Jane K. Howard

Leptin modulates the T-cell immune response and reverses starvation-induced immunosuppression

Nutritional deprivation suppresses immune function. The cloning of the obese gene and identification of its protein product leptin has provided fundamental insight into the hypothalamic regulation of body weight,. Circulating levels of this adipocyte-derived hormone are proportional to fat mass, but may be lowered rapidly by fasting, or increased by inflammatory mediators,. The impaired T-cell immunity of mice, now known to be defective in leptin (ob/ob) or its receptor (db/db),, has never been explained. Impaired cell-mediated immunity and reduced levels of leptin are both features of low body weight in humans. Indeed, malnutrition predisposes to death from infectious diseases. We report here that leptin has a specific effect on T-lymphocyte responses, differentially regulating the proliferation of naive and memory T cells. Leptin increased Th1 and suppressed Th2 cytokine production. Administration of leptin to mice reversed the immunosuppressive effects of acute starvation. Our findings suggest a new role for leptin in linking nutritional status to cognate cellular immune function, and provide a molecular mechanism to account for the immune dysfunction observed in starvation.

Enhanced leptin sensitivity and attenuation of diet-induced obesity in mice with haploinsufficiency of Socs3

Leptin is an adipocyte-derived hormone that regulates energy balance and neuroendocrine function primarily by acting on specific hypothalamic pathways. Resistance to the weight reducing effects of leptin is a feature of most cases of human and rodent obesity, yet the molecular basis of leptin resistance is poorly understood. We have previously identified suppressor of cytokine signaling-3 (Socs3) as a leptin-induced negative regulator of leptin receptor signaling and potential mediator of leptin resistance. However, due to the non-viability of mice with targeted disruption of Socs3 (ref. 6), the importance of Socs3 in leptin action in vivo was unclear. To determine the functional significance of Socs3 in energy balance in vivo we undertook studies in mice with heterozygous Socs3 deficiency (Socs3+/−). We report here that Socs3+/− mice display greater leptin sensitivity than wild-type control mice: Socs3+/− mice show both enhanced weight loss and increased hypothalamic leptin receptor signaling in response to exogenous leptin administration. Furthermore, Socs3+/− mice are significantly protected against the development of diet-induced obesity and associated metabolic complications. The level of Socs3 expression is thus a critical determinant of leptin sensitivity and obesity susceptibility in vivo and this molecule is a potential target for therapeutic intervention.

Requirement for leptin in the induction and progression of autoimmune encephalomyelitis.

Recent evidence indicates that leptin modifies T cell immunity, and may provide a key link between nutritional deficiency and immune dysfunction. To study the influence of leptin on autoimmunity, susceptibility to experimental autoimmune encephalomyelitis induced by immunization with a myelin-derived peptide was examined in leptin-deficient, C57BL/6J-ob/ob mice, with or without leptin replacement, and in wild-type controls. Leptin replacement converted disease resistance to susceptibility in the C57BL/6J-ob/ob mice; this was accompanied by a switch from a Th2 to Th1 pattern of cytokine release and consequent reversal of Ig subclass production. Our findings suggest that leptin is required for the induction and maintenance of an effective proinflammatory immune response in the CNS.

Attenuation of leptin and insulin signaling by SOCS proteins

Leptin and insulin are key hormones involved in the regulation of energy balance and glucose homeostasis. Development of resistance to the action of these hormones, which can occur with age, obesity and inflammation, appears to have a prime role in the pathogenesis of obesity and type 2 diabetes. Specific members of the suppressor of cytokine signaling (SOCS) family of proteins are now thought to have a role in the development of leptin and insulin resistance owing to their ability to inhibit leptin and insulin signaling pathways. In the case of leptin, current evidence suggests that SOCS3 appears to be of particular importance in the development of leptin resistance, whereas the ability to diminish insulin action has been described for several SOCS proteins (SOCS1, SOCS3, SOCS6 and SOCS7).

Absence of CC Chemokine Ligand 2 Does Not Limit Obesity-Associated Infiltration of Macrophages Into Adipose Tissue

Macrophage recruitment to adipose tissue in obesity contributes to enhanced adipose tissue inflammatory activity and thus may underlie obesity-associated metabolic dysfunction. Obese adipose tissue exhibits increases in CC chemokine ligand 2 (CCL2, or monocyte chemoattractant protein-1), an important macrophage-recruiting factor. We therefore hypothesized that elevated CCL2 may contribute to obesity-associated adipose tissue macrophage recruitment. Male 6-week-old CCL2−/− and wild-type mice (n = 11–14 per group) were fed standard and high-fat diets until 34 weeks of age. At 12–16 and 25–29 weeks of age, blood was collected for plasma glucose and hormone measurements, and glucose tolerance and insulin tolerance tests were performed. Adipose tissue was collected at 34 weeks for analysis of macrophage infiltration. Surprisingly, CCL2−/− mice on high-fat diet showed no reductions in adipose tissue macrophages. CCL2−/− mice on standard and high-fat diet were also glucose intolerant and had mildly increased plasma glucose and decreased serum adiponectin levels compared with wild-type mice. On high-fat diet, CCL2−/− mice also gained slightly more weight and were hyperinsulinemic compared with wild-type mice. Because macrophage levels were unchanged in CCL2−/− mice, the phenotype appears to be caused by lack of CCL2 itself. The fact that metabolic function was altered in CCL2−/− mice, despite no changes in adipose tissue macrophage levels, suggests that CCL2 has effects on metabolism that are independent of its macrophage-recruiting capabilities. Importantly, we conclude that CCL2 is not critical for adipose tissue macrophage recruitment. The dominant factor for recruiting macrophages in adipose tissue during obesity therefore remains to be identified.

