Rapamycin has suppressive and stimulatory effects on human plasmacytoid dendritic cell functions

Plasmacytoid dendritic cells at the forefront of anti-cancer immunity: rewiring strategies for tumor microenvironment remodeling

Plasmacytoid dendritic cells (pDCs) are multifaceted immune cells executing various innate immunological functions. Their first line of defence consists in type I interferons (I-IFN) production upon nucleic acids sensing through endosomal Toll-like receptor (TLR) 7- and 9-dependent signalling pathways. Type I IFNs are a class of proinflammatory cytokines that have context-dependent functions on cancer immunosurveillance and immunoediting. In the last few years, different studies have reported that pDCs are also able to sense cytosolic DNA through cGAS-STING (stimulator of interferon genes) pathway eliciting a potent I-IFN production independently of TLR7/9. Human pDCs are also endowed with direct effector functions via the upregulation of TRAIL and production of granzyme B, the latter modulated by cytokines abundant in cancer tissues. pDCs have been detected in a wide variety of human malignant neoplasms, including virus-associated cancers, recruited by chemotactic stimuli. Although the role of pDCs in cancer immune surveillance is still uncompletely understood, their spontaneous activation has been rarely documented; moreover, their presence in the tumor microenvironment (TME) has been associated with a tolerogenic phenotype induced by immunosuppressive cytokines or oncometabolites. Currently tested treatment options can lead to pDCs activation and disruption of the immunosuppressive TME, providing a relevant clinical benefit. On the contrary, the antibody-drug conjugates targeting BDCA-2 on immunosuppressive tumor-associated pDCs (TA-pDCs) could be proposed as novel immunomodulatory therapies to achieve disease control in patients with advance stage hematologic malignancies or solid tumors. This Review integrate recent evidence on the biology of pDCs and their pharmacological modulation, suggesting their relevant role at the forefront of cancer immunity.

Everolimus treatment enhances inhibitory immune checkpoint molecules' expression in monocyte-derived dendritic cells

BACKGROUND

Antigen-specific T-cell immunity is provided by dendritic cells (DCs), which are specialized antigen-presenting cells. Furthermore, they establish a link between innate and adaptive immune responses. Currently, DC modification is a new approach for the therapy of several disorders. During solid organ transplantation, Everolimus, which is a mammalian target of rapamycin (mTOR) inhibitor, was initially utilized to suppress the immune system's functionality. Due to the intervention of Everolimus in various signaling pathways in cells and its modulatory properties on the immune system, this study aims to investigate the effect of treatment with Everolimus on the maturation and expression of immune checkpoint genes in monocyte-derived DCs.

METHODS

To isolate monocytes from PBMCs, the CD14 marker was used via the MACS method. Monocytes were cultured and induced to differentiate into monocyte-derived DCs by utilizing GM-CSF and IL-4 cytokines. On the fifth day, immature DCs were treated with Everolimus and incubated for 24 h. On the sixth day, the flow cytometry technique was used to investigate the effect of Everolimus on the phenotypic characteristics of DCs. In the end, the expression of immune checkpoint genes in both the Everolimus-treated and untreated DCs groups was assessed using the real-time PCR method.

RESULTS

The findings of this research demonstrated that the administration of Everolimus to DCs led to a notable rise in human leukocyte antigen (HLA)-DR expression and a decrease in CD11c expression. Furthermore, there was a significant increase in the expression of immune checkpoint molecules, namely CTLA-4, VISTA, PD-L1, and BTLA, in DCs treated with Everolimus.

CONCLUSION

The findings of this study show that Everolimus can target DCs and affect their phenotype and function in order to shift them toward a partially tolerogenic state. However, additional research is required to gain a comprehensive understanding of the precise impact of Everolimus on the activation status of DCs.

Immunometabolic Signature during Respiratory Viral Infection: A Potential Target for Host-Directed Therapies

RNA viruses are known to induce a wide variety of respiratory tract illnesses, from simple colds to the latest coronavirus pandemic, causing effects on public health and the economy worldwide. Influenza virus (IV), parainfluenza virus (PIV), metapneumovirus (MPV), respiratory syncytial virus (RSV), rhinovirus (RhV), and coronavirus (CoV) are some of the most notable RNA viruses. Despite efforts, due to the high mutation rate, there are still no effective and scalable treatments that accompany the rapid emergence of new diseases associated with respiratory RNA viruses. Host-directed therapies have been applied to combat RNA virus infections by interfering with host cell factors that enhance the ability of immune cells to respond against those pathogens. The reprogramming of immune cell metabolism has recently emerged as a central mechanism in orchestrated immunity against respiratory viruses. Therefore, understanding the metabolic signature of immune cells during virus infection may be a promising tool for developing host-directed therapies. In this review, we revisit recent findings on the immunometabolic modulation in response to infection and discuss how these metabolic pathways may be used as targets for new therapies to combat illnesses caused by respiratory RNA viruses.

