[What if the origin of FAPs was contributing to their heterogeneity in muscle?] / Et si l'origine des progéniteurs fibro-adipeux contribuait à leur hétérogénéité dans le muscle ?

Fibro-adipogenic progenitors (FAPs) are resident mesenchymal stromal cells (MSCs) of skeletal muscle. They play a crucial role in muscle homeostasis and regeneration through their paracrine activity. Recent technological advances in single-cell RNA sequencing have allowed the characterization of the heterogeneity within this cell population. In this article, we will present the different subpopulations of FAPs under basal, injury, or degenerative conditions, as well as their associated functions in mice and humans. We will then discuss the potential extramuscular origin of a post-injury FAP population. Indeed, our recent work demonstrates that MSCs from adipose tissue, infiltrating the muscle, could contribute to FAP heterogeneity.

Title: Et si l'origine des progéniteurs fibro-adipeux contribuait à leur hétérogénéité dans le muscle ? Abstract: Les progéniteurs fibro-adipogéniques (FAPs), cellules stromales mésenchymateuses (CSMs) résidentes du muscle squelettique, jouent un rôle crucial dans l'homéostasie et la régénération musculaire via leur activité paracrine. Les avancées technologiques récentes dans le domaine du séquençage de l'ARN en cellule unique ont permis la description de l'hétérogénéité de cette population cellulaire. Dans cet article, nous présenterons les différentes sous-populations de FAPs en condition basale, lésionnelle ou de dégénérescence, ainsi que leurs fonctions associées chez la souris et l'homme. Nous discuterons ensuite de l'origine extra-musculaire possible d'une population de FAPs post-lésionnelle. En effet, nos travaux récents démontrent que des CSMs provenant du tissu adipeux et infiltrées dans le muscle pourraient participer à l'hétérogénéité des FAPs.

Adipose tissue is a source of regenerative cells that augment the repair of skeletal muscle after injury.

Fibro-adipogenic progenitors (FAPs) play a crucial role in skeletal muscle regeneration, as they generate a favorable niche that allows satellite cells to perform efficient muscle regeneration. After muscle injury, FAP content increases rapidly within the injured muscle, the origin of which has been attributed to their proliferation within the muscle itself. However, recent single-cell RNAseq approaches have revealed phenotype and functional heterogeneity in FAPs, raising the question of how this differentiation of regenerative subtypes occurs. Here we report that FAP-like cells residing in subcutaneous adipose tissue (ScAT), the adipose stromal cells (ASCs), are rapidly released from ScAT in response to muscle injury. Additionally, we find that released ASCs infiltrate the damaged muscle, via a platelet-dependent mechanism and thus contribute to the FAP heterogeneity. Moreover, we show that either blocking ASCs infiltration or removing ASCs tissue source impair muscle regeneration. Collectively, our data reveal that ScAT is an unsuspected physiological reservoir of regenerative cells that support skeletal muscle regeneration, underlining a beneficial relationship between muscle and fat.

A deep learning tool without muscle-by-muscle grading to differentiate myositis from facio-scapulo-humeral dystrophy using MRI.

PURPOSE: The purpose of this study was to assess the capabilities of a deep learning (DL) tool to discriminate between type 1 facioscapulo-humeral dystrophy (FSHD1) and myositis using whole-body muscle magnetic resonance imaging (MRI) examination without the need for visual grading of muscle signal changes. MATERIALS AND METHODS: A total of 40 patients who underwent whole-body MRI examination that included T1-weighted and STIR sequences were included. There were 19 patients with proven FSHD1 (9 men, 10 women; mean age, 47.7 ± 18.0 [SD] years; age range: 20-72 years) and 21 patients with myositis fulfilling European Neuromuscular Centre criteria and European League Against Rheumatism and American College of Rheumatology criteria (11 men, 10 women; mean age, 59.3 ± 17.0 [SD]; age range: 19-78 years). Based on thigh, calf, and shoulder sections a supervised training of a neural network was performed and its diagnostic performance was studied using a 5-fold cross validation method and compared to the results obtained by two radiologists specialized in musculoskeletal imaging. RESULTS: The DL tool was able to differentiate FSHD1 from myositis with a correct classification percentage respectively of 69 % (95% CI: 39-99), 75% (95% CI: 48-100) and 77% (95% CI: 60-94) when thigh only, thigh and calf or the thigh, calf, and shoulder MR images were analyzed. The percentages of correct classification of the two radiologists for these later MR images were 38/40 (95%) and 35/40 (87.5%), respectively; with no differences with DL tool correct classification (P = 0.41 and P > 0.99, respectively). Among the seven patients who were misclassified by the radiologists, the DL tool correctly classified six of them. CONCLUSION: A DL tool was developed to discriminate between FSHD1 and myositis using whole-body MRI with performances equivalent to those achieved by two radiologists. This study provides a proof of concept of the effectiveness of a DL approach to distinguish between two myopathies using MRI with a small amount of data, and no prior muscle signal changes grading.

