MicroRNAs in the regulation of TLR and RIG-I pathways

The innate immune system recognizes invading pathogens through germline-encoded pattern recognition receptors (PRRs), which elicit innate antimicrobial and inflammatory responses and initiate adaptive immunity to control or eliminate infection. Toll-like receptors (TLRs) and retinoic acid-inducible gene I (RIG-I) are the key innate immune PRRs and are tightly regulated by elaborate mechanisms to ensure a beneficial outcome in response to foreign invaders. Although much of the focus in the literature has been on the study of protein regulators of inflammation, microRNAs (miRNAs) have emerged as important controllers of certain features of the inflammatory process. Several miRNAs are induced by TLR and RIG-I activation in myeloid cells and act as feedback regulators of TLR and RIG-I signaling. In this review, we comprehensively discuss the recent understanding of how miRNA networks respond to TLR and RIG-I signaling and their role in the initiation and termination of inflammatory responses. Increasing evidence also indicates that both virus-encoded miRNAs and cellular miRNAs have important functions in viral replication and host anti-viral immunity.