The TSC-mTOR signaling pathway regulates the innate inflammatory response

The role of mTOR in memory CD8 T-cell differentiation

The mammalian target of rapamycin (mTOR) is an intracellular kinase that regulates cell growth and metabolism. Its specific inhibitor rapamycin is currently used in transplant recipients as an immunosuppressive drug to prevent allograft rejection. Studies have shown complex and diverse mechanisms for the immunosuppressive effects of rapamycin. The drug has been reported to inhibit T-cell proliferation, induce anergy, modulate T-cell trafficking, promote regulatory T cells, and also prevent maturation of dendritic cells as well as production of type I interferon. However, several other studies have paradoxically demonstrated immunostimulatory effects of rapamycin by improving antigen presentation and regulating cytokine production from macrophages and myeloid dendritic cells. Recently, it has been shown that rapamycin also exhibits immunostimulatory effects on memory CD8(+) T-cell differentiation. The drug improved both quantity and quality of memory CD8(+) T cells induced by viral infection and vaccination, showing that mTOR is a major regulator of memory CD8(+) T-cell differentiation. These discoveries have implications for the development of novel vaccine regimens. Here, we review the role of mTOR in memory CD8(+) T-cell differentiation and compare the effect of rapamycin among CD8(+) T cells, CD4(+) T cells, and dendritic cells. Also, we discuss potential application of these findings in a clinical setting.

Translation control: a multifaceted regulator of inflammatory response

A robust innate immune response is essential to the protection of all vertebrates from infection, but it often comes with the price tag of acute inflammation. If unchecked, a runaway inflammatory response can cause significant tissue damage, resulting in myriad disorders, such as dermatitis, toxic shock, cardiovascular disease, acute pelvic and arthritic inflammatory diseases, and various infections. To prevent such pathologies, cells have evolved mechanisms to rapidly and specifically shut off these beneficial inflammatory activities before they become detrimental. Our review of recent literature, including our own work, reveals that the most dominant and common mechanism is translational silencing, in which specific regulatory proteins or complexes are recruited to cis-acting RNA structures in the untranslated regions of single or multiple mRNAs that code for the inflammatory protein(s). Enhancement of the silencing function may constitute a novel pharmacological approach to prevent immunity-related inflammation.

mTOR and GSK-3 shape the CD4+ T-cell stimulatory and differentiation capacity of myeloid DCs after exposure to LPS

Prolonged inhibition of the kinase, mammalian target of rapamycin (mTOR), during myeloid dendritic cell (DC) generation confers resistance to maturation. Recently, however, mTOR inhibition immediately before Toll-like receptor ligation has been found to exert proinflammatory effects on myeloid cells, notably enhanced IL-12p40/p70 production. We show, for the first time, that mouse or human DCs generated under mTOR inhibition exhibit markedly enhanced IL-12p70 production after lipopolysaccharide (LPS) stimulation, despite impaired costimulatory molecule expression and poor T-cell stimulatory ability. Consistent with this finding, we reveal that increased IL-12p40 production occurs predominantly in CD86(lo) immature DCs. High IL-12p40/p70 production by CD86(lo) DC resulted from failed down-regulation of glycogen synthase kinase-3 (GSK-3) activity and could not be ascribed to enhanced Akt function. Despite high IL-12p70 secretion, rapamycin-conditioned, LPS-stimulated DCs remained poor T-cell stimulators, failing to enhance allogeneic Th1 cell responses. We also report that inhibition of GSK-3 impedes the ability of LPS-stimulated DCs to induce forkhead box p3 in CD4(+)CD25(-) T cells, as does the absence of IL-12p40/p70. Thus, GSK-3 activity in DC is regulated via signaling linked to mTOR and modulates their capacity both to produce IL-12p40/p70 and induce forkhead box p3 in CD4(+) T cells under inflammatory conditions.

