Adipocytes are the primary and most distinctive cell population within adipose tissue and are thought to be the cells most significantly altered by obesity. However, adipose tissue depots contain non-adipocyte populations, including endothelial cells, fibroblasts, adipocyte progenitors and resident macrophages. The function of these non-adipocytes in normal physiology and their alteration by obesity and aging remain largely unexplored. Recent work in our laboratory has revealed that adipose tissue macrophages (ATM's) are dramatically altered by obesity and may play an essential role in adipocyte tissue physiology. In mice, expression profiles of visceral adipose tissue reveal a strong positive correlation of macrophage transcripts with body mass and adipocyte size. Histological examination confirms increasing numbers of ATM's in perigonadal, omental and perirenal depots with increasing obesity. Remarkably, in severely obese (B6.V Lepop/op) mice, nearly 50 percent of cells within visceral adipose depots are macrophages. Obesity also alters the morphology of ATM's, leading to the appearance of multinucleated giant cells, reminiscent of those seen in chronic inflammatory conditions such as sarcoidosis and Wegner's granulomatosis. These alterations in ATM number and morphology correlate with previously reported production of pro-inflammatory cytokines and factors by obese visceral adipose tissue. Significant but less dramatic changes in ATM number and morphology are noted in subcutaneous depots. Macrophage deficient (B6C3Fe Csflop/op) mice show preferential accumulation of adipose mass in subcutaneous depots. We therefore hypothesize that obesity alters the number and activation state of ATM's that in turn modulate adipocyte development and metabolic function and may be responsible for the pro-inflammatory phenotype noted in patients with the metabolic syndrome. The aims of this proposal are to: (1) Phenotypically characterize adipose tissue macrophages from visceral and subcutaneous adipose tissue in lean and obese mice; (2) Develop in vitro systems to assay the metabolic and developmental effects of adipose tissue macrophages on adipocytes and (3) Characterize the in vivo effects of altered adipose tissue macrophage number and function on the hypertrophy and hyperplasia of adipocytes, and on systemic insulin sensitivity.