Interferon-gamma, a Th1 cytokine, regulates fat inflammation: a role for adaptive immunity in obesity

Adipose tissue inflammation: novel insight into the role of macrophages and lymphocytes

PURPOSE OF REVIEW

Obesity is associated with low-grade chronic inflammation in adipose tissue. This review presents an update on human and rodent studies analyzing the nature of fat-infiltrating immune cells, the time course of adipose tissue infiltration and underlying mechanisms.

RECENT FINDINGS

Intensive studies in rodents have shown that not only cells of the innate immune system traffic into adipose tissue but also various lymphocytes of the adaptive immunity are involved in inflammatory processes in fat. Several studies also provide insight in the order of appearance of macrophages and lymphocytes during the onset of obesity. Adipocytes and preadipocytes are also active players by their secretion of chemotactic adipokines.

SUMMARY

This review summarizes strong evidence for a link between the action of innate and adaptive immune systems in adipose tissue in the context of obesity and metabolism in rodents, but more studies in humans are necessary to relate this topic to human physiology. Targeting different immune cells at different stages of obesity may eventually lead to novel therapeutic approaches for the metabolic syndrome.

Obesity increases the production of proinflammatory mediators from adipose tissue T cells and compromises TCR repertoire diversity: implications for systemic inflammation and insulin resistance

Emerging evidence suggests that increases in activated T cell populations in adipose tissue may contribute toward obesity-associated metabolic syndrome. The present study investigates three unanswered questions: 1) Do adipose-resident T cells (ARTs) from lean and obese mice have altered cytokine production in response to TCR ligation?; 2) Do the extralymphoid ARTs possess a unique TCR repertoire compared with lymphoid-resident T cells and whether obesity alters the TCR diversity in specific adipose depots?; and 3) Does short-term elimination of T cells in epididymal fat pad without disturbing the systemic T cell homeostasis regulate inflammation and insulin-action during obesity? We found that obesity reduced the frequency of naive ART cells in s.c. fat and increased the effector-memory populations in visceral fat. The ARTs from diet-induced obese (DIO) mice had a higher frequency of IFN-gamma(+), granzyme B(+) cells, and upon TCR ligation, the ARTs from DIO mice produced increased levels of proinflammatory mediators. Importantly, compared with splenic T cells, ARTs exhibited markedly restricted TCR diversity, which was further compromised by obesity. Acute depletion of T cells from epididymal fat pads improved insulin action in young DIO mice but did not reverse obesity-associated feed forward cascade of chronic systemic inflammation and insulin resistance in middle-aged DIO mice. Collectively, these data establish that ARTs have a restricted TCR-Vbeta repertoire, and T cells contribute toward the complex proinflammatory microenvironment of adipose tissue in obesity. Development of future long-term T cell depletion protocols specific to visceral fat may represent an additional strategy to manage obesity-associated comorbidities.

Macrophages, inflammation, and insulin resistance

Obesity induces an insulin-resistant state in adipose tissue, liver, and muscle and is a strong risk factor for the development of type 2 diabetes mellitus. Insulin resistance in the setting of obesity results from a combination of altered functions of insulin target cells and the accumulation of macrophages that secrete proinflammatory mediators. At the molecular level, insulin resistance is promoted by a transition in macrophage polarization from an alternative M2 activation state maintained by STAT6 and PPARs to a classical M1 activation state driven by NF-kappaB, AP1, and other signal-dependent transcription factors that play crucial roles in innate immunity. Strategies focused on inhibiting the inflammation/insulin resistance axis that otherwise preserve essential innate immune functions may hold promise for therapeutic intervention.

T lymphocytes: a role in hypertension?

PURPOSE OF REVIEW

Low-grade inflammation has been shown to play a role in cardiovascular disease and specifically in hypertension. Circulating and tissue leukocytes and macrophages are a component of the mechanisms leading to inflammatory responses. Subsets of T lymphocytes have been implicated in the pathogenesis of hypertension and vascular remodeling. This is an area of active research and rapid development in cardiovascular and renal disease which offers great therapeutic potential.

RECENT FINDINGS

Recent data suggest that subsets of T lymphocytes, both effector T cells such as T-helper (Th)1 (interferon-gamma-producing) and Th2 lymphocytes [that produce interleukin (IL)-4], as well as Th17 (that produce IL-17), and T suppressor lymphocytes including regulatory T cells (Treg), which express the transcription factor forkhead box P3 (Foxp3), play critical roles in the development of angiotensin II, deoxycorticosterone salt-sensitive and Dahl salt-sensitive hypertension, and in the progression of vascular remodeling. As well, recent evidence suggests that the inflammatory response involving T lymphocytes may be triggered by oxidative stress in nuclei of the brain and associate with blood pressure elevation.

