The survival of antigen-stimulated T cells requires NFkappaB-mediated inhibition of p73 expression

The many-sided contributions of NF-κB to T-cell biology in health and disease

T cells (or T lymphocytes) exhibit a myriad of functions in immune responses, ranging from pathogen clearance to autoimmunity, cancer and even non-lymphoid tissue homeostasis. Therefore, deciphering the molecular mechanisms orchestrating their specification, function and gene expression pattern is critical not only for our comprehension of fundamental biology, but also for the discovery of novel therapeutic targets. Among the master regulators of T-cell identity, the functions of the NF-κB family of transcription factors have been under scrutiny for several decades. However, a more precise understanding of their pleiotropic functions is only just emerging. In this review we will provide a global overview of the roles of NF-κB in the different flavors of mature T cells. We aim at highlighting the complex and sometimes diverging roles of the five NF-κB subunits in health and disease.

Regulation of T Helper Cell Fate by TCR Signal Strength

T cells are critical in orchestrating protective immune responses to cancer and an array of pathogens. The interaction between a peptide MHC (pMHC) complex on antigen presenting cells (APCs) and T cell receptors (TCRs) on T cells initiates T cell activation, division, and clonal expansion in secondary lymphoid organs. T cells must also integrate multiple T cell-intrinsic and extrinsic signals to acquire the effector functions essential for the defense against invading microbes. In the case of T helper cell differentiation, while innate cytokines have been demonstrated to shape effector CD4⁺ T lymphocyte function, the contribution of TCR signaling strength to T helper cell differentiation is less understood. In this review, we summarize the signaling cascades regulated by the strength of TCR stimulation. Various mechanisms in which TCR signal strength controls T helper cell expansion and differentiation are also discussed.

Loss of Perp in T Cells Promotes Resistance to Apoptosis of T Helper 17 Cells and Exacerbates the Development of Experimental Autoimmune Encephalomyelitis in Mice

T helper 17 (Th17) cells are crucial for the pathogenesis of multiple sclerosis (MS) in humans and experimental autoimmune encephalomyelitis (EAE) in animals. High frequency of Th17 cells and low sensitivity to activation-induced cell death (AICD) are detected in MS patients. However, the mechanisms underlying apoptosis resistance of T cells remain unclear. Perp is an apoptosis-associated target of p53 and implicated in the development of cancers. Here, we show that loss of Perp in T cells does not affect Th1, Th17, or Treg cell differentiation, but does significantly increase the resistance of Perp^-/- Th17 cells to AICD and anti-Fas in Lck-Cre × Perp^fl/fl mice by inhibiting the caspase-dependent apoptotic pathway. Moreover, Lck-Cre × Perp^fl/fl mice exhibited earlier onset of EAE and severe spinal cord inflammation and demyelination, accompanied by increased levels of pro-inflammatory cytokines and enlarged population of Th17 cells. Therefore, Perp deletion promoted Th17 responses and exacerbated the development and severity of EAE.

p63 and p73 repress CXCR5 chemokine receptor gene expression in p53-deficient MCF-7 breast cancer cells during genotoxic stress

Many types of chemotherapeutic agents induce of DNA-damage that is accompanied by activation of p53 tumor suppressor, a key regulator of tumor development and progression. In our previous study we demonstrated that p53 could repress CXCR5 chemokine receptor gene in MCF-7 breast cancer cells via attenuation of NFkB activity. In this work we aimed to determine individual roles of p53 family members in the regulation of CXCR5 gene expression under genotoxic stress. DNA-alkylating agent methyl methanesulfonate caused a reduction in CXCR5 expression not only in parental MCF-7 cells but also in MCF-7-p53off cells with CRISPR/Cas9-mediated inactivation of the p53 gene. Since p53 knockout was associated with elevated expression of its p63 and p73 homologues, we knocked out p63 using CRISPR/Cas9 system and knocked down p73 using specific siRNA. The CXCR5 promoter activity, CXCR5 expression and CXCL13-directed migration in MCF-7 cells with inactivation of all three p53 family genes were completely insensitive to genotoxic stress, while pairwise p53+p63 or p53+p73 inactivation resulted in partial effects. Using deletion analysis and site-directed mutagenesis, we demonstrated that effects of NFkB on the CXCR5 promoter inversely correlated with p63 and p73 levels. Thus, all three p53 family members mediate the effects of genotoxic stress on the CXCR5 promoter using the same mechanism associated with attenuation of NFkB activity. Understanding of this mechanism could facilitate prognosis of tumor responses to chemotherapy.

Cell-Cycle Regulators and Cell Death in Immunity

Various cell death mechanisms are integral to host defense in both plants and mammals. Plant defense against biotrophic pathogens is associated with programmed cell death (PCD) of the infected cell. This effector-triggered PCD is partly analogous to pyroptosis, an inflammatory host cell death process that plays a crucial role in defense against microbial infections in mammals. Plant effector-triggered PCD also shares with mammalian apoptosis the involvement of cell-cycle regulators as signaling components. Here we explore the similarities between these different cell death programs as they relate to host defense and their relationship to the cell cycle.

The pharmacology of second-generation chimeric antigen receptors

Second-generation chimeric antigen receptors (CARs) retarget and reprogramme T cells to augment their antitumour efficacy. The combined activating and co-stimulatory domains incorporated in these CARs critically determine the function, differentiation, metabolism and persistence of engineered T cells. CD19-targeted CARs that incorporate CD28 or 4-1BB signalling domains are the best known to date. Both have shown remarkable complete remission rates in patients with refractory B cell malignancies. Recent data indicate that CD28-based CARs direct a brisk proliferative response and boost effector functions, whereas 4-1BB-based CARs induce a more progressive T cell accumulation that may compensate for less immediate potency. These distinct kinetic features can be exploited to further develop CAR-based T cell therapies for a variety of cancers. A new field of immunopharmacology is emerging.

Regulation of T cell differentiation and alloimmunity by the cyclin-dependent kinase inhibitor p18ink4c

Cellular proliferation in response to mitogenic stimuli is negatively regulated by the Cip/Kip and the Ink4 families of cyclin-dependent kinase (CDK) inhibitors. Several of these proteins are elevated in anergic T cells, suggesting a potential role in the induction or maintenance of tolerance. Our previous studies showed that p27kip1 is required for the induction of T cell anergy and transplantation tolerance by costimulatory blockade, but a role for Ink4 proteins in these processes has not been established. Here we show that CD4+ T cells from mice genetically deficient for p18ink4c divide more rapidly than wild-type cells in response to antigenic, costimulatory and growth factor signals. However, this gain of proliferative function was accompanied by a moderate increase in the rate of cell death, and was accompanied by an overall defect in the generation of alloreactive IFNγ-producing effector cells. Consistent with this, p18ink4c-deficient T cells were unable to induce graft-vs-host disease in vivo, and p18ink4c deficiency cooperated with costimulatory blockade to significantly increase the survival of fully mismatched allografts in a cardiac transplantation model. While both p18ink4c and p27kip1 act to restrict T cell proliferation, p18ink4c exerts an opposite effect from p27kip1 on alloimmunity and organ transplant rejection, most likely by sustaining T cell survival and the development of effector function. Our studies point to additional important links between the cell cycle machinery and the processes of T cell differentiation, survival and tolerance.

