BK Polyomavirus MicroRNA Levels in Exosomes Are Modulated by Non-Coding Control Region Activity and Down-Regulate Viral Replication When Delivered to Non-Infected Cells Prior to Infection

In immunosuppressed patients, -variants emerge carrying rearranged non-coding control-regions ( ) that increase early viral gene region (EVGR) expression and replication capacity. also encodes microRNAs, which have been reported to downregulate EVGR-encoded large T-antigen transcripts, to decrease viral replication in infected cells and to be secreted in exosomes. To investigate the interplay of and microRNAs, we compared archetype- and infection in cell culture. We found that laboratory and clinical -strains show higher replication rates but significantly lower microRNA levels than archetype virus intracellularly and in exosomes. To investigate whether or increased EVGR activity modulated microRNA levels, we examined the ( ) , which has an archetype -architecture but shows increased EVGR expression due to point mutations inactivating one Sp1 binding site. We found that microRNA levels following ( ) infection were as low as in -variants. Thus, rearrangements are not required for lower miRNA levels. Accordingly, Sp1 siRNA knock-down decreased microRNA levels in archetype infection but had no effect on ( )- or . However, replication was downregulated by exosome preparations carrying -microRNA prior to infection. To explore the potential relevance in humans, urine samples from 12 natalizumab-treated multiple sclerosis patients were analysed. In 7 patients, were detected showing high urine loads but low microRNAs levels, whereas the opposite was seen in 5 patients with archetype . We discuss the results in a dynamic model of replication according to activity and exosome regulation, which integrates immune selection pressure, spread to new host cells and emergence.