SUMMARY

These new data implicating T lymphocytes will eventually allow discovery of new therapeutic targets that may improve outcomes in cardiovascular and renal disease in humans.

A Western-like fat diet is sufficient to induce a gradual enhancement in fat mass over generations

The prevalence of obesity has steadily increased over the last few decades. During this time, populations of industrialized countries have been exposed to diets rich in fat with a high content of linoleic acid and a low content of alpha-linolenic acid compared with recommended intake. To assess the contribution of dietary fatty acids, male and female mice fed a high-fat diet (35% energy as fat, linoleic acid:alpha-linolenic acid ratio of 28) were mated randomly and maintained after breeding on the same diet for successive generations. Offspring showed, over four generations, a gradual enhancement in fat mass due to combined hyperplasia and hypertrophy with no change in food intake. Transgenerational alterations in adipokine levels were accompanied by hyperinsulinemia. Gene expression analyses of the stromal vascular fraction of adipose tissue, over generations, revealed discrete and steady changes in certain important players, such as CSF3 and Nocturnin. Thus, under conditions of genome stability and with no change in the regimen over four generations, we show that a Western-like fat diet induces a gradual fat mass enhancement, in accordance with the increasing prevalence of obesity observed in humans.

Inflammatory lipid mediators in adipocyte function and obesity

Survival of multicellular organisms depends on their ability to fight infection, metabolize nutrients, and store energy for times of need. Unsurprisingly, therefore, immunoregulatory and metabolic mechanisms interact in human conditions such as obesity. Both infiltrating immunoinflammatory cells and adipocytes play critical roles in the modulation of metabolic homeostasis, so it is important to understand factors that regulate both adipocyte and immune cell function. A currently favored paradigm for obesity-associated metabolic dysfunction is that chronic macronutrient and/or lipid overload (associated with adiposity) induces cellular stress that initiates and perpetuates an inflammatory cycle and pathophysiological signaling of immunoinflammatory cells and adipocytes. Many lipid mediators exert their biological effects by binding to cognate receptors, such as G-protein-coupled receptors and Toll-like receptors. This process is tightly regulated under normal physiological conditions, and any disruption can initiate disease processes. Observations that cellular lipid loading (associated with adiposity) initiates inflammatory events has encouraged studies on the role of lipid mediators. In this review, we speculate that lipid mediators act on important immune receptors to induce low-grade tissue inflammation, which leads to adipocyte and metabolic dysfunction.

Inflammation in atherosclerosis: transition from theory to practice

Inflammation drives the formation, progression, and rupture of atherosclerotic plaques. Experimental studies have demonstrated that an inflammatory subset of monocytes/macrophages preferentially accumulate in atherosclerotic plaque and produce proinflammatory cytokines. T lymphocytes can contribute to inflammatory processes that promote thrombosis by stimulating production of collagen-degrading proteinases and the potent procoagulant tissue factor. Recent data link obesity, inflammation, and modifiers of atherosclerotic events, a nexus of growing clinical concern given the worldwide increase in the prevalence of obesity. Modulators of inflammation derived from visceral adipose tissue evoke production of acute phase reactants in the liver, implicated in thrombogenesis and clot stability. Additionally, C-reactive protein levels rise with increasing levels of visceral adipose tissue. Adipose tissue in obese mice contains increased numbers of macrophages and T lymphocytes, increased T lymphocyte activation, and increased interferon-gamma (IFN-gamma) expression. IFN-gamma deficiency in mice reduces production of inflammatory cytokines and inflammatory cell accumulation in adipose tissue. Another series of in vitro and in vivo mouse experiments affirmed that adiponectin, an adipocytokine, the plasma levels of which drop with obesity, acts as an endogenous antiinflammatory modulator of both innate and adaptive immunity in atherogenesis. Thus, accumulating experimental evidence supports a key role for inflammation as a link between risk factors for atherosclerosis and the biology that underlies the complications of this disease. The recent JUPITER trial supports the clinical utility of an assessment of inflammatory status in guiding intervention to limit cardiovascular events. Inflammation is thus moving from a theoretical concept to a tool that provides practical clinical utility in risk assessment and targeting of therapy.

Natural killer T cells are involved in adipose tissues inflammation and glucose intolerance in diet-induced obese mice

BACKGROUND

Macrophage and lymphocyte infiltration in adipose tissue may contribute to the pathogenesis of obesity-mediated metabolic disorders. Natural killer T (NKT) cells, which integrate proinflammatory cytokines, have been demonstrated in the atherosclerotic lesions and in visceral adipose tissue.

