OBJECTIVE—To establish the mechanism of the phenotypic switch of adipose tissue macrophages (ATMs) from an alternatively activated (M2a) to a classically activated (M1) phenotype with obesity.

RESEARCH DESIGN AND METHODS—ATMs from lean and obese (high-fat diet–fed) C57Bl/6 mice were analyzed by a combination of flow cytometry, immunofluorescence, and expression analysis for M2a and M1 genes. Pulse labeling of ATMs with PKH26 assessed the recruitment rate of ATMs to spatially distinct regions.

RESULTS—Resident ATMs in lean mice express the M2a marker macrophage galactose N-acetyl-galactosamine specific lectin 1 (MGL1) and localize to interstitial spaces between adipocytes independent of CCR2 and CCL2. With diet-induced obesity, MGL1⁺ ATMs remain in interstitial spaces, whereas a population of MGL1⁻CCR2⁺ ATMs with high M1 and low M2a gene expression is recruited to clusters surrounding necrotic adipocytes. Pulse labeling showed that the rate of recruitment of new macrophages to MGL1⁻ ATM clusters is significantly faster than that of interstitial MGL1⁺ ATMs. This recruitment is attenuated in Ccr2^−/− mice. M2a- and M1-polarized macrophages produced different effects on adipogenesis and adipocyte insulin sensitivity in vitro.

CONCLUSIONS—The shift in the M2a/M1 ATM balance is generated by spatial and temporal differences in the recruitment of distinct ATM subtypes. The obesity-induced switch in ATM activation state is coupled to the localized recruitment of an inflammatory ATM subtype to macrophage clusters from the circulation and not to the conversion of resident M2a macrophages to M1 ATMs in situ.