Immunologic and dose dependent effects of rapamycin and its evolving role in chemoprevention

Rapamycin inhibits the mechanistic (formally mammalian) target of rapamycin (mTOR), an evolutionarily conserved intracellular kinase that influences activation of growth signaling pathways and immune responses to malignancy. Rapamycin has been found to have both immunosuppressant and immunostimulatory effects throughout the innate and adaptive responses based on the inhibition of mTOR signaling. While the immunosuppressant properties of rapamycin and mTOR inhibition explain rapamycin's success in the prevention of transplant rejection, the immunostimulatory characteristics are likely partially responsible for rapamycin's anti-neoplastic effects. The immunologic response to rapamycin is at least partially dependent on the dose and administration schedule, with lower doses inducing immunostimulation and intermittent dosing promoting immune function while limiting metabolic and immunosuppressant toxicities. In addition to its FDA-approved application in advanced malignancies, rapamycin may be effective as a chemopreventive agent, suspending progression of low-grade cancers, preventing invasive conversion of in situ malignancy, or delaying malignant transformation of established pre-malignant conditions.

Pharmacological inhibition of the mTOR pathway alters phenotype and cytokine expression in bovine monocyte-derived dendritic cells

Epidemiological studies have long demonstrated the association of nutrient status and immune dysfunction in dairy cows. Postpartum dairy cows experiencing a nutrient deficit show a propensity for increased inflammatory response, decreased pathogen clearance, and increased incidence of infectious disease. Studies in cows and other species show that the nutrient sensing mechanistic target of rapamycin (mTOR) signaling pathway could be one potential causal pathway connecting the deficit in nutrient availability and the heightened inflammatory response. Our objective was to investigate the effects of pharmacological mTOR pathway inhibition on phenotype and cytokine expression of bovine monocyte derived dendritic cells (moDC). We differentiated CD14⁺ monocytes from dairy cows (n = 14) into moDC in the presence or absence of first- or second-generation mTOR inhibitor rapamycin and PP242 (both 100 nM), respectively. On day seven cells were matured with E. coli lipopolysaccharide (LPS, 100 ng/mL) or left unstimulated to represent naïve moDC. Surface expression of CD14, CD40, CD80, and MHCII was measured via flow cytometry. We measured mRNA expression of IL10, IL12A, IL12B, and TNFα by rt-qPCR, and protein concentrations of IL-10 and TFN-α in cell culture supernatants with a bead-based multiplex assay. Cultures from ten cows successfully developed the moDC phenotype in culture without inhibitors, defined as increased surface expression of CD40, CD80, and MHCII compared with naïve moDC. Only data from these cows were considered for the results on effects of mTOR inhibitors. In naïve and mature moDC mTOR inhibition increased MHCII expression compared to controls. In mature moDC, in addition to MHCII, CD80 expression was increased compared with untreated LPS-stimulated controls. Expression of IL12A mRNA was upregulated in mature, mTOR inhibited moDC compared with untreated controls. In cell culture supernatants mTOR inhibition reduced IL-10 and increased TNF-α concentrations in naïve and mature moDCs compared with untreated controls. Overall rapamycin had a more consistent effect on altering phenotype and cytokine expression of moDC than PP242. In summary we observed an increased expression of co-stimulatory molecules and antigen presentation potential in mature moDC differentiated under mTOR inhibition, and a cytokine pattern that would potentially favor a Th1 type response. This study provides novel data indicating a role for mTOR signaling in bovine moDC phenotype and mediator profile. This proof-of-concept study demonstrates the role of the mTOR pathway in shaping the bovine immune response and may help to provide mechanistic insight and opportunities for modulation of the immune response during the nutrient deficit of early lactation.