Intranasally delivered siRNA targeting PI3K/Akt/mTOR inflammatory pathways protects from aspergillosis

Innate responses combine with adaptive immunity to generate the most effective form of anti-Aspergillus immune resistance. Although some degree of inflammation is required for protection, progressive inflammation may worsen disease and ultimately prevents pathogen eradication. To define molecular pathways leading to or diverting from pathogenic inflammation in infection, we resorted to dendritic cells (DCs), known to activate distinct signaling pathways in response to pathogens. We found that distinct intracellular pathways mediated the sensing of conidia and hyphae by lung DCs in vitro, which translate in vivo in the activation of protective Th1/Treg responses by conidia or inflammatory Th2/Th17 responses by hyphae. In vivo targeting inflammatory (PI3K/Akt/mTOR) or anti-inflammatory (STAT3/IDO) DC pathways by intranasally delivered small interfering RNA (siRNA) accordingly modified inflammation and immunity to infection. Thus, the screening of signaling pathways in DCs through a systems biology approach may be exploited for the development of siRNA therapeutics to attenuate inflammation in respiratory fungal infections and diseases.

Rapamycin inhibits cholangiocyte regeneration by blocking interleukin-6-induced activation of signal transducer and activator of transcription 3 after liver transplantation

Cholangiocyte proliferation is necessary for biliary recovery from cold ischemia and reperfusion injury (CIRI), but there are few studies on its intracellular mechanism. In this process, the role of rapamycin, a new immunosuppressant used in liver transplantation, is still unknown. In order to determine whether rapamycin can depress cholangiocyte regeneration by inhibiting signal transducer and activator of transcription 3 (STAT3) activation, rapamycin (0.05 mg/kg) was administered to rats for 3 days before orthotopic liver transplantation. The results indicated that cholangiocytes responded to extended cold preservation (12 hours) with severe bile duct injures, marked activation of the interleukin-6 (IL-6)/STAT3 signal pathway, and increased expression of cyclin D1 until 7 days after transplantation, and this was followed by compensatory cholangiocyte regeneration. However, rapamycin treatment inhibited STAT3 activation and resulted in decreased cholangiocyte proliferation and delayed biliary recovery after liver transplantation. On the other hand, rapamycin showed no effect on the expression of IL-6. We conclude that the IL-6/STAT3 signal pathway is involved in initiating cholangiocytes to regenerate and repair CIRI. Rapamycin represses cholangiocyte regeneration by inhibiting STAT3 activation, which might have a negative effect on the healing and recovery of bile ducts in grafts with extended cold preservation. Insights gained from this study will be helpful in designing therapy using rapamycin in clinical patients after liver transplantation.

Practical strategies for suppressing hypoxia-inducible factor activity in cancer therapy

The utility of anti-angiogenic strategies for cancer control is strongly compromised by hypoxia-driven phenotypic changes in cancer cells, which make cancer cells more invasive and more prone to give rise to metastases. A key mediator of this phenotypic shift is the transcription factor hypoxia-inducible factor-1 (HIF-1), which acts directly and indirectly to promote the epidermal-mesenchymal transition, boost cancer invasiveness, increase production of angiogenic factors, and induce chemoresistance. In some cancers, HIF-1 activity is constitutively elevated even in aerobic environments, making the cancer harder to treat and control. Practical strategies for suppressing HIF-1 activation may include the following: inhibiting NF-kappaB activation with salicylic acid and/or silibinin, which should decrease transcription of the HIF-1alpha gene; suppressing translation of HIF-1alpha mRNA with drugs that inhibit mTOR or topoisomerase I; supporting the effective activity of prolyl hydroxylases - which promote proteasomal degradation of HIF-1alpha under aerobic conditions - with antioxidant measures, alpha-ketoglutarate, and possibly dichloroacetate; promoting the O(2)-independent proteasomal degradation of HIF-1alpha with agents that inhibit the chaperone protein Hsp90; and blocking HIF-1 binding to its DNA response elements with anthracyclines. The utility of various combinations of these strategies should be tested in cancer cell cultures and rodent xenograft models; initial efforts in this regard have yielded encouraging results. Comprehensive strategies for suppressing HIF-1 activity can be expected to complement the efficacy of cancer chemotherapy and of effective anti-angiogenic regimens.

Transplantation: time to rethink immunosuppression by mTOR inhibitors?

