Human cytomegalovirus-encoded G protein-coupled receptor (GPCR), UL78, regulates viral reactivation.

Human cytomegalovirus (CMV) is a ubiquitous pathogen that establishes life-long, latent infection in hematopoietic cells. Immune-competent individuals are usually asymptomatic for disease. However, immune dysregulation in latently-infected individuals can result in viral reactivation, often causing further complications. Viral gene transcription during latency is restricted, although the CMV-encoded G-protein coupled receptor homologs, US28 and UL78, are expressed. We and others find US28 is critical for establishing and maintaining viral latency, in part, through regulating host cell signaling. How US28 switches from pro-latent to pro-lytic during reactivation, however, is unknown, though our findings herein reveal a role for UL78. Myeloid cells infected with a UL78 ORF deletion mutant maintain viral latency yet fail to efficiently reactivate. However, the UL78 G protein-coupling domain is not required for reactivation, suggesting UL78-mediated signaling is not critical for reactivation. Prior work revealed UL78 and US28 interact, resulting in altered US28-mediated signaling. Additionally, we showed US28 attenuates ERK phosphorylation during latency, while ERK is phosphorylated upon reactivation; however, the mechanism underlying this switch is unknown. Thus, we hypothesized the UL78:US28 interaction is important for altering US28-mediating signaling upon viral reactivation. We find US28 and UL78 interact during lytic infection of fibroblasts and colocalize in myeloid cells upon their differentiation. Further, reactivation in myeloid cells latently-infected with wild-type virus results in upregulated ERK phosphorylation, while parallel cultures infected with the UL78-deficient virus fail to do so. Our data reveal the first function for UL78 in myeloid cells, where it influences cellular signaling to switch from pro-latent to pro-lytic.