Fortunata Carbone

Regulatory T cell proliferative potential is impaired in human autoimmune disease

Human CD4+CD25highCD127−FoxP3+ regulatory T (Treg) cells suppress immune responses in vitro and in vivo. Reduced suppressive function and/or number of peripheral Treg cells has been previously reported in autoimmune disorders. Treg cells represent the most actively replicating compartment within the CD4+ cells in vivo, but they are hyporesponsive to classical T cell receptor (TCR) stimulation in vitro, a condition that is secondary to their overactive metabolic state. Here we report that proliferation of Treg cells after TCR stimulation is impaired in subjects with relapsing-remitting multiple sclerosis (RRMS) because of altered interleukin-2 (IL-2) secretion and IL-2 receptor (IL-2R)–signal transducer and activator of transcription 5 (STAT5) signaling. This is associated with decreased expression of the forkhead box P3 (FoxP3) 44- and 47-kDa splicing forms, overactivation of S6 ribosomal protein (a downstream target of the mammalian target of rapamycin, mTOR) and altered activity of the cyclin-dependent kinase inhibitor p27 (p27kip1) and extracellular signal–related kinases 1 and 2 (ERK1/2). The impaired capacity of Treg cells to proliferate in RRMS correlates with the clinical state of the subject, where increasing disease severity is associated with a decline in Treg cell expansion. These results suggest a previously unrecognized mechanism that may account for the progressive loss of Treg cells in autoimmune disease.

Immunological functions of leptin and adiponectin

Recent years have seen several advances in our understanding of the functions of adipose tissue regarding not only the energy storage, but also the regulation of complex metabolic and endocrine functions. In this context, leptin and adiponectin, the two most abundant adipocyte products, represent one of the best example of adipocytokines involved in the control of energy expenditure, lipid and carbohydrate metabolism as well as in the regulation of immune responses. Leptin and adiponectin secretion is counter-regulated in vivo, in relation to degree of adiposity, since plasma leptin concentrations are significantly elevated in obese subjects in proportion to body mass index while adiponectin secretion decreases in relation to the amount of adipose tissue. In this review we focus on the main biological activities of leptin and adiponectin on the lipid and carbohydrate metabolism and on their contribute in regulation of innate and adaptive immune responses.

Efficacy of Metreleptin in Obese Patients With Type 2 Diabetes: Cellular and Molecular Pathways Underlying Leptin Tolerance

OBJECTIVE Metreleptin has been efficacious in improving metabolic control in patients with lipodystrophy, but its efficacy has not been tested in obese patients with type 2 diabetes. RESEARCH DESIGN AND METHODS We studied the role of leptin in regulating the endocrine adaptation to long-term caloric deprivation and weight loss in obese diabetic subjects over 16 weeks in the context of a double-blinded, placebo–controlled, randomized trial. We then performed detailed interventional and mechanistic signaling studies in humans in vivo, ex vivo, and in vitro. RESULTS In obese patients with diabetes, metreleptin administration for 16 weeks did not alter body weight or circulating inflammatory markers but reduced HbA1c marginally (8.01 ± 0.93–7.96 ± 1.12, P = 0.03). Total leptin, leptin-binding protein, and antileptin antibody levels increased, limiting free leptin availability and resulting in circulating free leptin levels of ∼50 ng/mL. Consistent with clinical observations, all metreleptin signaling pathways studied in human adipose tissue and peripheral blood mononuclear cells were saturable at ∼50 ng/mL, with no major differences in timing or magnitude of leptin-activated STAT3 phosphorylation in tissues from male versus female or obese versus lean humans in vivo, ex vivo, or in vitro. We also observed for the first time that endoplasmic reticulum (ER) stress in human primary adipocytes inhibits leptin signaling. CONCLUSIONS In obese patients with diabetes, metreleptin administration did not alter body weight or circulating inflammatory markers but reduced HbA1c marginally. ER stress and the saturable nature of leptin signaling pathways play a key role in the development of leptin tolerance in obese patients with diabetes.