Several surprising findings indicate that pharmacological blocking of the multifunctional enzyme mTor fosters distinct differentiation programs in different immunocompetent cells. These data might lead to a striking change in our view of the role that mTor inhibition should have in immunosuppressive therapy for allogeneic transplant recipients.

Norepinephrine enhances the LPS-induced expression of COX-2 and secretion of PGE2 in primary rat microglia

Background

Recent studies suggest an important role for neurotransmitters as modulators of inflammation. Neuroinflammatory mediators such as cytokines and molecules of the arachidonic acid pathway are generated and released by microglia. The monoamine norepinephrine reduces the production of cytokines by activated microglia in vitro. However, little is known about the effects of norepinephrine on prostanoid synthesis. In the present study, we investigate the role of norepinephrine on cyclooxygenase- (COX-)2 expression/synthesis and prostaglandin (PG)E₂ production in rat primary microglia.

Results

Interestingly, norepinephrine increased COX-2 mRNA, but not protein expression. Norepinephrine strongly enhanced COX-2 expression and PGE₂ production induced by lipopolysaccharide (LPS). This effect is likely to be mediated by β-adrenoreceptors, since β-, but not α-adrenoreceptor agonists produced similar results. Furthermore, β-adrenoreceptor antagonists blocked the enhancement of COX-2 levels induced by norepinephrine and β-adrenoreceptor agonists.

Conclusions

Considering that PGE₂ displays different roles in neuroinflammatory and neurodegenerative disorders, norepinephrine may play an important function in the modulation of these processes in pathophysiological conditions.

Branched chain amino acids enhance the maturation and function of myeloid dendritic cells ex vivo in patients with advanced cirrhosis

An imbalance of plasma amino acids is observed in patients with advanced cirrhosis. The aim of this study was to investigate the influence of the extracellular amino acid imbalance on the function of myeloid dendritic cells (DCs) in patients with advanced cirrhosis. We made a serum-free culture medium consistent with the average concentration of plasma amino acids from healthy controls (HC, n = 25) or patients with advanced cirrhosis (LC, n = 43) to reflect more closely the actual environment of the living body. We compared the phenotypical and biological functions of blood dendritic cells antigen-positive dendritic cells (BDCA+ DCs) and monocyte-derived dendritic cells (MoDCs) from LC and HC with these media. After adding stimulants, the CD83 and CD86 expressions of DCs from LC were lower than those from HC. In both HC and LC, both CD83 and CD86 expressions of DCs stimulated under the cirrhotic medium were lower than under the control medium. This phenomenon was accompanied by a suppression of the mammalian target of rapamycin (mTOR)/S6K-signaling pathways. The interleukin 12 (IL-12) production in the cirrhotic medium was significantly lower than in the control medium and increased when valine or leucine was added to the medium. In patients with advanced cirrhosis, peripheral blood mononuclear cells stimulated in the autologous plasma after oral administration of branched-chain amino acid (BCAA) granules had significantly increased interferon gamma production.

CONCLUSION

In advanced cirrhosis, there is impairment of the function and maturation of DCs, which has been shown to be related to an imbalance in the extracellular amino acid profile. Elevating the extracellular concentration of BCAAs ex vivo in patients with advanced cirrhosis improved the function of DCs.

The multifunctional role of mTOR in innate immunity: implications for transplant immunity

The mammalian target of rapamycin (mTOR) is an evolutionary conserved serine-threonine kinase that senses various environmental stimuli in most cells primarily to control cell growth. Restriction of cellular proliferation by mTOR inhibition led to the use of mTOR inhibitors as immunosuppressants in allogeneic transplantation as well as novel anticancer agents. However, distinct inflammatory side effects such as fever, pneumonitis, glomerulonephritis or anemia of chronic disease have been observed under this treatment regime. Apart from the mere cell-cycle regulatory effect of mTOR in dividing cells, recent data revealed a master regulatory role of mTOR in the innate immune system. Hence, inhibition of mTOR promotes proinflammatory cytokines such as IL-12 and IL-1beta, inhibits the anti-inflammatory cytokine IL-10 and boosts MHC antigen presentation via autophagy in monocytes/macrophages and dendritic cells. Moreover, mTOR regulates type I interferon production and the expression of chemokine receptors and costimulatory molecules. These results place mTOR in a complex immunoregulatory context by controlling innate and adaptive immune responses. In this review, we discuss the clinical consequences of mTOR-inhibitor therapy and aim to integrate this recent data into our current view of the molecular mechanisms of clinically employed mTOR inhibitors and discuss their relevance with special emphasis to transplantation.

