The Immunology of Adipose Tissue

A close relationship exists between adipose tissue and the immune system. Recent discoveries have revised the concept of adipose tissue being dedicated merely for energy storage. Instead, adipose tissue is now considered an active immune organ with numerous roles in total physiological homeostasis. Adipose tissue has traditionally been classified into white- and brown-type adipose tissue, although a third type of inducible brownlike adipose tissue or "beige" has emerged. Chronic nutrient excess leads to the expansion and dysfunction of white adipose tissue, especially in the visceral compartment. Notably, cells of the immune system infiltrate visceral adipose tissue (VAT) during obesity, in an active process that tips the balance toward a state of low-grade chronic inflammation. This inflammatory process in VAT is driven by various subsets of immune cells and is a central mechanism connecting obesity with its metabolic complications. Brown and beige adipose tissues are involved in nonshivering thermogenesis, and because of their capacity to dissipate chemical energy in the form of heat, their function may impact energy balance. Emerging evidence indicates that many innate immune cells mediate the development of beige adipose tissue, which offers tremendous potential for the treatment of obesity and its complications. Here we summarize recent studies demonstrating the cross talk between immune cells in adipose tissues, with emphasis on the immunological mechanisms altered in obesity.