LAM/TREM2+ macrophages release extracellular vesicles and extracellular lipid droplets which modulate the phenotype of recipient macrophages and homeostasis of skeletal muscle cells

The polarization of tissue-resident macrophages is influenced by a variety of signals from the immune system and the local tissue environment, including nutrition. Although it is known that the quality and quantity of ingested lipids have a significant effect on the lipid composition of extracellular vesicles and their fate, it is unknown how the nutritional environment modifies the release and the function of macrophage-derived EVs. In this study, we used a combination of palmitate and oleate (1:2, FFA) to generate lipid-associated TREM2-expressing macrophages (LAM/TREM2+) in vitro. Using various electron microscopy techniques (TEM, SEM, CryoEM) and fluorophores, we found that FFA overload not only induces lipid storage in LAM/TREM2+ macrophages, but also alters their morphology and reduces the diversity and the number of the lipid-derived structures they release. In addition, LAM/TREM2+ macrophages accumulated lipid droplets (LDs) below the plasma membrane and we discovered for the first time that they export and disseminate full LDs into their environment, in addition to extracellular vesicles, by using a cellular pathway associated to CD81. The use of 14C-palmitate confirmed the presence of 14C-triacylglycerols in the large extracellular vesicle pellet. LAM/TREM2+ macrophage-derived EVs induced TREM2 and Il-10 expression in recipient M0 macrophages. These data provide potential insights into how dietary factors and metabolic perturbations can shape the functions of macrophage-derived EVs in the context of metabolic diseases such as diabetes and obesity. In addition, LAM/TREM2+ macrophage-derived EVs modulated insulin-sensitivity, mitochondrial oxidative capacity, lipid profiles and the expressions of genes encoding extracellular matrix components in recipient skeletal muscle cells. Although previously postulated but never demonstrated, these data also highlight the LAM/TREM2+ macrophage-derived EVs as important players in SkM tissue renewal and metabolic homeostasis.