MicroRNA fingerprints identify miR-150 as a plasma prognostic marker in patients with sepsis

BACKGROUND

The physiopathology of sepsis continues to be poorly understood, and despite recent advances in its management, sepsis is still a life-threatening condition with a poor outcome. If new diagnostic markers related to sepsis pathogenesis will be identified, new specific therapies might be developed and mortality reduced. Small regulatory non-coding RNAs, microRNAs (miRNAs), were recently linked to various diseases; the aim of our prospective study was to identify miRNAs that can differentiate patients with early-stage sepsis from healthy controls and to determine if miRNA levels correlate with the severity assessed by the Sequential Organ Failure Assessment (SOFA) score.

METHODOLOGY/PRINCIPAL FINDINGS

By using genome-wide miRNA profiling by microarray in peripheral blood leukocytes, we found that miR-150, miR-182, miR-342-5p, and miR-486 expression profiles differentiated sepsis patients from healthy controls. We also proved by quantitative reverse transcription-polymerase chain reaction that miR-150 levels were significantly reduced in plasma samples of sepsis patients and correlated with the level of disease severity measured by the SOFA score, but were independent of the white blood counts (WBC). We found that plasma levels of tumor necrosis factor alpha, interleukin-10, and interleukin-18, all genes with sequence complementarity to miR-150, were negatively correlated with the plasma levels of this miRNA. Furthermore, we identified that the plasma levels ratio for miR-150/interleukin-18 can be used for assessing the severity of the sepsis.

CONCLUSIONS/SIGNIFICANCE

We propose that miR-150 levels in both leukocytes and plasma correlate with the aggressiveness of sepsis and can be used as a marker of early sepsis. Furthermore, we envision miR-150 restoration as a future therapeutic option in sepsis patients.

mTOR: taking cues from the immune microenvironment

The ultimate outcome of T cell receptor recognition is determined by the context in which the antigen is encountered. In this fashion both antigen-presenting cells and T cells must integrate multiple environmental cues in the form of pathogen-associated molecular patterns, cytokines and accessory molecule signals. The mammalian target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase that plays a central role in integrating environmental signals critical to regulating metabolism and cell survival. In this paper we review the data demonstrating that mTOR integrates signals from the immune microenvironment and therefore facilitates the generation of the adaptive immune response. Specifically, we review the role of mTOR in promoting dendritic cell activation and maturation, in regulating full T cell activation versus anergy, and influencing the induction of regulatory T cells.

Signal transducer and activator of transcription 3 is required for abnormal proliferation and survival of TSC2-deficient cells: relevance to pulmonary lymphangioleiomyomatosis