Fighting the SARS-CoV-2 pandemic requires a global approach to understanding the heterogeneity of vaccine responses

Host genetic and environmental factors including age, biological sex, diet, geographical location, microbiome composition and metabolites converge to influence innate and adaptive immune responses to vaccines. Failure to understand and account for these factors when investigating severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine efficacy may impair the development of the next generation of vaccines. Most studies aimed at identifying mechanisms of vaccine-mediated immune protection have focused on adaptive immune responses. It is well established, however, that mobilization of the innate immune response is essential to the development of effective cellular and humoral immunity. A comprehensive understanding of the innate immune response and environmental factors that contribute to the development of broad and durable cellular and humoral immune responses to SARS-CoV-2 and other vaccines requires a holistic and unbiased approach. Along with optimization of the immunogen and vectors, the development of adjuvants based on our evolving understanding of how the innate immune system shapes vaccine responses will be essential. Defining the innate immune mechanisms underlying the establishment of long-lived plasma cells and memory T cells could lead to a universal vaccine for coronaviruses, a key biomedical priority.

CXCR4 blockade reduces the severity of murine heart allograft rejection by plasmacytoid dendritic cell-mediated immune regulation

Allograft-specific regulatory T cells (T_reg cells) are crucial for long-term graft acceptance after transplantation. Although adoptive T_reg cell transfer has been proposed, major challenges include graft-specificity and stability. Thus, there is an unmet need for the direct induction of graft-specific T_reg cells. We hypothesized a synergism of the immunotolerogenic effects of rapamycin (mTOR inhibition) and plerixafor (CXCR4 antagonist) for T_reg cell induction. Thus, we performed fully-mismatched heart transplantations and found combination treatment to result in prolonged allograft survival. Moreover, fibrosis and myocyte lesions were reduced. Although less CD3⁺ T cell infiltrated, higher T_reg cell numbers were observed. Noteworthy, this was accompanied by a plerixafor-dependent plasmacytoid dendritic cells-(pDCs)-mobilization. Furthermore, in vivo pDC-depletion abrogated the plerixafor-mediated T_reg cell number increase and reduced allograft survival. Our pharmacological approach allowed to increase T_reg cell numbers due to pDC-mediated immune regulation. Therefore pDCs can be an attractive immunotherapeutic target in addition to plerixafor treatment.

Modulation of the mTOR pathway plays a central role in dendritic cell functions after Echinococcus granulosus antigen recognition

Immune evasion is a hallmark of persistent echinococcal infection, comprising modulation of innate immune cells and antigen-specific T cell responses. However, recognition of Echinococcus granulosus by dendritic cells (DCs) is a key determinant of the host's response to this parasite. Given that mTOR signaling pathway has been described as a regulator linking metabolism and immune function in DCs, we reported for the first time in these cells, global translation levels, antigen uptake, phenotype, cytokine transcriptional levels, and splenocyte priming activity upon recognition of the hydatid fluid (HF) and the highly glycosylated laminar layer (LL). We found that LL induced a slight up-regulation of CD86 and MHC II in DCs and also stimulated the production of IL-6 and TNF-α. By contrast, HF did not increase the expression of any co-stimulatory molecules, but also down-modulated CD40 and stimulated the expression of the anti-inflammatory cytokine IL-10. Both parasitic antigens promoted protein synthesis through mTOR activation. The use of rapamycin decreased the expression of the cytokines tested, empowered the down-modulation of CD40 and also reduced splenocyte proliferation. Finally, we showed that E. granulosus antigens increase the amounts of LC3-positive structures in DCs which play critical roles in the presentation of these antigens to T cells.

Homozygous variant p. Arg90His in NCF1 is associated with early-onset Interferonopathy: a case report

BACKGROUND

Biallelic loss-of-function variants in NCF1 lead to reactive oxygen species deficiency and chronic granulomatous disease (CGD). Heterozygosity for the p.Arg90His variant in NCF1 has been associated with susceptibility to systemic lupus erythematosus, rheumatoid arthritis, and Sjögren's syndrome in adult patients. This study demonstrates the association of the homozygous p.Arg90His variant with interferonopathy with features of autoinflammation and autoimmunity in a pediatric patient.