The immunology of pregnancy: regulatory T cells control maternal immune tolerance toward the fetus.

Establishment and maintenance of pregnancy represents a challenge for the maternal immune system since it has to defend against pathogens and tolerate paternal alloantigens expressed in fetal tissues. Regulatory T (Treg) cells, a subset of suppressor CD4(+) T cells, play a dominant role in the maintenance of immunological self-tolerance by preventing immune and autoimmune responses against self-antigens. Although localized mechanisms contribute to fetal evasion from immune attack, in the last few years it has been observed that Treg cells are essential in promoting fetal survival avoiding the recognition of paternal semi-allogeneic tissues by maternal immune system. Several functional studies have shown that unexplained infertility, miscarriage and pre-clampsia are often associated with deficit in Treg cell number and function while normal pregnancy selectively stimulates the accumulation of maternal forkhead-box-P3(+) (FoxP3(+)) CD4(+) Treg cells with fetal specificity. Some papers have been reported that the number of Treg cells persists at elevated levels long after delivery developing an immune regulatory memory against fathers antigens, moreover these memory Treg cells rapidly proliferate during subsequent pregnancies, however, on the other hand, there are several evidence suggesting a clear decline of Treg cells number after delivery. Different factors such as cytokines, adipokines, pregnancy hormones and seminal fluid have immunoregulatory activity and influence the success of pregnancy by increasing Treg cell number and activity. The development of strategies capable of modulating immune responses toward fetal antigens through Treg cell manipulation, could have an impact on the induction of tolerance against fetal antigens during immune-mediated recurrent abortion.

Leptin-Induced mTOR Activation Defines a Specific Molecular and Transcriptional Signature Controlling CD4+ Effector T Cell Responses

The sensing by T cells of metabolic and energetic changes in the microenvironment can determine the differentiation, maturation, and activation of these cells. Although it is known that mammalian target of rapamycin (mTOR) gauges nutritonal and energetic signals in the extracellular milieu, it is not known how mTOR and metabolism influence CD4+CD25−FOXP3− effector T cell (Teff) responses. In this article, we show that leptin-induced activation of mTOR, which, in turn, controls leptin production and signaling, causes a defined cellular, biochemical, and transcriptional signature that determine the outcome of Teff responses, both in vitro and in vivo. The blockade of leptin/leptin receptor signaling, induced by genetic means or by starvation, leads to impaired mTOR activity that inhibits the proliferation of Teffs in vivo. Notably, the transcriptional signature of Teffs in the presence of leptin blockade appears similar to that observed in rapamycin-treated Teffs. These results identify a novel link between nutritional status and Teff responses through the leptin–mTOR axis and define a potential target for Teff modulation in normal and pathologic conditions.

Leptin Modulates the Survival of Autoreactive CD4+ T Cells through the Nutrient/Energy-Sensing Mammalian Target of Rapamycin Signaling Pathway

Chronic inflammation can associate with autoreactive immune responses, including CD4+ T cell responses to self-Ags. In this paper, we show that the adipocyte-derived proinflammatory hormone leptin can affect the survival and proliferation of autoreactive CD4+ T cells in experimental autoimmune encephalomyelitis, an animal model of human multiple sclerosis. We found that myelin olygodendrocyte glycoprotein peptide 35–55 (MOG35–55)-specific CD4+ T cells from C57BL/6J wild-type mice could not transfer experimental autoimmune encephalomyelitis into leptin-deficient ob/ob mice. Such a finding was associated with a reduced proliferation of the transferred MOG35–55-reactive CD4+ T cells, which had a reduced degradation of the cyclin-dependent kinase inhibitor p27kip1 and ERK1/2 phosphorylation. The transferred cells displayed reduced Th1/Th17 responses and reduced delayed-type hypersensitivity. Moreover, MOG35–55-reactive CD4+ T cells in ob/ob mice underwent apoptosis that associated with a downmodulation of Bcl-2. Similar results were observed in transgenic AND-TCR- mice carrying a TCR specific for the pigeon cytochrome c 88–104 peptide. These molecular events reveal a reduced activity of the nutrient/energy-sensing AKT/mammalian target of rapamycin pathway, which can be restored in vivo by exogenous leptin replacement. These results may help to explain a link between chronic inflammation and autoimmune T cell reactivity.