Anti-inflammatory therapy in type 1 diabetes

Type 1 diabetes (T1D) is a multi-factorial, organ-specific autoimmune disease in genetically susceptible individuals, which is characterized by a selective and progressive loss of insulin-producing β-cells. Cells mediating innate as well as adaptive immunity infiltrate pancreatic islets, thereby generating an aberrant inflammatory process called insulitis that can be mirrored by a pathologic autoantibody production and autoreactive T-cells. In tight cooperation with infiltrating innate immune cells, which secrete high levels of pro-inflammatory cytokines like IL-1β, TNFα, and INFγ effector T-cells trigger the fatal destruction process of β-cells. There is ongoing discussion on the contribution of inflammation in T1D pathogenesis, ranging from a bystander reaction of autoimmunity to a dysregulation of immune responses that initiate inflammatory processes and thereby actively promoting β-cell death. Here, we review recent advances in anti-inflammatory interventions in T1D animal models and preclinical studies and discuss their mode of action as well as their capacity to interfere with T1D development.

Intervention of PKC-θ as an immunosuppressive regimen

PKC-θ is selectively enriched in T cells and specifically translocates to immunological synapse where it mediates critical T cell receptor signals required for T cell activation, differentiation, and survival. T cells deficient in PKC-θ are defective in their ability to differentiate into inflammatory effector cells that mediate actual immune responses whereas, their differentiation into regulatory T cells (Treg) that inhibits the inflammatory T cells is enhanced. Therefore, the manipulation of PKC-θ activity can shift the ratio between inflammatory effector T cells and inhibitory Tregs, to control T cell-mediated immune responses that are responsible for autoimmunity and allograft rejection. Indeed, PKC-θ-deficient mice are resistant to the development of several Th2 and Th17-dependent autoimmune diseases and are defective in mounting alloimmune responses required for rejection of transplanted allografts and graft-versus-host disease. Selective inhibition of PKC-θ is therefore considered as a potential treatment for prevention of autoimmune diseases and allograft rejection.

Regulation of cell death and autophagy by IKK and NF-κB: critical mechanisms in immune function and cancer

The cellular response to survive or to undergo death is fundamental to the benefit of the organism, and errors in this process can lead to autoimmunity and cancer. The transcription factor nuclear factor κB (NF-κB) functions to block cell death through transcriptional induction of genes encoding anti-apoptotic and antioxidant proteins. This is essential for survival of activated cells of the immune system and for cells undergoing a DNA damage response. In Ras-transformed cells and tumors as well as other cancers, NF-κB functions to suppress apoptosis--a hallmark of cancer. Critical prosurvival roles for inhibitor of NF-κB kinase (IKK) family members, including IKKε and TBK1, have been reported, which are both NF-κB-dependent and -independent. While the roles of NF-κB in promoting cell survival in lymphocytes and in cancers is relatively clear, evidence has been presented that NF-κB can promote cell death in particular contexts. Recently, IKK was shown to play a critical role in the induction of autophagy, a metabolic response typically associated with cell survival but which can lead to cell death. This review provides an historical perspective, along with new findings, regarding the roles of the IKK and NF-κB pathways in regulating cell survival.

NF-κB in immunobiology

NF-κB was first discovered and characterized 25 years ago as a key regulator of inducible gene expression in the immune system. Thus, it is not surprising that the clearest biological role of NF-κB is in the development and function of the immune system. Both innate and adaptive immune responses as well as the development and maintenance of the cells and tissues that comprise the immune system are, at multiple steps, under the control of the NF-κB family of transcription factors. Although this is a well-studied area of NF-κB research, new and significant findings continue to accumulate. This review will focus on these areas of recent progress while also providing a broad overview of the roles of NF-κB in mammalian immunobiology.

Regulation of ΔNp63α by NFκΒ

ΔNp63α, the dominant negative isoform of the p63 family is an essential survival factor in head and neck squamous cell carcinoma. This isoform has been shown to be down regulated in response to several DNA damaging agents, thereby enabling an effective cellular response to genotoxic agents. Here, we identify a key molecular mechanism underlying the regulation of ΔNp63α expression in response to extrinsic stimuli, such as chemotherapeutic agents. We show that ΔNp63α interacts with NF-κΒ in presence of cisplatin. We find that NF-κΒ promotes ubiquitin-mediated proteasomal degradation of ΔNp63α. Chemotherapy-induced stimulation of NF-κΒ leads to degradation of ΔNp63α and augments trans-activation of p53 family-induced genes involved in the cellular response to DNA damage. Conversely, inhibition of NF-κΒ with siRNA-mediated silencing NF-κΒ expression attenuates chemotherapy induced degradation of ΔNp63α . These data demonstrate that NF-κΒ plays an essential role in regulating ΔNp63α in response to extrinsic stimuli. Our findings suggest that the activation of NF-κΒ may be a mechanism by which levels of ΔNp63α are reduced, thereby rendering the cells susceptible to cell death in the face of cellular stress or DNA damage.

Nuclear factor-kappaB inducing kinase is required for graft-versus-host disease

BACKGROUND

Donor T lymphocytes are directly responsible for graft-versus-host disease. Molecules important in T-cell function may, therefore, be appropriate targets for graft-versus-host disease therapy and/or prophylaxis. Here we analyzed whether nuclear factor-κ B inducing kinase might have a role in graft-versus-host disease.

DESIGN AND METHODS

We studied the expression of nuclear factor-κ B inducing kinase in human samples from patients with graft-versus-host disease. We also explored the effect of nuclear factor-κ B inducing kinase in a murine model of graft-versus-host disease using donor cells from aly/aly mice (deficient in nuclear factor-κ B inducing kinase) and C57BL/6 mice (control).

RESULTS

We detected expression of nuclear factor-κ B inducing kinase in T-lymphocytes in the pathological lesions of patients with acute graft-versus-host disease. Mice transplanted with aly/aly T lymphocytes did not develop graft-versus-host disease at all, while mice receiving C57BL/6 cells died of a lethal form of the disease. Deficiency of nuclear factor-κ B inducing kinase did not affect the engrafting ability of donor T cells, but severely impaired their expansion capacity early after transplantation, and aly/aly T cells showed a higher proportion of apoptosis than did C57BL/6 T cells. Effector T lymphocytes were the T-cell subset most affected by nuclear factor-κ B inducing kinase deficiency. We also detected lower amounts of inflammatory cytokines in the serum of mice receiving aly/aly T cells than in the serum of mice receiving C57BL/6 T cells.