Tumor suppressor complex TSC1/TSC2 represents a key negative regulator of mammalian target of rapamycin (mTOR)-S6 kinase 1 signaling. Mutational inactivation of TSC1 or TSC2, linked to a rare lung disease, lymphangioleiomyomatosis (LAM), manifests as neoplastic growth of smooth-muscle (SM)-like cells and cystic destruction of the lungs that induces loss of pulmonary function. However, the precise mechanisms of abnormal cell growth in LAM remain uncertain. Here, we demonstrate increased signal transducer and activator of transcription (STAT) 3 expression, phosphorylation, and nuclear localization in SM-like cells in LAM lungs and in TSC2-null xenographic tumors. Treatment of TSC2-null tumors with mTOR inhibitor rapamycin attenuated STAT3 expression and phosphorylation. Increased STAT3 level and activation were also observed in LAM-dissociated (LAMD) cell cultures compared with normal human bronchus fibroblasts (HBFs) from LAM patients. Although interferon (IFN)-gamma inhibited proliferation of HBFs, IFN-gamma treatment had little effect on proliferation of LAMD and TSC2-null cells. Re-expression of TSC2 or treatment with rapamycin inhibited IFN-gamma-induced STAT3 phosphorylation and synergized with IFN-gamma in inhibiting TSC2-null and LAMD cell proliferation. Reduction of STAT3 protein levels or activity using specific small interfering RNA or inhibitory peptide, respectively, decreased proliferation and induced apoptosis in TSC2-null and LAMD cells and sensitized cells to growth-inhibitory and proapoptotic effects of IFN-gamma. Collectively, our data demonstrate that STAT3 activation is required for proliferation and survival of cells with TSC2 dysfunction, that STAT3 impedes growth-inhibitory and proapoptotic effects of IFN-gamma, and that TSC2- and rapamycin-dependent inhibition of STAT3 restores antiproliferative effects of IFN-gamma. Thus, STAT3 may provide a novel therapeutic target for diseases associated with TSC1/TSC2 dysfunction.

Modulation of adaptive immunity with Toll-like receptors

The discovery of Toll-like receptors (TLRs), and their role in sensing infections represents one of the most seminal advances in immunology in recent years. It is now clear that TLRs play a fundamental role in innate recognition of microbes, and stimulate and tune the quality of the adaptive immune response. However, major knowledge gaps remain in our understanding of how TLRs regulate the development and persistence of T- and B-cell memory. Here, we review our current understanding of how TLR-signaling shapes the adaptive immune response, and highlight unanswered questions, the solution of which will be imperative in the rational exploitation of TLRs in vaccine design and immune therapy.

CD44 costimulation promotes FoxP3+ regulatory T cell persistence and function via production of IL-2, IL-10, and TGF-beta

Work by our group and others has demonstrated a role for the extracellular matrix receptor CD44 and its ligand hyaluronan in CD4(+)CD25(+) regulatory T cell (Treg) function. Herein, we explore the mechanistic basis for this observation. Using mouse FoxP3/GFP(+) Treg, we find that CD44 costimulation promotes expression of FoxP3, in part through production of IL-2. This promotion of IL-2 production was resistant to cyclosporin A treatment, suggesting that CD44 costimulation may promote IL-2 production through bypassing FoxP3-mediated suppression of NFAT. CD44 costimulation increased production of IL-10 in a partially IL-2-dependent manner and also promoted cell surface TGF-beta expression. Consistent with these findings, Treg from CD44 knockout mice demonstrated impaired regulatory function ex vivo and depressed production of IL-10 and cell surface TGF-beta. These data reveal a novel role for CD44 cross-linking in the production of regulatory cytokines. Similar salutary effects on FoxP3 expression were observed upon costimulation with hyaluronan, the primary natural ligand for CD44. This effect is dependent upon CD44 cross-linking; while both high-molecular-weight hyaluronan (HA) and plate-bound anti-CD44 Ab promoted FoxP3 expression, neither low-molecular weight HA nor soluble anti-CD44 Ab did so. The implication is that intact high-molecular weight HA can cross-link CD44 only in those settings where it predominates over fragmentary LMW-HA, namely, in uninflamed tissue. We propose that intact but not fragmented extracellular is capable of cross-linking CD44 and thereby maintains immunologic tolerance in uninjured or healing tissue.

m(en)TOR(ing) differentiating T helper cells

The decision of naive Th cells to assume the fate of effector or regulator Th cells heavily influences the outcome of adaptive immune responses. In this issue of Immunity, Delgoffe et al. (2009) use genetic approaches and demonstrate that mTOR kinase plays a critical role in this decision-making process.