CASE PRESENTATION

A 5-year old female of Indian ancestry with early-onset recurrent fever and headache, and persistently elevated antinuclear, anti-Ro, and anti-La antibodies was found to carry the homozygous p.Arg90His variant in NCF1 through exome sequencing. Her unaffected parents and three other siblings were carriers for the mutant allele. Because the presence of two NCF1 pseudogenes, this variant was confirmed by independent genotyping methods. Her intracellular neutrophil oxidative burst and NCF1 expression levels were normal, and no clinical features of CGD were apparent. Gene expression analysis in peripheral blood detected an interferon gene expression signature, which was further supported by cytokine analyses of supernatants of cultured patient's cells. These findings suggested that her inflammatory disease is at least in part mediated by type I interferons. While her fever episodes responded well to systemic steroids, treatment with the JAK inhibitor tofacitinib resulted in decreased serum ferritin levels and reduced frequency of fevers.

CONCLUSION

Homozygosity for p.Arg90His in NCF1 should be considered contributory in young patients with an atypical systemic inflammatory antecedent phenotype that may evolve into autoimmunity later in life. The complex genomic organization of NCF1 poses a difficulty for high-throughput genotyping techniques and variants in this gene should be carefully evaluated when using the next generation and Sanger sequencing technologies. The p.Arg90His variant is found at a variable allele frequency in different populations, and is higher in people of South East Asian ancestry. In complex genetic diseases such as SLE, other rare and common susceptibility alleles might be necessary for the full disease expressivity.

Type I Interferon Production of Plasmacytoid Dendritic Cells under Control

One of the most powerful and multifaceted cytokines produced by immune cells are type I interferons (IFNs), the basal secretion of which contributes to the maintenance of immune homeostasis, while their activation-induced production is essential to effective immune responses. Although, each cell is capable of producing type I IFNs, plasmacytoid dendritic cells (pDCs) possess a unique ability to rapidly produce large amounts of them. Importantly, type I IFNs have a prominent role in the pathomechanism of various pDC-associated diseases. Deficiency in type I IFN production increases the risk of more severe viral infections and the development of certain allergic reactions, and supports tumor resistance; nevertheless, its overproduction promotes autoimmune reactions. Therefore, the tight regulation of type I IFN responses of pDCs is essential to maintain an adequate level of immune response without causing adverse effects. Here, our goal was to summarize those endogenous factors that can influence the type I IFN responses of pDCs, and thus might serve as possible therapeutic targets in pDC-associated diseases. Furthermore, we briefly discuss the current therapeutic approaches targeting the pDC-type I IFN axis in viral infections, cancer, autoimmunity, and allergy, together with their limitations defined by the Janus-faced nature of pDC-derived type I IFNs.

Biology and therapeutic potential of interleukin-10

The authors review the molecular mechanisms regulating IL-10 production and response and describe classic and novel functions of IL-10 in immune and non-immune cells. They further discuss the therapeutic potential of IL-10 in different diseases and the outstanding questions underlying an effective application of IL-10 in clinical settings.

The cytokine IL-10 is a key anti-inflammatory mediator ensuring protection of a host from over-exuberant responses to pathogens and microbiota, while playing important roles in other settings as sterile wound healing, autoimmunity, cancer, and homeostasis. Here we discuss our current understanding of the regulation of IL-10 production and of the molecular pathways associated with IL-10 responses. In addition to IL-10’s classic inhibitory effects on myeloid cells, we also describe the nonclassic roles attributed to this pleiotropic cytokine, including how IL-10 regulates basic processes of neural and adipose cells and how it promotes CD8 T cell activation, as well as epithelial repair. We further discuss its therapeutic potential in the context of different diseases and the outstanding questions that may help develop an effective application of IL-10 in diverse clinical settings.

Metabolic Control of Dendritic Cell Functions: Digesting Information

Dendritic cells (DCs) control innate and adaptive immunity by patrolling tissues to gather antigens and danger signals derived from microbes and tissue. Subsequently, DCs integrate those environmental cues, orchestrate immunity or tolerance, and regulate tissue homeostasis. Recent advances in the field of immunometabolism highlight the notion that immune cells markedly alter cellular metabolic pathways during differentiation or upon activation, which has important implications on their functionality. Previous studies showed that active oxidative phosphorylation in mitochondria is associated with immature or tolerogenic DCs, while increased glycolysis upon pathogen sensing can promote immunogenic DC functions. However, new results in the last years suggest that regulation of DC metabolism in steady state, after immunogenic activation and during tolerance in different pathophysiological settings, may be more complex. Moreover, ontogenically distinct DC subsets show different functional specializations to control T cell responses. It is, thus, relevant how metabolism influences DC differentiation and plasticity, and what potential metabolic differences exist among DC subsets. Better understanding of the emerging connection between metabolic adaptions and functional DC specification will likely allow the development of therapeutic strategies to manipulate immune responses.

