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.