Lipid-associated macrophages in the tumor-adipose microenvironment facilitate breast cancer progression

The tumor-adipose microenvironment (TAME) is a universal microecosystem, that is characterized by the dysfunction of lipid metabolism, such as excessive free fatty acids (FFAs). Macrophages are the most abundant immune cell type within TAME, although their diversity in the TAME is not clear. We first reveal that infiltration of M2-like macrophages in the TAME is associated with poor survival in breast cancer. To explore lipid-associated alterations in the TAME, we also detected the levels of FFAs transporters including fatty acid binding proteins (FABPs) and fatty acid transport protein 1 (FATP1). The results indicated that expression of fatty acid transporters in the TAME is tightly linked to the function of macrophages and predicts survival in breast cancer. To explore the impact of FFAs transporters on the function of macrophages, we performed single-cell RNA sequencing (scRNA-seq) and spatial transcriptomics. Consequently, we identified a special subpopulation of macrophages defined as lipid-associated macrophages (LAMs), highly expressed macrophage markers (CD163, SPP1 and C1QC), genes involved in lipid metabolism (FABP3, FABP4, FABP5, LPL and LIPA) and some lipid receptors (LGALS3 and TREM2). Functionally, LAMs were characterized by a canonical functional signature of M2-like macrophages, lipid accumulation and enhancing phagocytosis, and they were mostly distributed in tumor-adipose junctional regions. Finally, the allograft cancer mouse models confirmed that LAMs depletion in the TAME synergizes the antitumorigenic effects of anti-PD1 therapy. In summary, we defined a novel subtype of macrophages in the TAME, that has unique features and clinical outcomes.