Physiological and pathological roles for microRNAs in the immune system

Key Points MicroRNAs (miRNAs) are expressed by cells that constitute the immune system, and they function by repressing specific mRNA targets at the post-transcriptional level. The biosynthesis of miRNAs involves several levels of regulation, some of which are influenced by inflammation. Specific miRNAs modulate haematopoietic cell development. miRNAs regulate both innate and adaptive immune responses. miRNA expression levels are dysregulated in diseases of immunological origin, such as cancer and autoimmunity. Altered miRNA expression can subsequently exacerbate disease severity. Research into miRNA is a recent development, and therefore many aspects of miRNA biology remain unexplored. Here, the authors review the fast-moving field of gene expression regulation by microRNAs. They describe how microRNAs influence many stages of innate and adaptive immune responses and how they might precipitate cancer and autoimmune disease if dysregulated. Mammalian microRNAs (miRNAs) have recently been identified as important regulators of gene expression, and they function by repressing specific target genes at the post-transcriptional level. Now, studies of miRNAs are resolving some unsolved issues in immunology. Recent studies have shown that miRNAs have unique expression profiles in cells of the innate and adaptive immune systems and have pivotal roles in the regulation of both cell development and function. Furthermore, when miRNAs are aberrantly expressed they can contribute to pathological conditions involving the immune system, such as cancer and autoimmunity; they have also been shown to be useful as diagnostic and prognostic indicators of disease type and severity. This Review discusses recent advances in our understanding of both the intended functions of miRNAs in managing immune cell biology and their pathological roles when their expression is dysregulated.