Browning Epicardial Adipose Tissue: Friend or Foe?

The epicardial adipose tissue (EAT) is the visceral fat depot of the heart which is highly plastic and in direct contact with myocardium and coronary arteries. Because of its singular proximity with the myocardium, the adipokines and pro-inflammatory molecules secreted by this tissue may directly affect the metabolism of the heart and coronary arteries. Its accumulation, measured by recent new non-invasive imaging modalities, has been prospectively associated with the onset and progression of coronary artery disease (CAD) and atrial fibrillation in humans. Recent studies have shown that EAT exhibits beige fat-like features, and express uncoupling protein 1 (UCP-1) at both mRNA and protein levels. However, this thermogenic potential could be lost with age, obesity and CAD. Here we provide an overview of the physiological and pathophysiological relevance of EAT and further discuss whether its thermogenic properties may serve as a target for obesity therapeutic management with a specific focus on the role of immune cells in this beiging phenomenon.

Endogenous Mobilization of Mesenchymal Stromal Cells: A Pathway for Interorgan Communication?

To coordinate specialized organs, inter-tissue communication appeared during evolution. Consequently, individual organs communicate their states via a vast interorgan communication network (ICN) made up of peptides, proteins, and metabolites that act between organs to coordinate cellular processes under homeostasis and stress. However, the nature of the interorgan signaling could be even more complex and involve mobilization mechanisms of unconventional cells that are still poorly described. Mesenchymal stem/stromal cells (MSCs) virtually reside in all tissues, though the biggest reservoir discovered so far is adipose tissue where they are named adipose stromal cells (ASCs). MSCs are thought to participate in tissue maintenance and repair since the administration of exogenous MSCs is well known to exert beneficial effects under several pathological conditions. However, the role of endogenous MSCs is barely understood. Though largely debated, the presence of circulating endogenous MSCs has been reported in multiple pathophysiological conditions, but the significance of such cell circulation is not known and therapeutically untapped. In this review, we discuss current knowledge on the circulation of native MSCs, and we highlight recent findings describing MSCs as putative key components of the ICN.

The Release of Adipose Stromal Cells from Subcutaneous Adipose Tissue Regulates Ectopic Intramuscular Adipocyte Deposition.

Ectopic lipid deposition (ELD) is defined by excess fat storage in locations not classically associated with adipose tissue (AT) storage. ELD is positively correlated with insulin resistance and increased risk of metabolic disorders. ELD appears as lipid droplets or adipocytes, whose cell origin is unknown. We previously showed that subcutaneous AT (ScAT) releases adipocyte progenitors into the circulation. Here, we demonstrate that triggering or preventing the release of adipocyte precursors from ScAT directly promoted or limited ectopic adipocyte formation in skeletal muscle in mice. Importantly, obesity-associated metabolic disorders could be mimicked by causing adipocyte precursor release without a high-fat diet. Finally, during nutrient overload, adipocyte progenitors exited ScAT, where their retention signals (CXCR4/CXCL12 axis) were greatly decreased, and further infiltrated skeletal muscles. These data provide insights into the formation of ELD associated with calorie overload and highlight adipocyte progenitor trafficking as a potential target in the treatment of metabolic diseases.

Surplus fat rapidly increases fat oxidation and insulin resistance in lipodystrophic mice.

