Immune modulation of brown(ing) adipose tissue in obesity

Obesity is associated with a variety of medical conditions such as type 2 diabetes and cardiovascular diseases and is therefore responsible for high morbidity and mortality rates. Increasing energy expenditure by brown adipose tissue (BAT) is a current novel strategy to reduce the excessive energy stores in obesity. Brown adipocytes burn energy to generate heat and are mainly activated upon cold exposure. As prolonged cold exposure is not a realistic therapy, researchers worldwide are searching for novel ways to activate BAT and/or induce beiging of white adipose tissue. Recently, the contribution of immune cells in the regulation of brown adipocyte activity and beiging of white adipose tissue has gained increased attention, with a prominent role for eosinophils and alternatively activatedmacrophages. This review discusses the rediscovery of BAT, presents an overview of modes of activation and differentiation of beige and brown adipocytes, and describes the recently discovered immunological pathways that are key in mediating brown/beige adipocyte development and function. Interventions in immunological pathways harbor the potential to provide novel strategies to increase beige and brown adipose tissue activity as a therapeutic target for obesity.