MCV-miR-M1 targets the host-cell immune response resulting in the attenuation of neutrophil chemotaxis

Virus-encoded miRNAs are emerging as key regulators of persistent infection and host-cell immune evasion. Merkel cell polyomavirus (MCPyV), the predominant aetiological agent of Merkel cell carcinoma (MCC), encodes a single miRNA, MCV-miR-M1, which targets the oncogenic MCPyV large T antigen (LT). MCV-miR-M1 has previously been shown to play an important role in establishment of long-term infection, however, the underlying mechanism is not fully understood. A key unanswered question is whether, in addition to auto-regulating LT, MCV-miR-M1 also targets cellular transcripts to orchestrate an environment conducive for persistent infection. To address this, we adopted an RNA-Seq-based approach to identify cellular targets of MCV-miR-M1. Intriguingly, bioinformatics analysis of transcripts that are differentially expressed in cells expressing MCV-miR-M1 revealed several genes implicated in immune evasion. Subsequent target validation led to the identification of the innate immunity protein, SP100, as a direct target of MCV-miR-M1. Moreover, MCV-miR-M1-mediated modulation of SP100 was associated with a significant decrease in CXCL8 secretion, resulting in the attenuation of neutrophil chemotaxis towards Merkel cells harbouring synthetic MCPyV. Based on these observations we propose that MCV-miR-M1 targets key immune response regulators to help facilitate persistent infection, which is a pre-requisite for cellular transformation in MCC.