Metabolism in Immune Cell Differentiation and Function

The immune system is a central determinant of organismal health. Functional immune responses require quiescent immune cells to rapidly grow, proliferate, and acquire effector functions when they sense infectious agents or other insults. Specialized metabolic programs are critical regulators of immune responses, and alterations in immune metabolism can cause immunological disorders. There has thus been growing interest in understanding how metabolic processes control immune cell functions under normal and pathophysiological conditions. In this chapter, we summarize how metabolic programs are tuned and what the physiological consequences of metabolic reprogramming are as they relate to immune cell homeostasis, differentiation, and function.

Dissecting the Multiplicity of Immune Effects of Immunosuppressive Drugs to Better Predict the Risk of de novo Malignancies in Solid Organ Transplant Patients

De novo malignancies constitute an emerging cause of morbidity after solid organ transplant (SOT), significantly affecting the long-term survival of transplant recipients. Pharmacologic immunosuppression may functionally impair the immunosurveillance in these patients, thereby increasing the risk of cancer development. Nevertheless, the multiplicity and heterogeneity of the immune effects induced by immunosuppressive drugs limit the current possibilities to reliably predict the risk of de novo malignancy in SOT patients. Therefore, there is the pressing need to better characterize the immune dysfunctions induced by the different immunosuppressive regimens administered to prevent allograft rejection to tailor more precisely the therapeutic schedule and decrease the risk of de novo malignancies. We herein highlight the impact exerted by different classes of immunosuppressants on the most relevant immune cells, with a particular focus on the effects on dendritic cells (DCs), the main regulators of the balance between immunosurveillance and tolerance.

Regulation of Dendritic Cell Immune Function and Metabolism by Cellular Nutrient Sensor Mammalian Target of Rapamycin (mTOR)

Dendritic cell (DC) activation is characterized by an acute increase in glucose metabolic flux that is required to fuel the high anabolic rates associated with DC activation. Inhibition of glycolysis significantly attenuates most aspects of DC immune effector function including antigen presentation, inflammatory cytokine production, and T cell stimulatory capacity. The cellular nutrient sensor mammalian/mechanistic Target of Rapamycin (mTOR) is an important upstream regulator of glycolytic metabolism and plays a central role in coordinating DC metabolic changes and immune responses. Because mTOR signaling can be activated by a variety of immunological stimuli, including signaling through the Toll-like Receptor (TLR) family of receptors, mTOR is involved in orchestrating many aspects of the DC metabolic response to microbial stimuli. It has become increasingly clear that mTOR's role in promoting or attenuating inflammatory processes in DCs is highly context-dependent and varies according to specific cellular subsets and the immunological conditions being studied. This review will address key aspects of the complex role of mTOR in regulating DC metabolism and effector function.

The IKK-related kinase TBK1 activates mTORC1 directly in response to growth factors and innate immune agonists

The innate immune kinase TBK1 initiates inflammatory responses to combat infectious pathogens by driving production of type I interferons. TBK1 also controls metabolic processes and promotes oncogene-induced cell proliferation and survival. Here, we demonstrate that TBK1 activates mTOR complex 1 (mTORC1) directly. In cultured cells, TBK1 associates with and activates mTORC1 through site-specific mTOR phosphorylation (on S2159) in response to certain growth factor receptors (i.e., EGF-receptor but not insulin receptor) and pathogen recognition receptors (PRRs) (i.e., TLR3; TLR4), revealing a stimulus-selective role for TBK1 in mTORC1 regulation. By studying cultured macrophages and those isolated from genome edited mTOR S2159A knock-in mice, we show that mTOR S2159 phosphorylation promotes mTORC1 signaling, IRF3 nuclear translocation, and IFN-β production. These data demonstrate a direct mechanistic link between TBK1 and mTORC1 function as well as physiologic significance of the TBK1-mTORC1 axis in control of innate immune function. These data unveil TBK1 as a direct mTORC1 activator and suggest unanticipated roles for mTORC1 downstream of TBK1 in control of innate immunity, tumorigenesis, and disorders linked to chronic inflammation.

