MicroRNA (miRNA) transfer via exosomes may mediate cell-to-cell communication. Interestingly, specific miRNAs are enriched in exosomes in a cell-type-dependent fashion. However, the mechanisms whereby miRNAs are sorted to exosomes and the significance of miRNA transfer to acceptor cells are unclear. We used macrophages and endothelial cells (ECs) as a model of heterotypic cell communication in order to investigate both processes. RNA profiling of macrophages and their exosomes shows that miRNA sorting to exosomes is modulated by cell-activation-dependent changes of miRNA target levels in the producer cells. Genetically perturbing the expression of individual miRNAs or their targeted transcripts promotes bidirectional miRNA relocation from the cell cytoplasm/P bodies (sites of miRNA activity) to multivesicular bodies (sites of exosome biogenesis) and controls miRNA sorting to exosomes. Furthermore, the use of Dicer-deficient cells and reporter lentiviral vectors (LVs) for miRNA activity shows that exosomal miRNAs are transferred from macrophages to ECs to detectably repress targeted sequences.

Expression of the mannose receptor (MRC1/CD206) identifies macrophage subtypes, such as alternatively activated macrophages (AAMs) and M2-polarized tumor-associated macrophages (TAMs), which are endowed with tissue-remodeling, proangiogenic, and protumoral activity. However, the significance of MRC1 expression for TAM's protumoral activity is unclear. Here, we describe and characterize miR-511-3p, an intronic microRNA (miRNA) encoded by both mouse and human MRC1 genes. By using sensitive miRNA reporter vectors, we demonstrate robust expression and bioactivity of miR-511-3p in MRC1(+) AAMs and TAMs. Unexpectedly, enforced expression of miR-511-3p tuned down the protumoral gene signature of MRC1(+) TAMs and inhibited tumor growth. Our findings suggest that transcriptional activation of Mrc1 in TAMs evokes a genetic program orchestrated by miR-511-3p, which limits rather than enhances their protumoral functions. Besides uncovering a role for MRC1 as gatekeeper of TAM's protumoral genetic programs, these observations suggest that endogenous miRNAs may operate to establish thresholds for inflammatory cell activation in tumors.