Abstract
Obesity-associated metabolic dysfunction is closely linked to chronic low-grade inflammation, or metaflammation, which is predominantly driven by changes in AT homeostasis. Macrophages, key components of the innate immune system, are central regulators of this inflammatory process. In lean AT, resident macrophages (AT-associated macrophages [ATMs]) exhibit an anti-inflammatory phenotype and support tissue homeostasis. However, during obesity, AT undergoes hypoxia, mechanical stress, and lipid overload, leading to immune cell infiltration and a phenotypic switch of ATMs toward a proinflammatory M1 profile. This shift contributes to systemic inflammation and obesity-associated metabolic risks. Here, we review the current understanding of macrophage polarization in obesity, highlighting the transcriptomic plasticity and functional heterogeneity of ATMs, their interactions within the AT microenvironment, and the formation of crown-like structures (CLSs) as a structural hallmark of AT inflammation. We also discuss the regulatory functions of transcription factors, such as hypoxia-inducible factor (HIF) 1α (HIF-1α) and peroxisome proliferator activated receptor gamma (PPARγ), that control the phenotypic switch of macrophages in healthy and obese ATs. Furthermore, we examined emerging macrophage subsets, such as CD9(+) and Trem2(+) lipid-associated macrophages (LAMs), and their dual roles in AT remodeling and inflammation. Understanding the complex network of macrophage activation in obese AT is essential for identifying therapeutic targets aimed at mitigating obesity-associated metabolic risk and restoring tissue function.