CONCLUSIONS

Our results show that nuclear factor-κ B inducing kinase has a role in graft-versus-host disease by maintaining the viability of activated alloreactive T lymphocytes.

NF-kappaB inhibits T-cell activation-induced, p73-dependent cell death by induction of MDM2

NF-κB is a key transcription factor involved in the regulation of T-cell activation and proliferation upon engagement of the T-cell receptor (TCR). T cells that lack the IκB kinase (IKKβ) are unable to activate NF-κB, and rapidly undergo apoptosis upon activation. NF-κB activation following T-cell receptor engagement induces the expression of Mdm2 through interaction with NF-κB sites in its P1 promoter, and enforced expression of Mdm2 protected T cells deficient for NF-κB activation from activation-induced cell death. In T cells with intact NF-κB signaling, ablation or pharmacologic inhibition of Mdm2 resulted in activation-induced apoptosis. Mdm2 coprecipitates with p73 in activated T cells, and apoptosis induced by inhibition of Mdm2 was p73-dependent. Further, Bim was identified as a p73 target gene required for cell death induced by Mdm2 inhibition, and a p73-responsive element in intron 1 of Bim was characterized. Our results demonstrate a pathway for survival of activated T cells through NF-κB-induced Mdm2, which blocks Bim-dependent apoptosis through binding and inhibition of p73.

Triple expression of B7-1, B7-2 and 4-1BBL enhanced antitumor immune response against mouse H22 hepatocellular carcinoma

OBJECTIVES

Costimulatory signals are essential for T-cell activation and hence play a very important role in antitumor immunity. B7 and 4-1BBL which belongs to tumor necrosis factor (TNF) family provide costimulatory interaction for T-cell activation and function. This study investigated the role of B7 and 4-1BBL in the amplification of tumor immunity by transduction of the B7-1, B7-2 and 4-1BBL into mouse hepatocellular carcinoma cell line H22.

METHODS

The tumorigenicity of H22 variants expressing either B7-1, B7-2 (H22/B7-1/B7-2) or 4-1BBL was compared with an H22 variant expressing B7-1, B7-2 and 4-1BBL (H22/B7-1/B7-2/4-1BBL). The study next investigated whether the combination of B7-1/B7-2 and 4-1BBL cell injection induced cytotoxic T lymphocyte (CTL) response and IL-2/IFN-γ secretion. The immune mechanisms underlying this combination treatment were then analyzed.

RESULTS

Syngeneic BALB/c mice injected with H22/B7-1/B7-2/4-1BBL cells that expressed elevated levels of B7-1, B7-2 and 4-1BBL showed a tumor development frequency of 50% compared with 100% in mice injected with the H22 parental line, H22/neo, H22/B7-1/B7-2 and H22/4-1BBL. Mice inoculated with H22 tumor cells expressing B7-1, B7-2 and 4-1BBL developed a strong cytotoxic T lymphocyte response and long-term immunity against wild-type tumor, suggesting a synergistic effect between the B7 and 4-1BBL costimulatory pathways. Results showed that H22/B7-1/B7-2/4-1BBL tumor vaccines probably protect the infiltrating lymphocytes from apoptosis and induce NF-κB activation to improve T-cell-mediated antitumor response.

CONCLUSIONS

In this study, the antitumor consequences of using B7-1, B7-2 and 4-1BBL gene transfer have demonstrated the therapeutic potential of gene therapy approach for hepatocellular carcinoma.

Inhibition of nuclear factor-kappa B differentially affects thyroid cancer cell growth, apoptosis, and invasion

Background

Nuclear factor-κB (NF-κB) is constitutively activated in many cancers and plays a key role in promoting cell proliferation, survival, and invasion. Our understanding of NF-κB signaling in thyroid cancer, however, is limited. In this study, we have investigated the role of NF-κB signaling in thyroid cancer cell proliferation, invasion, and apoptosis using selective genetic inhibition of NF-κB in advanced thyroid cancer cell lines.

Results

Three pharmacologic inhibitors of NF-κB differentially inhibited growth in a panel of advanced thyroid cancer cell lines, suggesting that these NF-κB inhibitors may have off-target effects. We therefore used a selective genetic approach to inhibit NF-κB signaling by overexpression of a dominant-negative IκBα (mIκBα). These studies revealed decreased cell growth in only one of five thyroid cancer cell lines (8505C), which occurred through a block in the S-G2/M transition. Resistance to TNFα-induced apoptosis was observed in all cell lines, likely through an NF-κB-dependent mechanism. Inhibition of NF-κB by mIκBα sensitized a subset of cell lines to TNFα-induced apoptosis. Sensitive cell lines displayed sustained activation of the stress-activated protein kinase/c-Jun NH2-terminal kinase (SAPK/JNK) pathway, defining a potential mechanism of response. Finally, NF-κB inhibition by mIκBα expression differentially reduced thyroid cancer cell invasion in these thyroid cancer cell lines. Sensitive cell lines demonstrated approximately a two-fold decrease in invasion, which was associated with differential expression of MMP-13. MMP-9 was reduced by mIκBα expression in all cell lines tested.

Conclusions

These data indicate that selective inhibition of NF-κB represents an attractive therapeutic target for the treatment of advanced thyroid. However, it is apparent that global regulation of thyroid cancer cell growth and invasion is not achieved by NF-κB signaling alone. Instead, our findings suggest that other important molecular processes play a critical role in defining the extent of NF-κB function within cancer cells.

Never say die: survival signaling in large granular lymphocyte leukemia

Large granular lymphocyte (LGL) leukemia is a rare disorder of mature cytotoxic T or natural killer cells. Large granular lymphocyte leukemia is characterized by the accumulation of cytotoxic cells in blood and infiltration in the bone marrow, liver, and spleen. Herein, we review clinical features of LGL leukemia. We focus our discussion on known survival signals believed to play a role in the pathogenesis of LGL leukemia and their potential therapeutic implications.

Roles of the NF-kappaB pathway in lymphocyte development and function

This article focuses on the functions of NF-kappaB that vitally impact lymphocytes and thus adaptive immunity. NF-kappaB has long been known to be essential for many of the responses of mature lymphocytes to invading pathogens. In addition, NF-kappaB has important functions in shaping the immune system so it is able to generate adaptive responses to pathogens. In both contexts, NF-kappaB executes critical cell-autonomous functions within lymphocytes as well as within supportive cells, such as antigen-presenting cells or epithelial cells. It is these aspects of NF-kappaB's physiologic impact that we address in this article.

Radiation-induced salivary gland dysfunction results from p53-dependent apoptosis

PURPOSE

Radiotherapy for head-and-neck cancer causes adverse secondary side effects in the salivary glands and results in diminished quality of life for the patient. A previous in vivo study in parotid salivary glands demonstrated that targeted head-and-neck irradiation resulted in marked increases in phosphorylated p53 (serine(18)) and apoptosis, which was suppressed in transgenic mice expressing a constitutively active mutant of Akt1 (myr-Akt1).

