Virus Entry is a Major Determinant of HCMV Latency in Monocytes

Human Cytomegalovirus (HCMV) infection can result in either productive or latent infection, the latter being the basis for the virus life-long persistence. Intriguingly, monocytes, which support latent infection, become permissive to productive infection upon differentiation to macrophages. However, the molecular factors explaining these differentiation-driven differences are not fully understood and have been so far attributed to chromatin-mediated repression of the viral genome. Here, by using metabolic labeling of newly synthesized RNA early in monocyte and macrophage infection, we discover a major early block in viral gene expression, and viral transcripts are barely detected in infected monocytes. By unbiasedly analyzing the changes between monocytes and their differentiated counterparts, we reveal that the levels of several cell surface proteins involved in HCMV entry are upregulated upon monocyte to macrophage differentiation, and correspondingly we uncover HCMV entry into monocytes compared to macrophages is extremely inefficient. Remarkably, ectopic expression of a canonical HCMV entry receptor in monocytes facilitates productive infection of these cells, demonstrating that given efficient viral entry, monocytes, like macrophages, have the capacity to support productive infection. Among the cell surface proteins that are upregulated upon monocyte differentiation are several integrins, which we show play an important role in HCMV entry into macrophages, partially explaining the differences in viral entry. Overall, our findings reveal that a previously unrecognized major barrier for productive infection in monocytes is entry, adding a critical layer to the paradigm of HCMV latency.