OBJECTIVE

To determine whether NKT cells are involved in glucose intolerance and adipose tissue inflammation in diet-induced obese mice.

METHODS AND RESULTS

Male beta(2)-microglobulin knockout (KO) mice lacking NKT cells and C57BL/6J (wild-type) mice were fed with a high-fat diet (HFD) for 13 weeks [corrected]. Body weight and visceral obesity were comparable between wild-type and KO mice. However, macrophage infiltration was reduced in adipose tissue and glucose intolerance was significantly ameliorated in KO mice. To further confirm that NKT cells are involved in these abnormalities, alpha-galactosylceramide, 0.1 microg/g body weight, which specifically activates NKT cells, was administered after 13 weeks of HFD feeding. alpha-Galactosylceramide significantly exacerbated glucose intolerance and macrophage infiltration as well as cytokine gene expression in adipose tissue.

CONCLUSIONS

NKT cells play a crucial role in the development of adipose tissue inflammation and glucose intolerance in diet-induced obesity.

CD11c expression in adipose tissue and blood and its role in diet-induced obesity

OBJECTIVE

To examine CD11c, a beta(2)-integrin, on adipose tissue (AT) leukocytes and blood monocytes and its role in diet-induced obesity.

METHODS AND RESULTS

High-fat diet-induced obese C57BL/6 mice, CD11c-deficient mice, and obese humans were studied. CD11c, leukocytes, and chemokines/cytokines were examined in AT and/or blood by flow cytometry, RNase protection assay, quantitative polymerase chain reaction, or enzyme-linked immunosorbent assay. Obese C57BL/6 mice had increased CD11c in AT and blood compared with lean controls. CD11c messenger RNA positively correlated with monocyte chemoattractant protein 1 in human visceral AT. Obese humans with metabolic syndrome had a higher CD11c level on blood monocytes compared with lean humans. Low-fat diet-induced weight loss reduced blood monocyte CD11c in obese mice and humans. Mouse and human monocyte CD11c levels and mouse AT CD11c messenger RNA correlated with insulin resistance. CD11c deficiency in mice did not alter weight gain but decreased inflammation, evidenced by a lower T-cell number and reduced levels of major histocompatibility complex class II, C-C chemokine ligand 2 (CCL5), CCL4, and interferon gamma in AT, and ameliorated insulin resistance and glucose intolerance associated with diet-induced obesity.

CONCLUSIONS

Diet-induced obesity increased CD11c in both AT and blood in mice and humans. CD11c plays an important role in T-cell accumulation and activation in AT, and contributes to insulin resistance associated with obesity.

Conditional disruption of Pkd1 in osteoblasts results in osteopenia due to direct impairment of bone formation

PKD1 (polycystin-1), the disease-causing gene for ADPKD, is widely expressed in various cell types, including osteoblasts, where its function is unknown. Although global inactivation of Pkd1 in mice results in abnormal skeletal development, the presence of polycystic kidneys and perinatal lethality confound ascertaining the direct osteoblastic functions of PKD1 in adult bone. To determine the role of PKD1 in osteoblasts, we conditionally inactivated Pkd1 in postnatal mature osteoblasts by crossing Oc (osteocalcin)-Cre mice with floxed Pkd1 (Pkd1(flox/m1Bei)) mice to generate conditional heterozygous (Oc-Cre;Pkd1(flox/+)) and homozygous (Oc-Cre;Pkd1(flox/m1Bei)) Pkd1-deficient mice. Cre-mediated recombination (Pkd1(Delta flox)) occurred exclusively in bone. Compared with control mice, the conditional deletion of Pkd1 from osteoblasts resulted in a gene dose-dependent reduction in bone mineral density, trabecular bone volume, and cortical thickness. In addition, mineral apposition rates and osteoblast-related gene expression, including Runx2-II (Runt-related transcription factor 2), osteocalcin, osteopontin, and bone sialoprotein, were reduced proportionate to the reduction of Pkd1 gene dose in bone of Oc-Cre;Pkd1(flox/+) and Oc-Cre;Pkd1(flox/m1Bei) mice. Primary osteoblasts derived from Oc-Cre;Pkd1(flox/m1Bei) displayed impaired differentiation and suppressed activity of the phosphatidylinositol 3-kinase-Akt-GSK3beta-beta-catenin signaling pathways. The conditional deletion of Pkd1 also resulted in increased adipogenesis in bone marrow and in osteoblast cultures. Thus, PKD1 directly functions in osteoblasts to regulate bone formation.

Gene profiling of human adipose tissue during evoked inflammation in vivo

OBJECTIVE

Adipose inflammation plays a central role in obesity-related metabolic and cardiovascular complications. However, few human adipose-secreted proteins are known to mediate these processes. We hypothesized that microarray mRNA profiling of human adipose during evoked inflammation could identify novel adipocytokines.

