Toward an Understanding of How Immune Cells Control Brown and Beige Adipobiology

Immune cells were recently found to have an unexpected involvement in controlling the thermogenic activity of brown and beige adipose tissue. Here, we review how macrophages, eosinophils, type 2 innate lymphoid cells, and T lymphocytes are linked to this process. In particular, the recruitment of alternatively activated macrophages and eosinophils is associated with brown fat activation and white fat browning. Conversely, pro-inflammatory immune cell recruitment represses the thermogenic activity of brown and beige adipose tissues via cytokines that inhibit noradrenergic signaling. Macrophages also influence the noradrenergic tone by degrading norepinephrine locally and by inhibiting sympathetic innervation over time. There is an increasingly recognized and complex role for immune cells in regulating the thermogenic activity of brown and beige adipose tissue. Villarroya et al. discuss the mechanistic basis behind this role, from changes in the sympathetic innervation to modulation of the norepinephrine pool in adipose tissues and alterations in local pro-inflammatory signaling.