Phenotypic switching of adipose tissue macrophages with obesity is generated by spatiotemporal differences in macrophage subtypes.
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.


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.
Lumeng
Carey N
CN
Life Sciences Institute, University of Michigan, Ann Arbor, MI, USA.
DelProposto
Jennifer B
JB
Westcott
Daniel J
DJ
Saltiel
Alan R
AR
D000273:Adipose Tissue / Q000166:cytology / Q000502:physiology*; D000818:Animals; D001212:Asialoglycoproteins / Q000172:deficiency / Q000235:genetics / Q000502:physiology; D004032:Diet; D005434:Flow Cytometry; D015870:Gene Expression; D037181:Lectins, C-Type / Q000172:deficiency / Q000235:genetics / Q000502:physiology; D008262:Macrophage Activation / Q000502:physiology*; D008264:Macrophages / Q000145:classification* / Q000166:cytology / Q000502:physiology*; D008565:Membrane Proteins / Q000172:deficiency / Q000235:genetics / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009765:Obesity / Q000235:genetics* / Q000503:physiopathology; D010641:Phenotype; D054390:Receptors, CCR2 / Q000172:deficiency / Q000235:genetics / Q000502:physiology
D001212:Asialoglycoproteins; C515335:Ccr2 protein, mouse; C064720:Clec10a protein, mouse; D037181:Lectins, C-Type; D008565:Membrane Proteins; D054390:Receptors, CCR2