OBJECTIVE: Surplus dietary fat cannot be converted into other macronutrient forms or excreted, so has to be stored or oxidized. Healthy mammals store excess energy in the form of triacylgycerol (TAG) in lipid droplets within adipocytes rather than oxidizing it, and thus ultimately gain weight. The 'overflow hypothesis' posits that the capacity to increase the size and number of adipocytes is finite and that when this limit is exceeded, fat accumulates in ectopic sites and leads to metabolic disease. METHODS: Here we studied the energetic and biochemical consequences of short-term (2-day) excess fat ingestion in a lipodystrophic (A-ZIP/F-1) mouse model in which adipose capacity is severely restricted. RESULTS: In wildtype littermates, this acute exposure to high fat diets resulted in excess energy intake and weight gain without any significant changes in macronutrient oxidation rates, glucose, TAG, or insulin levels. In contrast, hyperphagic lipodystrophic mice failed to gain weight; rather, they significantly increased hepatic steatosis and fat oxidation. This response was associated with a significant increase in hyperglycemia, hyperinsulinemia, glucosuria, hypertriglyceridemia, and worsening insulin tolerance. CONCLUSIONS: These data suggest that when adipose storage reserves are saturated, excess fat intake necessarily increases fat oxidation and induces oxidative substrate competition which exacerbates insulin resistance resolving any residual energy surplus through excretion of glucose.

Cholecystokinin is involved in triglyceride fatty acid uptake by rat adipose tissue.

The incorporation of plasma triglyceride (TG) fatty acids to white adipose tissue (WAT) depends on lipoprotein lipase (LPL), which is regulated by angiopoietin-like protein-4 (ANGPTL-4), an unfolding molecular chaperone that converts active LPL dimers into inactive monomers. The production of ANGPTL-4 is promoted by fasting and repressed by feeding. We hypothesized that the postprandial hormone cholecystokinin (CCK) facilitates the storage of dietary TG fatty acids in WAT by regulating the activity of the LPL/ANGPTL-4 axis and that it does so by acting directly on CCK receptors in adipocytes. We report that administration of CCK-8 (a bioactive fragment of CCK) to rats: (i) reduces plasma ANGTPL-4 levels; (ii) represses Angptl-4 expression in WAT and (iii) simultaneously enhances LPL activity in this tissue without inducing Lpl expression. In vivo CCK-8 effects are specifically antagonized by the CCK-2 receptor (CCK-2R) antagonist, L-365,260. Moreover, CCK-8 downregulates Angptl-4 expression in wild-type pre-adipocytes, an effect that is not observed in engineered pre-adipocytes lacking CCK-2R. These effects have functional consequences as CCK-8 was found to promote the uptake of dietary fatty acids by WAT, as demonstrated by means of proton nuclear magnetic resonance (1H-NMR). The efficacy of acute CCK-8 administration was not reduced after chronic CCK-8 treatment. Moreover, the effects of CCK-8 on WAT were not associated to the increase of circulating insulin. Our results show that cholecystokinin promotes lipid storage in WAT by acting on adipocyte CCK-2R, suggesting a pivotal role for CCK in TG homeostasis.

Role of Epicardial Adipose Tissue in Health and Disease: A Matter of Fat?

Epicardial adipose tissue (EAT) is a small but very biologically active ectopic fat depot that surrounds the heart. Given its rapid metabolism, thermogenic capacity, unique transcriptome, secretory profile, and simply measurability, epicardial fat has drawn increasing attention among researchers attempting to elucidate its putative role in health and cardiovascular diseases. The cellular crosstalk between epicardial adipocytes and cells of the vascular wall or myocytes is high and suggests a local role for this tissue. The balance between protective and proinflammatory/profibrotic cytokines, chemokines, and adipokines released by EAT seem to be a key element in atherogenesis and could represent a future therapeutic target. EAT amount has been found to predict clinical coronary outcomes. EAT can also modulate cardiac structure and function. Its amount has been associated with atrial fibrillation, coronary artery disease, and sleep apnea syndrome. Conversely, a beiging fat profile of EAT has been identified. In this review, we describe the current state of knowledge regarding the anatomy, physiology and pathophysiological role of EAT, and the factors more globally leading to ectopic fat development. We will also highlight the most recent findings on the origin of this ectopic tissue, and its association with cardiac diseases. © 2017 American Physiological Society. Compr Physiol 7:1051-1082, 2017.

Differential Hematopoietic Activity in White Adipose Tissue Depending on its Localization.