JAK-inhibitor tofacitinib suppresses interferon alfa production by plasmacytoid dendritic cells and inhibits arthrogenic and antiviral effects of interferon alfa

Tofacitinib is an oral Janus kinase inhibitor that is effective for the treatment of rheumatoid arthritis and shows encouraging therapeutic effects in several other autoimmune diseases. A prominent adverse effect of tofacitinib therapy is the increased risk of viral infections. Despite its advanced stage of clinical development, the modes of action that mediate the beneficial and adverse effects of tofacitinib in autoimmune diseases remain unclear. Interferon alfa (IFNα) produced by plasmacytoid dendritic cells (PDCs) is critically involved in the pathogenesis of many systemic autoimmune diseases and in immunity to viral infections. Using in vitro culture models with human cells, we studied the effects of tofacitinib on PDC survival and IFNα production, and on arthrogenic and antiviral effects of IFNα. Tofacitinib inhibited the expression of antiapoptotic BCL-A1 and BCL-XL in human PDC and induced PDC apoptosis. TLR7 stimulation upregulated the levels of antiapoptotic Bcl-2 family members and prevented the induction of PDC apoptosis by tofacitinib. However, tofacitinib robustly inhibited the production of IFNα by toll like receptor-stimulated PDC. In addition, tofacitinib profoundly suppressed IFNα-induced upregulation of TLR3 on synovial fibroblasts, thereby inhibiting their cytokine and protease production in response to TLR3 ligation. Finally, tofacitinib counteracted the suppressive effects of IFNα on viral replication. Tofacitinib inhibits PDC survival and IFNα production and suppresses arthrogenic and antiviral effects of IFNα signaling. Inhibition of the IFNα pathway at 2 levels may contribute to the beneficial effects of tofacitinib in autoimmune diseases and explain the increased viral infection rates observed during tofacitinib treatment.

Energy metabolic pathways control the fate and function of myeloid immune cells

The past decade has seen a significant interest in investigating the intracellular metabolism of cells of the immune system. This has increased the realization that immune cells endure metabolic reprogramming upon responding to pathogen-derived or inflammatory signals. More importantly, not only does this metabolic switch provide for the bioenergetic and biosynthetic demands but also it, in a highly specific manner, determines the cellular fate and function. In this review, we discuss the metabolic aspects that regulate the differentiation and function of myeloid cells, pivotal for both innate and adaptive immunity. The manipulation of these pathways can alter the function of these cells and therefore, could provide novel therapeutic approaches in cancer and other chronic inflammatory conditions.

TSG-6 Downregulates IFN-Alpha and TNF-Alpha Expression by Suppressing IRF7 Phosphorylation in Human Plasmacytoid Dendritic Cells

Proinflammatory cytokines such as TNF-α and type I interferons (IFN) are pathogenic signatures of systemic lupus erythematosus, and plasmacytoid dendritic cells (pDCs) play a major role by predominantly producing IFN-α. Given the rise of importance in identifying tumor necrosis stimulated gene 6 (TSG-6) as a key anti-inflammatory regulator, we investigate its function and its ability to counteract proinflammatory cytokine secretion by pDCs in vitro. CpG-A and R837 induced significant endogenous TSG-6 expression in the pDC cell-line GEN2.2. Following recombinant human TSG-6 treatment and CpG-A or R837 stimulation, significant reduction in IFN-α and TNF-α was observed in healthy donors' pDCs, and the same phenomenon was confirmed in GEN2.2. By CD44 blocking assay, we deduced that the suppressive effect of TSG-6 is mediated by CD44, by reducing IRF-7 phosphorylation. Our findings suggest that TSG-6 and its downstream signalling pathway could potentially be targeted to modulate proinflammatory cytokine expression in pDCs.

Screening Immunomodulators To Skew the Antigen-Specific Autoimmune Response

Current therapies to treat autoimmune diseases often result in side effects such as nonspecific immunosuppression. Therapies that can induce antigen-specific immune tolerance provide an opportunity to reverse autoimmunity and mitigate the risks associated with global immunosuppression. In an effort to induce antigen-specific immune tolerance, co-administration of immunomodulators with autoantigens has been investigated in an effort to reprogram autoimmunity. To date, identifying immunomodulators that may skew the antigen-specific immune response has been ad hoc at best. To address this need, we utilized splenocytes obtained from mice with experimental autoimmune encephalomyelitis (EAE) in order to determine if certain immunomodulators may induce markers of immune tolerance following antigen rechallenge. Of the immunomodulatory compounds investigated, only dexamethasone modified the antigen-specific immune response by skewing the cytokine response and decreasing T-cell populations at a concentration corresponding to a relevant in vivo dose. Thus, antigen-educated EAE splenocytes provide an ex vivo screen for investigating compounds capable of skewing the antigen-specific immune response, and this approach could be extrapolated to antigen-educated cells from other diseases or human tissues.