METHODS AND MATERIALS

Transgenic and knockout mouse models were exposed to irradiation, and p53-mediated transcription, apoptosis, and salivary gland dysfunction were analyzed.

RESULTS

The proapoptotic p53 target genes PUMA and Bax were induced in parotid salivary glands of mice at early time points after therapeutic radiation. This dose-dependent induction requires expression of p53 because no radiation-induced expression of PUMA and Bax was observed in p53-/- mice. Radiation also induced apoptosis in the parotid gland in a dose-dependent manner, which was p53 dependent. Furthermore, expression of p53 was required for the acute and chronic loss of salivary function after irradiation. In contrast, apoptosis was not induced in p53-/- mice, and their salivary function was preserved after radiation exposure.

CONCLUSIONS

Apoptosis in the salivary glands after therapeutic head-and-neck irradiation is mediated by p53 and corresponds to salivary gland dysfunction in vivo.

Antigen-mediated T cell expansion regulated by parallel pathways of death

T cells enigmatically require caspase-8, an inducer of apoptosis, for antigen-driven expansion and effective antiviral responses, and yet the pathways responsible for this effect have been elusive. A defect in caspase-8 expression does not affect progression through the cell cycle but causes an abnormally high rate of cell death that is distinct from apoptosis and does not involve a loss of NFkappaB activation. Instead, antigen or mitogen activated Casp8-deficient T cells exhibit an alternative type of cell death similar to programmed necrosis that depends on receptor interacting protein (Ripk1). The selective genetic ablation of caspase-8, NFkappaB, and Ripk1, reveals two forms of cell death that can regulate virus-specific T cell expansion.

Itch: a HECT-type E3 ligase regulating immunity, skin and cancer

The HECT-type E3 ubiquitin ligase (E3) Itch is absent in the non-agouti-lethal 18H or Itchy mice, which develop a severe immunological disease, including lung and stomach inflammation and hyperplasia of lymphoid and hematopoietic cells. The involvement of Itch in multiple signaling pathways and pathological conditions is presently an area of extensive scientific interest. This review aims to bring together a growing body of work exploring Itch-regulated biological processes, and to highlight recent discoveries on the regulatory mechanisms modulating its catalytic activity and substrate recognition capability. Our contribution is also an endeavor to correlate Itch substrate specificity with the pathological defects manifested by the mutant Itchy mice.

A gene signature-based approach identifies mTOR as a regulator of p73

Although genomic technologies have advanced the characterization of gene regulatory networks downstream of transcription factors, the identification of pathways upstream of these transcription factors has been more challenging. In this study we present a gene signature-based approach for connecting signaling pathways to transcription factors, as exemplified by p73. We generated a p73 gene signature by integrating whole-genome chromatin immunoprecipitation and expression profiling. The p73 signature was linked to corresponding signatures produced by drug candidates, using the in silico Connectivity Map resource, to identify drugs that would induce p73 activity. Of the pharmaceutical agents identified, there was enrichment for direct or indirect inhibitors of mammalian Target of Rapamycin (mTOR) signaling. Treatment of both primary cells and cancer cell lines with rapamycin, metformin, and pyrvinium resulted in an increase in p73 levels, as did RNA interference-mediated knockdown of mTOR. Further, a subset of genes associated with insulin response or autophagy exhibited mTOR-mediated, p73-dependent expression. Thus, downstream gene signatures can be used to identify upstream regulators of transcription factor activity, and in doing so, we identified a new link between mTOR, p73, and p73-regulated genes associated with autophagy and metabolic pathways.

Autoregulation of Th1-mediated inflammation by twist1

The basic helix-loop-helix transcriptional repressor twist1, as an antagonist of nuclear factor κB (NF-κB)–dependent cytokine expression, is involved in the regulation of inflammation-induced immunopathology. We show that twist1 is expressed by activated T helper (Th) 1 effector memory (EM) cells. Induction of twist1 in Th cells depended on NF-κB, nuclear factor of activated T cells (NFAT), and interleukin (IL)-12 signaling via signal transducer and activator of transcription (STAT) 4. Expression of twist1 was transient after T cell receptor engagement, and increased upon repeated stimulation of Th1 cells. Imprinting for enhanced twist1 expression was characteristic of repeatedly restimulated EM Th cells, and thus of the pathogenic memory Th cells characteristic of chronic inflammation. Th lymphocytes from the inflamed joint or gut tissue of patients with rheumatic diseases, Crohn's disease or ulcerative colitis expressed high levels of twist1. Expression of twist1 in Th1 lymphocytes limited the expression of the cytokines interferon-γ, IL-2, and tumor necrosis factor-α, and ameliorated Th1-mediated immunopathology in delayed-type hypersensitivity and antigen-induced arthritis.

The apoptotic pathway contributing to the deletion of naive CD8 T cells during the induction of peripheral tolerance to a cross-presented self-antigen

The maintenance of T cell tolerance in the periphery proceeds through several mechanisms, including anergy, immuno-regulation, and deletion via apoptosis. We examined the mechanism underlying the induction of CD8 T cell peripheral tolerance to a self-Ag expressed on pancreatic islet beta-cells. Following adoptive transfer, Ag-specific clone 4 T cells underwent deletion independently of extrinsic death receptors, including Fas, TNFR1, or TNFR2. Additional experiments revealed that the induction of clone 4 T cell apoptosis during peripheral tolerance occurred via an intrinsic death pathway that could be inhibited by overexpression of Bcl-2 or targeted deletion of the proapoptotic molecule, Bim, thereby resulting in accumulation of activated clone 4 T cells. Over-expression of Bcl-2 in clone 4 T cells promoted the development of effector function and insulitis whereas Bim-/- clone 4 cells were not autoaggressive. Examination of the upstream molecular mechanisms contributing to clone 4 T cell apoptosis revealed that it proceeded in a p53, E2F1, and E2F2-independent manner. Taken together, these data reveal that initiation of clone 4 T cell apoptosis during the induction of peripheral tolerance to a cross-presented self-Ag occurs through a Bcl-2-sensitive and at least partially Bim-dependent mechanism.

RelA/NF-kappaB recruitment on the bax gene promoter antagonizes p73-dependent apoptosis in costimulated T cells

The balance between antiapoptotic and proapoptotic proteins of the Bcl-2 family is critical in determining the fate of T cells in response to death stimuli. Proapoptotic genes, such as bax, are generally regulated by the p53 family of transcription factors, whereas NF-kappaB subunits can activate the transcription of antiapoptotic Bcl-2 members. Here, we show that CD28 activation protects memory T cells from irradiation-induced apoptosis by both upregulating bcl-xL and inhibiting bax gene expression. We found that p73, but not p53, binds to and trans-activates the bax gene promoter in irradiated T cells. The activation of RelA/NF-kappaB subunit in CD28 costimulated T cells and its binding onto the bax gene promoter results in suppression of bax transcription and decrease in both p73 and RNA polymerase II recruitment in vivo. RelA recruitment on the bax gene promoter is also accompanied by the lost of p300 binding and the parallel appearance of histone deacetylase-1-containing complexes. These findings identify RelA/NF-kappaB as a critical regulator of T-cell survival by affecting the balance of Bcl-2 family members.