RESEARCH DESIGN AND METHODS

Healthy human volunteers (n = 14) were treated with intravenous endotoxin (3 ng/kg lipopolysaccharide [LPS]) and underwent subcutaneous adipose biopsies before and after LPS. On Affymetrix U133Plus 2.0 arrays, adipose mRNAs modulated >1.5-fold (with P < 0.00001) were selected. SignalP 3.0 and SecretomeP 2.0 identified genes predicted to encode secreted proteins. Of these, 86 candidates were chosen for validation in adipose from an independent human endotoxemia protocol (N = 7, with 0.6 ng/kg LPS) and for exploration of cellular origin in primary human adipocytes and macrophages in vitro.

RESULTS

Microarray identified 776 adipose genes modulated by LPS; 298 were predicted to be secreted. Of detectable prioritized genes, 82 of 85 (96% [95% CI 90-99]) were upregulated (fold changes >1.0) during the lower-dose (LPS 0.6 ng/kg) validation study and 51 of 85 (59% [49-70]) were induced greater than 1.5-fold. Treatment of primary adipocytes with LPS and macrophage polarization to M1 proinflammatory phenotype increased expression by 1.5-fold for 58 and 73% of detectable genes, respectively.

CONCLUSIONS

We demonstrate that evoked inflammation of human adipose in vivo modulated expression of multiple genes likely secreted by adipocytes and monocytes. These included established adipocytokines and chemokines implicated in recruitment and activation of lymphocytes, adhesion molecules, antioxidants, and several novel genes with unknown function. Such candidates may represent biomarkers and therapeutic targets for obesity-related complications.

Interferon gamma attenuates insulin signaling, lipid storage, and differentiation in human adipocytes via activation of the JAK/STAT pathway

Recent reports demonstrate T-cell infiltration of adipose tissue in early obesity. We hypothesized that interferon (IFN) gamma, a major T-cell inflammatory cytokine, would attenuate human adipocyte functions and sought to establish signaling mechanisms. Differentiated human adipocytes were treated with IFNgamma +/- pharmacological inhibitors prior to insulin stimulation. [(3)H]Glucose uptake and AKT phosphorylation were assessed as markers of insulin sensitivity. IFNgamma induced sustained loss of insulin-stimulated glucose uptake in human adipocytes, coincident with reduced Akt phosphorylation and down-regulation of the insulin receptor, insulin receptor substrate-1, and GLUT4. Loss of adipocyte triglyceride storage was observed with IFNgamma co-incident with reduced expression of peroxisome proliferator-activated receptor gamma, adiponectin, perilipin, fatty acid synthase, and lipoprotein lipase. Treatment with IFNgamma also blocked differentiation of pre-adipocytes to the mature phenotype. IFNgamma-induced robust STAT1 phosphorylation and SOCS1 mRNA expression, with modest, transient STAT3 phosphorylation and SOCS3 induction. Preincubation with a non-selective JAK inhibitor restored glucose uptake and Akt phosphorylation while completely reversing IFNgamma suppression of adipogenic mRNAs and adipocyte differentiation. Specific inhibition of JAK2 or JAK3 failed to block IFNgamma effects suggesting a predominant role for JAK1-STAT1. We demonstrate that IFNgamma attenuates insulin sensitivity and suppresses differentiation in human adipocytes, an effect most likely mediated via sustained JAK-STAT1 pathway activation.

Receptors for tumor necrosis factor-alpha play a protective role against obesity and alter adipose tissue macrophage status

TNF-alpha signals through two receptors, TNFR1 and TNFR2. Our goals were: 1) determine the role of TNFRs in obesity and metabolic disease and 2) investigate whether TNFRs contribute to the link between obesity and adipose tissue macrophage infiltration and polarization. R1(-/-)R2(-/-) (RKO) and wild-type (WT) mice were fed standard chow or a high-fat/high-sucrose diet (HFHS) over 14 wk. Body composition, food intake, and energy expenditure were measured. Oral glucose tolerance and insulin sensitivity tests assessed glucose homeostasis. Adipose tissue and systemic inflammatory status were evaluated by quantifying plasma adipokine levels and macrophage-specific gene expression in fat. RKO mice were heavier (10%) and fatter (18%) than WT controls at 4 wk of age and were 26% heavier and 50% fatter than WT after 14 wk of HFHS diet feeding. Age- and diet-adjusted 24-h oxygen consumption, activity, and respiratory exchange ratio were significantly reduced in RKO mice. Obese RKO mice were markedly insulin resistant, suggesting that intact TNFR signaling is not required for the effect of obesity to impair glucose metabolism. Adipose tissue from HFHS-fed RKO mice exhibited increased macrophage infiltration, but compared with WT mice, macrophage phenotypic markers featured a predominance of antiinflammatory M2 over proinflammatory M1 cells. TNFRs play a physiological role to limit body weight and adiposity by modestly increasing metabolic rate and fatty acid oxidation, and they are required for obesity-induced activation of adipose tissue macrophages. Despite these effects, TNFRs are not required for obesity-induced insulin resistance.

