MicroRNAs grow up in the immune system

MicroRNA (miRNA) target predictions support a view in which each miRNA regulates translation and stability of several hundred messenger RNAs (mRNAs). Studies that overexpress individual miRNAs typically uncover relative subtle inhibition of the predicted targets. Accordingly, most miRNAs expressed in a given cell type may serve the function to broadly inhibit cell-type-inappropriate gene expression and deepen a pre-existing differentiation program. However, recent functional analyses of miRNAs in the immune system reveal that many cellular decisions are controlled by single miRNAs that entail significant downregulation of one or few target proteins. Investigations of these miRNA/mRNA pairs showed that miRNA-adjusted target protein levels are crucial at specific cellular transition points. Here, we will review recent advances in the regulation of the miRNA pathway and discuss how miRNAs control immune functions.