White adipose tissue (WAT) can be found in different locations in the body, and these different adipose deposits exhibit specific physiopathological importance according to the subcutaneous or abdominal locations. We have shown previously the presence of functional hematopoietic stem/progenitor cells (HSPC) in subcutaneous adipose tissue (SCAT). These cells exhibit a specific hematopoietic activity that contributes to the renewal of the immune cell compartment within this adipose deposit. In this study, we investigated whether HSPC can be found in visceral adipose tissue (VAT) and whether a putative difference in in situ hematopoiesis may be related to anatomical location and to site-specific immune cell content in VAT compared to SCAT. Therein, we identified for the first time the presence of HSPC in VAT. Using both in vitro assays and in vivo competitive repopulation experiments with sorted HSPC from VAT or SCAT, we showed that the hematopoietic activity of HSPC was lower in VAT, compared to SCAT. In addition, this altered hematopoietic activity of HSPC in VAT was due to their microenvironment, and may be related to a specific combination of secreted factors and extracellular matrix molecules expressed by adipose derived stromal cells. Our results indicate that WAT specific hematopoietic activity may be generalized to all adipose deposits, although with specificity according to the fat pad location. Considering the abundance of WAT in the body, this emphasizes the potential importance of this hematopoietic activity in physiopathological situations.

Enhanced glucose metabolism is preserved in cultured primary myotubes from obese donors in response to exercise training.

CONTEXT: It was suggested that human cultured primary myotubes retain the metabolic characteristics of their donor in vitro. OBJECTIVES: The aim of the present study was to investigate whether the metabolic responses to endurance training are also conserved in culture. DESIGN AND VOLUNTEERS: Middle-aged obese subjects completed an 8-week supervised aerobic exercise training program in which vastus lateralis muscle biopsies were collected before and after training. MAIN OUTCOME MEASURES: Anthropometric and blood parameters, as well as aerobic capacity, were assessed before and after training. Muscle biopsies were either used for Western blot analysis or digested to harvest myogenic progenitors that were differentiated into myotubes. Glucose oxidation, palmitate oxidation, and glycogen synthesis assays were performed on myotubes before and after training. Gene expression was assessed by real-time quantitative PCR. RESULTS: Our data indicate that in parallel of in vivo improvement of whole-body aerobic capacity and glucose metabolism, biopsy-derived primary myotubes showed similar patterns in vitro. Indeed, glucose oxidation, glycogen synthesis, and inhibition of palmitate oxidation by glucose were enhanced in myotubes after training. This was associated with consistent changes in the expression of metabolism-linked genes such as GLUT1, PDK4, and PDHA1. Interestingly, no difference in myogenic differentiation capacity was observed before and after training. CONCLUSION: Aerobic exercise training is associated with metabolic adaptations in vivo that are preserved in human cultured primary myotubes. It can be hypothesized that skeletal muscle microenvironmental changes induced by endurance training lead to metabolic imprinting on myogenic progenitor cells.

Native adipose stromal cells egress from adipose tissue in vivo: evidence during lymph node activation.

Adipose tissue (AT) has become accepted as a source of multipotent progenitor cells, the adipose stromal cells (ASCs). In this regard, considerable work has been performed to harvest and characterize this cell population as well as to investigate the mechanisms by which transplanted ASCs mediate tissue regeneration. In contrast the endogenous release of native ASCs by AT has been poorly investigated. In this work, we show that native ASCs egress from murine AT. Indeed, we demonstrated that the release of native ASCs from AT can be evidenced both using an ex vivo perfusion model that we set up and in vivo. Such a mobilization process is controlled by CXCR4 chemokine receptor. In addition, once mobilized from AT, circulating ASCs were found to navigate through lymph fluid and to home into lymph nodes (LN). Therefore, we demonstrated that, during the LN activation, the fat depot encapsulating the activated LN releases native ASCs, which in turn invade the activated LN. Moreover, the ASCs invading the LN were visualized in close physical interaction with podoplanin and ER-TR7 positive structures corresponding to the stromal network composing the LN. This dynamic was impaired with CXCR4 neutralizing antibody. Taken together, these data provide robust evidences that native ASCs can traffic in vivo and that AT might provide stromal cells to activated LNs.