Anti-inflammatory effects of nicotinic acid in adipocytes demonstrated by suppression of fractalkine, RANTES, and MCP-1 and upregulation of adiponectin

OBJECTIVE

A major site of action for the atheroprotective drug nicotinic acid (NA) is adipose tissue, via the G-protein-coupled receptor, GPR109A. Since, adipose tissue is an active secretory organ that contributes both positively and negatively to systemic inflammatory processes associated with cardiovascular disease, we hypothesized that NA would act directly upon adipocytes to alter the expression of pro-inflammatory chemokines, and the anti-inflammatory adipokine adiponectin.

METHODS AND RESULTS

TNF-alpha treatment (1.0ng/mL) of 3T3-L1 adipocytes resulted in an increase in gene expression of fractalkine (9+/-3.3-fold, P<0.01); monocyte chemoattractant protein-1 (MCP-1) (24+/-1.2-fold, P<0.001), 'regulated upon activation, normal T cell expressed and secreted' (RANTES) (500+/-55-fold, P<0.001) and inducible nitric oxide synthase (iNOS) (200+/-70-fold, P<0.05). The addition of NA (10(-4)M) to TNF-alpha-treated adipocytes attenuated expression of fractalkine (50+/-12%, P<0.01); MCP-1 (50+/-6%, P<0.01), RANTES (70+/-3%, P<0.01) and iNOS (60+/-16%). This pattern was mirrored in protein released from the adipocytes into the surrounding media. The effect on gene expression was neutralised by pre-treatment with pertussis toxin. NA attenuated macrophage chemotaxis (by 27+/-3.5%, P<0.001) towards adipocyte conditioned media. By contrast, NA, (10(-6)-10(-3)M) increased, in a dose-dependent manner, mRNA of the atheroprotective hormone adiponectin (3-5-fold n=6, P<0.01).

CONCLUSIONS

NA suppresses pro-atherogenic chemokines and upregulates the atheroprotective adiponectin through a G-protein-coupled pathway. Since adipose tissue has the potential to contribute to both systemic and local (perivascular) inflammation associated with atherosclerosis our results suggest a new "pleiotropic" role for NA.

Interplay between human adipocytes and T lymphocytes in obesity: CCL20 as an adipochemokine and T lymphocytes as lipogenic modulators

OBJECTIVE

Adipose tissue (AT) plays a major role in the low-grade inflammatory state associated with obesity. The aim of the present study was to characterize the human AT lymphocytes (ATLs) and to analyze their interactions with adipocytes.

METHODS AND RESULTS

Human ATL subsets were characterized by flow cytometry in subcutaneous ATs from 92 individuals with body mass index (BMI) ranging from 19 to 43 kg/m(2) and in paired biopsies of subcutaneous and visceral AT from 45 class II/III obese patients. CD3(+) ATLs were composed of effector and memory CD4(+) helper and CD8(+) cytotoxic T cells. The number of ATLs correlated positively with BMI and was higher in visceral than subcutaneous AT. Mature adipocytes stimulated the migration of ATLs and released the chemokine CCL20, the receptor of which (CCR6) was expressed in ATLs. The expression of adipocyte CCL20 was positively correlated with BMI and increased in visceral compared to subcutaneous adipocytes. ATLs expressed inflammatory markers and released interferon gamma (IFN gamma). Progenitor and adipocyte treatment with ATL-conditioned media reduced the insulin-mediated upregulation of lipogenic enzymes, an effect involving IFN gamma.

CONCLUSIONS

Therefore, crosstalk occurs between adipocytes and lymphocytes within human AT involving T cell chemoattraction by adipocytes and modulation of lipogenesis by ATLs.

