Indirect effects of leptin receptor deficiency on lymphocyte populations and immune response in db/db mice

Leptin acts in the periphery to protect thymocytes from glucocorticoid-mediated apoptosis in the absence of weight loss

Leptin is a member of the IL-6 cytokine family and is primarily produced by adipose tissue. At high enough concentration, leptin engages leptin receptors expressed in the hypothalamus that regulate a variety of functions, including induction of weight loss. Mice deficient in leptin (ob/ob) or leptin receptor (db/db) function exhibit thymic atrophy associated with a reduction in double-positive (DP) thymocytes. However, the mediator of such thymic atrophy remains to be identified, and the extent to which leptin acts in the periphery vs. the hypothalamus to promote thymocyte cellularity is unknown. In the present study, we first demonstrate that thymic cellularity and composition is fully restored in ob/ob mice subjected to adrenalectomy. Second, we observe that ob/ob mice treated with low-dose leptin peripherally but not centrally exhibit increased thymocyte cellularity in the absence of any weight loss or significant reduction in systemic corticosterone levels. Third, we demonstrate that reconstitution of db/db mice with wild-type bone marrow augments thymocyte cellularity and restores DP cell frequency despite elevated corticosterone levels. These and additional data support a mode of action whereby leptin acts in the periphery to reduce the sensitivity of DP thymocytes to glucocorticoid-mediated apoptosis in vivo. Strikingly, our data reveal that leptin's actions on thymic cellularity in the periphery can be uncoupled from its anorectic actions in the hypothalamus.

Adipose-immune interactions during obesity and caloric restriction: reciprocal mechanisms regulating immunity and health span

Increasing evidence suggests a tight coupling of metabolic and immune systems. This cross-talk mediated by neuroendocrine peptides as well as numerous cytokines and chemokines is believed to be responsible for integrating energy balance to immune function. These neuroendocrine-immune interactions are heightened during the state of chronic positive energy balance, as seen during obesity, and negative energy balance caused by caloric restriction (CR). Emerging evidence suggests that obesity may be associated with an immunodeficient state and chronic inflammation, which contribute to an increased risk of premature death. The direct interactions between expanded leukocyte populations within the adipose tissue during obesity and an increased number of adipocytes within an aging lymphoid microenvironment may constitute an important adaptive or pathological response as a result of change in energy balance. In stark contrast to obesity, CR causes negative energy balance and robustly prolongs a healthy lifespan in all of the species studied to date. Therefore, the endogenous neuroendocrine-metabolic sensors elevated or suppressed as a result of changes in energy balance may offer an important mechanism in understanding the antiaging and potential immune-enhancing nature of CR. Ghrelin, one such sensor of negative energy balance, is reduced during obesity and increased by CR. Ghrelin also regulates immune function by reducing proinflammatory cytokines and promotes thymopoiesis during aging and thus, may be a new CR mimetic target. The identification of immune effects and molecular pathways used by such orexigenic metabolic factors could offer potentially novel approaches to enhance immunity and increase healthy lifespan.

Cytokines, leptin, and stress-induced thymic atrophy

Thymopoiesis is essential for development and maintenance of a robust and healthy immune system. Acute thymic atrophy is a complication of many infections, environmental stressors, clinical preparative regimens, and cancer treatments used today. This undesirable sequela can decrease host ability to reconstitute the peripheral T cell repertoire and respond to new antigens. Currently, there are no treatments available to protect against acute thymic atrophy or accelerate recovery, thus leaving the immune system compromised during acute stress events. Several useful murine models are available for mechanistic studies of acute thymic atrophy, including a sepsis model of endotoxin-induced thymic involution. We have identified the IL-6 cytokine gene family members (i.e., leukemia inhibitory factor, IL-6, and oncostatin M) as thymosuppressive agents by the observation that they can acutely involute the thymus when injected into a young, healthy mouse. We have gone on to explore the role of thymosuppressive cytokines and specifically defined a corticosteroid-dependent mechanism of action for the leukemia inhibitory factor in acute thymic atrophy. We also have identified leptin as a novel, thymostimulatory agent that can protect against endotoxin-induced acute thymic atrophy. This review will highlight mechanisms of stress-induced thymic involution and focus on thymosuppressive agents involved in atrophy induction and thymostimulatory agents that may be exploited for therapeutic use.

Cells of the synovium in rheumatoid arthritis. Chondrocytes

Rheumatoid arthritis (RA) is one of the inflammatory joint diseases in a heterogeneous group of disorders that share features of destruction of the extracellular matrices of articular cartilage and bone. The underlying disturbance in immune regulation that is responsible for the localized joint pathology results in the release of inflammatory mediators in the synovial fluid and synovium that directly and indirectly influence cartilage homeostasis. Analysis of the breakdown products of the matrix components of joint cartilage in body fluids and quantitative imaging techniques have been used to assess the effects of the inflammatory joint disease on the local remodeling of joint structures. The role of the chondrocyte itself in cartilage destruction in the human rheumatoid joint has been difficult to address but has been inferred from studies in vitro and in animal models. This review covers current knowledge about the specific cellular and biochemical mechanisms that account for the disruption of the integrity of the cartilage matrix in RA.

