Virus-specific host miRNAs: antiviral defenses or promoters of persistent infection?

The discovery of host-microRNA (miRNA) targets in the genomes of many vertebrate viruses indicates that the corresponding miRNAs are a part of the host's innate antiviral defense. However, given that viruses evolve much faster than host miRNAs, it is surprising that viral variants lacking these 'antiviral' miRNA target sequences have not become established. We present an alternate view that miRNAs are among the host molecules that viruses co-opt to suppress their own replication to evade immune elimination and establish a persistent infection. The presence of host-miRNA targets in the genomes of rapidly evolving viruses probably reflects the adaptation of these viruses to the cellular miRNA milieu.