Genetic deficiency and pharmacological stabilization of mast cells reduce diet-induced obesity and diabetes in mice

Although mast cell functions classically relate to allergic responses1–3, recent studies indicate that these cells contribute to other common diseases such as multiple sclerosis, rheumatoid arthritis, atherosclerosis, aortic aneurysm, and cancer4–8. This study presents evidence that mast cells contribute importantly to diet-induced obesity and diabetes. White adipose tissues (WAT) from obese humans and mice contain more mast cells than WAT from their lean counterparts. Genetically determined mast cell deficiency and pharmacological stabilization of mast cells in mice reduce body weight gain and levels of inflammatory cytokines, chemokines, and proteases in serum and WAT, in concert with improved glucose homeostasis and energy expenditure. Mechanistic studies reveal that mast cells contribute to WAT and muscle angiogenesis and associated cell apoptosis and cathepsin activity. Adoptive transfer of cytokine-deficient mast cells established that these cells contribute to mice adipose tissue cysteine protease cathepsin expression, apoptosis, and angiogenesis, thereby promoting diet-induced obesity and glucose intolerance by production of IL6 and IFN-γ. Mast cell stabilizing agents in clinical use reduced obesity and diabetes in mice, suggesting the potential of developing novel therapies for these common human metabolic disorders.

Normalization of obesity-associated insulin resistance through immunotherapy

Progressive obesity and its associated metabolic syndromes represent a globally growing challenge, yet mechanistic understanding and current therapeutics are unsatisfactory. We discovered that CD4+ T-lymphocytes, resident in visceral adipose tissue (VAT), control insulin-resistance in diet-induced obese (DIO) mice and likely humans. DIO VAT-associated T cells display biased TCR-Vα repertoires suggesting antigen-specific expansion. CD4+ T-lymphocyte control of glucose homeostasis is compromised in DIO when VAT accumulates pathogenic IFNγ-secreting Th1 cells, overwhelming static numbers of Th2 (CD4+GATA-3+) and regulatory Foxp3+ T cells. CD4+ T cell transfer into DIO, lymphocyte-free RAG^null mice reversed weight gain and insulin resistance predominately through Th2 cells. Brief systemic treatment with αCD3 antibody or its F(ab′)₂ fragment, restores the Th1/Foxp3+ balance and reverses insulin resistance for months, despite continuing high-fat diet. The progression of obesity-associated metabolic abnormalities is physiologically under CD4+ T cell control, with expansion of adipose tissue-resident T cells that can be manipulated by immunotherapy.

Depot-specific differences in inflammatory mediators and a role for NK cells and IFN-gamma in inflammation in human adipose tissue

BACKGROUND

Adipose tissue is a primary in vivo site of inflammation in obesity. Excess visceral adipose tissue (VAT), when compared to subcutaneous adipose tissue (SAT), imparts an increased risk of obesity-related comorbidities and mortality, and exhibits differences in inflammation. Defining depot-specific differences in inflammatory function may reveal underlying mechanisms of adipose-tissue-based inflammation.

METHODS

Stromovascular cell fractions (SVFs) from VAT and SAT from obese humans undergoing bariatric surgery were studied in an in vitro culture system with transcriptional profiling, flow cytometric phenotyping, enzyme-linked immunosorbent assay and intracellular cytokine staining.

RESULTS

Transcriptional profiling of SVF revealed differences in inflammatory transcript levels in VAT relative to SAT, including elevated interferon-gamma (IFN-gamma) transcript levels. VAT demonstrated a broad leukocytosis relative to SAT that included macrophages, T cells and natural killer (NK) cells. IFN-gamma induced a proinflammatory cytokine expression pattern in SVF and adipose tissue macrophages (ATM). NK cells, which constitutively expressed IFN-gamma, were present at higher frequency in VAT relative to SAT. Both T and NK cells from SVF expressed IFN-gamma on activation, which was associated with tumor necrosis factor-alpha expression in macrophages.

CONCLUSION

These data suggest involvement of NK cells and IFN-gamma in regulating ATM phenotype and function in human obesity and a potential mechanism for the adverse physiologic effects of VAT.

Unexpected trafficking of immune cells within the adipose tissue during the onset of obesity

The primary inflammatory events occurring in the adipose tissue (AT) during high fat diet (HFD)-induced obesity are poorly defined. The present study was undertaken to characterize, in wild-type(+/+) and lymphocyte deficient RAG2(-/-) mice under HFD, the changes in AT immune cells by flow cytometry analyses. In (+/+) mice, early accumulation of AT B-cells was observed, followed by increased AT T-cell numbers and finally by the appearance of insulin resistance and AT macrophage accumulation. Lack of lymphocytes in the RAG2(-/-) mice did not affect the onset of obesity and the state of insulin resistance. However, a striking accumulation of AT NK cells and activated macrophages was detected. The present study demonstrates that AT is the site of an unexpected dynamic in innate and adaptive cells during diet-induced obesity and insulin resistance. Moreover it appears that early AT lymphocyte infiltration could be considered a protective process to temper adipose tissue inflammation.

