Adipose tissue macrophages: Regulators of adipose tissue immunometabolism during obesity

BACKGROUND: Adipose tissue macrophages (ATMs) are a well characterized regulator of adipose tissue inflammatory tone. Previously defined by the M1 vs M2 classification, we now have a better understanding of ATM diversity that departs from the old paradigm and reports a spectrum of ATM function and phenotypes in both brown and white adipose tissue. SCOPE OF REVIEW: This review provides an updated overview of ATM activation and function, ATM diversity in humans and rodents, and novel ATM functions that contribute to metabolic homeostasis and disease. MAJOR CONCLUSIONS: While the paradigm that resident ATMs predominate in the lean state and obesity leads to the accumulation of lipid-associated and inflammatory ATMs still broadly remains rigorously supported, the details of this model continue to be refined and single cell data provide new insight into ATM subtypes and states.