Leptin promotes cell survival and activates Jurkat T lymphocytes by stimulation of mitogen-activated protein kinase

Leptin (Ob) is a non-glycosylated peptide hormone that regulates energy homeostasis centrally, but also has systemic effects including the regulation of the immune function. We have reported previously that leptin activates human peripheral blood lymphocytes co-stimulated with phytohaemagglutinin (PHA) (4 microg/ml), which prevented the employment of pharmacological inhibitors of signalling pathways. In the present study, we used Jurkat T cells that responded to leptin with minimal PHA co-stimulation (0.25 microg/ml). The long isoform of leptin receptor is expressed on Jurkat T cells and upon leptin stimulation, the expression of early activation marker CD69 increases in a dose-dependent manner (0.1-10 nM). We have also found that leptin activates receptor-associated kinases of the Janus family-signal transucers and activators of transcription (JAK-STAT), mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3 kinase (PI3K) signalling pathways. Moreover, we sought to study the possible effect of leptin on cell survival and apoptosis of Jurkat T cells by culture in serum-free conditions. We have assayed the early phases of apoptosis by flow cytometric detection of fluorescein isothiocyanate (FITC)-labelled annexin V simultaneously with dye exclusion of propidium iodide (PI). As well, we have assayed the activation level of caspase-3 by inmunoblot with a specific antibody that recognizes active caspase-3. We have found that leptin inhibits the apoptotic process dose-dependently. By using pharmacological inhibitors, we have found that the stimulatory and anti-apoptotic effects of leptin in Jurkat T cells are dependent on MAPK activation, rather than the PI3K pathway, providing new data regarding the mechanism of action of leptin in T cells, which may be useful to understand more clearly the association between nutritional status and the immune function.

Leptin beyond body weight regulation--current concepts concerning its role in immune function and inflammation

Leptin, a 16 kDa non-glycosylated polypeptide produced primarily by adipocytes and released into the systemic circulation, exerts a multitude of regulatory functions including energy utilization and storage, regulation of various endocrine axes, bone metabolism, and thermoregulation. In addition to leptin's best known role as regulator of energy homeostasis, several studies indicate that leptin plays a pivotal role in immune and inflammatory response. Because of its dual nature as a hormone and cytokine, leptin can be nowadays considered the link between neuroendocrine and immune system. The increase in leptin production that occurs during infections and inflammatory processes strongly suggests that this adipokine is a part of the cytokines network which governs inflammatory/immune response and host defence mechanisms. Indeed, leptin plays a relevant role in inflammatory processes involving either innate or adaptive immune responses. Several studies have implicated leptin in the pathogenesis of autoimmune inflammatory conditions such as encephalomyelitis, type I diabetes, bowel inflammation and also articular degenerative diseases such as rheumatoid arthritis and osteoarthritis. Although the mechanisms by which leptin exerts its action as modulator of inflammatory/immune response are likely to be more complex than predicted and far to be completely depicted, there is a general consensus about its pivotal role as pro-inflammatory and immune-modulating agent. Here, we review the most recent advances on leptin biology with a particular attention to its adipokine facet, even though its role as metabolic hormone will be also addressed.

A role for leptin in sustaining lymphopoiesis and myelopoiesis

Although leptin is known for its regulation of food intake, it has many emerging roles in immune function. To better define the role of leptin in hematopoietic processes, a leptin-deficient obese mouse (ob/ob) and C57BL/6 lean wild-type controls were compared. Despite their large size and consumption of substantial amounts of nutrients, the ob/ob mice had only 60% as many nucleated cells in their marrow as controls. The greatest impact of leptin deficiency was on the B cell compartment that had 70% fewer cells, reducing the absolute number of pre-B and immature B cells to 21% and 12% of normal, respectively, and indicating a significant reduction in lymphopoiesis in ob/ob mice. Whereas the proportion of myeloids remained nearly normal in the obese mice, they also exhibited a reduction of 40% and 25%, respectively, in absolute numbers of granulocytes and monocytes. Seven days of provision of recombinant leptin promoted substantial lymphopoiesis, increasing the numbers of B cells in the marrow of the obese mice twofold, while doubling and tripling, respectively, the numbers of pre-B and immature B cells. Twelve days of supplementation brought these subpopulations to near-normal proportions. Leptin treatment also facilitated myelopoiesis such that the marrow of the obese mice contained normal numbers of monocytes and granulocytes after 7 days. Taken together, the data support an important role for leptin in sustaining lymphopoiesis and myelopoiesis.

The emerging role of adipokines as mediators of inflammation and immune responses

Interest in the biology of white adipose tissue (WAT) has increased dramatically since the discovery of leptin in 1994. The identification of the product of the gene obese (ob) threw light on the role of adipose tissue in the physiopathology of obesity-related diseases, and spurred the identification of numerous other adipokines, many of a pro-inflammatory nature. It has become increasingly evident that WAT-derived cytokines mediate between obesity-related exogenous factors (nutrition and lifestyle) and the molecular events that lead to metabolic syndrome and inflammatory and/or autoimmune conditions. Here, we review recent adipokine research, with particular attention to the roles of leptin, adiponectin, resistin, visfatin, apelin, vaspin and hepcidin in such conditions.

The leptin connection: regulatory T cells and autoimmunity

Leptin is a cytokine-like hormone with proinflammatory properties linked to autoimmune diseases. In this issue of Immunity, De Rosa et al. (2007) elucidate an important new role for leptin in the anergy and hyporesponsiveness of regulatory T cells.

Leptin: nourishment for the immune system

Leptin, a protein produced by adipocytes, exerts several functions, including modulation of the immune response. A report in this issue of the European Journal of Immunology describes for the first time the effect of either leptin receptor deficiency or blockade on murine dendritic cell (DC) maturation, survival and function. The study describes how leptin receptor deficiency/blockade delays DC maturation and promotes apoptosis, shifts the balance between pro- and anti-inflammatory cytokine production, and reduces the ability of DC to stimulate CD4(+) lymphocytes. These exciting novel data add an important piece of evidence to the picture of the role of leptin in immunity and inflammation and generate the possibility that many of the effects of leptin on T lymphocytes might be mediated through DC.