Reduced energy expenditure and increased inflammation are early events in the development of ovariectomy-induced obesity

Menopause, an age-related loss of ovarian hormone production, promotes increased adiposity and insulin resistance. However, the diet-independent mechanism by which loss of ovarian function promotes increased adipose tissue mass and associated metabolic pathologies remains unclear. To address this question, we monitored food intake and weight gain of ovariectomized (OVX) mice and sham OVX (SHM) mice for 12 wk. Although food intake was similar, OVX mice gained 25% more weight than SHM mice. Moreover, the OVX mice accumulated 4.7- and 4.4-fold more perigonadal and inguinal adipose tissue by weight, respectively, with 4.4-fold (perigonadal, P < 0.001) and 5.3-fold (inguinal, P < 0.01) larger adipocytes and no change in adipocyte cell number. OVX-induced adiposity was coincident with an 18% decrease in metabolic rate during the dark phase (P = 0.001) as well as an 11% decrease during the light phase (P = 0.03). In addition, ambulatory activity levels of OVX mice were decreased only during the dark phase (40%, P = 0.008). OVX mice displayed evidence of immune infiltration and inflammation in adipose tissue, because perigonadal and inguinal adipose depots from OVX mice had increased expression of TNFalpha, iNOS, CD11c, and other hallmarks of adipose tissue inflammation. In contrast, expression of the T cell marker CD3 (3.5-fold, P = 0.03) and Th1 cytokine interferon-gamma (IFNgamma) (2.6-fold, P = 0.02) were elevated in perigonadal but not sc fat. Finally, histology revealed OVX-specific liver hepatic steatosis, coincident with increased PPARgamma gene expression and downstream lipogenic gene expression. In summary, OVX in mice decreases energy expenditure, without altering energy intake, resulting in adipocyte hypertrophy, adipose tissue inflammation, and hepatic steatosis.

T cell-mediated inflammation in adipose tissue does not cause insulin resistance in hyperlipidemic mice

Obesity is associated with chronic inflammation in adipose tissue. Proinflammatory cytokines including tumor necrosis factor-alpha and interleukin-6 secreted by adipose tissue during the metabolic syndrome are proposed to cause local and general insulin resistance and promote development of type 2 diabetes. We have used a compound mutant mouse, Apoe(-/-)xCD4dnTGFbR, with dysregulation of T-cell activation, excessive production of proinflammatory cytokines, hyperlipidemia, and atherosclerosis, to dissect the role of inflammation in adipose tissue metabolism. These mice are lean, which avoids confounding effects of concomitant obesity. Expression and secretion of a set of proinflammatory factors including tumor necrosis factor-alpha, interferon-gamma, and monocyte chemoattractant protein-1 was increased in adipose tissue of Apoe(-/-)xCD4dnTGFbR mice, as was the enzyme 11beta-hydroxysteroid dehydrogenase type 1, which converts cortisone to bioactive cortisol. Interleukin-6, which has an inhibitory glucocorticoid response element in its promoter, was not upregulated. In spite of intense local inflammation, insulin sensitivity was not impaired in adipose tissue of Apoe(-/-)xCD4dnTGFbR mice unless exogenous interleukin-6 was administered. In conclusion, T-cell activation causes inflammation in adipose tissue but does not lead to insulin resistance in this tissue in the absence of interleukin-6.

Interleukin-17A inhibits adipocyte differentiation in human mesenchymal stem cells and regulates pro-inflammatory responses in adipocytes

The immune system is closely linked to human metabolic diseases. Serum levels of IL-6 increase with obesity and insulin resistance. Not only does IL-6 decrease the insulin sensitivity of human cells such as adipocytes, but it also regulates the lineage commitment of naïve T cells into interleukin (IL)-17A-producing CD4(+) T (Th17) cells. Although IL-17A exerts a variety of effects on somatic tissues, its functional role in human adipocytes has not been identified. In this work, we show that IL-17A inhibits adipocyte differentiation in human bone marrow mesenchymal stem cells (hBM-MSCs), while promoting lipolysis of differentiated adipocytes. We find that IL-17A increases both mRNA and protein secretion of IL-6 and IL-8 during adipocyte differentiation in hBM-MSCs. IL-17A up-regulates cyclooxygenase (COX)-2 gene expression and thereby increases the level of prostaglandin (PG) E(2) in differentiated adipocyes. The suppression of anti-adipogenic PGE(2) by COX inhibitors such as aspirin and NS-398 partially blocked the effect of IL-17A on adipocyte differentiation in hBM-MSCs. Therefore, IL-17A exhibits its inhibitory effect in part via the COX-2 induction in differentiated adipocytes. In addition, treatment with anti-IL-17A antibody neutralizes IL-17A-mediated effects on adipocyte differentiation and function. These results suggest that IL-17A plays a regulatory role in both the metabolic and inflammatory processes of human adipocytes, similar to other pro-inflammatory cytokines such as IL-1, IFNgamma, and TNFalpha.