Caspase-cleaved HPK1 induces CD95L-independent activation-induced cell death in T and B lymphocytes

Life and death of peripheral lymphocytes is strictly controlled to maintain physiologic levels of T and B cells. Activation-induced cell death (AICD) is one mechanism to delete superfluous lymphocytes by restimulation of their immunoreceptors and it depends partially on the CD95/CD95L system. Recently, we have shown that hematopoietic progenitor kinase 1 (HPK1) determines T-cell fate. While full-length HPK1 is essential for NF-kappaB activation in T cells, the C-terminal fragment of HPK1, HPK1-C, suppresses NF-kappaB and sensitizes toward AICD by a yet undefined cell death pathway. Here we show that upon IL-2-driven expansion of primary T cells, HPK1 is converted to HPK1-C by a caspase-3 activity below the threshold of apoptosis induction. HPK1-C selectively blocks induction of NF-kappaB-dependent antiapoptotic Bcl-2 family members but not of the proapoptotic Bcl-2 family member Bim. Interestingly, T and B lymphocytes from HPK1-C transgenic mice undergo AICD independently of the CD95/CD95L system but involving caspase-9. Knock down of HPK1/HPK1-C or Bim by small interfering RNA shows that CD95L-dependent and HPK1/HPK1-C-dependent cell death pathways complement each other in AICD of primary T cells. Our results define HPK1-C as a suppressor of antiapoptotic Bcl-2 proteins and provide a molecular basis for our understanding of CD95L-independent AICD of lymphocytes.

Life and death in peripheral T cells

During the course of an immune response, antigen-reactive T cells clonally expand and then are removed by apoptosis to maintain immune homeostasis. Life and death of T cells is determined by multiple factors, such as T-cell receptor triggering, co-stimulation or cytokine signalling, and by molecules, such as caspase-8 (FLICE)-like inhibitory protein (FLIP) and haematopoietic progenitor kinase 1 (HPK1), which regulate the nuclear factor-kappaB (NF-kappaB) pathway. Here, we discuss the concepts of activation-induced cell death (AICD) and activated cell-autonomous death (ACAD) in the regulation of life and death in T cells.

Recombinant adenoviral vectors can induce expression of p73 via the E4-orf6/7 protein

Despite the utility of recombinant adenoviral vectors in basic research, their therapeutic promise remains unfulfilled. Most engineered adenoviral vectors use a heterologous promoter to transcribe a foreign gene. We show that adenoviruses containing the cytomegalovirus immediate-early promoter induce the expression of the proapoptotic cellular protein TAp73 via the cyclin-dependent kinase-retinoblastoma protein-E2F pathway in murine embryonic fibroblasts. Cells transduced with these vectors also expressed high levels of the adenoviral E4-orf6/7 and E2A proteins. By contrast, adenoviruses containing the ubiquitin C promoter failed to elicit these effects. E4-orf6/7 is necessary and sufficient for increased TAp73 expression, as shown by using retrovirus-mediated E4-orf6/7 expression and adenovirus with the E4-orf6/7 gene deleted. Activation of TAp73 likely occurs via E4-orf6/7-induced dimerization of E2F and subsequent binding to the inverted E2F-responsive elements within the TAp73 promoter. In addition, adenoviral vectors containing the cytomegalovirus immediate-early promoter, but not the ubiquitin C promoter, cooperated with chemotherapeutic agents to decrease cellularity in vitro. In contrast to murine embryonic fibroblasts, adenoviruses containing the ubiquitin C promoter, but not the cytomegalovirus immediate-early promoter, induced both E4-orf6/7 and TAp73 in human foreskin fibroblasts, emphasizing the importance of cellular context for promoter-dependent effects. Because TAp73 is important for the efficacy of chemotherapy, adenoviruses that increase TAp73 expression may enhance cancer therapies by promoting apoptosis. However, such adenoviruses may impair the long-term survival of transduced cells during gene replacement therapies. Our findings reveal previously unknown effects of foreign promoters in recombinant adenoviral vectors and suggest means to improve the utility of engineered adenoviruses by better controlling their impact on viral and cellular gene expression.

Stabilization of p73 by nuclear IkappaB kinase-alpha mediates cisplatin-induced apoptosis

In response to DNA damage, p53 and its homolog p73 have a function antagonistic to NF-kappaB in deciding cell fate. Here, we show for the first time that p73, but not p53, is stabilized by physical interaction with nuclear IkappaB kinase (IKK)-alpha to enhance cisplatin (CDDP)-induced apoptosis. CDDP caused a significant increase in the amounts of nuclear IKK-alpha and p73alpha in human osteosarcoma-derived U2OS cells. Ectopic expression of IKK-alpha prolonged the half-life of p73 by inhibiting its ubiquitination and thereby enhancing its transactivation and pro-apoptotic activities. Consistent with these results, small interfering RNA-mediated knockdown of endogenous IKK-alpha inhibited the CDDP-mediated accumulation of p73alpha. The kinase-deficient mutant form of IKK-alpha interacted with p73alpha, but failed to stabilize it. Furthermore, CDDP-mediated accumulation of endogenous p73alpha was not detected in mouse embryonic fibroblasts (MEFs) prepared from IKK-alpha-deficient mice, and CDDP sensitivity was significantly decreased in IKK-alpha-deficient MEFs compared with wild-type MEFs. Thus, our results strongly suggest that the nuclear IKK-alpha-mediated accumulation of p73alpha is one of the novel molecular mechanisms to induce apoptotic cell death in response to CDDP, which may be particularly important in killing tumor cells with p53 mutation.

