Specific macrophage subsets accumulate in human subcutaneous and omental fat depots during obesity

Obesity is a chronic inflammatory disease associated with adipose tissue macrophage (ATM) activation. ATMs from lean mice contribute to tissue homeostasis by their M2-oriented polarization, whereas obesity leads to an increase of M1 inflammatory ATMs that underlies obesity-related metabolic disorders. In humans, studies characterizing ATMs and their functional status are limited. Here we investigated ATM phenotype in visceral (VAT) and subcutaneous (SAT) adipose tissue from healthy lean and obese individuals using two molecules previously identified as markers of M1-like and M2-like/tissue-resident macrophages, the C-type lectin CLEC5A and the scavenger receptor CD163L1, respectively. CD163L1 was expressed by the majority of ATMs, and CD163L1⁺ ATM density was greater with respect to cells expressing the pan-macrophage markers CD68 or CD11b. ATM counts in SAT, but not in VAT, increased in obese compared to lean individuals, measured with the three markers. Accordingly, CD163L1, CD68 and ITGAM gene expression was significantly enhanced in obese with respect to control individuals only in SAT. CLEC5A⁺ ATMs had a proinflammatory profile and were abundant in the lean VAT, but their density diminished in obesity. The only ATM subset that increased its counts in the obese VAT had a mixed M1-like (CD11c⁺ CD163^- CD209^- ) and M2-like (CLEC5A^- CD206⁺ ) phenotype. ATM expansion was dominated by a subset of M2-like macrophages (CD11c^- CLEC5A^- CD163⁺ CD206⁺ CD209⁺ ) in the obese SAT, with a minor contribution of a CD11c⁺ CLEC5A^- ATM subpopulation. Thus, both SAT and VAT seems to limit inflammation during obesity by differentially altering their ATM subset composition.