Diet and asthma--can you change what you or your children are by changing what you eat?

In this review we focus on dietary fat content and subsequent effects on asthma. According to the World Health Organisation over 300 million people currently have asthma. The majority of asthma cases are 'extrinsic' and result from inappropriate 'allergic' immune responses to inhaled environmental substances. Whilst some individuals are allergic to particular food components it is becoming clear that the content of the diet can more generally affect the health of the immune system. Components of maternal and early life diets have been reported to influence offspring immune function and asthma. There has been speculation that different types of dietary fat have pro- and anti-inflammatory effects but the results of various studies are contradictory. Asthma and obesity are two conditions that have almost simultaneously reached epidemic levels in some societies. There is evidence that diet-induced obesity alters immune function and there is little doubt that consumption of a high caloric diet with high fat content leads to obesity. However, there is conflicting information over whether and how obesity is linked to asthma in children and adults. Whilst obesity is to be avoided there is accumulating evidence that dietary fat per se does not necessarily predispose towards allergic symptoms.

High-fat feeding redirects cytokine responses and decreases allergic airway eosinophilia

BACKGROUND

Dietary fat intake has been associated with obesity and obesity in its turn with attenuated airway function and asthma, but it is unclear whether or how high-fat intake per se alters immune function relevant to development of allergic asthma.

OBJECTIVE

To use a non-obese mouse model of mild to moderate allergic asthma to compare effects of high-fat with isocaloric control-diet on allergic immune responses.

METHODS

C57BL/6 mice weaned and maintained on control (11% fat calories) or isocaloric high-fat diet (58% fat calories) were systemically sensitized with ovalbumin and challenged in the lungs. Allergic airway inflammation was assessed by measuring lung inflammation; serum antibodies; and, cytokines in serum, bronchoalveolar lavage (BAL) fluid and in supernatants of in vitro stimulated lung draining lymph node and spleen lymphocytes.

RESULTS

There was a significant reduction in lung eosinophilia and IL-5 in high-fat fed mice. Lung draining lymph node cells from these mice showed reduced pro-inflammatory cytokine (MCP-1 and TNF-alpha) release after ovalbumin re-stimulation and reduced release of IL-13 after concanavalin-A stimulation, indicating a general rather than just an antigen-specific change. There was no difference in IFN-gamma release. In contrast, pro-inflammatory cytokine release was increased from splenocytes. Decreased eosinophilia was not due to increased regulatory T cell or IL-10 induction in draining lymph nodes or spleen, nor to changes in antibody response to ovalbumin. However, decreased levels of serum and BAL eotaxin were found in high-fat fed animals.

CONCLUSIONS

The data indicate that high-fat dietary content redirects local immune responses to allergen in the lungs and systemic responses in the spleen and serum. These effects are not due to changes in regulatory T cell populations but may reflect a failure to mobilize eosinophils in response to allergic challenge.

Proinflammatory phenotype of perivascular adipocytes: influence of high-fat feeding

Adipose tissue depots originate from distinct precursor cells, are functionally diverse, and modulate disease processes in a depot-specific manner. However, the functional properties of perivascular adipocytes, and their influence on disease of the blood vessel wall, remain to be determined. We show that human coronary perivascular adipocytes exhibit a reduced state of adipocytic differentiation as compared with adipocytes derived from subcutaneous and visceral (perirenal) adipose depots. Secretion of antiinflammatory adiponectin is markedly reduced, whereas that of proinflammatory cytokines interleukin-6, interleukin-8, and monocyte chemoattractant protein-1, is markedly increased in perivascular adipocytes. These depot-specific differences in adipocyte function are demonstrable in both freshly isolated adipose tissues and in vitro-differentiated adipocytes. Murine aortic arch perivascular adipose tissues likewise express lower levels of adipocyte-associated genes as compared with subcutaneous and visceral adipose tissues. Moreover, 2 weeks of high-fat feeding caused further reductions in adipocyte-associated gene expression, while upregulating proinflammatory gene expression, in perivascular adipose tissues. These changes were observed in the absence of macrophage recruitment to the perivascular adipose depot. We conclude that perivascular adipocytes exhibit reduced differentiation and a heightened proinflammatory state, properties that are intrinsic to the adipocytes residing in this depot. Dysfunction of perivascular adipose tissue induced by fat feeding suggests that this unique adipose depot is capable of linking metabolic signals to inflammation in the blood vessel wall.