TGFbeta family members are key mediators in the induction of myofibroblast phenotype of human adipose tissue progenitor cells by macrophages.

OBJECTIVE: The present study was undertaken to characterize the remodeling phenotype of human adipose tissue (AT) macrophages (ATM) and to analyze their paracrine effects on AT progenitor cells. RESEARCH DESIGN AND METHODS: The phenotype of ATM, immunoselected from subcutaneous (Sc) AT originating from subjects with wide range of body mass index and from paired biopsies of Sc and omental (Om) AT from obese subjects, was studied by gene expression analysis in the native and activated states. The paracrine effects of ScATM on the phenotype of human ScAT progenitor cells (CD34(+)CD31(-)) were investigated. RESULTS: Two main ATM phenotypes were distinguished based on gene expression profiles. For ScAT-derived ATM, obesity and adipocyte-derived factors favored a pro-fibrotic/remodeling phenotype whereas the OmAT location and hypoxic culture conditions favored a pro-angiogenic phenotype. Treatment of native human ScAT progenitor cells with ScATM-conditioned media induced the appearance of myofibroblast-like cells as shown by expression of both α-SMA and the transcription factor SNAIL, an effect mimicked by TGFß1 and activinA. Immunohistochemical analyses showed the presence of double positive α-SMA and CD34 cells in the stroma of human ScAT. Moreover, the mRNA levels of SNAIL and SLUG in ScAT progenitor cells were higher in obese compared with lean subjects. CONCLUSIONS: Human ATM exhibit distinct pro-angiogenic and matrix remodeling/fibrotic phenotypes according to the adiposity and the location of AT, that may be related to AT microenvironment including hypoxia and adipokines. Moreover, human ScAT progenitor cells have been identified as target cells for ScATM-derived TGFß and as a potential source of fibrosis through their induction of myofibroblast-like cells.

PBX1: a novel stage-specific regulator of adipocyte development.

Although adipocyte terminal differentiation has been extensively studied, the early steps of adipocyte development and the embryonic origin of this lineage remain largely unknown. Here we describe a novel role for the pre-B-cell leukemia transcription factor one (PBX1) in adipocyte development using both mouse embryonic stem cells (mESCs) and human multipotent adipose-derived stem (hMADS) cells. We show that Pbx1(-/-) mESCs are unable to generate adipocytes, despite normal expression of neuroectoderm and neural crest (NC) markers. Early adipocyte lineage markers are not induced in Pbx1(-/-) mESCs, suggesting that Pbx1 controls the generation and/or the maintenance of adipocyte progenitors (APs) from the NC. We further characterize the function of PBX1 in postnatal adipogenesis and show that silencing of PBX1 expression in hMADS cells reduces their proliferation by preventing their entry in the S phase of the cell cycle. Furthermore, it promotes differentiation of hMADS cells into adipocytes and partially substitutes for glucocorticoids and rosiglitazone, two key proadipogenic agents. These effects involve direct modulation of PPARγ activity, most likely through regulation of the biosynthesis of PPARγ natural endogenous ligand(s). Together, our data suggest that PBX1 regulates adipocyte development at multiple levels, promoting the generation of NC-derived APs during embryogenesis, while favoring APs proliferation and preventing their commitment to the adipocyte lineage in postnatal life.

Adipose-derived stromal cells: cytokine expression and immune cell contaminants.

The present method describes an immunoselection/depletion approach to isolate the native human adipose tissue-derived progenitor cells that are free from endothelial cells and immune cells by the use of magnetic nanobeads and microbeads coupled to antibodies. Moreover, methods to isolate and to analyse the distinct cell populations that constitute the microenvironment of the human adipose tissue progenitor cells, i.e. mature adipocytes, endothelial cells, and macrophages, are mentioned.

Adipose tissue endothelial cells from obese human subjects: differences among depots in angiogenic, metabolic, and inflammatory gene expression and cellular senescence.

