Viral encoded microRNAs have been shown to play key roles in regulating gene expression and life cycle of human herpes viruses. Latency is one of the hallmarks of Human cytomegalovirus (HCMV or HHV5) life cycle and its control may have immense practical applications. The present study aims to identify HCMV encoded microRNAs during the latency phase of the virus. We used a highly sensitive newly developed real time PCR (RTPCR) assay that involves pre-amplification step before RTPCR to detect HMCV encoded microRNAs during its latency in purified monocytes and PBMC from HCMV IgG positive donors and in latently infected monocytic THP-1 cell lines. In contrast to lytic infection, in peripheral blood cells, during the latency phase, only eight HCMV encoded microRNAs were detected. Six originated from the UL region of the virus genome, and three from the US region. Reactivation of the virus from latency in monocytes obtained from the same donors, using dexamethasone, restored the expression of all known HCMV encoded miRNAs including those that were absent during latency. Establishing of latency in THP-1 cells resulted in the expression of the same eight HCMV miRNAs that are expressed in healthy donors, following gradual disappearance of five microRNAs. Interestingly, we could detect a shift in the abundance of the two arms of mir-US29 between the productive and latency stages of the virus, suggesting that the star "passenger" form of this microRNA is actively expressed during latency. Taken together, our study demonstrates that HCMV expresses both in vivo and in vitro only a subset of its microRNAs during its latency phase which may indicate that they play central role in maintenance and reactivation of latency.
