Obesity alters B cell and macrophage populations in brown adipose tissue

OBJECTIVE: The prevalence of obesity continues to rise, and it is understood that regulation of white adipose tissue (WAT) function is important to systemic metabolic homeostasis. Immune cells play a central role in the maintenance of WAT and their compositions change in number and inflammatory phenotype with the progression of obesity. Because of its energy-burning capabilities, brown adipose tissue (BAT) has become a focus of obesity research. While novel studies have focused on the function of brown adipocytes in thermogenesis, the tissue as a whole has not been immunologically characterized. METHODS: BAT immune cell populations were analyzed by flow cytometry and immunohistochemistry in diet-induced obese mice (3, 8, or 16 weeks of diet) and aged mice (1, 6–7, 10–15 months). RESULTS: Our data confirm the presence of macrophages and eosinophils, as previously reported, and also show that 20–30% of the immune cells in BAT are B cells. The number of B cells and eosinophils increases with diet-induced obesity while macrophages decrease. There is no change in number of any immune cell quantified with age. CONCLUSIONS: These studies reveal a novel finding of B220+ B cells in BAT, and show that BAT immune cell populations change in response to diet-induced obesity.