mTOR-Mediated Regulation of Dendritic Cell Differentiation and Function

Dendritic cells (DCs) are essential antigen-presenting cells that sample the extra- and intracellular milieu to process antigens for the instruction of T cell responses. The mammalian target of rapamycin (mTOR) network senses environmental cues and is important for numerous cellular processes. This review discusses how DCs use mTOR complexes (mTORC1 and 2) to adapt their cellular metabolism, transcriptional responses, and translation machinery to control DC development, antigen processing, cytokine production, and T cell stimulation. We present a spatiotemporal model suggesting that the mTOR network integrates pattern recognition and growth factor receptor activation with nutritional information from the cell and surrounding tissue to support T cell stimulation and tolerance. mTOR develops into a central player that regulates DC differentiation and immune functions.

Roles of mTOR complexes in the kidney: implications for renal disease and transplantation

The mTOR pathway has a central role in the regulation of cell metabolism, growth and proliferation. Studies involving selective gene targeting of mTOR complexes (mTORC1 and mTORC2) in renal cell populations and/or pharmacologic mTOR inhibition have revealed important roles of mTOR in podocyte homeostasis and tubular transport. Important advances have also been made in understanding the role of mTOR in renal injury, polycystic kidney disease and glomerular diseases, including diabetic nephropathy. Novel insights into the roles of mTORC1 and mTORC2 in the regulation of immune cell homeostasis and function are helping to improve understanding of the complex effects of mTOR targeting on immune responses, including those that impact both de novo renal disease and renal allograft outcomes. Extensive experience in clinical renal transplantation has resulted in successful conversion of patients from calcineurin inhibitors to mTOR inhibitors at various times post-transplantation, with excellent long-term graft function. Widespread use of this practice has, however, been limited owing to mTOR-inhibitor- related toxicities. Unique attributes of mTOR inhibitors include reduced rates of squamous cell carcinoma and cytomegalovirus infection compared to other regimens. As understanding of the mechanisms by which mTORC1 and mTORC2 drive the pathogenesis of renal disease progresses, clinical studies of mTOR pathway targeting will enable testing of evolving hypotheses.

New perspectives on mTOR inhibitors (rapamycin, rapalogs and TORKinibs) in transplantation

The macrolide rapamycin and its analogues (rapalogs) constitute the first generation of mammalian target of rapamycin (mTOR) inhibitors. Since the introduction of rapamycin as an immunosuppressant, there has been extensive progress in understanding its complex mechanisms of action. New insights into the function of mTOR in different immune cell types, vascular endothelial cells and neoplastic cells have opened new opportunities and challenges regarding mTOR as a pharmacological target. Currently, the two known mTOR complexes, mTOR complex (mTORC) 1 and mTORC2, are the subject of intense investigation, and the introduction of second-generation dual mTORC kinase inhibitors (TORKinibs) and gene knockout mice is helping to uncover the distinct roles of these complexes in different cell types. While the pharmacological profiling of rapalogs is advanced, much less is known about the properties of TORKinibs. A potential benefit of mTOR inhibition in transplantation is improved protection against transplant-associated viral infections compared with standard calcineurin inhibitor-based immunosuppression. Preclinical and clinical data also underscore the potentially favourable antitumour effects of mTOR inhibitors in regard to transplant-associated malignancies and as a novel treatment option for various other cancers. Many aspects of the mechanisms of action of mTOR inhibitors and their clinical implications remain unknown. In this brief review we discuss new findings and perspectives of mTOR inhibitors in transplantation.

Immunometabolism governs dendritic cell and macrophage function

Recent studies on intracellular metabolism in dendritic cells (DCs) and macrophages provide new insights on the functioning of these critical controllers of innate and adaptive immunity. Both cell types undergo profound metabolic reprogramming in response to environmental cues, such as hypoxia or nutrient alterations, but importantly also in response to danger signals and cytokines. Metabolites such as succinate and citrate have a direct impact on the functioning of macrophages. Immunogenicity and tolerogenicity of DCs is also determined by anabolic and catabolic processes, respectively. These findings provide new prospects for therapeutic manipulation in inflammatory diseases and cancer.

