CCR2 and obesity due to melanocortin 4 receptor deficiency: Consistent with this theory, Lumeng et al. demonstrated that in lean WAT, resident ATM are polarized toward an M2 state derived from CCR2− monocytes, whereas obesity leads to an increase in circulating CCR2+ monocytes, which are the source of the infiltrating M1 macrophages in obese WAT [13].