mTOR regulates memory CD8 T-cell differentiation

Memory CD8 T cells are a critical component of protective immunity, and inducing effective memory T-cell responses is a major goal of vaccines against chronic infections and tumours. Considerable effort has gone into designing vaccine regimens that will increase the magnitude of the memory response, but there has been minimal emphasis on developing strategies to improve the functional qualities of memory T cells. Here we show that mTOR (mammalian target of rapamycin, also known as FRAP1) is a major regulator of memory CD8 T-cell differentiation, and in contrast to what we expected, the immunosuppressive drug rapamycin has immunostimulatory effects on the generation of memory CD8 T cells. Treatment of mice with rapamycin following acute lymphocytic choriomeningitis virus infection enhanced not only the quantity but also the quality of virus-specific CD8 T cells. Similar effects were seen after immunization of mice with a vaccine based on non-replicating virus-like particles. In addition, rapamycin treatment also enhanced memory T-cell responses in non-human primates following vaccination with modified vaccinia virus Ankara. Rapamycin was effective during both the expansion and contraction phases of the T-cell response; during the expansion phase it increased the number of memory precursors, and during the contraction phase (effector to memory transition) it accelerated the memory T-cell differentiation program. Experiments using RNA interference to inhibit expression of mTOR, raptor (also known as 4932417H02Rik) or FKBP12 (also known as FKBP1A) in antigen-specific CD8 T cells showed that mTOR acts intrinsically through the mTORC1 (mTOR complex 1) pathway to regulate memory T-cell differentiation. Thus these studies identify a molecular pathway regulating memory formation and provide an effective strategy for improving the functional qualities of vaccine- or infection-induced memory T cells.

Involvement of mTOR kinase in cytokine-dependent microglial activation and cell proliferation

Neuroinflammation plays a prominent role in the pathophysiology of several neurodegenerative disorders, including Multiple Sclerosis. Reactive microglial cells are always found in areas of active demyelination as well as in normal-appearing white matter. Microglia contribute to initiating and maintaining brain inflammation, and once activated release pro-inflammatory mediators potentially cytotoxic, like nitric oxide (NO). It is now evident that the mTOR signaling pathway regulates different functions in the innate immune system, contributing to macrophage activation. More recently, mTOR has been found to enhance the survival of EOC2 microglia during oxygen-glucose deprivation and increase NO synthase 2 (NOS2) expression during hypoxia in BV2 microglial cell line, thus suggesting an involvement in microglial pro-inflammatory activation. In the present study, we detected mTOR activation in response to two different stimuli, namely LPS and a mixture of cytokines, in primary cultures of rat cortical microglia. Moreover, mTOR inhibitors reduced NOS activity and NOS2 expression induced by cytokines, but not those induced by LPS. The mTOR inhibitor RAD001, in combination with cytokines, also reduced microglial proliferation and the intracellular levels of cyclooxygenase. Under basal conditions mTOR inhibition significantly reduced microglial viability. Interestingly, mTOR inhibitors did not display any relevant effect on astrocyte NOS2 activity or cell viability. In conclusion, mTOR selectively controls microglial activation in response to pro-inflammatory cytokines and appears to play a crucial role in microglial viability; thus these drugs may be a useful pharmacological tool to reduce neuroinflammation.

The induction of IL-10 by zymosan in dendritic cells depends on CREB activation by the coactivators CREB-binding protein and TORC2 and autocrine PGE2

Stimulation of human monocyte-derived dendritic cells with the yeast extract zymosan is characterized by a predominant production of IL-10 and a strong induction of cyclooxygenase-2, but the molecular mechanisms underlying this response are only partially understood. To address this issue, the activation of transcription factors that may bind to the il10 proximal promoter was studied. Binding activity to Sp1, Sp3, NF-Y, and cAMP response element (CRE) sites was detected in the nuclear extracts of dendritic cells; however these binding activities were not influenced by zymosan. No binding activity to Stat1, Stat3, and c/EBP sites was detected. Notably, zymosan activated kappaB-binding activity, but inhibition of NF-kappaB was associated with enhanced IL-10 production. In sharp contrast, treatments acting on CREB (CRE binding protein), including 8-Br-cAMP, PGE(2), and inhibitors of PKA, COX, and glycogen-synthase kinase-3beta showed a direct correlation between CREB activation and IL-10 production. Zymosan induced binding of both P-CREB and CREB-binding protein (CBP) to the il10 promoter as judged from chromatin immunoprecipitation assays, whereas negative results were obtained with Ab reactive to Sp1, Sp3, c-Maf, and NF-Y. Zymosan also induced nuclear translocation of the CREB coactivator transducer of regulated CREB activity 2 (TORC2) and interaction of TORC2 with P-CREB coincidental with the association of CREB to the il10 promoter. Altogether, our data show that zymosan induces il10 transcription by a CRE-dependent mechanism that involves autocrine secretion of PGE(2) and a network of interactions of PKA, MAP/ERK, glycogen-synthase kinase-3beta, and calcineurin, which regulate CREB transcriptional activity by binding the coactivators CBP and TORC2 and inhibiting CBP interaction with other transcription factors.

