ligand mRNA in CD4+ T cells

CD40L (CD154) is regulated at the posttranscriptional level by an activation-induced process that results in a highly stable transcript at extended times of T cell activation. Transcript stability is mediated by polypyrimidine tract-binding protein (PTB)-containing complexes (complex I and II) that bind to three adjacent CU-rich sequences within the 3' untranslated region. To assess the role of PTB in the expression and distribution of CD40L mRNA, PTB was targeted using short hairpin RNA in both primary T cells and a T cell line that recapitulates the stability phase of regulated CD40L mRNA decay. PTB knockdown resulted in a marked decrease in the mRNA stability that resulted in lowered CD40L surface expression. PTB was also critical for appropriate distribution of CD40L mRNA between the nucleus and cytoplasm and in the cytoplasm between the cytosol and the translating polysomes. The activation-induced formation of PTB-specific ribonucleoprotein complexes was observed only with cytoplasmic and not nuclear PTB indicating functional differences in the protein defined by cellular localization. Finally, we observed that cytoplasmic and nuclear PTB isoforms were differentially modified relative to each other and that the changes in cytoplasmic PTB were consistent with activation-induced phosphorylation. Together this work suggests that differentially modified PTB regulates CD40L expression at multiple steps by 1) retaining CD40L mRNA in the nucleus, 2) directly regulating mRNA stability at late times of activation, and 3) forming a ribonuclear complex that preferentially associates with translating ribosomes thus leading to an enhanced level of CD40L protein.

The human glucocorticoid receptor as an RNA-binding protein: global analysis of glucocorticoid receptor-associated transcripts and identification of a target RNA motif

Posttranscriptional regulation is emerging as a key factor in glucocorticoid (GC)-mediated gene regulation. We investigated the role of the human GC receptor (GR) as an RNA-binding protein and its effect on mRNA turnover in human airway epithelial cells. Cell treatment with the potent GC budesonide accelerated the decay of CCL2 mRNA (t(1/2) = 8 ± 1 min versus 62 ± 17 min in DMSO-treated cells) and CCL7 mRNA (t(1/2) = 15 ± 4 min versus 114 ± 37 min), but not that of CCL5 mRNA (t(1/2)=231 ± 8 min versus 266 ± 5 min) in the BEAS-2B cell line. This effect was inhibited by preincubation with an anti-GR Ab, indicating that GR itself plays a role in the turnover of these transcripts. Coimmunoprecipitation and biotin pulldown experiments showed that GR associates with CCL2 and CCL7 mRNAs, but not CCL5 mRNA. These methods confirmed CCL2 mRNA targeting by GR in human primary airway epithelial cells. Association of the GR was localized to the 5' untranslated region of CCL2 mRNA and further mapped to nt 44-60. The collection of transcripts associated with GR, identified by immunoprecipitation of GR-mRNA complexes followed by microarray analysis, revealed 479 transcripts that associated with GR. Computational analysis of the primary sequence and secondary structures of these transcripts yielded a GC-rich motif, which was shown to bind to GR in vitro. This motif was used to predict binding of GR to an additional 7889 transcripts. These results indicate that cytoplasmic GR interacts with a subset of mRNA through specific sequences and can regulate turnover rates, suggesting a novel posttranscriptional role for GR as an RNA-binding protein.

Post-transcriptional regulons coordinate the initiation and resolution of inflammation

Transcriptional control mechanisms chart the course of the inflammatory response by synthesizing mRNAs encoding proteins that promote or inhibit inflammation. Because these mRNAs can be long-lived, turning off their synthesis does not rapidly stop or change the direction of inflammation. Post-transcriptional mechanisms that modify mRNA stability and/or translation provide more rapid and flexible control of this process and are particularly important in coordinating the initiation and resolution of inflammation. Here, I review the surprising variety of post-transcriptional control mechanisms that regulate the initiation and resolution of inflammation and discuss how these mechanisms are integrated to coordinate this essential process.

Lenalidomide treatment promotes CD154 expression on CLL cells and enhances production of antibodies by normal B cells through a PI3-kinase-dependent pathway

Chronic lymphocytic leukemia (CLL) involves a profound humoral immune defect and tumor-specific humoral tolerance that directly contribute to disease morbidity and mortality. CD154 gene therapy can reverse this immune defect, but attempts to do this pharmacologically have been unsuccessful. The immune-modulatory agent lenalidomide shows clinical activity in CLL, but its mechanism is poorly understood. Here, we demonstrate that lenalidomide induces expression of functional CD154 antigen on CLL cells both in vitro and in vivo. This occurs via enhanced CD154 transcription mediated by a Nuclear Factor of Activated T cells c1 (NFATc1)/Nuclear Factor-kappaB (NF-kappaB) complex and also through phosphoinositide-3 (PI3)-kinase pathway-dependent stabilization of CD154 mRNA. Importantly, CD154-positive CLL cells up-regulate BID, DR5, and p73, become sensitized to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis, and promote costimulatory activation of normal B cells to produce antibodies. In CLL patients receiving lenalidomide, similar evidence of CD154 activation is observed including BID, DR5, and p73 induction and also development of anti-ROR1 tumor-directed antibodies. Our data demonstrate that lenalidomide promotes CD154 expression on CLL cells with subsequent activation phenotype, and may therefore reverse the humoral immune defect observed in this disease. This study is registered at http://clinicaltrials.gov as NCT00466895.