Physiologically high concentrations of 17β-estradiol enhance NF-κB activity in human T cells

Estrogen has diverse effects on inflammation and immune responses. That pregnancy is associated with remission of some autoimmune diseases and exacerbation of others suggests that physiological fluctuation in estrogen levels could affect the immune responses in humans. However, the molecular basis for these phenomena is poorly understood. We hypothesized that fluctuations of estrogen levels modulate intracellular signaling for immune responses via estrogen receptors (ERs). In reporter assays, 17β-estradiol (E2) at a physiologically high concentration increased the activity of NF-κB in Jurkat cells stimulated by PMA/ionomycin or TNF-α. Overexpression and RNA interference experiments suggested that the effects were mediated through ERβ. Immunoprecipitation assay showed that both ERα and ERβ are directly associated with NF-κB in the cell nucleus. Using chromatin immunoprecipitation assay, we confirmed that ERα and ERβ associated with NF-κB and steroid hormone coactivators at the promoter region of NF-κB regulated gene. Considering that NF-κB regulates the expression of various genes essential for cell growth and death, estrogen could regulate the fate of T cells by affecting the activity of NF-κB. To determine whether E2 alters the fate of T cells, we investigated E2 actions on T cell apoptosis, a well-known NF-κB-mediated phenomenon. E2 increased apoptosis of Jurkat cells and decreased that of human peripheral blood T cells. Our results indicate that E2 at a physiologically high concentration modulates NF-κB signaling in human T cells via ERβ and affects T cell survival, suggesting that these actions may underlie the gender differences in autoimmune diseases.

p63 and p73 in human cancer: defining the network

The p53-related genes p63 and p73 exhibit significant structural homology to p53; however, they do not function as classical tumor suppressors and are rarely mutated in human cancers. Both p63 and p73 exhibit tissue-specific roles in normal development and a complex contribution to tumorigenesis that is due to their expression as multiple protein isoforms. The predominant p63/p73 isoforms expressed both in normal development and in many tumors lack the conserved transactivation (TA) domain; these isoforms instead exhibit a truncated N-terminus (DeltaN) and function at least in part as transcriptional repressors. p63 and p73 isoforms are regulated through both transcriptional and post-translational mechanisms, and they in turn regulate diverse cellular functions including proliferation, survival and differentiation. The net effect of p63/p73 expression in a given context depends on the ratio of TA/DeltaN isoforms expressed, on physical interaction between p63 and p73 isoforms, and on functional interactions with p53 at the promoters of specific downstream target genes. These multifaceted interactions occur in diverse ways in tumor-specific contexts, demonstrating a functional 'p53 family network' in human tumorigenesis. Understanding the regulation and mechanistic contributions of p63 and p73 in human cancers may ultimately provide new therapeutic opportunities for a variety of these diseases.

NF-kappaB and the immune response

One of the primary physiological roles of nuclear factor-kappa B (NF-kappaB) is in the immune system. In particular, NF-kappaB family members control the transcription of cytokines and antimicrobial effectors as well as genes that regulate cellular differentiation, survival and proliferation, thereby regulating various aspects of innate and adaptive immune responses. In addition, NF-kappaB also contributes to the development and survival of the cells and tissues that carry out immune responses in mammals. This review, therefore, describes the role of the NF-kappaB pathway in the development and functioning of the immune system.

MDM2 is required for suppression of apoptosis by activated Akt1 in salivary acinar cells

Chronic damage to the salivary glands is a common side effect following head and neck irradiation. It is hypothesized that irreversible damage to the salivary glands occurs immediately after radiation; however, previous studies with rat models have not shown a causal role for apoptosis in radiation-induced injury. We report that etoposide and gamma irradiation induce apoptosis of salivary acinar cells from FVB control mice in vitro and in vivo; however, apoptosis is reduced in transgenic mice expressing a constitutively activated mutant of Akt1 (myr-Akt1). Expression of myr-Akt1 in the salivary glands results in a significant reduction in phosphorylation of p53 at serine(18), total p53 protein accumulation, and p21(WAF1) or Bax mRNA following etoposide or gamma irradiation of primary salivary acinar cells. The reduced level of p53 protein in myr-Akt1 salivary glands corresponds with an increase in MDM2 phosphorylation in vivo, suggesting that the Akt/MDM2/p53 pathway is responsible for suppression of apoptosis. Dominant-negative Akt blocked phosphorylation of MDM2 in salivary acinar cells from myr-Akt1 transgenic mice. Reduction of MDM2 levels in myr-Akt1 primary salivary acinar cells with small interfering RNA increases the levels of p53 protein and renders these cells susceptible to etoposide-induced apoptosis in spite of the presence of activated Akt1. These results indicate that MDM2 is a critical substrate of activated Akt1 in the suppression of p53-dependent apoptosis in vivo.

NF-kappaB regulates the stability and activity of p73 by inducing its proteolytic degradation through a ubiquitin-dependent proteasome pathway

Nuclear factor kappa B (NF-kappaB), which exists as heterodimeric complexes composed of p50 and p65, has been shown to play an important role in cell survival processes. In the present study, we found for the first time that NF-kappaB has an ability to induce the ubiquitin-dependent proteasomal degradation of proapoptotic p73alpha. The activation of NF-kappaB in tumor necrosis factor alpha (TNF-alpha)-stimulated H1299 cells resulted in a significant reduction in the amounts of the endogenous p73alpha. Consistent with these results, TNF-alpha-mediated downregulation of p73alpha was observed in wild-type (WT) mouse embryonic fibroblasts (MEFs) but not in p65-deficient MEFs. Ectopic expression of NF-kappaB decreased a half-life of p73alpha by increasing its ubiquitination levels, and thereby inhibiting the transcriptional activity as well as proapoptotic function of p73alpha, whereas NF-kappaB had undetectable effects on p53. Immunoprecipitation experiments demonstrated that, under our experimental conditions, NF-kappaB does not bind to p73alpha in mammalian cultured cells. In contrast to WT p65, the COOH-terminal deletion mutant of p65 (p65DeltaC) failed to reduce the expression levels of p73alpha, suggesting that NF-kappaB-mediated proteolytic degradation of p73alpha requires the transcriptional activity of NF-kappaB. Taken together, our present results imply that NF-kappaB-mediated degradation of proapoptotic p73 is a novel inhibitory mechanism of p73 that regulates cell survival and death.

How T lymphocytes switch between life and death

While insufficient cell death of activated T cells can result in autoimmune disorders, elimination of too many T cells can lead to immunodeficiency. Therefore, T lymphocyte fate is highly regulated and requires that cells can switch from an apoptosis-resistant towards an apoptosis-sensitive state. This switch is tightly controlled by various effector molecules. Basically, two separate pathways control the fate of antigen-activated T cells: activation-induced cell death (AICD) and activated T cell autonomous death (ACAD). Autoreactive T lymphocytes are eliminated by restimulation via their T cell receptor (TCR) and undergo AICD involving death receptors (extrinsic pathway). In contrast, ACAD can lead to T cell deletion without TCR restimulation, and is determined by the ratio between anti- and pro-apoptotic Bcl-2 family members at the mitochondria (intrinsic pathway). While the extrinsic and the intrinsic pathway lead to caspase activation, non-caspase proteases (e.g., cathepsins) can be released by the lysosomes and might contribute to AICD as well as to ACAD. Activated T cells poses cell death escape mechanisms which are needed for survival of (memory) T cells, but are deleterious for autoimmune disorders or progression of T cell lymphomas.

