Novel role of bone morphogenetic protein 9 (BMP9) in innate host responses to HCMV infection

Herpesviruses modulate immune control to secure lifelong infection. The diverse mechanisms Human Cytomegalovirus (HCMV) employs in this regard can reveal unanticipated aspects of cellular signalling involved in antiviral immunity. Here, we describe a novel relationship between the transforming growth factor-beta (TGF-β) family cytokine bone morphogenetic protein 9 (BMP9) and HCMV infection. We identified cross-talk between BMP9-induced and type I interferon (IFN) receptor-mediated signaling, showing that BMP9 boosts the transcriptional response to and antiviral activity of IFNβ, and thereby enhances restriction of HCMV replication. We also show that BMP9 is secreted by human fibroblasts upon HCMV infection. However, HCMV infection impairs BMP9-induced enhancement of the response to IFNβ, indicating that this innate immune signalling role of BMP9 is actively targeted by HCMV. Indeed, we show that transmembrane proteins US18 and US20, which downregulate type I BMP receptors, are necessary and sufficient to cause inhibition of BMP9-mediated boosting of the antiviral response to IFNβ, so that HCMV lacking US18 and US20 is more sensitive to IFNβ. These results demonstrate that HCMV has a mutually antagonistic relationship with BMP9 and substantially extend the growing body of evidence that BMP signalling is an underappreciated modulator of innate immunity to viral infection.