Rapamycin-treated mature dendritic cells have a unique cytokine secretion profile and impaired allostimulatory capacity

Rapamycin (RAPA, sirolimus) is a recently introduced immunosuppressive agent. Its effect on the differentiation and antigen uptake of immature dendritic cells (iDCs) has been studied. However, whether it can also modulate the function of mature DCs (mDCs) is unknown. We investigated the effects of RAPA on rat bone marrow-derived DCs at different stages of maturation. RAPA affected maturation, increased apoptosis and reduced lipopolysaccharide (LPS)-induced IL-12 and IL-10 production in iDCs. However, mDCs were resistant to RAPA-induced apoptosis. RAPA-mDCs produced significantly less IL-10 and TNF-alpha when compared with mature DCs but similar amounts of IL-12. RAPA did not affect constitutive NF-kappaB activity, but inhibited allostimulatory activity in mature DCs. In conclusion, mDCs treated with RAPA are reprogrammed to produce a unique cytokine secretion profile and exhibit low allostimulatory capacity, which may play an important role in rapamycin-based immunomodulation.

Immunoregulatory functions of mTOR inhibition

The potent immunosuppressive action of rapamycin is commonly ascribed to inhibition of growth factor-induced T cell proliferation. However, it is now evident that the serine/threonine protein kinase mammalian target of rapamycin (mTOR) has an important role in the modulation of both innate and adaptive immune responses. mTOR regulates diverse functions of professional antigen-presenting cells, such as dendritic cells (DCs), and has important roles in the activation of effector T cells and the function and proliferation of regulatory T cells. In this Review, we discuss our current understanding of the mTOR pathway and the consequences of mTOR inhibition, both in DCs and T cells, including new data on the regulation of forkhead box P3 expression.

Immune regulation by rapamycin: moving beyond T cells

The mammalian target of rapamycin (mTOR) is a multifunctional kinase that promotes cell growth and division in response to growth factor and nutrient signals. Rapamycin exerts its potent immunosuppressive effects in part through direct effects on antigen-specific lymphocytes; however, rapamycin also modulates adaptive immunity through its effects on innate immune cells, including dendritic cells and macrophages. Studies have established rapamycin-sensitive functions of mTOR, downstream of Toll-like receptors, in shaping the cytokine response of myeloid cells and driving the production of interferon by plasmacytoid dendritic cells. These findings point to new strategies for boosting or suppressing specific immune responses.

The multiple facets of mTOR in immunity

The mammalian target of rapamycin (mTOR) is an evolutionarily conserved serine-threonine kinase that is known to sense the environmental and cellular nutrition status to control cell growth. In immunity, mTOR is essential for both the proper activation and subsequent proliferation of effector T cells, yet also restrains the development of regulatory T cells. However, in monocytes/macrophages and peripheral myeloid dendritic cells, mTOR restricts proinflammatory and promotes anti-inflammatory responses, whereas, in plasmacytoid dendritic cells, mTOR fosters type I interferon production. These results place mTOR in a novel immunoregulatory context that highlights the potential of mTOR inhibitors as both immunosuppressant and anti-cancer agents.

2008: Signaling breakthroughs of the year

This year's signaling breakthroughs extended from protein crystals to cells and subcellular structures to whole genomes. Nominations included research on synapses in brains and in B cells; the evolution and regulation of signaling networks; the identification of a new class of plant hormones; insights into the causes and treatments of disease, such as cancer and schizophrenia; and a possible way to stay in shape while avoiding exercise.