Role of adipokines signaling in the modulation of T cells function.

The field that links immunity and metabolism is rapidly expanding. Apparently non-immunological disorders such as obesity and type 2 diabetes have been linked to immune dysregulation, suggesting that metabolic alterations can be induced by or be consequence of an altered self-immune tolerance. In this context, adipose tissue produces and releases a variety of pro-inflammatory and anti-inflammatory factors, termed “adipokines,” which can be considered as the bridge between obesity-related exogenous factors, such as nutrition and lifestyle, and the molecular events leading to metabolic syndrome, inflammatory, and/or autoimmune conditions. In obesity, increased production of most adipokines impacts on multiple functions such as appetite and energy balance, modulation of immune responses, insulin sensitivity, angiogenesis, blood pressure, lipid metabolism, and so on. This report aims to discuss some of the recent topics of adipocytokine research and their related signaling pathways, that may be of particular importance as could lead to effective therapeutic strategies for obesity-associated diseases.

Obesity and susceptibility to autoimmune diseases

For decades, obesity has been considered to be the result of the complex interaction between genes and the environment and its pathogenesis is still unresolved. The discovery of hormones and neural mediators responsible for the control of food intake and metabolism at the hypothalamic level has provided fundamental insights into the complicated pathways that control food intake. However, the molecular basis for the association between obesity and low-degree chronic inflammation is still unknown. More recently, the discovery of leptin, one of the most abundant adipocyte-derived hormones, has suggested that nutritional status, through leptin secretion, can control immune self-tolerance modulating Treg suppressive function and responsiveness. Furthermore, recent experimental evidence has shown the presence of an abundant adipose tissue-resident Treg population responsible for the control of metabolic parameters and glucose homeostasis. Better knowledge of the intricate network of interactions among leptin-related energy regulation, Treg activities and obesity could lead to valuable strategies for therapeutic intervention in obesity and obesity-associated insulin resistance.

Leptin: The Prototypic Adipocytokine and its Role in NAFLD

Nonalcoholic fatty liver disease (NAFLD) is the hepatic manifestation of the metabolic syndrome, whose pathogenesis begins with the accumulation of liver fat and is followed by the development of necro-inflammation and fibrosis. Recent evidence indicates that adipocytokines, polypeptides secreted by the adispose tissue, might play an important role in the pathogeneic process and progression of NAFLD. In this review, we explore the role of leptin, and in part of other adipocytokines, in the interference with hepatic injury associated with fatty infiltration, in the modulation of steatosis and fibrosis, in both experimental models of the disease and in the clinical practice. We also discuss the potential use of leptin as non-invasive marker for differentiating simple fatty liver from NAFLD, and the possible novel therapeutic strategies aimed at interfering with the leptin axis to dampen chronic liver inflammation and NAFLD.

Leptin as immune mediator: Interaction between neuroendocrine and immune system.

Leptin is an adipocyte-derived hormone/cytokine that links nutritional status with neuroendocrine and immune functions. Initially described as an anti-obesity hormone, leptin has subsequently been shown to exert pleiotropic effects, being also able to influence haematopoiesis, thermogenesis, reproduction, angiogenesis, and more importantly immune homeostasis. As a cytokine, leptin can affect both innate and adaptive immunity, by inducing a pro-inflammatory response and thus playing a key role in the regulation of the pathogenesis of several autoimmune/inflammatory diseases. In this review, we discuss the most recent advances on the role of leptin as immune-modulator in mammals and we also provide an overview on its main functions in non-mammalian vertebrates.