The Transcription Factor T-bet Regulates Intestinal Inflammation Mediated by Interleukin-7 Receptor(+) Innate Lymphoid Cells

Summary Mice lacking the transcription factor T-bet in the innate immune system develop microbiota-dependent colitis. Here, we show that interleukin-17A (IL-17A)-producing IL-7Rα+ innate lymphoid cells (ILCs) were potent promoters of disease in Tbx21−/−Rag2−/− ulcerative colitis (TRUC) mice. TNF-α produced by CD103−CD11b+ dendritic cells synergized with IL-23 to drive IL-17A production by ILCs, demonstrating a previously unrecognized layer of cellular crosstalk between dendritic cells and ILCs. We have identified Helicobacter typhlonius as a key disease trigger driving excess TNF-α production and promoting colitis in TRUC mice. Crucially, T-bet also suppressed the expression of IL-7R, a key molecule involved in controlling intestinal ILC homeostasis. The importance of IL-7R signaling in TRUC disease was highlighted by the dramatic reduction in intestinal ILCs and attenuated colitis following IL-7R blockade. Taken together, these data demonstrate the mechanism by which T-bet regulates the complex interplay between mucosal dendritic cells, ILCs, and the intestinal microbiota.

Leptin interacts with glucagon-like peptide-1 neurons to reduce food intake and body weight in rodents

The adipose tissue hormone, leptin, and the neuropeptide glucagon‐like peptide‐1 (7–36) amide (GLP‐1) both reduce food intake and body weight in rodents. Using dual in situ hybridization, long isoform leptin receptor (OB‐Rb) was localized to GLP‐1 neurons originating in the nucleus of the solitary tract. ICV injection of the specific GLP‐1 receptor antagonist, exendin(9–39), at the onset of dark phase, did not affect feeding in saline pre‐treated controls, but blocked the reduction in food intake and body weight of leptin pre‐treated rats. These findings suggest that GLP‐1 neurons are a potential target for leptin in its control of feeding.

Leptin inhibits the anti‐CD3‐driven proliferation of peripheral blood T cells but enhances the production of proinflammatory cytokines

There is increasing evidence that leptin affects immune responses and that in the absence of leptin, immunity is suboptimal. Most data so far indicate that leptin increases proinflammatory immune responses by an effect on T cells and macrophages. Here we show that, under certain circumstances, leptin can inhibit T cell proliferative responses. Separation of the responding T cells into different subpopulations revealed an interesting heterogeneity of cellular behavior in that naïve and memory T cells were differentially affected by leptin. The anti‐CD3‐driven proliferation of memory T cells was inhibited by leptin, whereas that of naïve T cells was markedly enhanced. Despite the inhibition of proliferation of the memory T cells, their production of interferon‐γ was substantially increased. These data show that leptin can inhibit certain immune responses in vitro. However, despite this inhibition of proliferation, the production of proinflammatory cytokines is significantly enhanced by leptin. The findings demonstrated here show further complexity in the actions of leptin on the immune system.

Leptin potentiates experimental autoimmune encephalomyelitis in SJL female mice and confers susceptibility to males

SJL (H‐2s) female mice are more susceptible than males to experimental autoimmune encephalomyelitis (EAE) induced by immunization with myelin‐derived peptides. The reasons for this sexual dimorphism are unclear, but may include such factors as sex‐related differences in immune responsiveness, hormonal effects and sex‐linked genetic factors. Recent evidence indicates that leptin modifies T cell immunity promoting T helper (Th) 1 pro‐inflammatory immune responses. Circulating leptin levels show a marked sexual dimorphism, being higher in females than in males. In the present study, we investigated whether leptin treatment altered the course of relapsing‐remitting EAE, induced by the proteolipid protein peptide (PLP139–151), in SJL susceptible females and EAE‐resistant males. Administration of leptin to female SJL mice before or after disease onset significantly worsened the disease, with a concomitant increase in the PLP139–151‐specific delayed‐type hypersensitivity (DTH) reactivity and in vitro IFN‐γ secretion. Leptin treatment at priming with antigen or before disease onset rendered male SJL mice susceptible to EAE, with the appearance of PLP139–151‐specific DTH reactivity and a switch from a Th2 to Th1 pattern of cytokine release. Our findings indicate that leptin administration to susceptible females resulted in a more severe disease, and that reduced leptin levels in male SJL mice may contribute to the gender‐related differences in the induction phase of EAE.

Improved insulin sensitivity despite increased visceral adiposity in mice deficient for the immune cell transcription factor T-bet.

Summary Low-grade inflammation in fat is associated with insulin resistance, although the mechanisms are unclear. We report that mice deficient in the immune cell transcription factor T-bet have lower energy expenditure and increased visceral fat compared with wild-type mice, yet paradoxically are more insulin sensitive. This striking phenotype, present in young T-bet−/− mice, persisted with high-fat diet and increasing host age and was associated with altered immune cell numbers and cytokine secretion specifically in visceral adipose tissue. However, the favorable metabolic phenotype observed in T-bet-deficient hosts was lost in T-bet−/− mice also lacking adaptive immunity (T-bet−/−xRag2−/−), demonstrating that T-bet expression in the adaptive rather than the innate immune system impacts host glucose homeostasis. Indeed, adoptive transfer of T-bet-deficient, but not wild-type, CD4+ T cells to Rag2−/− mice improved insulin sensitivity. Our results reveal a role for T-bet in metabolic physiology and obesity-associated insulin resistance.