Evolving perspectives of mTOR complexes in immunity and transplantation

Since the discovery of Rapamycin (RAPA) and its immunosuppressive properties, enormous progress has been made in characterizing the mechanistic target of rapamycin (mTOR). Use of RAPA and its analogues (rapalogs) as anti-rejection agents has been accompanied by extensive investigation of how targeting of mTOR complex 1 (mTORC1), the principal target of RAPA, and more recently mTORC2, affects the function of immune cells, as well as vascular endothelial cells, that play crucial roles in regulation of allograft rejection. While considerable knowledge has accumulated on the function of mTORC1 and 2 in T cells, understanding of the differential roles of these complexes in antigen-presenting cells, NK cells and B cells/plasma cells is only beginning to emerge. Immune cell-specific targeting of mTORC1 or mTORC2, together with use of novel, second generation, dual mTORC kinase inhibitors (TORKinibs) have started to play an important role in elucidating the roles of these complexes and their potential for targeting in transplantation. Much remains unknown about the role of mTOR complexes and the consequences of mTOR targeting on immune reactivity in clinical transplantation. Here we address recent advances in understanding and evolving perspectives of the role of mTOR complexes and mTOR targeting in immunity, with extrapolation to transplantation.

Synergy between rapamycin and FLT3 ligand enhances plasmacytoid dendritic cell-dependent induction of CD4+CD25+FoxP3+ Treg

CD4(+)CD25(+)FoxP3(+) regulatory T cells (Treg) are critical elements for maintaining immune tolerance, for instance to exogenous antigens that are introduced during therapeutic interventions such as cell/organ transplant or gene/protein replacement therapy. Coadministration of antigen with rapamycin simultaneously promotes deletion of conventional CD4(+) T cells and induction of Treg. Here, we report that the cytokine FMS-like receptor tyrosine kinase ligand (Flt3L) enhances the in vivo effect of rapamycin. This occurs via selective expansion of plasmacytoid dendritic cells (pDCs), which further augments the number of Treg. Whereas in conventional DCs, rapamycin effectively blocks mammalian target of rapamycin (mTOR) 1 signaling induced by Flt3L, increased mTOR1 activity renders pDCs more resistant to inhibition by rapamycin. Consequently, Flt3L and rapamycin synergistically promote induction of antigen-specific Treg via selective expansion of pDCs. This concept is supported by the finding that Treg induction is abrogated upon pDC depletion. The combination with pDCs and rapamycin is requisite for Flt3L/antigen-induced Treg induction because Flt3L/antigen by itself fails to induce Treg. As co-administering Flt3L, rapamycin, and antigen blocked CD8(+) T-cell and antibody responses in models of gene and protein therapy, we conclude that the differential effect of rapamycin on DC subsets can be exploited for improved tolerance induction.

Dendritic cell metabolism

The past 15 years have seen enormous advances in our understanding of the receptor and signalling systems that allow dendritic cells (DCs) to respond to pathogens or other danger signals and initiate innate and adaptive immune responses. We are now beginning to appreciate that many of these pathways not only stimulate changes in the expression of genes that control DC immune functions, but also affect metabolic pathways, thereby integrating the cellular requirements of the activation process. In this Review, we focus on this relatively new area of research and attempt to describe an integrated view of DC immunometabolism.

Protein synthesis regulation, a pillar of strength for innate immunity?

Recognition of pathogen derived molecules by Pattern Recognition Receptors (PRR) induces the production of cytokines (i.e. type I interferons) that stimulate the surrounding cells to transcribe and translate hundreds of genes, in order to prevent further infection and organize the immune response. Here, we report on the rising matter that metabolism sensing and gene expression control at the level of mRNA translation, allow swift responses that mobilize host defenses and coordinate innate responses to infection.

The emerging role of mTOR signalling in antibacterial immunity

Mammalian target of rapamycin (mTOR) is a central regulator of cellular metabolic homeostasis that is highly conserved in evolution. Recent evidence has revealed the existence of a complex interplay between mTOR signalling and immunity. We review here the emerging role of mTOR signalling in the regulation of Toll-like receptor-dependent innate responses and in the activation of T cells and antigen-presenting cells. We also highlight the importance of amino-acid starvation-driven mTOR inhibition in the control of autophagy and intracellular bacterial clearance.