Background-Several studies have suggested that stem cells are present in the stroma-vascular fraction (SVF) of adipose tissue (AT). Methods and Results-To characterize the cell populations that compose the SVF of human AT originating from subcutaneous and visceral depots, fluorescence-activated cell sorter analysis was performed by use of fluorescent antibodies directed against the endothelial and stem cell markers CD31, CD34, CD133, and ABCG2.

Aims/hypothesis: Increased visceral white adipose tissue (WAT) is linked to the risk of developing diabetes. Methods/results: We showed by fluorescence activated cell sorting analysis that human visceral WAT contains macrophages, the proportion of which increased with obesity. Selective isolation of mature adipocytes and macrophages from human visceral WAT by CD14 immunoselection revealed that macrophages expressed higher levels of chemokines (monocyte chemotactic protein 1, macrophage inflammatory protein 1α, IL-8) and the adipokines resistin and visfatin than did mature adipocytes, as assessed by real-time PCR analysis. Moreover, resistin and visfatin proteins were found to be released predominantly by visceral WAT macrophages. Macrophage-derived secretory products stimulated phosphorylation of protein kinase B in human hepatocytes. Conclusions/interpretation: Resistin and visfatin might be considered to be proinflammatory markers. The increased macrophage population in obese human visceral WAT might be responsible for the enhanced production of chemokines as well as resistin and visfatin.

Obesity has been suggested to be a low-grade systemic inflammatory state, therefore we studied the interaction between human adipocytes and monocytes via adipose tissue (AT)-derived capillary endothelium. Cells composing the stroma-vascular fraction (SVF) of human ATs were characterized by fluorescence-activated cell sorter (FACS) analysis and two cell subsets (resident macrophages and endothelial cells [ECs]) were isolated using antibody-coupled microbeads. Media conditioned by mature adipocytes maintained in fibrin gels were applied to AT-derived ECs. Thereafter, the expression of endothelial adhesion molecules was analyzed as well as the adhesion and transmigration of human monocytes. FACS analysis showed that 11% of the SVF is composed of CD14 ؉ /CD31 ؉ cells, characterized as resident macrophages. A positive correlation was found between the BMI and the percentage of resident macrophages, suggesting that fat tissue growth is associated with a recruitment of blood monocytes. Incubation of AT-derived ECs with adipocyte-conditioned medium resulted in the upregulation of EC adhesion molecules and the increased chemotaxis of blood monocytes, an effect mimicked by recombinant human leptin. These results indicate that adipokines, such as leptin, activate ECs, leading to an enhanced diapedesis of blood monocytes, and suggesting that fat mass growth might be linked to inflammatory processes.

Background-Adipose tissue macrophages (ATMs) have become a focus of attention recently because they have been shown to accumulate with an increase in fat mass and to be involved in the genesis of insulin resistance in obese mice. However, the phenotype and functions of human ATMs are still to be defined. Methods and Results-The present study, performed on human subcutaneous AT, showed that ATMs from lean to overweight individuals are composed of distinct macrophage subsets based on the expression of several cell surface markers: CD45, CD14, CD31, CD44, HLA-DR, CD206, and CD16, as assessed by flow cytometry. ATMs isolated by an immunoselection protocol showed a mixed expression of proinflammatory (tumor necrosis factor-␣, interleukin-6 [IL-6], IL-23, monocyte chemoattractant protein-1, IL-8, cyclooxygenase-2) and antiinflammatory (IL-10, transforming growth factor-␤, alternative macrophage activation-associated cc chemokine-1, cyclooxygenase-1) factors. Fat mass enlargement is associated with accumulation of the CD206 ϩ /CD16 Ϫ macrophage subset that exhibits an M2 remodeling phenotype characterized by decreased expression of proinflammatory IL-8 and cyclooxygenase-2 and increased expression of lymphatic vessel endothelial hyaluronan receptor-1. ATMs specifically produced and released matrix metalloproteinase-9 compared with adipocytes and capillary endothelial cells, and secretion of matrix metalloproteinase-9 from human AT in vivo, assessed by arteriovenous difference measurement, was correlated with body mass index. Finally, ATMs exerted a marked proangiogenic effect on AT-derived endothelial and progenitor cells. Conclusions-The present results showed that the ATMs that accumulate with fat mass development exhibit a particular M2 remodeling phenotype. ATMs may be active players in the process of AT development through the extension of the capillary network and in the genesis of obesity-associated cardiovascular pathologies.

Obesity is considered a chronic low-grade inflammatory state. The white adipose tissue produces a variety of inflammation-related proteins whose expression is increased in obese subjects. The nonadipose cell fraction, which includes infiltrated macrophages, is a determinant source of inflammation-related molecules within the adipose tissue. Our working hypothesis is that macrophage infiltration affects fat expansion through a paracrine action on adipocyte differentiation. Human primary preadipocytes were then differentiated in the presence of conditioned media obtained from macrophages differentiated from blood monocytes. Preadipocytes treated by macrophage-conditioned medium displayed marked reduction of adipogenesis as assessed by decreased cellular lipid accumulation and reduced gene expression of adipogenic and lipogenic markers. In addition to this effect, the activation of macrophages by lipopolysaccharides stimulated nuclear factor kappaB signaling, increased gene expression and release of proinflammatory cytokines and chemokines, and induced preadipocyte proliferation. This phenomenon was associated with increased cyclin D1 gene expression and maintenance of the fibronectin-rich matrix. Anti-TNFalpha neutralizing antibody inhibits the inflammatory state of preadipocytes positioning TNFalpha as an important mediator of inflammation in preadipocytes. Strikingly, conditioned media produced by macrophages isolated from human adipose tissue exerted comparable effects with activated macrophages, i.e. decreased adipogenesis and increased inflammatory state in the preadipocytes. These data show that macrophage-secreted factors inhibit the formation of mature adipocytes, suggesting possible role in limiting adipose tissue expansion in humans.

This review summarizes recent data on the characterization of the macrophage population in fat tissue. Their origin, fate and activation will be considered. The potential involvement of adipose tissue macrophages in the development of insulin resistance and vascular pathologies, as well as in the control of adipose tissue growth and metabolism, will be examined.

ABSTRACT

Abstract. Sengenès C, Miranville A,