CD154 induces p73 to overcome the resistance to apoptosis of chronic lymphocytic leukemia cells lacking functional p53

Intravenous infusion of autologous chronic lymphocytic leukemia (CLL) cells transduced with an adenovirus encoding CD40-ligand (CD154) caused rapid reductions in leukemia-cell counts and lymphnode size. We hypothesized that CD40-ligation via CD154 sensitized CLL cells to death-receptor-mediated apoptosis. We found that CD154-expressing cells induced expression of CD95 and the BH3-interacting-domain death agonist (Bid) in CLL, regardless of whether the leukemia cells had functional p53. Such treatment also induced p73, a p53-related transcription factor regulated by c-Abl kinase, and enhanced the sensitivity to fludarabine (F-ara-A) of CLL cells lacking functional p53. Transduction of CLL cells with an adenovirus encoding p73 also induced Bid and CD95 and enhanced the sensitivity to F-ara-A of p53-deficient CLL cells. However, inhibition of c-Abl with imatinib suppressed CD154-induced expression of p73, p73-induced expression of Bid and CD95, and blocked the sensitization of p53-deficient CLL cells to CD95-mediated or F-ara-A-induced apoptosis. Conversely, CLL cells transduced with an imatinib-resistant c-Abl mutant could be induced by CD154 to express p73 and Bid even when treated with imatinib. These results indicate that CD154 can sensitize leukemia cells to apoptosis via the c-Abl-dependent activation of p73 and mitigate the resistance of p53-deficient CLL cells to anticancer drug therapy.

An adenoviral vector for probing promoter activity in primary immune cells

Functional analysis of the DNA regulatory regions that control gene expression has largely been performed through transient transfection of promoter-reporter constructs into transformed cells. However, transformed cells are often poor models of primary cells. To directly analyze DNA regulatory regions in primary cells, we generated a novel adenoviral luciferase reporter vector, pShuttle-luciferase-GFP (pSLUG) that contains a promoterless luciferase cassette (with an upstream cloning site) for probing promoter activity, and a GFP expression cassette that allows for the identification of transduced cells. Recombinant adenoviruses generated from this vector can transduce a wide range of primary immune cells with high efficiency, including human macrophages, dendritic cells and T cells; and mouse T cells transgenic for the coxsackie and adenoviral receptor (CAR). In primary T cells, we show inducible nuclear factor of activated T cells (NF-AT) activity using a recombinant pSLUG adenovirus containing a consensus NF-AT promoter. We further show inducible IL-12/23 p40 promoter activity in primary macrophages and dendritic cells using a recombinant pSLUG adenovirus containing the proximal human IL-12/23 p40 promoter. The pSLUG system promises to be a powerful tool for the analysis of DNA regulatory regions in diverse types of primary immune cells.

NF-kappaB-dependent regulation of the timing of activation-induced cell death of T lymphocytes

One of the mechanisms by which activated T cells die is activation-induced cell death (AICD). This pathway requires persistent stimulation via the TCR and engagement of death receptors. We found that TCR stimulation led to transient nuclear accumulation of the NF-kappaB component p65/RelA. In contrast, nuclear c-Rel levels remained high even after extended periods of activation. Loss of nuclear p65/RelA correlated with the onset of AICD, suggesting that p65/RelA target genes may maintain cell viability. Quantitative RNA analyses showed that three of several putative NF-kappaB-dependent antiapoptotic genes were expressed with kinetics that paralleled nuclear expression of p65/RelA. Of these three, ectopic expression only of Gadd45beta protected significantly against AICD, whereas IEX-1 and Bcl-x(L) were much less effective. We propose that the timing of AICD, and thus the length of the effector phase, are regulated by transient expression of a subset of p65/RelA-dependent antiapoptotic genes.

Siva-1 negatively regulates NF-kappaB activity: effect on T-cell receptor-mediated activation-induced cell death (AICD)

Ligation of TCRs on stimulated T cells leads to activation-induced cell death (AICD) resulting in the downregulation of immune responses, a process essential for T-cell homeostasis. In this study, using transformed T-cell lines such as Jurkat and Do11.10 as cellular models of TCR-mediated AICD, we have demonstrated that the proapoptotic protein Siva-1 is required for TCR-induced apoptosis. Knockdown of Siva-1 rendered T cells specifically resistant to anti-CD3 but not Fas-induced apoptosis. Further, we observed that in Siva-1 knockout Jurkat cells, TCR-mediated activation of the canonical and non-canonical limbs of the NF-kappaB pathway are significantly enhanced as reflected by elevated nuclear levels of p65 and RelB, respectively. In addition, loss of endogenous Siva-1 also resulted in the enhanced expression of NF-kappaB- responsive anti-apoptotic genes such as Bcl-xL and c-FLIP. Interestingly, the c-FLIP(short) was detected only in TCR-ligated Siva-1 knockdown Jurkat cells. These results demonstrate a significant role for endogenous Siva-1, through its inhibitory effect on NF-kappaB activity, in TCR-mediated AICD with implications in peripheral tolerance, T-cell homeostasis and cancer.

Controlling NF-κB activation in T cells by costimulatory receptors

Full and productive activation of T lymphocytes relies on the simultaneous delivery of T cell receptor (TCR)- and coreceptor-derived signals. In naïve T cells engagement of the TCR alone causes anergy, while TCR triggering of preactivated T cells results in activation-induced cell death. Costimulatory signals are prominently mirrored by the activation of NF-kappaB, which needs input from the TCR as well as from coreceptors in order to be fully activated and to fulfil its crucial function in the immune response. Coreceptor-generated signals tightly control the duration and amplitude of the NF-kappaB response. The activation of IkappaB kinase (IKK) complex at the contact zone between a T cell and an antigen-presenting cell offers the unique opportunity to study the spatial organization of IKK activation. Recent studies indicate that coreceptor pathways influence the threshold activities of many signalling mediators and thus act on multiple layers of the NF-kappaB pathway.

Expression of the Bcl-3 proto-oncogene suppresses p53 activation

While Bcl-3 expression in cancer was originally thought to be limited to B-cell lymphomas with a 14;19 chromosomal translocation, more recent evidence indicates that expression of this presumptive oncoprotein is significantly more widespread in cancer. However, an oncogenic role for Bcl-3 has not been clearly identified. Experiments presented here indicate that Bcl-3 is inducible by DNA damage and is required for the induction of Hdm2 gene expression and the suppression of persistent p53 activity. Furthermore, constitutive expression of Bcl-3 suppresses DNA damage-induced p53 activation and inhibits p53-induced apoptosis through a mechanism that is at least partly dependent on the up-regulation of Hdm2. The results provide insight into a mechanism whereby altered expression of Bcl-3 leads to tumorigenic potential. Since Bcl-3 is required for germinal center formation, these results suggest functional similarities with the unrelated Bcl-6 oncoprotein in suppressing potential p53-dependent cell cycle arrest and apoptosis in response to somatic hypermutation and class switch recombination.

