Background-Fat inflammation may play an important role in comorbidities associated with obesity such as atherosclerosis. Methods and Results-To first establish feasibility of fat transplantation, epididymal fat pads were harvested from wild-type C57BL/6J mice and transplanted into leptin-deficient (Lep ob/ob ) mice. Fat transplantation produced physiological leptin levels and prevented obesity and infertility in Lep ob/ob mice. However, the transplanted fat depots were associated with chronically increased macrophage infiltration with characteristics identical to those observed in fat harvested from obese animals. The inflammation in transplanted adipose depots was regulated by the same factors that have been implicated in endogenous fat inflammation such as monocyte chemoattractant protein-1. To determine whether this inflamed adipose depot could affect vascular disease in mice, epididymal fat depots were transplanted into atherosclerosis-prone apolipoprotein E-deficient ApoE Ϫ/Ϫ mice. Plasma from ApoE Ϫ/Ϫ mice receiving fat transplants contained increased leptin, resistin, and monocyte chemoattractant protein-1 compared with plasma from sham-operated ApoE Ϫ/Ϫ mice. Furthermore, mice transplanted with visceral fat developed significantly more atherosclerosis compared with sham-operated animals, whereas transplants with subcutaneous fat did not affect atherosclerosis despite a similar degree of fat inflammation. Treatment of transplanted ApoE Ϫ/Ϫ mice with pioglitazone decreased macrophage content of the transplanted visceral fat pad and reduced plasma monocyte chemoattractant protein-1. Importantly, pioglitazone also reduced atherosclerosis triggered by inflammatory visceral fat but had no protective effect on atherosclerosis in the absence of the visceral fat transplantation. Conclusions-Our

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