INTRODUCTION
Adipose tissue (AT) is a dynamic organ composed of lipid-storing adipocytes, adipocyte stem cells (ASCs), and resident immune cells. Macrophages are among the most abundant immune cells in AT and, depending on the physiological context, can maintain tissue function or drive inflammation. Prior studies have characterized inflammatory macrophages in obesity, but the identity and role of resident macrophages under steady state and their interaction with stromal populations is poorly understood. Recent evidence indicates that macrophage specialization may extend beyond tissue-level niches into more refined “subtissular” compartments. Whether such localization impacts AT macrophage identity and function has remained unresolved.
RATIONALE
We hypothesized that subtissular macrophage niches within AT serve as instructive hubs for ASC fate specification. Using spatial, transcriptomic, and functional analyses in mice and humans, we investigated whether specific macrophage subsets localized to defined AT compartments, interacted with progenitor cells, and regulated adipogenesis.
RESULTS
We defined three distinct AT macrophage (ATM) populations based on their subtissular localization and molecular signatures: parenchymal (pATMs), capsular (cATMs), and septal (sATMs). The sATMs, characterized by the expression of CD209b and LYVE1 (CD209b+LYVE1+), were selectively enriched in intralobular septa, a dense collagen- and hyaluronan-rich structure traversing adipocyte lobules, in close proximity to early CD26 expressing ASCs (ASCsCD26+). These sATMs were primarily embryonic-derived, long-lived, and distinct from monocyte-derived, inflammation-associated ATMs. In response to a high-fat diet (HFD) challenge, sATMs remained spatially restricted and resisted monocyte replacement whereas pATMs increased in number through both local proliferation and monocyte influx. Selective genetic depletion of sATMs using a Cd209b-driven DTR model enhanced thermogenesis, WAT beiging, increased oxygen consumption, and protection from HFD-induced obesity, independently of food intake. These mice also exhibited improved glucose tolerance and insulin sensitivity, along with reduced liver steatosis.
Single-cell and bulk RNA-seq of AT stromal fractions identified ASCsCD26+ as highly proliferative, early progenitors localized in the septa. Upon sATM depletion, ASCCD26+ frequency declined, and their transcriptional profiles shifted toward a thermogenic, brown/beige fate. Ligand-receptor interaction modeling pinpointed TGFβ1 as a signal derived from sATMs that acted on ASCs via TGFβR1/2. Specific deletion of Tgfb1 in LYVE1+ or TIM4+ macrophages in vivo recapitulated sATM depletion phenotypes. In vitro, ASCsCD26+ from TGFβ1-deficient mice showed impaired white adipogenic differentiation and greater capacity for thermogenic conversion.
We identified a conserved population of CD206+LYVE1+ human septal ATMs (hsATMs) in obese patients, expressing high levels of TGFB1 and residing adjacent to CD26+CD55+ early human ASCs (hASCsCD26+). Multiplex imaging confirmed close physical proximity between hsATMs and hASCsCD26+ within septal zones of human WAT.
CONCLUSION
Subtissular localization may be a critical determinant of macrophage function and positions the adipose septum as a discrete immunometabolic niche essential for tissue remodeling. Our findings define a spatially confined niche containing resident macrophages and stem cells within the septum of white adipose tissue that governs adipose tissue plasticity and energy balance. Septal ATMs provided localized TGFβ1 signals that direct early adipocyte progenitors toward white adipocyte differentiation while restraining thermogenic potential. Depletion or functional inactivation of this sATM niche shifted the ASC fate toward beige adipogenesis, enhanced energy expenditure, and conferred systemic metabolic benefits in obesity. This regulatory axis between septal resident macrophages and adipocyte progenitors develops understanding of immune-stromal cross-talk in metabolic tissues. Targeting the sATM–TGFβ1–ASC axis may offer therapeutic strategies to boost WAT beiging and counteract obesity and insulin resistance without triggering inflammation.
Spatially distinct macrophage niches govern adipogenesis.
White fat is organized into three anatomical compartments—septum, capsule, and parenchyma—each containing distinct populations of adipose tissue macrophages (ATMs). Septal macrophages (sATMs) are positioned in close contact with a population of early fat stem cells expressing CD26 (ASCsCD26+) within the septal niche. This spatial association enables sATMs to guide stem cell fate through TGFβ1 signaling, promoting the formation of energy-storing white fat cells. [Figure created using BioRender.com]