RelA regulates the survival of activated effector CD8 T cells

Nuclear factor of kappa B (NF-kappaB) transcription factors are critical regulators of T-cell activation and survival. The relative contribution of individual NF-kappaB members to these processes remains elusive. We investigated the role of RelA in the regulation of CD8 T-cell activation. We overexpressed, in mature CD8 T cells, a transactivation domain-deficient RelA molecule (p65TAD). We show that p65TAD forms homo- and heterodimers with p50 that bind kappaB sites and selectively inhibit RelA-dependent transactivation. Expression of p65TAD does not affect initial activation or cell cycle progression but induces the death of activated CD8 T cells in vitro and in vivo. However, the long-term survival of resting effector CD8 T cells seems not to be affected by p65TAD expression. Collectively, our results indicate that RelA is a critical regulator of survival of proliferating CD8 T cells but may be dispensable for the survival of resting effector T cells.

Activation or suppression of NFkappaB by HPK1 determines sensitivity to activation-induced cell death

Restimulation of the T-cell receptor (TCR) in activated T cells induces CD95 (Fas/Apo-1)-mediated activation-induced cell death (AICD). The TCR-proximal mechanisms leading to AICD are elusive. Here we characterize hematopoietic progenitor kinase 1 (HPK1) as a differentially regulated TCR-proximal signaling protein involved in AICD of primary T cells. We show that HPK1 is a functional component of the endogenous IkappaB kinase (IKK) complex and is crucial for TCR-mediated NFkappaB activation. While full-length HPK1 enhances IKKbeta phosphorylation, siRNA-mediated knockdown of HPK1 blunts TCR-mediated NFkappaB activation and increases cell death. We also demonstrate proteolytic processing of HPK1 into HPK1-C, specifically in AICD-sensitive primary T cells. The cleavage product HPK1-C sequesters the inactive IKK complex and suppresses NFkappaB upon TCR restimulation by binding to IKKalpha and IKKbeta. T cells of HPK1-C transgenic mice are sensitized towards TCR-mediated AICD. Consequently, preventing HPK1-C generation in primary T cells by siRNA-mediated knockdown results in decreased AICD. Thus, these results show a novel mechanism of sensitization of T lymphocytes towards AICD by suppression of NFkappaB, and propose that HPK1 is a life/death switch in T lymphocytes.

CD28 is required for induction and maintenance of immunological memory in toxin-reactive CD4+ T cells in vivo

We previously reported that Vbeta3+ CD4+ T cells maintained a protracted expansion, with the phenotypes of memory Th2 cells, for 30 days in C57BL/6 (B6) mice implanted with SEA-containing mini-osmotic pumps. In the present study, we followed the fate of Vbeta3+ CD4+ T cells in CD28-/- mice. Vbeta3+ CD4+ T cells increased to a degree similar to that of B6 Vbeta3+ CD4+ T cells until day 10 after implantation, then declined rapidly reaching the control level by 28 days. Remaining Vbeta3+ CD4+ T cells at that time did not exhibit memory phenotypes nor Th2-deviated responses. The rapid drop in Vbeta3+ CD4+ T cells in CD28-/- mice was attributable to upregulated induction of apoptosis owing to marginal inductions of Bcl-2 and Bcl-xL. Collectively, these data indicate CD28 to play critical roles in the generation and maintenance of SEA-reactive CD4+ T cells in vivo.

c-Rel-dependent priming of naive T cells by inflammatory cytokines

The intrinsic refractoriness of naive T cells for cytokine production is counteracted by cells of the innate immune system. Upon sensing danger via Toll-like receptors, these cells upregulate T cell costimulatory molecules and secrete cytokines that enhance T cell activation. We show that cytokine-mediated priming of naive T cells requires the NF-kappaB family member c-Rel. In resting naive cells c-Rel is associated primarily with IkappaBbeta, an inhibitory molecule that is not effectively degraded by TCR signals. Exposure of T cells to proinflammatory cytokines, TNF-alpha and IL-1beta, shifts c-Rel to IkappaBalpha-associated complexes that are readily targeted by the TCR. As a consequence, IL-2 and IFN-gamma mRNA are produced more quickly, and at higher levels, in cytokine-primed T cells. This mechanism does not operate in effector T cells where cytokine gene expression is c-Rel-independent. We propose that c-Rel plays a crucial role as a target of innate signals in T cells.

Activation of NF-kappaB by HTLV-I and implications for cell transformation

T-cell transformation by the human T-cell leukemia virus type I (HTLV-I) involves deregulation of cellular transcription factors, including members of the NF-kappaB family. In normal T cells, NF-kappaB activation occurs transiently in response to immune stimuli, which is required for antigen-stimulated T-cell proliferation and survival. However, HTLV-I induces persistent activation of NF-kappaB, causing deregulated expression of a large array of cellular genes, which in turn contributes to the induction of T-cell transformation. The HTLV-I transforming protein Tax functions as an intracellular stimulator of IkappaB kinase (IKK), a cellular kinase mediating NF-kappaB activation by diverse stimuli. Tax physically interacts with IKK and renders this inducible kinase constitutively active. By assembling different Tax/IKK complexes, Tax targets the persistent activation of both canonical and noncanonical NF-kappaB signaling pathways. Whereas Tax plays a primary role in HTLV-I-mediated NF-kappaB activation, recent studies reveal that the IKK/NF-kappaB signaling pathway is also activated in freshly isolated adult T-cell leukemia (ATL) cells that often lack detectable Tax expression. The mechanism underlying this Tax-independent pathway of NF-kappaB activation remains poorly understood. Clarifying the precise nature and consequences of the constitutive NF-kappaB activation in ATL cells is important for developing rational therapeutic strategies for this T-cell malignancy.

TCR pathway involves ICBP90 gene down-regulation via E2F binding sites

Antigen-induced cell death is essential for function, growth and differentiation of T-lymphocytes through legation of the T cell receptor. Since TCR-induced cell death occurs at late G1 checkpoint of the cell cycle and considering that ICBP90 is critical for G1/S transition, we studied the ICBP90 regulation through the TCR pathway in Jurkat cells. ICBP90 expression was strongly decreased after TCR triggering concomitantly to cyclin D3 and topoisomerase IIalpha expression decreases. Cell stimulation with PMA and/or calcium ionophore A23187 down-regulated ICBP90 expression. The decrease of ICBP90 protein and mRNA expressions was accompanied with cell growth arrest. A luciferase reporter assay demonstrated that activation of TCR pathways inhibit ICBP90 gene promoter activity. Three consensus E2F binding sites (called from E2F-a to E2F-c) were identified in the ICBP90 gene promoter and were subjected to mutations. The E2F-a, located in a highly active promoter fragment, shows a strong positive functional activity in proliferating cells. E2F-a and E2F-c binding sites are involved in the TCR-induced down-regulation of ICBP90 gene transcription. Altogether, our data demonstrate that TCR signaling pathways regulate ICBP90 gene expression through pRb/E2F complex. We propose that ICBP90 down-regulation is a key event in G1 arrest preceding T cell death.