Next-generation sequencing of microRNAs uncovers expression signatures in polarized macrophages.

microRNAs (miRNAs) are small noncoding RNAs that regulate gene expression at a posttranscriptional level and play a crucial role in the development of cells of the immune system. Macrophages are essential for generating inflammatory reactions upon tissue damage and encountering of invading pathogens, yet modulation of their immune responses is critical for maintaining tissue homeostasis. Macrophages can present different phenotypes, depending on the cytokine environment they encounter in the affected tissues. In this study, we have identified expression signatures of miRNAs that are differentially regulated during maturation of monocytes and polarization of macrophages by cytokines. We present a comprehensive characterization of miRNA expression in human monocytes and M1, M2a, and M2c polarized macrophages, using next-generation sequencing. Furthermore, we show that miRNA expression signatures are closely related to the various immune functions of polarized macrophages and therefore are involved in shaping the diverse phenotypes of these cells. The miRNAs identified here serve as markers for identification of inflammatory macrophages involved in the development of immune responses. Our findings contribute to understanding the role of miRNAs in determining the macrophage function in healthy and diseased tissues.