OBJECTIVE: Regional differences among adipose depots in capacities for fatty acid storage, susceptibility to hypoxia, and inflammation likely contribute to complications of obesity. We defined the properties of endothelial cells (EC) isolated from subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) biopsied in parallel from obese subjects. RESEARCH DESIGN AND METHODS: The architecture and properties of the fat tissue capillary network were analyzed using immunohistochemistry and flow cytometry. CD34(+)/CD31(+) EC were isolated by immunoselection/depletion. Expression of chemokines, adhesion molecules, angiogenic factor receptors, as well as lipogenic and senescence-related genes were assayed by real-time PCR. Fat cell size and expression of hypoxia-dependent genes were determined in adipocytes from both fat depots. RESULTS: Hypoxia-related genes were more highly expressed in VAT than SAT adipocytes. VAT adipocytes were smaller than SAT adipocytes. Vascular density and EC abundance were higher in VAT. VAT-EC exhibited a marked angiogenic and inflammatory state with decreased expression of metabolism-related genes, including endothelial lipase, GPIHBP1, and PPAR gamma. VAT-EC had enhanced expression of the cellular senescence markers, IGFBP3 and γ-H2AX, and decreased expression of SIRT1. Exposure to VAT adipocytes caused more EC senescence-associated ß-galactosidase activity than SAT adipocytes, an effect reduced in the presence of vascular endothelial growth factor A (VEGFA) neutralizing antibodies. CONCLUSIONS: VAT-EC exhibit a more marked angiogenic and proinflammatory state than SAT-EC. This phenotype may be related to premature EC senescence. VAT-EC may contribute to hypoxia and inflammation in VAT.

Evidence for epithelial-mesenchymal transition in adult human pancreatic exocrine cells.

It has been shown that adult pancreatic ductal cells can dedifferentiate and act as pancreatic progenitors. Dedifferentiation of epithelial cells is often associated with the epithelial-mesenchymal transition (EMT). In this study, we investigated the occurrence of EMT in adult human exocrine pancreatic cells both in vitro and in vivo. Cells of exocrine fraction isolated from the pancreas of brain-dead donors were first cultured in suspension for eight days. This led to the formation of spheroids, composed of a principal population of cells with duct-like phenotype. When cultivated in tissue culture-treated flasks, spheroid cells exhibited a proliferative capacity and coexpressed epithelial (cytokeratin7 and cytokeratin19) and mesenchymal (vimentin and alpha-smooth muscle actin) markers as well as marker of progenitor pancreatic cells (pancreatic duodenal homeobox factor-1) and surface markers of mesenchymal stem cells. The switch from E-cadherin to N-cadherin associated with Snail1 expression suggested that these cells underwent EMT. In addition, we showed coexpression of epithelial and mesenchymal markers in ductal cells of one normal adult pancreas and three type 2 diabetic pancreases. Some of the vimentin-positive cells were found to coexpress glucagon or amylase. These results point to the occurrence of EMT, which may take place on dedifferentiation of ductal cells during the regeneration or renewal of human pancreatic tissues.

Activin a plays a critical role in proliferation and differentiation of human adipose progenitors.

OBJECTIVE: Growth of white adipose tissue takes place in normal development and in obesity. A pool of adipose progenitors is responsible for the formation of new adipocytes and for the potential of this tissue to expand in response to chronic energy overload. However, factors controlling self-renewal of human adipose progenitors are largely unknown. We investigated the expression profile and the role of activin A in this process. RESEARCH DESIGN AND METHODS: Expression of INHBA/activin A was investigated in three types of human adipose progenitors. We then analyzed at the molecular level the function of activin A during human adipogenesis. We finally investigated the status of activin A in adipose tissues of lean and obese subjects and analyzed macrophage-induced regulation of its expression. RESULTS: INHBA/activin A is expressed by adipose progenitors from various fat depots, and its expression dramatically decreases as progenitors differentiate into adipocytes. Activin A regulates the number of undifferentiated progenitors. Sustained activation or inhibition of the activin A pathway impairs or promotes, respectively, adipocyte differentiation via the C/EBPß-LAP and Smad2 pathway in an autocrine/paracrine manner. Activin A is expressed at higher levels in adipose tissue of obese patients compared with the expression levels in lean subjects. Indeed, activin A levels in adipose progenitors are dramatically increased by factors secreted by macrophages derived from obese adipose tissue. CONCLUSIONS: Altogether, our data show that activin A plays a significant role in human adipogenesis. We propose a model in which macrophages that are located in adipose tissue regulate adipose progenitor self-renewal through activin A.

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.