Heterogeneity in extracellular vesicle secretion by single human macrophages revealed by super-resolution microscopy

The diverse origins, nanometre-scale and invasive isolation procedures associated with extracellular vesicles (EVs) mean they are usually studied in bulk and disconnected from their parental cell. Here, we used super-resolution microscopy to directly compare EVs secreted by individual human monocyte-derived macrophages (MDMs). MDMs were differentiated to be M0-, M1- or M2-like, with all three secreting EVs at similar densities following activation. However, M0-like cells secreted larger EVs than M1- and M2-like macrophages. Proteomic analysis revealed variations in the contents of differently sized EVs as well as between EVs secreted by different MDM phenotypes. Super resolution microscopy of single-cell secretions identified that the class II MHC protein, HLA-DR, was expressed on ∼40% of EVs secreted from M1-like MDMs, which was double the frequency observed for M0-like and M2-like EVs. Strikingly, human macrophages, isolated from the resected lungs of cancer patients, secreted EVs that expressed HLA-DR at double the frequency and with greater intensity than M1-like EVs. Quantitative analysis of single-cell EV profiles from all four macrophage phenotypes revealed distinct secretion types, five of which were consistent across multiple sample cohorts. A sub-population of M1-like MDMs secreted EVs similar to lung macrophages, suggesting an expansion or recruitment of cells with a specific EV secretion profile within the lungs of cancer patients. Thus, quantitative analysis of EV heterogeneity can be used for single cell profiling and to reveal novel macrophage biology.