Adipose tissue macrophages (ATMs) comprise resident macrophages present in adipose tissue. Besides adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells that includes pre-adipocytes, fibroblasts, vascular endothelial cells, and a large variety of immune cells. The latter ones are composed of mast cells, eosinophils, B cells, T cells and macrophages. The number of macrophages within adipose tissue differs depending on the metabolic status. As discovered by Rudolph Leibel and Anthony Ferrante et al. in 2003 at Columbia University, the percentage of macrophages within adipose tissue ranges from 10% in lean mice and humans up to 50% in obese leptin deficient mice, and up to 40% in obese humans. ATMs comprise nearly 50% of all immune cells in normal conditions, suggesting an important role in supporting normal functioning of the adipose tissue. Increased number of adipose tissue macrophages may correlate with increased production of pro-inflammatory molecules and might therefore contribute to the pathophysiological consequences of obesity (e.g. insulin resistance, type 2 diabetes), although is becoming recognized that in healthy conditions tissue-resident macrophages actively support a variety of critical physiological functions in nearly all organs and tissues, including adipose tissue.
